I. Histology of Bone Types Normal bone is lamellar - highly organised in mineralised plates, relatively hypocellular, and stress- oriented. It can be cortical or cancellous. Cortical bone makes up 80% of the skeleton, and is found in the outer shell of bone. It is composed of tightly-packed osteons or Haversian systems, made up of small concentric lamellar cylinders surrounding a central vascular channel, connected by Haversian (Volkmann’s) canals. These canals contain capillaries, arterioles, venules, nerves and possibly lymphatics. Lying between these osteons are interstitial lamellae. Fibrils often connect lamellae but do not cross cement lines, which form the outer border of osteons. The intraosseous circulation provides nutrition. Cortical bone has a slow turnover rate, a relatively high Young’s modulus, and a high resistance to bending and torsion. Cancellous bone is less dense and more elastic than cortical bone, has a smaller Young’s modulus, and a higher turnover rate. It is organised in trabecular struts, with lamellae running parallel to the trabeculae. It is found in the epiphyseal and metaphyseal regions of long bones and throughout the interior of short bones. Immature or pathologic bone is woven and more random, with more osteocytes than lamellar bone. It is the product of rapid bone formation, resulting in an irregular, disorganised pattern of collagen orientation and osteocyte distribution. It is found in embryonic and foetal development, and in healthy adults at ligament and tendon insertions. It also occurs in response to bony injury and dramatic changes in mechanical stimulation. It provides a temporary mechanical adjunct to allow bone to maintain or return quickly to its role as a structural support. Cellular Biology Osteoblasts form osteoid, the nonmineralised component of bone matrix. They differentiate from mesenchymal progenitor cells, and contain extensive endoplasmic reticulum with multiple cisternae, well- developed Golgi bodies, and numerous ribosomes and mitochondria, allowing for their abundant synthesis and secretion of matrix. Their differentiation in vivo is stimulated by various cytokines such as the interleukins, insulin-derived growth factor (IDGF) and platelet-derived growth factor (PDGF). They produce pro-α 1 collagen (a major component of osteoid), osteocalcin (in response to 1, 25(OH)2D3 ) and bone morphogenetic proteins. They initiate mineralization of osteoid material, possibly by modulating electrolyte fluxes between the extracellular fluid volume and osseous fluid. Osteoblasts are connected by numerous gap junctions, facilitating electrical/chemical communication between cells. Bone-lining cells are narrow, flattened cells which differentiate from osteoblasts but have fewer active organelles than osteoblasts. They envelop quiescent bone surfaces including endosteal, periosteal and intracortical surfaces. Their function is to encase the bone surface and moderate site-specific mineralization or resorption on activation by PTH. Osteocytes maintain bone, and comprise 90% of all cells in the mature skeleton. They originate as osteoblasts which have been trapped within osteoid formed by surrounding osteoblasts, forming a lacuna. They have a single nucleus, and an increased nucleus: cytoplasm ratio. Osteocytes are smaller in size and have fewer numbers of organelles than osteoblasts, therefore are not as active in matrix production. They maintain the cytoplasmic extensions of the osteoblasts, creating a large canalicular system - essentially a "syncytium". This system may transport biophysical data to cells within and at the surface of bone. Osteocytes play a role in controlling the extracellular concentration of calcium and phosphate - they are directly stimulated by Calcitonin and inhibited by PTH. Osteoclasts act in opposition to osteoblasts, and their role is to resorb bone. Thus bone formation and resorption are coupled. They are multinucleated, irregularly-shaped giant cells which arise from haematopoietic cell lines (monocyte progenitors form giant cells by fusion). Recognition and adherence to the bone surface is mediated via intracellular contractile proteins attached to integrins - this leads to the formation of an apical clear zone and ruffled border (thus increasing surface area). Vacuolar proton- ATPase pumps then localise to this ruffled border and act with intracellular carbonic anhydrase II to lower the pH of the extracellular bone compartment, thus forming a resorption pit, or Howship’s Lacuna. The lowered pH increases the solubility of hydroxyapatite crystals, and the exposed organic matrix is then digested by lysosomal enzymes. Osteoclasts have specific Calcitonin receptors, which are induced by 1, 25(OH)2D3 , PTH and TNF. Osteoprogenitor cells develop into osteoblasts. They are localised mesenchymal cells lining Haversian canals, endosteum and periosteum, pending the stimulus to differentiate into osteoblasts. Matrix Matrix is made up of organic components (40% dry weight in mature bone) and inorganic components (60% dry weight). 1. Organic Components a. Collagen Collagen is composed mainly of type I collagen, and provides bone’s tensile strength, comprising 90% of bone matrix. Collagen is composed of a triple helix of tropocollagen (two α 1 and one α 2 chains). Bone collagen molecules align themselves head to tail longitudinally, and with a quarter stagger laterally, to produce a collagen fibril. "Hole zones" (gaps) in the collagen fibril are located between the ends of molecules, while "pores" are located between the sides of parallel molecules - calcification is thought to occur within hole zones and pores. Cross-linking leads to decreased solubility and increased tensile strength. b. Proteoglycans Proteoglycans contribute to the compressive strength of bone. Their function is unclear, but they are thought to play a role in the reservation of space for bone development, the binding and availability of local growth factors, and the deposition and structuring of collagen fibrils. Inhibit mineralization. c. Osteocalcin Osteocalcin is produced by osteoblasts and makes up 10-20% of the collagenous protein of bone. It attracts osteoclasts, therefore its function is associated with bone remodelling. Increased synthesis is induced 1,25(OH)2D3 and inhibited by PTH. Levels in urine and serum are elevated in Paget ’s disease, renal osteodystrophy and hyperparathyroidism. Bone which is deficient in osteocalcin does not undergo resorption in vivo and is associated with premature closure of epiphyseal growth plates. d. Osteonectin Osteonectin is secreted by platelets, osteoblasts and osteoclasts. It is thought to play a role in the regulation of calcium or the organisation of material within the matrix, as it binds collagen, has a high affinity for both calcium and hydroxyapatite, and localises to crystal-producing matrix vesicles. e. Osteopontin Osteopontin mediates the attachment of cells to bone matrix, similar to integrins. It contains the Arg-Gly- Asp (RGD) amino acid sequence, which is preferentially recognised by cell surface integrin molecules. f. Growth Factors and Cytokines These occur in small amounts in bone matrix. They include Transforming Growth Factor b (TGF-b ), Insulin-like Growth Factor (IGF), Interleukins (IL-1, IL-6), Bone Morphogenic Proteins (BMP1-6) , Platelet- Derived Growth Factor (PDGF), Colony Stimulating Factors (CSFs), Heparin-Binding Growth Factors (HBGFs), Tumour Necrosis Factor α (TNF-a ), Prostaglandins (PGs) and Leukotrienes. i) Transforming Growth Factor b TGF-b is one of the most prevalent growth factors found in bone matrix. It is released during bone absorption, and enhances osteoblast activity (via elevated collagen synthesis), increases the bone apposition rate, and inhibits the differentiation of osteoclasts. Its activity is regulated by its conversion into an active peptide, which, in turn, is controlled by PTH. ii) Insulin-like Growth Factor In bone tissue, IGF-1 and IGF-2 are produced by fibroblasts and osteoblasts. Synthesis of IGF-1 is enhanced by PTH and PGE2 , and diminished by cortisol. IGF-1 increases bone apposition rates by increasing preosteoblast cell replication and osteoblastic collagen synthesis, and decreasing bone resorption. Overall, IGF seems to play a role in the maintenance of normal bone mass. iii) Interleukins IL-1 is a powerful stimulant of bone resorption. It is mitogenic for osteoclast precursors, and it promotes the proliferation and differentiation of committed precursors. Its action is potentiated by TNF-a , and it acts synergistically with PTH and PTH-related peptide. IL-6 is mainly responsible for the acute-phase protein response, and plays a major role as a paracrine growth factor in myeloma. It potentiates the bone-resorbing effects of IL-1 and TNF-a by stimulating early osteoclast lineage mitogenesis. Its synthesis is regulated by PTH, IL-1 and 1,25(OH)2D3, and is performed by osteoblasts. iv) Bone Morphogenic Proteins BMPs are members of the TGF-b superfamily of growth factors. They act on progenitor cells to induce differentiation into osteoblasts and chondroblasts. They are responsible for ectopic bone formation by certain tumour cells, epithelial cells and demineralised bone. BMPs appear to be stored with bone matrix and released with the resorptive activity that often follows injury. 2. inorganic components a. Calcium Hydroxyapatite [Ca10(PO4)6(OH)2] Know this formula. Calcium hydroxyapatite provides the compressive strength of bone. It makes up most of the inorganic matrix, and is responsible for mineralization of the matrix. (Mineralization is the transformation of hydroxyapatite from a soluble to a solid form, starting at multiple nucleation sites and then spreading by accretion, or crystal growth) Primary mineralization occurs in gaps in collagen, while secondary mineralization occurs at the periphery. b. Osteocalcium Phosphate (Brushite) Osteocalcium phosphate comprises the remainder of inorganic matrix. Bone Remodelling Bone remodelling is affected by mechanical function, according to Wolff’s Law, which attempts to predict bone adaptation in the face of an altered loading environment. Generally, remodelling occurs in response to stress, and responds to piezoelectric charges (compression causes negative potential, which stimulates osteoblast activity & bone formation; tension causes positive potential, leading to osteoclast stimulation). Bone is dynamic - coordinated osteoblast and osteoclast activity results in continuous remodelling of both cortical and cancellous bone throughout life. Cortical bone remodelling occurs by osteoclasts which tunnel through to the bone forming "cutting cones", followed by sheets of osteoblasts which deposit osteoid in lamellae. Cancellous bone remodelling involves osteoclast resorption of bone, followed by the deposition of osteoid by osteoblasts. Bone Circulation Anatomy Bones arnes receive blood well-supplied with arteries, receiving 5% of cardiac output under basal conditions. Long bud from periosteal arteries, nutrient arteries, and metaphyseal and epiphyseal arteries. Periosteal arteries enter the body of a bone at various points and supply the outer third of the cortex of the diaphysis. This is a low pressure system. Nutrient arteries are branches of major systemic arteries, and pass obliquely through the diaphyseal cortex to reach the medullary canal. Here they divide into longitudinally directed branches which supply at least the inner two-thirds of mature diaphyseal cortex. This is a high pressure system. Metaphyseal and epiphyseal arteries supply the ends of bone, and arise mainly from the periarticular vascular plexus. In growing bones they supply growth plates, so significant disruptions of blood flow disturb bone growth. Physiology Direction of flow In mature bone, arterial blood flows centrifugally from the high pressure nutrient arteries to the low pressure periosteal arteries. If a displaced fracture causes interruption of the nutrient artery system, the flow reverses as the periosteal system now predominates, so blood flow becomes centripetal. In developing bone, arterial flow is centripetal, because the periosteum is highly vascularized and is the major component of blood flow in bone. In mature bone, venous flow is centripetal - cortical capillaries drain to venous sinusoids, which then drain to emissary veins. Fluid compartments of bone are as follows: extravascular 65%, lacunar 6%, Haversian 6%, red blood cells 3%, other 20%. As with other tissues and organs, hypoxia, such as at high altitude, causes an increase in blood flow to bone, as does hypercapnia and sympathectomy. Fracture Healing After a bony injury, blood flow to the site initially decreases due to disruption of vascular structures. Blood flow then gradually increases over the following hours and days, peaking at around 2 weeks. By 3-5 months, flow has returned to normal. Fracture healing is largely reliant on bone blood flow - reaming of bone devascularises the central 50-80% of cortex, and thus is associated with most delayed vascularisation of all types of fixation. Regulation Blood flow to bone is regulated by humeral, metabolic and autonomic signals. The osseous vessels express various vasoactive receptors which may be exploited in the future by pharmacological agents for the treatment of bone diseases related to circulatory disturbances (eg. osteonecrosis, fracture nonunions). Tissues surrounding bone Periosteum is a dense connective tissue membrane which covers bone. It is composed of an outer fibrous layer, which is contiguous with joint capsules, and an inner, or cambium, layer which is loose, more vascular, and contains osteoblasts (if bone formation is in progress on the surface) and osteoblast precursors. If bone formation is not occurring, the outer layer is the main component of periosteum, and cells in the inner layer are sparse. Bone marrow Red marrow is the tissue in which blood cells develop, and is 40% water, 40% fat and 20% protein. In later stages of growth, and in the adult, when the rate of blood cell formation has decreased, red marrow slowly changes to yellow marrow. Yellow marrow is made up mostly of fat cells (80% fat, 15% water, 5% protein). Under the appropriate stimulus, yellow marrow can revert to red marrow. Enchondral bone formation/mineralization Cartilage Model Human bones are mostly preformed from hyaline cartilage, some from condensed mesenchyme, usually at 6 weeks. This model is gradually invaded by vascular buds, which bring in osteoprogenitor cells that differentiate into osteoblasts and form primary centres of ossification at around 8 weeks. The cartilage model grows through appositional growth (new bone is applied to the surface of existing bone leading to an increase in width of bone) and interstitial growth (growth and replacement by bone of deeper layers of epiphyseal growth plate, pushing the epiphysis and its overlying articular cartilage away from the metaphysis and diaphysis - leads to increased length of bone). Ossification thus spreads to replace the cartilage model. Marrow is formed by the resorption of the central cancellous bone and invasion of myeloid precursor cells, brought in by capillary buds. Secondary centres of ossification develop at the ends of bone, to form epiphyseal centres of ossification, which allow increase in length until the bone’s adult dimensions are attained. During the developmental stage, the epiphyses enjoy a rich arterial supply composed of an epiphyseal artery, metaphyseal arteries, nutrient arteries and perichondral arteries. Physis In immature long bones there are 2 growth plates: 1) horizontal (the physis), and 2) spherical (allowing the growth of the epiphysis; it has the same arrangement as the physis but is less organised).Note this .Physeal cartilage is classified into zones according to growth and function. Reserve zone - Here there is no evidence of cellular proliferation or active matrix production. There is decreased oxygen tension. Cells here store lipids, glycogen and proteoglycan aggregates for later growth. Therefore diseases such as lysosomal storage diseases (Gaucher’s) can affect this zone. Proliferative zone - The cartilage cells undergo division and actively produce matrix, and longitudinal growth occurs with chondrocytes forming columns. The oxygen tension here is increased, and there is also increased proteoglycan in the surrounding matrix which inhibits calcification. Defects in this zone (affecting chondrocyte proliferation and column formation) occur in achondroplasia. Hypertrophic zone - This may be subdivided into 3 zones: maturation, degeneration and provisional calcification. Here the cartilage cells are greatly enlarged (up to 5 times normal size), they have clear cytoplasm as a result of the glycogen accumulated, and the matrix is compressed into linear bands between the columns of hypertrophied cells. The cartilage cells accumulate calcium in mitochondria, then die, releasing calcium from matrix vesicles. Sinusoidal vessels bring osteoblasts, which use the cartilage as a template for bone formation. Metaphysis Here osteoblasts from progenitor cells accumulate on cartilage bars formed by physeal expansion. Mineralization of primary spongiosa (calcified cartilage bars) occurs, forming woven bone which is remodelled to form secondary spongiosa and a "cutback zone" at the metaphysis. Cortical bone is formed when physeal and intramembranous bone are remodelled in response to stress along the periphery of growing long bones. Periphery of the Physis. This has 2 main components: a) Groove of Ranvier - allow chondrocytes to travel to the periphery of the growth plate, resulting in lateral growth. b) Perichondrial Ring of LaCroix - dense fibrous tissue which anchors and supports the physis Mineralization Collagen hole zones (between ends of molecules) are seeded with calcium hydroxyapatite crystals, through branching and accretion. Hormone and Growth Factor Effects on the Growth Plate Hormones and growth factors affect the growth plate either directly or indirectly, through their effects on chondrocytes and matrix mineralization. Some factors are produced and act within the growth plate, while others are produced at a distant site. Intramembranous Ossification The flat bones of the skull, the mandible, and the clavicle ossify at least partly by intramembranous ossification. This occurs without a cartilage model, and occurs by aggregation of layers of connective tissue cells at the site of future bone formation, and their differentiation into osteoblasts. The osteoblasts then form a centre of ossification which expands by appositional growth. ii. bone injury and repair General Principles Bony response to injury consists of overlapping phases of inflammation, repair (soft callus then hard callus), and remodelling. Fracture healing is affected by systemic factors such as age, hormones and nutrition, and local factors such as degree of local trauma, type of bone affected, and infection. Inflammation (Haemorrhage/Granulation Tissue-minutes/hours) This begins immediately after the fracture, and is characterised by bleeding from the fracture site and surrounding tissues, causing haematoma formation, accompanied by oedema and pain. Lysosomal enzymes are released and tissue necrosis occurs - osteoclasts and macrophages remove necrotic bone and tissue debris from the fracture site. This is followed by the stimulation of proliferation of reparative cells such as osteoblasts and endothelial cells. Repair (Immature Callus/Mature Callus-weeks/months) Within 2 weeks, primary callus response occurs. If the bone ends are not in apposition to one another, soft (bridging) callus is formed around and between the fragments, reducing their mobility. This soft callus contains fibroblasts, proliferating osteoblasts and often chondroblasts, embedded in a matrix rich in collagen and glycoprotein, into which new blood vessels grow. Hard (medullary) callus supplements the bridging callus - the soft callus is gradually converted into woven bone, mainly by enchondral ossification. This stage is reached about 3 or 4 weeks after injury and continues until firm bony union occurs (around 2 or 3 months later for most adult bones). The amount of callus formation is indirectly proportional to the degree of immobilisation of the fracture. Remodelling (years) This stage overlap with hard callus formation and may continue for up to 7 years. It involves the gradual conversion of the woven bone of the hard callus to lamellar bone. It is considered complete when the site of the fracture can no longer be identified either structurally or functionally. It allows the restoration of bone to its normal configuration and shape, according to the stresses placed on it (Wolff’s Law). Growth Factors of Bone Bone Morphogenic Protein (BMP) BMP is osteoinductive - it acts on progenitor cells to induce differentiation into osteoblasts and chondroblasts. The target of BMP is the undifferentiated perivascular mesenchymal cell. BMP may be the main signal regulating skeletal formation and repair - it is known to induce bone formation de novo, following the same pathways as enchondral ossification. Transforming Growth Factor b TGF-b induces mesenchymal cell production of type II collagen and proteoglycans. It also enhances osteoblast activity, via increased collagen synthesis, as well as increasing the bone apposition rate and inhibiting the differentiation of osteoclasts. It is thought to regulate cartilage and bone formation in fracture callus. Insulin-like Growth Factor II IGF-II stimulates type I collagen, cellular proliferation and cartilage matrix synthesis. Platelet-Derived Growth Factor PDGF serves as a local cytokine regulator, attracting inflammatory cells to the fracture site. Hormonal Effects on Fracture Healing Fracture healing is increased by: growth hormone (by increasing callus volume), thyroid hormone/parathyroid hormone (by bone remodelling), and possibly also by Calcitonin (mechanism unknown). Cortisone however is known to decrease fracture healing by decreasing callus proliferation. Electricity and Fracture Healing Stress-generated potentials act as signals which regulate cellular activity. Examples include piezoelectric effect and streaming potentials. Streaming potentials arise when electrically charged fluid is forced over a tissue with a set charge. Piezoelectric effect refers to the displacement of charges in tissues which occur as a result of mechanical forces. Compression generates -ve charges and so bone healing. Transmembrane potentials are generated by cellular metabolism. Bone produces small electric potentials on its surface when an appropriate mechanical stress is exerted. It has been suggested that bone remodelling as a response to mechanical stress is mediated by these electric potentials, which then activate osteoclasts and osteoblasts. Therefore, devices have been invented with the aim of stimulating fracture repair by altering a variety of cellular activities of cartilage and bone cells. First used by Dwyer in Sydney. Various types of electrical stimulation have been used: Direct Current (DC) stimulates an inflammatory-like response. Alternating Current (AC) causes changes in cAMP accumulation, increases collagen synthesis, increases DNA synthesis and increases mineralization. Pulsed Electromagnetic Fields (PEMF) initiate calcification of fibrocartilage, but cannot induce the calcification of fibrous tissue. Bone Grafting Bone grafts provide a passive framework for host osteoblasts and osteoclasts (osteoconduction), and may provide active signals to the host response capable of influencing the process (osteoinduction). Autografts (tissue from the same individual), allografts (tissue transferred between members of the same species; donors must be screened for potential transmissible diseases) or xenografts (tissues transferred between species) can be used. Cancellous grafts are commonly used due to their porous nature, allowing rapid revascularisation, followed soon after by osteoblastic activity and mineralization, and later remodelling ("creeping substitution"). The incorporation process in cancellous bone is relatively rapid and complete compared to cortical grafts. They are used for grafting nonunions or cavitary defects. Cortical grafts are used to repair structural defects(stronger)and have a slower turnover. Revascularisation is slower than in cancellous grafts. Slow remodelling of Haversian systems is followed by a vigorous osteoclastic response, weakening the graft, then deposition of new bone, restoring strength. Osteoarticular allografts are often used in tumour surgery - these grafts are immunogenic, therefore they are usually subjected to long-term preservation, such as freezing or lyophilisation. This process destroys the viability of many of the chondrocytes. Tissue-matched fresh osteochondral grafts produce minimal immunogenic rejection and are well incorporated. Vascularized grafts do not undergo the incorporation process described for nonvascularized grafts. Instead they unite to the recipient-site skeleton by a process similar to fracture repair, and allow more rapid union with the preservation of most cells. Bone grafts may be: • fresh • fresh frozen - less immunogenic than fresh • freeze-dried - loses its structural integrity; least immunogenic • in bone matrix gelatin There are 5 recognised stages of graft healing: (as in fracture healing) 1) Inflammation - necrotic debris stimulated chemotaxis 2) Osteoblast differentiation - osteoblasts differentiate from precursors 3) Osteoinduction - osteoblasts and osteoclasts function 4) Osteoconduction - new bone is formed over the graft tissue 5) Remodelling - a process continuing for years Synthetic bone grafts are now in use, and are composed of calcium (as the phosphate, sulfate, carbonate, or coralline hydroxyapatite - thermoexchange process used to convert calcium carbonate skeleton to calcium phosphate), silicon (as silicate) or aluminium (as the oxide). Most recent is tantalum metallic mesh. iii. conditions of bone mineralization, bone mineral density, and bone viability Normal Bone Metabolism Calcium Calcium exists in 3 forms in the body: 1) as hydrated phosphate in the skeleton. Over 99% of the body’s calcium is stored here. 2) in the extracellular fluid. This accounts for less than 1% of total body calcium. The concentration here is maintained at a constant level, even at the expense of calcium in bone. Its function here is in the excitability of nerves and muscles, including the heart; in blood clotting; in membrane permeability; and in the activity of various enzymes. 3) inside cells. There is a low cytosolic calcium level which is tightly controlled. It plays a role in the functions of many enzymes. Absorption occurs mostly by an active transport system in the small intestine, which is Vitamin K dependent, and also by passive diffusion in the jejunum. It is 98% resorbed in the kidney, and is excreted in the faeces and urine. Calcium requirements are 800mg/day for Australian adults and 1000mg/day for females over the age of 50. Pregnant women require 1500mg/day in the 3rd trimester, and lactating women need 2000mg/day. There is a 700mg calcium turnover in and out of bone on a daily basis. Hypocalcaemia can lead to tetany, somnolence and areflexia, while hypercalcaemia can cause hyperreflexia and convulsions. Phosphate Phosphate not only plays a role in bone mineral, but also acts as a metabolite and buffer, and participates in enzyme systems. Around 85% of the body’s phosphate stores are in bone. Plasma phosphate is mostly unbound, and is resorbed in the proximal tubules of the kidney. The recommended daily intake is 1000-1500mg, and dietary intake is usually sufficient. Parathyroid Hormona PTH is a polypeptide chain secreted by the chief cells of the parathyroid glands, and its function is in the control of calcium ion concentration in the extracellular fluid. This is achieved by control of: a) calcium absorption from the gut; b) calcium excretion by the kidneys; and c) calcium release from bones. Decreased calcium levels in the extracellular fluid stimulate PTH release, which then acts at the intestine, kidneys and bone. Vitamin D3 Vitamin D3 is a naturally-occurring steroid that is derived from UV irradiation of 7-dehydrocholesterol. It is absorbed from the small intestine only when fat digestion and absorption are normal. In the liver it is hydroxylated to 25-OH D3, then it is further hydroxylated in the kidney either to 1,25(OH)2D3 (the active form), or to 24,25(OH)2D3 (an inactive metabolite). The active form has effects at the kidney, intestine and bone. Calcitonin Calcitonin is a large polypeptide secreted by the clear cells of the thyroid gland. Secretion is stimulated by increased calcium levels in the extracellular milieu. It decreases calcium concentration in the extracellular fluid by its effects at the intestine, kidney and bone (where it promotes calcium deposition). It may also play a role in fracture healing, and the treatment of osteoporosis. Other Hormones Oestrogen causes increased osteoblastic activity and inhibition of bone resorption. Deficiency, as in menopause, leads to decreased osteoblastic activity in the bones, decreased bone matrix, and decreased deposition of bone calcium and phosphate, thus causing osteoporosis. Corticosteroids increase resorption and impede fracture healing (↓ binding proteins → ↓ gut absorption of calcium; ↓ bone formation through inhibition of collagen synthesis) Thyroid hormones enhance osteoclastic bone resorption, leading to osteoporosis. Growth hormone increases gut calcium absorption more than its increase in urinary excretion, leading to a positive calcium balance. Growth factors such as PDGF and TGF-β play a role in bone and cartilage repair. Interaction Calcium and phosphate metabolism are influenced by various hormones and also by the levels of the metabolites themselves. Regulation of plasma levels is controlled in part by feedback mechanisms. It appears that peak bone mass occurs between the ages of 16 and 25 years, with higher peak bone mass in males and African-Americans. After this peak, bone is lost at a rate of 0.3-0.5% per year, with higher rates (2-3%) for untreated women during the 6th to tenth years after menopause. Conditions of Bone Mineralisation Hyperparathyroidism Excess PTH (80% Osteopaenia, Osteitis Kidney stone, weight loss, due to adenoma) fibrosa cystica hyperreflexia Familial syndromes Excess PTH (MEN- Osteopaenia Endocrine or renal I/II, renal) abnormality Hypocalcaemia Hypoparathyroidism Insufficient PTH Calcified basal ganglia Neuromuscular irritability, (idiopathic) tetany, cataracts PHP/Albright PTH receptor defect Exostoses, brachydactyly Short stature, obesity, short metacarpals & metatarsals Renal Chronic renal failure Rugger jersey spine Renal abnormalities osteodystrophy → ↓ phosphate excretion Rickets (Osteomalacia) Vit D deficient Insufficient Vit D (from Craniotabes, frontal Bony deformities (eg. genu diet or malabsorption) bossing, rachitic rosary, varum/valgum), hypotonia pathological fractures Vit D dependent Defective renal 1α Poor mineralisation Total alopecia hydroxylation Vit D resistant ↓ phosphate Poor mineralisation Short stature in childhood, reabsorption in renal hypotonia tubules Hypophosphatasia Low alkaline Poor mineralisation, Bone deformities, hypotonia phosphatase fractures Osteopaenia Osteoporosis ↓ bone mass Fractures (crush to Fractures, kyphosis vertebra; hip; radius) Scurvy Vit C deficiency → Thin cortices, poorly Fatigue, bleeding gums, abnormal collagen defined trabeculae, corner bruises, joint pain/ effusions sign Osteodense Paget’s Disease osteoclast activity + Areas of sclerosis & Bone pain, deformity, irregular bone radiodensity, "picture fractures, high output cardiac formation frame" vertebrae failure Osteopetrosis Abnormal osteoclasts Widened metaphysics, Aplastic anaemia, - cause unclear bone within bone hepatosplenomegaly Hypercalcaemia This can lead to such symptoms as lethargy, polyuria, constipation, disorientation, hyperreflexia, kidney stones, psychosis, and cardiac arrhythmias. Differential diagnoses: 1) Primary Hyperparathyroidism - There is overproduction of PTH, due to a parathyroid adenoma in 80% of cases. Elevated levels of PTH enhance urinary loss of phosphate (leading to hypophosphataemia) and bicarbonate (causing mild hyperchloremic metabolic acidosis. Osteoclastic stimulation results, causing enhanced bone resorption - this raises serum calcium and increases calcium excretion. Laboratory findings include elevated serum calcium, PTH, and urinary phosphate, and decreased serum phosphate. In long-standing hyperparathyroidism, one may see bony changes such as localised or generalised osteopaenia, osteitis fibrosa cystica (fibrous replacement of bone marrow), "brown tumour" (increased giant cells, RBC extravasation, haemosiderin staining and fibrous tissue haemosiderin, caused by subcortical bone resorption in the jaw) or chondrocalcinosis. Histological findings may include osteoblasts and osteoclasts active on both sides of trabeculae, areas of destruction, and wide osteoid seams. Surgical parathyroidectomy is curative. 2) Familial syndromes - Multiple Endocrine Neoplasia types I and II may have associated pituitary adenomas. Familial Hypercalciuric Hypercalcaemia, caused by poor renal clearance of calcium, may also result in hypercalcaemia. 3) Other causes of hypercalcaemia include malignancy (most common; include multiple myeloma and lymphomas), Addison’s Disease, hyperthyroidism, tuberculosis, Paget’s Disease, steroid administration and kidney disease. Hypocalcaemia This occurs due to deficiency or inadequate function of PTH or Vitamin D. Clinical features may include neuromuscular irritability (positive Chvostek’s or Trousseau’s signs, seizures, tetany), lenticular cataracts, fungal infections of the nails, pancreatic calcification and prolonged Q interval on ECG. Differential diagnoses: 1) Primary Hypoparathyroidism - This can be defined as an absolute or relative deficiency or inadequacy of PTH function. Diminished PTH action is associated with hypocalcaemia and hyperphosphataemia (urinary excretion is not enhanced due to lack of PTH). Skull radiographs may reveal basal ganglion calcification. 2) Pseudohypoparathyroidism (PHP) - This is a rare genetic disorder (X-linked dominant) causing a syndrome of PTH resistance due to blocking of PTH action at the cellular level. PTH levels are normal or elevated, with hypocalcaemia and hyperphosphataemia. There is a characteristic short stature, bony abnormalities (such as shortening of the metacarpals and metatarsals), brachydactyly, exostoses, obesity, and intellectual impairment. 3) Renal Osteodystrophy - This is a complex bone disorder in patients who have chronic renal failure. Renal impairment leads to an inability to excrete phosphate, with a compensatory decrease in serum calcium, which is usually adjusted by PTH (which increases urinary excretion of phosphate). However phosphate cannot be secreted, resulting in symptoms similar to hypoparathyroidism. Thus blood levels of PTH are greatly elevated. It is commonly associated with long-term haemodialysis. Radiographs may exhibit a "rugger jersey spine" appearance, associated with sclerosis in the region immediately beneath the vertebral endplates. 4) Rickets (Osteomalacia in adults) - In rickets there is inadequate mineralization of growing bone, causing changes in the physis (increased width and disorientation) and bone (cortical thinning and bowing). Osteomalacia is the adult equivalent of rickets. Rickets and Osteomalacia have various causes, outlined below: a) Vitamin D Deficiency Rickets This is rarely seen on the basis of dietary deficiency in developed countries, due to the fortification of dairy products with Vitamin D. It may be seen in children of Asian immigrants, in premature infants on prolonged total parenteral nutrition, in patients with dietary peculiarities, and those with malabsorption (sprue). A lack of exposure to sunlight can play a role in perpetuating rickets. In the Vitamin D deficient state, there is a reduction in the absorption of calcium and phosphate, leading to a compensatory secondary hyperparathyroidism. An initial increase in bone resorption is able to maintain normal serum calcium levels. Continued Vitamin D deficiency exacerbates the secondary hyperparathyroidism, causing a loss of phosphate and decreased ability to maintain serum calcium levels at normal. Laboratory findings include low Vitamin D levels, low normal calcium, low phosphate, and elevated PTH. Clinical features may include delayed closure of the fontanelles in the first year of life, leading to widened cranial sutures. There may be thickening of the skull (frontal bossing) and flattening of the occiput (craniotabes). An enlarged costochondral junction (rachitic rosary), posterior displacement of the sternum, and Harrison’s sulcus may be evident in the thorax. Bony deformities may appear on weight-bearing, such as genu varum or genu valgum, coxa vara, or lordosis of the spine. There may also be weakness and hypotonia of the muscles (causing a waddling gait), retarded bone growth (due to a defect in the hypotrophic zone with widened osteoid seams and physeal cupping), hypoplastic dental enamel, and a tendency to pathologic fractures. Treatment involves giving 5000 units of Vitamin D (as ergocalciferol) daily, and supplemental calcium in early stages of treatment (up to 3 g daily), to avoid hypocalcaemic symptoms. b) Hereditary Vitamin D-Dependent Rickets This rare condition is autosomal recessive, and is unresponsive to the usual physiological replacement doses of Vitamin D. It may be due to a defect in renal 1α -hydroxylation of 25-hydroxy Vitamin D , resulting in low levels of, or defective 1,25-(OH)2D3. Thus there is impaired calcium absorption, and there are typical features of low serum calcium, secondary hyperparathyroidism, and clinical features of Vitamin D deficiency (except the clinical features may be more florid, and may include total baldness). The metabolic abnormalities require treatment with high levels of Vitamin D - usual doses range from 20,000 to 100,000 units of Vitamin D daily, followed by a maintenance dosage of a 1,25-(OH)2D3 analogue. c) Familial Hypophosphataemic Rickets (Vitamin D-Resistant Rickets) This is an X-linked disorder characterised by hypophosphataemia due to decreased renal tubular reabsorption of phosphate. There may also be abnormal intestinal absorption of phosphate. Sufferers have a normal GFR and an impaired Vitamin D3 response. Clinical features are similar to those of other forms of rickets. Treatment requires phosphate replacements (1-4 g daily) and Vitamin D3. d) Hypophosphataemia This is an autosomal recessive disorder and may present in its more severe form as childhood rickets, and is less severe in adults. It is characterised by low alkaline phosphatase (an enzyme required for bone mineralization; it hydrolyses pyrophosphate to give inorganic phosphate - this step is thought to be required for initiating apatite formation in the matrix). Features resemble those of rickets. There is no satisfactory treatment, but phosphate therapy has been somewhat successful. Conditions of Bone Mineral Density Bone mass regulation depends on the relative rates of deposition and resorption of bone. Osteopaenia a) Osteoporosis Osteoporosis is characterised by diminution in bone mass, usually associated with loss of oestrogen in postmenopausal females. It leads to a tendency to fracture, commonly at sites of a large volume of trabecular bone, such as the vertebral body, proximal femur, or distal radius. Risk factors are Caucasian descent and north-western European background, early menopause, alcohol consumption, chronic smoking, patients on phenytoin (which impairs Vitamin D metabolism), low Vitamin D and low calcium diets, and breastfeeding. Lack of exercise may be an important influence on bone mass, especially during the growth phase. Cancellous bone is affected more severely. Patients may present with kyphosis or fracture of a vertebra (particularly crush fracture of T11-L1), hip or distal radius. Osteoporosis has been divided into two classifications: i) Type I Osteoporosis (Postmenopausal) - Mainly affects trabecular bone. Fractures of vertebra and distal radius are often seen. ii) Type II Osteoporosis (Age-related) - Occurs in patients aged over 75 years. It affects both cortical and trabecular bone, and is associated with poor calcium absorption. Fractures of hip and pelvis are common features. Initial laboratory tests should include serum and urine calcium, serum protein, inorganic phosphates, alkaline phosphatase and full blood count. Results of these are usually unremarkable in osteoporosis, but may help to eliminate hyperthyroidism, hyperparathyroidism, malignancy , Cushing’s syndrome and haematologic disorders as differential diagnoses. Plain radiographs are usually of no help, unless there is greater than 30% bone loss. Special studies may be done, including bone biopsy (to distinguish between osteoporosis and Osteomalacia), single and dual photon absorptiometry, quantitative computed tomography, and dual-energy X-ray absorptiometry (DEXA - most accurate and involves less radiation). Histologically, there is reduced bone mass and characteristic thinning of trabeculae, decreased size of osteons, and widened marrow and Haversian spaces. Treatment includes physical activity to prevent further bone loss, calcium supplements (more effective in Type II osteoporosis), oestrogen-progesterone therapy (for Type I osteoporosis - most effective if started within 6 years of menopause) and fluoride (inhibits bone resorption, and increases trabecular thickness and volume). Intramuscular calcimar (salmon calcitonin) injections have yielded some successful results in preventing bone loss in postmenopausal osteoporosis, but the effects may be transient and side-effects may occur. For patients at high risk of osteoporosis, the preventive regimen includes increased weight-bearing physical activity, dietary calcium intake of 1000-1500mg daily, and, if necessary, use of oestrogen therapy at menopause. Idiopathic Transient Osteoporosis of the Hip is an uncommon disorder, occurring most often in the third trimester of pregnancy. The patient complains of groin pain, limited range of movement, and localised osteopaenia, and the diagnosis is one of exclusion. Treatment involves analgesia and limited weight- bearing. b) Osteomalacia This may be considered clinically and histologically to be the adult counterpart of rickets. There is a defect in mineralization leading to a disproportionately large amount of unmineralised osteoid. It is caused by Vitamin D deficiency, such as from deficient diet, malabsorption, hepatic or pancreatic disease, gastric bypass surgery, renal osteodystrophy, or various drugs (Phenytoin - induces enzyme systems which enhance Vitamin D degeneration; aluminium-containing phosphate-binding antacids - aluminium deposits in bone prevent mineralization). Radiological appearance may be normal, or may show Looser’s zones (aka pseudofractures - radiolucencies occurring as bands at right angles to the cortex; occur at points of stress, and may become complete fractures), biconcave vertebral bodies, or trefoil pelvis. Diagnosis is by biopsy, which give histological appearance of increased width of unmineralised bone, blurred or discontinuous mineralization, and increased osteoid width. Treatment involves giving large doses of Vitamin D, and careful monitoring of serum calcium and phosphate (may decrease initially due to rapid remineralisation of bone). c) Scurvy Ascorbic acid is an essential nutrient required for the repair and growth of collagen, and for iron absorption. Deficiency leads to defects in collagen growth and repair, and impaired hydroxylation of collagen peptides. Dietary sources include rapidly growing fresh fruits and vegetables. Vitamin C deficiency may manifest as generalised fatigue, ecchymosis, gum bleeding, perifollicular haemorrhage, joint pain and effusions, and iron deficiency. Radiographs may show thin cortices, poorly-defined trabeculae, and metaphyseal clefts ("corner sign"). Histologically there may be granulation tissue replacing primary trabeculae, generalised subperiosteal haemorrhage, and widened zone of provisional calcification in the physis. Treatment is Vitamin C in doses of 100mg t.d.s. - this usually replenishes tissue stores within a week. d) Osteogenesis Imperfecta This is a heterogeneous group of disorders characterised by extreme bone fragility and multiple fractures. It is caused by abnormal collagen synthesis (failure of cross-linking), mainly due to a mutation in the genes producing type I collagen. e) Marrow Packing Disorders Osteopaenia can result from myeloma, leukaemia, and other disorders. Increased Osteodensity a) Paget’s Disease This is characterised by uncoordinated bone resorption and formation, ultimately leading to skeletal deformity (such as enlarged skull, bowing of long bones of the legs). There are three stages: 1) initial osteolytic phase, 2) mixed stage of osteolysis and osteogenesis, 3) burned-out sclerotic phase. Complications include sarcomatous change in an involved bone, and high-output cardiac failure due to the increased vascularity of the subcutaneous tissue overlying the involved bones. b) Osteopetrosis (Marble Bone Disease) This term refers to any of a group of disorders causing increased sclerosis of bones and, in the most severe autosomal recessive form, obliteration of the medullary canal in the long bones (due to decreased osteoclast and chondroclast function), widened metaphyses, "bone within a bone" appearance, hepatosplenomegaly and aplastic anaemia. Pathological fractures are common. This disorder may be the result of an immune abnormality (thymic defect). The autosomal dominant form ("Albers-Schönberg Disease") usually shows generalised osteosclerosis (including the "rugger jersey spine" appearance) most apparent in the skull. Histologically, osteoclasts lack the normal clear zone and ruffled border, and the marrow spaces are filled with necrotic calcified cartilage. In childhood, bone marrow transplantation of osteoclast precursors may be life-saving. Treatment may involve high doses of calcitriol, with or without steroids. c) Osteopoikilosis (Spotted Bone Disease) This is an autosomal dominant disorder characterised by numerous symmetrical islands of deep cortical bone in the medullary cavity and cancellous bone of long bones, pelvis and scapula. This is usually asymptomatic and there is no observed association with malignant change. Conditions of Bone Viability Osteonecrosis Osteonecrosis, or bone death, occurs as a result of either impaired blood supply (eg. due to trauma) or severe marrow and bone cell damage. The hip joint is commonly affected, causing eventual collapse and flattening of the femoral head. Other susceptible sites include the femoral condyles, head of humerus, capitulum, scaphoid, lunate and talus. It is associated with steroids and heavy alcohol consumption (both causing fatty infiltration → capillary compression), and also with blood dyscrasias (such as Sickle-cell Disease), decompression sickness (Caisson Disease), vasculitis, excessive radiation therapy, and Gaucher’s Disease (abnormal accumulation of glucocerebride in the reticuloendothelial system causes pressure on bone sinusoids, thus necrosis). a) Aetiology The aetiology of osteonecrosis is uncertain, but various factors have been implicated. These include vascular insults, and the enlargement of space-occupying marrow fat cells, causing ischaemia of adjacent tissues. b) Pathologic Changes There are four overlapping stages: 1) Bone death without structural change - Within 24 hours after infarction there is autolysis of osteocytes and necrosis of marrow. 2) Repair and early structural failure - Inflammation occurs, with a vascular reaction. New bone is laid down upon the dead trabeculae (visible on X-ray as increased bone density). 3) Major structural failure - A process of "creeping substitution" occurs, with resorption of necrotic trabeculae and remodelling. The bone is weakest during this phase, and collapse (crescent sign) and fragmentation may occur. 4) Articular destruction - Cartilage, deriving nourishment from synovial fluid, is preserved even in advanced osteonecrosis. However, severe distortion of the surface eventually results in cartilage destruction. c) Evaluation Detailed history-taking and physical examination (eg.↓ range of movement, pain, stiffness) are obvious first lines of evaluation. Other joints should be examined in order for early diagnosis of the disease process. In 50% of cases of idiopathic osteonecrosis, and in 80% of cases of steroid-induced osteonecrosis, disease is bilateral. Diagnosis is aided by MRI and bone scans, as well as radiography which shows distinctively increased bone density due to reactive new bone formation in the surrounding viable tissue. Femoral head pressure measurements may also be done - pressure greater than 30 mm Hg, or an increase of over 10 mm Hg with injection of 5ml of saline is considered abnormal. d) Treatment should aim to eliminate the cause if possible. Nontraumatic osteonecrosis of the proximal humerus and femoral condyle may show spontaneous improvement. In stages 1 and 2, weight-relief, splintage and surgical decompression of the bone may prevent bone collapse. If in stage 3 (ie. bone collapse has occurred), realignment osteotomy to shift stress to an undamaged area may relieve pain and prevent further bony injury. If in stage 4, treatment is the same as for osteoarthritis. e) Ficat’s classification of osteonecrosis of the hip Stage Pain Physical Bone Scan MRI Intraosseous Radiographs Treatment Examination Pressure 0 Nil Normal Normal Normal Normal None I Minimal ↓ int.rotation Nondiagnostic Early Normal Core changes decompression (?) II Moderate ↓ ROM Positive Positive Porosis/ Strut graft sclerosis III Advanced ↓ ROM Positive Positive Flat/crescent Hemiarthro- sign plasty IV Severe Pain Positive Positive Acetabular Total hip changes arthroplasty Osteochondrosis This refers to any of a group of disorders of one or more ossification centres in children, characterised by degeneration or aseptic necrosis followed by reossification. It may occur at traction apophyses in children and may be associated with trauma, inflammation of the joint capsule, or vascular insult/secondary thrombosis. It is pathologically similar to osteonecrosis in the adult. Common Osteochondrosis (OC) include: • Legg-Calvé-Perthes disease - OC of femoral head • Osgood-Schlatter disease - OC of tibial tuberosity • Sinding-Larsen-Johansen syndrome - OC of inferior patella • Sever’s disease - OC of calcaneus • Köhler’s disease - OC of tarsal navicular • Freiberg’s infarction - OC of metatarsal head A normal joint is designed to carry out a range of movements. Synovial joints have a dense fibrous capsule which may be reinforced by ligaments and muscles. The joint is lined by synovium and filled with synovial fluid for nutrition and lubrication of articular tissues. Articular cartilage is composed of connective tissue which is suited to distributing load and decreasing friction. Aspects of joints discussed here are: Cartilage, Articular Cartilage, Synovium Meniscus. Cartilage: Cartilage is a form of connective tissue composed of chondrocytes and a specialised extracellular matrix. This matrix consists of water, collagen, proteoglycans and other components such as adhesives and lipids. There are several different types of cartilage. Articular hyaline cartilage consists of a glassy and homogenous matrix with lacunae containing chondrocytes. Articular cartilage contains more collagen than other types of hyaline cartilage. It lines the bones of synovial joints and functions in load distribution and decreasing friction. The articular cartilage matrix is avascular, aneural and alymphatic relying on the process of diffusion to provide nutrients for the chondrocytes. Other types of cartilage include fibrocartilage, elastic cartilage, and physeal cartilage. Fibrocartilage contains abundant thick bundles of mostly type I collagen which can be seen with the light microscope. This type of cartilage is found at ligament and tendon insertions into bone, in menisci, intervertebral discs, the symphysis pubis, temporomandibular and sternoclavicular joints. Fibrocartilage provides good resistance to shear and compression forces. Elastic cartilage is characterised by the presence of elastic fibres within the matrix which increase elasticity in tissues such as the external ear and trachea. Physeal cartilage provides longitudinal growth to immature long bones. Articular Cartilage: Important properties of articular cartilage include: • Avascular (no blood vessels) • Aneural (no nerve fibers) • Alymphatic (no lymphatic vessels) • Very low friction on cartilage on cartilage motion • Self-renewing (maintenance and restoration of extracellular matrix) • With aging, loss of ability to maintain the extracellular matrix In regard to chondrocytes: • By cartilage volume, the cells only represent about 1%. • Chondrocytes are synthetic machines producing the extracellular matrix. o Intracellular organelles Endoplasmic reticulum Golgi apparatus • Chondrocytes do not have cell-to-cell contact in the extracellular matrix. • With aging, chondrocytes lose their synthetic abilities. • Chondrocytes respond to a number of stimuli: o Increase matrix production after sensing degradation of the matrix o Sense loads and increase matrix production o Respond to growth factors and anabolic stimuli Articular cartilage has three principal classes of macromolecules: • Collagen – 60% • Proteoglycans – 25% to 35% • Noncollagenous proteins/glycoproteins – 15% to 20% The three articular cartilage collagens that form cross bands are types II, IX, and XI. Of particular note: • Type XI binds to type II. • Type IX binds to the cross-banded fibrils in the superficial layer. • Type VI attaches to the matrix around the chondrocytes. • Type X is near the calcified layer and is probably involved in mineralization of the calcified layer. Noncollagenous proteins include: • Decorin and fibromodulin bind to type II collagen and likely stabilize the type II collagen network. Cartilage has a number of distinct zones. The superficial zone has a number of important characteristics: • Thinnest articular cartilage layer • Two layers: o Most superficial – fine collagen fibrils (lamina splendens) o Deep layer – flattened fibroblast-like chondrocytes (parallel to joint surface) • Forms a cartilage skin • Important chemical properties: o High collagen and low proteoglycan concentration o Fibronectin and water concentrations are highest in this zone • Great tensile stiffness and strength • Seals off the cartilage from the immune system The transitional zone lies between the superficial and middle zones of the articular cartilage. The following important points should be remembered: • The chondrocytes have a high concentration of synthetic organelles such as rough endoplasmic reticulum and Golgi apparatus. • The collagen fibers are larger than in the superficial zone. • The proteoglycan concentration is higher than the superficial zone. The chondrocytes in the calcified cartilage zone show the least metabolic activity. In contrast, the chondrocytes of the other areas are very active: • Superficial zone o Fine collagen fibrils (lamina splendens) o High collagen and low proteoglycan concentration o Fibronectin and water concentrations are highest in this zone • Transitional zone o The chondrocytes have a high concentration of synthetic organelles such as rough endoplasmic reticulum and Golgi apparatus. o The collagen fibers are larger than in the superficial zone. o The proteoglycan concentration is higher than the superficial zone. • Middle (radial or deep) zone o Largest diameter collagen fibrils o Highest proteoglycan content Other important points: Interleukin I has the potential to increase expression of matrix metalloproteinases that can dissolve the extracellular matrix. Type II collagen fibers resist tensile and shear deformation forces in the articular cartilage. In contrast, the glycosaminoglycan aggregates resist articular cartilage compression and fluid flow. Cyclic compressive loads have the ability to stimulate matrix synthesis – aggrecan core protein and the glycosaminoglycans. The characteristic findings in osteoarthritis are: • Asymmetric loss of the joint space • Subchondral sclerosis and cysts • Osteophyte formation Osteoarthritis As the cartilage degenerates, progressive bone remodelling occurs. The cause of osteoarthritis is unknown. From a chemical standpoint, one of the earliest findings is a decrease in the proteoglycan and an increase in the water content. One should remember: Constant type II collagen content Decreased proteoglycan concentration and decreased chain length Increased water content The decreased proteoglycan content results in increased permeability of the cartilage. A reduction of the stiffness makes the articular cartilage less able to bear loads. In the second stage, there is a cellular response – chondrocyte proliferation. Clusters of chondrocytes producing new matrix are visible. In this stage, there is nitric oxide and interleukin I production. These are catabolic factors that increase matrix metalloproteinase activity. Degradative enzymes break down types IX and XI collagen, which may compromise the stability of the type II collagen framework. In the last stage of osteoarthritis, there is reduced chondrocyte proliferation and function, which may be secondary to reduced ability to respond to anabolic factors (down regulation). There may be accumulation of molecules that bind to the anabolic factors (and keep the factors from the chondrocytes) such as decorin and insulin-dependent growth factor binding protein. Articular cartilage: Components Chondrocytes (5% wet weight) Chondroblasts which are derived from mesenchymal cells become trapped in lacunae and develop into chondrocytes. Chondrocytes are important in the control of matrix turnover through production of: • collagen • proteoglycans • enzymes for cartilage metabolism. Matrix Water (60-80% wet weight) • Articular cartilage is a highly hydrated material. Water distribution varies, making up 65% of wet weight at the deep zone and 80% at the surface. • Weight bearing capacity is made possible through regional changes in water content which allow deformation of the cartilage surface in response to stress. • Water provides nutrition and lubrication of cartilage. • Increases in water content lead to: o increased permeability o decreased strength o decreased Young’s modulus • • Water content decreases with normal ageing. • Water content increases in osteoarthritis Collagen (10-20% wet weight) – (image 1) • forms a cartilaginous framework which provides tensile strength. • 90-95% is type II collagen with increased Gly, Lys-OH, Pro-OH and hydrogen bonding. • small amounts of types V, VI, IX, X and XI collagen are present. Proteoglycans (10-15% wet weight) – (image 2) • half life of three months • provide compressive strength • regulate matrix hydration by providing a porous structure to trap and hold water • composed of subunits of glycosaminoglycans (GAG’s - disaccharide polymers) • o chondroitin-4-sulfate (decreases with age) o chondroitin-6-sulfate o keratin sulfate (increases with age) • GAG’s are bound to a protein core by sugar bonds to form a proteoglycan aggrecan molecule. • Aggrecan molecules are further stabilised by link proteins which bind them to hyaluronic acid to form a proteoglycan aggregate. Adhesives Molecular interactions between chondrocytes and collagen fibrils are mediated by fibronectin, chondronectin and anchorin CII. Lipids are present in cartilage but their function is unknown. Collagen molecules and proteoglycans interweave to form cartilage (image 3) Layers (image 4) Superficial Gliding Zone • abundant tangentially oriented collagen fibres • low proteoglycan concentration • high water content • discoid flattened cells, parallel to surface • low metabolic activity (proteoglycan synthesis low) Transitional Zone • thicker fibrils • oblique fibres • cells arranged singly or in pairs • high metabolic activity Radial Zone • increased collagen size • vertical (radial) orientation of fibres • high proteoglycan concentration Tidemark • Undulating barrier tangential to the surface which represents a plane of weakness. Calcified Zone • Hydroxyapatite crystals anchor the cartilage to the subchondral bone. Metabolism Collagen Synthesis of collagen takes place in stages at various intracellular and extracellular sites. Post- translational modifications occur in the rER and Golgi. Intracellular Events • mRNA messages are translated into polypeptide chains which are released into the cisternae of the rER. • The signal peptide is cleaved. • Lysine and proline residues are hydroxylated. • Hydroxylysine residues are glycosylated. • N-linked sugars are added to the terminal portion of the polypeptide. • Polypeptide chains form a triple helix molecule. • Procollagen is formed through intrachain and interchain disulfide bonds which stabilise the polypeptides and determine the shape of the molecule. • Procollagen is packed into secretory granules which move along microtubules to be released into the extracellular matrix Extracellular Events • Uncoiled terminal ends of procollagen are cleaved to form tropocollagen. • Tropocollagen molecules aggregate and lysine and hydroxylysine residues are crosslinked to form a collagen fibril. • Fibrils aggregate to form collagen fibres. Collagen catabolism is poorly understood, but enzymatic processes and mechanical factors may be involved. Proteoglycans The process of proteoglycan synthesis begins with translation of mRNA to form a protein core to which glycosaminoglycan chains are added. The resultant aggrecan molecules are transported to secretory vesicles and released into the extracellular matrix. Link proteins and hyaluronate from the cell membrane bind to the molecules forming proteoglycan aggregates. Proteoglycan catabolism depends on cleavage of globular domains resulting in non-aggregation of the resultant fragments. Regulation of Growth Cartilage synthesis is regulated by growth factors. Platelet-Derived Growth Factors (PDGF) • may have a role in healing cartilage lacerations Transforming Growth Factor Beta (TGF-b ) • stimulates proteoglycan synthesis • suppresses synthesis of type II collagen • prevents the degradative action of plasmin and stremolysin through stimulation of formation of plasminogen activator inhibitor-1 and tissue inhibitor of metalloproteinase (TIMP). Fibroblast Growth Factor (b-FGF) • Stimulates DNA synthesis in articular chondrocytes in adult articular cartilage. • may play a role in repair process. Insulin-Like Growth Factor-I (IGF-I) • stimulates DNA and cartilage matrix synthesis in both adult articular cartilage and immature growth plate cartilage. Changes With Ageing With ageing cartilage becomes hypocellular and has decreased elasticity. Chondrocytes • increase in size • increased lysosomal enzymes • cartilage becomes hypocellular (cells stop reproducing) Matrix • Proteoglycans o decrease in mass and size - decreased length of chondroitin sulfate chains o change in proportion - increased keratin sulfate • • Water content decreases • Protein content increases Healing of Articular Cartilage Deep lacerations • extend below the tidemark • heal with fibrocartilage • blunt trauma may cause osteoarthritic changes Superficial lacerations • above the tidemark • chondrocytes proliferate but do not heal • immobilisation leads to atrophy while continuous passive motion is beneficial to healing Synovium The interior surfaces of normal joints (except articular cartilage and menisci) are lined by a synovial membrane. Synovium is composed of vascularized connective tissue that lacks a basement membrane and contains two predominant cell types which reflect the function of the tissue. Type A cells - important in phagocytosis Type B cells - fibroblast-like cells which produce synovial fluid. There are other undifferentiated cells that have a reparative role. Type C cells may exist as an intermediate cell type. Synovial fluid produced by the synovium, is an ultrafiltrate of blood plasma containing in addition, hyaluronic acid, proteinase, collagenases and prostaglandins, but no red blood cells, clotting factors, or haemoglobin. Function • Nourishment of articular cartilage through diffusion. • Lubrication of the joint space: Joint lubrication can be divided into two types. It is likely that different types of lubrication become important in different types of movement. Boundary lubrication (slippery surfaces) is where in response to a load, the joint surfaces are separated by a mono/multi molecular layer of low shear strength material (hyaluronate-protein complexes in the synovial fluid). This allows sliding motion while preventing adhesions or abrasions. Fluid film lubrication is where fluid separates the joint surfaces. There are several different types. Hydrodynamic Lubrication • a wedge shaped film of fluid is pulled between two opposing surfaces • a modification of this type is seen in human joints: elasto-hydrodynamic o deformation of the surfaces also occur Squeeze Film Lubrication • Fluid forms a film after being forced from the articular surfaces subjected to a load. • The viscosity of the fluid increases so that a gel is formed which has an osmotic pressure equivalent to the pressure applied. Weeping (Hydrostatic) Lubrication • A film is formed between the articular surfaces as fluid leaks out of the cartilage. Boosted Lubrication (fluid entrapment) • Similar to squeeze film lubrication but avoids contact between the articular surfaces by trapping high viscosity gel in localised depressions. Flow characteristics: Flow is non-Newtonian (the viscosity coefficient µ is not a constant; the fluid is not linearly viscous). Its viscosity increases as the shear rate increases. Lubricin, a glycoprotein, is the key lubricating component of synovial fluid. Hyaluronan molecules in the knee become entangled and behave like an elastic solid during high strain activities (running, jumping). Meniscus The meniscus functions to deepen the articular surface of a number of synovial joints. By doing so it increases the contact area available for load distribution. These joints include: • Acromioclavicular • Sternoclavicular • Glenohumeral • Hip • Knee We will focus here on the meniscus of the knee joint. Anatomy The meniscus is a triangular, semilunar structure. Its peripheral border is attached to the joint capsule. In the knee, the medial meniscus is semicircular and the lateral meniscus is circular. Components The meniscus is composed of fibrocartilage. Cellular components These synthesise and maintain the extracellular matrix, and are responsible for anaerobic metabolism. Cells found are chondrocytes and fibroblasts, which are referred to as fibrochondrocytes. Fusiform cells: • in lacunae in superficial layer • resemble chondrocytes and fibroblasts • abundant ER and Golgi Ovoid cells: • surface and middle layer • abundant ER and Golgi Matrix components Collagen • primarily type I (55-65% dry weight), also types II, III, V, VI (5-10% dry weight) • Superficial layer - mesh like fibres oriented radially • Surface layer - (deep to superficial) collagen bundles aligned irregularly • Middle layer - (deep) parallel to circumferential fibres Elastin (0.6% dry weight) Proteoglycans → (1-3% Glycoproteins → dry weight) Adhesive glycoproteins include fibronectin, thrombospondin. Blood supply The geniculate arteries supply the menisci. The outer 25% of the menisci are supplied by a circumferentially arranged plexus, and the remaining 75% receive supply via diffusion. Tears that occur in the peripheral vascularised region (red zone) will heal via fibrovascular scar formation by fibrochondrocytes. Tears that occur in the central avascular regions (white zone), however, can't heal. Nerve supply The outer two-thirds of the menisci is innervated by type I and type II nerve endings which are concentrated in the anterior and posterior horns, with few fibres in the meniscal body. Non-Inflammatory Arthritides: Osteoarthritis Neuropathic Arthropathy Acute Rheumatic Fever Ochronosis Secondary Pulmonary Hypertrophic Osteoarthropathy Inflammatory Arthritides: Rheumatoid Arthritis Systemic Lupus Erythematosus Juvenile Rheumatoid Arthritis Relapsing Polychondritis Spondyloarthropathies Ankylosing Spondylitis Reiter’s Syndrome Psoriatic Arthritis Enteropathic Arthritis Crystal Deposition Disease Gout Chondrocalcinosis Calcium Hydroxyaptite Crystal Deposition Disease Infectious Arthritides Pyogenic Arthritis Tuberculous Arthritis Fungal Arthritis Lyme Disease Haemorrhagic Arthritides Haemophilic Arthropathy Sickle Cell Disease Pigmented Villonodular Synovitis Joint Fluid Analysis Arthroses Analysis 1. Non-inflammatory Arthritides • 200 WBCs, 25% PMNLs • Glucose and protein equal serum values • Normal viscosity (high) • Straw colour • firm mucin clot 2. Inflammatory arthritides • 2000-75000 WBCs, 50% PMNLs • Moderately decreased glucose (25mg/dl lower than serum glucose) • Low viscosity • Yellow-green colour • Friable mucin clot • Synovial fluid complement is decreased in RA and normal in ankylosing spondylitis 3. Infectious arthritides • > 80000 WBCs, ≥ 75% PMNLs • Positive gram stain (and cultures) • Low glucose (> 25mg/dl less than serum values) • Opaque fluid • Increased synovial lactate Non-inflammatory Arthritides Osteoarthritis (OA) Osteoarthritis (OA) is a degenerative joint disease characterised by progressive loss of articular cartilage with associated new bone formation and capsular fibrosis. It is the most common form of arthritis. Nearly everyone who lives long enough will be affected by OA. Its prevalence increases steeply with age and there may be a genetic predisposition. OA is classified as primary when it arises without an obvious cause, and secondary when it occurs following certain predisposing factors (such as previous trauma, congenital deformity, infection or a metabolic disorder). Aetiology On a cellular level, osteoarthritis may be due to failure of chondrocytes to repair damaged cartilage. Excessive stresses are applied to articular cartilage and there is an inadequate chondrocyte response, leading to degeneration of the articular cartilage. The disparity between the stress applied and the chondrocyte response may be due to: • abnormal loads over a small area of cartilage • weakening of the cartilage (not well understood) • abnormal support by subchondral bone Pathogenesis Current concepts on the pathogenesis of OA are based on the assumption that whatever the provoking cause, there is a final common pathway of changes in articular cartilage. It has been suggested that the initiating event is fatigue fracture of the collagen meshwork, is followed by increased hydration of the articular cartilage and loss of proteoglycans from the matrix into the synovial fluid. There is some evidence of increased collagenolytic activity but collagen loss may also be due to mechanical causes. Alternatively, it has been proposed that the initial lesions are microfractures of the subchondral bone following repetitive loading. Healing of these microfractures results in significant loss of resilience of the subchondral bone. A stress gradient develops in the adjacent articular cartilage. As the process evolves there is fibrillation of the cartilage, and deep clefts appear with reduplication and proliferation of chondrocytes within them. Proliferative changes also occur simultaneously at the joint margins with formation of osteophytes. Eventually articular cartilage is lost altogether in areas of maximum mechanical stress and the underlying bone becomes hardened and eburnated. There may be cyst formation. Changes in osteoarthritic cartilage: 1. Increased water content (in contrast to decreased water content in normal ageing) 2. This occurs early, and suggests some weakening of type II collagen 3. Changes in proteoglycans 4. There are shorter chains and shifts in the concentration of proteoglycans. The chondroitin/keratin ratio is decreased. 5. Collagen abnormalities 6. Disruption is caused by an increase in collagenase and proteoglycan-degrading enzyme concentration. 7. Proteoglycan binding 8. Proteoglycans bind to hyaluronic acid. This results from the action of proteolytic enzymes, due to increased prostaglandin E and decreased numbers of link protein. 9. Rate of synthesis 10. Rate of synthesis of DNA, collagen, and proteoglycans are increased 11. Increased levels of: 12. Cathepsins B and D, metalloproteinases (collagenase, gelatinase, and stromelysin). 13. Interleukin-1 14. Enhances enzyme synthesis and has a catabolic effect (causing cartilage degeneration). 15. GAGs and polysulfuric acid Clinical features Patients usually present after 40-50 years. The knee joint is most commonly affected. Pain is of an insidious onset, and progresses over months or years. Stiffness and swelling occur in later stages of the disease. Range of movement is reduced and there may be associated crepitus. Investigations and Diagnosis Diagnosis is made largely clinically by the presence of gradual onset of pain after activity, pattern of joint involvement and lack of significant soft-tissue swelling. Radiology may be useful and joint fluid analysis helps rule out other causes of arthritis. Radiology: • osteophytes • narrowing of joint space • subarticular sclerosis and bone cysts Joints that are commonly involved are the distal and proximal interphalangeal joints, and carpometacarpal joints of the hand. There is usually superolateral involvement of the hip joint and asymmetric involvement of the knee. Microscopic changes: • loss of superficial chondrocytes • cloning of chondrocytes (more than one chondrocyte per lacunae) • replication and breakdown of the tidemark • fissuring • cartilage destruction with eburnation of subchondral pagetoid bone Treatment • Protection of affected joints from overloading o weight loss o use of walking stick • • Exercise of supporting muscles around joints to avoid wasting. • Supportive measures such as pain relief by analgesics or NSAIDs. • Surgical treatment is indicated for patients with persistent symptoms and pain and ranges from arthroscopy to arthroplasty. • Realignment osteotomies may be done in younger patients to redistribute weight bearing load at the knee to prevent further damage. • Total joint arthroplasties for older patients (over 60) in advanced cases that are resistant to conservative treatment. Neuropathic Arthropathy Neuropathic arthropathy, also known as Charcot joint, is a rapidly progressive form of osteoarthritis caused by a disturbance in the sensory (position sense and pain) innervation of a joint. In the upper limbs it is most commonly associated with syringomyelia, followed next by Hansen’s disease. 25% of patients with syringomyelia get a Charcot joint. In the lower limbs, common causes include tabes dorsalis, peripheral neuropathies especially diabetes, cauda equina lesions, and congenital insensitivity to pain. Clinical features Include an unstable, swollen, yet painless joint. Effusions may be seen. Patients are typically older, and may present with haemarthrosis. Radiological changes include advanced destruction on both sides of the joint, joint distension due to increased amount of fluid, irregular bits of bone embedded in fibrous tissue, and heterotopic ossification. Treatment Involves stabilising the affected joints by limitation of activity and use of appropriate splintage, such as callipers and casts. Total joint arthroplasty and use of other orthopaedic hardware are contraindicated. Acute Rheumatic Fever Acute rheumatic fever is sometimes included in the inflammatory group of arthritides. Although was the most common cause of childhood arthritis, its prevalence in developed countries has progressively declined since the advent of antibiotics. Pathogenesis Acute Rheumatic Fever is triggered by infection with specific streptococcus. Molecular Genetics: I. Chromosome A. 23 pairs (46) in humans. B. Located in the nucleus of every cell. C. Contains minimum of 150,000 genes, however only some genes are expressed in a particular cell determining its unique qualities. D. contains deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). II. DNA A. Located inside the chromosome. B. Function is to regulate cellular activity by protein synthesis. It is also necessary for cell division, DNA replication, transcription and production of RNA. C. Contains 2 sugar molecules formed in double helix. The sugar molecules have one of adenine, guanine, cytosine, thymine and are linked to one another by hydrogen bonds. Adenine has 2 hydrogen bonds with thymine and guanine has 3 bonds with cytosine. III. Nucleotide A. consists of sugar molecule, phosphate and either adenine, guanine, cytosine and thymine. B. group of 3 nucleotides (codon) indicates a specific amino acid. C. due to pairing of adenine-thymine and cytosine-guanine, nucleotide sequences of one strand of DNA dictate the sequences of the other strand. IV. Gene A . parts of DNA (made up of a number of nucleotides). B. coding a specific protein. V. Transcription A. A process where DNA is transcribed to mRNA via RNA polymerase. B. One of steps in protein synthesis-important in regulation of cell activity. VI. Translation A. a process of sequencing amino acids (which are coded by mRNA) in order to build protein. VII. Regulating DNA A. gene promoter: initiate transcription. B. consensus sequences: binding sites for regulatory proteins. C. gene enhances: binding sites for transcription proteins. VIII. Biogenetic techniques A. Restriction enzymes: cut DNA at a location determined by a specific nucleotide sequences. The resulting fragments are called restriction fragments. B. Agarose gel electrophoresis: negatively charged DNA fragments are put in a gel and exposed to electrical field. As a result, there is separate of DNA fragments based on size - the smaller the gragments, the closer they are to the positive pole of electrical field. C. DNA ligation: attached human genes to plasmids in order to study the genes. D. Plasmid vectors: so called for the human genes about to be ligated to plasmids. The products of DNA ligation is called recombinant plasmid. E. Transformation: insertion of recombinant plasmid into bacterium which then replicates the recombinant plasmid hence increases the amount for study. F. Genomic library: a library of the entire genome, in the form of recombinant DNAs. Achieved by restricting the genes, which is then ligated with plasmid and transformed into E.coli which amplify the amount of recombinant genes and the genes are then isolated from the bacterial colony. G. Transgenic animals: animals with foreign genes in every cells of their body due to insertion and incorporation into chromosomes of those foreign genes into a single cell embryo. H. Southern Hybridization: technique to find a particular DNA sequence in mixed DNA. I. Northern Hybridization: technique to find a particular RNA in mixed RNA. J. Polymerase Chain Reaction (PCR) amplification: to increase the amount of specific DNA sequence in vivo by repeatedly synthesizing it. Used to screen DNA for gene mutation. IX. Effects on orthopaedics A. screening of genetic diseases such as osteogenesis Imperfecta, Marfan syndrome, chondrodystrophies, Vitamin D-resistant rickets and familial aortic aneurysm. B. bone tumours are a result of failure in regulating cell growth due to gene mutation. Immunology: I. Nonspecific Immune response A. inflammatory responses following a recognition of a foreign antigen. It consists of Histamine release (cause vasodilatation) and phagocytic cells that only recognize the antigen as non-self (macrophage, neutrophil, etc.) and respond by enzymatically digesting the foreign materials. B. It can be enhanced by complement system or subsided by anti- flammatory medication. II. Specific immune responses A. cell mediated and humoral immune responses. B. antigens: when recognized as foreign, starts an immune response. C. macrophages: eat the foreign body and presents antigen on its surface to T lymphocytes. D. T lymphocytes: originate in bone marrow but mature in thymus. It consists of helper T cells, suppressor T cells and killer T cells. Helper T cells produce cytokines to stimulate B cells, macrophages, killer T cells and suppressor T cells. Killer T cells also produce cytokines to stimulate suppressor T cells. Suppressor T cells are then damped down the immune response once the antigens are cleared. E. B lymphocytes: originate and mature in bone marrow. Differentiate into plasma cells and memory cells upon stimulation. Plasma cells reduce immunoglobulins (lg) against specific antigens. F. Immunoglobulins: 5 types-IgA (releases to and acts on mucosal surfaces), lgM (only inside blood circulation, largest, produced earliest by foetus and plasma cells at first encounter with new antigen), lgG (commonest, produced after lgM in response to new infection), lgD (acts as a receptor), lgE (attach to mast cells, involved in allergic reactions). G. Cytokines: a) glycoproteins produced by cell in response to foreign antigen. b) role in regulation of inflammatory and immune reaction. c) they are interferons, growth factors, colony stimulating factors, interleukins. H. Complement system a) 25 proteins. b) acts in sequence to amplify immune response. I. Immunogenetics a) Located on the short arm of chromosome 6 called Human Leukocyte antigens (HLA). b) make up the specificity of immune recognition. c) 6 class I loci and 14 class II loci. J. Transplantation a) allogenic grafting: tissue transplanted to another non genetically identical of the same species. b) xenografting: tissues transplanted to another of different species. c) Graft preparation: a) Reduce cellular response by freezing. b) Eliminate cellular response by freeze- drying (lyophilisation). K. Oncology-tumour cells have cell surface antigens that are: a) identical to the ones on normal cells indicating their origin, however they may lose some of these. b) maybe identical to those on flat cells. c) tumour associated. PREOP. CHECKS 1. Pulmonary complications A. Blood Gas: • pO2 = 7(FiO2 where pO2 is the normal value for a given FiO2 is the percent spired oxygen and pCOs is the value taken from the test result. • Aa gradient = normal pO2 - test result pO2. • Percent physiologic shunt = Aa gradient /20. B. Thromboembolism: • common problem especially with hip procedures. • Risk factors: a history of thromboembolism, obesity, malignancy, age, congestive heart failure, birth control pill use, varicose veins, smoking, general anaesthetics, increased blood viscosity, immobilization, paralysis d pregnancy. 1. Deep venous thrombosis (DVT): • venography: 97% accurate (70% for iliac veins). • 125 l-labelled fibrinogen: false positive at operative site. • impedance plethysmography: sensitivity is poor. • duplex ultrasonography (B mode): sensitivity for DVT proximal to bifurcation is 90%. • Doppler imaging: bedside tool. • Prophylaxis such as heparin, coumadin (warfarin), aspirin, dextran, pneumatic compression, enoxaparin are important in morbidity and mortality. Coumadin can be reversed with Vitamin K or fresh frozen plasma. • Treatment is by heparin then followed by warfarin therapy for the next 3 months. All thigh DVTs need treatment, but those below popliteal fossa may not. Patients with preoperative finding of DVT require vena cava filter. 2. Pulmonary embolism (PE): • suspected in postoperative patients with acute onset of pleuritic pain, tachypnoea (90%) and tachycardia (60%). • ECG shows right bundle branch block, right axis deviation in 25%, may show ST depression or T wave inversion lead III. • Chest X ray will hardly show hyperlucency and ABGs may show normal PaOs but this does not exclude PE. • V/Q scan may show the site of embolus but otherwise pulmonary angiography is the gold standard. • Treatment with heparin (continuous IV infusion) and its efficacy is monitored with partial thromboplastin time (PTT) for 7-10 days then followed by oral warfarin for 3 months which is monitored using prothrombin time (PT). 3. Coagulation: a) intrinsic pathway: initiated by factor XII touching the exposed collagen in blood vessels. monitored by PTT. b) extrinsic pathway: initiated y presence of thromboplastin in the circulation due to cellular injury, monitored by PT>. c) platelet function is measured by bleeding time test. Fibrinolytic system is responsible in dissolving clot by converting plasminogen to plasmin and plasmin dissolves the clot. C. Adult Respiratory Distress Syndrome (ARDS): • acute respiratory failure following pulmonary oedema. Initiated by trauma, shock, infection etc. • Aetiologies: pulmonary infection, sepsis, fat embolism, microembolism, aspiration, fluid overload, atelectasis, oxygen toxicity, pulmonary contusion, head injury. • manifested as tachypnoea, dyspnoea, hypoxemia, decreased lung compliance. Diagnosis can be made using ABGs following a long bone fracture. • 50% mortality rate. • Treatment is by ventilation with PEEP, but best avoided by early stabilization of long bone fractures. D. Fat embolism: • 24-72 hrs after incidence of long bone fractures in 3-4% of patients. • fatal in 10-15%. • manifested as tachypnoea, tachycardia, mental status changes and upper extremity petechiae. • aetiologies: bone marrow fat (mechanical theory), chylomicron changes due to stress (metabolic theory). • Ventilation-perfusion deficit resulted consistent with ARDS. • Treatment is by ventilation with high levels of PEEP, but can be prevented by early stabilization of the fracture. E. Pneumonia: • aspiration pneumonia in patients with decreased mentation, supine position and decreased GI motility. • Preventative measures: raising the bed’s head, use of antacids and metoclopramide • Treatment by IV antibiotics and pulmonary toilet. F. Pulmonary complications of orthopaedic disorders include scoliosis, Marfan syndrome. II. Non pulmonary complications A. Nutrition • sufficient nutrition prior to elective surgery, otherwise complications such as wound dehiscence, infection, pneumonia and sepsis can occur. • indicators: arm muscle circumference size (best indicator), energy panels, albumin levels, transferrin levels. • atrophy of the intestinal mucosa can occur from lack of enteral feeding which then causes bacterial translocation. Early elemental feeding through a jejunostomi tube helps multiple trauma patients from complications. • Full enteral or parenteral nutrition should be given since energy requirement is elevated during stress. Protein supplements are beneficial in patients at risk of developing multi organ failure. B. Myocardial infarction (MI) • identified by acute chest pain, radiation, EKG changes. Confirmed by elevated cardiac enzymes. • Risk factors include age, smoking, increased cholesterol, hypertension, aortic stenosis, history of coronary artery disease, diabetes, family history of heart problems. C. GI complications • ileus : treated with nasogastric suction and antacids. Common in diabetic with neuropathy. • upper GI bleeding: risk factors include history of ulcers, NSAID use and smoking. Treated by lavage, antacids and H2-blockers, vasopressin at left gastric artery may be required for more serious cases. • Ogilvie syndrome: can follow total joint replacement surgery. Signified by caecum distention. If caecum > 10 cm on abdominal X-ray, decompression thru colonoscopy is required. D. Decubitus ulcers • risk factors include advanced age, critical illness and neurologic impairment. • usually at sacrum, heels and buttocks. • increase the risk of infections and morbidity. Once there, debridement and sometimes soft tissue flaps are required for healing. • prevented by constant changing of position, special mattresses and treatment of systemic illness and malnutrition. E. Urinary tract infections (UTI) • most common nosocomial infections. • resulting in increased risk of infections at joint following TJA. • Preoperatively, should be treated sufficiently. Preoperative catherisation may reduce the rate of postoperative UTI. Catheter is removed 24 hrs postoperatively. F. Prostatic hyperplasia • results in urinary retention and increased risk of UTI. • Should be sought before the surgery based on history, physical examination especially PR and urine flow studies < 17 ml/s peak flow rate. G. Acute tubular necrosis • results in renal failure in trauma patients. • Early treatment includes alkalisation of urine. H. Genitourinary injury • Retrograde urethrogram is used to find lower urinary tract injury. • Risks increased with the use of NSAID. Should be suspected in patients with displaced anterior pelvic fractures. I. Shock • insufficient perfusion to vital tissues and organs. • Hypovolaemic shock: due to volume loss - resulting in decreased cardiac output (CO), increased peripheral vascular resistance (PVR) and venous construction. Treated by Ringer’s lactate fluid, then blood transfusion. Massive blood replacement requires fresh frozen plasma and platelets. Urine output shows the adequacy of fluid resuscitation. Insufficient fluid resuscitation can result in metabolic acidosis. • Cardiogenic shock: due to ineffective pumping - resulting in decreased CO, increased PVR, venous dilation. • Vasogenic shock: due to PE or pericardial tamponade - arteriolar constriction, venous dilation. • Neurogenic shock/ septic shock: due to blood pooling- arteriolar, capillary and venous dilation. J. Compartment syndrome III. Intraoperative consideration A. Anaesthesia • local anaesthesia may allow quicker recovery, decreased blood loss and fewer postoperative complications. • reduced blood loss with controlled hypotension during surgery - by nitroprussode, nitroglycerine, isoflurane. • Patients with neuromuscular disorders (Duchenne’s muscular dystrophy, arthrogryposis, osteogenesis imperfecta) should be suspected for malignant hyperthermia (autosomal dominant, hypermetabolic disorder of skeletal muscles) following the use of anaesthetics especially halothane and succinylcholine. Disease is marked by muscle rigidity, hypermetabolism, masseter muscle spasm, increased temperature and acidosis due to defect of cell membrane affecting calcium transport. For definitive diagnosis, muscle biopsy is required. Treated by dantrolene, electrolytes balance increased urinary output, respiratory support and cooling. B. Spinal cord monitoring • usually testing the posterior column. • Somatosensory cortical evoked potentials (SCEP) monitors spinal cord by evoking response from stimulation of peripheral areas. • Somatosensory spinal evoked potentials (SSEP) are more invasive but more sensitive. Preoperative recordings are compared to the ones measured at critical times during surgery. • The wake up test is the standard, done by patients responds in moving the peripheral parts according to commands upon lightening of anaesthesia. C. Torniquet • can result to injury of nerve and muscle underneath it. • prevented by careful application, wide cuffs, lower pressure (200 mmHg in upper extremity and 250 mmHg or 100-150 mmHg above systolic in lower extremity), double cuffs. IV. Other problems A. Pain control • acute implies the presence of potential tissue damage whereas chronic (3-6 months) does not. • Postoperative control of pain can be mediated at the nociceptors, decrease transduction of the nerve A and C fibers, dorsal column, spinothalamic tract, thalamus (local prostaglandin inhibitors, long acting local anaesthetics); increase modulation of brain stem centres or production of endogenous opiates (perispinal and systemic opiates). B. Transfusion • transfusion reactions * allergic reaction: most common, occurs at the end of transfusion. Chills, pruritus, erythema and urticaria results but usually resolved spontaneously. Pretreated by diphenhydramine/Benadryl and hydrocortisone in patients with history of allergy. * febrile reaction: occurs after the initial 100-300cc of packed RBCs transfusion. Chills and fevers due to antibody reaction to foreign WBCs. Treatment is as allergy. * haemolytic reaction: occurs early in the transfusion. Signified by chills, fever, tachycardia, chest tightness, flank pain. Treated by stopping the transfusion, IV fluids, appropriate lab studies and monitoring in intensive care. • transfusion risks * hepatitis, cytomegalovirus, HTLV-1 and HIV. * donors deferral of high risk individuals and more effective screening methods decrease the prevalence. • alternative to homologous blood transfusion * autologous deposition: At least Hb=11 and Hematocrit of 33%. Iron supplement during donation is routine. Storage of several units of blood prior to elective surgery with anticipated blood loss. Need adequate times between the donation and surgery. * cell saver- intraoperative autotransfusion: 400 ml blood loss to recover 250 ml. Can be used for only 4 hours at one time. * autotransfusion: postoperative drain recuperation is used. * acute preoperative normovolemic hemodilution: storage of autologous blood (replaced by crystalloids) preoperatively for immediate use of intra/postoperatively. * pharmacologic intervention: desmopression (ADH) analogue to increase level of factor VIII, recombinant erythropoietin to increase erythropoiesis and synthetic erythrocyte substitutes. * judicious use of blood products: platelet transfusion in massive bleeding or coagulopathies. Fresh frozen plasma for patients with massive bleeding and abnormal coagulation tests. Cryoprecipitate for haemophilia and for consumptive coagulopathies as a source of fibrinogen. C. Heterotopic ossification • seen after THA, in head injured patients and in elbow injuries patients. • Prophylaxis during THA: indomethacin. Diphosphonate does not prevent formation of osteoid matrix since the matrix calcifies after discontinuation of medication. Etidronate sodium prevents bone resorption at low dose and bone mineralization at high dose. Functions of the skeleton Bone, the material that makes vertebrates distinct from other animals, has evolved over several hundred million years to become a remarkable tissue. Bone is a material that has the same strength as cast iron, but achieves this while remaining as light as wood. The front leg of a horse can withstand the loads generated while this 1500-pound animal travels at 30 miles per hour. The upper arm is able to keep birds aloft through entire migrations, sometimes over 10,000 miles without landing. The antlers of deer, used as weapons in territorial clashes with other deer, undergo tremendous impacts without fracturing, ready to fight another day. At some point, unfortunately, forces of impact exceed even bone's ability to hold up. Falling on the ice, suffering a collision in a car or a tumble on the ski slopes can cause the bone to fail. While fractures are disastrous, bone - because it is a live tissue - almost instantly begins a healing process. Without question, bone is the ultimate biomaterial. It is light, strong, can adapt to its functional demands, and repair itself. Functions of the skeleton * Structural support for heart, lungs and marrow * Protection for brain, uterus, and other internal organs * Attachment sites for muscles allowing movement of limbs * Mineral reservoir for calcium and phosphorus * Defense against acidosis * Trap for some dangerous minerals such as lead Bone architecture There are two major kinds of bone, trabecular (spongy) and cortical. Trabecular bone gives supporting strength to the ends of the weight-bearing bone. The cortical (solid) bone on the outside forms the shaft of the long bone. This xray of a femur shows the thick cortical bone, and the trabecular bone which is arranged to withstand the stresses from usual standing and walking. Compressive stresses are those of the body weight pushing the bone down, and tensile stresses are from the muscles, pulling the bone apart. Photo courtesy of Clint Rubin Move mouse over image for labels Click image for a new window with larger view Magnified view of a cut surface of bone, showing the cortical bone and trabecular bone surrounded by marrow tissue (M). This is from the iliac crest, part of the pelvic bone. The actual width is about 1 cm. Further magnification demonstrates the organization of the cortical bone into Haversian systems, consisting of concentric layers of bone and a central canal which supplies blood. The small black dots are spaces that contain osteocytes. The boundaries between Haversian systems are the cement lines. Mineral reservoir In addition to its mechanical functions, the bone is a reservoir for minerals (a "metabolic" function). The bone stores 99% of the body's calcium and 85% of the phosphorus. It is very important to keep the blood level of calcium within a narrow range. If blood calcium gets too high or too low, the muscles and nerves will not function. In times of need, for example, during pregnancy, calcium can be removed from the bones. This process is carefully regulated by hormones and is discussed more completely in the section about hormones. Bone material properties Components of bone: * The organic matrix is composed primarily of the protein collagen which provides flexibility. 10% of adult bone mass is collagen. This is discussed further in the section on collagen and bone matrix * The mineral component is composed of hydroxyapatite, which is an insoluble salt of calcium and phosphorus. About 65% of adult bone mass is hydroxyapatite. * Bone also contains small amounts of magnesium, sodium, and bicarbonate. * Water comprises approximately 25% of adult bone mass. Changes with aging This graph shows values for bone mineral density at the hip in Caucasian men and women and African-American men and women. With aging, bone density decreases in all groups. This inevitable bone loss is frequently the cause of osteoporosis This image shows trabecular bone structure in the lower spine of a young adult compared to an osteoporotic elderly adult. Young adult Elderly Adult with osteoporosis Bone Cells There are two categories of bone cells. Osteoclasts are in the first category. They resorb (dissolve) the bone. The other category is the osteoblast family, which consists of osteoblasts that form bone, osteocytes that help maintain bone, and lining cells that cover the surface of the bone. Osteoclasts * . . . are large cells with many nuclei. * . . . share lineage with blood cells (especially macrophages). * Precursors circulate in the blood and bone marrow. * Mature osteoclasts are formed from fusion of the precursors. * This happens when RANK receptors on the osteoclast precursors are activated by the RANK-ligand which was secreted by osteoblasts. * Osteoprotegerin (OPG) is a factor in the marrow which also binds RANK-ligand, so it can help to regulate the osteoclast activation. * Osteoclasts resorb the bone. They form sealed compartments next to the bone surface and secrete acids and enzymes which degrade the bone. The edge next to the bone is called the ruffled border. * After they finish resorbing bone, they undergo apoptosis (programmed cell death, sometimes called 'cell suicide'). This process is regulated by proteins from other cells. Osteoblasts *. . . are cuboidal and columnar in shape with a central nucleus found on the bone surface. *Gap junctions with neighbouring osteoblasts allow cells to communicate with each other. *They come from bone marrow precursor cells. These precursors are capable of turning into either osteoblasts or fat cells, and various factors determine which kind of cells will be made. One of the factors is called Cbfa 1, which will cause the cell to differentiate into an osteoblast. *The job of osteoblasts is to make the proteins that will form the organic matrix of bone and to control mineralization of the bone *They have receptors for hormones such as vitamin D, oestrogen, and parathyroid hormone. *They secrete factors that activate osteoclasts (RANK-ligand) and other factors which communicate with other cells. *They secrete PHEX, a protein that helps to regulate the amount of phosphate excreted by the kidney. *When the team of osteoblasts has finished making new bone, some become surrounded with matrix and differentiate into osteocytes. Others will remain on the surface of the new bone and differentiate into lining cells. The rest undergo apoptosis (cell suicide) and disintegrate. Osteocytes * . . . live inside the bone and have long branches which allow them to contact each other as well as the lining cells on the bone surface. * . . . are in a perfect position to sense any mechanical strain on the bone. * . . . can secrete growth factors which activate the lining cells or stimulate the osteoblasts. * Their exact role is still under investigation, but probably the osteocytes direct bone remodelling to accomodate mechanical strain and repair fatigue damage. Lining cells * . . . are former osteoblasts which have become flat and pancake-shaped. * . . . line the entire surface of the bone. * . . . are responsible for immediate release of calcium from the bone if the blood calcium is too low. * . . . protect the bone from chemicals in the blood which dissolve crystals (such as pyrophosphate). * . . . have receptors for hormones and factors that initiate bone remodelling. Bone cell origins This diagram summarizes the origins and fates of the bone cells. Mesenchymal refers to cells which were deep within the embryo during early development; some of them remain in the bone marrow but do not form blood cells. The haematopoietic cells form the liquid part of the bone marrow, and some of them circulate with the blood. Local regulators Bone cells produce molecules (usually proteins) that communicate with other cells. These molecules are called growth factors and cytokines. They act on nearby cells, and thus are considered local regulators. These factors control cell division (proliferation), differentiation, and survival. Growth factors Bone morphogenetic proteins (BMPs): BMPs are produced in the bone or bone marrow. They bind to BMP receptors that are on mesenchymal stem cells within the bone marrow. This causes the cells to produce Cbfa 1, which is a factor that activates the DNA so proteins can be made -- a process known as gene transcription. When Cbfa 1 activates the genes, the cells differentiate into mature osteoblasts. Without Cbfa 1, the cells would turn into fat cells instead! Insulin-like growth factors (IGFs): These growth factors are produced by osteoblastic cells in response to several bone active hormones, such as parathyroid hormone and estrogens, or BMPs. IGFs accumulate in the bone matrix and are released during the process of bone remodelling by osteoclasts. IGFs stimulate osteoblastic cell replication -- in other words, they cause the osteoblasts to divide, forming new cells. They may also induce differentiation. Cytokines Interleukin-1 (IL-1), interleukin-6 (IL-6), and tumor necrosis factor (TNF) family of cytokines: These factors are produced by osteoblastic cells in response to systemic hormones or other cytokines. IL-6 can cause: * Bone marrow stem cells to differentiate into pre-osteoclasts * Changes in proliferation and differentiation of osteoblasts * Inhibition of apoptosis of osteoblasts RANKL (RANK-ligand) is a cytokine that stays on the surface of osteoblast-related cells. The cells make RANKL in response to systemic hormones (such as 1,25dihydroxyvitamin D3) and cytokines (such as IL-6). Cell contact between RANKL-expressing osteoblastic cells and RANK-expressing osteoclast precursors induces osteoclast development, as shown in the animation in the osteoclast section. Apoptosis The mature bone is always remodelling: the old bone is resorbed and replaced with new bone. A team of osteoblasts and osteoclasts move along the bone, dissolving and rebuilding. What happens to the cells when they have finished rebuilding an area of bone? The osteoclasts and most of the osteoblasts undergo a process called apoptosis, or cell suicide. They are not killed. There is no lack of oxygen or nutrients. There are no toxic materials. Instead, there are genes in the cell which can be activated, causing the cell to disintegrate. These genes (of course) are carefully regulated within the cell. The factors that regulate apoptosis are currently under investigation. Some are related to estrogens, or to interleukins. Medications which could modify apoptosis have the potential for treating or preventing osteoporosis. Collagen and Bone Matrix Mature bone is composed of proteins and minerals. Approximately 60% the weight of the bone is mineral, mainly calcium and phosphate. The rest is water and matrix, which is formed before the mineral is deposited, and can be considered the scaffolding for the bone. About 90% of the matrix proteins are collagen, which is the most abundant protein in the body. Collagen is very strong and forms bone, cartilage, skin, and tendons. How you would construct a strong but light material from only the following 3 ingredients: short threads, plaster of Paris, and glue? The best way would be to twist or braid the thread into strings, then glue the strings into ropes, then lay the ropes in a pattern and pour the plaster of Paris over them, the way concrete is poured over steel rebar. Collagen is formed as chains (short pieces of thread) which twist into triple helices (strings). These line up and are bonded together into ropes (fibrils). The fibrils then are arranged in layers, and mineral crystals will deposit between the layers. Below is a scanning electron image (B) that shows bone collagen fibrils in both longitudinal and cross sections. The back-scattered electron image (C) shows the regular patterns of collagen in layers in bone, which is why normal bone is called lamellar bone. The following table gives information about some of the other bone matrix proteins that seem most important. For some of these, the gene has been deleted in special populations of mice (called "knockout mice") to try to understand the function of the protein. NAME COMMENTS Effect of "knockout" Relatively abundant, may help regulate osteoblast Fibronectin Lethal differentiation Osteonectin "Bone connector" may regulate mineralization Osteoporosis Thrombospondin May inhibit bone cell precursors Dense bones Osteocalcin Binds calcium Bones seem normal Normal bones but Matrix-gla-protein Inhibits mineralization calcified blood vessels Other matrix proteins SIBLINGS - small integrin-binding ligand, N-linked glycoprotein family Bone sialoprotein Binds to integrins, may assist cancer cells - Resistance to PTH Increases angiogenesis (makes new blood vessels) which Osteopontin and removal of enhances bone resorption in some situations ovaries Matrix extracellular May induce a bone disease called osteomalacia - protein Proteoglycans - proteins with many attached sugars Biglycan Function uncertain Osteopenia Parathyroid hormone Parathyroid Hormone (PTH) is a peptide hormone produced by the parathyroid glands. It binds to receptors in the bone and kidney. A decrease in serum calcium concentration and an increase in serum phosphorous concentration stimulate PTH secretion. PTH also: * stimulates osteoclastic bone resorption indirectly to release calcium from bone. * stimulates bone formation that is coupled to bone resorption. * increases renal tubular reabsorption of calcium. * stimulates the renal production of 1,25 dihydroxyvitamin D to increase calcium absorption from the intestine. * enhances renal phosphate and bicarbonate excretion. Calcitonin Calcitonin is a peptide hormone produced by cells within the thyroid gland. Calcitonin secretion is stimulated by high blood calcium concentrations, and it acts as a physiologic antagonist to PTH. Osteoclasts have receptors for calcitonin, but the effects are transient. Calcitonin also: * inhibits osteoclast resorption * delays calcium absorption from the intestine * increases calcium urinary excretion Vitamin D 1,25 dihydroxyvitamin D is an active hormone which is produced by the kidney, under the control of PTH, from precursors of dietary vitamin D intake and UV skin- production of vitamin D. It is not really a vitamin, but the name was given many years ago, before anybody knew the function of this molecule. Vitamin D receptors are present in bone, kidney, intestines, and other cells. The chemical name is 1,25- dihydroxy-cholecalciferol, and it: * promotes gastrointestinal absorption of calcium and phosphorus. * is necessary for bone mineralization. * stimulates bone resorption when given in high doses. Gonadal steroids Gonadal steroids are produced by the ovaries and testes and are very important in maintaining bone balance. They are also important in normal growth and development and in the development of peak bone mass. The mechanism of action is unclear but receptors for oestrogen and androgen are found in bone. Estrogens: * are the principal circulating sex steroids in females. * are also necessary for bone strength in males. * help regulate the rates of bone formation and bone resorption. * decrease after menopause, contributing to development of osteoporosis. Androgens (such as testosterone): * are necessary for bone Growth hormone Growth Hormone (GH) is a growth promoting hormone produced by the pituitary gland. It is "anabolic", which means it stimulates bone formation. Growth hormone also: * stimulates the production of insulin-like growth factor 1 (IGF-1) by the skeleton. * is important in stimulating longitudinal growth. * Can increase bone mass when given to adults. Thyroid hormone Thyroid hormone is produced by the thyroid gland. Bone cells have receptors for thyroid. This hormone also: * is necessary for growth and maturation of the skeleton. * causes increased osteoclastic bone resorption and osteoporosis when levels are too high. Glucocorticoids Glucocorticoid (also called cortisol) is produced by the adrenal gland. Bone cells have receptors for glucocorticoid. This hormone is absolutely essential for life, but excess levels cause multiple deleterious effects on the skeleton. This steroid hormone: * decreases calcium absorption from the intestines. * inhibits bone formation. * increases bone resorption. * increases renal calcium excretion. * decreases sex steroid production. Summary of the effects of hormones on skeletal metabolism Increase Bone resorption Parathyroid hormone Glucocorticoids Thyroid Hormone Vitamin D metabolites in high doses Decrease Bone Resorption Calcitonin Gonadal steroids Increase Bone Formation Growth hormone Vitamin D metabolites Gonadal steroids Decrease Bone Formation Glucocorticoids The D Stitch: An Efficient Method to Facilitate Suture Removal Harkeerat Dhillon, MD, FRCS, MS (ORTH); Vaneet Randhawa, MD. ORTHOPEDICS January 09 The “D stitch” method is a simplified and improved method of suture removal that has shown promising results. In any surgical procedure involving finely placed sutures, suture removal becomes an intricate and tedious task. The “D stitch” is a means of facilitating this technique, reducing both the time necessary for suture removal as well as reducing discomfort to the patient. Materials and Methods In the development of the D stitch, a 4-0 nylon mattress suture and 2-0 nylon (D stitch) suture were typically used. Twenty-five patients were assessed by medical staff at the time of suture removal. This assessment took into account not only the level of difficulty in suture removal, but also the anxiety felt by the patient. On a difficulty scale, with 0/5 being very difficult and 5/5 being very easy, use of the D stitch revealed 11 scores of 5/5, 5 scores of 4/5, and 1 score of 3/5. Standard suture removal, however, used here as the “control,” yielded 1 case of 0/5, 2 cases of 1/5, and 5 cases of 2/5. Figure 1: Application of initial mattress suture (A, B). Surgical Technique At the time of wound closure, an initial mattress suture (or continuous, plain, transverse suture) is placed (Figure 1), in this case, using a 4-0 nylon suture. Before tying the knot, the D stitch is introduced using a 2-0 nylon suture that is placed along the incision, taking care that the 2-0 thread is situated between the two 4-0 threads of the mattress suture (Figure 2A). The rest of the incision is closed by repeating the above process with each suture, but care must be taken to ensure that the 2-0 nylon thread, used here as the D stitch, is positioned adjacent to the incision line between the suture knots (Figure 2B). On completion of the final suture, both ends of the D stitch are brought together and tied in a simple knot (Figure 2C), thus forming a “D” shape (Figure 2C). At the time of suture removal, the D stitch (Figure 2C) is cut and easily pulled out from beneath the incision-line sutures, thus leaving a gap between the suture knots and underlying incision line. This gap facilitates the removal of finely placed sutures by allowing the tip of the suture cutting scissor blades to be introduced into this gap. Figure 2: Introduction of the “D stitch” between the threads of the incision-line suture (A, B), followed by approximating and knotting of the D stitch and removal of sutures by cutting the D stitch, arrow (C). Figure 3: D stitch over a nonlinear incision (A, B). Principles of Bone Healing Our contemporary understanding of bone healing has evolved due to knowledge gleaned from a continuous interaction between basic laboratory investigations and clinical observations following procedures to augment healing of fractures, osseous defects, and unstable joints. The stages of bone healing parallel the early stages of bone development. The bone healing process is greatly influenced by a variety of systemic and local factors. A thorough understanding of the basic science of bone healing as well as the many factors that can affect it is critical to the management of a variety of musculoskeletal disorders. In particular, the evolving management of spinal disorders can greatly benefit from the advancement of our understanding of the principles of bone healing. Introduction Bone is a dynamic biological tissue composed of metabolically active cells that are integrated into a rigid framework. The healing potential of bone, whether in a fracture or fusion model, is influenced by a variety of biochemical, biomechanical, cellular, hormonal, and pathological mechanisms. A continuously occurring state of bone deposition, resorption, and remodelling facilitates the healing process. The success of many spine operations depends on the restoration of long-term spinal stability. Whereas spinal instrumentation devices may provide temporary support, a solid osseous union must be achieved to provide permanent stability. The failure of fusion to occur may result in the fatigue and failure of supporting instrumentation and persistence or worsening of symptoms. Understanding the basic biological and physiological principles of bone transplantation and healing will aid the spine surgeon in selecting the most effective techniques to achieve successful fusions. In this paper the anatomical, histological and biological features of this process will be reviewed. one Anatomy and Histology The cellular components of bone consist of osteogenic precursor cells, osteoblasts, osteoclasts, osteocytes, and the hematopoietic elements of bone marrow.[10,22] Osteoprogenitor cells are present on all nonresorptive bone surfaces, and they make up the deep layer of the periosteum, which invests the outer surface of bone, and the endosteum, which lines the internal medullary surfaces. The periosteum is a tough, vascular layer of connective tissue that covers the bone but not its articulating surfaces. The thick outer layer, termed the "fibrous layer," consists of irregular, dense connective tissue. A thinner, poorly defined inner layer called the "osteogenic layer" is made up of osteogenic cells. The endosteum is a single layer of osteogenic cells lacking a fibrous component. Osteoblasts are mature, metabolically active, bone-forming cells. They secrete osteoid, the unmineralized organic matrix that subsequently undergoes mineralization, giving the bone its strength and rigidity. As their bone-forming activity nears completion, some osteoblasts are converted into osteocytes whereas others remain on the periosteal or endosteal surfaces of bone as lining cells. Osteoblasts also play a role in the activation of bone resorption by osteoclasts. Osteocytes are mature osteoblasts trapped within the bone matrix. From each osteocyte a network of cytoplasmic processes extends through cylindrical canaliculi to blood vessels and other osteocytes. These cells are involved in the control of extracellular concentration of calcium and phosphorus, as well as in adaptive remodelling behaviour via cell-to-cell interactions in response to local environment. Osteoclasts are multinucleated, bone-resorbing cells controlled by hormonal and cellular mechanisms. These cells function in groups termed "cutting cones" that attach to bare bone surfaces and, by releasing hydrolytic enzymes, dissolve the inorganic and organic matrices of bone and calcified cartilage. This process results in the formation of shallow erosive pits on the bone surface called Howship lacunae. There are three primary types of bone: woven bone, cortical bone, and cancellous bone.[10,22] Woven bone is found during embryonic development, during fracture healing (callus formation), and in some pathological states such as hyperparathyroidism and Paget disease. It is composed of randomly arranged collagen bundles and irregularly shaped vascular spaces lined with osteoblasts. Woven bone is normally remodeled and replaced with cortical or cancellous bone. Cortical bone, also called compact or lamellar bone, is remodeled from woven bone by means of vascular channels that invade the embryonic bone from its periosteal and endosteal surfaces. It forms the internal and external tables of flat bones and the external surfaces of long bones. The primary structural unit of cortical bone is an osteon, also known as a haversian system. Osteons consist of cylindrical shaped lamellar bone that surrounds longitudinally oriented vascular channels called haversian canals. Horizontally oriented canals (Volkmann canals) connect adjacent osteons. The mechanical strength of cortical bone depends on the tight packing of the osteons. Cancellous bone (trabecular bone) lies between cortical bone surfaces and consists of a network of honeycombed interstices containing hematopoietic elements and bony trabeculae. The trabeculae are predominantly oriented perpendicular to external forces to provide structural support. Cancellous bone is continually undergoing remodelling on the internal endosteal surfaces. Bone Biochemistry Bone is composed of organic and inorganic elements. By weight, bone is approximately 20% water. The weight of dry bone is made up of inorganic calcium phosphate (65-70% of the weight) and an organic matrix of fibrous protein and collagen (30-35% of the weight).[10,19,21,22] Osteoid is the unmineralized organic matrix secreted by osteoblasts. It is composed of 90% type I collagen and 10% ground substance, which consists of noncollagenous proteins, glycoproteins, proteoglycans, peptides, carbohydrates, and lipids.[20,22] The mineralization of osteoid by inorganic mineral salts provides bone with its strength and rigidity. The inorganic content of bone consists primarily of calcium phosphate and calcium carbonate, with small quantities of magnesium, fluoride, and sodium. The mineral crystals form hydroxyapatite, which precipitates in an orderly arrangement around the collagen fibers of the osteoid. The initial calcification of osteoid typically occurs within a few days of secretion but is completed over the course of several months. Regulators of Bone Metabolism Bone metabolism is under constant regulation by a host of hormonal and local factors. Three of the calcitropic hormones that most affect bone metabolism are parathyroid hormone, vitamin D, and calcitonin. Parathyroid hormone increases the flow of calcium into the calcium pool and maintains the body's extracellular calcium levels at a relatively constant level. Osteoblasts are the only bone cells that have parathyroid hormone receptors. This hormone can induce cytoskeletal changes in osteoblasts. Vitamin D stimulates intestinal and renal calcium-binding proteins and facilitates active calcium transport. Calcitonin is secreted by the parafollicular cells of the thyroid gland in response to an acutely rising plasma calcium level. Calcitonin serves to inhibit calcium-dependent cellular metabolic activity. Bone metabolism is also affected by a series of proteins, or growth factors, released from platelets, macrophages, and fibroblasts. These proteins cause healing bone to vascularize, solidify, incorporate, and function mechanically. They can induce mesenchymal-derived cells, such as monocytes and fibroblasts, to migrate, proliferate, and differentiate into bone cells. The proteins that enhance bone healing include the BMPs, insulin-like growth factors, transforming growth factors, platelet derived growth factor, and fibroblast growth factor among others.[18,32] The most well known of these proteins are the BMPs, a family of glycoproteins derived from bone matrix. Bone morphogenetic proteins induce mesenchymal cells to differentiate into bone cells. Although typically present in only minute quantities in the body, several BMPs have been synthesized using recombinant DNA technology and are currently undergoing clinical trials to assess their potential to facilitate bone fusion in humans.[26-28] Other proteins influence bone healing in different ways. Transforming growth factor-regulates angiogenesis, bone formation, extracellular matrix synthesis, and controls cell-mediated activities. Osteonectin, fibronectin, osteonectin, and osteocalcin promote cell attachment, facilitate cell migration, and activate cells. Physiology of Bone Repair and Fusion The use of a bone graft for purposes of achieving arthrodesis is affected by each of the aforementioned anatomical, histological, and biochemical principles. Additionally, several physiological properties of bone grafts directly affect the success or failure of graft incorporation. These properties are osteogenesis, osteoinduction, and osteoconduction. Osteogenesis is the ability of the graft to produce new bone, and this process is dependent on the presence of live bone cells in the graft. Osteogenic graft materials contain viable cells with the ability to form bone (osteoprogenitor cells) or the potential to differentiate into bone-forming cells (inducible osteogenic precursor cells). These cells, which participate in the early stages of the healing process to unite the graft with the host bone, must be protected during the grafting procedure to ensure viability. Osteogenesis is a property found only in fresh autogenous bone and in bone marrow cells, although the authors of radiolabeling studies of graft cells have shown that very few of these transplanted cells survive. Osteoconduction is the physical property of the graft to serve as a scaffold for viable bone healing. Osteoconduction allows for the ingrowth of neovasculature and the infiltration of osteogenic precursor cells into the graft site. Osteoconductive properties are found in cancellous autografts and allografts, demineralized bone matrix, hydroxyapatite, collagen, and calcium phosphate. Osteoinduction is the ability of graft material to induce stem cells to differentiate into mature bone cells. This process is typically associated with the presence of bone growth factors within the graft material or as a supplement to the bone graft. Bone morphogenic proteins and demineralized bone matrix are the principal osteoinductive materials. To a much lesser degree, autograft and allograft bone also have some osteoinductive properties. Process of bone graft incorporation in a spinal fusion model is similar to the bone healing process that occurs in fractured long bones. Fracture healing restores the tissue to its original physical and mechanical properties and is influenced by a variety of systemic and local factors. Healing occurs in three distinct but overlapping stages: 1) the early inflammatory stage; 2) the repair stage; and 3) the late remodelling stage.[9,13] In the inflammatory stage, a hematoma develops within the fracture site during the first few hours and days. Inflammatory cells (macrophages, monocytes, lymphocytes, and polymorphonuclear cells) and fibroblasts infiltrate the bone under prostaglandin mediation. This results in the formation of granulation tissue, ingrowth of vascular tissue, and migration of mesenchymal cells. The primary nutrient and oxygen supply of this early process is provided by the exposed cancellous bone and muscle. The use of anti-inflammatory or cytotoxic medication during this 1st week may alter the inflammatory response and inhibit bone healing. During the repair stage, fibroblasts begin to lay down a stroma that helps support vascular ingrowth. It is during this stage that the presence of nicotine in the system can inhibit this capillary ingrowth.[11,23-25] A significantly decreased union rate had been consistently demonstrated in tobacco abusers.[2,3,6] As vascular ingrowth progresses, a collagen matrix is laid down while osteoid is secreted and subsequently mineralized, which leads to the formation of a soft callus around the repair site. In terms of resistance to movement, this callus is very weak in the first 4 to 6 weeks of the healing process and requires adequate protection in the form of bracing or internal fixation. Eventually, the callus ossifies, forming a bridge of woven bone between the fracture fragments. Alternatively, if proper immobilization is not used, ossification of the callus may not occur, and an unstable fibrous union may develop instead. Fracture healing is completed during the remodelling stage in which the healing bone is restored to its original shape, structure, and mechanical strength. Remodelling of the bone occurs slowly over months to years and is facilitated by mechanical stress placed on the bone. As the fracture site is exposed to an axial loading force, bone is generally laid down where it is needed and resorbed from where it is not needed. Adequate strength is typically achieved in 3 to 6 months. Although the physiological stages of bone repair in the spinal fusion model are similar to those that occur in long bone fractures, there are some differences. Unlike long bone fractures, bone grafts are used in spinal fusion procedures. During the spinal fusion healing process, bone grafts are incorporated by an integrated process in which old necrotic bone is slowly resorbed and simultaneously replaced with new viable bone. This incorporation process is termed "creeping substitution."[17,20] Primitive mesenchymal cells differentiate into osteoblasts that deposit osteoid around cores of necrotic bone. This process of bone deposition and remodelling eventually results in the replacement of necrotic bone within the graft. The most critical period of bone healing is the first 1 to 2 weeks in which inflammation and revascularization occur. The incorporation and remodelling of a bone graft require that mesenchymal cells have vascular access to the graft to differentiate into osteoblasts and osteoclasts. A variety of systemic factors can inhibit bone healing, including cigarette smoking, malnutrition, diabetes, rheumatoid arthritis, and osteoporosis. In particular, during the 1st week of bone healing, steroid medications, cytotoxic agents, and nonsteroidal anti-inflammatory medications can have harmful effects. Irradiation of the fusion site within the first 2 to 3 weeks can inhibit cell proliferation and induce an acute vasculitis that significantly compromises bone healing (SE Emery, unpublished data). Bone grafts are also strongly influenced by local mechanical forces during the remodelling stage. The density, geometry, thickness, and trabecular orientation of bone can change depending on the mechanical demands of the graft. In 1892, Wolff first popularized the concept of structural adaptation of bone, noting that bone placed under compressive or tensile stress is remodeled. Bone is formed where stresses require its presence and resorbed where stresses do not require it.[22,31] This serves to optimize the structural strength of the graft. Conversely, if the graft is significantly shielded from mechanical stresses, as in the case of rigid spinal implants, excessive bone resorption can potentially occur and result in a weakening of the graft. This potential disadvantage of instrumentation needs to be balanced with the beneficial effects that spinal fixation has on the fusion process. Bone Grafts The two types of bone grafts frequently used in spinal fusion are autografts and allografts. Autograft bone is transplanted from another part of the recipient's body. Allograft bone is transplanted from genetically nonidentical members of the same species. Both types of bone grafts are commonly used in spine surgery. The ideal bone graft should be: 1) osteoinductive and conductive; 2) biomechanically stable; 3) disease free; and 4) contain minimal antigenic factors. These features are all present with autograft bone. The disadvantages of autografts include the need for a separate incision for harvesting, increased operating time and blood loss, the risk of donor-site complications, and the frequent insufficient quantity of bone graft. The advantage of allograft bone is that it avoids the morbidity associated with donor-site complications and is readily available in the desired configuration and quantity. The disadvantages of allograft include delayed vascular penetration, slow bone formation, accelerated bone resorption, and delayed or incomplete graft incorporation. In general, allograft bone has a higher incidence of non-union or delayed union than autograft. Allografts are osteoconductive but are only weakly osteoinductive. Although transmission of infection and lack of histocompatibility are potential problems with allograft bone, improved tissue-banking standards have greatly reduced their incidence. Bone grafts can also be classified according to their structural anatomy: cortical or cancellous. Cortical bone has fewer osteoblasts and osteocytes, less surface area per unit weight, and contributes a barrier to vascular ingrowth and remodelling compared with cancellous bone. The advantage of cortical bone is its superior structural strength. The initial remodelling response to cortical bone is resorptive as osteoclastic activity predominates. Cortical grafts progressively weaken with time because of this bone resorption as well as slow, incomplete remodelling. Conversely, cancellous bone becomes progressively stronger because of its ability to induce early, rapid, new bone formation. When selecting a bone graft, the spine surgeon needs to consider the specific structural and biological demands that will be placed on the graft. If the graft is placed anteriorly in a compressive mode, cortical bone, either autogenic or allogenic, will be required. If placed posteriorly as a graft under tension with lower demands for structural support but also a lower probability of early vascular ingrowth, a cancellous autograft is preferred. Conclusions An understanding of the basic science of bone healing is critical to the consistent success of spinal fusion surgery. Although great advances have been made in the field of spinal instrumentation, it is only a solid osseous union that will ensure long-term spinal stability. Selection of the most appropriate bone graft material as well as careful attention to the principles of bone healing can greatly facilitate the potential for clinical success.