Joint Structure 1 1. Fibrous (synarthosis) i.e. sutures in skull 2. Cartilaginous (amphiarthrosis) i.e. ribs 3. Synovial (diatrhosis) i.e. limbs 2 3 Joint Structure Joint Histology • The articular cartilage (c) does not contain nerves (pain receptors), blood or lymphatic vessels. • The articular cartilage is firmly attached to the subchondral bone (asterisks). • The articular cartilage has little capacity for repair. • Nutrients for cartilage are provided by synovial fluid. Banks JB, Applied Veterinary Histology, William Wilkins Synovial Fossae (arrows): – Bilateral depression with no cartilage – Not clearly visible at birth – Lubrication? – Ooops!! (inexperienced pathologists may mistake these structures as a lesion) Fibrillation • Early degenerative change • Loss of proteoglycans • Degradation of collagen fibers • Increase water content • Dull yellowish appearance normal fibrillation Fibrillation • Fibrillation is an early degenerative change of the articular cartilage. • The causes of fibrillation are not specific and include trauma, irritation and chemical injury induced by enzymes released by leukocytes during inflammation. g • Fibrillation is biochemically characterized by loss of proteoglycans, degradation of collagen fibers and i fib t t t f d increase water content of chondrocytes. • Grossly, Grossly fibrillated cartilage appears dull and has a yellowish discoloration as shown in this picture (arrowheads). Fibrillation / HE Section • Microscopically, fibrillation is characterized by vertical fissures in cartilage (c). Note the fissure delineated illustration by the arrows in this illustration. • These cartilaginous fissures become filled with synovial fluid and eventually with g (g) granulation tissue (g). • As fibrillation progresses, the fissures expand through the entire thickness of the cartilage until the subchondral bone is h d d d to i l fluid. reached and exposed t synovial fl id • If the injury is severe, the surrounding cartilage becomes loose and detaches bone. from the subchondral bone Pieces of detached cartilage floating in the synovial space are called “joint mice.” Eburnation Eburnation is a degenerative condition characterized by the loss of the entire articular cartilage and exposure of the subchondral bone in one or more areas of the joint (arrowheads). friction, With mechanical friction the subchondral becomes polished and takes an ivory-like appearance, hence the name eburnation (ivory-like). The subchondral bone thickens, osteosclerosis. creating a condition known as osteosclerosis Clapping the affected joint with a knife or any th t l bj t ld l b other metal object would reveal an abnormal l sound (bone to metal) rather than the dull sound (metal on cartilage). This technique is routinely used during a postmortem i ti to fi b ti examination t confirm eburnation. Eburnation / Dog g p joint appears This is a femoral head showing eburnation. The superficial bone at the site of the j pp thicken, polished and smooth. This macerated specimen was obtained from a case of chronic degenerative joint disease in a g p y p dog with “hip dysplasia.” Cartilage Fibrillation and loss of attachment to the b h d l bone subchondral b Dog / Osteochondrosis An important degenerative change in articular cartilage associated with fibrillation is the loss of attachment to the bone. subchondral bone Pieces of degenerating cartilage are released into the joint cavity forming the so-called joint mice. This is a picture of the humeral head of a dog with chronic degenerative joint disease affecting the shoulder. The dog had lameness that started as a subtle gait abnormality. The dog was euthanatized. Note the fibrillation and deep fissures involving the articular cartilage of the humerus (arrowheads). Joint Mice / Shoulder Dog Joint mice are viable pieces of detached articular cartilage free floating in the synovial cavity. These pieces of detached cartilage are nourished by the synovial fluid and remain viable and continue to grow for long g g periods of time. Joint mice are a common sequel to degenerative j g , particularly joint diseases, p y osteochondrosis. This is a picture of the shoulder joint of a dog with chronic degenerative joint disease. Note the four pieces of free cartilage in the synovial space (arrowheads). Also note a large piece of degenerating articular cartilage in the head of the humerus (asterisk) which appears to be near detachment. Osteophytes Osteophytes are bone outgrowths derived from chondrification of fibrous tissue. It is a non-specific lesion which i f d in is found i many d ti degenerative and inflammatory conditions of joints. Osteoph te formation starts Osteophyte early as three days after injury but is only detected grossly around two weeks and radiographically in five weeks. Osteophytes are mainly found in the junctions of cartilage with periosteum or along the insertions of synovial capsules to the bone. e process of os eop y e The p ocess o osteophyte formation is known as osteophytosis. Osteop ytes Osteophytes If the source of articular injury ceases, osteophytes no longer grow but remain forever as multiple periarticular spurs of bone. These growths cause variable degree of joint deformity. This is the acetabulum of a cow with chronic joint disease. Note the massive numbers of osteophytes along the margins joint. and within the joint Osteophytes are sometimes seen as g joints of old incidental findings in the j animals. Osteophytes • Bone outgrowths at the junction of cartilage with periostium • Chondrofication and ossification of fibrous tissue • Joint deformity Pannus A pannus is a focal aggregate of granulation tissue that starts at the synovial membrane cartilage. and extends into the articular cartilage Pannus formation is typically associated to chronic synovial injury and synovits. Release of proteases from macrophages induces degradation of cartilage and chondromalacia. deg adat o o ca t age a d c o d o a ac a It is most commonly seen in immune- mediated arthritis. In the bottom picture note the formation of p g several pannus structures arising from the synovial membrane. Synovial Villous Hyperplasia Synovial villous hyperplasia is a non-specific change that results from persistent low-grade irritation of the synovial membrane. This synovial change is frequently accompanied by excessive production of synovial fluid (synovial effusion). Synovial villous hyperplasia is often found in chronic degenerative joint diseases. The synovial membrane is superficially covered by countless villous projections which are best demonstrated by immersing the joint in water as illustrated in this picture. Note th l t f th N t the velvety appearance of the synoviali l membrane with severe villous hyperplasia. The cartilage appears essentially normal. The histological view of synovial villous hyperplasia is illustrated in the next slide. Synovial Villous Hyperplasia A histological section of a synovial membrane showing severe villous hyperplasia. Note the large number of exophytic projections (asterisks) arising from the synovial membrane. Synovial villi are lined by hypertrophic synoviocytes. There are two types of synoviocytes. Synoviocytes type A are phagocytic, while synoviocytes type B are responsible for the production of the synovial fluid. Both types of synoviocytes proliferate rapidly in response to injury (reactive hyperplasia). th h b h h If there has been hemorrhage, synoviocytes type B phagocytize red blood cells and accumulate hemosiderin and the synovial membrane turns yellow (pigmented synovitis). Joint Response to Injury End-stage joint is a generic term used to describe several chronic articular changes in the same joint. End-stage joints show various degrees of fibrillation, eburnation, osteophytes, deformation, villous hypertrophy, pannus, capsule fibrosis and occasionally (fixation, immobility). ankylosis (fixation immobility) • Fibrillation • Eburnation • Osteophytes • Villous hyperplasia Joint i • J i t mice Capsular fibrosis • C l fib i • Pannus formation End Stage Joint End Stage Joint Note the large number of ostrophytes arising from the articular cartilage in this joint. Some osteophytes, particularly those in the synovial p y y junction are notably large (asterisk). The synovial membrane is hyperemic and thickened suggesting synovial villous hyperplasia (S). At the synovial-cartilage junction there are few small pannus formations (arrows). There are also some adhesions between cartilage and synovial membrane (arrowheads). Although not clearly shown in this picture, the articular capsule was fibrotic and thickened.