JOINTS AND JOINT TISSUE
Fibrous Joints They do not have synovial cavity. Two bones remain held together by thin layer of fibrous tissue or dense fibrous tissue or cement or sutures. (i) Sutures Synostoses – (Suture during infancy but fusion afterwards as in Frontal bone). Bones are thin and plate like, held together by inter digitations. e.g., skull bones. (ii) Gomphosis One bone remains embedded in the socket of other attached through fibres or cement layer. e.g., Thecodont teeth of humans. (iii) Shindylases One bone fits into slit of other. e.g., ethmoid bone into vomer. (iv) Syndesmosis Two bones are united by dense fibrous tissue. e.g., joint between skull bones and bones of upper jaw, distal ends of tibia and fibula. Cartilaginous Joints No synovial cavity, articulating bones are united by cartilage. (i) Synchondrosis Connecting material is hyaline cartilage. e.g., temporary joint between diaphysis and epiphysis of a long bone and permanent joint between true ribs and sternum. (ii) Symphysis Connecting material is broad flat disc of fibrocartilage. e.g., Intervertebral disc and symphysis pubis. Synovial Joints Are the Most perfect, freely movable and most common type of joints. They allow free movement in one or more directions. There is no direct contact between two bones, end is covered with hyaline cartilage cap (articular cartilage) and the whole structure is covered with ligament. The synovial cavity between two bones is lined with synovial membrane and is filled with synovial fluid secreted by this membrane. Synovial fluid acts as lubricant and shock absorber and also provides nourishment to articular cartilage. In old age stiffness of joints occur due to decrease in synovial fluid and erosion of cartilaginious part. Synovial membrane is composed of loose connective tissue with elastic fibres and a variable amount of adipose tissue. Synovial fluid also contains phagocytic cells and removes microbes and debris resulting from wear and tear of joints. It also contains Hyaluronic acid and interstitial fluid formed from blood plasma and is similar in appearance, consistency to uncooked egg white. When there is no movement, the fluid is quite viscous, but as movement increases, fluid becomes less viscous. Amount of synovial fluid varies in different joints of body, ranging from a thin viscous layer to about 3.5 ml of free fluid in large joint such as knee. It also removes metabolic wastes from the joint. Arthroscopy is examination of interior of a joint, usually knee by an arthroscope.
One interesting feature of some synovial joints is their ability to produce a cracking sound when pulled apart. (i) Articular discs (menisci) These are pads of fibrocartilage that lie between articular surfaces of some bones. These allow 2 bones of different shapes to fit tightly, these modify the shape of joint surfaces of the articulating bones. Articular discs help to maintain the stability of joints and direct the flow of synovial fluid to areas of greatest friction. (ii) Bursae Sac like structures situated between tendons and bones, muscles and bones, ligaments and bones, skin and bones. Their wall has connective tissue lined by synovial membrane, they are also filled with a fluid similar to synovial fluid. Inflammation of Bursa is called Bursitis.
Types of Synovial Joints Ball & Socket joint Articulate end of one bone is like a ball whereas other bone end is like a cup shaped socket. It permits triaxial movements, i.e., movement in three planes. e.g., Acetabulum of pelvic girdles and head of femur, glenoid cavity of pectoral girdles and head of humerus. Angula