Joints and Synovial Membranes
The connection between bones is made through articulations or joints, which may have the following functions, depending on the skeletal structure involved: to allow a degree of mobility, to prevent friction between bones, and, depending on the joint type, to absorb impact. Different articulations present different types of movement, such as gliding, pivotal, or hinge-types movement, or combined hinge-and-gliding, and ball-and-socket movements. The three main types of articulations are fibrous joints, synovial joints, and cartilaginous joints. In humans, the main articulated joints are found in the shoulders, elbows, wrists, hands, phalanges, feet, ankles, hip, and pelvis.
Fibrous and cartilaginous articulations connect bones through the juxtaposition of ligaments (either conjunctive tissue or cartilage), which are fixed on both connecting bones. Fibrous joints are found between cranial bones, between the roots of teeth and the adjacent alveolar walls, and between bones a great distance apart, such as the interosseous membranes of the forearms and of the legs. Cartilaginous joints occur with interposition of hyaline cartilage as in the spheno-occiptal synchondrosis, and as fibrocartilaginous synchondrosis, such as the symphysis pubis that unites the pubic bones.
Synovial articulations between two long bones such as those of the knees or the elbows allow a much greater motility than the fibrous and cartilaginous ones. They are constituted by the following elements: 1), a capsule filled with synovial fluid rich in hyaluronic acid, which is synthesized by the cells of the synovial membrane; 2), the joint cavity containing the capsule; and 3), articular cartilage lining the surface of the bones at the joint area. The synovial membranes line the internal portion of the synovial capsule and the tendon surfaces, where the synovial fluid acts, among other functions, as a lubricant. The synovial capsule is also known as synovial bursa, and along with other bursae such as the Achilles bursa, prepatellar bursa, and radiohumeral bursa, are also lined with synovial membranes. The bone extremities at the synovial joint contain a much softer type of bone known as subchondral bone. The articular cartilage is hyaline (i.e., transparent), and lines the subchondral bone surface, protecting it against friction. The synovial fluid, also termed synovia, is produced by dialysis from the plasma and enriched with hyaluronic acid synthesized in the joint by the synovial membrane cells. Inside the capsule, the synovia has the following functions: transport of nutrients to the joint tissues, removal of waste, and absorption of mechanical impact.
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