A joint is the location at which two bones make contact. Structurally, the joints are classified as:
Functionally, they can be classified as:
Synarthroses are joints with very little (if any) mobility. They can can be categorised by how the two bones are joined together:
Diarthroses (sometimes called synovial joints) are the most common and most moveable type of joint in the body.
The whole of a diarthrosis is contained by a ligamentous sac called the articluar capsule.
The surfaces of the two bones at the joint are covered in cartilage. The thickness of the cartilage varies with each joint, and sometimes may be of uneven thickness. Articular cartilage is multi-layered. A thin superficial layer provide a smooth surface for the two bones to slide against each other. Of all the layers, it has a the highest concentration of collagen and the lowest concentration of proteoglycans , making it very resistant to shear stresses. Deeper than that is an intermediate layer, which is mechanically designed to absorb shocks and distribute the load efficiently. The deepest layer is highly calcified, and anchors the articular cartilage to the bone.
In joints where the two surfaces do not fit snugly together, a meniscus or multiple folds of fibro-cartilage within the joint correct the fit, ensuring stability and the optimal distribution of load forces.
The synovium is a membrane that covers all the non-cartilaginous surfaces within the articular capsule. It secretes synovial fluid into the joint, which nourishes and lubricates the articular cartilage. The synovium is separated from the capsule by a layer of celluar tissue that contains blood vessels and nerves.
Synovial joints can be further grouped by their shape, which controls the movement they allow:
In cartilaginous joints (also known as synchondroses) bones are connected entirely by cartilage. In comparison to synovial joints, cartilaginous joints allow only slight movement. Examples of cartilaginous joints are the pubic symphysis, the joints between the ribs and the sternum, and the cartilage connecting the growth regions of immature long bones.
In fibrous joints bones are joined by tight and inflexible layers of dense connective tissue, consisting mainly of collagen fibers. In adults, these are not designed to allow any movement; however, in children, fibrous joints have not solidified and are movable. Examples of fibrous joints are:
See also: condyloprotector , arthritis.