Classifying Joints

A joint is a junction between two or more bone, but the sites of union of bones can have very different properties. We normally associate joints with movement between different bones of the skeleton such as the knee, elbow, shoulder, or hip, but not all joints permit movement.

Mobile joints are known as synovial joints, but the amount of movement permitted varies over a wide range. Non-mobile joints usually develop as growth sites during development and growth of the skeleton and they persist in different forms once growth is completed.

Joints that permit a wide range of free movement are called synovial joints. Characteristically, the bones are separated by a synovial cavity filled with a very small volume of synovial fluid. A thin fibrous capsule arranged like a cuff around the joint retains the synovial fluid and unites the bones. The capsule is often thickened locally to form ligaments. The bones that contribute to a synovial joint articulate with each other at the articular surfaces; these are usually reciprocally curved and covered with a layer of hyaline articular cartilage which has a low coefficient of friction. Synovial fluid, essentially a dialysate of blood plasma, is secreted into and resorbed from the synovial cavity by the synovial membrane, a vascular sheet of vascular connective tissue which lines the capsule and covers non-articulating areas of the bone within the joint cavity.

Goodness of fit or congruence between the articular surfaces forming a synovial joint determines its stability and range of movement. If the surfaces are congruent and fit closely, the joint is stable but may only have a limited range of movement. If the fit of the bones is not so close, the joint loses some stability, but increases its range of movement.

Various structures are found within synovial joints to improve the fit of the articular surfaces, hence the stability of the joint. The temporomandibular joint (TMJ) is formed between the base of the skull and mandible, the joint cavity of the TMJ is completely separated into upper and lower compartments by a fibrous articular disc, but they do not move independently in the TMJ. Muscle tendons may pass through a joint space, enclosed in their own lubricating sheath; the articular disc of the TMJ is an extension of the tendon of the lateral pterygoid muscle.

Limitation of movement at synovial joints is necessary to avoid damage to the joint and adjacent structures. The capsule and ligaments around synovial joints contain stretch receptors which feed data into the nervous system about their degree of tension and relative position. This type of sensory information is called proprioception, but we are generally unconscious of it. The tension or tone in the muscles around a joint offers passive resistance to stretch and reflex contraction occurs in response to stimulation of the stretch receptors of the ligaments and capsule. The ligaments and joint capsules also contain pain receptors which are stimulated by excessive movement of the joints to act as an additional alarm signal of potential damage.

Ligaments are well-defined bands of fibrous tissue connecting bones. Most are positioned to resist or limit the movement of a joint in a certain direction as well as their function as sensory receptors. Collateral ligaments are local thickenings of the joint capsule whereas accessory ligaments are completely isolated from the capsule.

Immobile Joints

Sutures are examples of joints with not movement.