Article: Joint

For other uses, see Joint (disambiguation).

A joint (articulation) is the location at which two bones make contact (articulate). Joints are constructed to both allow movement and provide mechanical support.

Classification

Structure and function of a joint are closely related.

Structural classification

Structurally, joints are classified as:

• fibrous - bones are connected by fibrous connective tissue.
• cartilaginous - bones are connected by cartilage.
• synovial - there is a space (synovial cavity) between the articulating bones.

Fibrous joints

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:

• Cranial sutures, joining the bones of the cranium.
• Gomphoses, the joints between the roots of the teeth and their sockets (or alveoli) in maxilla and mandible.

Cartilaginous joints

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. Another example is in the spinal column - the cartilaginous region between adjacent vertebrae.

Synovial joints

See synovial joint for details. (The term "Synovial joint" and "Diarthrosis joint" are often used interchangeably, although the first term refers to the structure and the second one to the function.)

Functional classification

Functionally, they can be classified as:

• synarthrosis - permit no movement.
• amphiarthrosis - permit little movement.
• diarthrosis - permit a variety of movements (e.g. flexion, adduction, pronation). Only synovial joints are diarthrosis.

Synarthrosis joints

Synarthroses are joints with very little (if any) mobility. They can be categorised by how the two bones are joined together:

• Syndesmoses are joints where the two bones are joined by one or more ligaments.
• Synchondroses are joints where the two bones are joined by a piece of cartilage.
• Synostoses are the fusion of two bones, to the point that they are practically one bone. In humans, the plates of the cranium, initially separate, fuse together as the child approaches adulthood. Children whose craniums fuse too early may suffer deformities and brain damage, as the skull does not expand properly to accommodate the growing brain - a condition known as craniostenosis.
• Amphiarthroses are slightly moveable joints where the two bone surfaces at the joint - both covered in hyaline cartilage - are joined by strands of fibrocartilage.

Amphiarthrosis joints

Most amphiarthrosis joints are cartilaginous. See above and amphiarthrosis for more details.

Diarthroses (Synovial, Diarthroidal joints)

See synovial joint for details.

See also

• Arthrology, the science concerned with the anatomy, function, dysfunction and treatment of joints.
• Kinesiology, the study of musculoskeletal movements.
• condyloprotector
• arthritis
• Replacement joint
• Joint lock