This article discusses zootomical anatomy. For information about cannabis "joints", see: spliff.
A joint is the location at which two bones make contact. They are three primary types of joints: snynovial, cartilaginous, and fibrous.
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2 Cartilaginous Joints
3 Fibrous Joints
Portions of bone which form a synovial joint are coated with articular cartilage and lubricated by synovial fluid; this reduces friction. Synovial joints are held together by ligaments. Most joints which produce substantial movement are synovial. The inner lining of synovial joints is called the synovium. The whole joint is contained in the joint capsule, which consists of a tough outer layer which helps to stabilise the joint, and a synovial membrane which produces synovial fluid. Synovial joints can be further grouped by their shape, which controls the movement they allow:
- Hinge joints, such as the elbow (between the humerus and the ulna). These joints act like a door hinge, allowing flexion and extension in just one plane.
- Ball and socket joints, such as the hip joint. These allow a wide arrange of movement.
- Condyloid (ellipsoid) joints, such as the knee. When the knee is extended there is no rotation, when it is flexed some rotation is possible. A condyloid joint is where two bones fit together with an odd shape (e.g. an ellipse), and one bone is concave, the other convex.
- Pivot joints, such as the elbow (between the radius and the ulna). This is where one bone rotates about another.
- Saddle joints, such as at the thumb (between the metacarpal and carpal). Saddle joints, which resemble a saddle, allow movement in a variety of directions.
- Gliding joints, such as in the carpals of the wrist. These joints allow a wide variety of movement, but not much distance.
Cartilaginous JointsIn comparison to synovial joints, cartilaginous joints do not allow a great deal of movement. An example is the pubic symphysis.
In adults, these are not designed to allow any movement; the bones of the skull are an example. However, in children, fibrous joints have not solidified and are movable.