Synovial joint

Synovial joints (or diarthroses, or diarthroidal joints) are the most common and most moveable type of joints in the human body. As with most other joints, synovial joints achieve movement at the point of contact of the articulating bones.

Structural and functional differences distinguish synovial joints from cartilagenous joints (synchondroses and symphyses) and fibrous joints (sutures, gomphoses, and syndesmoses). The main structural differences between synovial and fibrous joints is the existence of a capsule surrounding the articulating surfaces of a synovial joint and the presence of lubricating synovial fluid within that capsule (synovial cavity).

Structure

• articular capsule: The fibrous capsule is continuous with the periosteum of bone. It is also highly innervated but avascular (lacking blood and lymph vessels)
• articular cartilage: lines the epiphyses of joint end of bone. Provides the loading and unloading mechanism to resist load and shock
• synovial membrane: the inner layer of the fibrous articular capsule. The synovial membrane covers the lining of the synovial cavity where articular cartilage is absent.

Types

There are six types of synovial joints. Some are relatively immobile, but are more stable. Others have multiple degrees of freedom, but at the expense of greater risk of injury. In ascending order of mobility, they are:

Name	Example	Description
Gliding joints (or planar joints)	the carpals of the wrist	These joints allow only gliding or sliding movements.
Hinge joints	the elbow (between the humerus and the ulna)	These joints act like a door hinge, allowing flexion and extension in just one plane.
Pivot joints	the elbow (between the radius and the ulna)	This is where one bone rotates about another.
Condyloid joints (or ellipsoidal joints)	the wrist	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. Some classifications make a distinction between condyloid and ellipsoid joints.
Saddle joints	the thumb (between the metacarpal and carpal)	Saddle joints, which resemble a saddle, permit the same movements as the condyloid joints.
Ball and socket joints	the shoulder and hip joints	These allow a wide range of movement.

References

joints