A synovial joint, also known as diarthrosis, joins bones or cartilage with a fibrous joint capsule that is continuous with the periosteum of the joined bones, constitutes the outer boundary of a synovial cavity, and surrounds the bones' articulating surfaces. This joint unites long bones and permits free bone movement and greater mobility. The synovial cavity/joint is filled with synovial fluid. The joint capsule is made up of an outer layer of fibrous membrane, which keeps the bones together structurally, and an inner layer, the synovial membrane, which seals in the synovial fluid.
They are the most common and most movable type of joint in the body of a mammal. As with most other joints, synovial joints achieve movement at the point of contact of the articulating bones.
Synovial joints contain the following structures:
Many, but not all, synovial joints also contain additional structures:
The bone surrounding the joint on the proximal side is sometimes called the plafond, especially in the talocrural joint. A damage to this occurs in a Gosselin fracture.
The blood supply of a synovial joint is derived from the arteries sharing in the anastomosis around the joint.
There are seven 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:
(or gliding joint)
|carpals of the wrist, acromioclavicular joint||These joints allow only gliding or sliding movements, are multi-axial such as the articulation between vertebrae.|
|Hinge joints||elbow (between the humerus and the ulna)||These joints act as a door hinge does, allowing flexion and extension in just one plane|
|Pivot joints||atlanto-axial joint, proximal radioulnar joint, and distal radioulnar joint||One bone rotates about another|
(or ellipsoidal joints)
|wrist joint (radiocarpal joint)||A condyloid joint is a modified ball and socket joint that allows primary movement within two perpendicular axes, passive or secondary movement may occur on a third axes. Some classifications make a distinction between condyloid and ellipsoid joints; these joints allow flexion, extension, abduction, and adduction movements (circumduction).|
|Saddle joints||Carpometacarpal or trapeziometacarpal joint of thumb (between the metacarpal and carpal - trapezium), sternoclavicular joint||Saddle joints, where the two surfaces are reciprocally concave/convex in shape, which resemble a saddle, permit the same movements as the condyloid joints but allows greater movement.|
|Ball and socket joints
|shoulder (glenohumeral) and hip joints||These allow for all movements except gliding|
/ bicondyloid joints
|knee joint||condylar joint (condyles of femur join with condyles of tibia) and saddle joint (lower end of femur joins with patella)|
Main article: Anatomical terms of motion
The movements possible with synovial joints are:
The joint space equals the distance between the involved bones of the joint. A joint space narrowing is a sign of either (or both) osteoarthritis and inflammatory degeneration. The normal joint space is at least 2 mm in the hip (at the superior acetabulum), at least 3 mm in the knee, and 4–5 mm in the shoulder joint. For the temporomandibular joint, a joint space of between 1.5 and 4 mm is regarded as normal. Joint space narrowing is therefore a component of several radiographic classifications of osteoarthritis.
In rheumatoid arthritis, the clinical manifestations are primarily synovial inflammation and joint damage. The fibroblast-like synoviocytes, highly specialized mesenchymal cells found in the synovial membrane, have an active and prominent role in the pathogenic processes in the rheumatic joints. Therapies that target these cells are emerging as promising therapeutic tools, raising hope for future applications in rheumatoid arthritis.