Synovial joint

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. As with most other joints, synovial joints achieve movement at the point of contact of the articulating bones. They originated 400 million years ago in the first jawed vertebrates.

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 (French word for ceiling), especially in the talocrural joint. Damage to this structure is referred to as 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, therefore more stable. Others have multiple degrees of freedom, but at the expense of greater risk of injury. In ascending order of mobility, they are:

A multiaxial joint (polyaxial joint or triaxial joint) is a synovial joint that allows for several directions of movement. In the human body, the shoulder and hip joints are multiaxial joints. They allow the upper or lower limb to move in an anterior-posterior direction and a medial-lateral direction. In addition, the limb can also be rotated around its long axis. This third movement results in rotation of the limb so that its anterior surface is moved either toward or away from the midline of the body.