A joint capsule is a piece of tissue that surrounds a synovial joint. Its purpose is to hold the synovial fluid of the joint in place, as well as to provide an envelope for the entire joint. The joint capsule provides an important function to all synovial joints, but it can cause problems, such as frozen shoulder, osteoarthritis, and inflamed plica syndrome, when not functioning properly.

The most common type of joint in the human body is the synovial joint. Synovial joints contain synovial fluid that helps to lubricate movement. Fibrous joints, however, do not contain either synovial fluid or a joint capsule. Joints containing synovial fluid can perform a number of different actions, including abduction, extension, and rotation.

Synovial joints appear in the body in a number of different forms. For example, the elbow is a simple hinge joint, while the hip is a more complicated ball-and-socket joint that allows a greater range of movement. Joint capsules are present in all of these joints.

A joint capsule is made up of two separate layers. The first is an outer layer that contains a fibrous, colorless tissue. The second, inner layer is often called the synovial membrane. Both of these layers need to be in a healthy state in order for the joint to move as it should.

An example of a problem that can be caused by a shoulder joint capsule is frozen shoulder. Frozen shoulder is caused by the shoulder capsule undergoing a thickening, which then inhibits movement of the joint. The most common symptoms of frozen shoulder include the shoulder becoming stiff and painful. In many cases, the shoulder will follow a predictable progression where it will "freeze up" before entering the "frozen" stage. This malady can last for up to a year before the shoulder begins to free up.

Plica syndrome is caused by the joint capsule in the knee — it occurs when the synovial tissue surrounding the joint becomes inflamed. In many cases, this is due to repeated strain being placed on the knee during activities such as running or jumping. There are some cases, however, where a direct impact injury may cause the joint capsule to become inflamed. Plica syndrome is often difficult to diagnose, as some of the symptoms are similar to other, more common knee injuries, such as patellofemoral syndrome and IT band syndrome. If the injury cannot be managed, then surgery is occasionally necessary to remove the plica.