Glossary

Joint capsule

Quick answer

The knee joint capsule is a tough, flexible sleeve of fibrous tissue that wraps around the joint, from the femur to the tibia. It holds in the joint fluid, is lined by the synovium and is thickened in places to form ligaments, as Dr. Pablo Gelber, knee surgeon at Aman Hospital in Doha, describes it.

What it is

The capsule has two layers: an outer fibrous layer that gives strength, and an inner synovial lining that makes joint fluid. At the front it blends with the kneecap and the tendons, and at the sides and back it is reinforced by ligaments. At the front it forms a pouch above the kneecap, the suprapatellar pouch, where fluid often collects when the knee swells.

The capsule also contains nerve endings that help with balance and position sense.

Why it matters for patients

The capsule needs to stretch and fold as the knee moves. After an injury or surgery, it can thicken and scar, which is one of the causes of stiffness and difficulty straightening or bending fully, sometimes called arthrofibrosis. Early, guided movement after surgery helps to prevent this.

When fluid builds up in the knee, it can push out through a weak point in the back of the capsule into a fluid sac behind the knee, called a Baker's cyst. Tightness of the posterior capsule can also stop the knee from fully straightening, which affects walking and the load on the joint.

Frequently asked questions

Can a tight capsule be stretched?

Often, yes. Physiotherapy with regular, controlled stretching improves many cases of stiffness. If movement does not improve, a surgeon may consider manipulation or arthroscopic release.

Is a Baker's cyst a problem with the capsule?

It is fluid from the knee that has collected behind it through a natural opening in the capsule. Treating the cause of the extra fluid usually matters more than the cyst itself.