The knee joint capsule is the structure surrounding the knee, made up of ligaments, bone, and fluid-filled cavities. It allows the full knee to have flexion, or bending motion, due to the folds within the capsule.

The joint capsule is made up of the patella (kneecap), which is within the anterior (front) capsule, the tibia (shinbone), and the femur (thigh bone). The capsule is held together with ligaments (fibrous connective tissue) that help with the range of motion.

The capsule contains synovial fluid, which is found in the cavities of highly mobile joints called synovial joints. This fluid circulates around the patella, tibia, and femur. It helps lubricate and provide nutrients to the joint.

The joint’s posterior aspect, or back portion, is stronger and thicker. It makes the person, when standing, more stable and better able to balance. This is important because the knee is generally unstable due to its bony configuration.