Joint capsule

In anatomy, a joint capsule or articular capsule is an envelope surrounding a synovial joint. Each joint capsule has two parts: an outer fibrous layer or membrane, and an inner synovial layer or membrane.
Membranes.
Each capsule consists of two layers or membranes:
On the inside of the capsule, articular cartilage covers the end surfaces of the bones that articulate within that joint.
The outer layer is highly innervated by the same nerves which perforate through the adjacent muscles associated with the joint.
Fibrous membrane.
The fibrous membrane of the joint capsule is attached to the whole circumference of the articular end of each bone entering into the joint, and thus entirely surrounds the articulation. It is made up of dense irregular connective tissue. It's a long spongy tissue.
Clinical significance.
Frozen shoulder (adhesive capsulitis) is a disorder in which the shoulder capsule becomes inflamed.
Plica syndrome is a disorder in which the synovial plica becomes inflamed and causes abnormal biomechanics in the knee.