Joint capsule of the thumb
The joint capsule of the thumb is fibrous and durable, and allows for great mobility. There are nine interphalangeal joints in each hand, two on each finger and one in the thumb. The thumb joint has two collateral ligaments as well as the capsule, which is lined by a synovial membrane. The collateral ligaments are called the anterior and posterior ligaments. They are responsible for reinforcing the thumb. Flexion and extension occur parallel to the palm of the hand. As flexion occurs, there is medial rotation of the metacarpal. The joint capsule of the thumb is similar to that of the fingers. The only notable difference being the articular surface of the metacarpal head is smaller and consequently less movement is available. It is reinforced by two cartilage covered sesamoid bones on its volar surface. The sesamoids also increase the leverage actions and there is very little hypertension compared with the fingers. The ulnar side of the joint is most susceptible to trauma.