The shoulder is the most flexible joint in the body. But it does have limits. If the joint is pushed past these limits, the shoulder joint may move too much. This is called shoulder instability.
The shoulder is a shallow ball-and-socket joint. The humeral head is the ball of the top of the upper arm bone (humerus). The glenoid is a shallow socket on the shoulder blade. The humeral head rests on the glenoid and is held in place by strong tissues. These include:
Labrum. This is a circle of cartilage. It covers the rim of the socket.
Capsule. This is a sheet of ligaments and other tough tissues. It encloses the joint and connects the ball to the socket.
Rotator cuff. This is a group of four muscles and tendons. These stretch between the ball and shoulder blade.
Shoulder instability can occur when any of these structures are weakened or torn, and the ball doesn’t stay centered in the socket. Multidirectional shoulder instability means the ball moves too far in more than one direction (forward, back, or down). This extra movement can lead to discomfort, pain, and problems using the shoulder.