The labrum is a rim of cartilage that lines the socket of the shoulder and adds stability to the shoulder joint. The shoulder joint itself is made up of the glenoid (the socket) and the humeral head (the ball), from the top of the upper arm bone (the humerus). The labrum deepens this fairly shallow socket by up to 50%, increasing shoulder stability. It also serves as an attachment site for ligaments and tendons in the shoulder. Along with the rotator cuff and muscles around the shoulder blade, the labrum adds stability to a very mobile shoulder joint.