Similar to the hip, the shoulder joint is a ball-and-socket joint. The ball of the upper arm bone is held in place within the socket of the shoulder blade. However, the shoulder socket itself is extremely shallow and unstable. The bones of the shoulder are not held in place adequately, thereby requiring extra support. To help compensate for this instability, the shoulder joint has a cuff of cartilage (labrum) that forms a cup for the end of the arm bone to move within. The labrum wraps around the shallow shoulder socket, thus making the socket deeper. In addition, the biceps muscle tendons of the upper arm attaches to the shoulder just above the labrum. This arrangement makes the shoulder much more stable and allows for a very wide range of movements.
A shoulder or arm injury may cause a labrum tear, but the labrum itself may simply become brittle with age and may fray and tear as part of the aging process.