Dr. Armin Tehrany, Orthopedic surgeon of Mount Sinai Hospital, explains the nature of the shoulder joint and compares it to a traditional ball-and-cup joint like the hip.

Doctor Tehrany: I'd like to think of the shoulder joint kind of like a basketball on a dish. As opposed to the hip joint which is truly a ball and socket joint the shoulder is not really a ball and socket joint. So if this is the hip the hip's got a nice cup as a socket that ball can stay in. It's much less likely to slip out or dislocate. But I like to think of the shoulder joint kind of like a basketball on a dish, big basketball, small dish. So the problem with that is it's much more prone to moving around a lot. Much more prone to slipping out especially if you're younger and much more prone to slipping up when you're older and as a result causing a tear in the Rotator Cuff. The good thing about that is the shoulder is more mobile so you can have more motion than you can with a hip. The problem is it's more likely to cause trouble.