My friends have had hip and knee replacements, but none have had shoulder replacements. Why would this be?
Knee and hip replacements are much more common because we bear weight on these joints all the time and they can wear out after a while. However, shoulder replacements aren't rare. Because the shoulder is a ball and socket type of joint, the shoulder is anchored down with muscles, tendons, and ligaments. They must all stay healthy for the shoulder to move around and give you enough strength to lift things.

Some people develop problems with their shoulders because of repetition over the years or through over use, as with ball players. Other problems, like arthritis or trauma to the shoulder can cause deterioration, leading to the need for a replacement.