The top end of your upper arm bone is shaped like a ball. Muscles and ligaments hold this ball against a cup-shaped part of the shoulder bone. Surgeons usually replace the top of the upper arm bone with a long metal piece, inserted into your upper arm bone that has a rounded head. If the cup-shaped surface of your shoulder bone that cradles your upper arm bone is also damaged, doctors smooth it and then cap it with a plastic or metal and plastic piece.
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