http://injuredshoulder.com/ | The labrum is definitely the cup-shaped rim of cartilage that lines and reinforces the ball and socket joint of your shoulder, which includes the glenoid - the shallow shoulder socket - as well as the head, or ball, of your upper arm bone referred to as the humerus. The labrum is definitely the attachment site for your ligaments and supports the socket and ball joint together with the rotator cuff tendons and muscles.