The labrum is the articular surface (the area where the ball of the arm bone or humerus connects to the actual joint surface or glenoid cavity *the socket of the shoulder). Its like looking at the Universal Joint on a car crankshaft. The surface on the inside of the socket is where the labrum forms a secure ‘cuff’ for the bony humerus to insert into. Like a ring of fibre that makes the articulation more secure, it helps to create a deeper rim. Like the rubber ring on a preserving jar.