How is biomechanical knowledge useful for understanding shoulder disorders?

Shoulder has a wide range of motion. Contact point between the glenoid and the humeral head, tension of soft tissue, and alignment of muscles connecting the bones drastically change in its range of motion. Mechanism stabilizing shoulder joint is influenced by arm positioning. Assessment of joint stability should always take arm positioning into consideration. In fact, there is a subtle instability which can only be detected when the arm is elevated. The glenoid dysplasia might be a cause for its instability. These findings can be detected only by 3D computer generated images except in some typical cases. Generally, joint stability is judged by location of the humeral head center in the glenoid. More intricate approach is necessary for some disorders. Analysis of contact area and other glenohumeral relation can clarify instability which cannot be detected through investigating location of joint center. The studies using those parameters showed that the joint is inherently unstable with the arm externally rotated at 90 degrees of elevation, and that joint positioning would be more apt to be affected from the cocking to the acceleration phase during throwing activity. We think such instability might lead to throwing disorders.