In contrast to secondary impingement, which is due to superior instability of the humeral head, intrinsic narrowing of the supraspinatus outlet with compression of the subacromial bursa and supraspinatus tendon can also cause pain. This condition is referred to as primary impingement and is not specific to any phase of the throw. It may be caused by acquired structural changes, such as a hooked acromion morphology  and acromioclavicular arthrosis
, or by congenital changes, such as an accessory acromial ossicle  or a thick coracoacromial ligament . Primary impingement is commonly seen in football quarterbacks, because of the direct overhead motion of the throw, and in other overhead athletes, such as tennis players during the serve and freestyle swimmers.
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