Os acromiale in a baseball catcher.

Throwing injuries of the shoulder can result from an acute traumatic event or chronic overuse. Shoulder impingement has multiple etiologies; the most common being rotator cuff weakness/overuse and resultant glenohumeral instability. However, an uncommon cause of shoulder impingement syndrome is that of a nonfused os acromiale. There are three centers of ossification in the acromion which are usually completely fused by 18 yr of age. The most common site of nonunion is between the meso-acromion and meta-acromion. Os acromiale is reported at a rate of 14/1000 (1.4%) and is bilateral in approximately 62% of cases. The classic diagnosis is radiographically defined with both AP and axillary lateral views, and a contralateral comparison view may be helpful. Computerized axial tomography also aids in the diagnosis. Most os acromiale are asymptomatic. However, if recalcitrant impingement syndrome and/or rotator cuff tears are found in association with os acromiale, then surgical fusion or resection of the ossicle is recommended.