Arthroscopic excision of unstable os acromiale associated with impingement syndrome: a case report.

Os acromiale is a rare anatomical variant that is caused by failure of fusion of the acromial apophysis and is usually asymptomatic. We report a case of impingement syndrome of the left shoulder secondary to unstable os acromiale, which was initially overlooked and confirmed only during arthroscopic examination. Arthroscopic excision of the unstable fragment was successful without residual dysfunction of the deltoid muscle.