The so-called Os acromiale is an anatomic variation that can be found in app. 3-8% of the population. Usually, the 4 ossification centres at the acromion merge till 25 years of age. In the case of an Os acromiale, this merging remains incomplete or doesn't happen at all. If there is a hypermobile Os acromiale, it can dislocate caudally by traction of the M. deltoideus and block the subacromial space.
(Case Os acromiale courtesy of Dr. John Hunter, radiopaedia.org)
