Relationship between os acromiale and AC joint anatomical position

Most of the orthopaedic literature on os acromiale is focused on the clinical implications, such as impingement and rotator cuff tears; although os acromiale is present in 8% of subjects, scarce information is reported on the causes that may predispose patients to it. Our aim is to investigate whether the origin of os acromiale is related to the position of the acromioclavicular (AC) joint. The acromions of 211 volunteers (control group) and 33 subjects without or with os acromiale were radiographically (axillary view) classified by the Edelson and Taitz method. The method distinguishes the acromion into three types on the basis of the distance between the anterior aspect of the acromion and the AC joint. Of 33 subjects with os acromiale, 11 were without pain. We have compared the frequency of the three types of acromion observed in the two investigated cohorts. Half of the acromions of the control group (52.1%) had the articular facet of the AC joint on the acromion tip, whereas in 45.4% the facet tip was located distally. On the other hand, of 33 subjects with os acromiale, 18.1% and 81.1%, respectively, had the AC joint lying on or distal to the acromion tip. Our data suggest that the greater the distance of the AC joint from the anterior edge of the acromion, the higher the likelihood of an os acromiale.