Relationship between os acromiale and AC joint anatomical position

Aims Most of the orthopaedic literature on os acromiale is focused on corresponding clinical implication, such as impingement syndrome and rotator cuff tear; whilst, although it is present in 8% of subjects, scarce information is reported on the causes that may predispose to it. Our aim is to investigate whether the origin of os acromiale is related to position of AC joint. Methods The acrornions of 21 1 volunteers (control group) and 33 subjects, respectively, without or with os acromiale have been radiographically (axillary view) classified in accordance to the Edelson and Taitz' method. The latter distinguishes the acromion in three types on the basis of the distance between the anterior aspect ofthe acromion and AC joint. Out of 33 subjects with os acromiale, 11 were shoulder painless. We have compared among them the frequencies of the three types of acromion observed in the two investigated cohorts. Results Half (52.l%) of the acromions of the control group had the articular facet for the AC joint on the acromion tip whilst in 45.4% facet was tip distally located, On the other hand, out of 33 subjects with os acromiale. 18.1% and 81.1% had, respectively, AC joint lying on or distally to the acromion tip. Conclusions. Our data suggest that the longer is the distance of AC joint from the anterior edge of the acromion, the higher is the possibility that an os acromiale origin. lt is plausible that ossitication centers are more stable, during anterior deltoid action, whether are comprised in the AC joint, whilst they more suffer the mechanical traction when belong to acromion whose articular facet is tip distally located. Out of control group, 4.9% had an os acromiale. These data confirm the previously reported finding that os acromiale may be occasionally observed in asymptomatic subject.