[The incidence and role of the os acromiale in the acromiohumeral impingement syndrome].

Os acromiale (an apparently supernumerary bone) corresponds to persistence of the lateral end of acromion ossification nucleus over 18 years of age. Os acromiale is articulated with the clavicle and the base of acromion; its incidence is high and often bilateral. A rarer variant is secondary os acromiale, lying between acromion and great humeral tuberosity. Both variants predispose to the onset of pain due to acromiohumeral impingement. Rotator cuff tears are frequently associated due to direct injury to subacromial compartment structures and to overtraction of acromiocoracoid ligament following functional overuse. Three hundred and ninety-eight patients were examined (313 males and 85 females, aged 12 to 71 years): 265 of them underwent conventional radiography with specific projections for the shoulder and 133 underwent CT. Dysplasia was observed in 9.55% of cases (38 patients), which is in agreement with reports by other authors. CT demonstrates the presence of os acromiale better than specific radiographic projections, without requiring painful patient positioning. Axial CT scans allow the depiction of subacromial compartment structures that cannot be demonstrated by US. The evident and frequent association of arthrotic-degenerative acromioclavicular alterations even after os acromiale assimilation confirms the close relationship of the latter bone to rotator cuff conditions, especially to rotator cuff tears. In our opinion, the correct description of this finding has high prognostic value in the subjects likely to experience functional shoulder overuse and allows the correct etiopathogenesis of pain. Therefore CT, which is an elective imaging method especially suited to demonstrate calcium-density structures, thanks to its high spatial and contrast resolution, can be used concurrently with MR imaging because it is less expensive and more easily available.