Os acromiale: population differences and their etiological significance

The os acromiale is an accessory bone resulting from failure of the acromial apophysis to fuse to the scapula. It is a relatively frequent defect affecting 4-18% of most large skeletal series. The etiology of os acromiale is poorly understood, and two competing hypotheses have been proposed: (1) that the accessory bone represents a genetic defect, and (2) that it results from mechanical stress on the developing acromion. In order to assess the evidence for these two hypotheses, os acromiale frequencies from a South African cadaver sample (n = 494) were compared to frequencies from a medieval Danish archaeological sample (n = 532). The South African frequency (18.2%) was significantly higher than the medieval Danish frequency (7.7%, p < 0.0001). These results mirror those of another study comparing African- and Euro-Americans, and tend to support the genetic hypothesis. However, a left side bias (72%) was also found among the South Africans (p = 0.013), lending some support to the mechanical stress hypothesis. Sex and age biases were also examined but were not found. In order to further test the mechanical hypothesis, the medieval Danes were divided into groups of higher and lower socioeconomic status based upon cemetery type. Lower status individuals were presumed to have engaged in strenuous labor activities more often and at a younger age than higher status individuals. However, comparison of these two groups failed to demonstrate a significant frequency difference (p = 0.105). Based on the results of this study, it is suggested that a third etiology combining a genetic predisposition for os acromiale with mechanical stress as the proximate cause should also be considered.