Prevalence of os acromiale in Japanese population: Comparison between patients with shoulder pain and asymptomatic subjects.

Background: Os acromiale, a condition in which the acromial apophysis fails to fuse, and its prevalence varies across ethnic groups. Although higher rates have been reported in African and Caucasian populations, data are scarce for Asian populations, particularly for Japanese individuals. This study aimed to determine the prevalence of os acromiale in a Japanese population and its association with rotator cuff-related symptoms. Methods: We retrospectively identified 2221 shoulders of 1411 Japanese subjects aged over 25 years who underwent computed tomography (CT) of the shoulder at our institution between September 2010 and March 2023. The shoulders were divided into an asymptomatic group, which included 1386 shoulders of 693 patients with no documented history of shoulder pain who underwent chest CT for non-orthopedic reasons, and a symptomatic group, which included 836 shoulders of 800 patients who underwent arthroscopic surgery for rotator cuff tears or tendinopathy. Os acromiale was identified using three-dimensional CT and classified by location. Results: Os acromiale was identified in 16 shoulders (0.72 %), 10 of which were classified as pre-acromial and 6 as meso-acromial. The prevalence was significantly higher in the symptomatic group than in the asymptomatic group (1.56 % vs 0.22 %, p = 0.002). No significant difference was observed according to sex or laterality. Conclusion: The prevalence of os acromiale in Japanese was 0.72 %. The prevalence of this condition is markedly lower in this population than in African and Caucasian populations. However, its significantly higher occurrence in symptomatic patients suggests a potential role in shoulder pain associated with rotator cuff lesions. These results emphasize the importance of recognizing os acromiale as a possible anatomical contributor to shoulder symptoms in clinical practice.