Prevalence and factors associated with os acromiale: a multicenter study.

Background: Os acromiale is an accessory bone resulting from the nonunion of the acromial ossification center, with a reported prevalence of approximately 2%. While os acromiale is often asymptomatic, when symptomatic, it has been reported to be strongly associated with rotator cuff injuries. This multicenter study aimed to determine the prevalence of and the factors associated with os acromiale in the Japanese population. Methods: We analyzed 6,842 shoulder magnetic resonance imaging scans for shoulder disabilities obtained at 10 facilities between April 2018 and March 2023. The average age was 63.7 years, with 3,483 male and 3,359 female shoulders. This study assessed the prevalence of os acromiale, its location (pre-acromion, meso-acromion, and meta-acromion), size (length, width, and thickness), acromion shape (square tip, intermediate, and cobra), and the association between os acromiale and rotator cuff injuries. Statistical analysis was performed using Fisher's exact test with a significance level set at Results: A total of 76 cases of os acromiale (1.1%) were identified. There were 58 shoulders with pre-acromion, 18 with meso-acromion, and none with meta-acromion. The dimensions were as follows: pre-acromion (length 7.5 mm, width 10.5 mm, and thickness 5.3 mm), and meso-acromion (length 20.1 mm, width 20.8 mm, and thickness 8.5 mm). Acromial shapes included a square tip in 13 shoulders, intermediate tip in 38 shoulders, and cobra shape in 25 shoulders. The presence of os acromiale was significantly associated with rotator cuff injury ( Conclusion: The study supports previous findings that os acromiale is associated with rotator cuff injuries. In addition, the prevalence of os acromiale in the Japanese population was lower than that in non-Asian populations, and the size of os acromiale tended to be smaller than in European populations.