Prevalence of Os Acromiale in Thai Patients With Shoulder Problems: A Magnetic Resonance Imaging Study

BACKGROUND: Os acromiale is an anatomic variant in which the acromial ossification center fails to fuse. It may lead to painful shoulder conditions and surgical intervention may be required for treatment. The prevalence of os acromiale in South Koreans (0.7%) has previously been the representative number for the Asian population, and it is lower than that in the Black and White populations. PURPOSE: To determine the prevalence of os acromiale in Thai patients with shoulder disabilities who underwent magnetic resonance imaging (MRI). STUDY DESIGN: Cross-sectional study; Level of evidence, 3. METHODS: We retrospectively reviewed the MRI scans of patients with shoulder disabilities between January 2019 and May 2021 at a single institution. The relationship of sex, age, and shoulder diagnosis to the presence of os acromiale was then analyzed. RESULTS: A total of 940 MRI scans in 848 patients were evaluated, of which 20 shoulders were detected to have os acromiale, a prevalence of 2.13%. The mean age of the patients was 60.25 years, with 60% being female and 40% male. Of the 20 os acromiale cases, 17 (85%) were preacromiale, 1 (5%) mesoacromiale, and 2 (10%) meta-acromiale. The associated diagnoses were as follows: 75%, full-thickness rotator cuff (RC) tear; 10%, partial-thickness RC tear; 5%, RC tendinopathy with symptomatic os acromiale; 5%, anterior shoulder instability with full-thickness RC tear; and 5%, frozen shoulder. Age, sex, and associated shoulder disability were not associated with the presence of os acromiale. CONCLUSION: In Thai patients with shoulder problems who required MRI evaluation, the prevalence of os acromiale was 2.13%. This is lower than the prevalence from other ethnic groups but slightly higher than that from the same ethnic group (Korea). There was no relation between the presence of os acromiale and shoulder pain, regardless of diagnosis.