Prevalence of Os Acromiale in an Indigenous East African Population: A Computerized Tomography Scan-based Study

Mordicai Atinga1 , Njalalle Baraza1 , Sarah Wambui2 , Wachira Joseph Thiong’o3 , Angela Atinga4

1. Department of Orthopaedic Surgery, Aga Khan University Hospital, Nairobi, Kenya

2. Department of Radiology, Aga Khan University Hospital, Nairobi, Kenya

3. University of Nairobi, College of Health Sciences, Nairobi, Kenya

4. Department of Interventional Radiology, Sunnybrook Hospital, Toronto, Ontario, Canada

Correspondence to: Dr. Wachira Joseph Thiong’o; Email:

Received: 26 Feb 2021; Revised: 20 Aug 2021; Accepted: 1 Sep 2021; Available online: 7 Oct 2021 


Background: Failure of fusion to any of the acromion physes at maturity may lead to an os acromiale. A radiological review into the prevalence of os acromiale in an indigenous East African population was performed using computer tomography (CT) imaging. The study aimed to demonstrate a lower prevalence of os acromiale in an urban population compared with cadaveric studies. Methods: This was a cross-sectional study with consecutive sampling. CT scans of both shoulders obtained for non-shoulder pathology were used for the analysis. CT scanning was performed at a tertiary referral hospital from January 2019 to July 2020. Results: Nine hundred eighty-two CT studies were eligible for the review. There were 148 os acromiale identified that were either pre-acromiale or mesoacromiale. The study found a 15% prevalence of os acromiale among an ethnic East African population, of which almost 40% were meso-acromiale and 60% were pre-acromiale, with no meta-acromiale. Conclusion: The prevalence of os acromiale found in this study is similar to that observed in earlier cadaveric studies. 


Keywords: Os acromiale, Acromion, Pre-acromiale, Meso-acromiale, Meta-acromiale, Cadaveric studies

Ann Afr Surg. 2021; 19 (1): 4–7


Conflicts of Interest: None
Funding: None
© 2022 Author. This work is licensed under the Creative Commons Attribution 4.0 International License. 


An os acromiale is an anatomical variant wherein there is failure of fusion of the acromion physes (1). The acromion fuses from three physes, a process that is normally complete by 18 years old (2). The failure of fusion can occur at any physis, and os acromiale may be classified based on the location as pre-, meso-, or a metaacromiale (1). Prevalence studies based on cadaveric or radiological assessment have shown a rate between 0.7% and 18%, with the higher rate in black African populations (3–5). The etiology of os acromiale has been predominantly hypothesized as being genetic, environmental, or possibly a combination of the two (3).

Click to view Figure 1. Image showing acromial physes and the resultant anatomical regions relevant to os acromiale (adopted from with permission)

Most cases of os acromiale are asymptomatic (6). However, the presence of an os acromiale in a patient with rotator cuff pathology increases surgical complexity in subacromial decompression, and separately, the unstable os acromiale may also require fixation (7–9). In clinical practice, an os acromiale is diagnosed radiologically with axial radiographs, computer tomography (CT), or magnetic resonance (MR) imaging. If missed, this can lead to suboptimal or failed surgical treatment for shoulder pathology. Given the high prevalence reported in the literature (3,10), this study sought to elucidate the prevalence in an indigenous East African population as the surgical management of shoulder pathology accelerates. It was hypothesized that the etiology of os acromiale was environmental, and the incidence of os acromiale would be lower in our urban patient population, who are possibly exposed to less physical strains. The study aimed to determine the prevalence of os acromiale in an asymptomatic black East African population based on CT imaging and to test the hypothesis that this would be lower than previously published results from cadaveric studies.

Materials and methods

This cross-sectional study was performed in a tertiary referral hospital. Consecutive CT chest studies performed for non-shoulder pathology between January 2019 and September 2020 were identified from our local picture archiving and communication system. This was to allow at least 800 eligible scans for assessment similar to previous studies (4, 11). The studies were deemed eligible for inclusion if both shoulder girdles, including the acromion, were visible, if the investigated patients were older than 18 years, and if the imaged patients were of black African ethnicity. All CT imaging was performed on either a Somatom Definition flash 256 dual-slice CT scanner (Siemens, Munich, Germany) or a Discovery 64-slice scanner (GE, Boston, MA, USA). All the images were retrospectively reviewed by a single radiologist with a special interest in musculoskeletal radiology. The presence of an os acromiale was assessed on images reconstructed under a bone algorithm. Multiplanar reformats were utilized where necessary. Demographic data collected included age and sex of the patients, presence or absence of an os acromiale, and, if present, the classification into pre-, meso-, or metaacromiale (Figure 1). The prevalence was calculated based on the number of CT scans with an os acromiale (unilateral and/or bilateral) as percentage of the total number of CT