What is Os Acromiale?
Definition and Pathophysiology
Os acromiale is a failure of fusion between one or more ossification centers of the acromion process of the scapula, resulting in a separate, unfused bone fragment at the anterior portion of the acromion. 1, 2
- The acromion normally develops from multiple ossification centers that fuse during skeletal maturation, typically by age 22 years 3
- When fusion fails to occur, the resulting separate ossicle is termed os acromiale 1
- The most common type is the meso-acromion, which represents the unfused anterior portion of the acromion 4, 5
Epidemiology and Prevalence
- Os acromiale occurs in approximately 8.0% of the general population based on anatomical studies of 1,198 skeletons 3
- The condition is bilateral in 33.3% of cases 3
- Significantly more common in Black individuals (13.2%) compared to White individuals (5.8%) 3
- More frequent in men (8.5%) than women (4.9%), though this difference is less pronounced 3
- Radiographic and anatomical studies report frequencies ranging from 1-15% depending on the population studied 3
Clinical Significance and Symptoms
- Os acromiale is a common cause of shoulder pain that should be considered in the differential diagnosis of subacromial impingement 1
- Pain results from several mechanisms: 4
- Motion and instability at the unfused fragment site
- Impingement from the mobile fragment
- Concomitant rotator cuff pathology
- Symptoms are often associated with subacromial pathology, though the presence of os acromiale alone does not necessarily cause symptoms 3
Diagnostic Approach
- Standard shoulder radiographs including anteroposterior and axillary views are the initial imaging modality 3
- CT scan or MRI are often helpful to confirm the diagnosis when radiographs are inconclusive 1
- Axillary radiographs may show a circumferential line suggestive of an acromial joint, though some cases represent fused os acromiale that can be mistaken for free ossicles 3
Important Clinical Distinction
- Care must be taken to differentiate os acromiale from a normal immature acromion in younger patients 3
- Persistent acromial apophyses can be present up to age 21 years and should not be confused with true os acromiale 3
- Some specimens demonstrate solidly fused os acromiale with a distinct circumferential line that appears radiographically identical to unfused fragments 3