Figure 1. Anteroposterior radiograph of the shoulder.
Figure 2. Outlet view of shoulder.
Figure 3. Axillary view of shoulder.
A 48-year-old man presents to the office with left shoulder pain that has been present for 3 months. The patient notes that the pain is worse when reaching over his head. He denies any recent injury to the shoulder. On physical examination, the patient has significant tenderness to palpation over the acromion and mildly positive impingement signs. Radiographic images of the shoulder show an apparent os acromiale (Figures 1-3).
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There are 3 ossification centers in the acromion: the pre-acromion (anterior), meso-acromion (middle), and meta-acromion (posterior). The acromion is formed when the centers have completely fused, usually by the age of 15 to 18 years, but can occur as late as 25 years.1 Os acromiale is a failure of the primary ossification centers of the acromion to fuse to the basi-acromion.1,2
The majority of os acromiale are asymptomatic and found incidentally on radiographs.1 The shoulder can become symptomatic with motion at the non-union site (most commonly the meso-acromion) or dynamic impingement as the unfused fragment moves with shoulder motion.2
Physical examination findings include shoulder pain with overhead motion and pain directly over the superior acromion. Three-view radiographs should be taken including an anteroposterior (AP), outlet, and axillary views of the shoulder.1 Os acromiale is often missed on AP radiographs and is most often seen on axillary view. Magnetic resonance imaging (MRI) is the most commonly used image modality to confirm the diagnosis and help determine if another pathology is present. Computed tomography is a good option to help visualize the unfused acromion.1,2
Conservative treatment is recommended for at least a 6-month period before surgery is considered. Surgical options include excision for symptomatic small fragments, arthroscopic decompression (and rotator cuff repair if present), and/or open fusion (open reduction and internal fixation).1,2
Dagan Cloutier, MPAS, PA-C, practices in a multispecialty orthopedic group in the southern New Hampshire region and is editor in chief of the Journal of Orthopaedics for Physician Assistants.
1. You T, Frostick S, Zhang WT, Yin Q. Os acromiale: reviews and current perspectives. Orthop Surg. 2019;11(5):738-744. doi:10.1111/os.12518
2. Hurst SA, Gregory TM, Reilly P. Os acromiale: a review of its incidence, pathophysiology, and clinical management. EFORT Open Rev. 2019;4(8):525-532. doi:10.1302/2058-5241.4.180100