Working Diagnosis:
Rotator cuff tear secondary to mechanical impingement of acromion with complication of mobile os acromiale.

Treatment:
Initial treatment including physical therapy, non-steroidal anti-inflammatory drugs as needed, and cortisone injection in subacromial space resulted in mild improvement.

Rapid return of symptoms necessitated surgical intervention via shoulder arthroscopy. During shoulder arthroscopy, patient was noted to have a small anterior acromial spur, severe rotator cuff tearing, and a mobile os acromiale. The surgeon made note that the distal acromion was "mobile on palpation, allowing significant inferior motion and therefore significantly increased impingement upon the rotator cuff."

Surgical treatment was performed with removal of the distal acromion and rotator cuff repair.

Outcome:
The patient reported complete resolution of pain and related symptoms. He has regained full shoulder strength and range of motion with no complications. He was able to return to his full pre-injury activity level and experiences no restrictions or disabilities.

Author's Comments:
Os acromiale is present in approximately 8% of the population. There are four ossification centers or growth plates of the acromion and the failure to fuse results in an os acromiale. Most os acromiale are asymptomatic, but some may severely limit a patient's shoulder function because of pain. Pain may be from irritation over the os itself, from movement of the unfused bones, or from muscle, tendon, or bursal irritation or impingement due to resulting acromion motion. Failure of fusion may occur at any of the ossification centers, but most commonly occur at the meso-meta border. Case Photo #5 Case Photo #6 Case Photo #7

First line therapy includes NSAIDS, activity modification, physical therapy, and subacromial corticosteroid injections. If non-surgical methods fail to improve symptoms, surgical intervention such as the removal of small fragments located in the anterior acromion, debridement of the acromial undersurface, and open reduction and internal fixation are available.

Rotator cuff injuries are often multi-factorial in nature, and a combination of mechanical and functional impairments increase risk of rotator cuff tendonopathy. During initial radiographic evaluation os acromiale is often not commented on or discounted as a "normal variant." However, they may contribute, alone or in combination with other factors to cause shoulder pain.

Editor's Comments:
The acromiom often fuses between ages 15 and 18, but complete union can occur as late as age 25. About 30-60% of os acromiale are bilateral. As the author noted, pain generation from os acromiale may be directly at the nonunion site or secondary to dynamic impingement. A diagnostic subacromial injection may be useful in assessing the source of pain. Os acromiale is initially treated conservatively as described above. The most common surgical methods for refractory cases are internal fixation (60%), excision (27%), and acromioplasty (13%). In addition, 59% of surgically treated cases have concomitant rotator cuff repair and 25% have distal clavicle excision. All techniques have excellent outcomes.

References:
1. Sammarco VJ. Os Acromiale: frequency, anatomy, and clinical implications. The Journal of Bone and Joint Surgery 2000; 82:394-400.
2. Mathew L. Busbee, MD. (Jan 11, 2010), SportsMD.
3. Park JP, Lee JK, Phelps CT. Os acromiale associated with rotator cuff impingement. MR imaging of the shoulder. Radiology 1994; 193:255-257.

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