Os acromiale, the joining of the acromion to the scapular spine by fibrocartilaginous tissue rather than bone, is an anatomic variant that has been reported in approximately 8% of the population worldwide. Although it is often an incidental finding, os acromiale has been identified as a contributor to shoulder impingement symptoms and rotator cuff tears.

In many cases, this is an incidental finding, is not a cause of pain, and can be "ignored." However, When a symptomatic os acromiale exists,  surgical intervention is considered. Options include os acromiale excision, open reduction and internal fixation, and arthroscopic decompression.

Excision usually is reserved for small to midsized fragments (preacromion) or after failed open reduction and internal fixation. Persistent deltoid dysfunction may result from excision of a large os acromiale. If the Os results in impingement, but is stable, it can be "decompressed" during arthroscopic acromioplasty. But, when it is a large fragment, and unstable, then Open reduction and internal fixation is required---which preserves large fragments (meso- and meta-acromiale) while maintaining deltoid function. Cannulated screw fixation has been shown to result in good union rates. This has historically required the  use of a structural bone graft taken from your ilac crest (pelvis bone).Although synthetic alternatives are commonly used today.
Appropriate management requires consideration not only of the xray finding, but more importantly, the physical examination. If the Os is tender, and if there is evidence of inflammation on the MRI, this may indicate that it needs to be stabilized. Arthroscopic assessment may also be helpful. The video below shows an os acromiale--the scope is in the lateral portal.

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