Working Diagnosis:
Symptomatic os acromiale with compensatory peri-articular dysfunction and pain

Treatment:
We performed a diagnostic/therapeutic US-guided injection of the bony abnormality with 0.5 cc 1% lidocaine without epi and 20 mg Kenalog which resulted in immediate and complete pain resolution for 5 days. Given the significant improvement in pain, she underwent arthroscopic subacromial decompression with partial acromioplasty and open ossicle excision with orthopedics.

Outcome:
She is currently recovering well post-operatively. She is progressing in physical therapy and reporting significant reduction of presurgical symptoms, with progressive return to desired level of physical activity.

Author's Comments:
We suspected that the heterotopic ossification was the primary pain generator which resulted in the compensatory scapulothoracic dysfunction. We should suspect underlying structural abnormalities, such as os acromiale in young patients without injury, presenting with years of pain and compensatory mechanics.

Editor's Comments:
An os acromiale is one of the most common causes of chronic anterior shoulder pain, particularly in the patient with an immature skeleton. However, an os ocromiale can be present and asymptomatic in many. This case presents an elegant use of a diagnostic injection by delivering local anesthetic under guidance to confirm a diagnosis before making a surgery decision.
Additionally pain around the scapula and pain associated with scapulothoracic motion can be difficult to understand or pinpoint. However, it is worthwhile to spend some extra time and clinical expertise evaluating the function or dysfunction of the scapulothoracic joint in an effort to make the best treatment plan for any pain in the shoulder region.

References:
Hurst SA, Gregory TM, Reilly P. Os acromiale: a review of its incidence, pathophysiology, and clinical management. EFORT Open Rev. 2019 Aug 9;4(8):525-532. doi: 10.1302/2058-5241.4.180100. PMID: 31538003; PMCID: PMC6719606.
Eckl, L., & Scheibel, M. (2023). Surgical treatment of a symptomatic os acromiale by arthroscopy-assisted double-button fixation: A case report. Archives of Orthopaedic and Trauma Surgery, 143(1), 423-428. https://doi.org/10.1007/s00402-022-04341-4

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