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Subdeltoid lipoma: a case with symptoms mimicking glenohumeral instability and subacromial impingement.

Lipoma is the most frequently occurring benign soft tissue tumor in the shoulder and the axillary region in middle-aged and older persons, yet few such lipoma cases have been associated with clinical symptoms. A 38-year-old right-handed man presented with an enlarged feeling and a painful back-and-forth popping in his left shoulder. Although moderate tenderness of the subacromial bursa and bicipital groove existed, an obvious impingement sign was absent. Also not evidenced were signs of neurological deficits, limited range of motion, or any physical findings suggestive of instability. Magnetic resonance imaging showed a homogenous tumor in the subdeltoid that was isointense relative to the subcutaneous fat and fluid collection in the hypertrophic subacromial bursa. As the tumor was considered from the clinical and imaging findings to be attributable to all clinical symptoms, it was resected en bloc with a satisfactory result. Histopathologically, the tumor showed typical features of a simple lipoma. To our knowledge, the present case is the first of a subdeltoid intermuscular lipoma of which mechanism developing symptoms was preoperatively surmised from imaging. The symptom-mimicking shoulder instability was assumed to be produced by the back-and-forth snapping of the lipoma beneath the deltoid muscle. The mechanism for developing the subacromial impingement-like symptom was surmised to derive from the middle deltoid fibers pressuring the lipoma to push up into the subacromial space. This case is presented to emphasize the careful reading of imaging in considering the pathomechanism of the attributing symptoms.