Intra-articular fibroma of tendon sheath arising in the acromioclavicular joint

Fibroma of the tendon sheath, a rare benign soft tissue tumor that most often occurs in the distal upper extremities (hands and wrist), is exceedingly rare to present as an intraarticular mass. Presented here is the first case in the English literature, to our knowledge, of a fibroma of the tendon sheath arising in the acromioclavicular joint. The patient presented with recurrent shoulder pain with activity without antecedent trauma. Radiographs were essentially normal. MR images demonstrated a lobulated, heterogeneous mass with contrast enhancement arising from the acromioclavicular joint. Following surgical resection, histopathology revealed hypocellular collagen matrix with spindle-shaped fibroblasts, confirming the diagnosis of fibroma of tendon sheath. The imaging features of the fibroma of the tendon sheath and a brief review of the literature are presented.