Soft Tissue Osteoma of the Hand Mimicking an Aggressive Soft Tissue Tumor: A Case Report

Background: Soft tissue osteoma (extraskeletal osteoma) is an exceedingly rare benign osteogenic tumor that originates entirely within soft tissues without continuity with the underlying skeleton. Its occurrence in the upper extremity, especially the hand, is exceptionally unusual. Because its clinical presentation is nonspecific and its magnetic resonance imaging (MRI) features can closely mimic aggressive soft tissue neoplasms, preoperative diagnostic confusion may occur. Case Presentation: A 28-year-old man presented with a painful, firm palpable mass in the left hypothenar space accompanied by mild tenderness and localized hypoesthesia along the ulnar digital nerve territory. MRI revealed an intramuscular lesion located in the abductor digiti minimi muscle displaying heterogeneous signal intensity, a peripheral low-signal rim, peritumoral soft tissue edema, and heterogeneous contrast enhancement, prompting high suspicion for an aggressive process such as synovial sarcoma or aggressive fibromatosis. After an ultrasound-guided core needle biopsy failed because of marked tumor hardness, surgical exploration was undertaken with oncologic resection in mind. Intraoperatively, the lesion showed a distinct capsule and a clear tissue plane without gross invasion of adjacent structures, allowing complete excision with preservation of the capsule and neurovascular structures. Frozen-section diagnosis was attempted but deferred because of dense calcification, and permanent histopathologic examination subsequently confirmed soft tissue osteoma. Postoperatively, the neurological symptoms resolved completely, and no clinical evidence of recurrence was observed at the six-month follow-up. Conclusions: Soft tissue osteoma should be included in the differential diagnosis of calcified or ossified indeterminate soft tissue masses of the hand. Awareness of this entity and its potential imaging pitfalls may help avoid overly aggressive management when evaluating indeterminate calcified or ossified soft tissue masses of the hand.