Extraosseous osteochondroma of the hypothenar region: a case report and literature review

BACKGROUND: Osteochondroma (OC) is the most common benign bone tumor, yet hand involvement is uncommon and extraosseous presentations are rarer still. The hypothenar compartment is an exceptionally unusual site, and lack of corticomedullary continuity can mimic soft-tissue and parosteal lesions. We report an extraosseous OC of the hypothenar region with adjacent ulnar artery thrombosis, provide a focused literature synthesis, and advance a microtrauma-related pathogenesis. CASE PRESENTATION: A 24-year-old Afghan male construction worker presented with a 10-year history of a slowly enlarging, firm, minimally painful mass in the left hypothenar region. Examination showed a bony-hard, immobile mass without neurovascular deficit. Radiographs and MRI demonstrated a 4.0 × 3.5 × 2.0 cm well-circumscribed ossified soft-tissue lesion with no corticomedullary continuity and no malignant features. En bloc excision was performed; intraoperatively a thrombosed ulnar artery was noted adjacent to the mass, with nerves preserved. Histopathology revealed a thin hyaline cartilage cap with abrupt endochondral ossification to mature trabecular bone containing marrow elements and no cytologic atypia, confirming extraosseous OC. Recovery was uneventful; at two months the wound was well-healed with full range of motion and no evidence of recurrence. CONCLUSIONS: This case exemplifies a rare presentation of OC in the hypothenar compartment, extending the recognized anatomic spectrum of extraosseous OC and highlighting the potential for palmar soft tissues to harbor osteocartilaginous lesions without corticomedullary continuity. Clinicoradiologic and pathologic correlation was decisive, as an orderly hyaline cartilage cap with endochondral ossification to mature trabecular bone established osteochondroma and distinguished it from the principal ossified extraosseous mimics, particularly BPOP/Nora lesion, soft-tissue chondroma, myositis ossificans, and surface osteosarcoma. The coexisting ulnar artery thrombosis, in the setting of repetitive manual loading, supports a biologically plausible link between microtrauma, local vascular injury, metaplastic chondrogenesis, and endochondral ossification in soft tissue. The literature synthesis indicates excellent outcomes after complete excision with no recurrence across short to midterm follow up. Clinically, firm palmar masses without corticomedullary continuity should prompt consideration of OC, confirmation by histology when imaging is indeterminate, and treatment with complete excision including the cartilage cap.