Multilobed Osteochondroma of the Posterior Humeral Midshaft: A Rare Morphological Variant.

Osteochondroma, or osseocartilaginous exostosis, is the most common benign bone tumor, accounting for approximately 30%-35% of benign bone lesions and predominantly arising from the metaphyseal regions of long bones in young males. Mid-diaphyseal origin and multilobed morphology are exceptionally uncommon and may present diagnostic and surgical challenges. We report a rare case of a solitary, multilobed osteochondroma arising from the posterior humeral midshaft in an 18-year-old male. The patient presented with a 1-year history of a gradually enlarging, painless mass over the lateral aspect of the left upper arm, without functional limitation or neurovascular symptoms. Physical examination revealed a firm, immobile, nontender mass with intact distal neurovascular status. Magnetic resonance imaging demonstrated a pedunculated osseous lesion measuring 6.5 × 3.3 × 2.3 cm, exhibiting continuity with the humeral medullary cavity and a distinctive three-lobed configuration, including a bilobed stalk. The cartilage cap thickness measured approximately 4 mm, with no surrounding soft tissue abnormalities. Surgical excision was performed via a posterior longitudinal approach, with careful identification and preservation of the radial nerve. Intraoperatively, three distinct pedunculated bony stalks arising from the posterior humeral midshaft were excised en bloc. Histopathological analysis confirmed a benign osteochondroma, showing mature trabecular bone capped by hyaline cartilage without atypia or malignant features. To the best of our knowledge, no previous report has specifically described this combination of posterior humeral mid-diaphyseal location and multilobed morphology. This case expands the anatomical and morphological spectrum of osteochondroma and highlights the critical role of advanced imaging in preoperative planning, particularly for lesions in proximity to major neurovascular structures. Recognition of such atypical presentations is essential to ensure accurate diagnosis, safe surgical management, and avoidance of iatrogenic complications.