Beyond the Biopsy: Macrodystrophia Lipomatosa Unmasked After False Osteochondroma Diagnosis.

Introduction: Macrodystrophia lipomatosa (MDL) is a rare, non-hereditary congenital disorder of localized gigantism caused by disproportionate proliferation of fibroadipose tissue in all mesenchymal elements of the affected extremity. Its clinical, radiological, and histopathological features can closely mimic those of osteochondroma - the most common benign bone tumor - leading to misdiagnosis, inappropriate surgery, and rapid recurrence. We report a case in which a recurrent foot swelling in a child, twice confirmed as osteochondroma on histopathology, was revised to MDL and successfully managed with staged reconstruction. Case Report: A 9-year-old male presented in 2024 with a progressively enlarging, recurrent swelling over the medial aspect of the right foot associated with overlying skin changes. The swelling had first been noted when the child began walking. Initial excision at an outside institution had yielded a histopathological diagnosis of osteochondroma arising from the head of the first metatarsal. Recurrence occurred within 2-3 months. Repeat biopsy in September 2020 again reported osteochondroma; a skeletal survey showed no additional lesions. By 2024, continued enlargement with soft-tissue involvement, friable overlying skin, and radiological features prompted a revised diagnosis of MDL. A two-stage surgical approach was undertaken. Stage one comprised wide local excision, polymethylmethacrylate (bone cement) spacer placement with K-wire stabilization of the first metatarsal, and soft-tissue reconstruction using an anterolateral thigh free flap. Stage two involved removal of the cement, reconstruction of the first ray with a fibular strut graft fixed with a foot plate and intramedullary K-wire, and simultaneous flap debulking. The patient progressed from non-weight-bearing to full weight-bearing at 1 month postoperatively. Results: At 20 months of follow-up, the child ambulates independently and performs all activities of daily living without restriction. There is no clinical or radiological evidence of recurrence. A mild limitation in running and toe movement persists. Conclusion: MDL can reliably masquerade as osteochondroma on both biopsy and imaging, particularly in the foot. Rapid post-excision recurrence with progressive soft-tissue involvement should trigger magnetic resonance imaging reassessment and reconsideration of the diagnosis. A staged reconstructive approach - wide excision, free flap coverage, and biological bone reconstruction - achieves durable functional recovery in most cases.