Case Report: Atypical femoral fractures in adult-onset hypophosphatasia treated with low-dose asfotase alfa and adjunctive teriparatide therapy.

Hypophosphatasia is a rare inherited metabolic bone disease caused by low activity of tissue-nonspecific alkaline phosphatase. Adult-onset hypophosphatasia is increasingly recognized, but fracture patterns and optimal management remain unclear. We report a 52-year-old woman who presented with progressive pain and bilateral atypical femoral fractures. She was diagnosed with hypophosphatasia based on low alkaline phosphatase, markedly elevated pyridoxal 5'-phosphate, a history of metatarsal fracture, premature tooth loss, and pathogenic ALPL variants. Low-dose asfotase alfa was started because standard-dose therapy was not feasible. After 6 months, callus formation was observed, and pain improved. However, 6 months later, the pain recurred, and radiographs showed a progressive fracture, suggesting insufficient fracture healing. Staged intramedullary fixation was performed for both femurs. Two perioperative femoral fractures occurred during the course of surgical management. Therefore, adjunctive teriparatide (20 µg daily for six months) was initiated while continuing low-dose asfotase alfa. At 5-year follow-up, radiographic union of all femoral fractures was achieved, and the patient remained ambulatory without assistance and had no fracture-related pain. This case shows that adult-onset hypophosphatasia can cause severe skeletal fragility and may require individualized multidisciplinary management, including biochemical diagnosis, long-term radiographic surveillance, cautious surgical planning, and selective use of medication.