Bilateral atypical femoral subtrochanteric fractures in a premenopausal patient receiving prolonged bisphosphonate therapy: evidence of severely suppressed bone turnover.

We report a case of bilateral atypical femoral fractures that occurred in a patient who had been taking bisphosphonate long-term. A 36-year-old premenopausal female diagnosed with systemic lupus erythematosus and dermatomyositis had been treated with glucocorticoid and alendronate (5 mg/day) to prevent glucocorticoid-induced osteoporosis. She was taken to our hospital because she could not walk immediately after falling down from the standing position. A plain radiograph showed a subtrochanteric fracture of the left femur. Four months later, she fell again and sustained a contralateral subtrochanteric fracture. For each fracture, a femoral intramedullary nail was inserted. Delayed union was detected in both sides, and revision surgery with an iliac bone graft was required for implant breakage in the right side. Histomorphometric findings for the ilium revealed remarkably decreased osteoid volume with no osteoclasts and a minimally eroded surface, suggesting that bone turnover was severely suppressed. However, histology of the delayed union site revealed callus formation and some osteoclast appearance, suggesting that fracture healing was occurring. In total, it took 29 months (left) and 24 months (right) until fracture healing was achieved, showing delayed union. This case is extremely rare in that patient who presented with atypical femoral fractures in spite of her premenopausal status. The bone histomorphometric findings from this case suggest that severely suppressed bone turnover is associated with atypical femoral subtrochanteric fracture and can cause delayed union in patients treated with alendronate long-term.