Bone Formation Parameters of the Biopsied Ilium Differ between Subtrochanteric and Diaphyseal Atypical Femoral Fractures in Bisphosphonate-Treated Patients

: Atypical femoral fractures (AFFs) are defined as atraumatic or low-trauma fractures located in the subtrochanteric or diaphyseal sites. Long-term bisphosphonates (BPs) are administered to prevent fragility fractures in patients with primary osteoporosis or collagen diseases who are already taking glucocorticoids (GCs). Long-term BP use is one of the most important risk factors for AFFs. Its pathogenesis is characterized by severely suppressed bone turnover (SSBT), but whether the characteristics of patients are different regarding to location of fracture site remains unknown. In this study, we compared the characteristics and bone histomorphometric findings between subtrochanteric and diaphyseal sites in patients with BP-associated AFFs. Nine women with BP-associated AFFs were recruited, including 3 with systemic lupus erythematosus, 2 with rheumatoid arthritis, 2 with primary osteoporosis, 1 with polymyalgia rheumatica, and 1 with sarcoidosis. Patients were divided into the subtrochanteric group (n = 5; average age, 52 years; BP treatment, 5.9 years) and the diaphyseal group (n = 4; average age, 77 years; BP treatment, 2.6 years). Compared with the diaphyseal group, the subtrochanteric group had significantly higher daily GC doses (average, 10.9 vs. 2.3 mg/day) and significantly lower serum 25-hydroxyvitamin-D levels (17.8 vs. 25.6 ng/mL). Bone histomorphometry of the biopsied iliac bone showed SSBT in 3 cases (subtrochanteric, n = 1; diaphyseal, n = 2). Osteoid volume and trabecular thickness were significantly lower in the subtrochanteric group than in the diaphyseal group. Bone formation was inhibited more severely in subtrochanteric than in the diaphyseal group due to the higher GC doses used.