Association of atypical femoral fractures with bisphosphonate use by patients with varus hip geometry.

BACKGROUND There is increasing evidence associating "atypical" femoral fractures with prolonged exposure to bisphosphonate therapy. The cause of these fractures is unknown and likely multifactorial. This study evaluated the hypothesis that patients with primary osteoporosis who sustain atypical femoral fracture(s) while on chronic bisphosphonate therapy have a more varus proximal femoral geometry than patients who use bisphosphonates for primary osteoporosis but do not sustain a femoral fracture. METHODS The femoral neck-shaft angle was measured on the radiographs of 111 patients with atypical femoral shaft fracture(s) and thirty-three asymptomatic patients; both groups were on chronic bisphosphonate therapy. Patients with characteristic lateral cortical thickening, stress lines, and thigh pain were included in the fracture group. RESULTS The mean neck-shaft angle of the patients who sustained atypical femoral fracture(s) while taking bisphosphonates (case group) differed significantly from that of the patients on bisphosphonate therapy without a fracture (129.5° versus 133.8°; p < 0.001). Fifty-three (48%) of the patients in the case group had a neck-shaft angle that was lower than the lowest angle in the control group (128°). Side-to-side comparison in patients with a unilateral pathologic involvement and an asymptomatic contralateral lower limb did not demonstrate any significant difference between the neck-shaft angles in the two limbs. CONCLUSIONS Patients on chronic bisphosphonate therapy who presented with atypical femoral fracture(s) had more varus proximal femoral geometry than those who took bisphosphonates without sustaining a fracture. Although no causative effect can be determined, a finding of varus geometry may help to better identify patients at risk for fracture after long-term bisphosphonate use.