Background: Atypical femur fractures (AFF) arise in the subtrochanteric and diaphyseal regions. Because of this unique distribution, we hypothesized that patients with AFF demonstrate specific geometrical variations of their femur whereby baseline tensile forces applied to the lateral cortex are higher and might favor the appearance of these rare stress fractures, when exposed to bisphosphonates.
Methods: Subjects who sustained AFF, as defined by the ASBMR task force, were recruited. Using the EOS low irradiation 2D3D X-ray scanner, bilateral lower extremities examinations were obtained in the upright weight-bearing position. EOS permits 3D surface images of high resolution from simultaneous two-plane images. We compared the participants femur geometrical parameters to those of a normal reference cohort and examined differences between those who sustained diaphyseal vs subtrochanteric AFF.
Results: We identified 25 subjects (21 women; mean age 67 (S.D. 9) years; 23 Caucasian, 2 Asian) with AFF. All were exposed to bisphosphonates (average cumulative duration of use of 10.6 (S.D. 4.6) years). There were 38 AFF (13 bilateral, 15 complete and 23 incomplete; 28 diaphyseal and 10 subtrochanteric). Compared with reference values, our subjects tended to have shorter lower limbs (femur 39.9 S.D. (2.2) cm and tibia 33.9 S.D. (2.2) cm), lesser femur neck-shaft angle (125.5° S.D. (6.5)), wider hip knee shaft angle (7.0° S.D. (1.8)) and higher femoral torsion (15.1° S.D. (10.8)). Compared to women with diaphyseal fractures, those with subtrochanteric fractures had a lesser femur neck-shaft angle (122.8° S.D. (3.8) vs 127.9° S.D. (6.8); P=0.09) and longer femoral offset (4.2 S.D. (0.2) cm vs 3.8 S.D. (0.6) cm; P=0.08).
Conclusion: Our data support that subjects with AFF exhibit femur geometry that results in higher mechanical load on the lateral femur, particularly in women with subtrochanteric fractures; this may play an important role in the pathogenesis of AFF.
18 May 2013 - 22 May 2013