Distribution of atypical fractures and cortical stress lesions in the femur: implications on pathophysiology.

INTRODUCTION: Some authors have hypothesised that atypical femur fractures occur due to tensile mechanism of failure. We studied the distribution of such lesions along the femur shaft to determine if they concentrate in regions that are subject to tensile loading.
METHODS: From May 2004 to March 2010, radiological reviews of 48 patients aged 69 +/- 10.4 (range 47-92) years with atypical femoral fractures and lesions were performed. The absolute distance of each lesion from the greater trochanter and the ratio of the distance of each lesion from the greater trochanter expressed as a percentage of the entire femur length were measured.
RESULTS: All periosteal reactions and cortical stress lesions occurred in the lateral cortex. There were 35 right femoral lesions (28 complete fractures and seven cortical stress reactions), with a median distance of 108.3 +/- 54.0 (range 67.0-270.4) mm from the greater trochanter and a median ratio of 23.9 +/- 11.7 (range 15.7-58.6) percent of the entire femoral length. There were 38 left femoral lesions (27 complete fractures and 11 cortical stress reactions), with a median distance of 109.9 +/- 43.1 (range 73.6-246.2) mm from the greater trochanter and a median ratio of 24.4 +/- 9.1(range 16.3-51.1) percent of the entire femoral length.
CONCLUSION: Based on previously established femoral shaft loading characteristics, atypical lesions were clustered at the region of maximal tensile loading. No lesion occurred in regions that were subject to compressive loading. This unique distribution supports a tensile mechanism of failure in such lesions.