Purpose: Anatomically, the femoral bone shows sagittal and coronal bowing. Pronounced bowing would affect femoral implant surgery accuracy, but factors associated with degrees of three-dimensional (3D) femoral bowing have not yet been identified. This study measured 3D femoral bowing and examined the correlation between 3D femoral bowing and risk factors. Methods: 3D femoral bone models of unaffected legs from 125 patients were reconstructed from computed tomography data. A trans-epicondylar axis (TEA)-based coordinate system was introduced into bone models and the reproducibility of this coordinate system was evaluated. The cross-sectional contours of the femoral canal were extracted along the vertical axis of the TEA-based coordinate system, and the angles of 3D femoral bowing were examined. Results: The reproducibility to set the coordinate was very high. Total, sagittal, and coronal femoral bowing were 8.9 ± 1.8°, 8.7 ± 1.8°, and 0.1 ± 1.9°, respectively. There was no significant difference in the degrees of femoral bowing between males and females. Total, coronal, and sagittal femoral bowing correlated with age, height, and femoral length, but only femoral coronal bowing correlated with bone mineral density (BMD) of the femur and lumbar spine. Conclusions: We introduced a highly reproducible method for measuring femoral sagittal and coronal bowing by constructing a femoral 3D coordinate system. Anterior femoral bowing was the dominant direction of bowing in our Japanese cohort, whereas the degree of lateral bowing correlated with age, height, femoral length, and BMD. Our results suggest that lateral femoral bowing could increase with aging and decreasing BMD.
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