Variability of human femoral geometry and its implications on nail design

The objectives of this study are to present a new method of quantifying variability in human femoral geometry and to use this data to optimize intramedullary nail geometry for a better fit within the femoral canal.

Computed tomography (CT) scans of forty intact adult human femora were divided according to race, sex, and age, and were reconstructed digitally into three-dimensional solid models. Geometric features were then measured and compared among groups using ANOVA. An average geometric model was generated, from which an optimal intramedullary (IM) nail curvature function was derived. Insertion of the derived optimal nail and of two currently-available commercial nails into the femur having the highest degree of curvature was then simulated via finite element methods.

Substantial variability in femoral curvature was observed among the population sample. Sex was found to correlate most strongly with femoral size; males tend to have larger femora than females. Although the average femoral radius of curvature for African Americans was slightly higher than for Caucasians, the difference was not statistically significant. Curvature did not vary across the sample by age, sex or race. Finite element analysis results simulating IM nail insertion using the geometrically-optimized nail showed a substantial decrease in von Mises stress when compared to tested commercially-available IM nails. Fracture was predicted within the posterior canal wall for commercially available nails, as the generated stresses exceeded the allowable stress of cortical bone, but not for the geometrically-optimized nail.

Considering variation in femoral geometry among patients when designing implantable fixation devices may be important. Femoral size differs between adults by sex, with males being larger than females. Pattern of curvature, however, did not differ, although there may be slight differences between European and African Americans. As such, the proposed average femoral curvature function calculated here may be sufficient for the entire population. According to finite element analysis, insertion-induced stresses in the femur were within the allowable range for the geometrically-optimized nail and appear more desirable than in other common nail designs.