3D Morphometric Analysis of Femoral Isthmus Dimensions and Their Influence on Modular Kinked Revision Stem Design in Total Hip Arthroplasty.

The development of stems for femoral revision surgeries requires an understanding of how distal fixation can be achieved, especially in those cases where the curvature of the diaphysis is not negligible, and a straight stem could result in three-point fixation. To this extent, a series of parameters linked to the femoral canal isthmus were identified and their correlation, as well as the potential difference between White and Asian ethnicities, were investigated. The anatomical features of 215 three-dimensional femur models were automatically computed. Statistical inference was performed leveraging the frequentist and Bayesian frameworks. A moderate positive correlation was found between the three-dimensional proximal-isthmus axis angle α and the distance between the isthmus cross-section and proximal-isthmus axis tilting point D_TI (p < 0.001, B₁₀ = 1.1 · 10⁵, ρ = 0.35). White subjects had both significantly longer distance between the head center and isthmus cross-section D_HI (p < 0.001, B₁₀ = 626.57, ∆µ = 10.18 mm) and longer distance between the head center and tilting point D_HT (p < 0.001, B₁₀ = 1.2 · 10⁵, ∆µ = 12.33 mm) with respect to Asian ones. Among all parameters, the correlation between α and D_TI is the most relevant and should be considered in designing a kinked revision stem to determine the best trade-off between fixation area and mechanical stability. The difference between White and Asian patients found for D_HI and the correlation between α and D_HI might have no practical effect in case of modular stem design. The remaining parameters had neither statistically significant correlation/difference nor practical impact on the design. These findings allow to reduce the design variables, thus leading to a more effective design process.