A centroid iterative method for fitting the femoral neck axis in rotational osteotomy: a finite element analysis and biomechanical investigation

The femoral neck axis (FNA) is an important reference in femoral neck rotational osteotomy, which is a hip preservation procedure. The purpose of this study was to propose a method for determining the FNA with high accuracy and reliability and to evaluate the effect of FNA determination accuracy on the stress and strain distribution in the proximal femur. Femoral computed tomography data from 50 patients were reconstructed, and the FNA was fitted by the centroid iterative method. The fitting accuracy was evaluated in terms of the distance between the femoral head centre and the FNA. The reliability was assessed by intraclass correlation coefficient (ICC). Stress-strain distributions of the native femur model and rotational osteotomy models with accurate FNA and deviated FNA were simulated by finite element analysis and digital image correlation methods. The distance between the femoral head centre and the FNA was 1.24 ± 0.35 mm. The intra- and interobserver reliability was high, with ICC values of 0.960 and 0.924, respectively. The maximum von Mises stress was 25.76 MPa, 50.82 MPa, and 93.24 MPa for the native femur model, accurate FNA model, and deviated FNA model, respectively. The finite element strain distributions were linearly correlated with digital image correlation results. The centroid iterative method for FNA fitting has high accuracy and reliability. Accurate determination of the FNA reduces stress concentration in the proximal femur and decreases the risk of subsequent fracture and non-union of the osteotomy surface.