3D femoral neck anteversion measurements based on the posterior femoral plane in ORTHODOC® system

This study provides a robust measuring method of the femoral neck anteversion angle for use in a total hip replacement pre-planning program. The femora of 24 patients (69.3 +/- 6.3 years old) were CT-scanned and converted into three-dimensionally volume-rendered models in ORTHODOC (R) (ISS Inc., CA, USA) which is the pre-planning software for ROBODOC surgery. The Mod.ISS method (the modified ISS method), designed by authors, measures the anteversion angle of the proximal-most femoral neck confluence on the plane perpendicular to the femoral mechanical axis. 3D FNC method proposed by the authors of the present study involves measurement of the anteversion angle of three-dimensional femoral neck center on a plane perpendicular to the posterior femoral plane and parallel to the posterior condylar axis. Here, we found that interobserver reproducibility was 1.8 degrees(SD = 1.3) for the Mod.ISS method and 2.(4) over circle (SD = 1.9) for the proposed 3D FNC method. The anteversion angle of the local femoral neck axis was measured as theta= 25.3(L/D) -5.4 in L/D = 0.1 - 0.6, where L/D is distance (L) from the proximal-most neck confluence along the femoral mechanical axis, normalized with respect to the diameter of the femoral head (D). At L/D = 0.5, the anteversion angle of the femoral neck axis was coincident with the average femoral neck anteversion determined by the 3D FNC method. We conclude that the 3D FNC method is a gold standard for measuring the femoral neck anteversion applicable during both pre-operative and post-operative stages, because its femoral neck center can be determined in three-dimensional space during both stages.