Automated extraction of the femoral anatomical axis for determining the intramedullary rod parameters in total knee arthroplasty.

Abstract

The automated extraction of anatomical reference parameters may improve speed, precision and accuracy of surgical procedures. In this study, an automated method for extracting the femoral anatomical axis (FAA) from a 3D surface mesh, based on geometrical entity fitting, is presented. This was applied to conventional total knee arthroplasty, which uses an intramedullary rod (FIR) to orient the femoral prosthesis with respect to the FAA. The orientation and entry point of a FIR with a length of 200 mm are automatically determined from the FAA, as it has been shown that errors in these parameters may lead to malalignment of the mechanical axis. Moreover, the effect of partially scanning the leg was investigated by creating reduced femur models and comparing the results with the full models. Precise measurements are obtained for 50 models by using a central and two outer parts, with lengths of 20 and 120 mm, which correspond to 58% of the mean femoral length. The deviations were less than 2 mm for the FAA, 2.8 mm for the FAA endpoints and 0.7° and 1.3 mm for the FIR orientation and entry point. The computer-based techniques might eventually be used for preoperative planning of total knee arthroplasty.