Geometric analysis of potential error in using femoral intramedullary guides in total knee arthroplasty

Proper component orientation is necessary for the long-term success of a to tal knee arthroplasty. Femoral component placement has used jigging systems that are based on intramedullary and extramedullary guides. The intramedul lary system has been shown to foster more accurate and reproducible placeme nt of the femoral component. This study analyzed and quantified the potenti al error of the intramedullary guide system. Geometric data of the human fe mur were obtained from radiographs of 45 cadaver femora in the anteroposter ior and lateral views. The correct entry point of the intramedullary rod in the distal femur is the midaxial line of the femoral canal on anteroposter ior and lateral radiographs. The position of this point in relation to the mediolateral and anteroposterior dimensions of the distal femur was calcula ted as a ratio. The average calculated ratios were anteroposterior, 0.53 of fset medially; lateral, 0.32 offset anterior. Mathematical models were cons tructed to quantify the potential varus valgus and flexion extension error of the guide rods. Variables investigated were entry point location, guide rod diameter, guide rod length, and rotation. The surgeon must be cognizant of these potential sources of error to maximize the accuracy of the femora l intramedullary jigging system.