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 total knee arthroplasty. Femoral component placement has used jigging systems that are based on intramedullary and extramedullary guides. The intramedullary system has been shown to foster more accurate and reproducible placement of the femoral component. This study analyzed and quantified the potential error of the intramedullary guide system. Geometric data of the human femur were obtained from radiographs of 45 cadaver femora in the anteroposterior and lateral views. The correct entry point of the intramedullary rod in the distal femur is the midaxial line of the femoral canal on anteroposterior and lateral radiographs. The position of this point in relation to the mediolateral and anteroposterior dimensions of the distal femur was calculated as a ratio. The average calculated ratios were anteroposterior, 0.53 offset medially; lateral, 0.32 offset anterior. Mathematical models were constructed 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 femoral intramedullary jigging system.