A New Method for Calculating Femoral Anterior Cortex Point Location and Its Effect on Component Sizing and Placement

BACKGROUND: Variation in anterior femoral cortex morphology can cause improper component placement and alignment. When surgical inaccuracies occur, the mechanical properties of the distal femur may be altered, which could result in lower surgical success rates and an increased chance of postoperative complications. QUESTIONS/PURPOSES: The purpose of the study is to come up with a reproducible computational algorithm to simulate what the surgeon does in the operating room. This method could help in surgical preplanning, patient-specific instruments, and implant design. From there, we evaluated (1) the angular difference between reference alignment axes; and (2) whether the location of the anterior cortex point and alignment axes had an effect on implant placement and amount of bone resected in seven implant systems. METHODS: We analyzed 470 femurs from white and black individuals. Two points were defined using automatic three-dimensional landmarking: sizing point and femoral resection anterior cortex (FRAC) point. Alignment axes including the transepicondylar, posterior condylar, distal anatomical (DAA), and mechanical axes (MAs) were automatically calculated and used along with the resection point to define the anterior reference plane. Two mechanical axes were defined for the purpose of this study: MA-1 is a virtual construct used in navigated surgeries defined as the axis joining the center of the femoral head and the knee center and MA-2 was calculated as the axis joining the center of the femoral head and distal exit point of the DAA. Amounts of anterior, posterior, and distal resected bone were calculated along with the difference in orientation between the alignment axes. RESULTS: The mean angular difference between transepicondylar axis and posterior condylar axis (PCA) was 5.44° ± 2.99°. All seven implant families showed more total bone resection on both the lateral and medial sides when the implants were aligned using MA-2 and PCA+3 of external rotation (PCA+3) when compared with using MA-1 and PCA+3 (p < 0.01). Using MA-2 and PCA+3 as an alignment method reduced the amount of bone resection on both medial and lateral anterior surfaces from 1 to 2 mm. CONCLUSION: The FRAC point is a key landmark in the placement and sizing of the femoral component. Improper sizing, notching, undercutting, or overstuffing can occur based on selecting the highest or lowest cortex point. CLINICAL RELEVANCE: Balanced placement, prevention of notching, and anterior and posterior cut balancing were accomplished when using the suggested cortex point.