Increased Intercondylar Femoral Box Cut-to-Femur Size Ratio During Posterior-Stabilized Total Knee Arthroplasty Increases Risk for Intraoperative Fracture

BACKGROUND: Iatrogenic intraoperative fractures are preventable complications in total knee arthroplasty. As press-fit fixation becomes more popular, further investigation into risk factors is needed. Some authors have suggested that smaller femurs may be at higher risk in posterior-stabilized constructs owing to industry designs trending toward larger, constant box sizes that increase the amount of bone resection relative to bone stock. METHODS: Finite element analysis (FEA) was used to investigate the effect of insertion of posterior-stabilized femoral components on stress distributions in small femurs and whether common bony preparation techniques could further affect risk for intraoperative fracture. The FEA results were validated with mechanical testing by loading to failure with varying resection depths of the distal femur and varying lateralization of the box cut. RESULTS: With a standard distal resection depth and neutral box position, a decrease in femur size led to an increase in maximal von Mises stresses by 43.6% medially and 44.3% laterally. Box lateralization and increased distal resection depth had minimal changes on the maximal stresses (3.3% medially and −0.4% laterally) on average-sized femurs while having a much larger effect on the stress distribution in small femurs (118.3% medially and 6.7% laterally). CONCLUSIONS: A subset of intraoperative femur fractures is potentially preventable. Small femur sizes, especially ones that would require increased distal resection or change in implant positioning, may benefit from an alternative design without the need for a cam/post mechanism.