Posterior condylar bone resection and femoral implant thickness vary by up to 3mm across implant systems: implications for flexion gap balancing.

Abstract

The purpose of this study was to evaluate the thickness of medial and lateral posterior femoral condylar bone resected with five implant systems using posterior referencing jigs set at 3° of external rotation. The hypothesis was that posterior condylar resection thickness on the medial side would be equal to the thickness of the femoral implant posteriorly, regardless of implant system. Posterior referencing femoral sizers were used on right femur sawbones models for five different implant systems. Each sawbones model was sized using a femoral sizer for the specific implant system. Sizing guides were set at 3° of external rotation for the right femur. Each system's 4-in-1 cutting block was then used to make posterior condylar cuts. The thicknesses of the cut bones were measured using a manual calliper. The amount of bone resected from both medial (P = 0.0004) and lateral (P < 0.0001) posterior condyles differed significantly across the five implant systems. The mean thickness of bone resected from the posteromedial femoral condyle ranged from 9.4 ± 0.5 to 12.4 ± 0.9mm. The mean thickness of the posterolateral condyle cut ranged from 6.7 ± 0.6 to 10.2 ± 0.3mm. The difference in thicknesses between the bone resection from the posteromedial condyle and the implant thickness of the posterior condyles ranged from 0.6 to 2.9mm. The thickness of bone removed from the posterior femoral condyles varied by up to 3mm across the five TKA implant systems. For each system, the posteromedial condyle resection was larger than the thickness of the posterior condyle of the actual implant. As the difference between the posterior bone resection and the implant thickness increases, the flexion gap will likely loosen and should be accounted for during gap balancing. In commonly used knee implant systems, resected bone is greater than implant thickness and may lead to flexion instability.