Morphological evaluation of the sagittal plane femoral load-bearing surface in computer-simulated virtual total knee arthroplasty implantation at different flexion angles.

PURPOSE: To examine the effect of implantation of the femoral component of a total knee arthroplasty (TKA) system in 0°, 3°, and 6° of flexion on the sagittal plane morphology of the femoral load-bearing surfaces. It was hypothesized that increasing the flexion angle would result in undersizing of the anterior surface without changing the flexion gap.
METHODS: Computer simulation of a TKA using three-dimensional models of 10 healthy knees, matched to three different sized femoral components. Size discrepancy in the sagittal plane anterior, distal, and posterior joint surfaces between the native and prosthetic knees was calculated at 0°, 3°, and 6° of flexion.
RESULTS: The required component size varied with the angle of implantation: 0°, size 3/size 4 (N = 7/3), 3°, size 3 (N = 10); and 6°, size 2/size 3 (N = 4/6). Component undersizing ranged between 4.4-6.3 mm at the anterior lateral surface, with a significant difference between 0° and 6° (p < 0.05), and 1.2-3.5 mm at the anterior medial surface. Component oversizing of the distal surface of the lateral condyle (2.9 mm) and undersizing of the medial surface of the posterior condyle (1.6-2.3 mm) were comparable at all three flexion angles of component implantation.
CONCLUSIONS: Increasing the flexion angle of implantation increased the incidence of using a smaller size of femoral component without significant interference with the flexion gap. However, the effect of a smaller femoral component on undersizing of the anterior surface of the condyle and the impact on the extensor mechanism need to be considered.