Does knee stability in the coronal plane in extension affect function and outcome after total knee arthroplasty?

Purpose: The aim of the present study was to clarify whether varus–valgus laxities under static stress in extension, femoral condylar lift-off during walking, and patient-reported outcomes after total knee arthroplasty (TKA) were correlated with each other. Methods: Ninety-four knees, which had undergone posterior-stabilized TKA, were analysed. The varus–valgus laxity during knee extension was measured using a stress radiograph. New Knee Society Score (KSS) questionnaires were mailed to all patients. Correlations between the values of stress radiographs and KSS were analysed. Additionally, continuous radiological images were taken of 15 patients while each walked on a treadmill to determine condylar lift-off from the tibial tray using a 3D-to-2D image-to-model registration technique. Correlations between the amount of lift-off and either the stress radiograph or the KSS were also analyzed. Results: The mean angle measured was 5.9 ± 2.7° with varus stress and 5.0 ± 1.6° with valgus stress. The difference between them was 0.9 ± 2.8°. Varus–valgus laxities, or the differences between them, did not show any statistically significant correlation with either component of the KSS (p > 0.05). The average amount of femoral condylar lift-off during walking was 1.4 ± 0.8 mm (medial side) and 1.3 ± 0.6 mm (lateral side). The amount of lift-off did not correlate with either varus–valgus laxities or the KSS (p > 0.05). Conclusions: No correlations were found among varus–valgus laxities under static stress in extension, femoral condylar lift-off during walking, or patient-reported outcomes after well-aligned TKA. This study suggests that small variations in coronal laxities do not influence lift-off during walking and the patient-reported outcomes. Level of evidence: IV.