No influence of coronal laxity and alignment on lift-off after well-balanced and aligned total knee arthroplasty

Purpose: In vivo fluoroscopic analyses have revealed the kinematics after total knee arthroplasty (TKA), including femoral condylar lift-off. This study asked whether differences in static varus-valgus laxity or coronal limb alignment after TKA affect lift-off under weight-bearing conditions. It was hypothesised that there is a correlation between coronal laxity or alignment and lift-off during walking. Methods: The current study analysed nineteen subjects undergoing cruciate-retaining TKA performed by the measured resection technique. The varus-valgus laxity at knee extension was measured using a 150 N stress radiograph. The mechanical axis was measured using a full-standing radiograph. Continuous radiological images were taken while the subject walked on a treadmill, and the images during single-leg stance were analysed to determine the lift-off using a 3D-to-2D image-to-model registration technique. Results: The average angle in varus/valgus stress was 6.8 ± 1.8°/6.6 ± 2.1°. No statistically significant differences were observed between the varus and valgus laxity. The average amount of lift-off was 0.7 ± 0.4 mm. The static varus-valgus laxity (n. s.) or the differences in the laxities (n. s.) on the stress radiograph did not influence lift-off. The weight-bearing ratio was achieved within the middle third of the knee in 90 % of subjects. Two outliers with valgus alignment (68 ± 1 %) demonstrated no significant difference in lift-off in comparison with the majority of the subjects (46 ± 9 %). Conclusion: The static coronal laxity and alignment did not influence the lift-off under dynamic weight-bearing conditions after well-balanced and aligned cruciate-retaining TKA. Measured resection technique can produce sufficient coronal stability and alignment without significant lift-off during walking. Level of evidence: IV.