Varus-valgus stability at 90A° flexion correlates with the stability at midflexion range more widely than that at 0A° extension in posterior-stabilized total knee arthroplasty

Midflexion stability can potentially improve the outcome of total knee arthroplasty (TKA). The purpose of this study was to evaluate the correlation between varus-valgus stability at 0A degrees of extension and 90A degrees of flexion and that at the midflexion range in posterior-stabilized (PS)-TKA. Forty-three knees that underwent PS-TKA were evaluated. Manual mild passive varus-valgus stress was applied to the knees, and the postoperative maximum varus-valgus stability was measured every 10A degrees throughout range of motion, using a navigation system. Correlations between the stability at 0A degrees, 90A degrees of flexion, and that at each midflexion angle were evaluated using Spearman's correlation coefficients. The stability of 0A degrees modestly correlated with that of 10A degrees-20A degrees, but it did not significantly correlate with that of 30A degrees-80A degrees. However, the stability of 90A degrees strongly correlated with that of 60A degrees-80A degrees, modestly correlated with that of 40A degrees-50A degrees, weakly correlated with that of 20A degrees-30A degrees, and did not correlate with that of 10A degrees. The present study confirmed the importance of acquiring stability at 90A degrees flexion to achieve midflexion stability in PS-TKA. However, initial flexion stability did not strongly correlate with the stability at either 0A degrees or 90A degrees. Our findings can provide useful information for understanding varus-valgus stability throughout the range of motion in PS-TKA. Attention to soft tissue balancing is necessary to stabilize a knee at the initial flexion range in PS-TKA.