Effect of Posterior Laxity on Knee Flexion Angle After Cruciate-Retaining Total Knee Arthroplasty

It is still controversial whether anteroposterior laxity or posterior tibial slope (PTS) after total knee arthroplasty (TKA) affects maximum flexion angle. The purpose of this study was to evaluate the relationship between PTS and tibial anteroposterior laxity after TKA and to examine the effect of these factors on knee flexion angle after TKA. This study enrolled 96 knees in 80 patients undergoing cruciate-retaining TKA (CR-TKA). Anterior translation (AT) and posterior translation (PT) were measured separately at a flexion of 90 degrees using stress radiographs. Total anteroposterior translation (APT) was defined as the sum of AT and PT. PTS was measured in a lateral view. In univariate analysis, postoperative flexion angle was positively correlated with preoperative flexion (R = 0.51; p < 0.01), PT (R = 0.48; p < 0.01), and APT (R = 0.48; p < 0.01), whereas it was negatively correlated with body mass index (BMI; R = - 0.43; p < 0.01) and PTS (R = - 0.24; p = 0.02). In multivariate analysis, postoperative flexion angle was negatively correlated with BMI (β = - 0.243; p < 0.01), whereas it was positively correlated with preoperative flexion (β = 0.406; p < 0.01) and PT (β = 0.279; p < 0.01). A negative correlation was found between PTS and PT (R = - 0.39, p < 0.01). An increase in posterior laxity at 90 degrees of flexion significantly increased postoperative flexion. An increase in PTS was associated with decreased posterior laxity; however, it had no effect on postoperative flexion.