The effect of hamstring tightness on intraoperative extension gap in posterior stabilized total knee arthroplasty

This study aimed to determine the factors related to intraoperative extension gap (EG) in patients who underwent posterior-stabilized total knee arthroplasty (TKA). A total of 106 TKAs in 84 patients were retrospectively reviewed. Only patients who underwent the same method of bone resection were included consecutively. Bilateral popliteal angle (BPA) was used as an indicator of hamstring tightness. EG and extension space angle were measured using an offset type tensor. The associations between patient variables and EG were analyzed using multivariable linear regression and Pearson's correlation coefficients. The average EG was 12.9 ± 2.1 mm, and the average extension space angle was 2.8° ± 3.2°. BPA was greater than flexion contracture in most cases (94.3%), and no difference was found in only six cases (5.7%). According to multivariable linear regression analysis which was conducted after modifying the BPA into a categorical variable by 5°, EG was correlated with BPA (p < 0.001). Pearson’s correlation coefficient between EG and BPA was − 0.674 (p < 0.001). No other factors were significantly correlated with intraoperative EG. The present study found that popliteal angle is a different entity from flexion contracture, and that it is a predictable factor for EG in osteoarthritis patients. Smaller BPAs led to larger EG in patients who underwent the same degree of bone resection.