Significant effect of the posterior tibial slope and medial/lateral ligament balance on knee flexion in total knee arthroplasty

The intra-operative femorotibial joint gap and ligament balance, the predictors affecting these gaps and their balances, as well as the postoperative knee flexion, were examined. These factors were assessed radiographically after a posterior cruciate-retaining total knee arthroplasty (TKA). The posterior condylar offset and posterior tibial slope have been reported as the most important intra-operative factors affecting cruciate-retaining-type TKAs. The joint gap and balance have not been investigated in assessments of the posterior condylar offset and the posterior tibial slope. The femorotibial gap and medial/lateral ligament balance were measured with an offset-type tensor. The femorotibial gaps were measured at 0A degrees, 45A degrees, 90A degrees and 135A degrees of knee flexion, and various gap changes were calculated at 0A degrees-90A degrees and 0A degrees-135A degrees. Cruciate-retaining-type arthroplasties were performed in 98 knees with varus osteoarthritis. The 0A degrees-90A degrees femorotibial gap change was strongly affected by the posterior condylar offset value (postoperative posterior condylar offset subtracted by the preoperative posterior condylar offset). The 0A degrees-135A degrees femorotibial gap change was significantly correlated with the posterior tibial slope and the 135A degrees medial/lateral ligament balance. The postoperative flexion angle was positively correlated with the preoperative flexion angle, gamma angle and the posterior tibial slope. Multiple-regression analysis demonstrated that the preoperative flexion angle, gamma angle, posterior tibial slope and 90A degrees medial/lateral ligament balance were significant independent factors for the postoperative knee flexion angle. The flexion angle change (postoperative flexion angle subtracted by the preoperative flexion angle) was also strongly correlated with the preoperative flexion angle, posterior tibial slope and 90A degrees medial/lateral ligament balance. The postoperative flexion angle is affected by multiple factors, especially in cruciate-retaining-type TKAs. However, it is important to pay attention not only to the posterior tibial slope, but also to the flexion medial/lateral ligament balance during surgery. A cruciate-retaining-type TKA has the potential to achieve both stability and a wide range of motion and to improve the patients' activities of daily living.