Abstract
Introduction
Total knee arthroplasty (TKA) with a computer-assisted navigation system has been developed to improve the accuracy of the alignment of osteotomies and implantations. One of the most important goals of TKA is to improve the flexion angle. Although accurate soft tissue balancing has been recognized as an essential surgical intervention influencing flexion angle, the direct relationship between post-operative flexion angle and intra-operative soft tissue balance during TKA, has little been clarified. In the present study, therefore, we focused on the relationship between them in cruciate-retaining (CR) TKA with a navigation system.
Materials and methods
The subjects were 30 consecutive patients (2 men, 28 women), who underwent primary CR TKA (B. Braun Aesculap, e-motion) between May 2006 and December 2009. TKAs were performed using a image-free navigation system (OrthoPilot; B. Braun Aesculap, Tuttlingen, Germany). All cases were osteoarthritis with varus deformity. Average patient age at the time of surgery was 74.0 years (range, 62-86 years). After all bony resections and soft tissue releases were completed appropriately using a navigation system with tibia-first gap technique, a tensor was fixed to the proximal tibia and the femoral trial was fitted. Using the tensor that is designed to facilitate soft tissue balance measurements throughout the range of motion with a reduced patello-femoral (PF) joint and femoral component in place, the joint component gap and ligament balance (varus angle) were measured after the PF joint reduced and femoral component in place (Fig.1). Assessments of joint component gap and ligament balance were carried out at 0°, 30°, 60°, 90°, 120° flexion angle, which were monitored by the navigation system. Joint component gap change values during 30°- 0°, 60°- 0°, 90°- 0°, 120°- 0° flexion angle were calculated. The correlation between post operative flexion angles and pre-operative flexion angle, intra-operative joint component gaps, joint component gap change values and ligament balances were assessed using linear regression analysis.
Results
Pre- and post-operative knee flexion angle averaged 120.5 ± 15.4° and 121.2 ± 8.3°. Pre-operative flexion angle was positively correlated with post-operative flexion angle (R = 0.408, P = 0.025). Average joint component gaps were 13.7, 17.1, 17.5, 16.0 and 15.3 mm at 0, 30, 60, 90 and 120° of flexion, respectively. Average ligament balances were 1.8, 1.3, -0.3, -4.2 and -4.9° in varus at 0, 30, 60, 90 and 120° of flexion, respectively. Average joint component gap changes were 3.4, 3.8, 2.4 and 1.6 mm at each range of motion between 30–0, 60-0, 90–0 and 120–0° of flexion, respectively. Joint component gap at 90° flexion (R = 0.473, P =0.008) and joint component gap change value 90–0° (R = 0.495, P =0.005) showed positive correlations with post-operative knee flexion angle (Fig.2). The other factors assessed in this study showed no correlation with post-operative knee flexion angle.
Discussion
We performed an intra-operative assessment of soft tissue balance using the tensor in CR TKA with a navigation system. As previously reported, pre-operative flexion angle showed a positive correlation with post-operative flexion angle. Of note, the results showed smaller joint component gap at 90° flexion and joint gap change value 90–0° led to less flexion angle in CR TKA. Compared to posterior-stabilized TKA, CR TKA often results in tightened flexion gap. In such a situation, the results suggest that creation of relative larger flexion gap is important for acquisition of better flexion angle.
