Soft tissue laxity is highly variable in patients undergoing total knee arthroplasty

Background One major goal of total knee arthroplasty (TKA) is to achieve balanced medial and lateral gaps in flexion and extension. While bone resections are planned by the surgeon, soft tissue laxity is largely intrinsic and patient-specific in the absence of additional soft tissue releases. We sought to determine the variability in soft tissue laxity in patients undergoing TKA. Methods We retrospectively reviewed 113 patients undergoing TKA. Data on preoperative knee deformity were collected. Data from a dynamic intraoperative stress examination were collected by a robotic tracking system to quantify maximal medial and lateral opening in flexion (85-95 degrees) and extension (-5-20 degrees). T-tests were used to assess the differences between continuous variables. Results A valgus stress opened the medial compartment a mean of 4.3 +/- 2.3 mm (0.0-12.4 mm) in extension and 4.6 +/- 2.3 mm (0.0-12.9 mm) in flexion. A varus stress opened the lateral compartment a mean of 5.4 +/- 2.4 mm (0.3-12.6 mm) in extension and 6.2 +/- 2.5 mm (0.0-13.4 mm) in flexion. The medial compartment of varus knees opened significantly more in response to valgus stress than valgus knees in both extension (5.2 mm vs. 2.6 mm; P < 0.0001) and flexion (5.4 mm vs 3.3 mm; P < 0.0001). The lateral compartment of valgus knees opened significantly more in response to varus stress than varus knees in both extension (6.7 mm vs. 4.8 mm; P < 0.0001) and flexion (7.4 mm vs. 5.8 mm; P = 0.0003). Conclusions Soft tissue laxity is highly variable in patients undergoing TKA, contributing anywhere from 0-13 mm to the post-resection gap. Only a small part of this variability is predictable by preoperative deformity. These findings have implications for either measured-resection or gap-balancing techniques. Level of Evidence Level III.