Abstract
Introduction
Achieving a well-balanced midflexion and flexion soft tissue envelope is a major goal in Total Knee Arthroplasty (TKA). The definition of soft tissue balance that results in optimal outcomes, however, is not well understood. Studies have investigated the native soft tissue envelope in cadaveric specimen and have shown loosening of the knee in flexion, particularly on the lateral side. These methods however do not reflect the post TKA environment, are invasive, and not appropriate for intra-operative use. This study utilizes a digital gap measuring tool to investigate the impact of soft tissue balance in midflexion and flexion on post-operative pain.
Methods
A prospective multicenter multi-surgeon study was performed in which patients underwent TKA with a dynamic ligament-balancing tool in combination with a robotic-assisted navigation platform. All surgeries were performed with APEX implants (Corin Ltd., USA) using a variety of tibia and femur first techniques. Gap measurements were acquired under load (average 80 N) throughout the range of motion during trialing with the balancing tool inserted in place of the tibial trial. Patients completed KOOS pain questionnaires at 3months±2weeks post-op. Linear correlations were investigated between KOOS pain and coronal gap measurements in midflexion (30°–60°) and flexion (>70°). T-tests were used to compare outcomes between categorical data.
Results
92 patients underwent TKA and completed questionnaires, with an average age of 68±9 years, 51% left and 57% female. No significant correlations were found between the medial and lateral gap size in midflexion or flexion and post-operative pain. Significant correlations were found between the absolute difference in the medial and lateral gaps in midflexion (r=−0.3, p=0.005) and flexion (r=−0.27, p=0.01) indicating knees with a more balance soft tissue profile reported improved pain outcomes at 3-months. Knees with less than 1mm difference in ML gap reported improved pain scores compared to those with greater gap differences in flexion (Δ=6.6, p=0.03).
A significant correlation was found between the difference in average gaps in midflexion and flexion, and post-op pain (r=0.22, p=0.04) in which knees that were looser in flexion than midflexion reported improved pain outcomes. When dichotomizing these results in to looser or tighter in flexion compared to midflexion, knees that were looser reported significantly improved pain outcomes (Δ=8.2, p=0.02).
Discussion and Conclusion
Improved outcomes correlated with a symmetrically balanced coronal midflexion gap and looser flexion space is consistent with the soft tissue balance of the native knee. The lack of a correlation between lateral loosening in flexion and improved outcomes may be a result of greater congruency between the femoral component and tibial insert than the native knee, preventing medial pivot lateral posterior condyle rollback motion. These results indicate that targeting the native soft tissue profile may not result in optimal outcomes when performing a TKA with a neutral tibial resection and an externally rotated femoral component. Further investigation is required to determine if these results hold with a larger data set and the effect on functional outcomes at both 3-months and longer follow-up periods.
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