Summary

An Inverse Kinematic approach to total knee replacement, utilising the BalanceBot technology, achieves excellent soft tissue balance when compared to a simulated kinematic approach.

Abstract

Introduction

Achieving soft tissue balance in total knee arthroplasty (TKA) is critical for successful outcomes. While kinematic alignment (KA) maintains the native joint line, KA may not achieve consistent soft tissue balance. Restricted KA (rKA) and restricted inverse KA (riKA) are potential improvements to the conventional KA approach. The aim of this study is to compare how riKA and rKA achieve and reproduce soft tissue balance by analyzing joint laxity, mediolateral (ML) balance, and the tibial recut rate required to achieve extension balance.

Methods

50 consecutive TKAs were retrospectively reviewed from a single surgeon. Four were excluded due to collateral soft tissue deficiency (1), missing radiographic data (2), and missing log file data (1).
A tibia-first riKA technique with robotic ligament tensioning was used. A restricted anatomical tibial resection was performed within 3° varus and 1° valgus. The tensioner was then inserted into the joint to collect laxity data through a range of motion. This data was used in predictive gap-planning software. The femoral component was virtually positioned to achieve rectangular extension gaps, while allowing lateral flexion laxity. Laxity was defined as the gap between the resected tibia and the planned femoral component.
rKA was simulated for each riKA case post hoc by using standard femoral wear levels recorded from preoperative AP long leg radiographs. As dictated by the study design, the riKA and rKA tibial resections were identical. rKA femoral resections were unrestricted in varus/valgus and rotation and defined as the implant thickness after accounting for distal femoral wear.
In extension and flexion, medial and lateral resection differences were calculated between riKA and rKA. rKA laxity was calculated by subtracting the resection difference from riKA laxity. ML balance was calculated as medial minus lateral laxity. Laxity, ML balance, and % of knees requiring tibial recut due to 1 and 2 mm ML imbalance were compared. Statistical analyses included Wilcoxon rank-sum, Levene’s, and chi-squared tests where appropriate

Significance

set at p=0.05).

Results

In extension, rKA laxity was tighter (p<0.01) and more variable (p<0.01) medially (mean±SD: 10.0±2.3 vs. 10.9±1.2mm) and more variable laterally (10.1±2.7 vs. 10.6±1.1mm, p<0.0001) compared to riKA. In flexion, rKA laxity was greater (p<0.001) and more variable (p<0.05) medially (12.5±2.9 vs. 11.3±1.9mm) and laterally (14.6±3.3 vs. 11.9±1.9mm) compared to riKA. rKA balance in extension was more variable than riKA (0.0±2.9mm vs. 0.3±0.8mm, p<0.0001) with more knees having >1 mm (67% vs 11%, p<0.0001) and >2 mm (48% vs 4%, p<0.0001) of imbalance. rKA flexion balance was more laterally imbalanced (p<0.0001) and more variable (p<0. 01) than riKA (-2.1±3.2 vs. -0.6±2.2mm).

Conclusions

Laxity and ML balance were less reproducible with rKA, with approximately half and two thirds of rKA patients requiring a tibial recut to achieve ML extension balance within 2 and 1 mm, respectively. Surgical technique significantly affects laxity and balance, and objective intraoperative soft tissue assessment along with predictive balancing allows for consistently achieving desirable laxity and balance.