Summary

Joint balance and laxity targets were identified for improved pain scores at 2-year post; No association was found between alignment and outcome, indicating joint balance may have a greater impact on outcome than alignment.

Abstract

Introduction

Sensor augmented robotic assisted surgical platforms can quantitatively achieve a balanced joint in total knee arthroplasty (TKA). Attempts to define optimal joint balance have met with some success, however, the confounding impact of component alignment and relevance of these targets on 2-year outcomes has not been investigated. In this study we investigate if intra-operative joint balance is associated with midterm outcomes and aim to define optimal balance targets for KOOS pain scores 2 years post-TKA. Additionally, we investigate how final alignment impacts outcome.

Methods

212 patients were enrolled in a prospective cohort and received robot assisted posterior-cruciate-ligament sacrificing TKA with an ultra-congruent tibial insert utilizing a tibia-first gap-balancing approach. Demographics were captured pre-operatively and KOOS pain questionnaires were captured at 2-years post-op (747±101 days). Joint laxity throughout flexion was measured under a load of 70–90N during trialing. All tibial and femoral resection angles were recorded. Quadratic correlations between intra-operative joint gaps, alignment, and 2-year KOOS pain scores were investigated and informed threshold values for improved outcomes. Laxity is defined as the gaps between TKA components under load, and balance is the medial gap minus the lateral gap. Mann-Whitney-U tests were used to compare groups. The proportion of knees which satisfy the Patient Acceptable Symptom State (PASS) (87.5 points) is used to determine the clinical utility of the targets for achieving improved patient outcomes.

Results

Demographics of the population are: 58%F, 67±8 years, BMI of 32±5 kg/m2, coronal deformity of 5.2°±6.2° varus. Joint balance and laxity correlated significantly with KOOS Pain score at 2-years throughout flexion. Balance and laxity targets with maximum and minimum thresholds for improved pain scores were identified in extension (med. laxity: -1 to 2.5 mm, balance: 2.5 mm med. tight to 1.5 mm lat. tight), midflexion (avg. laxity: 0 to 2.5 mm, balance: 1.5 mm med. tight to 1.0 mm lat. tight) and flexion (avg. laxity: 0 to 2.5 mm, balance: 2.0 mm med. tight to 1.5 mm lat. tight) and are shown in Table 1. When all targets are satisfied, a higher pain score was achieved compared to those which did not (93.8 vs 88.0, p=0.0001). The proportion of knees which satisfied the PASS criterion was highest in knees which satisfied all targets (83%) and reported a 26% improvement compared to knees which did not satisfy all targets (p=0.008). No associations were identified between femoral or tibio-femoral component alignment and outcome at 2 years post-op.

Conclusions

Joint balance and laxity targets were identified for improved pain scores at 2-year post. No association was found between alignment and outcome, indicating joint balance may have a greater impact on outcome than alignment.