Comparison of Total Knee Arthroplasty Soft-Tissue Balancing between Surgeon Feel and the Modified Pendulum Knee Drop (PKD) Assessment

The purpose of this study was to (1) compare the accuracy and precision of a surgeon's manual soft-tissue balancing compared to the modified pendulum knee drop (PKD) tests and (2) evaluate the generalizability of the modified PKD test across a range of polyethylene thicknesses and surgical plans. Methods Robotic-assisted TKAs were performed on an advanced knee simulator (AKS) model. In the first arm of this study, four high-volume arthroplasty surgeons conducted manual laxity assessments on the AKS, repeated with a knee stocking using the modified PKD test with inserts from nine to 14 millimeters (mm). A total of 15 blinded trials were performed per surgeon. Cohen's kappa statistics were calculated for the manual and modified PKD groups. For the second arm, two independent surgeons developed two surgical plans for the same deformity and the PKD test was used to evaluate tibial inserts ranging from nine to 14 mm to quantify laxity changes. A total of three trials were conducted per tibial insert for each surgical plan, totaling 48 trials. Analyses of variance tests were used to assess the significance between surgical plans based on stiffness values and knee excursion with Tukey post hoc analyses to locate differences. Results Surgeons were able to identify the correct polyethylene thickness 65% of the time and the modified PKD test 96.7% of the time (P < 0.001). The average intra-rater kappa was 0.58 when surgeon-only and 0.95 when the surgeon used the PKD test. There was no improvement in accuracy across the three rounds for each surgeon. Across both surgical plans, smaller inserts exhibited increased laxity, while larger inserts demonstrated increased stiffness. Across both surgical plans, there was a progressive increase in stiffness with larger inserts. Discussion High-volume arthroplasty surgeons have variability in their soft-tissue balancing, which is improved with the use of the PKD test, and 2) the use of a modified PKD test can reproducibly give quantitative data on TKA stiffness across surgical plans.