Objective Quantification of Soft-Tissue Balance in Total Knee Arthroplasty: Expanded Validation of the Pendulum Knee Drop Test Across Multiple Surgeons.

Abstract

Despite increasing evidence showing major diversity in subjective balancing approaches, soft-tissue balance during total knee arthroplasty (TKA) remains primarily based upon empirical surgeon assessment. A quantitative technique for describing knee stiffness and laxity after TKA was previously introduced: the modified pendulum knee drop (PKD) test. This study expands upon prior validation work by evaluating the reproducibility of surgeon-directed balancing assessments compared with the modified PKD test across a larger cohort of arthroplasty surgeons. Robotic-assisted TKAs were performed on an advanced knee simulator model previously validated for ligament balancing investigations. After completing blinded polyethylene insert identification trials using manual balance assessments, eight high-volume arthroplasty surgeons underwent additional testing with the modified PKD test. During several testing cycles, polyethylene inserts ranging from 9 to 14 mm were randomly assigned and assessed. For both manual and PKD-assisted evaluations, accuracy, delta error, and intra- and interobserver reliability were computed. Stiffness and laxity behavior were described using oscillatory excursion measurements and logarithmic decrement values. Surgeons correctly identified polyethylene insert thickness manually in 60% of trials and within 1 mm in 84.2% of trials. In contrast, PKD-assisted balancing resulted in 97% correct insert identification (<i>p</i> < 0.001). The mean delta between surgeon-estimated and actual polyethylene thickness was 0.68 mm during manual assessment versus 0.05 mm with the PKD test (<i>p</i> < 0.001). Intraobserver reliability improved from a kappa of 0.61 during manual balancing to 0.96 with PKD assessment, while interobserver reliability improved from 0.38 to 0.97, respectively. The PKD test maintained highly reproducible thickness-independent stiffness ranking patterns across nearly all trials. High-volume arthroplasty surgeons demonstrated substantial variability during subjective soft-tissue balancing assessments, whereas the modified PKD test provided highly reproducible, objective biomechanical characterization of stiffness and laxity during TKA. These expanded findings further validate the PKD methodology as a reproducible tool to quantify soft-tissue knee laxity during robotic-assisted TKA.