Validation of a Pendulum Knee Drop Technique to Quantify Knee Stiffness in Total Knee Arthroplasty

A quantitative approach for intraoperative evaluation of knee stiffness during total knee arthroplasty (TKA) is the pendulum knee drop (PKD) technique. By assessing the PKD technique's capacity to reliably and sensitively identify controlled variations in knee stiffness across operators and testing situations, this study aimed to verify the method. Robotic-assisted TKA was performed on a validated advanced knee simulator (AKS) model. Different thicknesses of polyethylene inserts were used to introduce controlled changes in knee stiffness. Inertial measurement units were used for PKD testing in two experimental settings with various operators and sensor systems. Stiffness was measured using log decrement ratios. Sensitivity to small changes in stiffness and reproducibility was evaluated. Across multiple trials and operators, the PKD technique demonstrated low variability and high reproducibility. Incremental increases in stiffness were consistently detected across both 1- and 2-mm insert thickness variations. Log decrement ratios were comparable between operators for equivalent insert conditions (p > 0.05), supporting interobserver reliability. Strong relationships were observed between oscillatory behavior and stiffness parameters. In a controlled TKA model, the PKD technique demonstrated high sensitivity and reproducibility in detecting incremental changes in knee stiffness, supporting its potential as an objective intraoperative adjunct for soft tissue balancing. Insert thickness variation was used as a controlled perturbation to validate the measurement system rather than representing a primary outcome. In a controlled TKA setting, the PKD test is a sensitive and repeatable way to measure knee stiffness. This method may provide a consistent and objective adjunct to subjective intraoperative evaluation.