(L-466) Effect of Foot Progression Angle on Knee Medial Compartment Loading During Stepping on an Elliptical Trainer

Introduction: Medial knee osteoarthritis (KOA) is a progressive joint disease with a rare chance of cartilage repair. Therefore, slowing the rate of cartilage loss is the best strategy for dealing with the condition. Peak knee adduction moment (pKAM) is an indicator of medial knee loading and a surrogate measure of the presence and progression of cartilage damage in the medial compartment of the knee. Previous studies have shown that gait modification strategies, including changes in the foot progression angle (FPA), can reduce pKAM. However, due to subject-to-subject variability in response to these interventions, these studies were limited in detecting the accurate FPA that induces the an optimal reduction in pKAM for each subject. Therefore, the objective of this pilot study was to explore the subject specific FPA for reducing pKAM in healthy adults using a custom robotic elliptical trainer that applies various controlled slow perturbations in the transverse plane to the knee while the subject makes steps without noticing the perturbations. It should be noted that elliptical stepping produces less impact on the knee than overground walking.

Methods: Apparatus - A customized elliptical trainer with a six-degree-of-freedom (6-DOF) goniometer and multi-DOF controlled stepping was used to determine knee moments during stepping. A 6-axis force/torque sensor was mounted underneath each footplate to measure "footplate reaction" forces and moments. The footplates of this customized elliptical trainer were modified and can rotate in the transverse plane. Procedure - A total of 22 healthy subjects with no history of lower limb injury participated in this pilot study. A typical stepping session started with regular stepping. Then, the footplates were fixed at a 10° external rotation (toe-out), and subjects started stepping in this position. While subjects were stepping, the footplates started rotating 10° inward (toe-in) with a slow velocity until it reached the neutral position. Lastly, the footplates rotated outward until it reached 10° internal rotation. All these modifications in the footplate position were done without the subjects' knowledge. These footplate perturbations were done to both footplates simultaneously. Measurements of the knee moment were done in real-time on the right side. The slope of a linear fit for changes in the pKAM during footplate transition was calculated. Also, pKAM for each toe-in/toe-out position was measured. Statistical analysis - The last ten stepping cycles in each condition (footplate position) were chosen for analysis. One-Samplet-test was used to compare the slope of the linear fit with zero slope. Wilcoxon signed ranks test was utilized to investigate differences in pKAM between different conditions for each subject. The significance level was set at 0.05.
RESULTS: The slope of changes in the pKAM during footplate transition was significantly different from zero (P < 0.05, Figure 1), with a positive slope towards the toe-out position. Among 22 subjects, 18 of them (81.82%) reduced their pKAM with toe-in by 20.24% and 4 subjects (18.18%) reduced their pKAM during toe-out condition by 15.16%. However, the results of the Wilcoxon signed ranks test only showed a significant difference for the magnitude of pKAM between different footplate positions in the first group (toe-in group) (P < 0.05, Figure 3). The pKAM values (mean±std) observed for the neutral, toe-out, and toe-in foot positioning were 1.82±0.70, 2.26±0.77, and 1.50±0.69 in the toe-in group, and 2.08±1.73, 1.87±1.78, and 2.57±1.47 in the toe-out group respectively.

DISCUSSION: In this pilot study, we evaluated potential foot positioning during elliptical stepping for the rehabilitation of patients with medial KOA. Specifically, we evaluated whether stepping with modified foot positioning reduces peak KAM during stepping on our customized elliptical trainer. It has been reported that toe-in or toe-out gait patterns alleviate KAM in healthy individuals and individuals with KOA during overground walking. Similarly, the transition of our elliptical footplates in transverse plane (form toe-out to toe-in position) showed reduction in the pKAM. Therefore, these modifications in foot positionings are a useful approach for identifying optimal strategies in reducing pKAM during stepping, with the subjects walking without noticing the foot positioning changes. This is an advantage over identifying the strategies during overground walking, during which the subjects make the foot positioning changes voluntarily and thus may change the walking and knee joint loading patterns. Therefore, the identified foot positioning strategies can be used in rehabilitation of medial tibiofemoral KOA.