The relationship of ankle and foot kinematics with knee loading and foot center of pressure responses to a flexible footwear intervention

Abstract

Purpose: Flexible footwear has been associated with reduction of the knee adduction moment (KAM), an indicator dynamic medial knee loading. The mechanisms underlying this reduction, although unclear, may be in part through medialization of the foot center of pressure (COP), which has been associated with reduction in the KAM. Likewise, ankle and foot kinematics likely influence these relationships. Here, we evaluate the association of detailed foot kinematics with the response of knee loading and foot COP to a flexible footwear intervention. Methods: Participants with symptomatic medial compartment knee OA (KL 2 and 3) were evaluated at baseline using simultaneous barefoot COP analyses and 3-D gait analyses which included detailed foot kinematic modeling. All participants were provided flexible shoes (Dr. Comfort Flex-OA, Mequon, WI) and instructed to wear them at least 6 hours/day, 6 days/week. Gait and COP analyses were repeated after 12 and 24 weeks of wearing the shoes. Gait testing consisted of five trials each while barefoot, in subjects “own shoe”, and in the flexible shoe. During barefoot trials, plantar pressure distribution was acquired simultaneously by mounting a pressure platform onto a force plate and leveling the stacked assembly with the walkway. COP was quantified by determining the Medial to Lateral Pressure Index (MLPI), normalized to foot width. Paired samples t-tests were used to evaluate changes in KAM and foot COP. Spearman correlations were used to evaluate the association between KAM reduction and medialization of foot COP over 24 weeks with baseline foot kinematics as well as the change in kinematics after 24 weeks of wearing flexible shoes. Results: 28 participants were evaluated (mean age 59±7 years, 21 women). At 24 weeks, the KAM in the flexible shoe was reduced compared with their own shoes (2.88±0.99 vs 2.73±0.81%BW*Ht, p=0.030). Overall there did not appear to be significant medialization in foot COP and medialization was not related to KAM reduction. Reduction in KAM after 24 weeks was directly associated with overall range from foot supination to pronation during barefoot walking at baseline (rho=0.426, p=0.024) and flatter foot as measured by maximal medial arch angle after 24 weeks of wearing flexible shoes (rho=0.472, p=0.023). Medialization in foot COP after 24 weeks was directly associated with more maximal forefoot supination during barefoot walking at baseline (rho=0.405,p=0.036) and more rearfoot eversion by 24 weeks (rho=0.506, p=0.012). Conclusions: These results suggest that ankle and foot kinematic factors rather than medialization of foot COP are responsible for reducing medial knee loading with flexible footwear. Overall foot mobility during walking, in particular forefoot supination and pronation as well as lower medial arch by 24 weeks was associated with greater medial knee load reduction with use of flexible footwear. In contrast, medialization of foot COP alone did not affect medial knee loading. Other related foot kinematic variables including extent of forefoot supination and more rearfoot eversion were related to medialization in foot COP. This study demonstrates that dynamic foot kinematics are related to knee loading responses and plantar pressure changes with use of flexible footwear.