Effect of knee unloading shoes on regional plantar forces in people with symptomatic knee osteoarthritis – an exploratory study

BACKGROUND: Knee ‘unloading’ footwear can reduce the external knee adduction moment in people with knee osteoarthritis, yet effects of these shoes on regional plantar forces are unknown. We evaluated the effects of unloading shoes on in-shoe regional plantar forces, and whether measures of foot posture and/or mobility moderate these effects in people with symptomatic knee osteoarthritis. METHODS: In this exploratory study 21 participants underwent testing while wearing knee unloading shoes (ASICS GEL-Melbourne OA) and conventional shoes in random order. Peak total forces were compared across conditions for: lateral heel, medial heel, lateral forefoot, and medial forefoot. Arch index, centre of pressure position and medial-lateral heel peak force ratio were also evaluated. Foot posture, foot mobility magnitude and navicular drop were separately added to the mixed linear model to investigate if these modified the effect of footwear on outcomes. RESULTS: Unloading shoes significantly increased lateral heel and lateral forefoot force (12.9 and 20.2% respectively, all P < 0.001), with concurrent decreases in the medial heel (8.9%, P = 0.001) and medial forefoot (9.9%, P = 0.005). Unloading shoes significantly shifted the centre of pressure anteriorly (4.7%, P < 0.001) and laterally (5.6%, P = 0.034), but did not affect the arch index (8.7%, P = 0.093). Foot posture, foot mobility magnitude and navicular drop did not moderate the effect of footwear on outcomes. CONCLUSION: Compared to conventional shoes, unloading shoes caused a lateral shift in foot pressure and force patterns. Although these effects were not moderated by foot posture, FMM or navicular drop, variability in the individual increases in lateral heel force suggests participant characteristics other than foot posture may play a role. TRIAL REGISTRATION: ACTRN12613000851763. Registered 02 August 2013.