Effect of knee unloading shoes on regional plantar pressures in people with symptomatic knee osteoarthritis

Abstract

Purpose: Knee ‘unloading’ footwear can reduce joint loading in people with knee osteoarthritis, yet effects of these shoes on regional plantar pressures are unknown. Measurement of regional plantar pressures permits an increased understanding of how unloading footwear changes regional plantar loading, and may yield important insights into why some people with knee osteoarthritis respond to unloading footwear, while others do not. We evaluated the effects of unloading shoes on in-shoe regional plantar pressures, and whether measures of foot posture and/or mobility moderate these effects in people with symptomatic knee osteoarthritis. Methods: Twenty-one participants with medial compartment knee osteoarthritis underwent in-shoe plantar pressure measurements (Novel Pedar) while wearing knee unloading shoes (ASICS GEL-Melbourne OA) and standard conventional walking shoes (ASICS) in random order. The knee unloading shoes were commercially available unloading walking shoes with triple-density midsoles (stiffer laterally than medially) and mild (5-degree) lateral wedge insoles attached to the underside of the sock liners. Peak total forces were compared across conditions for: entire foot, lateral heel, medial heel, lateral forefoot, and medial forefoot. Arch index, center of pressure position and medial-lateral heel peak force ratio were also evaluated. Foot posture (neutral or pronated) was assessed using the Foot Posture Index. Vertical and medial-lateral mobility of the midfoot was assessed with the foot mobility magnitude. Foot mobility was also assessed using the navicular drop test. 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: Patient characteristics are presented in Table 1. The effects of unloading shoes on in-shoe regional plantar pressures, relative to conventional shoes, are summarized in Table 2. Unloading shoes significantly increased total, lateral heel and lateral forefoot force (+6.4%, +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). As a result of these changes, the medial-lateral heel force ratio also decreased significantly with unloading shoes (−20.0%, P < 0.001). Unloading shoes significantly shifted the center 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). Whilst all but three participants demonstrated an increase in lateral heel force with unloading shoes relative to conventional shoes (Figure 1), there was a large degree of individual variation in response. Despite this, no significant interaction effect was observed for foot posture, foot mobility magnitude and navicular drop on the effect of footwear on regional plantar pressures. Conclusions: Compared to conventional shoes, unloading shoes cause a lateral shift in foot pressure and force patterns. These findings may explain why a small proportion of people with knee osteoarthritis experience foot problems when wearing unloading footwear. Foot posture/mobility did not moderate effects of footwear on plantar pressures.Table 1Characteristics of participants (n = 21)Age, years63.4 (7.0)Male, n (%)9 (43)Body mass index, kg/m229.8 (3.6)Mild radiographic disease (grade 2), n (%)16 (29)Moderate radiographic disease (grade 3), n (%)19 (43)Severe radiographic disease (grade 4), n (%)16 (29)Foot Posture Index Classification, n (%)(Highly) supinated (scores less than 0)0 (0)Normal (scores from 0 to +5)9 (43)(Highly) pronated (scores greater than +5)12 (57)Foot mobility magnitude, mm9.6 (3.8)Navicular drop, mm7.6 (3.1)Pain while walking (NRS)23.7 (3.0)Physical function (WOMAC)318.5 (13.2)Values are reported as mean (SD) unless otherwise indicated.1Using the Kellgren-Lawrencegrading system; NRS = numerical rating scale; WOMAC = Western Ontario and McMasterUniversities OsteoarthritisIndex. 2Ranges from 0 to 10 (higher scores indicate worse pain); 3Ranges from 0 to 68 (higher scores indicate worse function). Open table in a new tab Table 2The effect of unloading shoes on plantar forces relative to conventional shoes.Plantar force measurementsConventional shoes, Mean (SD)Unloading shoes, Mean (SD)Difference (95%CI)P valuePeak foot force, N806.8 (152.7)858.7 (155.8)52.0 (37.6 to 66.4)<0.001Peak lateral heel force, N233.0 (68.3)263.0 (69.6)30.0 (18.1 to 42.0)<0.001Peak medial heel force, N229.4 (62.5)208.9 (57.7)−20.5 (−32.3 to −8.7)0.001Peak lateral forefoot force, N204.1 (44.4)245.4 (52.3)41.3 (21.5 to 61.0)<0.001Peak medial forefoot force, N246.1 (74.8)221.8 (63.9)−24.3 (−41.2 to −7.4)0.005Arch index (midstance)0.23 (0.04)0.21 (0.03)−0.01 (−0.03 to 0.00)0.093CoP x-position [ML](25%stance), mm53.0 (5.0)55.5 (4.9)2.5 (1.4 to 3.6)<0.001CoP y-position [AP](25%stance), mm78.9 (15.5)83.3 (14.7)4.4 (0.3 to 8.5)0.034Medial - lateral heel peakforce ratio1.0 (0.3)0.8 (0.3)−0.2 (−0.3 to −0.1)<0.001CoP = center of pressure Open table in a new tab Values are reported as mean (SD) unless otherwise indicated.1Using the Kellgren-Lawrencegrading system; NRS = numerical rating scale; WOMAC = Western Ontario and McMasterUniversities OsteoarthritisIndex. 2Ranges from 0 to 10 (higher scores indicate worse pain); 3Ranges from 0 to 68 (higher scores indicate worse function). CoP = center of pressure