Over recent years, there has been increasing research attention focussed on the role of nonsurgical foot interventions, such as footwear and shoe insoles, in the management of people with knee OA . Given that knee OA is likely to be mechanically driven , and that footwear and insoles have been shown to influence knee joint loading [37••], these rehabilitation strategies may offer great potential in the management of patients with knee OA.
ACR clinical guidelines conditionally recommend medially wedged insoles for patients with valgus knee OA, based on beneficial findings regarding pain and function in a RCT and a single systematic review [38, 39]. Given that no new research regarding medial-wedged insoles for knee OA has been published recently, the ACR recommendations regarding this orthotic intervention reflects the body of knowledge as it currently stands.
In contrast to OARSI and EULAR guidelines which suggested that laterally wedged insoles could benefit people with medial knee OA [2, 4, 5], the ACR guidelines were unable to form a recommendation regarding the use of lateral-wedge insoles . The American Academy of Orthopaedic Surgeons (AAOS) guideline, published in 2008, takes a more definitive stance and recommends that lateral heel wedges NOT be prescribed for people with medial knee OA . The discrepancy in recommendations regarding lateral wedges is largely related to the conflicting findings between biomechanical studies and clinical trials. Early research into lateral wedges was primarily confined to biomechanical evaluations. A recent systematic review (of literature published until January 2011) evaluated the effect of insoles on the external KAM in people with medial knee OA [37••]. This review demonstrated that lateral wedging was associated with a reduction in the peak KAM in ten out of the twelve studies included in the review (mean reductions ranging from 2 to 12 %). Most new biomechanical evaluations of lateral wedges published since 2011 agree with these findings [41–43]. Unfortunately, the assumption that the load-reducing effects of lateral wedges will translate into clinically meaningful reductions in pain in people with knee OA has not been borne out in the clinical trials conducted to date [44–47]. Of note, the latest RCT, involving 200 people with medial knee OA and published in BMJ in 2011 [48••], demonstrated that lateral wedges worn over 12 months do not significantly reduce pain or slow structural disease progression (as measured via cartilage volume on MRI). On the basis of the latest research findings, it would appear that lateral wedges offer no clinical benefit to patients with knee OA.
Given the disappointing findings of clinical trials evaluating lateral wedges, attention has recently turned to alternative types of insoles for managing knee OA. Shock-absorbing insoles have been evaluated for their effects on biomechanics, pain and function in one small study in 16 people with knee OA [49•]. This study found no immediate effect of shock-absorbing insoles on any parameter of KAM; however, late stance peak KAM did reduce from baseline to follow-up after 1 month of wearing the insoles. There is currently no evidence that a reduction in late stance peak KAM is clinically relevant. Whilst this study showed that shock-absorbing insoles significantly reduced pain and improved function after 1 month of wear, it is not clear whether these changes in symptoms are due to a real treatment effect or a placebo response. Clinical trials are needed to evaluate the efficacy of shock-absorbing insoles for knee OA and to provide evidence for informing future clinical guidelines.
Research has shown that medial knee OA is associated with a flattened (pronated) foot posture . Furthermore, recent data has linked the pronated foot type to a greater frequency of knee pain and medial tibiofemoral cartilage damage in older people . Whilst medial arch supports could constitute a logical treatment choice for these patients, the causal relationship between flattened foot posture and knee OA is yet to be established. It is possible that medial arch supports may cause a medial shift in the centre of pressure, thereby increasing the distance between the ground reaction force and the knee centre and increasing the KAM during gait [52, 53], and could be potentially harmful rather than beneficial. There is currently limited research about the effects of medial arch supports in people with knee OA. One study  evaluated a combination orthotic comprising both a medial arch support and lateral wedging. However, because this study involved a combination orthotic and utilised a control condition that also comprised medial arch supports, no conclusions about the independent biomechanical effects of medial arch supports can be drawn. In contrast, a recent study [55••] evaluated the independent immediate effects of medial arch supports on the KAM in 21 people with medial knee OA, as well as effects on pain during gait analysis. Findings showed that the medial arches had no significant effect on the KAM, nor resulted in any immediate changes in pain. Further research in this area is needed, particularly with a focus on longer-term follow-up periods, in order to assist clinicians in deciding whether medial arch supports may be of benefit for patients with knee OA.
Clinical guidelines from OARSI recommend that every patient with knee OA should receive advice concerning appropriate footwear . This recommendation was based on expert opinion given the lack of RCT evidence at the time the guidelines were developed. Regular off-the-shelf footwear increases parameters of medial knee load compared to barefoot walking in people with knee OA by 7–14 % depending on shoe type . The types of shoes that are more likely to increase medial knee load include those with high heels, and those that promote foot stability rather than mobility [56–59, 60•]. It appears that lightweight, flat and flexible footwear options may be optimal for people with knee OA. This is supported by data from recent biomechanical studies [61, 62] that evaluated an inexpensive, flat canvas shoe with a flexible rubber sole (Moleca, Calcados Beira Rio). In elderly women with knee OA, these shoes resulted in lower knee loads during stair descent and walking compared to heeled shoes. In fact, the knee loads experienced with the flexible shoe approximated those of barefoot walking. Thus, biomechanical evidence suggests clinicians should assess the footwear of patients with knee OA, particularly those with medial compartment disease and/or varus malalignment who are at risk of higher knee loads (and more rapid structural disease progression [26, 27]), and advise them regarding optimal footwear choices to minimise loading across the knee.
Over the past few years, there has been much interest in the development and evaluation of novel innovative footwear, specially designed to reduce knee loads in people with knee OA. Shoes with “variable stiffness” soles, where the lateral sole is stiffer compared to the medial side, have been shown to reduce the KAM compared to shoes with a constant stiffness sole [63, 64]. In an RCT comparing variable stiffness shoes to constant stiffness control shoes, the variable stiffness shoes resulted in significant within-group pain reductions at 6 months (average 37 % pain reduction compared to 19 % in control group); however, the change in symptoms was not significantly different between groups at either 6 months or 1 year [65, 66••]. The nonsignificant findings of this study may be related to the small sample size. A “mobility” shoe specially engineered to mimic barefoot walking and incorporating a flexible grooved sole, can reduce the peak KAM by 8 % compared to self-selected walking shoes , however the efficacy of this novel shoe style in treating symptoms associated with knee OA has not yet been evaluated. Commercially available unstable shoes (Masai Barefoot Technology®), which have a multilayered sole that mimics walking on soft uneven ground, have been suggested as a treatment option for people with knee OA given their potential to improve gait stability and reduce joint load . Recent kinematic and kinetic evaluations of these shoes have shown they reduce peak KAM by up to 13 % in overweight males (who are at risk of developing knee OA) . However an RCT, which compared unstable shoes to control walking shoes over 12 weeks in a sample with symptomatic knee OA, showed no significant differences between groups regarding change in pain, balance, knee range of motion or ankle strength . Thus at present it is not clear what role innovative shoe designs may play in managing symptoms associated with knee OA.