Footwear and insole design features that reduce neuropathic plantar forefoot ulcer risk in people with diabetes: a systematic literature review

Background In people with diabetes, offloading high-risk foot regions by optimising footwear, or insoles, may prevent ulceration. This systematic review aimed to summarise and evaluate the evidence for footwear and insole features that reduce pathological plantar pressures and the occurrence of diabetic neuropathy ulceration at the plantar forefoot in people with diabetic neuropathy. Methods Six electronic databases (Medline, Cinahl, Amed, Proquest, Scopus, Academic Search Premier) were searched in July 2019. The search period was from 1987 to July 2019. Articles, in English, using footwear or insoles as interventions in patients with diabetic neuropathy were reviewed. Any study design was eligible for inclusion except systematic literature reviews and case reports. Search terms were diabetic foot, physiopathology, foot deformities, neuropath*, footwear, orthoses, shoe, footwear prescription, insole, sock*, ulcer prevention, offloading, foot ulcer, plantar pressure. Results Twenty-five studies were reviewed. The included articles used repeated measure (n = 12), case-control (n = 3), prospective cohort (n = 2), randomised crossover (n = 1), and randomised controlled trial (RCT) (n = 7) designs. This involved a total of 2063 participants. Eleven studies investigated footwear, and 14 studies investigated insoles as an intervention. Six studies investigated ulcer recurrence; no study investigated the first occurrence of ulceration. The most commonly examined outcome measures were peak plantar pressure, pressure-time integral and total contact area. Methodological quality varied. Strong evidence existed for rocker soles to reduce peak plantar pressure. Moderate evidence existed for custom insoles to offload forefoot plantar pressure. There was weak evidence that insole contact area influenced plantar pressure. Conclusion Rocker soles, custom-made insoles with metatarsal additions and a high degree of contact between the insole and foot reduce plantar pressures in a manner that may reduce ulcer occurrence. Most studies rely on reduction in plantar pressure measures as an outcome, rather than the occurrence of ulceration. There is limited evidence to inform footwear and insole interventions and prescription in this population. Further high-quality studies in this field are required.

Diabetic peripheral neuropathy (DPN) is the central risk factor for the development of plantar foot ulceration [13]. Over 30% of persons with diabetes will develop DPN [14], the incidence increasing with age [15,16]. DPN can affect the autonomic, sensory and motor nervous systems. Sensory neuropathy interrupts the protective feedback mechanism of touch and pain [17]. Motor neuropathy results in compromised muscle innervation, reduction in strength, and combined with limited joint mobility, the development of foot deformities. These deformities may lead to an increase in plantar foot pressures, particularly in the forefoot [18][19][20][21]. Autonomic neuropathy leads to diminished sweating and changes to skin perfusion, leading to dry skin and hyperkeratosis. As skin integrity is compromised, patients are more susceptible to trauma which may precipitate a diabetic foot ulcer [21][22][23][24].
Neuropathic ulcers in diabetic feet occur mostly at the plantar forefoot [11,25,26] and correspond to areas of peak plantar pressure [27]. Bennetts et al. [28] demonstrated that most peak pressure areas are located in the forefoot regions in this population. Limited range of motion at the forefoot joints is also likely to contribute to the peak plantar pressures (PPP) observed in this region [29]. For this reason, plantar pressure mapping is used to guide footwear and insole manufacture and judge their effectiveness [30].
Reducing plantar pressures is considered a key factor for wound healing and prevention of ulcer recurrence [31,32]. Footwear and insoles are an essential treatment modality for offloading these pressures [33,34]. The desired offloading threshold should be > 30% reduction in dynamic in-shoe plantar pressure from the baseline or < 200 kPa to ensure ulcer-free survival at the forefoot [35]. This systematic review aimed to summarise and evaluate the evidence for footwear and insole features that reduce pathological plantar pressures and the occurrence of diabetic neuropathy ulceration at the plantar forefoot in people with diabetic neuropathy.

Methods
The systematic search was performed according to the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) Statement [36].

Search strategy
In July 2019, six electronic databases were searched (Medline, Cinahl, Amed, Proquest, Scopus, Academic Search Premier) using medical subject headings followed by a keyword subject heading. The search period was from 1987 to July 2019. The search terms can be seen in Fig. 1 and Supplementary file 1.

Eligibility criteria
All studies included in the systematic review were obtained from full-text peer-reviewed journals published in English. Studies that did not use footwear or insole as a mode of intervention for long term offloading were excluded. Letters to the editor, opinion pieces, conference proceedings, and editorials were also excluded. All study designs except systematic reviews and case reports were eligible for inclusion. The titles and abstracts of the articles were screened by one reviewer (SA). Full-text articles were reviewed based on the following criteria: i, participants were adult (> 18 years), had diabetes; ii, all or some of the participants had neuropathy and foot deformity, history of plantar forefoot ulcers but no Charcot foot, history of heel ulcer or active foot ulcers; iii, studies used footwear or insoles as a long-term offloading intervention; iv, the outcome of the study was either (re)occurrence of forefoot ulcer or change in forefoot plantar pressure outcomes; v, the footwear or insole interventions had to be sufficiently described to be able to draw useful conclusions; vi, conventional materials and manufacturing techniques were used; and vii, closed-in footwear was used. The reference lists of studies obtained through the database search were also searched to identify relevant citations.

Quality assessment
Quality assessment was performed independently by two reviewers (SA and AB). The quality assessment form was adapted from the McMaster Critical Review Form -Quantitative Studies [37].
Study characteristics are shown in Tables 1 and 2.

Participants and settings
The participants were over 18 years of age, and the sample sizes ranged from 10 to 299. All participants in treatment groups had diabetes, and the majority had neuropathy. Participants had active or healed plantar foot ulcers, amputation, foot deformities, increased barefoot plantar pressure, or peripheral vascular disease. Most (88%) of the studies recruited participants from developed countries within high-risk foot clinics and 12% from developing countries [63]. Study duration ranged from a single session to 5 years.

Outcome measures
Eighteen studies [30, 38-42, 44, 46-55, 64] measured PPP as the primary outcome, and the majority measured this inshoe. Most of the studies [30,38,39,47,50,52,53,57,64] used 200 kPa as an upper threshold to classify the intervention as successful offloading the foot. The remaining studies compared a baseline pressure assessment without the intervention to peak pressure reductions with the interventions. PTI and Force Time Integral (FTI) had also been assessed as a parallel outcome measure in some studies [40, 48-51, 53, 55]. Other studies [50,51,55,64] also measured contact area and soft tissue thickness (STT) [46,48] as a parallel outcome. Some single parameters measured by the studies were maximum force, contact area [64], and walking convenience [42]. One study [41] reported foot-sole hardness as an indicator and reduction in shore hardness value. Six studies [56][57][58][59][60][61] reported ulcer recurrence as a primary outcome measure and another study [62] reported on ulcerative and non-ulcerative lesions as the primary outcome. Three studies [57,58,60] measured patient adherence in their study as a secondary outcome.

Reductions in forefoot plantar pressure
Arts et al. [38] reported on the effectiveness of footwear and insole design based on the algorithm proposed by Dahmen et al. [68]. The rate of pressure reduction was lower at the metatarsals area (29-50%) compared to midfoot (81%) and known ulcer location (62%) [38] when footwear and insoles are designed according to Dahmen's algorithm.
Non-weight-bearing (NWB) casting technique yields more effective custom-made insoles to offload the hallux region and semi-weight-bearing (SWB) casting technique is more effective to offload 1-3 metatarsal heads (MTHs) [55]. The NWB insoles also yield the highest arch support comparing to insoles made by other casting techniques [55].
Insoles designed based on foot shape and plantar pressure data are more effective to offload the forefoot region compared to insoles designed based on foot shape only [49,54,62]. The outcome can be between 32 to 21% improvement from shape-only and traditionally manufactured insoles out of polypropylene base [49].
Custom-made insoles with multi-density, softer materials have demonstrated improved forefoot offloading compared to higher-density EVA (55°shore A). Extra arch support, metatarsal pads, a plastazote top cover, and local cushioning can further reduce plantar forefoot pressure [42,64]. Metatarsal pad, local cushion and a plastazote top cover can reduce peak pressure by 14 to 15.9% on their own. A plastazote top cover combined with a metatarsal pad and local cushioning reduces 24 and 22% PPP at the forefoot [39].

Reductions in ulcer recurrence
López-Moral et al. [60] explored the effect of two rocker soles: semi-rigid (Wellwalk technology with Vibram Strips) and rigid on the recurrence of ulceration. By using, a rigid rocker sole the risk of re-ulceration at the forefoot was reduced by 64% when compared with semirigid rocker sole footwear.
Busch et al. [56] examined the effect of two different footwear (Lucro stock diabetic footwear versus regular retail footwear) with insoles on ulcer relapse of 92 participants with high-risk neuropathic feet at 12 and 42 months. The footwear was available in three different widths with differing features: rocker bottom outsoles and soft upper with three layers. This combined footwear and insoles reduced ulcer relapse by 45% compared with standard footwear within the first year.
Rizzo et al. [61] compared a treatment group who were given therapeutic footwear designed as per Dahmen et al. [57,68] and custom-made insoles to a control group who received standard footwear. The participants were assessed for ulcer occurrence and relapse at 12, 36 and 60 months. Ulcer relapse rates were significantly lower (11.5% versus 38.6% at 12 months, 17.6% versus 61% at 36 months and 23.5% versus 72% at 60 months) in the treatment group than controls.
Lavery et al. [59] examined the effect of shearreducing insoles on ulcer recurrence when compared with standard insoles in the same style of footwear. Shear-reducing insoles were 3.5 times less likely to create ulcers in the study participants compared to the standard insoles, although, both insole types demonstrated equivalent plantar pressure reduction [69].
In another study [57] based on the algorithm proposed by Dahmen et al. [57,68] the treatment group received custom-made footwear that was adjusted following inshoe pressure analysis. Controls received custom-made footwear without the in-shoe pressure analysis. The primary outcome was ulcer relapse after 18 months. The outcomes were not significantly different due, in part, to variance in patient adherence.

Discussion
Footwear and insoles are complex biomechanical interventions due to variance in design, materials, manufacturing methods, individual preferences and rates of adherence. This complexity is compounded when it is considered alongside the range of foot pathologies that co-exist with diabetes. Forefoot structural deformities are prevalent in this patient group [11,12] increasing inshoe plantar pressure at the metatarsal heads. The importance of footwear and insoles in offloading PPP for preventing plantar forefoot foot ulceration is well documented [70,71]. However, the specifications of design parameters and materials that can reduce PPP at the forefoot area are not precise. Reduction of PPP is one of the major factors to reduce the risk of ulcer occurrence and recurrence. This review explores the identification of critical design features and materials used in footwear and insole manufacturing that can reduce PPP at the forefoot and prevent ulcer occurrence and recurrence. Summery of those features that are available in the literature has been presented in Appendix 1 and 2.
Several studies have suggested rocker sole profile as the most recommended design to offload PPP at the forefoot [30,39,51,52,56,60,61]. The studies showed strong evidence for the rocker sole with evidence pointing towards specific variations of the rocker sole: such as apex position, apex angle, rocker angle and rigidity of sole materials. An RCT [60] showed that a rocker sole configuration with the pivot point under the metatarsal heads and rigid sole materials improve plantar pressure offloading at the forefoot compared to rocker sole made with semi-rigid materials. In a 6 month follow-up, the plantar ulcer recurrence rate was 23 and 64% among the experimental and control group where sole rigidity was the only variant. Preece et al. [52] and Praet et al. [51] compared apex position and rocker angle for rocker sole design in their studies. They recommended an apex position at 52-63% of shoe length and rocker angle of 20-23 0 to provide effective offloading at the forefoot (< 200 kPa), finding it more effective than any other lower or higher values of those respective parameters.
Arts et al. [38] in the Netherlands and Rizzo et al. [61] in Italy tested the effect of footwear design suggested by the consensus-based algorithm proposed by Dahmen et al. [68]. The key footwear design features in Dahmen algorithm are based on medical conditions. For example, the recommendations for a person with diabetes and history of neuropathic ulcers are footwear with a high upper (above ankle boots), stiffened tongue and leg uppers, rigid rocker soles with early pivot point. Both studies used above-ankle boots with custom-made insoles to offload pressure at the forefoot area. Both studies found that footwear and insoles designed according to this algorithm, are effective in offloading the neuropathic diabetic foot. However, Arts and colleagues [38] found that the algorithm is not as effective for footwear specifications to offload plantar pressure at the metatarsal heads.
There is a lack of guidance in the literature on footwear modifications that offload the forefoot. Footwear modification (also known as footwear customisation or optimisation) is common in both prefabricated and fully custom-made footwear. Most frequent footwear modifications are a re-configuration of rocker sole profile, such as early or significant pivot point (rocker angle) and stiffening the outer sole [30,39]. Footwear modification success (≤200 kPa) is least at the forefoot [38,39]. Bus et al. [30] recommended in-shoe plantar pressure analysis as an effective tool to guide the modifications for offloading the target regions in the neuropathic foot.
Insole modification features include local cushioning, replacing top covers with plastazote and applying a new or re-positioning existing metatarsal bars and metatarsal domes [30,39,47,61], removing plugs, and adding arch supports [61,64]. These are the most effective (PPP reduced ≤200 kPa) modifications in offloading or reducing PPP in targeted regions [30,39]. The targeted regions were determined by the history of ulceration or from PPP measurements data. These modifications in the insole are proven to be effective in offloading plantar pressure at an optimal level. However, they are least effective in offloading pressure at the metatarsal heads [38,39].
Pedorthists commonly use a higher upper height in their treatment of neuropathic forefoot ulcers. Dahmen et al. [68] and Diabetic Foot Australia (DFA) guideline [34] support such practice. However, Praet et al. [51] showed that high-ankle boots did not influence plantar pressure offloading when compared with low cut footwear. The authors suggest that although high-ankle boots do not change plantar pressures, they may reduce shear forces inside the shoe at the forefoot by increasing contact area around the ankle. Considering these findings, further studies assessing high-ankle boots will help to inform clinicians working in this field.
Many design features were not examined in the literature. Higher quality research is required to scientifically examine other important footwear design parameters, including heel height, toe height, upper materials, sole materials, heel counters, and closure systems for this therapeutic target.
In practice, the use of custom-made insoles over prefabricated devices needs to be considered in relation to cost versus benefit. Paton and colleagues [50] used two different insoles, made out of EVA and Poron, and compared cost as well. Custom devices were 18% higher cost in delivery than prefabricated insoles. The main difference was where the foot was cast to make the insoles, or insoles were selected from stock. There was no significant difference in PPP reduction between the two types of insoles. Custom-made insoles were, however, found to reduce PTI more than prefabricated insoles and lasted longer [50]. Customised devices may be preferred in practice as they account for structural changes in the diabetic foot, which is likely the reason that they reduce PTI more than prefabricated devices. Other studies [30,40,46,48,53,55] that compared PPP reduction capacity of the custom-made insoles with prefabricated insoles and not examined the cost, those found custom-made insoles to be more effective in pressure offloading in almost every region of the foot.
Most common insole base materials are EVA with the hardness of 50-55 0 Shore A and 30-35 0 Shore A [47,53] and the latter material showed improved performance in offloading PPP. However, the medium-density EVA base (30-35 0 Shore A) insoles need more frequent replacement than the higher density EVA group insoles due to material fatigue.
PPT or Poron as mid-layer [56] and top cover materials either MCR, plastozote or microfiber are effective in plantar forefoot pressure offloading. PPT or Poron is also used as a top cover in some insole designs [56,64]. Use of a leather top cover is of limited benefit due to its poor pressure reduction capacity [39].
None of the studies looked at the prevention of initial neuropathic plantar forefoot ulcer occurrence rather than a subsequent recurrence ulcer. Additionally, studies did not assess forefoot ulceration in isolation, but whole foot ulceration. PPP reduction in different regions requires different types of offloading. Further, different footwear and insole design features show differences in pressure reduction efficacy in different regions of the foot. The articles relied on in-shoe plantar pressure measurement data as a predictor of ulceration. However, other factors such as co-morbidity and lack of adherence to treatment also contribute to ulcer occurrence.
Plantar tissue stress incorporates vertical plantar pressure, horizontal shear pressure, and the frequency at which it is applied [73]. The reliance on plantar pressures as a predictor of ulceration may, therefore, be only one part of the picture. Lavery et al. [59,69] reported that two different insoles (shear-reducing and standard insoles) with equivalent plantar pressure reduction capacity could have a significantly different outcome in ulcer recurrence where shear-reducing is the only differentiation factor. Shear-reducing insoles had 3.5 times higher ulcer prevention capacity than the standard insoles in the study participants. Since design features are likely to influence footwear function, and therefore, adherence, it is important to consider which features may prevent ulceration.
There is limited data in the literature to determine the efficacy of footwear in preventing ulcer occurrence. Preece et al. [52] and Martinez-Santos et al. [47] explored the efficacy of footwear and insole design features, but could not make any recommendations for preventing ulcer occurrence.
In this review, the articles were excluded if the participants had heel ulcer, Charcot foot or any active, dorsal foot ulcers, and these might limit the representation of complete diabetic foot conditions. This may limit the footwear and insole feature recommendations for those feet that have those conditions. Heterogeneity in study designs, interventions, outcome measures and footwear and insoles design features make it also very difficult to come into a conclusion. Greater variations in participant's inclusion criteria and foot deformities, footwear and insole types, their measuring, casting and designing techniques, in-shoe pressure analysis systems may result in inconsistent data. Hence, we can not make a clear comparison or pool data to analyse further.
Because of the need to customise to the individual, the success of custom-made footwear as an intervention in offloading the plantar foot is dependent on the knowledge and skills of the prescribers and manufacturers [30,40,55]. The studies in this review used a variety of skilled practitioners in these roles such as orthopaedic shoemakers, pedorthists depending on the region. The presence of these practitioners in the interdisciplinary team approach in high-risk foot services is increasingly recognised ( [34], http://nadc. net.au/foot-network/).
Several studies [30,42,50,57,60,61] explored patient satisfaction and adherence to wearing footwear and insoles. Patient adherence to wearing therapeutic footwear is vital to ensure improved offloading and ulcer prevention [57,60,61]. No difference was found in patients' perceptions of custom-made versus prefabricated insoles [50]. Adding arch support and large metatarsal domes to basic insoles reduces patient adherence and walking comfort, despite evidence that these features improve pressure offloading [42].
Studies did not report the factors that influence adherence to therapy, which also limits the application of our findings. Consideration of patient expectations, effective education on footwear and activity-specific device designs are limited in the literature. Studies also did not consider geographical and socioeconomic factors. Most studies [30, 38, 39, 42, 47, 48, 50-53, 56, 57, 60, 61] were carried out in developed countries [63] with climates conducive to using ankle-high boots. Also, the practicality of these ankle-high boots for countries with warmer climates needs revisiting concerning patient adherence.
There was no study to take a personalised-treatment approach to focus on an individual's need or preference to increase adherence. Footwear is a very personal item, and a pre-study participant's feedback on their future footwear is crucial as opposed to only post-study feedback as adherence plays a vital role in an individual's outcome [51,57,58,60]. Study designs like the N-of-1 or single-patient-trial design [74,75] may bridge the gap in the literature.
Appropriate footwear design that takes into consideration the needs of low-income countries and those with warmer climates are limited in the literature, even though the prevalence of diabetes tend to be higher among the populations in these regions [76].

Conclusion
There is limited evidence to inform footwear and insole interventions, especially in conjunction with inshoe plantar pressure reduction. The available evidence supports the identification of footwear and insole design and modification parameters that can influence forefoot plantar pressure reduction. Prevention of ulcer occurrence or recurrence at the plantar forefoot region in diabetic patients is limited. Further research is needed to improve care for people with diabetic foot ulceration.