Biomechanical factors play an important role in diabetic foot disease. Elevated dynamic plantar pressures significantly increase the risk of foot ulceration in patients who have lost protective sensation due to peripheral neuropathy . In fact, patients with high barefoot plantar pressures have a threefold to fourfold increased chance to develop a foot ulcer compared with patients with low plantar pressures [2, 3]. These increased pressure levels are caused primarily by structural modifications, such as foot deformity, plantar tissue quality loss and limited joint mobility, which are frequently observed in diabetic patients [4-7].
Reducing high foot pressures (i.e. offloading) is one of the main goals in healing and preventing foot ulcers. For this purpose, many different methods have been designed and used. The capacity to offload the diabetic foot is variable across these methods (~20%-80% peak pressure reduction compared with a control condition) and is strongly associated with the efficacy to heal plantar foot ulcers . Thus, with better offloading, more foot ulcers are healed in a shorter time.
Despite the important role of offloading in diabetic foot disease, several aspects are underexposed. First, the importance of offloading compared with other contributing factors in the aetiology and management of diabetic foot ulcers is not clear. Secondly, the use of offloading modalities in clinical practice is diverse and often not in agreement with evidence-based guidelines. Thirdly, the efficacy and role of offloading in healing complicated (i.e. ischaemic and/or infected) and non-plantar foot ulcers has not been clearly defined. Finally, in the currently available literature, ulcer prevention is underexposed compared with ulcer treatment.
These four issues require the attention of clinicians and scientists and can be expressed, in random order of importance, as four priorities:
Offloading should be studied in a broader perspective of contributing factors to foot ulceration.
The existing gap between evidence-based guidelines and clinical practice in the use of offloading treatment needs to be bridged.
The efficacy and role of offloading in healing complicated and non-plantar foot ulcers should be better defined.
A larger focus should be put on the prevention of diabetic foot ulcers.
Offloading studied in a broader perspective
Increased plantar foot pressures increase the risk for ulceration, but a general or individual pressure threshold above which an ulcer develops or below which an ulcer heals has not yet been identified. A barefoot dynamic peak plantar pressure of 700 kPa proved to be 70% sensitive and 65% specific for the development of foot ulceration in high-risk diabetic patients in one study . In another study, a barefoot peak pressure of 875 kPa was found to be 64% sensitive and 46% specific for foot ulceration . These results mean that still a significant number of patients develop a foot ulcer despite having lower than threshold pressures, and a significant number of patients do not ulcerate despite having higher than threshold pressures. Therefore, accurately predicting clinical outcome based on measured barefoot peak pressure is difficult.
The limitation of these analyses is that they are based solely on the measurement of the peak value of the normal component of barefoot pressure in a laboratory setting. Other pressure components and variables are not taken into account but may be important. Because patients normally wear shoes or other offloading devices, the in-shoe plantar pressure is one of them. Although ulcer prediction studies based on measured in-shoe plantar pressures do not exist, one cross-sectional study measured an average in-shoe peak pressure of ~200 kPa in patients who did not re-ulcerate for a longer period of time while wearing prescribed therapeutic footwear . This pressure value could therefore be seen as indicative for ulcer survival, although it should be noted that inter-subject variability was large, and data collection was not prospective. Another pressure component that may be important in ulcer development is shear . Ulcers often develop underneath callus which is mediated by shear. Unfortunately, the shear pressure component cannot be measured with the currently available commercial pressure measurement systems, and therefore its role in ulcer formation remains to be identified. Finally, other parameters than peak pressure, such as pressure–time integral or peak pressure gradient may prove to be important when assessed in prospective clinical studies . In conclusion, a more comprehensive pressure analysis may have the potential to improve ulcer prediction in the diabetic foot.
Apart from plantar foot pressure, non-biomechanical factors likely determine clinical outcome. These include patient behavioural factors such as the type and intensity of daily physical activity and adherence to prescribed treatment. The number of foot steps taken during the day determines the accumulated stress on the foot, which has been shown to discriminate between ulcer and ulcer-free patients [14, 15]. Increased variability in activity over the day is another outcome that has been associated with ulcer development . These findings represent ambulatory data, whereas studies show that diabetic foot patients spend twice as much time standing than walking during the day . The longer application of moderate pressures during standing could also prove to be a mechanism that increases ulcer risk. When it comes to treatment adherence, it is easy to understand that not even the best footwear or offloading device or instruction given (e.g. do not walk barefoot at any time) will be effective if it is not used or adhered to. The value of continuous, uninterrupted pressure relief can be appreciated from studies showing more effective healing of foot ulcers when using non-removable instead of removable offloading treatment [18, 19]. Adherence to using removable offloading treatment or prescribed footwear while being ambulatory can be as low as 25–28% [20, 21]. Therefore, treatment adherence is an important issue in ulcer development and healing in this patient group.
To better understand why a patient gets a foot ulcer, heals from an ulcer, or remains free from ulceration, all above-mentioned factors, in addition to disease-related factors, should be considered and assessed (Figure 1). Preferably, this should be done using objective and quantitative tools to improve the validity of outcomes [22-24]. Although a general or individual pressure threshold for ulceration is not very likely to emerge, improved prediction based on a combined assessment of disease-related, biomechanical and behavioural parameters should be possible.
In 2007, the International Working Group on the Diabetic Foot (IWGDF) developed evidence-based guidelines on the effectiveness of footwear and offloading interventions to prevent and heal foot ulcers and reduce plantar pressure in diabetes [25, 26]. Most evidence is available on the use of offloading in healing neuropathic plantar foot ulcers. The guidelines recommend the use of non-removable offloading, such as a total contact cast (TCC), as first-choice treatment option . If casting is not available, below-the-knee walkers are recommended. Preferably, these are made irremovable to force treatment adherence and improve outcome [19, 27, 28]. The evidence base for the use of half-shoes, forefoot offloading shoes and cast shoes is still small, but these modalities are recommended when below-the-knee devices are contra-indicated. The guideline is clear in that conventional or standard therapeutic footwear should not be used for ulcer treatment in diabetes, as there are many more effective modalities. Finally, to better define the role of surgical offloading compared with conservative treatment in ulcer healing, more studies are needed.
Current daily foot practice deviates significantly from these evidence-based recommendations. A nationwide survey in the USA showed that only 2% of the 901 centres surveyed use the TCC as primary offloading method to treat diabetic foot ulcers . In 46% of centres, the TCC is not used at all, and 58% do not consider the TCC the gold standard treatment. The most commonly applied treatment method is modification of the footwear, for which no evidence exists. A recent retrospective analysis of 264 diabetic patients with a foot ulcer confirmed these findings, showing that only 6% of patients were treated with a TCC . In a large prospective follow-up study conducted in 14 specialized foot centres throughout Europe (Eurodiale), only 18% of the approximately 600 studied patients with a plantar foot ulcer were treated with a TCC and 17% with cast shoes . Striking was the large range in the use of casting across centres and countries: from 0% to 60% of cases. Also in this study, most ulcers were treated with (temporary) footwear.
These data show a large gap between available evidence and current foot practice, which may be explained by existing barriers to follow guidelines. With physicians, this may include the lack of awareness or familiarity with the guideline, disagreement with the guideline, lack of outcome expectancy with using the guideline, inertia to previous practice and ease of use of the guideline . Patients may refuse to use certain modalities for practical or cosmetic reasons. Specific to the use of TCC, issues may be the lack of trained technicians to apply the cast, lack of reimbursement, possible secondary lesions, immobility and inconvenience . Nevertheless, regardless of these existing barriers, in the treatment of neuropathic plantar foot ulcers, poor offloading should be considered poor treatment and should therefore be discouraged for the benefit of the patient.
Clearly, this gap needs to be bridged. Recently, several suggestions have been made on how to achieve this . First, professional societies should adopt and implement the guidelines of the IWGDF, which are specific, evidence based, agreed upon by representatives from 80 different member states and easy to use. The IWGDF could help to increase awareness within these societies through these national representatives. Secondly, practitioners may have to change expectations on what entails adequate time to healing. It seems that targets for ulcer healing are not often set, even though the evidence shows that neuropathic ulcers should heal within 6–8 weeks. Using such targets may stimulate the use of effective offloading modalities. Thirdly, the financial responsibility for treating foot ulcers may have to be reconsidered. This could include reimbursement limits for non-healing neuropathic ulcers or a bonus when quick healing is achieved. Fourthly, requirements for measureable and effective offloading could be introduced. In Germany, such requirements have been introduced for footwear prescription in secondary prevention practice, under penalty of no reimbursement. These requirements may not be achievable in each centre. Therefore, specialized referral centres, with the equipment, trained personnel and knowledge in place could be established. Finally, parallel to improving ‘the effectiveness of what we preach’, we need to improve ‘the efficacy of what we practice’. Lack of evidence does not necessarily imply lack of effectiveness. Therefore, prescribers are urged to demonstrate the efficacy of their currently used non-evidence-based methods so that their use in clinical practice can be supported or discouraged based on the data provided. Eventually, the goal is a better use of evidence-based approaches in offloading treatment that will benefit the patient.
Complicated and non-plantar foot ulcers
The available evidence on the use of offloading for ulcer treatment is almost entirely related to the treatment of non-complicated plantar neuropathic foot ulcers [25, 26]. Evidence is scarce on complicated and non-plantar foot ulcers, even though, in contrast to earlier times, these ulcers currently represent the majority of diabetic foot ulcers treated in multidisciplinary settings [33, 34]. Dorsal foot ulcers can have a biomechanical aetiology, for example through ill-fitting footwear. Offloading these wounds may seem relatively easy to achieve by providing enough room inside the shoe or device, but evidence is lacking. The treatment of ischaemic and/or infected neuropathic ulcers is more difficult than with purely neuropathic ulcers, for which good offloading and debridement often suffice. One study showed that, whereas neuropathic ulcers and mildly infected/ischaemic ulcers can be treated effectively with casting (69–90% healing rates), treatment outcome for plantar ulcers that are infected and ischaemic is poor (only 36% healing rate) . Offloading is important in treating these wounds, but additional procedures such as antibiotic therapy or revascularization interventions are required to achieve proper healing. The relative contribution of these interventions to healing is unknown. Clearly, more evidence for the use of offloading treatment in healing complicated and non-plantar wounds needs to be established.
As mentioned above, the evidence base to support the use of offloading is largest for the treatment of foot ulcers. The evidence base for ulcer prevention is limited and inconsistent [25, 26]. In fact, for primary prevention, it is non-existent. High ulcer recurrence rates are still found in diabetes  and show that we are ineffective in keeping ulcers healed. Different reasons may explain why it is so difficult to prevent ulcer recurrence (Table 1). Several reasons concern the role of prescribed therapeutic footwear. Although the use of therapeutic footwear for ulcer prevention is well accepted in clinical practice, the available evidence is not yet convincing. This is likely related to the fact that evidence-based guidelines or standardized protocols for diabetic footwear prescription are non-existent. Some consensus protocols on this topic have been published, but their clinical effectiveness is unknown [37, 38]. The consequence is a wide diversity of methods, materials and designs used in footwear prescription which does not improve consistency in outcome. Next to this, information on footwear offloading efficacy is nearly always lacking, which increases the chance of sub-optimal footwear being used.
Table 1. Why is it so difficult to prevent foot ulcer recurrence in diabetes?
Patients do not sense they have a foot problem due to the existing peripheral neuropathy
The precipitating factors that caused the previous ulcer (i.e. neuropathy, deformity, high pressure and patient behaviour) are not removed
Visits to the specialized clinic or health care professional are less frequent than during ulcer treatment, sometimes even absent
Transition from an offloading device for healing to footwear for prevention inevitably increases plantar pressure
Patients wear footwear which is removable, so adherence to prescribed treatment may be a problem
Footwear design and evaluation is still mostly dependent on the clinical experience and skills of the diabetic foot team and a trial-and-error approach. A more quantitative approach, however, can lead to improved offloading which may reduce the risk for ulceration. For example, customized insoles which are designed and manufactured using barefoot pressure analysis, three-dimensional foot shape measurement and computer-assisted methods can reduce peak pressures with 30% when compared with traditionally designed and manufactured custom insoles . Another example is the use of in-shoe plantar pressure measurement in footwear evaluation. In-shoe pressure analysis can be used as a tool to guide modifications to the footwear with the goal to reduce pressure at high-risk regions. Using this approach, a 30% reduction in peak pressure compared with baseline can be achieved . These quantitative approaches represent innovative methods that are expected to become more integrated in diabetic footwear practice. The clinical efficacy of these methods is currently being tested and will provide more insight in the role of prescribed therapeutic footwear and offloading in the prevention of foot ulceration in diabetic patients.
As one of the reasons that explain why it is difficult to prevent ulcer recurrence, non-adherence to wearing prescribed footwear seems to be particularly a problem inside the house and with footwear that is perceived as unattractive by the patient [21, 40]. Studies in patients with other foot pathologies than diabetes show that the usability and acceptance of prescribed footwear are important components that determine adherence to use [41, 42]. Therefore, ways to improve adherence could include the prescription of multiple pairs of footwear for use inside and outside the house, the design of more attractive footwear without loss of functionality and better education and communication strategies. Motivational interviewing could be a promising technique in this regard .
Clearly, a much stronger focus on the prevention of foot ulcers in diabetes is needed. The role of footwear and above-mentioned innovative methods to improve footwear efficacy and adherence should be central in this focus. They should be evaluated relative to other preventative strategies such as podiatric care, adequate screening and follow-up of patients and early recognition of signs of foot disease [44, 45]. Such evaluations may lead to solutions with which more foot complication in diabetes can be prevented.
Although biomechanical factors play an important role in the development and management of foot ulceration in patients with diabetes, several aspects related to offloading need priority. Offloading should no longer be studied as a single entity in the management of foot complications but should be analysed in a broader perspective of contributing factors that determine clinical outcome, which include patient behavioural factors. Clearly, the existing gap between evidence-based recommendations and clinical practice in the use of offloading for ulcer treatment needs to be bridged, and several ways to do so have been presented. Our knowledge about the use of offloading in treating complicated and non-plantar foot ulcers is limited and needs to be expanded because these ulcers are dominant in current multidisciplinary foot practice. Finally, a larger focus should be put on the prevention of foot ulcers, in particular on assessing the efficacy of therapeutic footwear and its relative role in comparison with other preventative strategies. The focus on these issues is needed to further improve our understanding of the role of offloading and to improve clinical outcome in the management of the diabetic foot.