Is it the foot— or the footwear? Considerations for protecting a diabetic foot
In diabetes, assessing the mechanical properties of the patient’s soft tissues and the properties of footwear and orthoses comes before drafting a management plan.
By Nachiappan Chockalingam, PhD
Prescription footwear and foot orthotics are standard interventions for any type of diabetic foot disease. We lack knowledge, however, on how to select appropriate materials for footwear and orthotics for our patients who have diabetes; generally, cushioning materials with which these devices are constructed are selected based on the clinician’s experience and on anecdotal evidence. In this article, I look at promising advances being made by researchers in developing quantitative evidence to inform the management of diabetic foot disease.
Limited tools to address major sequelae of diabetes
The complications of diabetes include a range of foot pathologies:
ulcers, as a result of injury and trauma
musculoskeletal problems, leading to shape deformities
neuropathic osteoarthropathy, a degenerative disorder of the joints within the foot.
Typically, people with diabetes who have neuropathy are susceptible to injury and mechanical trauma; because of the loss of sensation, they might not notice resulting damage to soft tissues for a long time.
Many people who have diabetic neuropathy also have peripheral artery disease; compromised circulation in the lower extremity impairs their ability to fight infection and puts them at higher risk of foot ulcers. Previous reports suggest that the lifetime incidence of foot ulcer in diabetes can be as high as 25%.1 Ulcers tend to lead to infection and, in some cases, to amputation.
Although several clinical management and prevention modalities—eg, regulation of the blood glucose level, a structured screening process, patient education, and footwear and/or orthotics—have been described, there is a paucity of information on the effectiveness of such measures.
We don’t know a lot about how to select appro- priate materials for footwear and orthotics to protect the feet of our patients with diabetes. New research might give us the tools to make good decisions and achieve better outcomes.
Understanding tissue mechanics through laboratory modeling
Given the multifactorial nature of diabetic foot ulcer, there is a clear need to understand the tissue mechanics of this complication, with an eye to developing effective clinical management tools using footwear or orthotics. An earlier study,2 while outlining the importance and the mechanical behavior of the heel pad within the foot during walking, attempted to quantify the stress–strain relationship in this structure—a relationship that is paramount to understanding mechanical characteristics of the tissues in question.
A later study3 then utilized an ultrasound-based indentation device to estimate the energies that were input into, and returned from, the heel pad. A nonlinear visco-elastic model used within this study helped to quantify the elastic and viscous model components; this quantification correlated significantly with maximum strain. This kind of information is essential to develop computational models, which will lead to a better understanding of structures within the foot.
Finite element modeling is a computational tool used in engineering and the physical sciences to understand and predict structural behavior. This tool has been used to understand the bio- mechanics of the foot, its internal structures, and supportive devices such as footwear and orthotics, but the true contribution of finite element modeling to our understanding specific clinical conditions and the development of clinical management schemes remains questionable.
A principal reason for this gap in knowledge—between the laboratory and the clinic—is that the finite element method is used mainly within the research environment, not at the point of delivery of clinical assessment and management. There is a clear need for clinically applicable finite-element models that will not only help researchers develop novel diagnostic techniques but also pave the way for novel methods of treatment planning and optimization.
An overview of the modeling techniques employed within the field of foot biomechanics was provided by Behforootan and colleagues,4who lament the lack of an integrated modeling system that could be used directly in a clinical scenario. Their report outlines the key challenges related to acquisition of 1) accurate data to create the necessary geometry and 2) information on patient-specific material properties with which to run models.
To address some of these needs, Chatzistergos and co-workers5 developed a clinically viable, noninvasive method of assessing the mechanical properties of the heel pad. Their study, in addition to validating the method they proposed, investigated the effect of nonlinear mechanical behavior of the heel pad on its ability to distribute loads uniformly during foot contact.
The results of Chatzistergos’s work highlighted the fact that the function of the heel pad is influenced by 1) its overall deformability and 2) the nonlinear nature of its mechanical behavior. If deformability is constant, changes in heel-pad biomechanics—leading to mechanical behavior that is more nonlinear, so to speak, in nature—can compromise the ability of the heel pad to distribute plantar loading in a uniform manner, thus making it more prone to overloading and trauma. The technique developed within this study was validated against in vivo data, and proved to be able to assess the visco-hyperelastic behavior of the heel pad accurately.
Although specialist prescription footwear and orthoses are used widely by people with diabetes to redistribute pressure, there is a dearth of information on the optimum cushioning properties of this equipment. Another set of studies attempted to develop a numerical, subject-specific method that can be used to investigate the cushioning properties of different insole materials.6,7 The reported procedures involved generation of 2-dimensional, subject-specific finite element models of the heel pad, based on ultrasound indentation. These models are used to inverse-engineer the material properties of the heel pad and simulate contact between plantar soft tissue and a flat insole.
After this modeling procedure was validated, the researchers utilized it to investigate the importance of measuring plantar soft-tissue stiffness, thickness, and loading for proper selection of insole material. The results indicated that heel-pad stiffness and thickness influence plantar pressure but not optimum insole properties. On the other hand, loading appears to significantly influence optimum insole material properties. The results of this study highlight the fact that the parameters that affect the loading of plantar soft tissues, such as body mass and level of physical activity, should be carefully considered during selection of insole material.
Although body mass and physical activity are important, one needs to understand the correlation between mechanical properties of the heel pad of people with diabetes and the clinical parameters that are normally used to monitor their health and risk of ulceration.8,9 Another investigation that attempted to address this matter used the same ultrasound-based indentation device to measure the thickness and stiffness of patients’ heel pads. 5 The investigators correlated the results of this mechanical testing with clinical assessment measures, including the ankle–brachial index and the vibration perception threshold.
The study found that people who have diabetes and elevated levels of triglycerides and fasting blood glucose are more likely to have stiffer heel pads. Increased stiffness could limit the ability of tissues to evenly distribute loads, making them more vulnerable to trauma and ulceration. This study was the first to identify a link between the biochemical profile of people with diabetes and the mechanical properties of their plantar soft tissues; subsequent studies,2 by other groups, verified these results.
Naemi and colleagues,10 after establishing the mechanical parameters of the heel pad, investigated whether assessment of the mechanical properties of plantar soft tissue can increase the accuracy of predicting diabetic foot ulceration. The study employed ultrasound elastography to investigate plantar soft-tissue stiffness and thickness along with other clinical parameters in 40 patients with diabetic neuropathy but without foot ulcers. At 1-year follow-up, 7 patients had developed foot ulceration.
Using a predictive model, the results indicated that patients with higher plantar soft-tissue thickness and lower stiffness in the area of the first metatarsal bone had an increased risk of ulceration. When investigators added the plantar soft-tissue stiffness and thickness to the model, results improved in terms of specificity (by 3%), sensitivity (by 14%), predictive value (5%) and strength of prognosis (1%).
The study makes it clear that the mechanical properties of plantar soft tissue can be used to improve the predictability of ulceration in patients with a moderate-to-high risk of ulceration. Understanding tissue behavior undoubtedly helps clinical management and development of new prognostic and diagnostic tools and interventions. But one needs to be pragmatic: How, then, can we apply this knowledge to improve available interventions and, thus, avoid foot ulceration.
Optimizing cushioning materials, patient by patient
A recent study6 investigated the biomechanical properties of a set of cushioning materials to establish the basis for patient-specific material optimization. This is an important departure from current empirical evidence—a new direction that will substantially help the existing prescription process. The study used bespoke cushioning materials, which exhibited similar mechanical behavior but were of varying stiffness. In addition to performing mechanical testing, researchers asked participants to walk on these materials to test their ability to reduce pressure.
Overall, results showed a reduction in plantar pressure by optimization of the stiffness of the cushioning materials. In addition, researchers correlated optimum stiffness and body mass for individual subjects. This is the first study to provide quantitative data to support the importance of stiffness optimization in cushioning materials and sets the stage for development of methods to inform optimum material selection in the clinic.
Progress toward better outcomes
The studies reviewed in this article are a step towards developing quantitative evidence to inform the clinical management of diabetic foot disease—from the development of screening guidelines to evidence-based prognostic and diagnostic tools. In addition, the reports discussed here that looked at bespoke materials and their biomechanical properties help pave the way for patient-specific interventions in diabetic foot care.
For example, one might need different materials for a given patient’s right foot and left foot, depending on his (her) biomechanics and level and type of physical activity; findings of current studies are that materials for walking might need to be stiffer than materials for standing. Development of quantitative evidence will give clinicians a clear idea of a given patient’s needs, rather than depending on experience and empirical evidence provided by peers.
To sum up: Clinicians and patients should think about both the mechanical properties of soft tissues and the properties of footwear/orthoses before planning or choosing any clinical management plan.
Nachiappan Chockalingam, MSc, PhD, CEng, CSci, PFHEA, is Professor of Clinical Biomechanics at Staffordshire University, Stoke-on-Trent, Staffordshire, UK.