As Metin Yavuz, PhD, notes, well-known scholars such as Cavanagh, Armstrong, Bakker and Bus classified the evidence to support the use of footwear for the ulceration as “meager.”1 Dr. Yavuz suggests that therapeutic footwear utilized in earlier studies may not have been tested for performance prior to researchers conducting the studies. He adds that all the footwear and orthotics in such studies focused on reducing plantar pressure and did not account for other factors such as shear forces or the activity level of patients. He says that may be why the footwear failed to prevent ulcer recurrence.
Ahmet Erdemir, PhD, also cites the importance of examining other factors such as the long-term performance of footwear, patient adherence and activity levels.
Brian Davis, PhD, concurs that one should not overlook shearing forces when noting the moderate success of therapeutic footwear. While it is likely frictional forces are a key variable, he acknowledges several other reasons for skin breakdown. These factors include alterations in collagen properties, autonomic nervous dysfunction that alters sweating responses, diminished microcirculation and superimposed aging.
Dr. Erdemir calls plantar pressure a “convenient candidate” that can identify the risk of ulceration and aid in testing the performance of footwear interventions since one can easily measure this variable for barefoot conditions and also via in-shoe testing. While he notes there is some association with peak plantar pressures and plantar ulceration, Dr. Erdemir adds the possibility of contact shear, pressure gradient and internal stresses as mechanical variables being responsible for foot ulceration.2-4 As he explains, it is not clear that interventions aimed at plantar pressure reduction also reduce these mechanical variables.
Dr. Yavuz notes that Lavery and colleagues called foot pressure a “poor tool” in predicting ulcers.5 As he explains, foot ulcers do not necessarily occur at peak pressure sites but may occur at sites that experience normal magnitudes of pressure.6,7 On the other hand, Dr. Yavuz has co-authored studies that suggest shear forces also play a major role in ulceration.2,8
Since the etiology of diabetic foot ulcers is multifactorial, Dr. Yavuz says focusing on only one or two factors will oversimplify the problem.
“I personally think it is time that we approach the problem from a broader perspective and try to understand the exact mechanism that leads to tissue breakdown. Once we know the exact pathology, we can then design optimal and more effective orthotics,” emphasizes Dr. Yavuz.
While it is possible to design orthotics to reduce plantar shear, there are some constraints, according to Dr. Erdemir. When it comes to tackling plantar pressures, Dr. Erdemir says the vertical force must be distributed evenly under the foot in order to support the foot during gait. When it comes to footwear, he tries to distribute pressure evenly so peak pressures will be close to mean pressure.
For plantar shear, Dr. Erdemir notes a horizontal force under the foot is necessary for adequate propulsion. He says physicians must also ensure even distribution of horizontal force to prevent high peak shear stress. Insole solutions to distribute plantar pressures may not necessarily have the same effect on contact shear. In addition, without in-shoe measurement capabilities to evaluate in-shoe shear, the performance of footwear on reduction of contact shear remains unknown, according to Dr. Erdemir.
Furthermore, Dr. Erdemir notes that dynamic stability during walking without adequate shear forces may be an issue. He suggests that computational modeling studies are likely to be helpful to identify potential orthotic designs.
Although insoles that target pressure work in a straightforward way, Dr. Davis says in the case of shear, it may be the relationships between neighboring regions that are important. As he explains, if an insole is designed to reduce skin shear in region A, then increased shear is likely in region B. Depending on the direction of shear, Dr. Davis notes this could either result in skin “bunching” and/or skin “stretching” in other regions. If one designs an insole to reduce friction over the entire plantar surface, he says slippage will occur within the shoe and the patient may experience skin damage on the dorsal surface of the toes.
Dr. Yavuz notes a significant dilemma. While it may be possible to reduce shear forces under the foot with an orthotic design, he says this would come at the expense of increasing the repetition count of mechanical factors. He notes that shear forces, particularly propulsive shear forces, are associated with speed and step length during normal gait. If there is a constant cadence, Dr. Yavuz says the higher the propulsive shear forces under the foot, the longer steps the patient takes and the faster the patient walks. Therefore, if patients wear orthotics that reduce shear forces under the foot, Dr. Yavuz says they will need to take a greater number of steps to traverse a certain distance.
Dr. Yavuz cites Brand’s research that the repetition count of mechanical forces is as important as their magnitudes in tissue breakdown.9 As he explains, if a patient takes more steps every day to go to work when wearing such orthotics, his or her foot may still ulcerate even if the shear forces under the foot are lower.
However, Dr. Yavuz cautions against drawing sharp conclusions in this matter without knowing at what force (magnitude x repetition) combinations the skin will break down. It is crucial to obtain such data, at least from animal or cadaver models, so one can design diabetic orthotics optimally, according to Dr. Yavuz.
Dr. Yavuz notes he and other researchers are working on such smart orthotics, which will record variables like pressure, shear, temperature, humidity and number of repetitions within the shoe, and alert patients when they reach thresholds for those variables. He says Michael Mueller, PhD, PT, was working on a funded research project regarding the development of such a device. Although
Dr. Yavuz does not know the end result of this particular project, he feels the mission is not too far away from being accomplished.
Dr. Erdemir notes researchers must overcome two hurdles to make smart orthotics. The first is acquiring the technology to measure the aforementioned variables. While daily logging of plantar pressures is not necessarily an issue, Drs. Erdemir and Davis say in-shoe measurement of shear, temperature and humidity will likely require considerable engineering research in the near future.
“The inside of a person’s shoe is a harsh environment and getting electronic circuitry to withstand the daily demands imposed by human locomotion is a very challenging problem,” notes Dr. Davis.
Dr. Yavuz feels it is relatively easier to measure pressure, temperature, humidity and activity in shoe conditions. He feels the challenge is in quantifying shear forces under the foot. Dr. Yavuz is currently collaborating with a
medical device design company to achieve this goal of measuring plantar shear force distribution in shod. However, Drs. Yavuz and Erdemir concur that risk thresholds for these parameters remain elusive.
“Do we really know what magnitudes and numbers of repetition of pressure, shear, temperature and humidity put someone at risk?” questions Dr. Erdemir. “We are in need of significant amount of biomechanical and clinical studies to establish this information.”
Dr. Davis is skeptical about the potential cost of smart orthoses.
“While this is, in principle, an attractive concept, I think the reality is that the cost of a system like this would make it unappealing to both patients and healthcare providers,” notes Dr. Davis.
Dr. Davis says the key to insurance coverage is validating the cost-effectiveness of new solutions such as smart orthoses. He says double-blind studies are necessary to compare orthotics with smart orthotics, and longitudinal studies are necessary for matched patient cohorts with the endpoint of a documented reduction in ulcer incidence. Dr. Davis notes such studies are relatively expensive as they require well over 100 patients per group, with each group usually needing to be tracked for 12 to 24 months.
“With this said, there is probably no easier approach to reduce ulcers than one that focuses on the use of appropriate footwear,” offers Dr. Davis.
If the profession confirms that smart orthotics are more effective than present orthotic devices, Dr. Yavuz advocates that insurance companies should cover them. Given the high cost of the diabetic foot and amputations, he says if smart orthotics have reasonable purchase and maintenance costs, they can present a “win-win situation for all parties,” including patients, healthcare providers and insurance companies.
Dr. Erdemir is uncertain that insurance companies will cover smart orthotic devices. However, he feels establishing the efficacy of orthotics to prevent foot ulceration and an analysis of cost savings associated with prevention via footwear prescription are necessary steps that can encourage policy makers and insurance companies to adopt such practices.
Dr. Davis is the Vice President and Director of the Medical Device Development Center at the Austen BioInnovation Institute in Akron, Ohio.
Dr. Erdemir is the Director of the Computational Biomodeling Core and a Staff Scientist in the Department of Biomedical Engineering/ND-20 at Lerner Research Institute in Cleveland, Ohio.
Dr. Yavuz is an Assistant Professor in the Department of Basic Sciences at the Ohio College of Podiatric Medicine.
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2. Yavuz M, Erdemir A, Botek G, et al. Peak pressure and shear locations under the foot: relevance to diabetic patients. Diabetes Care 2007; 30(10):2643-5.
3. Mueller MJ, Zou D, Lott DJ. Pressure gradient as an indicator of plantar skin injury. Diabetes Care 2005 Dec;28(12):2908-12.
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