Temperature and Pressure Regulating Insoles for Prevention of Diabetic Foot Ulcers

Abstract

Diabetic foot ulcers (DFU) pose a major threat to the US healthcare system as well as patients and their families. The high ulcer recurrence rates indicate that existing preventive measures are not effective. A new generation of multimodal preventive devices may reduce ulceration and amputation rates. Since previous research has revealed that tissue maintained at cooler temperatures is more resistant to breaking down, the evaluated technology may prevent foot ulceration. The purpose of the study reported herein was to test previously designed Temperature and Pressure Monitoring and Regulating Insoles (TAPMARI) in diabetic neuropathic and healthy subjects. A cooling unit, a mini water pump, a battery pack and a microcontroller (or simply thermostat) were placed inside a box, attached to subjects’ calf, which provided cooling inside the shoe. The microcontroller was set at 28°C. A total of 8 subjects provided informed consent; 3 of whom had diabetic neuropathy. Subjects used the instrumented shoe on the right foot and the matching control shoe on the left and walked on a treadmill for 5 minutes at self-selected speeds. Baseline and post-walking thermographs were obtained with a thermal camera. At the 2-hour midpoint subjects again walked on the treadmill for 5 minutes at self-selected speeds. Second baseline and post-walking thermographs were captured. Plantar pressure distributions were also quantified. The TAPMARI successfully regulated foot temperatures at or below the target temperature. The mean baseline temperature of the right (regulated) and left (control) feet were 28.1±1.9 °C (mean ± standard deviation), for all subjects. The mean temperatures at the end of the study were 25.9±2.5°C (right) and 31.7±1.6°C (left) in all subjects. In the diabetic neuropathy group, the final mean temperatures were 27.5±2.4°C (right) and 31.6±0.8°C (left), which indicated that the temperature goal was met inside the instrumented shoe. By regulating temperatures TAPMARI may reduce the metabolic demands in the foot and prevent cell autolysis by eliminating the imbalance between oxygen demand and supply. This study warrants further development and testing of TAPMARI as well as investigating the clinical effectiveness in preventing DFU.

1. INTRODUCTION

According to the 2017 American National Diabetes Statistics Report, 108,000 lower extremity amputations had to be performed on patients with diabetes (1). Foot ulcers that occur because of diabetic neuropathy (DN) often lead to infection and eventually amputation. The reason behind these dramatic amputation rates may be tied to lack of effective preventive measures. A well-known medical proverb indicates that the best treatment is prevention. Effective preventive measures can only be developed after a comprehensive understanding of a clinical problem. To date, the exact pathology of diabetic foot ulceration remains to be solved. While many previous investigators held high foot pressures responsible for DFU, it would only be an oversimplification to focus only on plantar pressures. The National Pressure Ulcer Advisory Panel has recognized other contributing factors such as interface shear stress and microclimate as key causes in pressure injuries as early as in 2009 (2). A number of animal studies revealed that tissue maintained at lower temperatures is more resistant to breaking down under static loading (3, 4). These studies explored the occurrence of pressure ulceration under static mechanical loading and at various tissue temperatures. Their results indicated that skin maintained at the 25–30°C band did not ulcerate under test conditions, whereas skin breakdown occurred starting at 35°C. Another finding was that the higher the tissue temperature the more severe the skin breakdown (3, 4). Previous reports by Yavuz et al (5–8) indicate that plantar temperatures in both healthy individual and diabetic patients can easily rise to these risk levels after a short load-bearing exercise, particularly due to plantar shear.

While most of the existing reports on the significance of temperature as a causative factor for DFU come from literature focused on pressure injury (ulcer), primarily from animal studies, we believe high skin/tissue temperatures play a major factor in DFU as well. Therefore, we have developed a novel shoe insole that can regulate plantar temperatures to a preset range, while passively mitigating pressure concentrations. We call this system the Temperature and Pressure Monitoring and Regulating Insoles or simply TAPMARI. The objective of this study was to evaluate the feasibility of the Temperature and Pressure Monitoring and Regulating Insole to maintain skin temperatures within a set range, which was previously reported to preserve tissue in animal studies.

2. METHODS

2.1. Subjects

Eight individuals (one male, seven female; median age of 45 (27 to 80) years), five healthy and three with diabetic neuropathy (DN) were recruited. Subjects’ demographics are shown in Table 1. Peripheral neuropathy in diabetic subjects was determined using a Biothesiometer (Bio-Medical Instrument Co, OH) and based on the 25V diagnostic criterion. Ankle-Brachial Index of each diabetic patient was quantified with the help of a portable Doppler Ultrasound unit (Summit Doppler LifeDop 250 ABI System, CT).

Table 1.

Demographics of subjects enrolled in the study. Values are median (range), where applicable.

	Healthy Control Subjects	Subjects with Diabetic Neuropathy	Overall
N of subjects	5	3	8
Gender	4 Female - 1 Male	3 Female	7 Female - 1 Male
Age (range)	28.7 (27–34)	71.2 (56–80)	33.2 (27–80)
BMI (range)	22.6 (21.9–27.8)	26.8 (23.4–31.4)	24.0 (21.9–31.4)
Ankle-Brachial Index – ABI	N/A	0.92 (0.68–1.18)*	N/A
Duration of diabetes in years	N/A	27 (24–45)	N/A
Type I / Type II diabetes	N/A	1/2	3
Vibration perception in Volts	N/A	34.7 (12–51)	N/A

^*Data available in two subjects

2.2. Insole design

A multilayer insole was instrumented with silicone piping that was placed in midsole. The upper layer was manufactured from thermally conductive silicone. A cooling element, a mini water pump, a rechargeable battery and a microcontroller (i.e., a thermostat) was placed inside an enclosure which was attached to subjects’ calf. The pump circulated water through the cooling element, which chilled the water and then through the silicone piping. The microcontroller was set at 28°C (±1°C), which has been shown to preserve tissue against breaking down (3, 4).

2.3. Experimental setup

Subjects wore the instrumented shoe on the right foot and the matching control shoe on the left for 4 hours or until the battery powering the cooling element was depleted. After being fitted with the shoes, subjects walked on a treadmill for 5 minutes at self-selected speeds. A baseline and post-walking thermograph was obtained with an infrared thermal camera (650sc, FLIR, OR). Subjects were then allowed to leave the lab but were instructed to return back to the lab after two hours. At the two-hour midpoint subjects again walked on the treadmill for 5 minutes at self-selected speeds. Second baseline and post-walking thermographs were captured. The FLIR Tools+ software was used to process thermal data. Plantar pressures were also quantified using the Pedar in-shoe pressure system (Novel USA, MN) during treadmill walking. The study was approved by the North Texas Regional IRB and each subject signed an informed consent form.

3. RESULTS

Results indicated that the Temperature and Pressure Monitoring and Regulating Insole was successful in regulating plantar temperatures at or below the target temperature. The mean baseline temperature of the right (regulated) and left (control) feet were 28.1±1.9°C, for all subjects (100%). The mean skin temperatures at the end of the study were 25.9 ±2.5°C (right) and 31.7±1.6°C (left) in all subjects (100%). In DN group, the final mean temperatures were 27.5±2.5°C (right) and 31.6±0.8°C (left), which indicated that the temperature goal was met inside the instrumented shoe. Data also indicated that skin temperatures in the control foot increased by approximately 3.7°C after two bouts of treadmill walking. On the other hand, in the diabetic neuropathy group, we observed plantar temperatures as high as 30.8°C in certain regions of the right foot, particularly the midfoot. This indicates that the design needs to be improved in the midfoot area. The mean peak pressure captured in Temperature and Pressure Monitoring and Regulating Insole feet was 211±8 kPa (mean ± standard deviation) in the diabetic neuropathy group, whereas this value was 247±47 kPa in the control foot. This indicates a relatively good pressure distribution by the Temperature and Pressure Monitoring and Regulating Insoles as 200kPa and lower values have been recommended for diabetic footwear (9). A representative pressure profile of a subject with Diabetic Neuropathy is shown in Figure 1.

Figure 1.

(a) TAPMARI in-use. Representative image of a subject with Diabetic Neuropathy (b) Peak Pressure Distribution. Thermographs of Baseline (c) and time at 4 hours (d). R: Regulated shoe, C: Control shoe. Scale bar for (b) 0 to 500kPa, (c) and (d) 22 to 40°C.

4. DISCUSSION

The incidence of re-ulceration after an ulcer heals is 50–80% if no prevention services are provided (10). When therapeutic shoes and insoles, education and close monitoring are provided 30–40% of high risk patients will still develop an ulcer within a year (10). Many of these ulcers lead to infection, amputation, and reduction in patients’ quality of life and burdening the US Healthcare System. Therefore, we believe that a multimodal approach in a novel shoe and insole that addresses both high pressures and temperatures can minimize the high rate of ulcer recurrence observed in patients with diabetes.

We have previously reported two types of increases in skin temperature in diabetic foot (6). Patients with diabetic neuropathy have warmer feet compared to healthy individuals (7). We also have demonstrated that weight-bearing activity results in a further increase in temperatures (5). Unfortunately, patients with diabetes are usually prescribed rugged shoes for protection, which are primarily made of synthetic and thick materials which serve as heat insulators, similar to insulated mugs that keep hot beverages warm for a long time. We have observed that temperature of the feet may reach 37°C after load-bearing activity such as walking (11). Previous animal studies indicated that tissue at ≥35°C has a much higher risk for breaking down. The most likely reason is the associated increase in metabolic rate: the warmer the tissue the higher the metabolic rate. In patients with restricted blood circulation, a rise in metabolic rate might be problematic since the demand for oxygen will increase with elevated temperatures. The imbalance between oxygen demand and supply, if substantial, may lead to cell autolysis. TAPMARI has the potential to regulate metabolic rates, via regulating foot temperatures and mitigate this negative outcome.

It can be questioned that the cooling action of the TAPMARI may lead to vasoconstriction and reduce blood perfusion and oxygen saturation in the tissue. Similarly, it may be suggested that warming the diabetic foot may be more beneficial in order to create a vasodilation effect and increase blood perfusion. While this is true for tissue that is not under mechanical stress, in mechanically loaded tissue (such as during standing or walking), a local increase in temperature has a minimal vasodilation effect, since mechanical stress usually occludes the vessels (12). This makes it very difficult for vasodilatation to occur, which has been confirmed in at least one previous study (13).

Our results revealed that in diabetic patients TAPMARI provided an approximately 4°C temperature relief between the contralateral feet (27.5°C vs 31.6°C). Given that a 1°C increase in tissue temperatures lead to approximately 10% increase in the metabolic rate (14), it can be assumed that TAPMARI prevented a 40% increase in metabolic rate in the right foot, and hence regulated the demand for oxygen in the tissue.

5. CONCLUSION

Temperature regulation of the diabetic foot combined with pressure reduction on the sole of the foot may be quite beneficial in preventing diabetic foot ulcers. We believe that, TAPMARI has the potential to provide better preventive care for diabetic patients, after rigorous clinical testing.