Methodology for use of a neuroprosthetic to reduce plantar pressure: applications in patients with diabetic foot disease.

Foot-related complications remain among the most common causes of hospitalization, ulceration, amputation, and reduced productivity in people with diabetes.1–3 The consistent entry point in this process involves neuropathy, which, with repetitive normal and shear stress over a prominent area, may lead to inflammation, ulceration, infection, and ultimately, amputation. Due to glycosylation of soft tissues and reduced activity, people with diabetes are at higher risk for developing foot-related deformities, which amplify this cycle.4–6 This is particularly true in the Achilles tendon,4–8 which leads to equinus deformity, increasing forefoot pressure. In addition to nonsurgical options, such as shoes, insoles, and braces, clinicians have employed prophylactic and curative procedures to prevent or heal wounds, respectively.9,10 Clinical researchers have suggested lengthening of the Achilles tendon or gastrocnemius complex as a potentially effective means of reducing pressure, improving healing time, and potentially reducing risk for ulceration or reulceration.11–17 Extremity-focused neuroprosthetic devices have increased in popularity in the treatment of various neurological conditions, most notably in treatment of foot drop following cerebrovascular events. The purpose of this brief report was to evaluate the ability of a neuroprosthetic or functional electrical stimulator18–20 (WalkAide®, Innovative Neurotronics, Austin, TX) to modulate plantar pressure in the foot. The functional electrical stimulation (FES)-based intervention include a single channel electrical stimulator that causes dorsiflexion through stimulating peroneal nerve. The stimulator is linked to a computer via Bluetooth® and also includes a heel tilt sensor for triggering. The authors designed a repeated-measures human performance laboratory study to evaluate four different device settings through in-shoe pressure measurements using the F-Scan® (Tekscan, Boston, MA) in-shoe plantar pressure measurement system. The FES device was connected to the right foot for the study subject. The study was conducted on a single healthy volunteer, and five gait cycles per setting were used for data collection. The study subject walked, shod condition, at normal gait speed with set stimulation parameters per the WalkAide device setting. Plantar foot contact area and the vertical ground reaction force were collected at 100 Hz. Three outcomes were extracted using this toolbox, including (1) regression factor index (RFI), (2) magnitude of second peak plantar pressure, and (3) relative location of second peak pressure as a percentage of the stance phase. For extracting the outcomes, we calculated the average of at least four steps taken from the middle of the walking trials, during gait steady state, when each person's gait was closest to a steady gait for each foot. To summarize the algorithm, maximum pressure for each time sample was extracted, thereby creating a ‘peak pressure’ pattern for each step. Then, the time of toe-off and heel strike was detected by a peak detection algorithm, which estimated the gradient of peak pressure pattern. This allows separating each cycle as well as estimating the stance duration. The pattern of peak pressure was then time-normalized by setting heel contact to 0%, toe-off to 100%, and linearly interpolating to a fixed number of samples. This process was repeated for a certain number of steps (in this study, minimum four steps), and then the mean trajectory was computed for use in subsequent analysis. The magnitude of location of the second peak pressure as a percentage of stance time was estimated using the mean trajectory and a peak detection algorithm. This approach allows moderating the effect of variable stance duration in multiple gait cycles across pressure profiles of different steps. Estimation of RFI was explained in detail in our previous publication.21 Using the approach explained earlier, the dynamic of plantar loading was quantified while using four different device settings: standard drop foot, extended drop foot, maximum pulse width intensity, and soft pulse. The objective was to explore the best setting, which would yield the best plantar loading pattern. We assumed that an idea pattern represents an RFI closer to +1, a lower peak pressure magnitude, a better symmetry between right and left stance duration, and an earlier occurrence of second plantar pressure. Table 1 summarizes the repeated-measures test22 for ‘baseline’ condition (standard drop foot setting) for repeatability of plantar pressure and gait parameters assessment. Table 1 Summary of the Repeated-Measures Tests for Plantar Pressure and Gait Parameters The standard drop foot setting was found to achieve the best results for pressure reduction based on the three outcomes studied, as described above. The authors noted a peak pressure reduction of 34% at the great toe, 11% at the first metatarsal head, and 19% for lesser toes. Second peak of plantar pressure had a 28% reduction from baseline for this setting. The location of the second peak was 86% versus 83% of stance time for baseline versus intervention. The second peak amplitude decreases 28% for the standard drop foot setting, versus 60% for the extended drop foot setting. This is equivalent to a dose response in a drug study. Higher reduction in the affected foot is available at the cost of reduction in the RFI factor and longer stance duration. Mobility is an important daily requirement, so much so that any disruption to it severely degrades our perceived quality of life. Evidence suggests that surgical interventions such as Achilles tendon lengthening achieve 27–32% pressure reduction at the forefoot region.11,23 This effect is temporary and may lead to high baseline pressures within 9 months of treatment. Commercially available FES systems, such as the one presented in this study (WalkAide), make it possible for an increasingly growing number of clinical researchers to evaluate the mobility training paradigm for diabetic lower extremity disorders. We have provided efficacy data for the FES device (WalkAide) for nonsurgical pressure reduction equivalent to surgical interventions. This is achieved by using electrical stimulation to fire upon foot strike to reduce pressure in the forefoot by maximizing available dorsiflexion. This report presents results from a single case study on a healthy subject and provides a methodology for data analysis using objective outcomes. Further research is required to understand the benefits of this technology and proposed methodology in patients with diabetes and lower extremity complications.