Novel In-Shoe Exoskeleton for Offloading of Forefoot Pressure for Individuals With Diabetic Foot Pathology

Abstract

Introduction: Infected diabetic foot ulcers are the leading cause of lower limb amputation. This study evaluated the ability of in-shoe exoskeletons to redirect forces outside of body and through an exoskeleton as an effective means of offloading plantar pressure, the major contributing factor of ulceration.

Methods: We compared pressure in the forefoot and hind-foot of participants (n = 5) shod with novel exoskeleton footwear. Plantar pressure readings were taken during a 6-m walk at participant's self-selected speed, and five strides were averaged. Results were taken with Achilles exotendon springs disengaged as a baseline, followed by measurements taken with the springs engaged.

Results: When springs were engaged, all participants demonstrated a decrease in forefoot pressure, averaging a 22% reduction ( P < .050). Patient feedback was universally positive, preferring the exotendon springs to be engaged and active.

Conclusions: Offloading is standard of care for reducing harmful plantar pressure, which may lead to foot ulcers. However, current offloading modalities are limited and have issues. This proof-of-concept study proposed a novel offloading approach based on an exoskeleton solution. Results suggest that when the novel exoskeletons were deployed in footwear and exotendon springs engaged, force was successfully transferred from the lower leg through the exoskeleton-enabled shoe to ground, reducing load on the forefoot. The results need to be confirmed in a larger sample. Another study is warranted to examine the effectiveness of this offloading to prevent diabetic foot ulcer, while minimizing gait alteration in daily physical activities.

Keywords: diabetic foot ulcer; exoskeleton; offloading; passive wearable robot; wound healing.

Figure 1.

An ambulatory total contact cast. Photo by David G. Armstrong.

Figure 2.

(left) External concept sketch showing how an exoskeleton offloading device can be concealed without negative aesthetics within a high-topped shoe. In this concept, the molded rubber exotendon spring is shown in dark gray to demonstrate suitability for low-cost manufacturing approaches. (right) Same model shoe revealing how the exoskeleton is hidden within the footwear’s construction layers.

Figure 3.

The ulceration/amputation lifecycle showing progression of the disease, its reoccurrence, and the subsequent path to possible amputation.

Figure 4.

Illustration of how proposed interventions can interrupt the ulceration/amputation lifecycle to maximize ulcer-free days and extend DFU remission.

Figure 5.

(left) Shoe with no offloading, with 200N of force mediated by the forefoot. (right) How 75N of force harvested from shin in dorsi-flexion can establish a reciprocal upward 75N pull on an exotendon spring parallel to the Achilles tendon anchored at the heel. This upward force leveraged across a fulcrum proximal to the ankle delivers a 50N downward force to ground through the exoskeleton under the forefoot, offloading a portion of the forefoot’s 200N force to leave a remainder of 150 N (a reduction of 25%).

Figure 6.

Two photos of prototype exoskeleton footwear being worn bilaterally, right trouser leg elevated to reveal the yoke and spring (pink elastic cord coils), and Boa reel. Note that the exoskeleton is present and active on both limbs, but the design makes it aesthetically hidden under the trousers and does not significantly alter the appearance of the footwear, which is another benefit.

Figure 7.

Five participants numbered along the horizontal axis with the average percentage change in plantar pressure along the vertical axis; during a 6-m walk (at least 5 strides averaged): blue bars show change in forefoot pressure (downward trajectory shows offloading), while red bars show change in hind-foot pressure.

Figure 8.

Comparison of elapsed 6-m walk times (time in seconds on the vertical axis). Subjects 2 through 5 with a history of diabetes. Four out of five participants walked faster with springs engaged.

Figure 9.

Sample of forefoot pressure (measured in kilopascals along vertical axis) across the stance phase of the gait cycle (measured in seconds along horizontal axis) No spring shown in light green line (upper), full spring in dark green line (lower). Note: peak offloading at heel-lift and a reduction in elapsed time of the stance phase of the gait cycle. (Subject 4, six stances averaged.)