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.

Introduction to the Prevalence and Cause of the Disease

Diabetic foot ulcer (DFU) develops as a result of neuropathy and its associated loss of protective sensation in the foot which, following mechanical stress, leads to blistering and or callus and subsequent tissue loss.¹ It is the leading proximate cause of lower limb amputation.² DFU is a major burden on individuals, their families and the health care system in developed and developing countries, and affects approximately 15-25% of those with diabetes.^2-5 In 2012 alone, the total costs related to diabetes were $245 billion, a 41% increase since 2007.⁵ Approximately one-third of all diabetes-related costs in the United States were spent on DFUs, with two-thirds of these costs incurred in inpatient settings.⁵

The most significant endpoint of DFU is lower extremity amputation (LEA), affecting more than 10% of cases. The vast majority of these amputations are considered preventable⁶ yet the lack of effective primary and secondary preventative interventions exacerbates the problem. This is particularly true for people that have healed a wound (diabetic foot remission), where recurrence of foot wounds is 40%, 60%, 80%, and 100% at 1, 3, 5, and 10 years, respectively.¹

Foot ulceration is multifaceted; the most significant factors are elevated plantar pressures and shear forces in a neuropathic insensate foot, especially at the forefoot.⁷ Obesity, aging, frailty, foot deformities and limited joint mobility are all associated with the patient population and create further abnormal gait patterns which tend to place even higher loads and shear forces on the forefoot and great toe.^8-13

Overview: Prevention of Initial Ulceration

The current standard of care for the preventing initial onset of foot ulcers includes “screening, risk classification, regular foot care, diabetic shoes and insoles, and diabetic foot education.”¹⁴ Researchers have demonstrated that these interventions can be effective in the prevention of DFU,⁶ but the increasing prevalence of DFU and amputation¹⁵ is a clear indicator that much more innovation is required to understand the human factors that influence disease prevention to make a difference in the overall population health.

Diabetic shoes, arch supports, and custom orthotic insoles can help better distribute pressure, from forefoot to midfoot while standing, thereby lowering peak pressure at the forefoot. Years of deployment and several research studies have found no significant long-term benefits when they were used,^16,17 emphasizing the need for innovation to prevent DFU, preserve independence, and alleviate this global health care burden.

Overview: Supporting Diabetic Foot Remission

Once patients have experienced an initial foot ulcer, there are two types of offloading devices employed. Total contact casts (TCCs) and removable cast walkers (RCWs) are often deployed for several weeks following interventions such as wound debridement and closure as well as minor amputations; diabetic shoes are then deployed for long-term support of diabetic foot remission (preventing recurrence). Use of TCCs has been repeatedly demonstrated to improve wound healing;¹⁸ however, they are used in fewer than 25% of surgeries, due to difficulty in proper application and lack of reimbursement.¹⁹ Figure 1 shows a TCC deployed on a patient. The attributes of the TCC provide excellent isolation but can also introduce issues, including:

Figure 1.

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

1. the unilateral configuration creates an asymmetry in heal elevation that can impact the health of the knees, hip and back

2.  the total isolation of the foot required for successful initial healing can lead to muscle wasting over time

These factors have the paradox that they foster healing in the initial weeks after surgery, but can lead to significant gait issues when deployed too long. This is problematic because this population is predisposed to gait issues, and further exacerbation can remain with the patient for many months, increasing forefoot pressure and thereby accelerate recurrence of the ulcer.^12,13

Innovation

The innovation being pursued is a novel exoskeleton capable of harvesting pretibial forces and transmitting them external to the body across the ankle joint to offload forefoot pressure. The innovative design mimics the biomechanics of the ankle joint and Achilles tendon in a configuration that offers the comfort and aesthetics of traditional footwear. Once proven, this mechanism can be deployed into the construction of a range of footwear to meet patients’ needs: from simple low-cost house shoes to more highly customized and adjustable designs. An artist’s rendering of the initial concept is shown in Figure 2.

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.

Overview of the Disease Lifecycle and Goal of the Innovative Design

To illustrate the nature of how the disease progresses, we have developed a fictitious representation of an ulceration/amputation lifecycle in Figure 3.

Figure 3.

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

This illustration is intended to highlight the high frequency of reulceration, 40% within 12 months of treatment^1,20,21 for people in diabetic foot remission, placing them at higher risk for future amputation.

Figure 4 shows the intended value proposition of the proposed exoskeleton footwear that can interrupt this lifecycle (A) by providing prophylactic benefit to prevent or delay initial onset of ulceration “Δ onset” and (B) to maximize ulcer-free days of diabetic foot remission “Δ recurrence” after treatment.

Figure 4.

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

This proof-of-concept study has been designed to evaluate early prototypes of exoskeleton enabled footwear and their ability to provide offloading, especially while walking. This pilot study will allow an estimate of the sample size required to validate effectiveness of exoskeleton footwear for effectively reducing plantar pressure under regions of interest while minimizing alteration in gait or balance.

Introduction to the Prototype Device

The exoskeleton acts as a passive (non–battery powered) low-cost wearable robot. Its intention is to harvest horizontal leg force from the lower shin, externalize it into the exoskeleton, translate it across an exotendon spring running parallel to the Achilles tendon, and then deliver that force as a downward vector immediately below the forefoot.

Figure 5 shows two images. On the left, without deploying an exotendon spring, the foot bears its full load. On the right, deploying the exotendon spring enables a contribution of force by the footwear (downward red arrow) offsetting the force required from the user (downward blue arrow). In this way, force is routed from the leg to the ground. Not only does this offload the forefoot, but also offloads the plantar fascia, the ankle, and the Achilles tendon—all of which are subject to injury and which benefit from offloading during rehabilitation.

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%).

In the prototypes tested, shown in Figure 6, a diabetic tennis shoe was partially dismantled, and a semirigid endoskeleton was inserted between the layers of footwear upper, heel cup and arch. This endoskeleton provided rigidity to the shoe to enable it to transfer forces to the ground, while still being comfortable, breathable, and aesthetically pleasing. A rotatable yoke was positioned above the ankle and allowed to rotate via bilateral hinges colinear with the ankle joint. The rotating yoke harvests force from a cushioned shin strap and thereby applies lift to the exotendon spring. In this prototype, springs were created from elastic cord, making the control of spring rate easier. The exotendon spring was adjustably anchored at the heel of the shoe through a Boa® ratcheting reel (Boa Technologies, Boulder, CO). Because of the ankle hinge, the upward pull on the exotendon creates a torque in the shoe, which delivers a downward force into the ground below the forefoot. As a result of this supplemental external force on the ground through the exoskeleton, a commensurate amount of force is offloaded from the forefoot.

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.

Methods

Subject Recruitment

All subjects were recruited from Southern Arizona Limb Salvage Alliance (SALSA) at the University of Arizona Health System, or affiliated and local clinics in Tucson, Arizona, USA. The study received local institutional review board approval from the University of Arizona. Each participant signed an informed consent prior to participating.

For the purpose of this proof-of-concept study, only ambulatory subjects with minimum risk of foot problems were recruited. Since the protocol included a walking test with unproven offloading efficacy, to minimize risk, only ambulatory subjects (with or without diabetes) and without any major foot risk (eg, those with active foot ulcers, those with foot deformity such as Charcot, and those with recent healed ulcers [over last 6 months]) were recruited.

Testing Method

For this part of the clinical trial, participants were asked to walk 6 m at their own self-selected speed; timing was measured by a stopwatch.

Participants’ plantar pressure was recorded at 100 Hz using computerized pressure insoles (F-Scan™, TekScan Inc, South Boston, MA). Participants wore the same exotendon footwear for all activities and repeated at least three passes under three separate test conditions:

1. no exotendon springs engaged

2. medium exotendon springs engaged (5 to 35 N)

3. strong exotendon spring engaged (10 to 150 N)

For the purpose of the proof of concept, it was felt that the journal article would be more clear if we simply compared conditions 1 and 3. In this way the effect can be more clearly quantified. The overlaps in spring force simply represent the range of force from when the exotendon spring first engages to when the spring is near maximum dorsi-flexion.

Plantar pressures were measured under regions of interest including hindfoot, midfoot, and forefoot. The measured peak plantar pressure values under regions of interest for at least 5 consecutive strides were averaged for final statistical analysis and between condition comparisons. We hypothesized that spring engagement is correlated to dorsiflexion, and that the level of offloading increases with dorsiflexion.

Analysis Plan

Plantar Pressure data was gathered through the TekScan F-Scan™ system. One-way analysis of variance (ANOVA) and Wilcoxon signed-rank test (pairwise) were used for between conditions comparison. In addition, the between condition differences were quantified by estimating Cohen’s d effect size and represented as d in the results section. Values of the Cohen’s d ranging from 0.20 to 0.49 indicate small, from 0.50 to 0.79 indicate medium, from 0.80 to 1.29 indicate large, and above 1.30 indicate very large effects.²²

All statistical analyses were performed using IBM Statistical Package for Social Sciences (SPSS Version 23, IBM, Chicago, IL, USA), with a significance level of P < .050 and two-tailed analysis.

Results

Out of 10 participants who satisfied inclusion and exclusion criteria and were recruited for the purpose of this study, 5 completed the study and are included in this analysis (age range = 38-60, mean = 49.6, SD = 10.9 years). The group ranged in BMI from 31 to 38 kg/m², with an average of 35.8 kg/m² (SD = 4.43). Participants included 1 male and 4 females; of these 5 participants there was 1 healthy-normal volunteer and 4 with diabetes mellitus who were enrolled in the study. The reasons for not completing the study were based on inability to fit the width of the two available sizes of prototype footwear (n = 3); 2 participants were unable to present for multiple scheduled testing dates (n = 2).

Plantar Pressure

Plantar pressure was measured during the 6-m walk to evaluate the impact of engaging the exotendon spring during gait. Comparing the same subject as their own control and wearing the same footwear, participants’ plantar pressures were consistently reduced during tests when the exotendon springs were engaged. To help isolate participant’s weight out of the equation, the change in plantar pressure was calculated as a percentage from the baseline reading with no springs engaged. Figure 7 shares results from the five participants. Change from baseline ranged from a 3.6% to a 36.2% reduction in forefoot plantar pressure, with an average reduction of 22.1% (SD = 13.3%, P = .043), shown by the blue bars; and the change in hindfoot pressure ranged from an 8.2% reduction to a 42.5% increase with an average increase of 18.0% hindfoot plantar pressure (SD = 18.0%), shown in the red bars.

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.

Holding the footwear/insole the same was essential to maintaining consistency in instrumentation. Using separate shoes for a baseline may have introduced significant risk for changes in sensor placement, lacing tightness, footbed anomalies (including sole stiffness, insole stiffness, and other simple variations in manufacturing as well as “break-in” differences over time) that could register changes in pressure.

Walking Speed

As a pilot study, it was important to understand whether engagement of the exotendon springs altered gait speed. Participants conducted the 6-m walk at least three times for each spring strength condition. Times to complete each 6-m pass were recorded. Figure 8 includes these results. Of the four participants with diabetes, three conducted the 6-m walk faster; change in elapsed time ranged from 21.9% slower to 25.1% faster (SD = 20.1%). As there are insufficient data to establish a statistically significant result, future studies will need to investigate further. The data, as well as clinical observation of the participants for safety assurance, showed no evidence that the novel exoskeleton device hinders gait speed.

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.

Gait Cycle

Offloading was evaluated across the gate cycle. At heal strike, there was no pressure on the forefoot, and no offloading. Offloading reached its peak at heel-lift. A representative averaging of 6 stances is shown in Figure 9, highlighting the relationship between dorsiflexion angle during the stance phase of the gait cycle and offloading.

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.)

The proof-of-concept study did not endeavor to fully analyze gait (eg, stride length, dynamic stability, stride symmetry, range of motion, or isokinetic forces); its aim was to quantify the ability for the exoskeleton to offload pressure from the forefoot while maintaining basic gait characteristics. Future studies will include full gait analysis, which will include a baseline.

Patient Feedback

During clinical testing, all participants were interviewed for level of comfort and acceptability of the new exoskeleton system. No negative comments were noted. All subjects claimed a positive experience and ease of using this new footwear solution. One of the participants with a past history of DFU preferred to walk in the shoes with springs fully engaged, commenting, “The exotendon springs give me more stability.”

Forecasting Number of Patients for Future Studies

Analysis of the effect size using Cohen’s d test resulted in a value of 0.65, which is considered as a medium effect size.²³

Knowing this effect size, we can now forecast the minimum number of participants required future studies. Based on an effect size (Cohen’s d) of 0.65, our minimum number of participants for clinical validation will be at least 40, assuming a power of 80% and an alpha of 5%.

Discussion

Studies have clearly established links between areas of high plantar pressure to ulceration in individuals with diabetes.^14,17 Although current diabetic footwear, insoles, and other orthotics have been shown to better distribute plantar forces across the foot, they have not been effective at reducing the frequency of occurrence or recurrence.^16,17 With the ongoing increase in the incidence of obesity, metabolic disorders, and diabetes, strong innovation is needed to simply compensate for the growth in the patient population. This innovation must interrupt the ulceration/amputation lifecycle.

The results of this study suggest that forces can be harvested by an exoskeleton from the shin, externalized outside the body, and delivered into the ground under the forefoot, thereby offloading commensurate force and pressure from the forefoot. This study supports the proof-of-concept hypothesis that passive and adjustable offloading-based exoskeleton footwear can offer effective offloading with minimum alteration of gait speed. Another study is warranted to examine superior benefit of the proposed offloading approach in preventing or managing DFUs.

Reduction in load also influences lateral and vertical plantar shear forces. Shear forces are directly related to both vertical load and the rate at which the vertical load escalates. A reduction of load and loading rate will have a direct impact on shear, even without the inclusion of further shear-reducing elements within the exoskeleton design. Future studies should seek to quantify this impact, as shear forces have a significant impact on DFU progression. :

There are two major vectors critical in interrupting the ulceration/amputation lifecycle:

1. Delaying and preventing initial ulceration occurrence. The offloading exoskeleton/exotendon approach explored within this study may be translated into a wide range of off-the-shelf footwear, including sandals and house shoes as a way to keep costs low and allow the solution to scale up to large volumes, thereby reducing price points and making it widely affordable. In this way, large adoption may enable a prophylaxis prior to initial onset.

2. Maximizing ulcer-free days and diabetic foot remission after treatment. The proposed offloading approach may also be configured with highly adjustable anchors to simplify a custom fit and levels of offloading assistance. The solution may be deployed as an alternative to diabetic shoes and as a new alternative that allows clinicians another option after patients are removed from a TCC.

Human Factors Assessment

Adherence is a critical issue with TCCs and other offloading devices. Comfort, style, and breathability are key factors that influence adherence and differentiate the novel exoskeleton. Further human factors studies are essential to refining the design to optimize adherence. The positive patient feedback offered spontaneously by affected patients speaks to potential future unanticipated benefits of exoskeleton/exotendon technology and future studies should evaluate whether the device can increase stability and confidence in gait. After walking with the prototype footwear, patients say that they like it better with springs fully engaged and active. Participants with the most unsteady gait were most vocal and preferred the exotendon springs fully engaged.

Positive patient feedback opens the door to investigate whether the proposed exoskeletal footwear approach may be integrated with powered motors to provide increased propulsive force for users. The world of rehabilitation robots, wearable robots and wearable sensors will open the door for advanced technology to be deployed. There are ample reasons to study the proposed exoskeleton footwear in collaboration with a wearable robotics research lab so that powered drives and sensors can be evaluated on whether they can further improve on the passive exotendon springs currently deployed in our prototypes.

Possible Risks to the Pretibial Area (Shin)

The proposed device creates a potential risk of skin irritation at the shin, however, there are multiple factors that make risks to the pretibial area an order of magnitude smaller than the risks of common surgical offloading approaches, the extended use of TCCs, the danger of recurrent DFU, or the morbidity associated with amputation. These factors include:

• - Sensation: The pretibial area is closer to the core and typically retains sensation even after the distal region of the foot has become nonsensate, allowing users to easily make adjustments or depower the device in the event of irritation, long before it progresses to chaffing

• - Leverage: The exoskeleton can be designed to reach to various elevations of the shin. Those at higher risk of pretibial irritation can be fit with devices that reach higher on the shin. The leverage afforded is relative to the fulcrum as measured by the distance from the ankle joint. Proposed exoskeleton design configurations harvest forces 2 to 4 times further away from the ankle joint than the forefoot, lowering the forces harvested by 2 to 4 times that required for offloading.

• - Pressure: The area established by the yoke to wrap the frontal area of the shin can be designed in the exoskeleton to have a surface area larger than the surface area of the metatarsal ridge and toes, further reducing the pressure required for operation.

• - Secondary Amendments: There are a number of hydrogel sheets, closed cell foams, and low shear fabrics that can be employed to protect the pretibial area. These approaches are well known in the field of wearable robotics and can be deployed between the skin and the yoke to further isolate the user from harm.

Future studies should quantify these pretibial pressures and evaluate proactive means to reduce risk to this area.

Possible Risks to the Hindfoot

As shown in Figure 7, the proposed device changed hindfoot plantar pressure, lowering it in one subject and increasing it in 4 subjects. Increases in hindfoot plantar pressure can increase the risk of ulceration in the hindfoot region, implying that the device may not be suited for all patients. However, this trade-off is common in clinical practice as a means of preventing further amputations which start most frequently at the toes; these approaches include heel-lifts, as well as surgical intervention to the Achilles tendon.

Conclusions

The results of this pilot study suggest that exoskeleton-based footwear has the potential to harvest forces from the pretibial (shin) area and translate them to offload forces under the forefoot. This suggests that the exoskeletal system externalized forces outside of the ankle and foot and delivered force directly to ground, offloading plantar pressure—a critical contributor to DFU.

The amount of offloading was relative to the degree of tension within the exotendon springs. Increasing the tension in the exotendon springs did increase forefoot offloading. This off-loading did not significantly alter gait performance or speed.

Patients with a past history of diabetic foot complications responded favorably to the engagement of the exotendon springs.

This exoskeleton may be deployed in a variety of footwear—from sandals to sneakers and boots—enabling it to be brought to market as both a low-cost off-the-shelf product as well as a highly customized adjustable product. It may be deployed as a passive system, simply relying on the exotendon spring to store and release energy, or it may be integrated with motorized wearable robotic solutions with embedded sensors that may offer increased diagnostic and foot-health monitoring capabilities.

Further study in a larger sample size is warranted to better understand the efficacy of the exoskeleton footwear as well as the human factors that influence patient adherence and acceptance.