A Novel Shear Reduction Insole Effect on the Thermal Response to Walking Stress,Balance,and Gait

Abstract

Background:

Shear stresses have been implicated in the formation of diabetes-related foot ulcers. The aim of this study was to evaluate the effect of a novel shear-reducing insole on the thermal response to walking, balance, and gait.

Method:

Twenty-seven diabetes peripheral neuropathy patients were enrolled and asked to take 200 steps in both intervention and standard insoles. Thermal foot images of the feet were taken at baseline (1) following a 5-minute temperature acclimatization and (2) after walking. Testing order was randomized, and a 5-minute washout period was used between testing each insole condition. Sudomotor function was also assessed. Gait and balance were measured under single and dual task conditions using a validated body worn sensor system.

Results:

The mean age was 65.1 years, height was 67.3 inches, weight was 218 pounds, and body mass index was 33.9, 48% were female, and 82% had type 2 diabetes. After walking in both insole conditions, foot temperatures increased significantly in standard insoles. The intervention insole significantly reduced forefoot and midfoot temperature increases (64.1%, P = .008; 48%, P = .046) compared to standard insoles. There were significant negative correlations with sudomotor function and baseline temperatures (r = .53-.57). The intervention demonstrated 10.4% less gait initiation double support time compared to standard insoles (P = .05). There were no differences in static balance measures.

Conclusions:

We found significantly lower forefoot and midfoot temperature increases following walking with shear-reducing insoles compared to standard insoles. We also found improvements in gait. These findings merit future study for the prevention of foot ulcer.

Keywords

autonomic dysfunction foot biomechanics foot complications foot foot wear orthotics shear friction

Reducing shear or side-to-side stresses on the plantar surfaces of feet in patients with diabetes is of particular interest lately. Conflicting clinical trial data of custom pressure-reducing footwear suggest that other offloading or footwear approaches may be necessary.^1,2 Gait lab studies suggest shear stresses are in a different location and magnitude versus age-matched controls.³ Shear stresses in the foot also demonstrate a large association with thermal changes in response to walking in patients with diabetes and peripheral neuropathy.⁴ Currently, there are no commercially available products to detect shear inside the shoe. However, relative temperature changes using thermometry may represent antecedent inflammatory changes from shear and pressure occurring prior to clinical expression of foot ulceration at the skin level. Purposeful “cooling” of the foot in clinical trials resulted in effect sizes of 4- to 10-fold reduction in reulceration.^5-7 Autonomic neuropathy can also affect thermoregulation of foot temperature and may be a more sensitive measure⁸ for predicting foot ulcer.⁹ In this study we measured sudomotor function for small fiber autonomic neuropathy.^10-12

Shear-reducing insoles that permit side-to-side motion in the device to reduce shear at the skin and deeper tissue level may make patients feel unsteady and less confident with walking. Therefore, in this study, we measured gait and balance changes under single and dual task conditions to explore for gait adaptations. In addition, these measurements may indicate whether reduction in shear force in forefoot area may alter gait and balance, which may lead to increased risk of falling in this high risk population. The aim of this study was to build on our prior preliminary testing of a shear-reducing insole¹³ to describe the thermal response to walking stress, gait changes, and balance in patients with diabetes and peripheral neuropathy.

Methods

This cross-sectional study was conducted at the University of Michigan Orthotics and Prosthetics Center. Twenty-seven consecutive subjects with diabetes (type 1 or type 2) presenting for prescription shoe and insole fitting were asked to enroll. Subjects were included if they had diabetes, peripheral neuropathy (DPN),¹⁴ and either preulcerative callus or history of foot ulcer. Subjects were excluded if they were unable to independently walk 100 feet or had lower extremity prosthesis, active cellulitis, foot ulcer, or Charcot foot. Eligible subjects signed an informed consent and the study received ethical approval from the Institutional Review Board. The trial was registered at ClinicalTrials.gov (NCT01844479).

Subjects’ feet were digitized using AMFIT® (Amfit Incorporated, Vancouver, WA, USA) for standard insoles as well as the Dynamic Foot Orthoses (DFO, designed and developed by PA). The DFO has a rolling link mechanism at the distal third of the foot to reduce sliding friction and torque at the metatarsal heads in addition to decreasing compressive forces. Frictional resistance is reduced at the (1) skin using a Rubbatex® neoprene rubber top cover with 4-way stretch darlex® (Richardson Products Incorporated, Frankfort, IL, USA) on both sides and (2) deeper tissues using a silicone layer that slides on firm density ethyl-vinyl acetate base material lined with ballistic nylon (Figure 1). Bench testing revealed 270% average decreased shear stiffness for the DFO compared to standard insoles at each foot region during a simulated gait cycle.¹³ Standard insoles were fabricated using firm density plastazote® and PPT® bi-lam (American Plastics, Arlington, TX, USA). All subjects were given standardized extra depth shoes with semi rigid rocker sole (Dr. Comfort®, DJO, UK) and wore a lightweight sock.

Figure 1.

Dynamic Foot Orthoses (DFO).

Prior to testing, all subjects had debridement of preulcerative callus. There was a 5-minute acclimatization period without weight-bearing, shoes, or socks prior to baseline thermal imaging and standardized walking stress test.¹⁵ In random sequence, each insole condition was tested for gait during habitual speed,¹⁶ balance in tandem stance with eyes open and closed,¹⁷ and a thermal image was taken after 200 steps of walking stress. To explore whether reducing shear force may impact risk of falling, gait was also examined during dual task (walking while counting backward), which is a more sensitive paradigm than single task (walking alone) to evaluate risk of falling.¹⁸ Subjects had a standard washout and acclimatize period of 5 minutes prior to the second footwear testing. Testing of sudomotor dysfunction by measuring electrochemical sweat conductance with a Sudoscan™ (Impeto Medical, Paris, France) was also performed¹² to explore for an association between autonomic neuropathy and thermal response to walking. The Sudoscan test takes approximately 2 minutes to perform and demonstrates a low coefficient of variation of 7% at the foot level.^11,12 Spatiotemporal gait and balance were assessed using a validated wearable sensors technology (LEGsys™, Biosensics LLC, Cambridge, MA, USA).¹⁹ Gait was quantified by stride velocity, stride length, and stride time; stance and double stance phases as a percentage of stride time; and gait speed variability (estimated using coefficient and variation of stride velocity). We estimated gait during both gait initiation (ie, acceleration phase) and gait steady state.²⁰ Gait initiation is believed to be more implicated in shear force development due to acceleration to steady state gait. Balance was assessed in the static condition. Balance measures were center of mass sway (COM) in anteriorposterior (COM_AP), mediolateral (COM_ML), and area (COM_APxML).¹⁷ Thermal imaging was conducted with a Fluke® Ti25 thermal imager (Fluke Corporation, Everett, WA, USA). A purpose-designed image processing toolbox¹⁵ was used to isolate each foot from the thermal image and to extract plantar temperature in 3 anatomical regions (eg, hind-, mid-, and forefoot). This toolbox estimates 5th, 50th, and 95th temperature percentiles at each region. For this study, we analyzed only the 50th percentile value. Thermal imaging interrater reliability has been previously described as good²¹ with intraclass correlation coefficients ranging from .82 to .97.²²

The sample size was estimated based on our prior work measuring gait using another shear-reducing insole. We found a significantly shorter gait initiation (average 30%, P = .036, 95% CI = −2.2, –0.6 steps). Assuming power of 80% and alpha = .05, 26 subjects would be needed to observe a significant improvement in gait initiation by reducing shear force.

Statistical Analysis

Comparison across walking task (single task and dual task) for each walking condition (type of footwear) was done with repeated measures ANOVA 2 × 2 test, and pairwise main effect or interaction comparisons were done using a Sidak adjustment. Spearman correlation coefficient was used to examine correlation between thermal response and sudomotor function. For all tests an alpha level of .05 was considered statistically significant. All calculations were made using SPSS® version 21 (SPSS, Chicago, IL, USA) or Matlab (MathWorks, v7.4).

Results

Our sample of patients included 82% with type 2 diabetes and 18% with type 1 with an average age of 65.1 years, height of 67.3 inches, weight of 218 pounds, and body mass index of 33.9, with 48% being female and 52% being male.

After walking in both insole conditions, foot temperatures increased with significant increases observed in standard insoles only. The DFO reduced forefoot (64.1%, P = .008) and midfoot (48%, P = .046) temperature increases compared to standard insoles (Figure 2). There were significant negative correlations with sudomotor function and baseline temperatures (r = .53-.57) (Table 1). No correlation was found between thermal response to walking and subject’s BMI for both insole conditions (r < .10, P > .600). On the same note, the observed difference for thermal response to walking between 2 insole conditions was independent on gender (P = .649). Although a negative correlation was observed between thermal response to walking and gait speed (r = –.52, P = .008), walking speed didn’t affect the observed difference in thermal response to walking between 2 insole conditions (P = .290). In Figure 3, we found a significant correlation with gait speed and temperature for the standard insole condition (P = .002) but not for the DFO condition (P = .06). We also explored for a correlation with gait variability and temperature and found no significant correlation for the standard insole condition (P = .098) or DFO (P = .542).

Figure 2.

Temperature change in degrees Celsius after walking 200 steps. DFO, Dynamic Foot Orthoses; SCI = standard control insoles.

Table 1.

Sudomotor Function Negative Correlation With Thermal Response Percentile at Baseline by Foot Region.

	Percentile temperature in foot region of interest
	5%	50%	95%
Forefoot	.2433-.3171	.2090-.324	.1388-.3124
Midfoot	.4494-.5340	.4878-.5736	.5389-.5454
Heel	.5498-.5696	.5494-.5622	.5496-.5526

Values in bold indicate significant P values less than .05.

Figure 3.

Correlation of gait parameters and insole condition. (a) Correlation of gait speed by insole condition. (b) Correlation of gait variability by insole condition.

Under single task conditions, there was a trend for all gait parameters to improve (Table 2). The DFO demonstrated 10.4% less double support time during gait initiation compared to standard insoles (P = .05). Under dual task conditions, there was also a trend toward all gait parameters improving. For example, we found a 32% reduction in gait variability with DFO (P = ns) (Table 3). Similar trend was observed for balance data. During eyes-open wearing DFO, COM_APxML, COM_AP, and COM_ML were reduced by 46%, 11%, and 18%, respectively, compared to standard insoles (P = ns). However, during the eyes-closed condition, there were no insole differences (data not shown).

Table 2.

Between Groups Comparison for Gait Data During Single Task Condition.^a

	STD	DF	Mean difference DF-STD	P value	95% CI lower bound	95% CI upper bound
Gait initiation, steps	4.53 ± 0.63	4.32 ± 0.58	−0.21 (4.6%)	.83	−2.23	1.81
SV at gait initiation, m/s	1.06 ± 0.05	1.10 ± 0.04	0.04 (3.8%)	.09	−0.008	0.094
SV during steady state, m/s	1.08 ± 0.05	1.10 ± 0.04	0.03 (2.8%)	.33	−0.030	0.083
STL at gait initiation, m	1.24 ± 0.04	1.26 ± 0.04	0.02 (1.6%)	.25	−0.016	0.056
STL, m	1.25 ± 0.04	1.27 ± 0.04	0.02 (1.6%)	.43	−0.072	0.032
GCT at gait initiation, s	1.24 ± 1.17	1.17 ± 0.03	−0.07 (5.6%)	.13	−0.167	0.024
GCT, s	1.21 ± 0.06	1.16 ± 0.03	−0.04 (3.3%)	.26	−0.034	0.117
COM_AP, deg2	4.56 ± 0.35	4.44 ± 0.37	−0.12 (2.6%)	.74	−0.840	0.609
COM_ML, deg2	4.60 ± 0.48	4.61 ± 0.36	0.01 (0%)	.98	−0.618	0.635
DS at gait initiation	31.6 ± 2.4	28.3 ± 1.3	–3.3 (10.4%)	.05	–6.634	0.003
DS at gait steady state, %	30.5 ± 2.5	28.4 ± 1.5	−2.0 (6.6%)	.18	−1.052	5.131
Gait variability, %	6.8 ± 2.3	5.5 ± 2.1	−1.3 (19%)	.14	−3.086	0.483

AP, anterior posterior; COM, center of mass (defined by sacrum range of motion during each stride); DS, double stance phase; GCT, gait cycle time (mean value); ML, medial lateral; STL, stride length (mean value); SV, stride velocity (mean value).

Adjusted by age.

Table 3.

Between Groups Comparison for Gait Data During Dual Task Condition.^a

	STD	DF	Mean difference DF-STD	P value	95% CI lower bound	95% CI upper bound
Gait initiation, steps	3.58 ± 0.53	3.42 ± 0.58	−0.16 (4.5%)	.84	−1.819	1.503
SV at gait initiation, m/s	1.00 ± 0.05	0.99 ± 0.05	−0.02 (2.0%)	.60	−0.094	0.056
SV during steady state, m/s	1.01 ± 0.05	1.00 ± 0.04	−0.01 (0.9%)	.80	−0.096	0.075
STL at gait initiation, m	1.21 ± 0.04	1.21 ± 0.04	0.00 (0.0%)	1.0	−0.049	0.049
STL, m	1.22 ± 0.04	1.22 ± 0.04	−0.00 (0.0%)	.89	−0.053	0.046
GCT at gait initiation, s	1.25 ± 0.04	1.31 ± 0.06	0.06 (5.0%)	.13	−0.018	0.135
GCT, s	1.23 ± 0.04	1.26 ± 0.05	0.03 (2.4%)	.42	−0.047	0.107
COM_AP, deg2	4.47 ± 0.52	3.92 ± 0.24	−0.55 (12.3%)	.24	−1.493	0.395
COM_ML, deg2	4.73 ± 0.65	3.99 ± 0.30	−0.74 (15.6%)	.14	−1.749	0.266
DS at gait initiation	31.5 ± 2.2	29.8 ± 1.3	−1.6 (5%)	.24	−4.288	1.130
DS at gait steady state, %	30.9 ± 2.3	28.2 ± 1.2	−2.6 (8.4%)	.16	−6.378	1.141
Gait variability, %	7.45 ± 2.69	5.08 ± 0.97	−2.37 (31.8%)	.23	−6.347	1.604

Adjusted by age.

Discussion

We found that a novel shear-reducing insole significantly reduced temperature increases over baseline in the forefoot (64%) and midfoot (48%) after walking 200 steps compared to standard insoles. This may suggest that thermal response to walking could be used as an indirect test to evaluate magnitude of change in shear force via change in insoles or shoes interface. The magnitude of differences take on more clinical significance when scaling the reduced temperature increases over 7754 steps per day with twice as much spent standing compared to walking.²³ Our findings differ from those of Hall et al, who found no thermal differences using an infrared thermometer in healthy subjects walking on a treadmill for 6 minutes. It should be noted that Hall et al studied healthy subjects and used an infrared thermometer rather than thermal imaging.²⁴ We explored for other gait conditions that would be expected to increase shear forces during walking, such as gait speed and gait variability. We found no significant correlations with gait speed and gait variability with temperature in the DFO condition; however, gait speed was significant for the standard insole condition and approached significance for the DFO. Taken collectively with our prior findings from measurement of shear stiffness,¹³ the DFO appears to reduce shear in the foot, and temperature change may represent a surrogate measure of shear in response to walking stress.¹⁵

Surprisingly, we found trends for improvements in all gait parameters using the DFO under single and dual task conditions with a significant decrease in double support time during gait initiation. Gait and balance were measured to assess for maladaptive gait strategies secondary to more movement permitted by the freely moving top cover of the DFO. We suspected movement might increase fear of falling causing subjects to adopt a more conservative gait strategy. This was not borne out in our data. One potential explanation could be the DFO’s contoured arch could have contributed to improved gait parameters.¹⁶ Our prior work demonstrated that custom foot orthoses reduced gait variability and medial and lateral COM displacement¹⁶ that may be the result of improved proprioceptive feedback.^16,25 The custom foot orthoses tested in these studies used top covers that were glued to the underlying contoured shell and they did not allow freedom movement within top cover layers as the DFO in the present study.

Interestingly, we found a moderate to good negative correlation²¹ between sudomotor function, regulated by the small unmyelinated sympathetic fibers, and thermal response of walking (r = .53-.61). Autonomic nerve fibers are also involved in body’s ability to regulate skin temperature. Sudomotor function is thought be a sensitive measure where abnormalities are present in 32% without clinical evidence of neuropathy.⁸ In a multivariate study, sudomotor function demonstrated larger associations with diabetes-related foot ulcer over monofilament insensitivity, vibration perception threshold, and neuropathy disability scores.⁹ The Sudoscan has a low coefficient of variation of 7% at the foot level^11,12 that is 3- and 4-fold lower than Neuropad (22%) and vibration perception threshold testing (33%).¹² Increases in regional foot temperatures have been associated with subsequent ulcer formation with thermal monitoring holding promise for foot ulcer prevention.²⁶ When regional temperature increases are detected, they can be significantly reduced with purposeful cooling.^5-7,27 When taken together with biomechanical causes of increased temperature,²⁸ thermal monitoring appears to hold promise as an objective intermediate outcome measure for footwear trials.¹⁵

Strengths of our study include good measurement properties for the instrumentation used.^{8-12,17,19,21,22} We also allowed for a thermal wash-out period¹⁵ between testing and randomized the insole testing sequence. We assessed for gait and balance changes under single and dual task conditions. There are also limitations to our study. The cross-sectional design does not permit causal inference. While detecting thermal changes between feet have demonstrate large effect sizes for ulcer recurrence,^5-7,26 there are no accepted thermal thresholds for development of new ulcers.

Conclusions

In conclusion, we found significant reductions in forefoot and midfoot temperature increases after known walking stress using a novel shear-reducing insole when compared to standard insoles. We also found sudomotor function demonstrated significant correlations with the thermal response after walking. Future work should focus on the efficacy of the DFO for reducing foot ulcers. Future footwear studies should also consider measuring thermal and sudomotor function changes.

Footnotes

Abbreviations

AP, anterior posterior; COM, center of mass; DFO, dynamic foot orthotic; DPN diabetes-related peripheral neuropathy; ML, medial lateral.

Declaration of Conflicting Interests

The author(s) declared the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article: PA disclosed the technology to the Office of Technology Transfer at the University of Michigan. PA was not involved with the data collection or analysis of the data.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The project described was supported by grant P30DK020572 (Michigan Diabetes Research Center) from the National Institute of Diabetes and Digestive and Kidney Diseases.

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