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. Author manuscript; available in PMC: 2013 Feb 2.
Published in final edited form as: J Biomech. 2011 Dec 12;45(3):619–622. doi: 10.1016/j.jbiomech.2011.11.004

Spatial relationships between shearing stresses and pressure on the plantar skin surface during gait

Samantha Stucke 1, Daniel McFarland 1, Larry Goss 2, Sergey Fonov 2, Grant R McMillan 2, Amy Tucker 4, Necip Berme 3, Hasan Cenk Guler 3, Chris Bigelow 3, Brian L Davis 1,*
PMCID: PMC3264746  NIHMSID: NIHMS342425  PMID: 22169152

Abstract

Based on the hypothesis that diabetic foot lesions have a mechanical etiology, extensive efforts have sought to establish a relationship between ulcer occurrence and plantar pressure distribution. However, these factors are still not fully understood. The purpose of this study was to simultaneously record shear and pressure distributions in the heel and forefoot and to answer whether: (i) peak pressure and peak shear for anterior-posterior (AP) and medio-lateral (ML) occur at different locations, and if (ii) peak pressure is always centrally located between sites of maximum AP and ML shear stresses. A custom built system was used to collect shear and pressure data simultaneously on 11 subjects using the 2-step method. The peak pressure was found to be 362 kPa ±106 in the heel and 527 kPa ± 123 in the forefoot. In addition, the average peak shear values were higher in the forefoot than in the heel. The greatest shear on the plantar surface of the forefoot occurred in the anterior direction (mean and std dev: 37.7 ±7.6 kPa), whereas for the heel, peak shear on the foot was in the posterior direction (21.2 ±5 kPa). The results of this study suggest that the interactions of the shear forces caused greater “spreading” in the forefoot and greater tissue “dragging” in the heel. The results also showed that peak shear stresses do not occur at the same site or time as peak pressure. This may be an important factor in locating where skin breakdown occurs in patients at high-risk for ulceration.

Keywords: Biomechanics, Bioinstrumentation, Diabetic, Plantarsurface, Ulceration

1.Introduction

Diabetic foot ulcers continue to be a burden on the US healthcare system with an annual cost of approximately $6 billion (Frykberg et al. 2006). Based on the hypothesis that diabetic foot lesions have a mechanical etiology, extensive efforts have sought to establish a relationship between ulcer occurrence and plantar pressure distribution. In most of the pressure distribution studies peak pressure parameters were chosen as a possible ulcer predictor. However the existing longitudinal studies have yielded only moderate correlations between peak pressure and the occurrence of diabetic foot lesions (Armstrong et al. 1998, Lavery et al. 2003, Veves et al. 1992).

Surprisingly, only one research group has examined whether all plantar ulcers developed in the follow-up period matched the baseline peak pressure sites. Veves et al. (1992) reported that only 38% of the ulcers developed under the peak pressure area. As an outcome, foot pressure has been labeled as a “poor tool” in the prediction of diabetic ulcers and where they would occur (Lavery et al. 2003).

Recently, investigators have examined shear stresses and their distribution under diabetic feet in more detail. One such study (Yavuz et al. 2007) showed that shearing stresses and peak pressures do not typically occur at the same location. What has not been studied is whether this discrepancy between peak shear locations is due to tissue being “spread” radially as a result of pressure between the foot and ground, or whether it is due to a “dragging” effect where the forward (or backward) motion of the foot causes tissue to become bunched in front of (or behind) the site of maximum pressure. Therefore the purpose of this study was to simultaneously record shear and pressure distributions in the heel and forefoot and answer the questions whether: (i) peak pressure and peak shear for anterior-posterior (AP) and medio-lateral (ML) occur at different locations, and if (ii) peak pressure is always centrally located between sites of maximum AP and ML shear stresses. Answers to these questions would shed light on the complex spatial and temporal interactions between shear and pressure acting on the plantar surface of the foot.

2.Research Design and Methods

Shear and Pressure data during walking were collected on 11 volunteers (7Males/4Females, mean age 38 ± 14.5 years), none of whom had (i) gross foot deformities (minor clawing of the toes was permissible), (ii) prior foot surgeries nor (iii) foot pain. The protocol was explained to the volunteers before their participation and each signed an informed consent form which was approved by the Institutional Review Board. Detailed patient characteristics are given in Table 1.

Table 1.

Characteristics of the subjects

Control
No of subjects 11
Gender 7 Male/ 4 Female
Age (years) 38 ± 14.5 (20-61)
Weight (lb.) 180 ± 23.4 (150-215)
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Values are presented as the mean ± standard deviation, with the range in parentheses.

The custom built shear and pressure system consists of a reusable 40×58×0.17 cm surface stress sensitive film (S3F) (Fonov et al. 2005, 2007) sensitive to pressure and shear mounted on a 60×60 cm 6-component force plate that can obtain ground reaction forces (Figure 1a). The device was mounted in an 8’ × 2’ walkway in such a manner that the top of the stress sensitive film was flush with the surrounding walking surface (Figure 1b). Shear and pressure data were collected on the left foot, using the 2-step method (McPoil et al. 1999). Subjects initiated walking with the right foot and took a series of 3 steps; data were collected from the subjects’ 2nd step. Three bare foot trials were collected for each subject.

Figure 1a.

Figure 1a

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Representation of the custom built shear and pressure system. The glass plate and polymer film are removed to permit viewing of the internal components. A total of four cameras are used to image the film, although only two can be seen in these views. The mirrors direct the cameras’ field of view upward to the film. The light bars provide the illumination for imaging and for excitation of the fluorescent probe in the film.

Figure 1b.

Figure 1b

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Close up view of a subject stepping on to the shear and pressure platform.

The output of the device consisted of three data vectors; vertical, AP and ML shear forces. The process of measuring pressure and shear stress is accomplished in three steps. First, the normal and tangential displacements of the film are optically measured. These displacements are then converted to pressure and shear stress distributions using a physical stress-strain model of the film. Finally the force plate measurements are used to validate and, if necessary, rescale the pressure and shear stress distributions. Note that anterior, posterior, medial and lateral shear acting on the foot, are designated as Sant, Spos, Smed and Slat, respectively. The Cartesian coordinates for the locations of the peak stresses occurring in the forefoot and heel were determined for each dataset. Difference in peak pressure and shear location errors were quantified by calculating Euclidian distances (D) between peak stresses.

3. Results

The mean peak pressure values in the heel and forefoot were found to be 362 ±106 kPa and 527 ± 123 kPa respectively, which corresponds well with previous studies (Cavanagh et al. 1987). Mean peak shear values in the forefoot were higher than in the heel. The peak average shear acting on the plantar surface of the forefoot was directed anteriorly (37.7 ±7.6 kPa) whereas the minimum average shear was in the posterior direction (17.6 ±5.7 kPa). For the heel the peak average shear acting on the plantar surface occurred in the posterior direction (21.2 ±5 kPa) and the minimum average shear in the anterior direction (8.3 ±2.8 kPa) (Figure 2a).

Figure 2a.

Figure 2a

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Average shear values for the heel and forefoot

In all 11 subjects peak pressure and peak shear for AP and ML occurred at different locations in the heel and forefoot. In the heel, the peak pressure site, on average, was 24.8mm away from Sant, 17.37mm from Spos, 20.93mm from Smed and 22.94mm from Slat. In the forefoot, the peak pressure site, on average, was located 22.79mm away from Sant, 29.66 mm from Spos, 24.26 mm from Smed and 26.67mm from Slat, (Figure 2b). In addition the peak AP shear values not only occurred at different locations than the peak pressure values, but also at different times. In the heel, the peak AP shear values occurred prior to the peak pressure value 60.61% of the time, while in the forefoot, it occurred afterwards 57.58% of the time (Figures 3a and b).

Figure 2b.

Figure 2b

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Graphic representation of the distances from average peak pressure to average shear in the Heel and Forefoot.

Figure 3.

Figure 3

Figure 3

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Time histories for shear and pressure for a single trial for a representative subject. (a) Heel, (b) Forefoot.

The corresponding shear forces acting in the immediate vicinity of the peak pressure location were examined. The number of occasions when there were outwardly directed forces on either side of the peak pressure location was tabulated; this number of occurrences indicated a “spreading” effect. Conversely, the number of times shearing stresses were unidirectional and were before or after the peak pressure location, indicated situations where “dragging” occurred (Figures 4a and b). This data showed that in the forefoot shear forces tended to cause tissue to have a “spreading” effect. However under the heel the AP shear forces created a “dragging” effect. Overall, “spreading” occurred in both directions in the forefoot (94% ML, 67% AP). In the heel region “spreading” occurred 82% of the time in the ML direction, while “dragging” occurred in 82% of the time in the AP direction.

Figure 4a.

Figure 4a

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Pressure and shear locations in the forefoot of a representative subject, showing the main cause of shear in all directions is due to a radial “spreading” effect.

Figure 4b.

Figure 4b

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Pressure and shear locations in the heel of a representative subject, showing the main cause of shear in the AP direction is due to a “dragging” effect.

4. Discussion

This study focused on both the magnitudes of 3D skin stresses acting on the plantar surface of the foot and whether pressure was more likely to be associated with radial “spreading” shear stresses or traction that is due to skin “dragging” at the interface between foot and ground. These results are important as the occurrences of skin breakdown are much higher in the forefoot then in the heel (Caselli et al. 2002) and the higher shear values might help explain why skin breakdown occurs more in the forefoot than in the heel (Mueller et al. 2005).

In addition, it is important to note that the peak shear values were always at different locations than the peak pressure values. This might help explain why foot pressure alone is a poor predictor of ulceration (Lavery et al. 2003). The results of this study also suggest that peak shear values not only occur at different locations than peak pressure, but also at different times, which also corresponds well with previous studies (Perry et al. 2002).

As expected, the maximum AP shears experienced by the heel and the forefoot are in opposite directions, which is most likely caused by the heel strike and push-off. We have shown that the interactions of the shear forces caused greater “spreading” in the forefoot (Figure 4a) and greater “dragging” in the heel. The peak shear values in the ML direction straddle the peak pressure location 94% of the time. This also occurs in the AP direction 67% of the time. Since the peak pressure is located directly in between both of the peak shear values and the forces are moving in opposite directions about the peak pressure location radial “spreading” occurs. The heel however, is subject to a “dragging” effect in the AP direction (Figure 4b). Similar to the forefoot, the peak shear values in the ML direction straddle the peak pressure location 82% of the time. However, the peak shear values in the AP direction occur prior to the peak pressure location and in the same direction (anterior to posterior) 82% of the time producing a “dragging” effect, which occurs during heel strike. These findings give us a more complete look at the stresses acting on the plantar foot.

In conclusion, the results of this study show that peak shear stresses do not occur at the same site or time as peak pressure and that shear stress may be an important factor in locating where skin break down may occur on the plantar foot. Shear stresses are higher in the forefoot where skin breakdown is most prevalent, compared to the heel. The ability to locate peak shear and pressure on the plantar foot furthers our understanding of the factors that play a role in skin breakdown.

Acknowledgements

The authors would like to acknowledge the Wound Care Center at Akron General Medical Center for their assistance in subject recruitment.

This research was possible due to support from National Institutes of Health (Grant # 4R44DK084844-02), awarded to Bertec Corporation with Cenk Guler as the Principal Investigator. The authors are indebted to the substantial contributions by Cenk. While his untimely death has certainly affected the pace of the project, his significant role in developing new biomechanical technologies will continue to serve as an inspiration to the rest of the team. Moreover, his legacy will continue by way of all the students he impacted through his teachings of biomechanics, dynamics, and gait analysis.

Footnotes

Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Conflict of interest statement

ABIA does not have conflict of interest whereas the technology used in this paper was developed by Bertec and ISSI and commercialization of this system will benefit Bertec and ISSI.

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