Plantar Soft Tissue Material Properties

Introduction

_{Figure 1 High plantar pressure regions prone to ulceration}

    Foot ulceration is one of the catastrophic complications associated with diabetes mellitus that may eventually require amputation of the affected limb if not treated in time. In order to prevent the need for amputation from arising, a better understanding of the underlying causes of foot ulceration is crucial. Our research aims to understand the differences between diabetic and healthy plantar tissue at the cellular/matrix level and see how differences found at the cellular level correlate to differences in plantar tissue mechanical properties that ultimately affect foot function. Since ulceration typically occurs at regions prone to higher plantar pressures (Fig. 1), we are interested in the cellular and mechanical tissue characteristics at these sensitive locations in the foot. Further, since most changes in tissue properties associated with diabetes occurs over long periods of time, we are also examining the effect of age on tissue properties. The long-term goal of this research is to reduce the incidence of lower extremity ulceration and amputation by applying the knowledge gained to alter the stress distribution beneath diabetic feet through improved foot orthoses and possibly to initiate a cellular/matrix level treatment that could alter the tissue mechanical properties. While we are currently in the process of obtaining the properties of diabetic tissue; the methods and results presented herein are from an initial study whereby we obtained mechanical properties of healthy young tissue. (see Journal of Biomechanics 40 (2007) 2975–2981 for full details).

Methods

_{Figure 2 Plantar tissue specimen locations}

      Eleven fresh frozen non-diabetic, young (36.4 ± 8.4 years) cadaveric feet were screened for foot abnormalities and thawed prior to dissection. 2 cm x 2 cm soft tissue specimens were dissected free from the bone and skin from the 6 locations of interest (Fig. 2). Initial thickness was measured after each specimen

_{Figure 3 Testing apparatus with the top (A) and bottom (B) platens, specimen (C), heater (D), environmental chamber (E)}

Results

_{Figure 4 Average stress relaxation response and QLV fit from average coefficients for specimen locations; ca = subcalcaneal, ha = subhallucal, la = lateral submidfoot}

    The QLV dummy data created from the mean coefficients for each area closely coincided with the average data for all locations (e.g. Fig. 4), indicating that averaging the coefficients for each location was a good approximation of the average data.

    All areas exhibited a non-linear stress–strain response for all frequencies, with an extended toe region up to approximately 30% strain, (e.g. Fig. 5), and increased stress with increased frequency.

    There were significant differences in the stress between all locations, with the subcalcaneal (89.574.0 kPa) having the highest values and the 3rd submetatarsal (70.374.0 kPa) having the lowest values. Further, the stress increased with frequency.

_{Figure 5 Stress-strain response of typical 5th submetatarsal area}

    The subcalcaneal location (0.8370.03 MPa) had a significantly larger modulus than the other 5 locations (ranging from 0.6770.03 to 0.7470.03MPa, Fig. 6a). The modulus showed frequency dependence as it was significantly higher at 1Hz and at 10 Hz (Fig. 6b).

    Energy loss also varied significantly across locations, with the subcalcaneal location having the least energy loss  (36.073.0%), followed by the 5th submetatarsal (43.073.0%). The energy loss also increased significantly at higher frequencies.

_{Figure 6 Modulus as a function of location and frequency; * = significant difference from all others}

Discussion

    The purpose of this study was to explore the material properties of the plantar soft tissue in young, healthy specimens at the subhallucal, the 1st, 3rd and 5th submetatarsal, the lateral submidfoot and the subcalcaneal locations. QLV models of the soft tissue showed the calcaneus and 5th metatarsal areas required more time than other areas to achieve steady state. Data from triangle wave experiments demonstrated that the material properties of the plantar soft tissue were dependent on specimen location and testing frequency. The subcalcaneal tissue had different material properties from the other locations, with a significantly greater modulus and less energy loss. Tissue from all locations were found to be rate sensitive, as the peak stress, modulus and energy loss were significantly greater for the 1 and 10 Hz tests. The general results from this study - that the subcalcaneal tissues are different from tissues obtained at other locations and that the all the plantar soft tissues are strain rate sensitive- will guide the development of material models to account for location specific differences as well as strain rate dependence of the tissue. The data collected here can be employed to generate geometry independent computational models of the plantar soft tissue for use in complex finite element models of the foot. These sophisticated, validated models of the foot that in turn can be used to addressed complicated questions not easily answered with living subjects, such as: ‘‘What is the shear stress between the skin and fat layer beneath the metatarsal heads during turning?’’ and ‘‘What are the optimal orthotic properties to reduce stress beneath the metatarsal heads?’’ Furthermore, application of the methods presented herein can also be used to characterize differences between healthy and diabetic and older tissue to increase the fundamental understanding of the disease process. Implementing the knowledge gained will reduce the burden of ulceration and amputation on diabetic patients.

Conclusions

We have demonstrated that the plantar soft tissue has different compressive material properties at the subcalcaneal area and is frequency sensitive across all locations. These results have important implications for computational foot models, as material models of the plantar soft tissue need to account for location and frequency.

Acknowledgements

This work was supported in part by the Department of Veterans Affairs, Rehabilitation Research & Development Service Grant nos. A2362P and A2661C and is currently being supported by NIH Grant no. 1R01DK075633.

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