Mechanical Modeling of Healthy and Diseased Calcaneal Fat Pad Surrogates

Abstract

The calcaneal fat pad is a major load bearing component of the human foot due to daily gait activities such as standing, walking, and running. Heel and arch pain pathologies such as plantar fasciitis, which over one third of the world population suffers from, is a consequent effect of calcaneal fat pad damage. Also, fat pad stiffening and ulceration has been observed due to diabetes mellitus. To date, the biomechanics of fat pad damage is poorly understood due to the unavailability of live human models (because of ethical and biosafety issues) or biofidelic surrogates for testing. This also precludes the study of the effectiveness of preventive custom orthotics for foot pain pathologies caused due to fat pad damage. The current work addresses this key gap in the literature with the development of novel biofidelic surrogates， which simulate the in vivo and in vitro compressive mechanical properties of a healthy calcaneal fat pad. Also, surrogates were developed to simulate the in vivo mechanical behavior of the fat pad due to plantar fasciitis and diabetes. A four-part elastomeric material system was used to fabricate the surrogates, and their mechanical properties were characterized using dynamic and cyclic load testing. Different strain (or displacement) rates were tested to understand surrogate behavior due to high impact loads. These surrogates can be integrated with a prosthetic foot model and mechanically tested to characterize the shock absorption in different simulated gait activities, and due to varying fat pad material property in foot pain pathologies (i.e., plantar fasciitis, diabetes, and injury). Additionally, such a foot surrogate model, fitted with a custom orthotic and footwear, can be used for the experimental testing of shock absorption characteristics of preventive orthoses.

Effect of loading history on material properties of human heel pad: an in-vivo pilot investigation during gait

“…The average primary/ unloaded heel pad thicknesses reported in previous studies were ranged from 11.5 to 19.1 mm in healthy adults [ 9 , 22 , 32 , 33 ]. In addition, the primary heel pad thickness was proven to be related with the gender, age and physiques of subjects [ 34 , 35 ].…”

Expansion potential of auxetic prosthetic skin grafts: a review

“…[102]. In addition, different linear isotropic and non-linear hyperelastic materials models have been used to estimate the biaxial results [137][138][139][140].…”

Mechanical Characteristics of Diabetic and Non-Diabetic Plantar Skin

“…Current skin surrogates are often developed for use with a singular or limited range of use in mind (Bostan et al, 2016;Nachman and Franklin, 2016), such as to test the skin care industry, or for use in surgical simulations and thus often neglect the plantar aspect of the foot or provide simplified mechanical properties (Singh et al, 2022). Chanda (2018) recognised this need and have begun to develop elastomer surrogates alongside the development of calcaneal fat pad surrogates (Chanda and McClain, 2019) but plantar skin surrogate development is still under-served. The development of surrogates to recreate mechanical characteristics of skin and underlying tissue of the plantar foot allows for biofidelic test bed creation.…”