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.
Highlights
The foot is the most distal part of the human leg and functions to transmit our weight to the ground, balance our posture, and assist in locomotion [1]
The force versus strain plots corresponding to the biomechanical behavior of healthy calcaneal heel pads tested in vivo were reproduced (Figure 4)
Similar plots for calcaneal heel pads tested in vivo for patients with plantar pain and diabetes were reproduced (Figure 4)
Summary
The foot is the most distal part of the human leg and functions to transmit our weight to the ground, balance our posture, and assist in locomotion [1]. The heel is the first point of contact between the body and the ground, and is the primary absorber of the ground reaction force. The calcaneus, or heel bone, is supported and cushioned by the calcaneal fat pad that functions as a shock absorber (see Figure 1a). The average heel fat pad is 18 mm thick, and is composed of honeycombed fat globules organized into a bilayer: the macrochamber and microchamber (Figure 1b). These globules are held together by a collagenous septum. The organization of the fat pad into macrochambers and microchambers affects the nonlinear anisotropic material behavior of the heel pad [2]. Microchambers have been found to experience very little deformation under loading conditions and have a modulus of elasticity ten times greater than that of Biomimetics 2019, 4, 1; doi:10.3390/biomimetics4010001 www.mdpi.com/journal/biomimetics
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