Medical Engineering & Physics 34 (2012) 1253–1259 Contents lists available at SciVerse ScienceDirect Medical Engineering & Physics jou rnal h omepa g e: www.elsevier.com/locate/medengphy Investigation on the load-displacement curves of a human healthy heel pad: In vivo compression data compared to numerical results C.G. Fontanella a,∗ , S. Matteoli b , E.L. Carniel a , J.E. Wilhjelm b , A. Virga c , A. Corvi c , A.N. Natali a a Centre of Mechanics of Biological Materials, University of Padova, Via F. Marzolo 9, 35131 Padova, Italy b Department of Electrical Engineering, Technical University of Denmark, Ørsteds Plads, DK-2800, Kgs. Lyngby, Denmark c Department of Mechanics and Industrial Technologies, University of Florence, via S. Marta 3, 50139 Florence, Italy a r t i c l e i n f o Article history: Received 29 June 2011 Received in revised form 12 December 2011 Accepted 14 December 2011 Keywords: Heel pad analysis Experimental tests Visco-elasticity Numerical model a b s t r a c t The aims of the present work were to build a 3D subject-specific heel pad model based on the anatomy revealed by MR imaging of a subject’s heel pad, and to compare the load–displacement responses obtained from this model with those obtained from a compression device used on the subject’s heel pad. A 30 year-old European healthy female (mass = 54 kg, height = 165 cm) was enrolled in this study. Her left foot underwent both MRI and compression tests. A numerical model of the heel region was developed based on a 3D CAD solid model obtained by MR images. The calcaneal fat pad tissue was described with a visco-hyperelastic model, while a fiber-reinforced hyperelastic model was formulated for the skin. Numerical analyses were performed to interpret the mechanical response of heel tissues. Different loading conditions were assumed according to experimental tests. The heel tissues showed a non-linear visco-elastic behavior and the load–displacement curves followed a characteristic hysteresis form. The energy dissipation ratios measured by experimental tests (0.25 ± 0.02 at low strain rate and 0.26 ± 0.03 at high strain rate) were comparable with those evaluated by finite element analyses (0.23 ± 0.01 at low strain rate and 0.25 ± 0.01 at high strain rate). The validity and efficacy of the investigation performed was confirmed by the interpretation of the mechanical response of the heel tissues under different strain rates. The mean absolute percentage error between experimental data and model results was 0.39% at low strain rate and 0.28% at high strain rate. © 2011 IPEM. Published by Elsevier Ltd. All rights reserved. 1. Introduction The human heel pad is a complex structure consisting of a fat pad with micro- and macro-chambers separated by an intricate fibro- elastic septation. Individual fat cells and fat globules are sealed off by delicate membranes [1], creating a so-called honeycomb con- figuration [2]. The structure of the septation is capable of bonding fat tissue from single compartments and hence such compartments are resistive to compressive loads [3,4]. The heel pad acts as an effi- cient shock absorber, smoothing the effects of impact forces during gait. The heel pad exhibits non-linear visco-elastic behavior [5,6], like most soft biological tissue does. Due to the visco-elastic nature of the heel pad and its capability to deform under load- ing, when a loading/unloading cycle is applied a load–displacement curve is obtained [7] showing a characteristic hysteresis [8,9]. The ∗ Corresponding author at: University of Padova, Centre of Mechanics of Biological Materials, Via F. Marzolo 9, I-35131 Padova, Italy. Tel.: +39 049 827 5605; fax: +39 049 827 5604. E-mail address: chiaragiulia.fontanella@unipd.it (C.G. Fontanella). mechanical behavior of the heel pad is determined by the mechan- ical responses of the adipose tissues and skin and by the interaction phenomena between these two tissues. Under compression load- ing, the heel pad initially has low stiffness, then the collagen fibers of the fat pad and skin come under tension, limiting the defor- mation of the tissue and resulting in an increase of the stiffness [10–12]. A way to perform a loading/unloading cycle and deter- mine the mechanical behavior of heel pad is to use a mechanical compression device [13–15,16–21]. The analysis of hysteresis curves obtained from experimental tests on human heel pad allows for quantitative and qualitative investigation of the mechanical response of the tissue. Knowl- edge of the biomechanical properties of healthy and diseased heel pads may be used for screening patients (e.g. diabetic subjects, heel pain subjects, etc.) and for the prevention of pathologies. The experimental approach can be investigated and integrated with a computational model of the heel pad. Such a model would allow a better understanding of the stress–strain relationship of the tis- sues, in order to evaluate phenomena that are not measurable with sufficient accuracy by means of experimental tests. Finite ele- ment modeling of the soft tissues of the foot would pave the way for understanding stress-related injuries (e.g. plantar fasciitis and 1350-4533/$ – see front matter © 2011 IPEM. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.medengphy.2011.12.013