The effect of heel-pad thickness and loading protocol on measured heel-pad stiffness and a standardized protocol for inter-subject comparability

Background: Heel-pad stiffness is an important parameter in clinical assessments of the lower limb and is usually quantified by the slope of the force-deformation curve. However, the data produced is affected by the geometry of the heel, thus making inferences about tissue behaviour difficult.

Method: With the use of finite element analysis the aim of this study is to explore the possibility of expressing heel-pad stiffness in terms of stress-strain data. An axisymmetric, non-linear and time-dependent representation of the heel was created. The material model, incorporating non-linearity and viscoelasticity, was based on a series of experiments involving healthy, cadaveric specimens and loading at different loading rates (0, 175 and 350 mm/s). The conditions of an in vivo study were then replicated and stress-strain data of the model were compared. Good agreement was achieved (error <5%) at higher strains (>0.2). Probe diameter, loading rate and heel-pad thickness were then varied and heel-pad stiffness, expressed in terms of both force-deformation and stress-strain characteristics, reported.

Findings: In terms of the force-deformation characteristics, thin heels are consistently stiffer than thick heels. In terms of stress-strain characteristics, thicker heels are stiffer than thin heels using small probes whereas thinner heels are stiffer than thick heels using large probes. It was possible to predict stress-strain data of the heel-pad that are least-dependent of heel-pad thickness using large probes and slow-rising loads.

Interpretation: It is suggested that stress-strain curves derived from large probes under slow loads would provide the most robust and standardized measure of heel-pad stiffness.

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