Elsevier

Journal of Biomechanics

Volume 53, 28 February 2017, Pages 191-195
Journal of Biomechanics

Short communication
Spatial-dependent mechanical properties of the heel pad by shear wave elastography

https://doi.org/10.1016/j.jbiomech.2017.01.004Get rights and content

Abstract

The heel pad plays an important role in gait, and its biomechanical behavior and functionality are determined by its specialized architecture and mechanical properties. The purpose of this study was to apply supersonic shear wave elastography, an ultrasound-based noninvasive modality that can quantitatively estimate the shear stiffness of the tissue, to investigate the spatial-dependent mechanical properties of the heel pad. Measurements were conducted in 40 heel pads of 20 normal participants aged between 20 and 30 years by shear wave elastography. The continuous change in local shear stiffness of the heel pad along the depth direction of the heel pad was measured. The result showed that the mechanical properties of the heel pad were highly heterogeneous but followed a simple and specific pattern that local heel pad shear stiffness was highest beneath the plantar skin and decreased continuously with increasing depth. This finding provides a better understanding of the heel pad biomechanics and basis for further investigation of the heterogeneous properties of the heel pad.

Introduction

The heel pad plays an important role during locomotion in functioning as a shock absorber for reducing impact force transmitted to the human body and a load-carrying system for resisting compressive loading (Ker et al., 1989, Naemi et al., 2016a, Rome, 1998). Histologically, the heel pad is a highly specialized structure formed by a matrix of fibrous connective tissues arranged in septa containing closely packed fat tissues (Blechschmidt, 1934, Jahss et al., 1992), and it is this specialized architecture determines its biomechanical behavior (Jørgensen, 1985, Miller-Young et al., 2002, Naemi and Chockalingam, 2013) and gives it such great shock-absorbing and load-carrying capabilities (De Clercq et al., 1994, Rome, 1998); the resilient fat tissues are responsible for cushioning while the strong fibrous septa structure are designed to resist compressive loading (Naemi and Chockalingam, 2013, Rome, 1998). Injury, disease and aging deteriorate the architecture and mechanical properties of the heel pad and its functionality (Kwan et al., 2010, Natali et al., 2012, Rome, 1998). Understanding normal and pathological heel pad properties is important for developing improved diagnosis and treatment strategies, and designing methods to facilitate restoration of functionality.

It was not until the study by Blechschmidt that people began to understand the heel pad was actually heterogeneous in nature and arranged into a superficial microchamber and a deep macrochamber layer (Blechschmidt, 1934). Hsu and his colleagues identified the microchamber and macrochamber layers by B-mode ultrasonography and measured their stiffness by an ultrasound-based mechanical device (Hsu et al., 2007, Hsu et al., 2009). They applied a compression force to the heel pad and monitored the corresponding deformations of the respective layers by ultrasonography. The stiffness of each layer can then be obtained through dividing the magnitude of force by the deformation. Their studies provided a foundation for understanding the heterogeneous properties of the heel pad.

Supersonic shear wave elastography (SWE), an ultrasound-based modality that can noninvasively quantify the stiffness of the tissue by estimating its shear elastic modulus (Bercoff et al., 2004), provides an appropriate tool to explore the heterogeneous architecture and mechanical properties of the heel pad in more detail. It uses an acoustic radiation force to generate shear waves propagating in the tissue, and measures the shear wave speed that can be used to estimate the tissue shear modulus by the equation μ=ρcs2, where ρ is the tissue density typically assumed as 1000 kg/m3, cs is the shear wave speed and μ is the tissue shear modulus. Shear modulus is a measure of the tissue stiffness along the shear direction, referred to as “stiffness” in the following text. SWE can freely measure the local stiffness of a region of interest (ROI) throughout the tissue as long as ultrasound waves can be successfully generated and captured, therefore is capable of providing a more detailed evaluation. In literature, SWE has been applied to investigate stiffness of the heel pad and its respective layers (Lin et al., 2015), and spatial-dependent stiffness of the Achilles tendon (DeWall et al., 2014).

The purpose of this study was to explore the spatial-dependent mechanical properties of the heel pad by investigating the continuous change in local stiffness of the heel pad along the depth direction by SWE.

Section snippets

System

SWE measurement was performed by the Aixplorer® ultrasound system (SuperSonic Imagine, Aix-en-Provence, France) with a linear array transducer (SuperLinear SL15-4). The tissue preset and SWE optimization settings of the system were assigned as general and penetration, respectively. Other settings were defaults. The SWE mode displays tissue stiffness by a color elasticity map superimposed on a corresponding B-mode image. The color elasticity map represents stiffness by colors ranged from red

Results

For each measurement line, local heel pad stiffness decreased continuously with increasing depth from the plantar skin toward the calcaneus (Fig. 2). The maximum stiffness value (23.2±4.1, 23.5±4.5, and 23.7±4.0 kPa for the three measurement lines, respectively) occurred at the ROI (0–1 mm) nearest to the plantar skin, and the minimum (6.2±2.0, 5.7±2.3, and 4.5±2.5 kPa) occurred at the deepest ROI (9–10 mm). This continuous decreasing trend could be described by an exponential function with a

Discussion

Previously, it has been shown that the heel pad is heterogeneous in nature, and is arranged into a superficial microchamber layer containing small fat chambers and a deep thick macrochamber layer containing big fat chambers (Blechschmidt, 1934, Buschmann et al., 1995). The microchamber is much thinner and stiffer than the macrochamber (Hsu et al., 2007, Hsu et al., 2009). The present study applied SWE to further demonstrate that the mechanical properties of the heel pad behaved in a more

Conflict of interest statement

The authors declare that there is no conflict of interest relevant to this study.

Acknowledgements

The authors acknowledge the financial support provided by the Ministry of Science and Technology of Taiwan (Grant number: MOST 103-2314-B-002-167-MY3).

Cited by (17)

  • Increased exposure to loading is associated with increased plantar soft tissue hardness in people with diabetes and neuropathy

    2022, Diabetes Research and Clinical Practice
    Citation Excerpt :

    A recent in vivo and computational analysis has revealed that specific changes in the mechanical behaviour of plantar soft tissue can significantly undermine the tissue’s ability to fulfil its mechanical role making it more vulnerable to overload injury and ulceration [5]. These findings point to a direct relationship between plantar soft tissue biomechanics and the risk for ulceration and highlight the importance of tissue biomechanics for reliable risk assessment and effective prevention of diabetic foot ulceration [5–12]. In a seminal study on the effect of diabetes on plantar soft tissue biomechanics, Piaggesi et al.(1999) [13] observed that people with diabetes and peripheral neuropathy tend to have harder plantar soft tissues compared to their non-diabetic or diabetic non-neuropathic counterparts.

  • Plantar Soft Tissue

    2022, Foot and Ankle Biomechanics
  • Finite Element Modeling

    2022, Foot and Ankle Biomechanics
  • Plantar Soft Tissue Characterization Using Reverberant Shear Wave Elastography: A Proof-of-Concept Study

    2022, Ultrasound in Medicine and Biology
    Citation Excerpt :

    Shear wave speed (SWS) has been used to assess the in vivo non-linear mechanical behavior of the heel pad with a commercially available SWE method (Chatzistergos et al. 2018). However, this application is limited as unidirectional shear wave propagation is not fully achieved because plantar soft tissue consists of a multilayer structure where, close to the bone, it generates excess reflections from its surface (Lin et al. 2017a, ; Wu et al. 2018). The novel elastography technique based on the application of a reverberant shear wave field (Parker et al., 2017,Parker et al., 2011 ) leverages wave reflections produced by internal inhomogeneities, organ boundaries, hard structures and physiological activity through application of multiple external vibration sources.

  • Effects of water immersion on sensitivity and plantar skin properties

    2018, Neuroscience Letters
    Citation Excerpt :

    Not only plantar sensitivity, but also plantar mechanical properties of the skin have been investigated. There are studies examining plantar skin mechanical properties (e.g. [7]), plantar tactile sensitivity (e.g. [8]), or both (e.g. [9]). Lin et al. [7] found that plantar shear stiffness is highest at the level of the skin, and decreases with increasing depth of the tissue.

View all citing articles on Scopus
View full text