Short communicationSpatial-dependent mechanical properties of the heel pad by shear wave elastography
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 , where is the tissue density typically assumed as 1000 kg/m3, is the shear wave speed and 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).
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