Internal strain estimation for quantification of human heel pad elastic modulus: A phantom study

Karen Holst, Hervé Liebgott, Jens E. Wilhjelm, Svetoslav Nikolov, Søren T. Torp-Pedersen, Philippe Delachartre, Jørgen Arendt Jensen

Research output: Contribution to journalJournal articleResearchpeer-review

Abstract

Shock absorption is the most important function of the human heel pad. However, changes in heel pad elasticity, as seen in e.g. long-distance runners, diabetes patients, and victims of Falanga torture are affecting this function, often in a painful manner. Assessment of heel pad elasticity is usually based on one or a few strain measurements obtained by an external load-deformation system. The aim of this study was to develop a technique for quantitative measurements of heel pad elastic modulus based on several internal strain measures from within the heel pad by use of ultrasound images. Nine heel phantoms were manufactured featuring a combination of three heel pad stiffnesses and three heel pad thicknesses to model the normal human variation. Each phantom was tested in an indentation system comprising a 7MHz linear array ultrasound transducer, working as the indentor, and a connected load cell. Load-compression data and ultrasound B-mode images were simultaneously acquired in 19 compression steps of 0.1mm each. The internal tissue displacement was for each step calculated by a phase-based cross-correlation technique and internal strain maps were derived from these displacement maps. Elastic moduli were found from the resulting stress–strain curves. The elastic moduli made it possible to distinguish eight of nine phantoms from each other according to the manufactured stiffness and showed very little dependence of the thickness. Mean elastic moduli for the three soft, the three medium, and the three hard phantoms were 89kPa, 153kPa, and 168kPa, respectively. The combination of ultrasound images and force measurements provided an effective way of assessing the elastic properties of the heel pad due to the internal strain estimation.
Original languageEnglish
JournalUltrasonics
Volume53
Issue number2
Pages (from-to)439-446
ISSN0041-624X
DOIs
Publication statusPublished - 2013

Keywords

  • Elastic modulus
  • Strain maps
  • Force gauge
  • Heel pad

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