title: The design and validation of a magnetic resonance imaging-compatible device for obtaining mechanical properties of plantar soft tissue via gated acquisition.
abstract: Changes in the mechanical properties of the plantar soft tissue in people with diabetes may contribute to the formation of plantar ulcers. Such ulcers have been shown to be in the causal pathway for lower extremity amputation. The hydraulic plantar soft tissue reducer (HyPSTER) was designed to measure in vivo, rate-dependent plantar soft tissue compressive force and three-dimensional deformations to help understand, predict, and prevent ulcer formation. These patient-specific values can then be used in an inverse finite element analysis to determine tissue moduli, and subsequently used in a foot model to show regions of high stress under a wide variety of loading conditions. The HyPSTER uses an actuator to drive a magnetic resonance imaging-compatible hydraulic loading platform. Pressure and actuator position were synchronized with gated magnetic resonance imaging acquisition. Achievable loading rates were slower than those found in normal walking because of a water-hammer effect (pressure wave ringing) in the hydraulic system when the actuator direction was changed rapidly. The subsequent verification tests were, therefore, performed at 0.2 Hz. The unloaded displacement accuracy of the system was within 0.31%. Compliance, presumably in the system's plastic components, caused a displacement loss of 5.7 mm during a 20-mm actuator test at 1354 N. This was accounted for with a target to actual calibration curve. The positional accuracy of the HyPSTER during loaded displacement verification tests from 3 to 9 mm against a silicone backstop was 95.9% with a precision of 98.7%. The HyPSTER generated minimal artifact in the magnetic resonance imaging scanner. Careful analysis of the synchronization of the HyPSTER and the magnetic resonance imaging scanner was performed. With some limitations, the HyPSTER provided key functionality in measuring dynamic, patient-specific plantar soft tissue mechanical properties.
authors.affiliation: RR&D Center of Excellence for Limb Loss Prevention and Prosthetic Engineering, VA Puget Sound Health Care System, Seattle, WA, USA Department of Mechanical Engineering, University of Washington, Seattle, WA, USA.
authors.forename: Evan D
authors.initials: ED
authors.lastname: Williams
authors.affiliation: RR&D Center of Excellence for Limb Loss Prevention and Prosthetic Engineering, VA Puget Sound Health Care System, Seattle, WA, USA Department of Mechanical Engineering, University of Washington, Seattle, WA, USA.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Stebbins
authors.affiliation: Department of Mechanical Engineering, University of Washington, Seattle, WA, USA Department of Orthopaedics & Sports Medicine, University of Washington, Seattle, WA, USA.
authors.forename: Peter R
authors.initials: PR
authors.lastname: Cavanagh
authors.affiliation: Department of Radiology, University of Washington, Seattle, WA, USA.
authors.forename: David R
authors.initials: DR
authors.lastname: Haynor
authors.affiliation: Department of Radiology, University of Washington, Seattle, WA, USA.
authors.forename: Baocheng
authors.initials: B
authors.lastname: Chu
authors.affiliation: RR&D Center of Excellence for Limb Loss Prevention and Prosthetic Engineering, VA Puget Sound Health Care System, Seattle, WA, USA.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Fassbind
authors.affiliation: RR&D Center of Excellence for Limb Loss Prevention and Prosthetic Engineering, VA Puget Sound Health Care System, Seattle, WA, USA Department of Mechanical Engineering, University of Washington, Seattle, WA, USA.
authors.forename: Vara
authors.initials: V
authors.lastname: Isvilanonda
authors.affiliation: RR&D Center of Excellence for Limb Loss Prevention and Prosthetic Engineering, VA Puget Sound Health Care System, Seattle, WA, USA Department of Mechanical Engineering, University of Washington, Seattle, WA, USA Department of Orthopaedics & Sports Medicine, University of Washington, Seattle, WA, USA wrledoux@uw.edu.
authors.forename: William R
authors.initials: WR
authors.lastname: Ledoux
mesh_terms: D001696:Biomechanical Phenomena / Q000502:physiology*; D004867:Equipment Design; D005528:Foot / Q000502:physiology*; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000295:instrumentation* / Q000379:methods; D008954:Models, Biological; D016138:Walking / Q000502:physiology*
keywords: Heel pad; diabetes; magnetic resonance imaging–compatible; mechanical properties