Obtaining Material Properties of the Plantar Soft Tissue for a Patient-Specific Finite Element Model of the Foot

Abstract

<italic>Background</italic>: People with diabetes account for just over 8% of the US population, but they undergo approximately 63% of all non-traumatic lower limb amputations, or 65,700 amputations per year. Eighty-five percent of those amputations are preceded by a foot ulcer. Diabetes has been shown to increase the stiffness of the plantar soft tissue in cadaveric samples, which could cause shifting of the magnitude and/or location of peak stresses within the foot. The purpose of this research project was to develop a magnetic resonance imaging (MRI) compatible, dynamic loading device that used cardiac-gated MRI imaging to obtain <italic>in vivo </italic>force versus deformation data for the plantar soft tissue. The resulting data will be used as inputs to an inverse finite element (FE) analysis to solve for soft tissue material properties in a patient-specific FE model. <italic>Methods </italic>: A computer-controlled, MRI-compatible loading device was designed, built, and tested in the laboratory to quantify performance parameters and ensure safe operation of the device before proceeding to pilot human studies. A test subject was loaded by the device while tissue thickness changes were measured via an ultrasound transducer attached to the loading device. Finally, an MRI pilot study was completed in which the heel of a test subject was dynamically loaded in compression at a 0.2 Hz rate inside an MRI while three-dimensional (3-D) images were obtained. <italic>Results</italic>: The amount of test subject movement and shift within the apparatus under loading were determined during ultrasound testing, as was a displacement calibration curve for normal human soft tissue. Force on the foot and displacement of the soft tissue during 12 phases of loading and unloading were obtained from the MRI pilot study, from which the stiffness of the plantar skin and adipose tissue was calculated to be 55 N/mm. <italic>Conclusion </italic>: An MRI-compatible, computer-controlled loading device was successfully designed, built, and used in a pilot human study. As the first device of its kind, it will prove beneficial to the research and medical communities by increasing understanding of the initiation of foot ulcers in people with diabetes.