August 2012

High-tech heel pad tissue analysis: Orthotic implications

By Jordana Bieze Foster

The use of technology to improve orthotic design is not limited to CAD-CAM applications, as demonstrated by University of Salford researchers in two presentations at the Orthotics Technology Forum.

Daniel Parker, a doctoral student in the university’s School of Health Sciences, and colleagues have developed a machine that replicates the forces that occur under the heel during the gait cycle and assesses tissue response to those forces.

The Soft Tissue Response Imaging Device, affectionately known as STRIDE, operates with the patient standing on a platform with his or her foot braced to limit motion. A cylindrical column positioned under the heel and driven by an actuator applies force to the tissue, and tissue changes are assessed using ultrasound and a linear variable displacement transducer.

Variables that can be measured or calculated from STRIDE data include stress, strain, compressibility, energy dissipation, and stiffness.

“The tissue characterization can be used to identify differences between individuals,” Parker said. “We are currently looking at older versus younger individuals to see if there are tissue changes over time in people who don’t have pathology.”

Other potential applications include the ability to adjust the material properties of implants (to better mimic tissue) or orthotic devices (to improve tissue response) and to adjust the positioning of such devices to address key areas.

“The benefit is to inform the development of clinical or performance interventions,” Parker said.

Additional applications may be possible by combining the benefits of STRIDE with those of finite element analysis (FEA), a type of computer modeling in which an object is segmented into elements so the model can calculate the effect of an applied force on each of those elements rather than on the object as a whole.

Nafiseh Ahanchian, also a doctoral student in the School of Health Sciences, has used some of the STRIDE findings to develop an FEA model of heel pad behavior. The initial model included anatomical data from magnetic resonance imaging scans about the different tissue structures within the heel and values from the medical literature for such variables as shear modulus and change in high-strain behavior. The calculated displacement was then compared to the actual experimental displacement and the relevant variables adjusted such that the model results better approximated the experimental results.

“This model can be used to examine a large number of footwear designs, without the burden of a high volume of experimentations, to predict stress in the heel pad under varied shoe or insole conditions,” Ahanchian said.  “Understanding the behavior of the heel pad might assist with investigating the mechanical functionality of the foot and the design of footwear.”

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