Method includes complex rotations
By Emily Delzell
A surrogate biomodel of a child’s lower extremities, in particular the ability to model movement along multiple axes of rotation, could help improve researchers’ biomechanical understanding of bracing for clubfoot.
The multicenter team of investigators who developed the biomodel reported on their testing of the complex surrogate in September in the Journal of Pediatric Orthopaedics (JPO).
Clinicians now achieve high rates of clubfoot deformity correction using the Ponseti method (See, “Clubfoot conversion: Nearly all surgeons adopt Ponseti method”); once corrected, however, children’s feet retain a stubborn tendency to relapse. Children who stop postcorrection brace use at 2 years have relapse rates of up to 56%; among children who wear braces until they are aged at least 4 years, the relapse rate is 11%, study authors reported.
“Dr. [Ignacio] Ponseti revolutionized treatment of children with clubfoot by turning a debilitating surgery into a gentle manipulation. His method, however, relies heavily on patient compliance to wear a brace for up to five years after correction,” said study lead author Andrew DiMeo Sr, PhD, director of industrial relations and assistant professor of the practice in the University Joint Department of Biomedical Engineering at North Carolina State University and the University of North Carolina at Chapel Hill.
In an attempt to improve both the bracing itself as well as patient compliance, DiMeo and colleagues designed a surrogate biomodel with the biomechanics of a 5-year-old child, featuring complete lower extremity anatomy with joint articulation and kinematic capabilities.
Investigators based the model on anthropometric data detailed in a reference text of normal human measurements.
“We then used the Ponseti Training Model as starting point, converting the physical model into image data using x-ray computed tomography, converting that image data into solid models, scaling the solid models to match that of an average 5-year-old, then using rapid prototyping to convert the scaled solid model back into a physical form,” DiMeo explained.
The biomodel’s components include skeletal structure constructed from acrylonitrile butadiene styrene (ABS) resin and muscle-tendon systems and ligaments made from springs, cables, and elastic cord. The JPO paper validated the biomodel’s reliability and repeatability for measuring forces applied by different braces, demonstrating an error rate between 1% and 6%.
Co-investigator Jose Morcuende, MD, PhD, associate professor of orthopedic surgery at the University of Iowa in Iowa City, told LER that problems of compliance with clubfoot bracing, which are largely dictated by patient comfort, are compounded by a lack of evidence and consensus regarding optimal brace para- meters, including spring lengths and loads applied to muscle-tendon systems. The biomodel, DiMeo said, can characterize and a compare a number of brace parameters, including bar width, external rotation, and dorsiflexion angle.
“We have clinical experience showing us what works and what doesn’t, but we’ve not had a good way of testing the best position to produce an optimal stretch without having difficulties with the knees and the hips,” he said. “Dr. DiMeo was able to demonstrate, for example, that a too-wide bar results in a knock-kneed effect. This is something we’re pretty well aware of clinically but haven’t been able to test in the lab until now.”
DiMeo, he noted, is preparing a second paper that details the use of the biomodel to compare an ankle foot orthosis and a flexible bar brace as well as different bracing parameters.
“The surrogate model allows unbiased comparisons between braces and brace parameters. For example, the standard of care suggests a brace be worn with the bar length set to shoulder width,” DiMeo said. “What happens if the width is set slightly wider or narrower? These subtle brace adjustments can have an impact on how well correction is maintained.”
Morcuende noted lessons gleaned from the biomodel will help clinicians determine which brace types best prevent children from experiencing relapse and further treatment.
“This is the first biomechanical model to test the different bracing parameters for clubfoot and is a unique, complex model that I think will be very helpful to test the available braces and to narrow down which ones are working and which ones are not,” he said.
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DiMeo AJ, Lalush DS, Grabt E, Morcuende JA. Development of a surrogate biomodel for the investigation of clubfoot bracing. J Pediatr Orthop 2012;32(7): e47-e52.