Segment Model Valid Method for Determining Total Power of Transtibial Structures

When compared with an anatomically relevant biomechanical model, a unified deformable segment model is a valid generalized method for determining the quantity of total power of transtibial structures, according to results from a recent study.

“When we design prosthetics and orthotics, what we’re trying to do, ideally, is replicate the function of natural ankle/foot systems. Many of the advanced technologies that are being used now don’t mimic anatomical structures like ankle joints and feet, but they may be spring-like devices that do not have specific joints,” Steven Stanhope, PhD, professor at the University of Delaware and principal in Intelligent Digital Manufacturing LLC, told O&P Business News. “As a result of that, it’s been remarkably difficult to be able to compare the power storage and return characteristics generated by some prosthetic systems to the natural systems. For the past couple of decades, scientists have been writing about those difficulties, so we decided to roll up our sleeves and see if we can solve it.”

UD vs. AR

Eleven healthy participants were enrolled in a fully-instrumented gait analysis where estimates of total transtibial power derived by a unified deformable (UD) segment model — a hybrid segment comprising a proximal rigid component with an in-series distal deformable component — were compared with those derived by an anatomically relevant (AR) model, comprising anatomically congruent segments and joints. Researchers used a six-camera motion capturing system to collect kinematic data and a strain gage force platform to collect kinetic data.


Steven Stamhope


The researchers obtained the estimates of total power via the AR model by the summation of the total ankle joint power, which was predominately negative for approximately the first 76% of stance and positive for the remainder of stance, and the distal foot segmental power, which showed a period of prominent negative power during early and late stance. After combining the two measures, study results showed that AR was characterized by a period of negative power for approximately the first 80% of stance, followed by a period of positive power for the remainder of stance.

According to study results, total power derived via the UD model demonstrated similar patterns to AR throughout most of stance, with a period of negative power for approximately the first 80% of stance, followed by a period of positive power for the remainder of stance. Researchers found the maximum deviation between the two methods occurred during late stance, where UD was consistently higher than AR. Overall, the main advantage of the UD segment model is that it did not require the definition of an ankle joint or foot structures.

“We should now be able to measure accurately the mechanical power profiles of amputee or prosthetic ankle/foot systems, perhaps even higher than the ankle, and relate that information directly back to how normal individuals function,” Stanhope said. “We hope that will really enhance the clinicians’ ability in the future to select the most appropriate system.”

A bigger initiative

Currently, Stanhope and colleagues are in the process of submitting papers that look at network ratios and determining the ratio of positive power to negative power in natural ankle/foot systems. They have also been working on a project to create function customized AFOs funded by the Defense Advanced Research Projects Agency. According to Stanhope, all of these studies connect to a larger initiative being worked on at the University of Delaware.

“This study, along with a whole series of studies that are related to this, relate to an initiative we’ve had here at the University of Delaware, which is on the rapid manufacture of personalized rehabilitation devices,” Stanhope said. “The concept is to be able to do intelligent digital manufacturing for mass customization of devices and leverage 3-D printing technologies such that we can customize through direct digital manufacturing techniques, not only to fit the function of the device for an individual but also to try to improve the objectivity with which we actually design and manufacture the devices. It’s part of a much broader initiative.”— by Casey Murphy

For more information:
Takahashi KZ. J Biomech. 2012;45:2662-2667.

Disclosure: Stanhope is employed at Intelligent Digital Manufacturing, LLC.

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