April 1, 2015

Study: Experimental and Computational Analysis of Composite AFOs

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An international team of researchers conducted a study of a software-based tool that can predict the mechanical behavior of composite material AFOs and thermoplastic AFOs to improve orthotists’ efficiency when fabricating composite material AFOs, such as those made from carbon fiber. The authors noted that posterior leaf spring AFOs—with low energy storage and energy return capabilities—are generally made with thermoplastics because of the material’s easy molding and rapid fabrication capabilities. However, carbon fiber AFOs provide improved ankle plantarflexor power, plantarflexor ankle joint moment, walking speed, stride length, and decreased energy cost, but can require costly design modifications.

The software described in the study used finite element analysis (FEA) modeling to predict AFO performance under different loading conditions, such as walking or running, and possible failure and fatigue life prior to fabrication. For carbon fiber orthoses, the tool would allow an orthotist to pre-establish design factors such as fiber orientation, thickness, and number and type of plies, thereby improving AFO performance and reducing manufacturing costs.

Three AFOs, two made of carbon fiber composite material and one made of thermoplastic material, were fabricated and then mechanically tested to produce force-displacement data. The FEA models were validated by comparing the predictions with mechanical testing data performed under the same stresses and strains. The study’s simulation results show that FEA models produced accurate predictions for both types of AFOs, and that carbon fiber AFOs perform better than those fabricated from thermoplastic.

“By creating a tool to predict energy storage and release based on a patient’s physical characteristics and gait parameters, AFO prescription could be more standardized and the function of AFOs increased for many individuals,” the study says. “We conclude that highly accurate FEA models will allow orthotists to improve CF [carbon fiber] AFO fabrication without wasting resources (time and money) on trial-and-error fabrications that are expensive and do not consistently improve AFO and user performance.”

The study was published in Volume 51-10, 2014, of the Journal of Rehabilitation Research & Development.

(Editor’s note: Although Volume 51-10 is dated 2014, it was published online in March 2015.)

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