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Research Papers

Assessment of a Virtual Functional Prototyping Process for the Rapid Manufacture of Passive-Dynamic Ankle-Foot Orthoses

[+] Author and Article Information
Elisa S. Schrank

Department of Kinesiology and
Applied Physiology,
University of Delaware,
5 Innovation Way,
Suite 300,
Newark, DE 19711
e-mail: schranke@udel.edu

Richard Moore

Advanced Design and Manufacturing Division,
U.S. Army Edgewood Chemical
Biological Center,
Edgewood, MD 21010

Steven J. Stanhope

Department of Mechanical Engineering,
Biomechanics and Movement Science
Interdisciplinary Program,
Department of Kinesiology and
Applied Physiology,
Department of Biomedical Engineering,
University of Delaware,
Newark, DE 19711

1Corresponding author.

Contributed by the Bioengineering Division of ASME for publication in the Journal of Biomechanical Engineering. Manuscript received January 30, 2013; final manuscript received June 6, 2013; accepted manuscript posted June 17, 2013; published online September 20, 2013. Assoc. Editor: Kenneth Fischer.

J Biomech Eng 135(10), 101011 (Sep 20, 2013) (7 pages) Paper No: BIO-13-1058; doi: 10.1115/1.4024825 History: Received January 30, 2013; Revised June 06, 2013; Accepted June 17, 2013

Passive-dynamic ankle-foot orthosis (PD-AFO) bending stiffness is a key functional characteristic for achieving enhanced gait function. However, current orthosis customization methods inhibit objective premanufacture tuning of the PD-AFO bending stiffness, making optimization of orthosis function challenging. We have developed a novel virtual functional prototyping (VFP) process, which harnesses the strengths of computer aided design (CAD) model parameterization and finite element analysis, to quantitatively tune and predict the functional characteristics of a PD-AFO, which is rapidly manufactured via fused deposition modeling (FDM). The purpose of this study was to assess the VFP process for PD-AFO bending stiffness. A PD-AFO CAD model was customized for a healthy subject and tuned to four bending stiffness values via VFP. Two sets of each tuned model were fabricated via FDM using medical-grade polycarbonate (PC-ISO). Dimensional accuracy of the fabricated orthoses was excellent (average 0.51 ± 0.39 mm). Manufacturing precision ranged from 0.0 to 0.74 Nm/deg (average 0.30 ± 0.36 Nm/deg). Bending stiffness prediction accuracy was within 1 Nm/deg using the manufacturer provided PC-ISO elastic modulus (average 0.48 ± 0.35 Nm/deg). Using an experimentally derived PC-ISO elastic modulus improved the optimized bending stiffness prediction accuracy (average 0.29 ± 0.57 Nm/deg). Robustness of the derived modulus was tested by carrying out the VFP process for a disparate subject, tuning the PD-AFO model to five bending stiffness values. For this disparate subject, bending stiffness prediction accuracy was strong (average 0.20 ± 0.14 Nm/deg). Overall, the VFP process had excellent dimensional accuracy, good manufacturing precision, and strong prediction accuracy with the derived modulus. Implementing VFP as part of our PD-AFO customization and manufacturing framework, which also includes fit customization, provides a novel and powerful method to predictably tune and precisely manufacture orthoses with objectively customized fit and functional characteristics.

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Figures

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Fig. 2

(a) Schematic of experimental stiffness testing device; (b) PD-AFO aligned in experimental stiffness testing device; and (c) CAD model assembly of PD-AFO, cuff insert, and testing rod with applied constraints and loads used during the virtual bending stiffness determination

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Fig. 3

Virtual bending stiffness determination using FEA. Load magnitude, distance from AJC to load application and FEA-determined vertical displacement of testing rod used to calculate the virtual bending stiffness.

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Fig. 4

Influence of strut scale factor on strut profile. PD-AFO strut thickness scaled by a strut scale factor of (a) 50%, (b) 100%, and (c) 130%.

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Fig. 5

Fabricated PD-AFOs. Orthoses A–D in alphabetical order from right to left.

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Fig. 6

Representative graph of experimental bending stiffness testing results. Experimentally derived bending stiffness calculated as the ratio of the moment to angle at 20 deg deformation for each of the six trials performed within the experimental testing session shown. Experimental bending stiffness calculated as average of the six trials.

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Fig. 7

Bending stiffness prediction accuracy. Experimental bending stiffness (Exp.) and virtual bending stiffness (Virt.) measures for all eight fabricated PD-AFOs.

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Fig. 8

Optimized bending stiffness prediction accuracy. Experimental bending stiffness measures (Exp.) compared to original virtual bending stiffness (Orig.) using manufacturer provided elastic modulus and optimized virtual bending stiffness (Opt.) using derived elastic modulus for orthoses A2, B2, C2, and D2.

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