A quantitative analysis of optimum design for rigid ankle foot orthoses: The effect of thickness and reinforcement design on stiffness

Panagiotis E. Chatzistergos (Lead / Corresponding author), Nicola Eddison, Evangelia Ganniari-Papageorgiou, Nachiappan Chockalingam

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Abstract

Background: An ankle foot orthosis (AFO) which is prescribed to be rigid should only deform a small amount to achieve its clinical goals. Material thickness and the design of reinforcing features can significantly affect AFO rigidity, but their selection remains based on anecdotal evidence. Objectives: To quantify the effect of these parameters on AFO stiffness and to set the basis for quantitative guidelines for the design optimisation of rigid AFOs. Study design: Experimental and computational study. Methods: A polypropylene AFO was produced according to UK standard practice and its stiffness was experimentally measured for 30Nm of dorsiflexion. Its geometry and mechanical characteristics were utilised to create a finite element (FE) model of a typical AFO prescribed to be rigid. Following validation, the model was used to quantify the effect of material thickness and reinforcement design (i.e., reinforcement placement, length) on stiffness. A final set of AFO samples was produced to experimentally confirm key findings. Results and conclusions: For a specific AFO geometry and loading magnitude, there is a thickness threshold below which the AFO cannot effectively resist flexion and buckles. FE modelling showed that stiffness is maximised when reinforcements are placed at the anterior-most position possible. This key finding was also experimentally confirmed. The stiffness of an AFO reinforced according to standard practice with lateral and medial ribbing was 4.4 6 0.1 Nm/degree. Instructing the orthotic technician to move the ribbings anteriorly increased stiffness by 22%. Further stiffening is achieved by ensuring the reinforcements extend from the footplate to at least two-thirds of the AFO’s total height.

Original languageEnglish
Pages (from-to)204-212
Number of pages9
JournalProsthetics and Orthotics International
Volume48
Issue number2
Early online date13 Jun 2023
DOIs
Publication statusPublished - 1 Apr 2024

Keywords

  • AFO
  • dorsiflexion
  • finite element modeling
  • orthoses
  • stiffness

ASJC Scopus subject areas

  • Health Professions (miscellaneous)
  • Rehabilitation

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