Considerations for optimising the design of rigid ankle-foot orthoses for children with cerebral palsy.

Donald, Sean

Abstract

Cerebral Palsy (CP) is the leading cause of childhood motor disabilities, resulting in a
variety of gait impairments. Ankle-foot Orthoses (AFOs) are key to managing these im-
pairments, aiming to facilitate weight-bearing activities and normalise lower limb biome-
chanics. To optimise their performance, the Optimal Segment Kinematic Alignment
Approach to Rehabilitation (OSKAR) has been recommended[1], [2], which focuses on
manipulating shank kinematics, to normalise Ground Reaction Force (GRF) alignment
and external joint moments. However, the ability to control and alter shank kinematics
depends on the stiffness of a rigid Ankle Foot Orthosis (AFO), which is influenced by its
design. Despite this, there are no clinical guidelines on how to tailor the design of rigid
AFOs to the characteristics of the individual.
Therefore, this research aims to explore the impact of rigid AFO design factors on
stiffness, before applying this knowledge to move towards an optimised rigid AFO de-
sign. To achieve these aims, Finite Element (FE) models, simulating the mechanical
behaviour of AFOs designed for a 5-, 10- and 15-year-old child, were validated against
data from a novel test rig. Then they were utilised in two primary research studies.
The first study investigated the mechanical properties of AFOs designed according to
current practice, to determine the impact of thermoplastic rigidity and thickness, trim
line design and ribbing on AFO stiffness. This demonstrated that AFO thickness and
trim lines design had the biggest impact on AFO stiffness, whilst challenging some as-
pects of clinical practice, such as the placement of ribbing and the use of through the
malleoli trim lines.
These findings were then applied to develop a novel reinforcement and move towards an
optimal rigid AFO design. This process was directed by design criteria which covered
the mechanical, clinical and user requirements. It evaluated the effects of a prototype
design, which has the potential to be applied clinically to stiffen rigid AFOs, whilst being
more aligned to the needs of the user and clinician.
As a result, this thesis advanced the understanding of rigid AFO design for children
with CP. Furthermore, by integrating FE analysis with mechanical and clinical design
criteria, it outlined a new framework for rigid AFO design that links fundamental biome-
chanical principles to clinical applications, which with further refinement, could be used
to optimise the design of rigid AFOs for clinical practice.

• Thesis Type: Thesis
• Acceptance Date: Jan 21, 2025
• Online Publication Date: Mar 27, 2025
• Deposit Date: Feb 5, 2025
• Publicly Available Date: Apr 28, 2025
• Keywords: Cerebral Palsy, AFO, Gait
• Award Date: Mar 27, 2025