Data technologies for lower limb orthosis design and assessment

Abstract

Lower limb orthoses, such as ankle foot orthoses (AFOs) or foot orthoses (FOs), can reduce pain, manage deformity and improve mobility for individuals with a range of musculoskeletal and neurological conditions. However, bioengineering research into orthosis design has been limited, with little consideration of how devices affect interfacing skin and soft tissues. Regarding FOs, plantar pressure metrics have been considered, but how they relate to the underlying tissues is unclear. Additionally, their effectiveness for people living with foot deformities, e.g. people with rheumatoid arthritis (RA), is not well understood. Thus, improved understanding of how tissues interact with orthoses, in terms of the mechanical and thermal boundary and tissue damage risk, is needed. This would be key to ensuring devices are effective and designed to meet user needs. This research aimed to assess how lower limb orthosis design parameters affect soft tissue health and variation between individuals. A multi-modal approach was taken, combining experimental testing of AFOs and computational modelling of AFOs and FOs. In vivo pilot testing with five healthy participants examined plantar pressure metrics and microclimate with two AFO designs and two sock materials, during stationary standing and gait. The AFO with a more compliant material caused lower peak interface pressures and pressure gradients. Greater elevations in temperature and humidity occurred where the AFOs were more encompassing. Cotton socks controlled temperature better but humidity was higher than with bamboo socks. Simplified forefoot computational models were developed to assess orthosis design parameters in two scenarios: five healthy individuals wearing AFOs, and 13 individuals with RA wearing FOs. Plantar pressures and shear strains were assessed for the effects on superficial and deep soft tissues, respectively. As with the experimental testing, softer interface materials resulted in lower interface pressures for both AFOs (-30 to -49%) and FOs (EVA vs Poron materials mean: -1.9 to -2.2%, p<0.05), but shear strain generally increased or was unchanged (AFOs: -4.4 to 4.3%; FOs mean: 3.3-5.3%, p<0.01). Total contact FOs reduced all pressure and strain metrics compared to flat FOs. An AFO undersized across its width produced similar pressure and strain predictions to a normally-fitted AFO, likely due to the specific conditions modelled. Interparticipant variability was high across all test conditions, and was often more prominent than intra-participant differences observed across designs. While design-related trends were generally consistent between all participants, there were exceptions. For the RA computational study, correlations were assessed between model predictions and clinical and morphological data, to determine whether the models could distinguish between condition presentations and severity, and thus determine individual FO requirements. BMI and first metatarsal head (MH1) curvature correlated with pressure and shear strain model predictions (BMI: 0.600<r<0.652, p<0.05; MH1 curvature: 0.529<r<0.574, 0.040<p<0.063), but scores relating to pain and intermetatarsal bursitis did not. Relationships between model predictions and either tissue depth under MH1 or sesamoid bone offset could not be established, due to the small sample size and MR dataset limitations. These studies represent novel methods to assess AFO and FO design, including the first FE modelling of a population with RA and comparison with clinical data. Important considerations for future orthosis research and clinical application have also been highlighted, such as differences in superficial plantar pressure and deeper shear strains trends, and high inter-participant variability. Future model development, to consider dynamic gait or other vulnerable foot regions, could further aid orthosis assessment. By optimising orthosis design, devices may be tailored to improve effectiveness and user comfort, and reduce tissue damage risk.

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Identifiers

ORCID for Emily Sarah Kelly: orcid.org/0000-0001-5776-6857

ORCID for Alexander Dickinson: orcid.org/0000-0002-9647-1944

ORCID for Pete Worsley: orcid.org/0000-0003-0145-5042

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