Abstract

Foot orthoses are prescribed to reduce forefoot plantar pressures and pain in people with rheumatoid arthritis. Computational modelling can assess how the orthoses affect internal tissue stresses, but previous studies have focused on a single healthy individual. This study aimed to ascertain whether simplified forefoot models would produce differing biomechanical predictions at the orthotic interface between people with rheumatoid arthritis of varying severity, and in comparison to a healthy control. The forefoot models were developed from magnetic resonance data of 13 participants with rheumatoid arthritis and one healthy individual. Measurements of bony morphology and soft tissue thickness were taken to assess deformity. These were compared to model predictions (99th% shear strain and plantar pressure, max. pressure gradient, volume of soft tissue over 10% shear strain), alongside clinical data including body mass index and Leeds Foot Impact Scale–Impairment/Footwear score (LFIS-IF). The predicted pressure and shear strain for the healthy participant fell at the lower end of the rheumatoid models’ range. Medial first metatarsal head curvature moderately correlated to all model predicted outcomes (0.529 < r < 0.574, 0.040 < p < 0.063). BMI strongly correlated to all model predictions except pressure gradients (0.600 < r < 0.652, p < 0.05). There were no apparent relationships between model predictions and instances of bursae, erosion and synovial hypertrophy or LFIS-IF score. The forefoot models produced differing biomechanical predictions between a healthy individual and participants with rheumatoid arthritis, and between individuals with rheumatoid arthritis. Models capable of predicting subject specific biomechanical orthotic interactions could be used in the future to inform more personalised devices to protect skin and soft tissue health. While the model results did not clearly correlate with all clinical measures, there was a wide range in model predictions and morphological measures across the participants. Thus, the need for assessment of foot orthoses across a population, rather than for one individual, is clear.

Highlights

  • Rheumatoid arthritis (RA) is frequently characterized by deformities at the metatarsophalangeal (MTP) joints, frequently with erosion and subluxation of the first and fifth MTP joints (Bowen et al, 2011)

  • In participants with RA for over 10 years, there was an even split of people with low vs moderate to high foot impairment (LFIS-IF threshold of 7 (Turner et al, 2006))

  • Comparing to a single healthy individual does not confirm the models would distinguish between cohorts of healthy vs RA, but the results do provide another point of comparison for condition severity

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Summary

Introduction

Rheumatoid arthritis (RA) is frequently characterized by deformities at the metatarsophalangeal (MTP) joints, frequently with erosion and subluxation of the first and fifth MTP joints (Bowen et al, 2011). Through inflammation or adventitial bursae formation as a response to friction and high pressures (Bowen et al, 2010; Van Hul et al, 2011). Associated with these morphological changes are pain, reduced foot function, and risk of soft tissue wounds (van der Leeden et al, 2006; Bowen et al, 2011). The most common sites of ulceration in the RA foot are the dorsal aspect of hammer toes (48%), the metatarsal heads (32%), and the medial aspect of the first MTP joint, with many ulcers reoccurring (Firth et al, 2008)

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