Analysis and comparison of wrist splint designs using the finite element method: Multi-material three-dimensional printing compared to typical existing practice with thermoplastics

Rheumatoid arthritis (RA) is a chronic disease affecting the joints. Treatment can include immobilisation of the affected joint with a custom-fitting splint, which are typically fabricated by hand from Low Temperature Thermoplastic (LTT), but the approach poses several limitations. This study focused on the evaluation, by Finite Element Analysis (FEA), of Additive Manufacturing (AM) techniques for wrist splints in order to improve upon the typical splinting approach. An AM splint, specifically designed to be built using Objet Connex multi-material technology and a virtual model of a typical splint, digitised from a real patient-specific splint using 3D scanning, were modelled in Computer-Aided Design software. Forty FEA simulations were performed in Flexion-Extension and Radial-Ulnar wrist movements to compare the displacements and the stresses. Simulations have shown that for low severity loads, the AM splint has 25%, 76% and 27% less displacement in the main loading direction than the typical splint in Flexion, Extension and Radial respectively, while Ulnar values were 75% lower in the traditional splint. For higher severity loads, the Flexion and Extension movements resulted in deflections that were 24% and 60% respectively lower in the AM splint. However, for higher severity loading the Radial defection values were very similar in both splints and Ulnar movement deflection was higher in the AM splint. A physical prototype of the AM splint was also manufactured and was tested under normal conditions to validate the FEA data. Results from static tests showed maximum displacements of 3.46mm, 0.97mm, 3.53mm, and 2.51mm Flexion, Extension, Radial and Ulnar directions respectively. According to these results, the present research argues that, from a technical point of view, the AM splint design stands at the same or even better level of performance in displacements and stress values in comparison to the typical LTT approach and is therefore a feasible approach to splint design and manufacture.