PSI - Issue 42

Theodoros Marinopoulos et al. / Procedia Structural Integrity 42 (2022) 903–910 T. Marinopoulos et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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is essential to investigate their mechanical performance to ensure safety during their use. Technological developments such as additive manufacturing (AM) are gaining ground in healthcare and prosthetics (Chen et al., 2016) with the aim to assist the faster product delivery and easier customisation of healthcare equipment (Jin et al., 2015). With limited reports on the mechanical behaviour of such products is important to investigate their capabilities and mechanical compliance in depth. Since prosthetic sockets must bear same loads (as other parts of a prosthesis) and despite being bespoke products and hence not coming directly under standardized regulations, BS EN ISO 10328 (BSi BRITISH STANDARDS, 2006) have been used to investigate their mechanical compliance (Goh et al., 2002; McGrath et al., 2017). These standards detail the load- bearing capacity for the prosthesis’ components. Static and dynamic conditions are explained and two loading conditions are described related to the heel-strike (early stance) and toe-off (late stance) (Gerschutz et al., 2012; Owen and DesJardins, 2020) scenarios. Previous studies were limited mainly to below-knee cases and conventionally manufactured sockets; still, significant strength variation was reported (Gariboldi et al., 2022). The load-bearing capacity of the sockets ranged from 1836 N (Current et al., 1999) to 13132 N (Pousett et al., 2019), meaning that even some of the conventional products failed to meet the requirements. AM has been recently introduced in healthcare, and the relevant studies in the area of prosthetics are still quite limited (Herbert et al., 2005; Nguyen et al., 2018). Reports so far confirmed that 3D printed prosthetic sockets could safely meet the requirements described in the IS guidelines, although conventionally manufactured sockets usually outperformed them (Pousett et al., 2019). Infill in AM was reported to have a negligible effect (Campbell et al., 2018). While the additives and color of the filament were found to affect the mechanical performance of the final material (Hanon and Zsidai, 2021; Wittbrodt and Pearce, 2015), no report on these factors was produced for prosthetic-socket applications. So, there is a common understanding that mechanical performance of prosthetic sockets is generally under investigated. In this study above-knee prosthetic sockets were 3D printed and tested according to BS EN ISO 10328 in order to assess their ultimate strength. The load requirements were adjusted for pediatric applications as discussed previously (Marinopoulos et al., 2022). An in-house developed testing rig was used to perform the tests, with loads matching those of Condition I (heel-strike) of BS EN ISO 10328:200. Sockets were manufactured using a desktop extrusion 3D printer the material from different manufacturers to address the effect of the filament. Finite-element models were used to analyze the stress distribution in the prosthetic sockets and assist with the redesign process. 2. Methodology 2.1. Socket design and manufacturing A paediatric socket designed from the scanned geometry of an above-knee residual limb was used in this study (Fig. 1a). Since the top part of the socket bears a minimal load, it was removed. The distal end of the socket was redesigned to accommodate for a nuts-and-bolts mechanism to be attached to an off-the-shelf pyramid. A void was introduced between the limb and the distal part of the socket to allow for better load distribution on the prosthetic. This design will be referred to as original (Fig. 1b). To 3D print the socket, CURA slicing software was used. The wall number was set to 3 and 100% infill was used to be as close as possible to the bulk-material properties. A lines infill pattern was selected, with the layer height set at 0.2 mm. A 0.4 mm nozzle was used in AM with a constant travel speed of 60 mm/m. Sockets were printed in PLA from two different manufacturers denoted aPLA and bPLA, respectively. Five original sockets were printed from each manufacturer’s material. 2.2. Experimental framework The printed original sockets were tested according to guidelines of BS EN ISO 10328:2006. In these standards, two loading conditions are described, referring to different stances during walking. Condition I, corresponding to the heel strike, was selected as the one experiencing the highest loads. The ultimate strength criterion was selected, with revised load-bearing level of 3890 N, as in (Marinopoulos et al., 2022).