Comparative analysis of the structural behaviour of prosthetic sockets tested with three mock limbs of different compliance

This thesis is developed within the framework of the Pro-Olympia project, in collaboration with INAIL, and focuses on the structural behaviour of transtibial prosthetic sockets. Within the research activities, the expert group of the American Orthotic & Prosthetic Association (AOPA) – Socket Guidance Workgroup (SGW) identified three critical artificial residual limb shapes to be used in laboratory testing of prosthetic sockets: Bulky-Long, Bulky-Short, and Long Slender. This thesis aims to evaluate how these different shapes, each manufactured with varying levels of compliance (Rigid, Hybrid, and Compliant), influence the mechanical response of prosthetic sockets during experimental testing. The experimental testing was conducted at the Machine Laboratory of the Department of Industrial Engineering (DII) at the University of Padua. Three Bulky-Long mock limbs were fabricated in Rigid, Hybrid, and Compliant compositions, respectively. In parallel, twelve composite sockets were manufactured at the INAIL Prosthetic Centre following the same fabrication protocol to ensure comparability. Each socket–mock limb assembly was instrumented using a combination of measurement systems: strain gauges to quantify local strains, reflective markers for motion-capture analysis of global kinematics, and pressure sensors placed at selected anatomical locations during preliminary donning and doffing trials. The assemblies were then subjected to two categories of mechanical tests: (i) ultimate tests, in which sockets were loaded to failure, and (ii) short cyclic tests, designed as preliminary steps toward future high-cycle fatigue testing. Results from the ultimate tests demonstrated a clear dependence of failure load and failure mode on mock limb compliance. Sockets tested with the Rigid mock limb reached higher failure loads (approximately 7000 N) but exhibited failure at the distal pyramid adapter, suggesting unrealistic support conditions and an overestimation of socket strength. In contrast, sockets tested with the Hybrid and Compliant mock limbs failed at lower loads (around 5000 N) and showed more physiologically consistent failure patterns, such as anterior shell buckling or distal attachment detachment. Strain gauge data confirmed that deformation during failure localizes primarily in the distal region and predominantly along the transverse direction. Short cyclic tests revealed further distinctions among mock limbs: while some strain gauge locations exhibited consistent behaviour across all compositions, others, such as at the mid patellar tendon (MPT), showed opposite strain patterns when tested with the Compliant limb. These findings highlight the significant influence of mock limb stiffness on load transmission and socket deformation. Motion capture analyses verified the alignment stability of the test setup and confirmed negligible variations in lever arm across mock limb types. Some limitations were identified throughout the project, including sensor saturation in pressure sensors measurements and control-system constraints preventing precise loading profiles during cyclic tests. These findings highlight the need for improved Proportional–Integral–Derivative (PID) controller tuning, extended strain gauge mapping, and expanded mock limb fabrication to include the remaining shapes (Long Slender and Bulky Short). Additionally, since fatigue tests according to ISO 10328 require three million loading cycles and are extremely time-consuming, future research could benefit from a multicentric testing approach involving laboratories worldwide. Overall, the results of this work demonstrate that mock limb compliance plays a crucial role in socket mechanical behaviour and should therefore be carefully considered when designing standardized testing protocols. A unified international guideline for mock limbs would standardize socket testing and improve overall prosthetic quality.