Mechanical evaluation of 3D-printed above-knee prosthetic sockets using a biofidelic hyperelastic silicone limb surrogate.

Abstract

Rigid limb surrogates transfer full mechanical loads directly to the prosthetic socket during structural testing, causing premature distal failure and underestimating of actual load capacity. In contrast, soft-compliant surrogates buffer applied loads and enables realistic structural deformation. This study investigates: (i) the influence of rigid Plaster of Paris (PoP) versus biofidelic hyperelastic silicone surrogates on the structural load capacity of 3D-printed transfemoral prosthetic sockets; and (ii) orientation-dependent structural deformation and failure behaviors arising from asymmetric transfemoral anatomy. Sockets encapsulated with PoP and silicone surrogates were evaluated under ((ISO 10328:2016) 2016 Prosthetics-Structural testing of lower-limb prostheses-Requirements and test methods (International Organization for Standardization)) static loading (Condition II) at two loading alignments: toe-off and toe-towards-lateral (90° laterally rotated) configurations. Initial testing with 3D-printed bespoke commercial (BC) connectors induced premature distal connector failure; however, under toe-towards-lateral loading, socket/silicone assemblies exhibited a 4.47-fold greater displacement and a 72.46% higher ultimate force than socket/PoP assemblies. Sockets with an enlarged BC connector encapsulated in the silicone surrogate showed distinct force-displacement hysteresis loops under toe-towards-lateral loading, indicating energy dissipation. Crucially, enlarging the BC connector shifted the failure site from interlayer delamination at the distal end under toe-off to trans-layer fracture across the lateral socket wall. Conversely, socket/PoP assemblies consistently failed at the enlarged BC distal connector regardless of loading orientation. Finite element analysis confirmed non-compliant, uniform contact pressure in socket/PoP assemblies, whereas the silicone surrogate produced compliant pressure gradients. Loading orientation significantly affected the structural deformation (p< 0.05) of socket/silicone assemblies: toe-towards-lateral loading induced two-fold greater deformation and an 18.95% higher ultimate force than under toe-off loading due to the thicker lateral silicone resulting from asymmetric limb geometry. These findings demonstrate that a biofidelic hyperelastic surrogate buffers stress and redistributes contact pressure similar to biological soft tissue, establishing a more accurate pre-clinical framework for evaluating 3D-printed prosthetic socket strength.