Biomechanical Design, Additive Manufacturing, and Proof-of-Concept Evaluation of a 3D-Printed Transradial Prosthesis with a Manual Ratchet-Type Locking Mechanism
Rodrigo Valdés-Cienfuegos, Pamela Torres-Rodríguez, José Antonio Rodríguez-Calderón, Alán Eduardo Monroy-Domínguez, Ángel Eduardo Medellin Serafín, Emmanuel López-Neri, Odin Ramírez-Fernández, Sergio Arturo Gama Lara
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Source: Crossref
Published: Oct 12, 2026
DOI: 10.3390/biomechanics6040094
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Background/Objectives: Upper-limb loss represents a critical clinical and socioeconomic challenge globally. While myoelectric devices offer multi-articulated grasping, their high cost restricts access in low-resource settings. This work presents the preliminary biomechanical design, Fused Deposition Modeling (FDM) manufacturing, and proof-of-concept evaluation of a custom transradial prosthesis (EVA) with a manual, incremental ratchet-type tensioning mechanism. Methods: Virtual CAD assembly was parametrically customized from user anthropometry. Finite Element Analysis (FEA) incorporating transversely isotropic material models (E_x = E_y = 2150 MPa, E_z = 1620 MPa) and transient dynamic impact simulations evaluated structural stresses and contact transitions. Physical prototypes were FDM-printed in Polyethylene Terephthalate Glycol (PETG) and Thermoplastic Polyurethane (TPU). Grip forces were experimentally measured using piezoresistive sensors, while transmission torque (M_knob = 1.37 N·m) was analytically derived. Ratchet reliability was tested over 1000 continuous cycles, and an able-bodied simulator pilot study (N = 5) evaluated usability (Box and Block Test, Donning/Doffing, System Usability Scale). Results: Peak static von Mises stress (45.0 MPa) remained below the PETG yield limit (47–60 MPa), whereas transient impact stress reached 53.8 MPa. Safe grip thresholds without slippage were established up to 5.3 kg for wood, 3.4 kg for iron, and 2.1 kg for glass. Experimental grip force measured 122.5 ± 4.2 N. Failure at 7.5 kg followed a two-stage sequence: hinge micro-delamination (6.5 kg) followed by internal return spring snapping. Cyclic ratchet testing showed 0.08 mm pawl wear and 0.2–0.4 mm back-slip with 90.9% retained grip force. Simulator testing yielded a Box and Block score of 14.2 ± 1.8 blocks/min, donning time of 28.5 ± 4.2 s, and a SUS score of 78.5/100. Conclusions: The manual ratchet mechanism provides an ultra-lightweight (344 g) and affordable (~$50 USD) assistive alternative. However, manual contralateral actuation occupies the sound hand (100% involvement during adjustment), precluding true bimanual independence. Near-term mitigations, including shoulder-driven pull-cables and sEMG solenoids, are proposed.
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