Multidimensional evaluation of a two-level-of-compliance quasi-passive prosthetic wrist: functional, biomechanical, cognitive and user-experience outcomes in trans-radial limb loss.

backgroundAdvances in upper limb prosthetics have expanded functional capabilities for individuals with trans-radial limb loss, yet compensatory movements remain common due to mobility limitations of current prosthetic devices. This study evaluated the functional performance of a quasi-passive prosthetic wrist with two degrees of freedom and adjustable compliance operating according to the Synchronous Hand-Wrist Operation principle (compliant during reaching and automatically stiffening at grasp onset), focusing on clinical function, compensatory strategies, cognitive workload and user experience.

methodsSix male prosthetic users (53 ± 13 years) completed two experimental sessions on consecutive days. Session one included familiarization and clinical evaluation using the Southampton Hand Assessment Procedure (SHAP). In session two, compensatory strategies were assessed during six standardized manual tasks using motion capture (Vicon, UK) and compared against age- and sex-matched healthy controls. Prosthetic users performed all procedures with the device in compliant-while-reaching (ON) and full-stiff modes (OFF) in randomized order. Clinical, spatio-temporal and kinematic outcomes were analyzed using linear mixed-effect models. Subjective evaluation included the NASA-Task Load Index (NASA-TLX), System Usability Scale (SUS) and a custom questionnaire, assessing the overall experience with the device in its intended compliant operating mode.

resultsIn the SHAP, the ON condition yielded significantly lower mean hand functionality scores than OFF (55.0 vs. 58.3, ~ 5% decrease; p = 0.034) and longer completion times in 4/26 items (+ 1.4 s on average, + 18-44%; p < 0.05). Biomechanical analysis showed increased reaching phase duration in ON across all tasks, with minimal increases in total hand trajectory. Task-specific kinematic adaptations were evident: when wrist deviation during grasping occurred in ON, reduced compensations were observed at more proximal joints, bringing joint ranges of motion closer to healthy controls. Additional motor strategies were required, particularly in tasks involving surface interaction or complex hand-object alignment. Subjective evaluations indicated moderate cognitive workload (NASA-TLX: 48/100), above-average usability (SUS: 74/100) and overall positive acceptance.

conclusionsA multi-DoF quasi-passive prosthetic wrist can reduce proximal compensations, although introducing longer execution times and novel motor demands. Functional benefits are strongly task-dependent. These findings stress the importance of balancing added mechanical complexity with usability to optimize clinical outcomes and user satisfaction.