Impact of the choice of upper limb prosthesis mechanism on kinematics and dynamic quality

Upper limb prostheses can greatly improve the condition of amputees. However, prosthetic mechanisms have different topologies and there is no consensus on the choice of an appropriate mechanism. This paper evaluates the impact of prosthetic mechanism topology on the prosthesis' performance during daily tasks. The proposed multibody model is compared to four open-loop and one closed-loop existing mechanisms according to: (1) consumed energy, (2) global and local movement reconstruction errors during inverse kinematics, (3) movement smoothness, which reflects the dynamic appearance of the prosthesis, also called 'dynamic cosmesis'. Flexion-extension (FE) and pronation-supination (PS) tasks were studied in 15 healthy subjects. All parameters identified at least one group difference (p < 0.0001) in both tasks. Most closed-loop mechanisms (50% in FE and 100 % in PS) including the proposed model were among the most energy-efficient mechanisms. Out of all models, the proposed model was the most energy efficient in FE (2.07 +/- 0.69 KJ) and in PS (0.25 +/- 0.16 KJ). This model also reproduced the studied movements with the lowest errors (1.39 +/- 0.2 mm in FE and 1.38 +/- 0.25 mm in PS), especially at the forearm level. The results show that the wrist plays a major role in motion smoothness and that two series mechanisms have exhibited a poor dynamic cosmesis because of their higher jerk cost ((1.73 +/- 0.30) x 1010) in FE and (9.29 +/- 17) x 1013 in PS tasks)). Finally, the mechanism topology affects the performance of upper limb prostheses and represents a novel aspect in the prostheses design which can be applied to exoskeleton design.