Compliant Prosthetic Wrists Entail More Natural Use Than Stiff Wrists During Reaching, Not (Necessarily) During Manipulation.

Developing an artificial arm with functions equivalent to those of the human arm is one of the challenging goals of bioengineering. State-of-the-artprostheses lack several degrees of freedom and force the individuals to compensate for them by means of compensatory movements, which often result in residual limb pain and overuse syndromes. Passive wristsmay reduce such compensatory actions, nonethelessto date their actual efficacy, associated to conventional myoelectric hands is a matter of debate. We hypothesized that a transradial prosthesiswould allow a simpler operation if its wrist behaved compliant during the reaching and grasping phase, and stiff during the holding andmanipulation phase. To assess this, we compared a stiff and a compliant wrist and evaluating the extent of compensatory movements in the trunk and shoulder, with unimpaired subjects wearing orthoses, while performing nine activities of daily living taken from the southampton hand assessment procedure. Our findings show indeed that the optimal compliance for a prosthetic wrist is specific to the phase of the motor task: the compliant wrist outperforms the stiff wrist during the reaching phase, whereas the stiff wrist exhibits more natural movements during the manipulation phase of heavy objects. Hence, this paper invites rehabilitation engineers to develop wrists with switchable compliance.

A passive wrist with switchable stiffness for a body-powered hydraulically actuated hand prosthesis.

State of art upper limb prostheses lack several degrees of freedom (DoF) and force the individuals to compensate for them by changing the motions of their arms and body. Such movements often yield to articulation injuries, nonetheless these could be prevented by adding DoFs, for instance, an articulated passive wrist. Available stiff or compliant wrists with passive flexion/extension and/or radial/ulnar deviation are sub-optimal solutions. Indeed, stiff wrists induce the individuals wearing them to perform exaggerated compensatory movements during the reaching phase while compliant wrists proved to be unpractical while manipulating heavy objects. Here we present a wrist capable of combining the benefits of stiff and compliant wrists. It is provided with two switchable levels of passive compliance that are automatically selected. The prototype was integrated in a body-powered hydraulic hand prosthesis and actuated using the same hydraulic circuit of the hand. Detailed analysis of the parameters that affect the compliance, the critical load and the performance of the prosthesis are presented.

Independent Long Fingers are not Essential for a Grasping Hand.

The human hand is a complex integrated system with motor and sensory components that provides individuals with high functionality and elegant behaviour. In direct connection with the brain, the hand is capable of performing countless actions ranging from fine digit manipulation to the handling of heavy objects. However the question of which movements mostly contribute to the manipulation skills of the hand, and thus should be included in prosthetic hands, is yet to be answered. Building from our previous work, and assuming that a hand with independent long fingers allowed performance comparable to a hand with coupled fingers, here we explored the actual contribution of independent fingers while performing activities of daily living using custom built orthoses. Our findings show that, when an opposable thumb is present, independent long fingers provide a measureable advantage in performing activities of daily living only when precision grasps are involved. In addition, the results suggest that the remarkable grasping skills of the human hand rely more on the independent abduction/adduction of the fingers than on their independent flexion/extension. These findings are of interest to the designers of artificial hands, including biomimetic prostheses and exoskeletons.

Is it Finger or Wrist Dexterity That is Missing in Current Hand Prostheses?

Building prostheses with dexterous motor function equivalent to that of the human hand is one of the ambitious goals of bioengineers. State of art prostheses lack several degrees of freedom (DoF) and force the individuals to compensate for them by changing the motions of their arms and body. However, such compensatory movements often result in residual limb pain and overuse syndromes. Significant efforts were spent in designing artificial hands with multiple allowed grasps but little work has been done with regards to wrist design, regardless the fact that the wrist contributes significantly to the execution of upper limb motor tasks. We hypothesized that a single DoF hand with wrist flexion/extension allowed function comparable to a highly performant multi DoF hand without wrist flexion/extension. To assess this we compared four emulated architectures of hand-wrist prostheses using the Southampton Hand Assessment Procedure and evaluating the extent of compensatory movements with unimpaired subjects wearing ortheses. Our findings show indeed that shifting the dexterity from the hand to the wrist could preserve the ability of transradial amputees in performing common tasks with limited effect on the compensatory movements. Hence, this study invites rehabilitation engineers to focus on novel artificial wrist architectures.

Exploiting arm posture synergies in activities of daily living to control the wrist rotation in upper limb prostheses: A feasibility study.

Although significant technological advances have been made in the last forty years, natural and effortless control of upper limb prostheses is still an open issue. Commercially available myoelectric prostheses present limited Degrees of Freedom (DoF) mainly because of the lack of available and reliable independent control signals from the human body. Thus, despite the crucial role that an actuated wrist could play in a transradial prosthesis in terms of avoiding compensatory movements, commercial hand prostheses present only manually adjustable passive wrists or actuated rotators controlled by (unnatural) sequential control strategies. In the present study we investigated the synergies between the humeral orientation with respect to the trunk and the forearm pronation/supination angles during the execution of a wide range of activities of daily living, in healthy subjects. Our results showed consistent postural synergies between the two selected body segments for almost the totality of the activities of daily living under investigation. This is a promising result because these postural synergies could be exploited to automatically control the wrist rotator unit in transradial prostheses improving the fluency and the dexterity of the amputee.