Federico Montagnani

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.

Preliminary design and development of a two degrees of freedom passive compliant prosthetic wrist with switchable stiffness

Amputees wearing a transradial prosthesis are compelled to perform compensatory movements of the shoulder, elbow and trunk, while reaching objects in the environment, due to the lack of degrees of freedom (DoF) in their artificial arm. These unnatural movements yield to articulation injuries in the long run, which could be reduced/eliminated by adding an articulated compliant wrist. Several passive compliant wrists are commercially available, however they are manually adjustable and present a unique level of stiffness. These features make them not very functional when it comes to use them in a broad range of daily activities. In this paper we present the design, preliminary development and characterization of an innovative two DoFs prosthetic wrist. The movements allowed to the hand mounted on the wrist are flexion/extension and abduction/adduction. The wrist is provided with two switchable levels of passive compliance that can be selected by means of a linear actuation system. A preliminary prototype was developed and characterized in terms of stiffness response.

Non-Back-Drivable Rotary Mechanism with Intrinsic Compliance for Robotic Thumb Abduction/Adduction

Duplicating the complexity of the human thumb with a robotic one is a difficult task for engineers, when taking into consideration the necessary miniaturization and robustness requirements. Miniaturization is required to fit all the components within the size of a human hand; however, the reduction in size of the components affects the strength of the transmission. Several robotic hands with active thumbs were designed. Most of these had the abduction/adduction joint rigidly connected to the electrical motor and gearhead. This is a non-optimal solution because although miniaturized, the transmission is back-drivable and thus the motor gearhead is exposed to shocks. In this paper, we present the design of a non-back-drivable rotary mechanism with intrinsic compliance for a robotic thumb ab/adduction joint. The mechanism enables switch off of the power supply once a desired posture is stable, thus avoiding accidental dangerous releases of the grasped object, absorbs the impact forces generated at the instan...

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.