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Abstract
Dexterous hand movement is possible due to closed loop control dependent on efferent motor output and afferent sensory feedback. This control strategy is significantly altered in those with upper limb amputation as sensations of touch and movement are inherently lost. For upper limb prosthetic users, the absence of sensory feedback impedes efficient use of the prosthesis and is highlighted as a major factor contributing to user rejection of myoelectric prostheses. Numerous sensory feedback systems have been proposed in literature to address this gap in prosthetic control; however, these systems have yet to be implemented for long term use. Methodologies for communicating prosthetic grasp and touch information are reviewed, including discussion of selected designs and test results. With a focus on clinical and translational challenges, this review highlights and compares techniques employed to provide amputees with sensory feedback. Additionally, promising future directions are discussed and highlighted.
Acknowledgements
The authors wish to thank Michael Rory Dawson and Dr Ming Chan for their assistance and support.
Financial & competing interests disclosure
J Schofield has received PhD salary funding through the NIH (Grant#:R01NS0817102. K Evans has received MSc salary funding through NSERC-CGSM and AITF scholarship. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.
No writing assistance was utilized in the production of this manuscript.
Key issues
The basis of hand movement is closed-loop motor control comprised a dynamic interplay between motor output and sensory input. The loss of an upper extremity significantly alters this closed-loop control strategy as sensations of touch and movement are inherently lost.
The lack of sensory feedback in prostheses increases the cognitive demand placed on the user as operation of the prosthesis requires continuous conscious attention.
Sensory feedback systems employ instrumentation (or sensors) at the prosthetic level to detect an external stimulus. This instrumentation in turn drives the output of a haptic feedback device (also termed tactor) that conveys information about the external stimulus to the prosthetic user.
Grasp and touch sensory feedback systems can be divided into three categories: substitution, modality matched and somatotopically matched methods.
Sensory substitution categorizes a group of feedback systems that apply a feedback signal that is not matched in modality to the stimulus occurring at the prosthesis and includes vibrational, electrotactile, auditory and other less common feedback types.
Modality matching refers to a feedback signal that is congruent to the external stimulus detected by the prosthetic sensor and includes mechanotactile feedback and other multimodal feedback types.
Somatotopic matching refers to methods in which the feedback signal is perceived as being anatomically matched in location to where the stimulus is being applied to the prosthesis and includes peripheral nerve stimulation, natural phantom mapping and targeted reinnervation.
The ideal feedback system would combine the benefits of modality and somatotopic-matching systems to allow the participant to feel a relevant stimulus at the correct location on their missing limb.
In future clinical trials, it will be important to integrate feedback devices in to a prosthetic socket and perform functional tasks beyond grip force discrimination and simple object manipulation to comprehensively evaluate a feedback system’s efficacy.