Phantom finger phenomena and the effects of toe-to-finger transplantation

Electro-cutaneous stimulation on the palm elicits referred sensations on intact but not on amputated digits

OBJECTIVE

Grasping and manipulation control critically depends on tactile feedback. Without this feedback, the ability for fine control of a prosthesis is limited in upper limb amputees. Early studies have shown that non-invasive electro-cutaneous stimulation (ES) can induce referred sensations that are spread to a wider and/or more distant area, with respect to the electrodes. Building on this, we sought to exploit this effect to provide somatotopically matched sensory feedback to people with partial hand (digital) amputations.

APPROACH

For the first time, this work investigated the possibility of inducing referred sensations in the digits by activating the palmar nerves. Specifically, we electrically stimulated 18 sites on the palm of non-amputees to evaluate the effects of sites and stimulation parameters on modality, magnitude, and location of the evoked sensations. We performed similar tests with partial hand amputees by testing those sites that had most consistently elicited referred sensations in non-amputees.

MAIN RESULTS

We demonstrated referred sensations in non-amputees from all stimulation sites in one form or another. Specifically, the stimulation of 16 of the 18 sites gave rise to reliable referred sensations. Amputees experienced referred sensations to unimpaired digits, just like non-amputees, but we were unable to evoke referred sensations in their missing digits: none of them reported sensations that extended beyond the tip of the stump.

SIGNIFICANCE

The possibility of eliciting referred sensations on the digits may be exploited in haptic systems for providing touch sensations without obstructing the fingertips or their movements. The study also suggests that the phenomenon of referred sensations through ES may not be exploited for partial hand prostheses, and it invites researchers to explore alternative approaches. Finally, the results seem to confirm previous studies suggesting that the stumps in partial hand amputees partially acquire the role of the missing fingertips, physiologically and cognitively.

Painful foot neuromas after toe-to-thumb transfer

Reconstruction of the thumb by transfer of a toe has evolved technically to the point that this complex procedure can result in a mobile, sensate, and aesthetically pleasing digit that contributes to an almost-normally functioning hand. Donor site deformity is well recognized, primarily as it relates to the appearance of the foot after transfer of the hallux to the thumb position and stiffness of the remaining portions of the big toe. The present report describes donor site disability related to painful neuromas of the superficial and deep peroneal nerves and the common plantar digital nerve to the first webspace. Salvage of the disabled donor foot is possible by applying techniques used to treat painful neuromas of the upper extremity, neuroma resection, and muscle implantation. The specific techniques used in treating this painful foot donor site after toe-to-thumb transfer are described.