Restoration of Genuine Sensation and Proprioception of Individual Fingers Following Transradial Amputation with Targeted Sensory Reinnervation as a Mechanoneural Interface

Abstract

Background/Objectives: Tactile gnosis derives from the interplay between the hand's tactile input and the memory systems of the brain. It is the prerequisite for complex hand functions. Impaired sensation leads to profound disability. Various invasive and non-invasive sensory substitution strategies for providing feedback from prostheses have been unsuccessful when translated to clinical practice, since they fail to match the feeling to genuine sensation of the somatosensory cortex. Methods: Herein, we describe a novel surgical technique for upper-limb-targeted sensory reinnervation (ulTSR) and report how single digital nerves selectively reinnervate the forearm skin and restore the spatial sensory capacity of single digits of the amputated hand in a case series of seven patients. We explore the interplay of the redirected residual digital nerves and the interpretation of sensory perception after reinnervation of the forearm skin in the somatosensory cortex by evaluating sensory nerve action potentials (SNAPs), somatosensory evoked potentials (SEPs), and amputation-associated pain qualities. Results: Digital nerves were rerouted and reliably reinnervated the forearm skin after hand amputation, leading to somatotopy and limb maps of the thumb and four individual fingers. SNAPs were obtained from the donor digital nerves after stimulating the recipient sensory nerves of the forearm. Matching SEPs were obtained after electrocutaneous stimulation of the reinnervated skin areas of the forearm where the thumb, index, and little fingers are perceived. Pain incidence was significantly reduced or even fully resolved. Conclusions: We propose that ulTSR can lead to higher acceptance of prosthetic hands and substantially reduce the incidence of phantom limb and neuroma pain. In addition, the spatial restoration of lost-hand sensing and the somatotopic reinnervation of the forearm skin may serve as a machine interface, allowing for genuine sensation and embodiment of the prosthetic hand without the need for complex neural coding adjustments.

Keywords: electroencephalogram; hand amputation; limb map; neuroma prophylaxis; neuropathic pain; pattern recognition; phantom hand; phantom limb map; phantom limb pain; sensory feedback; targeted muscle reinnervation; targeted sensory reinnervation; transradial amputation.

Figure 1

(A) Recipient nerves on the forearm; (B) LM (=phantom hand with fingers 1–5) after reinnervation. (C) Drawing of the amputation level and preparation of the median and ulnar nerves. (D) Microsurgical separation of the two fascicles of the median nerve and the two branches of the ulnar nerve. (E) Transposition of the separated two median nerve fascicles and two ulnar branches with performance of ulTSR I-III and TMR below the elbow joint.

Figure 2

End-to-end re-coaptation and RPNI wrapped around the coaptation site as neuroma prevention.

Figure 3

Experimental setup for SEP measurement. (Left) EEG cap attached and setup of electrodes at stimulation areas. (Middle) Electrode placement for stimulation of thumb, index, and little finger. (Right) Stimulation setup for thumb, index ,and little finger on the healthy hand.

Figure 4

(A) Self-drawn LM by the patient is shown on the left forearm stump of patient 4 and on the right forearm of patient 6, both 5 months after undergoing ulTSR. For patient 6, the entire limb map is visible by rotating the forearm into a supinated position. (B) LM drawn by patient 3 on the right forearm 5 months after ulTSR. Perception of the ice pad as a cold sensation on the lateral edge of the LM corresponding to the thumb.

Figure 5

SEPs obtained from electrocutaneous stimulation applied on thumb, index and little fingers on the healthy hand (first row) and thumb, index and little finger area on the impaired side (second row) from three subjects (first column P01, second column P02 and third column P03). They are displayed after averaging groups of four channels as denoted by the colorcoded boxes on the topographical maps (red: FC5, CP5, C3, T7—blue: FC1, C3, CP1, Cz—purple: FC2, Cz, CP2, C4—brown: FC6, C4, CP6, T8). The topographical maps depict the spatial distribution of the electrical activity across the scalp at the time point of maximum negative SEP magnitude (denoted in textboxes within each subplot). The impaired side of each subject, as well as the number of trials used for averaging are shown on top of each subplot.