Transcutaneous Electrical Nerve Stimulation at the Wrist as a Method to Restore Sensory Feedback for Individuals with Partial Hand Amputation

INTRODUCTION

Partial hand amputation accounts for an estimated 92% of all upper-limb amputations¹ and results in up to a 90% loss of total upper extremity ability and a 54% decrease in whole-person ability². Sensory feedback is critical for dexterous hand function, yet there are limited methods available to restore sensory function to individuals with partial hand amputation.

Artificial sensory feedback via mechanical or vibrotactile stimulation only evokes sensation locally at the site of the stimulation, which impedes use of the residual hand or prosthesis. Here we explore the use of transcutaneous electrical nerve stimulation (TENS) of the nerves at the wrist to evoke distally referred sensations from the amputated digit(s).

METHODS

Participants:

Thirty-eight intact individuals and one individual with partial hand amputation participated in this study. Participants were 55% male and between the ages of 18 and 52. Informed consent and experimental protocols were carried out in accordance with the University of Utah Institutional Review Board.

TENS:

Stimulation was delivered using a non-invasive neural stimulator³ and surface electrodes placed around the wrist. Pulse frequency (PF) varied between 12.5–225 Hz, pulse width (PW) between 0.170–0.230 ms, and pulse amplitude (PA) between 2.52–4.98 mA.

Experimental Design:

Participants reported the location of sensations evoked by different electrode placements around the wrist. Participants also reported the perceived intensity of the sensations on a free-magnitude scale in response to changes in stimulation PF, PW, and PA.

RESULTS

TENS at the wrist elicited localized distally referred sensations on the digits. Isolated sensations were achieved in the palm and four of the five digits using only changes in electrode positioning (Fig. 1A). Consistent with prior work^4,5, increasing PF, PW, or PA resulted in more intense sensations (Fig. 1B). Findings were consistent between the intact participants and the individual with partial hand amputation; sensations were evoked on the phantom digit using similar electrode placements and stimulation parameters as used with intact individuals.

Fig. 1.

Distally referred sensations were localized on the digits using the electrode placements shown in the boxes (A). Increasing PF, PW, or PA increased sensation intensity (B).

DISCUSSION

TENS can be delivered non-invasively at the wrist to create intuitive, distally referred sensations in the hand. The location of the sensations can be determined by the electrode placement, and the intensity of the sensations can be modulated using PF, PW, or PA. It has been suggested that TENS’s greatest barrier to achieving localized sensation is the spillover of the referred sensation into adjacent projected map areas⁶. Here we show TENS at the wrist can produce isolated sensations with no spillover. However, identifying the specific stimulation parameters to achieve these ideal sensations can be time consuming. We turned away 35% of participants because we could not identify the necessary stimulation parameters within 1 hour. Future work will attempt to create a heatmap of common electrode placements that produce functionally relevant isolated sensations.

CONCLUSION

TENS of the nerves at the wrist provides a unique opportunity to restore anatomically congruent sensory feedback after a partial hand amputation without interfering with the residual hand or prosthesis.

CLINICAL APPLICATIONS

A bracelet or watchband with integrated TENS could be worn by individuals with partial hand amputation. Forces exerted on a sensorized prosthesis could be wirelessly transmitted to the TENS unit to evoke a sensation on the phantom digit with a corresponding intensity.