Sensory capacity of reinnervated skin after redirection of amputated upper limb nerves to the chest

Abstract

Targeted reinnervation is a new neural-machine interface that has been developed to help improve the function of new-generation prosthetic limbs. Targeted reinnervation is a surgical procedure that takes the nerves that once innervated a severed limb and redirects them to proximal muscle and skin sites. The sensory afferents of the redirected nerves reinnervate the skin overlying the transfer site. This creates a sensory expression of the missing limb in the amputee's reinnervated skin. When these individuals are touched on this reinnervated skin they feel as though they are being touched on their missing limb. Targeted reinnervation takes nerves that once served the hand, a skin region of high functional importance, and redirects them to less functionally relevant skin areas adjacent to the amputation site. In an effort to better understand the sensory capacity of the reinnervated target skin following this procedure, we examined grating orientation thresholds and point localization thresholds on two amputees who had undergone the targeted reinnervation surgery. Grating orientation thresholds and point localization thresholds were also measured on the contralateral normal skin of the targeted reinnervation amputees and on analogous sites in able-bodied controls. Grating orientation thresholds for the reinnervated skin of the targeted reinnervation amputees were found to be similar to normal ranges for both the amputees' contralateral skin and also for the control population. Point localization thresholds for these amputees were found to be lower for their reinnervated skin than for their contralateral skin. Reinnervated point localization thresholds values were also lower in comparison to homologous chest sites on the control population. Mechanisms appear to be in place to maximize re-established touch input in targeted reinnervation amputees. It seems that sound sensory function is provided to the denervated skin of the residual limb when connected to afferent pathways once serving highly functionally relevant regions of the brain. This suggests that tactile interface devices could be used to give a physiologically appropriate sense of touch to a prosthetic limb, which would likely help with better functional utilization of the prosthetic device and possibly help to more effectively integrate the device with the user's self-image.

Figure 1

Diagrams of testing regions on the chests of the targeted reinnervation amputees. (A) TR1, Male bilateral shoulder disarticulation amputee. (B) TR2, female unilateral short trans-humeral amputee (functional shoulder disarticulation). The dashed line surrounding the light grey area on the left chest of the amputees denotes the extent of sensation that is referred to the missing limb. Within the light grey area the amputees feel a mix of referred hand sensation and native chest sensation. The white area denotes where the targeted reinnervation amputees feel only referred hand sensation with no native chest sensation. The crossed arrows show the vertical and horizontal axes of the grating placements for the grating orientation task. The Y shaped lines denote the placement of the grids used for the point localization task.

Figure 2

Grating orientation thresholds in millimeters for the reinnervated (grey) and normal chest (white) for each targeted reinnervation amputee and the control population (white). There were 15 individuals in the control population. Error bars indicate 1 SD. This test employed a two-interval-forced-choice staircase routine to converge at a threshold value corresponding to 70.7% correct responses.

Figure 3

Point localization thresholds in millimeters for the reinnervated and normal chest for TR1 (A and B), TR2 (C and D) and controls (E). There were six individuals in the control population. Touch stimuli were applied with a 10 g Semmes-Weinstein monofilament first to the central reference point then to a point on an arm of the grid. The percentage of times the subject reported a difference was plotted as a function of the distance from the centre of the grid. A psychometric curve was fit to the data with the 75% difference point taken as threshold.