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Targeted Reinnervation

Background

Currently, the most widely used upper extremity prosthetic devices are controlled via myoelectric signals from an agonist/antagonist pair of muscles remaining after amputation. These signals are quantified via surface electromyography, and because there is only one source, can only volitionally move one joint of the prosthetic device at once. In addition to the frustration of only single degrees of motion at a time, these devices rely on the action of muscles not accustomed to the type of control required for successful hand maneuvering (i.e., use of the shoulder muscles as the myoelectric signal source would result in awkward and non-intuitive control of the prosthetic hand). Thus, the usability of any prosthetic limb could be greatly improved by incorporating simultaneous movement of multiple joints and by making operation more intuitive. Targeted reinnervation surgery provides modalities for these modifications.

Summary of Procedures

There are two types of Targeted Reinnervation procedures: targeted muscle reinnervation (TMR) and targeted sensory reinnervation (TSR).

TMR

First proposed and researched by Todd Kuiken, Ph.D., targeted reinnervation surgery is a highly specialized procedure that allows amplification of the myoelectric signals of residual nerves from an amputated limb. This amplification is accomplished via transfer of the nerves into a target muscle rendered biomechanically nonfunctional after limb amputation. To optimize the strength and purity of the surface electromyography signal, layers of subcutaneous tissue over the target muscle are removed, and the target muscle itself is denervated prior to reinnervation by the residual nerves.

TSR

Also proposed by Todd Kuiken, Ph.D., TSR inserts residual nerves on a target muscle. However, instead of resulting in amplified myoelectric signal output to the prosthetic device, TSR allows for sensory input.

Case Studies

In this particular case study, a 24 year old female underwent the targeted reinnervation procedure, with her musculocutaneous, median, ulnar, and radial nerves transferred to her pectoralis muscle. This part of the procedure is specifically called targeted muscle reinnervation (TMR). Unlike other targeted reinnervation procedures which focused only on TMR, this procedure included targeted sensory reinnervation (TSR).

Once her prosthetic device was programmed to read the signals resulting from the TMR instead of a signal from a single set of muscles, the female patient’s ability to perform certain tasks greatly improved. This was due not only to the fact that both the elbow and hand could be moved simultaneously, but also to the intuitive nature of the control system-she would envision herself flexing her elbow and the prosthetic would respond analogously. If this improvement were not impressive enough, the feedback offered by TSR has even further exciting implications for prosthetic technology. With sensory feedback being provided to the TSM target area via stimulation from the prosthetic device, this technology could provide a sense of temperature, texture, graded pressure-modifications resulting in more ‘normal’ use of the limb.

Implications

For obvious reasons, incorporation of sensory data into prosthetic technologies is the logical next step towards creating a device that more closely approximates real limb function. The results of TSM in this case study showed evidence that all cutaneous sensory modalities were present in the target area, each with perception thresholds close to those on the un-amputated limb. Of course, further research is required to fully realize the intricacies of this system. As discussed in lecture, convergence and divergence, along with cortical plasticity effects must be considered. As noted by the authors, more insight into the density of skin-receptors and their types is required to model afferent-receptor interaction. Also, the surface area of the target area might have been too small, considering the vast disparity in the number of fibers innervating the fingers versus the pectoral surface.