Broadband Prosthetic Interfaces: Combining Nerve Transfers and Implantable Multichannel EMG Technology to Decode Spinal Motor Neuron Activity

Abstract

Modern robotic hands/upper limbs may replace multiple degrees of freedom of extremity function. However, their intuitive use requires a high number of control signals, which current man-machine interfaces do not provide. Here, we discuss a broadband control interface that combines targeted muscle reinnervation, implantable multichannel electromyographic sensors, and advanced decoding to address the increasing capabilities of modern robotic limbs. With targeted muscle reinnervation, nerves that have lost their targets due to an amputation are surgically transferred to residual stump muscles to increase the number of intuitive prosthetic control signals. This surgery re-establishes a nerve-muscle connection that is used for sensing nerve activity with myoelectric interfaces. Moreover, the nerve transfer determines neurophysiological effects, such as muscular hyper-reinnervation and cortical reafferentation that can be exploited by the myoelectric interface. Modern implantable multichannel EMG sensors provide signals from which it is possible to disentangle the behavior of single motor neurons. Recent studies have shown that the neural drive to muscles can be decoded from these signals and thereby the user's intention can be reliably estimated. By combining these concepts in chronic implants and embedded electronics, we believe that it is in principle possible to establish a broadband man-machine interface, with specific applications in prosthesis control. This perspective illustrates this concept, based on combining advanced surgical techniques with recording hardware and processing algorithms. Here we describe the scientific evidence for this concept, current state of investigations, challenges, and alternative approaches to improve current prosthetic interfaces.

Keywords: EMG; TMR; myoelectric prosthesis; nerve transfers; prosthetic control; prosthetic interface; targeted muscle reinnervation.

Figure 1

TMR and hyper-reinnervation: Top: Physiologically, peripheral nerves typically innervate multiple muscles via different motor fascicles. The fascicles' motor neurons are located in the motor neuron columns of the spinal cord. Each motor neuron innervates a certain number of muscle fibers, termed the muscle unit. After amputation, these motor neurons and fascicles remain intact without any function. Bottom: During TMR surgery, amputated nerves are transferred to replace the target muscle's original motor nerve. The donor nerve is typically a multi-fascicular nerve that includes a higher number of motor neurons. Consequently, the targeted muscle is hyper-reinnervated by more motor neurons which form smaller muscle units. Additionally, the individual motor fascicles could form fascicular territories within the muscle that could potentially contract independently from each other.

Figure 2

Broadband prosthetic interface: During TMR, the amputated ulnar nerve was rerouted to the short/medial head of the biceps to provide additional EMG signals. Using multichannel EMG electrodes, single motor unit activity can be decoded from the EMG data and the neural drive of the ulnar nerve to the intrisic hand musculature estimated. This information could allow the delivery of extremely precise control signals, ultimately with the same accuracy as physiologically reached.