Functional electrical stimulation of motor nerves or muscles has been used for decades to restore movements in paralyzed patients who suffered a spinal cord injury, for example. The idea is based on the seminal findings by Luigi Galvani in the 18th century. He could induce movements in frog legs when exposing them to electric voltages – at the time, provided by naturally occurring lightning or simple “batteries” made up of different metals.
The principle has been implemented in a variety of different neurotechnological approaches to restore motion, in particular for arm and hand movements. These are particularly important when it comes to restoring a certain degree of independence to tetraplegics in everyday life. However, such approaches have always been hampered by the size of the electrodes used for stimulation which were rather bulky, so that they stimulated the whole nerve or muscle at the same time. Thus, they typically could only induce a single kind of simple movement. Such simple movements like elbow extensions are of limited use for everyday actions. To produce more complex, useful movements such as grasping movements a stimulation combined from several electrodes must be used. This need has resulted in spatially bulky devices, tedious procedures for setting them up and, in the case of implanted electrodes, complicated and risky surgical procedures.
An interdisciplinary team of engineers and clinicians at the University of Montpellier in France has now taken an alternative route to functional electrical stimulation. In a clinical study with tetraplegic patients, they implanted a multi-contact nerve cuff electrode by CorTec instead of the classical ring type electrodes around the radial or median nerves. A total of 12 electrode contacts were placed at different positions along and around the nerve. By steering the stimulation currents through different combinations of the 12 electrode contacts the authors were able to reproducibly elicit an astonishing variety of different movements, including the elbow, wrist and fingers. In this current steering procedure the electrical currents seem to take different routes through the nerve and thus activate different functional compartments of the nerve called fascicles that are connected to different muscles or muscle subdivisions.
The results of the French researchers impressively demonstrate that by applying such multi-contact electrodes, it could be possible to build much more versatile and flexible systems for functional electrical stimulation that efficiently and robustly produce a variety of movements through a single implanted electrode. Such systems could help not only to minimize the surgical risks, but above all to maximize the practical benefits for patients and improve their quality of life.
Wafa Tigra, Christine Azevedo, Jacques Teissier, Anthony Gelis, Bertrand Coulet, Jean-Louis Divoux, David Guiraud (2019): “Implanted Nerve Electrical Stimulation allows to Selectively Restore Hand and Forearm Movements in Patients with a Complete Tetraplegia”. BioRxiv, doi: https://doi.org/10.1101/534362