Functional Electrical Stimulation (FES) of Denervated Muscles: Existing and Prospective Technological Solutions

Recent experimental and clinical work gives strong evidence that functional electrical stimulation (FES) is a powerful tool for regeneration, functional restoration and maintenance of denervated musculature, a fact that for various reasons was not recognized in the past. On reason was the lack of associated technology that in comparison to existing FES equipment for nerve stimulation has to meet various completely different demands. Most of the few existing stimulators for denervated muscles provided by industry or published in conjunction with scientific studies are not sufficient for muscle restoration and maintenance due to their limited range of electrical parameters obviously with respect to the presently too restrictive EU regulations for stimulation devices. In order to be effective a stimulator for activation of denervated skeletal muscles via surface electrodes requires biphasic long-duration impulses with a pulse width between 10 and 300ms and amplitudes of up to ± 100 V respectively ± 250 mA. These demands rise safety issues in the design and application of both the stimulator and the electrodes. In Vienna a sufficiently working prototype equipment was developed and clinically tested. The alternative of an implantable stimulation system is not jet available for clinical application, but an experimental prototype system was tested in an experimental study on sheep for up to 18 months. Stimuli of similar pulse width are required, but due to the direct electrode attachment to the muscle with less amplitude. Specific solutions are needed in conjunction with power supply, end stage and electrode design to induce a suitable electrical field while avoiding tissue damage.