Neuroprostheses may provide the most promising gains in arm and hand function to individuals with SCI (Kilgore et al. 2018). Neuroprostheses utilize functional electrical stimulation or myoelectrically controlled systems to move prostheses or robotic end effectors. This is achieved through stimulation of residual motor nerves via transcutaneous, percutaneous, or implanted electrodes (Krucoff et al. 2016). Transcutaneous stimulation utilizes electrodes placed on the surface of the skin to stimulate a motor point of the muscle of interest (Baker et al. 1993; Mortimer 1981) while percutaneous and fully implanted electrodes are placed under the skin or in the muscle to stimulate the motor nerve of the muscle of interest (Cameron et al. 1997; Hoshimiya & Nanda 1989).
A variety of neuroprosthetic systems exist including the Handmaster-NMS-1, BGS, and ETHZ-ParaCare systems. All have been applied successfully as rehabilitation tools to restore grasping function in individuals with SCI. However, the most widely used neuroprosthesis for grasping is the Freehand system. Generally, to control the neuroprosthesis, individuals use an on/off switch or apply analog sensors to generate a desired command. There is usually a time delay of one or two seconds from command issue to grasp execution. Therefore, the speed that an individual can grasp and release objects is somewhat limited. Besides the technological drawbacks of neuroprostheses, an important barrier contributing to the use of neuroprostheses (or lack thereof) is the commercial availability of the device. Despite demonstrated improvements in upper extremity function and QOL following stroke or SCI, only one device is commercially available (Venugopalan et al. 2015). For a full list of the benefits and drawbacks of neuroprostheses, please refer to Table 10.
Neuroprostheses can increase independence, reduce the need for other assistive devices, and decrease the time it takes to carry out activities of daily living (Kilgore et al. 2018). As such, neuroprotheses are typically used to complete tasks such as eating, drinking and personal hygiene. It is important to note that neuroprostheses are distinct from brain computer interfaces. Neuroprostheses connect any part of the nervous system to a device, whereas BCIs connect the brain with a computer and/or robotic system (Krucoff et al. 2016).
With advances in the technological capacity of neuroprostheses, many studies have examined their use in individuals with SCI. As such, the methodological details and results from 18 studies are presented in Table 11.
A multitude of studies have investigated the feasibility and efficacy of neuroprostheses for SCI rehabilitation. Based upon the literature, a variety of neuroprostheses exist including myoelectrically controlled neuroprostheses, the Freehand system, Ness H200, and the EHTZ Paracare system. Despite several differences between these systems, all studies demonstrated that use of the system was feasible and more importantly, efficacious. All of the neuroprostheses used resulted in significant positive functional outcomes for individuals with SCI. However, the commercial unavailability of these devices impacts clinical use greatly.
The Freehand System results in significant positive functional outcomes for individuals with tetraplegia, however, there is limited opportunity for standardized clinical use at this time as the device is not commercially available. In addition, most patients need to undergo multiple surgeries for the implantation of electrodes and other various components of the device in order to gain optimal use of the system. This represents another barrier to the wide spread application of the Freehand System.
The NESS H200 developed by Nathan et al., and produced by Neuromuscular Electrical Stimulator Systems, Ra’anana, Israel is the only commercially available upper limb surface FES system (Ragnarsson 2008; Venugopalan et al. 2015). It has been FDA approved for use with individuals with stroke and SCI. It is predominantly used as an exercise tool for stroke subjects and is commercially available in a limited number of countries (Popovic et al. 2002). The NESS H200 has three surface stimulation channels used to generate grasping function in tetraplegia and stroke subjects. One channel is used to stimulate the extensor digitorum communis muscle at the volar side of the forearm. The second channel stimulates the flexor digitorium superficialis and profundus muscles. The third stimulation channel generates thumb opposition. The system is controlled with a push button that triggers hand opening and closing functions. The system is easy to don and doff. However, there are some limitations in its design: the rigid arm splint does not provide enough flexibility of the electrodes for stimulation of the finger flexors for grasp, and it is a stiff orthosis that fixes the wrist joint angle and prevents full supination of the forearm (Popovic et al. 2002).
The ETHZ-Para Care System was developed collaboratively between ParaCare, the University Hospital Zurich, the Rehabilitation Engineering Group at Swiss Federal Institute of Technology Zurich and Compex SA, Switzerland. The system was designed to improve grasping and walking function in SCI and stroke patients. Surface stimulation FES system is programmable, with four stimulation channels and can be interfaced with any sensor or sensory system. The system provides both palmar and lateral grasps. The device has some reported disadvantages that include a lengthy time to don and doff (seven to ten minutes), and it is not commercially available. The next generation of the device will be called the Compex Motion (Popovic et al. 2001; Popovic et al. 2006). The Compex Motion device is currently available in clinical trials with approximately 80 units available. The Compex Motion stimulator was designed to serve as a hardware platform for the development of diverse FES systems that apply transcutaneous (surface) stimulation technology. One of the main advantages in this system is that it is easily programmable (Popovic et al. 2006).
In summary, neuroprostheses are a promising rehabilitative therapy for SCI. Use of a variety of systems demonstrates significant improvements in hand function and quality of life. However, the lack of commercial availability and invasiveness of surgery are deterrents to its clinical use. Future research should focus on developing an affordable and easily accessible neuroprosthesis system.
There is level 4 evidence (from two pre-post tests: Kilgore et al. 2018 and Kilgore et al. 2008) that a surgically implanted neuroprosthesis significantly improves grip strength/pinch force to enhance hand function and ADLs in individuals with SCI.
There is level 4 evidence (from five pre-post studies: Peckham et al. 2001; Taylor et al. 2001; Hobbey et al. 2001; Carroll et al. 2000; Mulcahey et al. 1997) that the implanted Freehand System results in positive increases in grip strength, grasping and overall independence.
There is level 4 evidence (from two pre-post studies: Alon and McBride 2003; Snoek et al. 2000) that with sufficient practice using the NESS H200 neuroprosthesis, individuals with SCI may regain grasp, hold and release abilities.
There is level 4 evidence (from eight case series: Mulcahey et al. 2004; Memberg et al. 2003; Taylor et al. 2002; Bryden et al. 2000; Wuolle et al. 1999; Kilgore et al. 1997; Smith et al. 1994; Smith et al. 1996) that the implanted Freehand System increases grip strength, grasping, ADL and function, and overall independence.
There is level 4 evidence (from one case series: Mangold et al. 2005) that the ETHZ-ParaCare neuroprosthesis is flexible (non-surgical) and has significant positive outcomes in rehabilitation and the ability to perform daily living tasks.