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Second Generation Neuroprosthesis: Myoelectrically Controlled

Second generation of neuroprosthesis (NP) being developed at Case Western Reserve , Cleveland, OH, USA, provides control of grasp, forearm pronation and elbow extension through the use of electromyographic signals generated by voluntary musculature used to control the various functions of the NP. The system consists of an implanted stimulator-telemeter (IST-12), implanted electrodes for stimulation and recording, an external control unit and a transcutaneous inductive link. The EMG signals can be obtained from two independent muscles. The IST-12 is capable of stimulating 12 different muscles. The design feature of the IST-12 enables the size of the implanted components to remain small and provides the opportunity for customized control algorithms and stimulation patterns. The NP functions controlled via EMG signal includes grasp pattern selection (2-4 grasp patterns), opening and closing the hand in a proportional manner, turning the system on and off, turning elbow extension on and off, and the ability to lock and unlock the hand so that the grasp can be maintained in a fixed position without the need for continued control input. One EMG channel is used to control grasp opening and closing and is generally placed on the most distal UE muscle under voluntary control, typically the extensor carpi radialis longus (ECRL) and brachioradialis (Br). The second EMG channel is used to provide state or logic commands such as system on/off and selection of grasp pattern. The latter channel is placed on a more proximal muscle such as trapezius and platysma. All of the control signals are derived from ipsilateral muscles, enabling bilateral function to be provided by implementing a second system in the contralateral limb (Kilgore et al. 2008).

Augmentive surgical procedures (arthrodeses, tendon transfers and tendon synchronization using side by side repair) to the hand and arm are often performed at the same time of the implantation to provide improved hand function when the IST-12 is not being used and to further optimize the system with electrically stimulated transfers (Kilgore et al. 2008).

Further research is also being completed on an implantable stimulator and wearable external controller (Micropulse) at the Cleveland FES Centre, Cleveland, USA. The controller, under going benching testing, is worn on the wrist and wirelessly communicates with the implantable stimulator (Wheeler & Peckham 2009).

Another device being developed is the myoelectrically controlled functional electrical stimulation (MeCFES) in Denmark. This system consists of an amplifier, a signal processor and single-channel stimulator that allows the user to proportionally control the stimulus intensity to reinforce the tendosis grip in subjects with C6-7 tetraplegia.

Table: Second Generation Neuroprosthesis : Myoelectrically Conrolled

The study by Kilgore et al. (2008) studied the first three individuals to receive a myoelectrically controlled NP (IST-12) designed for hand and arm function. The IST-12 can stimulate 12 different muscles versus the first generation NP (Freehand System) which only stimulated 8 muscles. The additional electrodes provided the user with improved hand function (activation of the intrinsic muscles), wrist extension, improved reach (triceps function) and improved shoulder stability through activation of the shoulder musculature. The IST-12 provided each of the three subjects with significantly increased pinch force and grasp function which resulted in increased independence with ADL functions. All three study subjects used the device at home on a regular basis. The durability of the implanted components was comparable to long-term results from the first generation NP, an incidence failure of<2%.

Conclusion

There is level 4 evidence (from one pre-post study; Kilgore et al. 2008) that the use of the IST-12, a second generation neuroprosthesis, combined with augmented surgical procedures (arthrodesis, tendon transfers and tendon synchronization) improved pinch force, grasp function and the functional abilities of individuals with cervical level spinal cord injuries.

  • The IST-12 neuroprosthesis, a second generation, myoelectrically controlled implantable device appears to have a positive effect on pinch and grasp functions which result in increased independence with activities of daily living.