Targeted reinnervation enables amputees to control motorized prosthetic devices and regain sensory feedback. The method was developed by Dr. Todd Kuiken at Northwestern University and Rehabilitation Institute of Chicago and Dr. Gregory Dumanian at Northwestern University Division of Plastic Surgery.
Overview Targeted reinnervation has an efferent and an afferent component. Targeted muscle reinnervation is a method by which a spare muscle (the target muscle) of an amputated patient is denervated (its original nerves cut and/or de-activated), then reinnervated with residual nerves of the amputated limb. The resultant EMG signals of the targeted muscle now represent the motor commands to the missing limb, and are used to drive a motorized prosthetic device. Targeted sensory reinnervation is a method by which skin near or over the targeted muscle is denervated, then reinnervated with afferent fibers of the remaining hand nerves. Therefore, when this piece of skin is touched, it provides the amputee with a sense of the missing arm or hand being touched.
Motivation Several methods exist that seek to achieve advanced control of motorized neural prosthetics. Chronic brain implants record neuronal signals from the motor cortex, while methods such as EEG and fMRI obtain motor commands non-invasively. The recorded signals are decoded into electrical signals, and input into assistive devices or motorized prosthetics. Traditional myoelectric prostheses utilize surface EMG signals from the remains of the amputated limb. For example, a patient may flex a shoulder muscle in order to generate EMG signals that may be used to send “bend elbow” command to the prosthesis. However, there are shortcomings to all of these methods. Chronic implants fail over a period of time because neuronal signal degrade due to tissue immune response to foreign bodies. EEG and fMRI do not obtain as strong signals as direct electrode implant. Traditional myoelectric prostheses are unable to provide multiple control signals simultaneously, thus only one action can be performed at a time. They are also unnatural to use because the users have to use muscles (such as shoulder) that are not normally involved with lower arm functions to control lower arm functions (such as opening and closing hands). The solution to these problems could include a completely different concept of neural interface.
Advantages Targeted reinnervation does not require any implants. Therefore, it does not have the issue of tissue foreign body response as chronic brain implant technology does. The targeted muscle acts as a natural amplifier for the neuronal signals produced by the transferred residual nerves. This is an advantage over technologies like EEG and fMRI that utilize weaker signals. With targeted reinnervation, multiple yet independent EMG signals can be produced, thus multiple functions of the artificial limb can be controlled simultaneously. For example, the patient would be able to perform actions such as throwing a ball relatively gracefully, exhibiting simultaneous control of elbow and hand. The control is also intuitive to the patient because the EMG signals are generated by transferred residual limb nerves, unlike traditional myoelectric prosthetics where EMG signals have to be generated by muscles normally not involved in arm or wrist functions. Also, existing commercially available myoelectric prostheses, such as powered wrists, elbows can be used. There is no need to develop specific prostheses for targeted reinnervation. By means of nerve transfer, targeted reinnervation can also provide sensory feedback, which has not been achieved by any other form of prosthetics aforementioned.
Methods
Targeted Muscle Reinnervation The goal of targeted muscle reinnervation is to transfer multiple nerves into separate regions of the targeted muscle, record multiple yet independent signals from the muscle regions, and to use the EMG signals to control a motorized prosthesis sophisticated enough to process multiple control signals.
Surgical Procedure The requirement to transplant multiple nerves into a muscle region originated from a hypothesis that hyper-reinnervation, by which an excessive amount of motor neurons transferred to a muscle, can increase the reinnervation of muscle fibers hence improving the recovery of paralyzed muscles. The hypothesis was tested on rat skeletal muscles and the result indicated that hyper-reinnervated muscles recovered more muscle mass and strength and more number of motor units were formed. The first surgical patient was a bilateral shoulder disarticulation amputee. Both arms were entirely amputated at the shoulder level, with only the shoulder blades remaining. The pectoral muscles were chosen targets because they were close to the shoulder, and they were also biologically non-functional due to detachment from the amputated arm. The pectoral muscles were first denervated by cutting the original nerves that innervate them. The proximal ends of the original nerves were ligated to prevent them from reinnervating the pectoral muscle. Then the remnant arm nerves (brachial plexus) were transferred into the pectoral muscles. The musculocutaneous nerve was transferred to the clavicular head of the pectoralis major muscle; the median nerve was transferred to the upper sternal of the pectoralis major muscle; the radial nerve was transferred to the lower sternal head of the pectoralis major muscle. The pectoralis minor muscle was translocated from under the pectorialis major muscle to the lateral chest wall, so that its EMG signals would not interfere with those of the pectoralis major muscle, and it is also a fourth muscle target. The ulnar nerve was then transferred to the moved pectoralis minor muscle. The musculocutaneous, median, radial, and ulnar nerves (brachial plexus) were sewn onto the distal ends of the original pectoral muscle nerve fascicles and onto the muscle itself. Subcutaneous fat over the pectoral muscle was removed so that the electrodes can be as close to the muscle as possible to obtain optimal EMG signals.
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