Targeted reinnervation enables amputees to control motorized prosthetic devices and regain sensory feedback.
[2] Several methods exist that seek to achieve advanced control of motorized neural prosthetics.
[3] Traditional myoelectric prostheses utilize surface EMG signals from the remains of the amputated limb.
[5] 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.
Chronic implants fail over a period of time because neuronal signal degrade due to tissue immune response to foreign bodies.
[4] Traditional myoelectric prostheses are unable to provide multiple control signals simultaneously, thus only one action can be performed at a time.
The targeted muscle acts as a natural amplifier for the neuronal signals produced by the transferred residual nerves.
[1] For example, the patient would be able to perform actions such as throwing a ball relatively gracefully, exhibiting simultaneous control of elbow and hand.
By means of nerve transfer, targeted reinnervation can also provide sensory feedback, which has not been achieved by any other form of prosthetics aforementioned.
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.
During training session, the patient was sitting in an upright position and shown each of the 27 normal movements (such as shoulder adduction/abduction, hand open/close, elbow flexion/extension etc.)
[10] After surgery, the patient was fitted with his pre-surgery body-powered prosthesis on the right side and an experimental myoelectric prosthesis consisted of a Griefer terminal device, a power wrist rotator, a Boston digital arm, and an LTI-Collier Shoulder joint on the left side.
[6] The performances of these two prostheses were compared with a box-and-blocks test, where the patient was allowed 2 minutes to move one-inch cubes from one box to another, over a short wall.
[6] To fully utilize the multiple signals provided by targeted reinnervation, an experimental prosthesis was constructed with added power components: a TouchEMAS shoulder, a humeral rotator, and a hand capable of opening and closing with wrist flexion/extension function.
With this six-motor prosthesis, the patient could control multiple joints at the same time and perform new tasks that could not be accomplished with other prostheses, such as reaching out to pick up objects and putting on a hat.
[2] Since then, areas of the pectoral muscle have been mapped to parts of arm and hand according to patient's description of touch sensations he felt.
With this discovery, the team set out to perform nerve transfer surgery specifically aimed to reinnervate sensory feedback.
[2] This technique has been dubbed “transfer sensation”, and it has the potential of providing useful sensory feedback, such as pressure sensing, to help the patient judge the amount of force to be exerted.
[2] After surgery, the patient was asked to identify the chest areas with most prominent sensation of individual digits, which were then mapped onto a diagram.
[2] A Neurotip neurometer was used to determine the sensibility of sharpness and dullness at 20 sites distributed throughout the targeted muscle (the chest).
[2] However, instead of normal pressure sensing, she perceived tingling in response to touch on the targeted chest skin.
[1] The muscle region was not reinnervated as expected, but instead turned bluish after mobilization, possibly due to a congestion of vascular supply.
[1] This could also prompt the production of more sophisticated prosthetic devices with more degrees of freedom, such as the six-motor experimental prosthesis mentioned above.
[1] Much work is still to be done to translocate the sensory feedback from the reinnervated target muscle to the actual prosthesis, or to construct prostheses that are capable of providing appropriate stimuli to the reinnervated target muscle according to the external stimuli received, so that the sensory feedback of the arm comes from its native physical position.
Beginning in 2016, the Applied Physics Laboratory at Johns Hopkins began working with a patient having undergone both targeted muscle reinnervation and osseointegration of a titanium port to test and perfect their design for the Modular Prosthetic Limb funded by DARPA[11]