TARGETED MUSCLE REINNERVATION (TMR)
Targeted Muscle Reinnervation (TMR) is a surgical technique that was conceptualized and successfully carried out by Todd A. Kuiken and Gregory A. Dumanian, as a means of providing more conscious control to the prosthetic arms used by upper limb amputees.
Targeted Muscle Reinnervation (TMR) is a surgical technique that was conceptualized and successfully carried out by Todd A. Kuiken and Gregory A. Dumanian, as a means of providing more conscious control to the prosthetic arms used by upper limb amputees. This revolutionary surgical procedure allows the amputee to use their own residual nerves and muscles to actively control their prosthesis through targeted reinnervation.
The residual nerves in the amputee’s limb are transferred to muscles that are no longer functioning due to the amputation. The nerves that innervated the different areas of the arm prior to the amputation are transferred with the purpose of reinnervating the muscles and stimulating muscular contractions to allow the amputee to use them as new control sites for controlling the prosthetic arm. The non-functioning muscle is divided into different segments and the nerve is surgically relocated, so that the neural information from the nerve can be used to conduct muscular contractions in the severed muscle. Once the nerves grow within the muscles and reinnervate them, these areas begin to set up a ‘targeted’ channel of communication in a way that electric signals can be picked up from these muscles through the electrodes of the bionic arm. These EMG signals are then amplified and processed to intuitively control the terminal device and the prosthetic elbow unit.
TMR surgery is generally performed for amputees at the higher levels, that is, transhumeral and shoulder disarticulation levels as these levels usually witness the maximum prosthesis rejection rates and TMR can offer an innovative solution to solve the problems that arise regarding the prosthetic control for higher level amputees. The purpose of performing a TMR procedure is usually to set up new control sites that will help in opening and closing of the prosthetic terminal device as well as for elbow flexion and extension. The nerves that are commonly involved in TMR include the ulnar, median, radial and musculocutaneous nerves. For transhumeral amputees, the TMR procedure creates at least 2 control sites and for shoulder disarticulation amputees, it can create up to 4 control sites.
Amputees with below the elbow amputations usually have some amount of residual muscles in the forearm that can produce the required signals when commanded by the brain to control the prosthesis for opening and closing the terminal device. However, ongoing research is attempting to develop the TMR technology for transradial amputees to help them control multi functional hands with ease.
TMR is usually indicated for amputees who are not able to successfully control their prosthetic devices and have long residual limbs as these limbs contain nerves that are long enough to be relocated. TMR is not an option for patients with brachial plexus injuries or avulsive proximal nerves injuries because the nerves that are being used must be functionally and cortically connected to be effective. Some other contraindications include residual limbs with excessive scars or skin grafts and for residual bones with heterotopic ossification.
Fitting a Bionic Arm Following Targeted Muscle Reinnervation
Although challenging, prosthetic fitment after TMR has proved to be quite successful and helpful for upper limb amputees, allowing them to easily and intuitively control their bionic arms in a more natural and realistic manner. Stabilization of the reinnervated nerve can take up to 6 months following the surgery, and control site selection must be attempted only after this stabilization has been achieved.
Advantages of TMR for Bionic Hand Fitment
- Since TMR allows for selecting up to 6 control sites and use 6 electrodes to control the bionic arm, the functions and movements carried out by the arm are quicker and smoother as compared to convention control strategies.
- This also makes the prosthetic control more intuitive and allows the amputee to control and move multiple joints simultaneously, just like in a normal anatomical limb.
- A major benefit of TMR is that it manages neuroma formation and phantom limb sensations by providing a different space for the neural axons to grow into.
- TMR allows for pattern recognition prosthetic control by allowing the amputee to use multiple grips and grasps since the nerve used for reinnervation is likely to contain motor neurons that had earlier innervated the hand muscles. This eliminates the need to constantly switch between different motions.
- Unlike implantable chips and sensors that are sometimes rejected by the body, TMR makes use of the body’s own nerves and muscles and thus, eliminates the possibility of break down or rejection.
- Moreover, certain clinicians have also reported that following TMR surgeries, some users have reported slight sensations and sensory feedback, which is being explored further to help the amputees to regain some amount of feeling while using the prosthetic arms.