Targeted Muscle Reinnervation (TMR)
TMR is a procedure for arm amputees that reassigns nerves, giving an amputee the opportunity to use a prosthesis that allows for elbow, wrist, and hand movements. The procedure is performed differently depending on the type of arm amputation. TMR is paired with extensive rehabilitation allowing the patient to learn how to use the advanced prosthesis. It should not be confused with an another TMR acronym that stands for Trans Myocardial laser Revascularization, a procedure by which a physician uses a laser to make holes in the heart to relieve the pain of severe angina.
The goal of TMR is to achieve a higher level of function and more precise control, especially in upper extremity amputations at the elbow or higher. Coordinating motions such as elbow flexing and hand gripping are possible with TMR.
Each time a movement occurs, such as bending the elbow or twisting the wrist, signals go from the brain through the nervous system to the appropriate muscle that then performs the movement. When the arm (specifically the shoulder and below) is amputated, the nerves that once allowed for the control of the hand, wrist, and elbow are lost. The nerves damaged in arm amputees include the musculocutaneous nerve (controls upper arm muscles such as the coracobrachialis, biceps and medial brachialis), median nerve (controls the forearm, wrist and hand movement), radial nerve, (controls hand open and wrist-up movements) and ulnar nerve (controls grasping motion).
There are two different types of arm amputations: everything below the shoulder has been amputated (shoulder disarticulation amputation) and everything below the top of the elbow has been amputated (above-elbow amputation).
Traditionally, most motorized artificial limbs have been controlled by electrical signals (electromyogram-EMG) from pairs of muscles in the amputated limb. This allows for isolated but not coordinated motion.
Targeted Muscle Reinnervation (TMR) is an innovative surgical procedure developed in the early 2000s that provides easier, more intuitive prosthesis control for individuals with above-elbow or shoulder amputations. This technology works by reassigning nerves that once controlled the arm and hand, giving an above-elbow or shoulder amputee freedom to control their prosthesis. There are so many nerves and muscles in the body that reassigning one will have little to no effect on daily functions.
With TMR, there are 5 movements the prosthesis can do: elbow up, elbow down, hand open, hand close, and twist wrist. All these movements are controlled by the brain, thanks to the relocated nerves. In the past, above-elbow and shoulder amputees had significantly less control over their prosthetics and could at most move their elbow up and down. Being fitted with a TMR prosthesis completely changes their lives by giving them more control over their prosthesis and increases their independence.
TMR is best suited for above-elbow and shoulder amputees because they have completely lost all nerves and muscles that control the arm and hand. In the case of below-elbow amputees, they still have some of the muscles and nerves left in their forearm which fire when the brain signals their hand to open or close. These signals enable the prosthetic device to operate with normal brain thoughts.
TMR is only available for amputees who:
- Amputated above the elbow or at the shoulder within the last 10 years
- Have stable, soft tissues
- Are willing to take part in rehabilitation
There are two different ways to perform the procedure – one for above-elbow amputees and one for shoulder amputees. For above-elbow amputees, the musculocutaneous nerve, which controls shoulder and elbow movement and is located in the middle of the arm, is removed from one of the two bicep muscles. The median nerve, which controls the forearm, wrist and hand movement and is located in the inner upper part of the arm, is removed and placed where the musculocutaneous nerve originally was.
For shoulder disarticulation amputees, instead of rewiring muscles from the upper arm, three of the chest muscles (pectoralis major, the pectoralis minor, and serratus) are rewired. When rewiring the chest muscles, an original nerve must be removed from each and replaced with the musculocutaneous nerve, median nerve, ulnar nerve, and radial nerve. The procedure itself lasts approximately four to six hours. Successful TMR means that amputees now have a physiologic conduit to the brain for controlling a prosthetic arm and hand, using their body’s own nerves and muscles.
The procedure itself lasts approximately two hours. If the transfer is successful, within two to six months the reassigned nerve will grow into the muscle, arborize, and find endpoints. Once the muscle is reinnervated and begins getting signals, the patient will need to build up the strength of those signals by exercising that muscle. When they are able to generate a signal that can be read by the sensors, that signal is used for prosthetic rehabilitation. After a successful transfer, when the patient thinks about moving his hand, the rewired muscle will move, allowing the hand to move. The muscle that the nerves were removed from will not function since the nerve controlling it has been removed. Fortunately, there are plenty of other muscles and nerves that perform the same or similar action to the nerves and muscles that were deactivated.
Recovery is a reasonably long process due to the rehabilitation. For trauma patients, TMR adds no extra length to their hospital stay. For those who have the TMR surgery at a later date, the hospital stay time varies based on the type of amputation. Above-elbow amputees need a day, while shoulder disarticulation amputees need more time since the surgery is more complicated. After TMR, the patient can get fitted for a myoelectric-controlled prosthesis – an externally powered artificial limb that is controlled with the electrical signals generated naturally by your own muscles. Until the prosthesis is ready, the patient can wear their regular prosthesis. To use the prosthesis, the patient will have to go through focused rehabilitation that involves coordinative and neuromuscular training. The rehabilitation teaches the patients how to use the prosthesis as well as helps with the healing process.
Surgeons at Florida Orthopaedic Institute are fully trained and skilled in TMR. If you, or someone you know, is a potential candidate for TMR surgery, please make an appointment for a consultation.