Targeted Muscle Re-innervation, or simply TMR, is a novel procedure that involves the rewiring of the body’s peripheral nerves. While nerve transfers have been around for decades, they have mainly been in upper extremity nerve injuries. The TMR procedure is taking this concept of nerve transfers and applying it to amputees. The procedure was initially described by Drs. Todd Kuiken and Gregory Dumanian at Northwestern University in Chicago, Illinois as a method to create an amplified signal for myoelectric prosthetics of the upper extremity following an amputation. The concept revolves around taking the severed nerve endings in the shoulder area and rerouting them to much smaller motor nerves in the pectoralis major muscle. The pectoralis major muscle is able to be utilized for this nerve transfer since it is not a function muscle in the sense that it does not have an arm to move in the amputee, but is still a very viable muscle. Following the nerve transfer, the nerves that previously went to the arm will now innervate portions of the pectoralis major muscle to allow an electrical signal to be picked up through the skin. While the procedure was developed mainly for re-routing the motor nerves, there was a secondary benefit of improvement in the pain that the patients were experiencing from the cut nerve endings. The thought is that that the sensory component of the amputated nerves followed the motor component into the muscle as well, which essentially gives the sensory nerves a place to go and a job to do. Over time, the sensation of pain will decrease as the sensory fibers grow into the muscle. This procedure is now being applied to patients with amputation of both the upper and lower extremities for the sole purpose of managing the pain and has shown a significant benefit for this purpose in several studies thus far.
In order to better understand pain in the peripheral nervous system, we need to run through some basic concepts. The peripheral nervous system includes all the nerves once they leave the brain or spinal cord, which are referred to the central nervous system. These peripheral nerves have two main categories: motor nerves, which are responsible for muscle movement; and sensory nerves, which allow the sensation of touch, pressure, pain, etc. So, all nerves basically have a job to do, some will carry an impulse from your brain to a muscle for movement, while others carry various signals from the external environment to the brain to give the perception of sensation. When an amputation occurs, all these nerves going to the distal amputated part are severed, leaving these nerves without a job to do. The motor nerves no longer have a muscle to contract, and the sensory nerves no longer have the skin to provide input from the external environment. The cut sensory nerve trunk, then sends out small nerve endings to try and search for something to attach to; however, without a distal target, these nerve endings will form a ball of nerve tissue, called a neuroma. This neuroma can be thought of as a cut power line that is sparking on the ground, and when the neuroma is pressed, it will produce a jolt of pain. In the case of amputees, this can also produce phantom limb pain since that sensory nerve use to represent a portion of the body that is no longer present.
The TMR procedure has been one of the greatest advancements in the last century for the care of amputees since the amputation procedures have remained relatively unchanged during this time. Previously little thought was given to the cut nerve endings during or after the amputation. Once the limb was amputated and the incision healed, the surgeon had nothing else to really offer the patient. The patients would often go on to develop chronic pain, and the prosthetists, physiatrists, or pain management physicians were left to come up with solutions for the pain. Often this would include multiple prosthetic modifications, several pain medications, nerve blocks, and spinal cord stimulators to name a few. These are often very well-trained professionals, who were working within their respective skill sets since addressing the nerves surgically is still a foreign concept for many surgeons. As more physicians become aware of the benefits of this procedure in regards reducing pain in the amputee population, it will become more readily available in more centers. Currently it is often limited to major cities or academic medical centers; however, I am working with several other physicians across the country to train and educate physicians in this procedure.