The Science Behind TMR
The Science Behind Targeted Muscle Reinnervation (TMR)
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The brachial plexus nerves carry all the motor control information needed to move the arms, hands, and fingers. This vast trove of control information becomes inaccessible after arm amputation. TMR provides access to this information by enabling severed arm nerves to reinnervate redundant target muscles, which serve as natural biological amplifiers of these neural control signals. TMR has been performed successfully as long as six years after amputation of an arm.
Two basic features of neuromuscular physiology underlie the concept of TMR:
(1) Studies in animals and people have shown that even after limb amputation, the brain continues to transmit control signals through the severed residual nerves. This continues for long time periods—perhaps indefinitely [1-3].
(2) Animal studies have shown that severed nerves can grow into (reinnervate) different (non-native) muscle, and that this muscle contracts in response to signals from the new nerve .
Basic Principles of TMR
Successful TMR depends on two key factors:
(1) Good reinnervation of target muscles.
(2) Generation of strong, independent EMG signals.
For successful target muscle reinnervation:
Large brachial plexus nerves (8-10 mm diameter) are transferred to much smaller (up to 1 mm) motor nerve branches. Such hyper-reinnervation makes it more likely that all available motor endplates are reinnervated, thus ensuring good muscle recovery.
In a rat reinnervation model, bigger nerves (with more
motor neurons) ensure better muscle recovery.
(Click image to enlarge)
- The target muscle is denervated prior to nerve transfer. This ensures that the target muscle only contracts in response to intended arm movement, and that the maximum number of motor endplates are available for reinnervation by the transferred nerve.
For strong, independent EMG signals:
Separate target muscles to prevent crosstalk.
Separating adjacent target muscles creates more
independent EMG signals. (Click image to enlarge)
- Remove fat overlying the target muscle.
1. Gordon T, Stein RB, Thomas CK. Innervation and function of hind-limb muscles in the cat after cross-union of the tibial and peroneal nerves. The Journal of physiology. May 1986;374:429-441.
2. Davis LA, Gordon T, Hoffer JA, Jhamandas J, Stein RB. Compound action potentials recorded from mammalian peripheral nerves following ligation or resuturing. The Journal of physiology. Dec 1978;285:543-559.
3. Dhillon GS, Lawrence SM, Hutchinson DT, Horch KW. Residual function in peripheral nerve stumps of amputees: implications for neural control of artificial limbs. The Journal of hand surgery. Jul 2004;29(4):605-615; discussion 616-608.
4. Elsberg CA. Experiments on Motor Nerve Regeneration and the Direct Neurotization of Paralyzed Muscles by Their Own and by Foreign Nerves. Science. Mar 30 1917;45(1161):318-320.
Todd A. Kuiken, MD, PhD, began studying nerve transfers with the intention of producing new EMG signals for control of myoelectric prosthetic arms while in graduate school. Years of animal work and EMG simulation studies resulted in the first human nerve transfer surgery intended to improve prosthesis control, in 2002. The technique, called targeted muscle reinnervation (TMR), was successful and has since become an established clinical procedure, benefiting many patients across the US and overseas.
Dr. Kuiken leads an interdisciplinary team that includes physicians, prosthetists, therapists, neuroscientists, engineers, software developers, graduate students, and post-doctoral researchers at the Center for Bionic Medicine within the Rehabilitation Institute of Chicago. This combination of clinical and research expertize provides a unique environment in which to understand and develop TMR and to translate research data into clinical applications. Four integrated research groups within the Center for Bionic Medicine seek to study the functional and sensory benefits of TMR, to develop lighter, more functional prosthetic devices, and to design control systems to capitalize on the vast neural information made available by TMR. TMR has continued to evolve and improve, in particular with recent collaborative research on pattern recognition control. Dr. Kuiken has continued to lead efforts to understand and capitalize on the potential of TMR to provide improved prosthetic function.
Dr. Kuiken received a BS in biomedical engineering from Duke University, and a PhD in biomedical engineering and an MD from Northwestern University. He completed a residency in physical medicine and rehabilitation at the Rehabilitation Institute of Chicago and Northwestern University Medical School. In addition to leading the Center for Bionic Medicine, Dr. Kuiken is Director of Amputee Services at the Rehabilitation Institute of Chicago. He is also a Professor in the Departments of Physical Medicine and Rehabilitation, Surgery, and Biomedical Engineering at Northwestern University. Dr. Kuiken is the recipient of many awards and honors for his work on TMR and is an internationally respected leader both in research and the clinical care of people with limb loss.