Modern prostheses offer patients who have had a hand amputated much greater capability in everyday life than was possible with previous prosthetic reconstructive techniques. Redundant nerves from the amputated extremity can be surgically transferred to provide a much better connection between the patient’s body and the prosthesis. This technique has proven to be successful, although the specific reasons for its success were not fully understood. A team of researchers led by Konstantin Bergmeister and Oskar Aszmann from the Division of Plastic and Reconstructive Surgery and the Christian Doppler Laboratory for Recovery of Limb Function at MedUni Vienna, demonstrated, in an animal model, that the key to success lies in the muscle undergoing a change of identity triggered by the donor nerve.
Bionic prostheses are mentally controlled, in that they register the activation of residual muscles in the limb stump. Theoretically it should be possible for the latest generation prostheses to execute the same number of movements as a healthy human hand. However, the link between man and prosthesis is not yet capable of controlling all mechanically possible functions, because the interface between man and prostheses is limited in terms of signal transmission. "If we could solve this problem, the latest prostheses could actually become an intuitively operated replacement that functions just like a human hand," underscore the researchers.
To enable the prosthesis to move at all, nerves have to be surgically transferred during the amputation procedure to increase the total number of muscle control signals. This involves connecting amputated peripheral nerves to residual muscles in the amputation stump. This method is very successful, because these muscles regenerate after a few months to provide better control of the prosthesis. However, until now, it was not clear what specific changes this nerve transfer technique produces in muscles and nerves.
Previously unknown neurophysiological effects discovered
As part of an experimental study conducted over several years, a research team led by Konstantin Bergmeister and Oskar Aszmann from MedUni Vienna’s Division of Plastic and Reconstructive Surgery (Head: Christine Radtke) and Christian Doppler Laboratory for Recovery of Limb Function have now shown that this nerve transfer technique has previously unidentified neurophysiological effects. These result in more accurate muscle contractility and much more finely controlled muscle signals than previously thought.
It was also found that muscles take on the identity of the donor nerves, that is to say the function of the muscle from which the nerve was originally harvested. This means that muscles can be modified very specifically to achieve the desired control of the lost extremity. This information will now be used in follow-up studies to refine the surgical technique of nerve transfer and adapt it more accurately to fine control systems. The vision of an intuitively controlled prosthesis that can perform all the natural manual functions could become a reality within the next few years.
"Peripheral nerve transfers change target muscle structure and function." K. D. Bergmeister, M. Aman, S. Muceli, I. Vujaklija, K. Manzano-Szalai, E. Unger, R. A. Byrne, C. Scheinecker, O. Riedl, S. Salminger, F. Frommlet, G. H. Borschel, D. Farina, O. C. Aszmann, Sci. Adv. 5, eaau2956 (2019).
The study, which has now been published in prestigious journal Science Advances, was conducted as a joint international project by Imperial College London and partners at MedUni Vienna. It was financed by the ERC Advanced Grant DEMOVE (D.F. contract #: 267888) and by the Christian Doppler Research Association of the Austrian Federal Ministry for Science, Research and Economic Affairs.