September 27, 2013
A Step Forward for Artificial Limbs
Four years ago Zac Vawter was in a motorcycle collision that severely injured his right leg. He underwent an amputation at the knee, thereby becoming one of the more than one million amputees living in the U.S. Over half of the amputations in the U.S. are due to vascular disease – poor circulation caused by diabetes, smoking, and high cholesterol. Just under half are due to trauma.
Modern leg prostheses that replace both the knee and ankle joint use motors that power each joint and multiple sensors that inform the prosthesis about its orientation, movement, and load. The prosthesis software has several modes or programs that it can execute – walking on a level surface, climbing stairs, even running. To switch modes the user typically uses a remote control on a key fob. So a user might arrive at the bottom of a flight of stairs, push a button to get the prosthesis into “climbing stairs” mode, and then at the top push another button to go back to “walking on level ground” mode.
This week’s issue of the New England Journal of Medicine (NEJM) published a report of a novel prosthesis interface that is much safer and more intuitive. A similar interface has been used to control prosthetic arms, but this has been the first application in a prosthetic leg. Zac Vawter was the subject who worked with the research team and helped them customize and test the prosthesis.
The novel interface uses electrodes that measure electrical activity in the user’s thigh. Prior to using the prosthesis the researchers made multiple recordings from the electrodes as Vawter was told to move his (absent) knee and ankle in various directions. From these recordings software was used to decode his intentions from the electrode signals. The results are a very natural interface. For example when Vawter tries to bend his ankle to point his toes up, his prosthesis senses this and executes his will.
This allows Vawter to go from walking to climbing stairs to descending a ramp by simply thinking about what he wants his prosthesis to do; no manual switching; no remote control. Take a look at the videos in the NEJM report and this YouTube video to appreciate how natural and fluid his gait appears.
Many improvements will be needed before such a prosthetic can be put into production. The current prototype is too heavy, too big, and too noisy. And the researchers say that the software that decodes the electrode signals must be further refined.
This has the potential of making many amputees much more mobile. Of course, as treatment for diabetes, high cholesterol, and vascular disease improves, there will be many fewer amputees in the future. Driverless cars and improved worker safety may decrease the number further.
It will be exciting to watch this technology mature. In the meantime, I wish Mr. Vawter all the best, and I advise anyone who’ll listen not to ride motorcycles.
Man Controls Artificial Leg Using Only His Brain, Researchers Say (WSJ)
The amputation statistics I cited are from the Amputee Coalition website.
Important legal mumbo jumbo: