A US man who was paralysed from the waist down following a snowmobile accident five years ago is now able to stand and walk with assistance again following pioneering surgery.
The treatment involved implanting a spinal cord stimulator in Jered Chinnock's back below the injury site, followed by physical therapy over a 43-week period.
Now with the stimulator turned on, Mr Chinnock can think about and intentionally step with the help of a front-wheeled walker, with trainers providing occasional assistance and support.
It is the first time that chronically-dormant human spinal cord circuits have been reactivated enough to enable a patient to stand and walk again.
"What this is teaching us is that those networks of neurons below a spinal cord injury still can function after paralysis," said Dr Kendell Lee, co-principal investigator, neurosurgeon and director of the Mayo Clinic's Neural Engineering Laboratories.
Mr Chinnock, from Minnesota, who is now 29 years old, injured his spinal cord in the middle of his back in a snowmobile accident in 2013.
He was left with complete loss of movement, function and sensation below the position of the injury - effectively from the middle of his torso down.
In 2016, following 22 weeks of physical therapy which was unsuccessful in promoting recovery, he had an electrode implanted near his spinal cord which then allowed neurons to receive signals when he wanted to stand up or take a step.
The electrode, a long-existing product used for treating pain, is connected to a pulse generator that lies beneath Mr Chinnock's abdomen skin and communicates with an external controller wirelessly.
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Over the course of 113 return visits to the Mayo Clinic over 43 weeks, Mr Chinnock began to increase his voluntary ability to take steps with assistance.
Eventually he became capable of walking over ground using a front-wheeled walker and step on a treadmill placing his arms on support bars to help with balance.
When the electrode stimulation was off, however, Mr Chinnock remained paralysed.
Because he still cannot feel his legs, the researchers place mirrors at his legs so he can see what is happening.
In total, during individual therapy sessions Mr Chinnock has been able to take up to 331 steps over 102 metres, walking for 16 minutes at an average speed of 0.2 metres per second.
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"This technology is not to regrow the nerves and in fact the device we used to stimulate the nerve is well below the level of the injury, and so we know this is unlikely to have regrown the nerve," said Dr Lee.
However, researchers are not entirely sure exactly what caused the seemingly miraculous result.
"Now I think the real challenge starts, and that is understanding how this happened, why it happened, and which patients will respond," said Dr Kristin Zhao, co-principal investigator and director of the Mayo Clinic's Assistive and Restorative Technology Laboratory.
The results of the research, carried out by the Mayo Clinic in collaboration with University of California Los Angeles, are published in the journal Nature Medicine.
The research surgery has since been carried out on another patient, but the scientists have not revealed whether similar results have been recorded.
