Surgeons from China and Italy claimed that two studies published Wednesday add evidence to their ability to treat "irreversible" spinal-cord injuries and a related controversial aspiration to perform the world’s first human head transplant.

Xiaoping Ren and Sergio Canavero said the new work they published in a scientific journal showed that monkeys and dogs were able to walk again after their spinal cords were "fully transected" during surgery and then put back together again. The neurosurgeons described the results as medically "unprecedented."

The highly experimental procedures took place at Harbin Medical University in China. Both studies were supported by video evidence and published in Surgical Neurology International, a peer-reviewed medical journal based in the United States.

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Canavero, who is based in Turin, Italy, and has a reputation in the global medical community as something of a sensationalist, said that for too long neurological surgeons have "stuck to the view that a severed spinal cord cannot be mended in any way, a mantra uncritically repeated over and over."

He said that the two new papers published Wednesday, plus a previous study published last year in Surgery, a separate peer-reviewed medical journal based in the United States, "completely reject this view."

In an email exchange from Harbin in northern China, Xiaoping said the findings were proof that human trials should be initiated and he noted that while this particular "grand achievement" took place in China, he was open to doing it "anywhere in the world."

Still, even if the alleged advance – it involves applying a polyethylene glycol substance, or PEG, "to mend" severed or injured spinal cords – withstands further scrutiny and is ultimately capable of helping patients with spinal cord injuries and paralysis it is likely to be met with skepticism. That's partly because of the two researchers' past claims that they want to use the technique with PEG as a basis for human head transplants.

While the researchers have tested head transplants, with some success, on small animals including mice and dogs, it's a concept that raises raises profound ethical, psychological and surgical questions. It also calls to mind Mary Shelley’s 1831 novel Frankenstein, about a scientist who creates a creature from body parts.

"The Americans did not understand," Canavero told USA TODAY two years ago as he announced that he would soon perform the world’s first human head transplant in China because medical communities in the United States and Europe would not permit him to do it there. From space exploration to climate-change science, China has indicated it intends to lead, not follow, the U.S. in all the major scientific and technological frontiers over the coming decades.

More:Italian doctor says world's first human head transplant 'imminent'

Canavero estimated the procedure would cost up to $100 million and involve several dozen surgeons and specialists. He said the donor would be the healthy body of a brain-dead patient matched for build with a recipient's disease-free head.

The researcher said he would simultaneously sever the spinal cords of the donor and recipient with a diamond blade. To protect the recipient's brain from immediate death before it is attached to the body, it would be cooled to a state of deep hypothermia.

Michael Sarr, a former surgeon at the Mayo Clinic in Minnesota, and editor of the journal Surgery, told USA TODAY in 2017 that Canavero's procedure was radical.

Doctors "have always been taught that when you cut a nerve, the 'downstream side,' the part that takes a signal and conducts it to somewhere else, dies," he said.

"The 'upstream side,' the part that generates the signal, dies back a little – a millimeter or two – and eventually regrows. As long as that 'downstream' channel is still there, it can regrow through that channel, but only for a length of about a foot."

This is why, he said, if you amputate your wrist and then re-implant it and line the nerves up well, you can recover function in your hand. But if your arm gets amputated at the shoulder, it won't be re-implanted because it will never lead to a functional hand.

"What Canavero (would) do differently is bathe the ends of the nerves in a solution that stabilizes the membranes and put them back together," Sarr said. "The nerves will be fused, but won't regrow. And he will do this not in the peripheral nerves such as you find in the arm, but in the spinal cord, where there's multiple types of nerve channels."

Most medical experts say the procedure would be a long shot, but even if the operation works, the biggest obstacle may not be the science itself.

"There are too many risks at this point to go ahead with it," Assya Pascalev, a biomedical ethicist at Howard University in Washington, D.C., said in 2017.

"The first heart transplant, hand transplant, facial transplant: all were met with serious reservations. There are also regulatory concerns. China does not have the same ethical standards and requirements that the United States and Europe have."