The techniques have shown some promise in animal studies. But the medical team, led by Polish and English doctors, also emphasized that the results would “have to be confirmed in a larger group of patients sustaining similar types of spinal injury” before the treatment could be considered truly effective. The case report was published in the journal Cell Transplantation.

The history of spinal injury treatment is studded with false hope and miracle recoveries that could never be replicated, experts said. In previous studies, scientists experimented with some of the same methods used on Mr. Fidyka, with disappointing results.

Depending on their injuries, patients may show what appears to be spontaneous progress without any special treatments.

“You can see surprising improvements in patients engaging in rehab, even long after the injury,” Dr. Tuszynski said.

And it is often not clear how much spinal cord tissue is spared after an injury, experts said.

Mr. Fidyka’s injury was severe by any measure. His spinal column was cut nearly in half at midback, the two pieces connected by a bit of scar tissue. Working in Poland, the medical team first removed one of the two olfactory bulbs from behind his nasal cavity. The olfactory bulbs are rich in cells that support nerve function and help repair damage, and the doctors cultured those cells in the lab. They then injected the cells into spinal tissue at the site of the injury. Surgeons also grafted tissue from the patient’s ankle to help bridge the nerve-repair cells across from one side of the spinal cut to the other — about a third of an inch.