A type of eye injury that caused blindness and was previously believed to be irreversible has been cured in human trials using stem cells.

The cornea is the the outer layer of the eye that focuses light and acts as a protective layer. Although the cornea is full of cells that help to repair injuries, if the damage done to it is too severe, it can leave patients with a blindness that was previously believed to be irreversible.

That belief was recently challenged in a clinical trial published in Nature Communications. The trial used a new procedure, called cultivated autologous limbal epithelial cells (or CALEC), that involves doctors taking healthy stem cells from the patient’s other eye, growing them in a lab for weeks and then transplanting them into the damaged cornea.

The study was led by Harvard Medical School and Massachusetts Eye and Ear researchers and surgeons. The researchers reported a 92% success rate, with 50% of the 14 patients reporting fully restored corneas within three months, and 79% by 12 months. The cornea restoration returned sight to some of the patients, although not completely, and no patients reported major side effects.

“Our first trial in four patients showed that CALEC was safe and the treatment was possible,” principal investigator Ula Jurkunas, MD, associate director of the Cornea Service at Mass Eye and Ear and professor of Ophthalmology at Harvard Medical School, said in a news release. “Now we have this new data supporting that CALEC is more than 90% effective at restoring the cornea’s surface, which makes a meaningful difference in individuals with cornea damage that was considered untreatable.”

The operation has room to grow, but is believed to be an exciting step forward in curing a disorder and injury once thought to be a permanent sentence for blindness.

“We feel this research warrants additional trials that can help lead towards FDA approval,” Jurkunas said in the release. “While we are proud to have been able to bring a new treatment from the lab bench to clinical trials, our guiding objective was and always will be for patients around the country to have access to this effective treatment.”