Myopia, or short-sightedness, is almost twice as common in the US and Europe today than it was 50 years ago. It has also become an important problem in some Eastern Asian countries where it affects as much as 70 to 90% of the population. Although glasses and contact lenses are the first choice for many people, permanent vision correction by refractive surgery is becoming more popular, even though the procedure is not without risk. A team of researchers at Columbia University in New York has now developed a new, safe and non-invasive technique that makes use of a femtosecond laser to produce a low-density plasma in the cornea. The plasma generates reactive oxygen species that then react with collagenous tissue in the treated area to form cross-links and alter the refractive power of the eye.
In myopia, incoming light focuses in front of, rather than on the retina. Permanent corrective surgery techniques such as laser-assistedin situ keratomileusis (LASIK) and photorefractive keratectomy (PRK) work by reshaping the curvature of the cornea by laser ablating a portion of corneal tissue. Although they work well, these procedures can weaken the cornea and even lead to post-surgical complications. They are not suitable for patients with a weak cornea either.
New non-invasive approach
Researchers led by Sinisa Vukelic have now developed a new, safer approach in which a low-density plasma produced by a low-power ultrafast laser generates an ionization field within biological issue. Although the laser is powerful enough to produce the plasma inside the set focal volume, it does not have enough energy to damage biological tissue, explains Vukelic.
When applied to collagenous tissue (which is present in the cornea), this plasma ionizes water molecules in the region of interest. This ionization produces reactive oxygen species that then interact with collagen fibrils to form chemical bonds, or cross-links. These change the mechanical properties of the treated tissue, so changing the overall macrostructure (or curvature) of the cornea.
Changing the refractive power of the eye
“If we carefully tailor these changes, we can adjust the corneal curvature and thus change the refractive power of the eye,” Vukelic tells Physics World. “And since the process is photochemical, the induced changes remain stable.”
The researchers tested out their technique on pig eyes in the lab and rabbit eyes in vivo. Pig eyes are frequently used in ophthalmic ex vivo research, explains Vukelic, and rabbits as a model for correcting refractive errors.
The technique could not only be used to treat myopia but also conditions like hyperopia, astigmatism and keratoconus, he adds. “This non-surgical alternative to refractive surgery has fewer side effects and might also be suitable for patients with thin corneas, topographical abnormalities and dry eyes, since it would not disrupt the cornea or cause nerve damage.”
The team, reporting its work in Nature Photonics, says that it is now busy trying to understand how laser-induced cross-links deform the cornea. “We are also looking to bring the technique to the patient and start clinical trials in the near future,” says Vukelic.