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Femtosecond laser-induced refractive index change may lead to paradigm shift in refractive correction
The treatment causes no tissue disruption and can be applied to the cornea as well as IOLs and contact lenses.
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A novel approach to refractive correction uses the femtosecond laser to change the refractive index of the cornea without cutting, ablating or removing tissue.
“LIRIC (laser-induced refractive index change) is a revolutionary technology because it’s a much less invasive approach to conventional refractive surgery. By staying below the optical breakdown — or tissue damage — threshold, LIRIC avoids cutting or ablating tissue and there is no need for epithelial debridement, flap creation or lenticular extraction. Animal studies have also shown that LIRIC does not affect the corneal nerves, so patients should be less susceptible to dry eye. All these things will result in a safer first-line option for patients,” Scott M. MacRae, MD, said in an interview with Ocular Surgery News.
A group of researchers at the University of Rochester has been working at this technique for more than 12 years. The first studies in human corneas have proved successful, and new studies are currently ongoing. Refractive error, presbyopia and high-order aberrations can be addressed not just on the cornea but on IOLs and contact lenses.
“It is potentially the perfect method for fine-tuning the results of IOL implantation in case of residual refractive error,” MacRae said.
Noninvasive low-energy treatment
Ophthalmologists have been educated to address refractive errors or aberrations through shape change or with spectacles, contact lenses, IOLs or corneal surgery. LIRIC is a different approach, not acting on the shape but on the refractive index. The mechanism of action depends upon the target material, such as the cornea or IOL, and associated laser parameters.
“Fundamentally, any material is a cocktail of ingredients. Individual ingredients have an associated refractive index. The LIRIC laser locally alters the ingredients’ ratio, resulting in a refractive index modification. For example, with corneal stroma, the treatment decreases local water content, causing densification,” MacRae said.
An ultrafast, low femtosecond pulse energy laser is used, specially developed for this procedure. While an excimer laser spot size is about 1 mm, the spot of this laser is 1 µm.
“It is like comparing a whale to a sea horse in terms of pulse size. The pulse energy levels are well below the stromal tissue disruption threshold and are between 1:60 to 1:400 less than the flap cutting femtosecond pulse energies. We are not disrupting the cornea but just remodeling it, densifying the system. In this way we can change sphere and cylinder, can do wavefront correction, and we can also do a presbyopic correction either bifocal or trifocal. We can do whatever you can do in an optical system using refractive indexing,” MacRae said.
The procedure is carried out in just more than 1 minute by docking the laser into the patient’s eye. The treatment time is too long, MacRae said, and the group is currently working at shortening it.
One further advantage is that the procedure can be repeated.
“If John needs 1 D of myopic correction when he is 16 and then gets another diopter of myopia when he is 23, we can perform the additional correction,” he said.
Because there is no disruption of the tissue, no antibiotics or anti-inflammatory drops are required after the procedure.
The studies
Twenty-seven eyes were treated in the first-in-human study at the end of 2018, and the patients will be monitored for at least 12 months. A bifocal treatment for presbyopia was performed, with an addition of 2.5 D over a 3-mm optical zone.
Three-month data show an excellent safety profile. Immediately after treatment and up to the present day, all eyes were clear with no signs of inflammation, scarring or opacity. Patients exhibited a significant benefit to distance corrected near vision with no degradation of distance vision. The next step will be the correction of myopia.
Studies have also been carried out on IOLs and contact lenses, in which 9.5 D of refractive index change was achieved.
“That was our starting point, when one of the physicists of the University of Rochester came back from the lab and said, ‘Hey, I can change the refractive index of glass and plastics,’” MacRae said.
The work done on lens material was also instrumental to the achievement of predictable results in human corneas.
“We first wrote the myopic LIRIC cornea pattern into soft contact lenses. Measuring through-focus visual performance in normal patients wearing these contacts gave us an idea of what to expect, and the results corresponded to what we predicted. In the future, we’ll use this information to guide us in our first-in-human myopic LIRIC studies,” he said.
The refractive patterns obtained in animal models, human eyes and artificial lenses have undergone thorough examination for light scatter, wavefront aberrations and image quality, and no negative alterations were found.
“The other beauty of this is that no significant corneal damage was found when we looked at corneal nerves adjacent to the area of treatment. If this is true, it is unlikely that there will be significant dry eye as a consequence. Even with SMILE there is some dryness, but not with LIRIC,” MacRae said. – by Michela Cimberle
- For more information:
- Scott M. MacRae, MD, can be reached at University of Rochester Medical Center, 100 Meridian Centre, Suite 125, Rochester, NY 14618-3926; email: scott_macrae@urmc.rochester.edu.
Disclosure: MacRae reports he is a consultant for Clerio Vision.