Adaptive optics ... the key?

Desktop adaptive optics system by Imagine Eyes (Orsay, France)

Research, companies focus on developments in this flourishing area

There is probably not a refractive surgeon on the planet who hasn’t dealt with this scenario: A post-LASIK patient with 20/20 uncorrected visual acuity, minimal higher-order aberrations (HOAs) and a normal corneal topography map. From the standpoint of all currently used clinical measurements, the outcome is great. Still, the patient is miserable.
Researchers at the Cleveland Clinic, Cleveland, are embarking on a project that could provide refractive surgeons as well as their patients with the ability to truly understand what visual outcomes following refractive surgery would be. In the process, they could help to cut down on some of the more frustrating cases in refractive surgery.
The research involves use of a desktop adaptive optics system that’s been developed by a French company, Imagine Eyes (Orsay, France). The system under development uses a Hartmann-Shack aberrometer, but it also contains a deformable mirror enabling it to measure and modify aberrations.

What is adaptive optics?

Developed for use in astronomy as a way of reducing the costs of high-powered telescopes, adaptive optics is a technique designed to compensate for imperfections, whether they are in the eye’s optical system or in space, in order to sharpen the image and improve contrast. According to the Center for Adaptive Optics, University of California at Santa Cruz, there is no way to avoid the imperfections in the cornea and lens in living beings. So, adaptive optics represents one of the best options for looking at retinal tissue. Adaptive optics can also compensate for the micro-fluctuations that occur in eye muscles, making it unnecessary to dilate the eye to complete an examination.
Although there are a number of adaptive optics systems types in use around the world, the basic principle is relatively similar. All adaptive optics systems have three main elements: a wavefront sensor, command and control algorithms (software), and a mechanism for wavefront correction, typically a deformable mirror. A deformable mirror is capable of changing shapes through the use of actuators that push and pull on the mirror. In the case of the Imagine Eyes system, there are 52 actuators that push and pull on an 8 × 8 array. The system has a resolution of 1,024 centroids; this compares to approximately 200 centroids in most wavefront systems currently on the market, while the system developed by Wavefront Sciences (Advanced Medical Optics, AMO, Santa Ana, Calif.) has 800 centroids.
In vision research, a laser spot is focused on the retina. Next, the light from the retina is analyzed by a wavefront sensor, which then sends a command to the actuators to change the shape of the deformable mirror. In the case of astronomy, an adaptive optics system will update the mirror’s shape several hundred times in just one second.
In ophthalmology, adaptive optics is expected to play an increasingly important role in both retinal imaging and in refractive surgery outcomes diagnostics. “The primary focus of wavefront aberrometry, in the beginning, was to use it for research to understand about aberrations and how they affected vision,” said Ronald Krueger, M.D., medical director, Department of Refractive Surgery, Cole Eye Center, Cleveland Clinic. “Then we came up with wavefront-guided treatments, and that opened up a whole new realm of laser vision correction.”
In addition to the adaptive optics system developed by Imagine Eyes, adaptive optics can be found in confocal scanning laser ophthalmoscopy and optical coherence tomography. Cambridge, Mass.-based company Boston Micromachines announced earlier this year that it had created a new deformable mirror capable of “ultra-high resolution” retinal imaging. The company said that the development of this deformable mirror would help to make adaptive optics commercially available in imaging instruments.
Currently available wavefront aberrometers are able to measure the level and type of lower- and HOAs in order to create a customized ablation and measure the changes in aberrations following surgery. However, they cannot provide a real-time visual simulation of what outcomes from a LASIK procedure or phakic IOL implantation will look like—or if a patient’s brain will accept the changes made to the optical system.

True refractive outcome?

In the case of the Imagine Eyes adaptive optics system, the visual simulator allows researchers to cancel out all of the errors in the eye’s optical system, both lower and higher order, leaving them with just the neurological component of someone’s vision.
“We’ve never been able to take those two components and split them apart,” Dr. Krueger said. “If we have a way of canceling out all of the optics, it opens up a whole new avenue in vision correction that’s not just dealing with optics, but also the neurological acceptance. It opens up a new avenue in vision correction.”
Validation work presented at this year’s Association of Research in Vision and Ophthalmology (ARVO) meeting looked at the visual effects of adding or taking away different aberrations. In the study, subjects would have their refraction, cylinder, and HOAs measured by the unit. Then, the researchers would completely correct all refractive errors and HOAs, before adding one type of HOA back into the visual system, to see how something like coma impacts the vision.
“High spherical aberrations were the biggest detriment to vision,” said Dr. Krueger. “That tells you that high myopic laser corrections, which tend to cause high levels of induced spherical aberrations, are the ones that are going to disturb patients the most.”
While symmetrical aberrations, like spherical defocus and spherical aberrations, have the most negative impact on vision, asymmetric aberrations such as coma, trefoil, and astigmatism tend to have less of an impact. According to Dr. Krueger, this is where the brain plays a role in vision.
“You may look at a convolved image and say, ‘Oh, that coma looks horrible,’ but the brain’s ability to see it and resolve it into a letter, that is different,” said Dr. Krueger. “This is what we’re starting to learn more about.”
Imagine Eyes president Nicolas Chateau said another interesting point of their research was that the results varied greatly from patient to patient. Subjects also underwent testing where just the lower-order aberrations were corrected and then all the aberrations. The results were then compared. “When we looked closely at the results, we saw that for some subjects there was an improvement of three lines, while for others there was just a very small improvement,” said Mr. Chateau. He maintains that this variance cannot be detected with a traditional wavefront aberrometer that uses convolution principles. This involves taking a mathematical formulation to express what a patient is seeing. Adaptive optics enables the actual testing of the impact of various aberrations on vision.

Adaptive optics in the future

When wavefront technology was introduced some seven years ago, there seemed to be just a handful of refractive surgeons who could see the value of creating a customized laser treatment. With the introduction of adaptive optics, the complexity is going to increase even further.
“We’re going to start talking about neurological processing and everything else,” said Dr. Krueger. “It will likely be the more research-oriented clinicians who get involved in the beginning.”
Still, it’s the potential to use it with a wider application that intrigues Dr. Krueger. “You could take a skeptical patient and show them what a typical –9 D LASIK correction looks like with residual spherical aberration, and then show them what the vision would look like if he or she was implanted with a phakic IOL,” he said. “It would enable you to talk about the risks and the benefits of the two approaches from a very particular perspective.”
Particularly with the growing interest in presbyopic surgery, adaptive optics could play an important role in helping patients understand what their vision would be like after a specific treatment, said Mr. Chateau and Dr. Krueger.
“It is known from experiences with multifocal contact lenses that it is very difficult to predict the acceptance of a presbyopic correction. We feel the preview function (on this system) could be very useful,” said Dr. Krueger.
Certainly from a research perspective, the interest in adaptive optics is at a high. At this year’s ARVO meeting, there were 500 presentations, posters, and workshops devoted to the subject. The vast majority focused on retinal imaging. Dr. Krueger notes that this is because with adaptive optics, the resolution is the same going into the eye as it is coming out of the eye. This allows for the capture of very high-resolution, 3-D images of the retina.
“We need to continue to do research like we’ve been doing and, in the process, continue to make adaptive optics more and more user friendly so that you create something that is easy to use and requires just a few, simple steps,” he said.

Editors’ note: Dr. Krueger has no financial interests related to his comments.

Contact Information
Chateau: nchateau@imagine-eyes.com, +33 1 64 86 15 66
Krueger: KRUEGER@ccf.org, +1 216 445-8585