Seeing the Future: The Promise of Refractive Index Shaping

American Academy of Ophthalmology (AAO) 2017 Annual Meeting

H. Burkhard BD Dick, MD, PhD


December 15, 2017

A Dream That May Soon Become Reality

The old adage "Hindsight is easier than foresight," or more specifically (and probably coined by an ophthalmologist), "Hindsight is 20/20," holds oh so true for a number of situations in cataract surgery.

Every cataract surgeon certainly has had his or her share of patients who opted for a multifocal intraocular lens (IOL) and then could not cope with the visual experience it provided.

Another matter is refractive error. As the saying goes, today's cataract patients are always refractive patients. The expectations for postoperative visual quality are usually high. Many patients expect at least perfect distance vision while being content with some additional correction for reading and other near-distance activities. Missing the target refraction by one diopter (or even less) might lead to significant dissatisfaction and to the search for measures to deal with the residual refractive error.

How great would it be if this could be done in a noninvasive manner, or as minimally invasive as possible? This dream, shared by so many surgeons, may not be too far away from becoming reality. At least, this was the impression that George O. Waring IV, MD, left on me during his presentation[1] on refractive index shaping (RIS) at this year's American Academy of Ophthalmology meeting.

It reminded me of our own experience with a technique that tries to correct residual refractive errors: the light-adjustable lens. This IOL includes a proprietary photoreactive silicone macromer in a medical-grade silicone polymer matrix. Applying the appropriate wavelength of light through a defined spatial irradiance profile onto the light-adjustable IOL polymerizes the macromers in the exposed region, ultimately producing a change in shape and predictable power. Equilibrium is achieved after 24 hours. By controlling the irradiation dose and spatial irradiance profile, the refractive power of the light-adjustable IOL is modified to add or subtract spherical power, or to eliminate astigmatic error.

In a study conducted at our clinic,[2] 96% of 21 eyes were within ± 0.50 diopter of the intended refractive outcome and 81% within ± 0.25 diopter 1 year after implantation and postoperative light-induced fine tuning. Until the final "lock-in," the patient has to wear special ultraviolet protective glasses to prevent any unintended alteration of the IOL power.

Transformative Technique

Dr Waring presented the next generation of postoperative refractive adjustment, which is performed by our preferred instrument in cataract surgery: the femtosecond laser.

Applying this laser to a hydrophilic acrylic IOL not only selectively changes polarity and refractive index but also allows for the creation of 3D structures within an IOL. This means an IOL can be transformed from a multifocal lens into a monofocal lens, and vice versa.

With the new technique of RIS, developed by Josef Bille and coworkers, the high-repetition-rate femtosecond laser pulses are directed to a designated area to create a "lens" inside an IOL. RIS changes the refractive characteristics of the polymeric material without cutting the material. The height of the implanted IOL is not altered.

In a process called "phase wrapping," a convex lens is reduced by the femtosecond laser to a thin layer of approximately 50 μm in thickness, creating multiple refractive zones. The different phase levels are created by controlling the energy per pulse and focal spot. This whole process can be visualized by the fluorescence microscope. RIS has been successfully performed on both hydrophobic and hydrophilic IOLs in vivo.[3,4]

Dr Waring presented a number of procedures done in the model eye, during which the refractive index was changed by about 3.5 diopters. Fortunately, missing the target refraction by that much is a rare event for most surgeons. He reported that spherical adjustments were possible, and also that astigmatism was successfully "treated" in these models. This sounds encouraging and gives us an additional option to treat astigmatism. Of course, this is something we strive to combat during the operation—in the case of femtosecond laser cataract surgery, for instance, by performing arcuate incisions.

Almost breathtaking was the reported ease with which monofocality could be switched to multifocality, if desired, and multifocality to monofocality.

Almost breathtaking was the reported ease with which monofocality could be switched to multifocality, if desired, and multifocality to monofocality. This could represent a better way out if vision problems related to a multifocal IOL persist. While many patients get used to the different foci, the blurring of the intermediate vision (depending on the IOL type), the lowered contrast, and even halo and glare, others will not be able to overcome these problems. For these patients, a reversal from multifocal to monofocal using the femtosecond laser might be preferable to an explantation and thus represent a completely new surgical intervention.

There is another potentially rewarding long-term aspect of this new and exciting technology: RIS might be used to adjust the power of an IOL implanted during childhood because of congenital or juvenile cataract, and done so repeatedly, whenever deemed necessary, for an entire lifetime.

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