New developments in optics promise sharper vision, fewer aberrations

Corneal inlays, aberrometers, aspheric IOLs and new corneal topographers represent the cutting edge of optical technology.

ByJack T. Holladay, MD, MSEE, FACS

The first topic that is gaining attention in optics is that of pinhole optics. We have discussed the AcuFocus ACI 7000 corneal inlay before. The trial for that is on hold in the United States right now because the developer wants to fine-tune the optics, including the thickness and number of holes, so that when they complete phase 3, they will have exactly the inlay that they want.

Jack T. Holladay

Another interesting new intracorneal device for presbyopia is the InVue corneal inlay from Biovision.

This is a small hydrophilic lens – about 15-µm to 20-µm thick – measuring about 3 mm in diameter.

The device is implanted about 200 µm into the cornea of the patient’s nondominant eye to create monovision. The center of the lens has no power, but rather it features a tiny 0.2-mm pinhole for nutritional purposes, and near vision is corrected by the lens’ outer rim with plus power.

The next topic is pixelate optics, which involves inserting a thin pixelated plate into the middle of an IOL. The device can lend an electrical charge that can change the index of refraction up to +4 D within the optic.

A battery will be about the size of a capsular tension ring. The index of refraction of pixels can be changed, which in turn will change the power of the IOL. The change in power will be controlled by pixels that are dedicated to recognizing contrast, similar to the autofocus feature on digital cameras.

This concept is already applied in telescopes, but instead of using reflective optics like a mirror, this visual application relies on refractive optics.

The large prototype already works and is used to replace deformable mirrors; the challenge will be to miniaturize the battery, circuit and pixel grid down to the size of a wafer that fits between the two halves of a standard IOL.

I think this is still probably 6 to 8 years away, but it is the future because it does not involve multifocality. It could take different forms: It could use the ciliary body for electrical impulse and the capsule for support, or it could use the optical recognition system for focusing and the bag only for support.

New aberrometers

There are two new aberrometers coming out, one of which is the aberrometer from Wave-Tec Vision Systems. This has been working for about a year but is still being fine-tuned. The WaveTec device can measure refractive error before and after IOL implantation, thereby eliminating any large refractive surprises.

This device measures astigmatism after IOL implantation as well, guiding the placement of limbal relaxing incisions or toric IOL implantation. It gives feedback to the surgeon for optimal orientation.

The WaveTec device uses what is called a Talbot interference pattern. It is not like Shack-Hartmann or Tscherning, or the Nidek OPD scan that uses streak retinoscopy or sciascopy. It is an entirely different technique. This will really be a boon for surgeons.

The OQAS (Optical Quality Analysis System) device, manufactured by Visiometrics, was developed by Pablo Artal. What this brings to the table is the ability to measure and quantitate light scatter. If a patient has a cataract that scatters light but there are no other significant aberrations, current wavefront aberrometers will not measure forward light scatter.

Therefore, most devices significantly overestimate visual performance because the reduced optical performance that comes from that forward light scatter is not accounted for. That is one of the main reasons why we have a hard time correlating wavefront results with visual performance results.

This is just now becoming available in the United States.

Aspheric IOLs

We have had three choices in aspheric IOLs in the United States from Alcon, Advanced Medical Optics and eyeonics, and there is no question they have improved the visual performance of our patients in terms of contrast sensitivity and visual acuity.

More recently, we have found that it is likely that some hyperasphericity, exaggerated negative spherical aberration, when the lens is even more prolate than the average for the normal eye to make it perfect, improves near vision. The principle is similar to presbyLASIK.

The concept takes into account that the pupil constricts when you are looking up close. This principle allows you to have an aspheric optic where the surface creates a little hyperopia in the periphery and a little myopia in the center of the dilated pupil. But when the pupil constricts and you look up close, you become nearsighted. So you get much better near vision than if there were no spherical aberration.

This is the basis of presbyLASIK, of hyperopic LASIK and hyperaspheric IOL technology, such as the new hyperaspheric optic from AMO, as well as the crystalens HD from eyeonics.

Central corneal optics

Accurate measurement of central anterior and posterior corneal optics is critical after refractive surgery.

There are several units now available or coming out that do this. There is the Pentacam (Oculus), the Orbscan (Bausch & Lomb) the Galilei Dual Scheimpflug Analyzer (Ziemer), and a combination of the Visante OCT and the Humphrey Atlas corneal topographer (Carl Zeiss Meditec).

We are learning more and more that measuring the back surface of the cornea allows us to better determine in patients who have had previous refractive surgery, and in those with no previous surgery, what the real power of the cornea is.

We have always assumed that when we measure the front surface, that the back surface is a constant relationship to the ratio of the radius that we measure in the front (approximately 82.2% back to front ratio). That is just not true. After refractive surgery, it is not even close.

All of these technologies allow us to do a better job of measuring the cornea. They also enable us to pick up keratoconus, pellucid marginal degeneration and other thinning disorders because studies have shown that these disorders are detectable on the back surface before they are on the front surface.

Carl Zeiss Meditec is coming out with a combination of Visante OCT and its topographer, called the Humphrey Atlas. The Visante measures thickness down to a micron, and the topographer measures the front surface down to a micron. So by integrating the two devices we will be able to have all that information tied together on one network.

Also, Schwind has a new laser, the Amaris system, that has a tilt in its algorithm to track the eye in four to five dimensions. The eye moves not just side to side, but it also rotates. When we speak of motion, we need to describe it in six dimensions.

You need three to describe where it is, and you need three more for rotation. We are not talking about regular three dimensions we often think of. We are talking about when something is in motion. It is like in baseball. When you throw a knuckleball, you can describe where that ball is at any point in space as it goes from the pitcher’s hand to the plate. But if the pitcher puts some spin on the ball, there is no way to describe that unless you have some way to measure rotation.

If you have six dimensions, you can describe any pitch. The point is that the eye is never like a knuckleball; it is a curveball. Whether adding more dimensions will translate into better optical performance, and whether we get better results, we do not know yet.

Clearly, we know we were deficient in monitoring the rotation around the X and Y axis. By adding these parameters, we are truly describing the motion of the eye as we are attempting to track it.

For more information:

Jack T. Holladay, MD, MSEE, FACS, can be reached at Holladay LASIK Institute, Bellaire Triangle Building, 6802 Mapleridge, Suite 200, Bellaire, TX 77401; fax: 713-668-7336; e-mail: holladay@docholladay.com. Dr. Holladay is a consultant for Advanced Medical Optics, Nidek and Oculus.