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Optics key to developments in phakic, aphakic and keratorefractive surgery
OSN Optics Section Editor Jack T. Holladay, MD, MSEE, FACS, discusses wavefront technology and phakic and aphakic IOLs.
Optics is important in all subspecialties in ophthalmology. Today, the hot topics are in cataract and refractive surgery because they deal surgically with the optical portion of the visual pathway. Recent advances in diagnostic equipment using wavefront technology have made improvements in IOL design and keratorefractive surgery possible.
The visual pathway can be divided into an optical and a sensory system. Everything from the retina to the occipital cortex would be the sensory component. The optical component can be evaluated using wavefront aberrometry, and the entire visual system with visual acuity measurements and contrast sensitivity. In keratorefractive surgery (LASIK, PRK, EpiLASIK, etc.), we are doing a good job at correcting refractive errors and attaining good visual acuities, but we are not doing so well with mesopic contrast sensitivity. We still induce higher-order aberrations, the most important of which are spherical aberration and coma, and we reduce the mesopic contrast sensitivity.
Solving the problem of presbyopia
Our major goal for the next decade is to solve the problem of presbyopia without any compromises. Multifocal lenses and accommodating IOLs are interim steps to the solution of presbyopia. Multifocal lenses cause a reduction in retinal image contrast, and accommodative IOLs have been shown not to move significantly. Their performance is primarily due to smaller optic sizes (4.5 mm), which increase the depth of field, just the same as making the aperture smaller in a camera. The tradeoff with these lenses will become clear when the retinal surgeons complain that they are not able to see the peripheral retinal adequately to repair holes and detachments.
Modified prolate IOLs
A significant improvement with the performance of monofocal IOLs has come with modified prolate IOLs such as the Tecnis lens by Advanced Medical Optics. The Food and Drug Administration has allowed the manufacturer to make claims that the lens improves night driving and reduces the risk for pseudophakic drivers and those with whom they share the road. The improvement in retinal contrast is 40%, and this translates into a 0.5 second improvement in reaction time for the driver when detecting a pedestrian at night and trying to stop. For comparison, the third rear red light mandated by federal regulations improves reaction time by 0.3 seconds. The modified prolate lens should have a significant effect in reducing accidents, deaths and the excessive costs of car repairs. My prediction is that within 3 to 5 years most lenses will have a modified prolate surface because it will have such significant economic effects on driving, especially for the elderly.
Centration and tilt
Centering IOLs and excimer laser surgery on the visual axis are critical for a good optical result. In order to achieve the benefits of the modified prolate IOL, the surgeon must implant the lens with 0.5 mm or less of decentration and less than 7° of tilt. Fortunately, our current cataract surgery with capsulorrhexis and viscoelastics exceeds these values by a factor of at least two to three.
Aphakic IOLs, whether in the bag, sulcus or anterior chamber angle, are co-linear with the cornea, and phakic IOLs are co-linear with the cornea and crystalline lens. Iris-supported lenses centered over the pupil are decentered by angle kappa, about 0.3 to 0.5 mm. These lenses will induce significant coma because they are not along the optical axis of the eye.
The cornea and the crystalline lens are on one optical axis. The eye is tilted 5.2° (angle alpha), it induces a slight coma, and it induces about a 0.25 D of astigmatism, so the cornea has a little astigmatism the other way to make the entire optical system spherical.
As soon as we center the IOL over the pupil, we cover the pupil so we get rid of edge glare but increase coma. The problem with that is we are tilted with respect to the two optical elements in the system (cornea and crystalline lens) that automatically induce coma. It also induces a lot of other higher-order aberrations, which are worse than the patient had before surgery.
IOL calculations
Because these new technologies in IOLs are striving to make individuals free of glasses, there is a necessity to be more exact with IOL calculations and consequently measure the eye more accurately than we have done in the past. The IOL Master by Carl Zeiss Meditec, using optical interference as opposed to ultrasound, has made a significant impact in improving the accuracy of biometry because it is only limited by the wavelength of light, which is a million times shorter than the wavelength of ultrasound. It is also improving the precision of axial length measurements from 100 µm with ultrasound to 10 µm with light. It also makes no assumptions about retinal thickness, as does ultrasound.
Excimer laser surgery
Excimer laser surgery is usually centered over the pupil and, like IOLs centered over the pupil, also induces coma. The original laser treatments do not compensate for the oblique incidence of light as the laser beam moves more peripherally on the cornea, which has resulted in oblate corneas and large amounts of induced spherical aberrations. The improvement in wavefront-guided ablations has little to do with wavefront. The companies have compensated for the reduced energy in the periphery and embedded this in the wavefront software. They could have put this in their standard software and not required the purchase of wavefront aberrometer or wavefront cards.
If we center the laser on the optical axis rather than the pupil and compute a treatment that is proper, the ablation depth becomes much deeper. As we move forward, the companies will be able to calculate what is needed over the optical axis and then just limit the ablation to the part over the pupil. This technique will significantly reduce induced coma and improve our optical outcomes.
New instruments that are now available are the Artemis Digital High Frequency System by Ultralink LLC, which gives much higher resolution of the anatomy of the anterior segment that traditional B-scan. Another instrument, the OCT by Carl Zeiss Meditec, uses light to image the anterior segment. A system called the Pentacam from Oculus is a system that uses a Scheimpflug to image the anterior segment. The slit rotates 360° and is recorded from a 45° angle. The posterior surface of the cornea is imaged well, and the resolution may solve the problem of accurately measuring the cornea when cataract surgery is necessary after refractive surgery.
Challenges and improvements
Most of the higher-order aberrations above spherical aberration and coma are in the crystalline lens. The lens aberrations are not stable, and trying to correct these on the cornea does not work. If there are significant aberrations in the lens, the lens should be removed. The other problem with wavefront is that when you make the modifications on the front of the cornea and you send the new ray back, it goes through the crystalline lens through a different path, which means it is not going to do the same thing it did the first time. In the future, we will be moving back to topographically guided ablations. This is already happening in Europe and has had much greater success than wavefront.
Maloney on cataract surgery
In the November 15 issue of Ocular Surgery News, William F. Maloney, MD, will discuss the state of cataract surgery today.
For Your Information:
- Jack T. Holladay, MD, MSEE, FACS, a clinical professor of ophthalmology, can be reached at 5420 Dashwood St., Suite 207, Houston, TX 77081; 713-668-7337; fax: 713-668-7336; e-mail: holladay@docholladay.com.