Refractive correction is not one-size-fits-all

In this report from the OSN Section Editor Summit, Jack T. Holladay, MD, MSEE, FACS, discusses the correction of spherical aberration and lens-based refractive surgery.

Issue: June 15, 2006

ByJack T. Holladay, MD, MSEE, FACS

A note from the editors:
Ocular Surgery News convened its annual Section Editor Summit in Las Vegas in March. In this fourth installment of reports from the OSN Section Editor Summit, Optics, Refraction and Contact Lenses Section Editor Jack T. Holladay, MD, MSEE, FACS, gives an update on his subspecialty.

We have said in these overviews previously that optics is not a subspecialty of ophthalmology, but it is a subject that touches all the subspecialties. Today I would like to talk about optics in cataract and refractive surgery.

We know there is positive spherical aberration in the cornea. To correct this, the simple thing to do would be to make an IOL that is equal and opposite to the average spherical aberration of the population, which is about 0.27 µm, placing a crisp, clear image on the retina.

When we are young, we have negative spherical aberration in the crystalline lens that is almost equal and opposite to the positive aberrations in the cornea. As we get older, the aberration in the crystalline lens becomes more positive and results in halos around lights in low-light conditions.

To keep the images crisp and clear, we need to reduce these spherical aberrations. The newer aspheric IOLs help treat those aberrations.

But another key is better diagnostic procedures. Today our refractive surgery requires better diagnostic surgical procedures, but we should be measuring not just the ocular wavefronts, but also the topography wavefronts. Those higher-order aberrations in the crystalline lens that are above coma and spherical aberration change every year. If you correct those, you are going to have to re-treat that patient every year.

We need to measure the topography and correct all of the ocular aberrations that are from the cornea and spherical aberrations and coma if they are in the lens. If the aberration is in the crystalline lens, and most of the aberrations above coma are, it is better to do refractive lensectomy and remove the aberrations with the crystalline lens removal. So you need to measure both the cornea and the crystalline lens. You also want to match the spherical aberration of the cornea with the proper negative spherical aberration in the IOL to give patients the best results.

If the aberration is not in the crystalline lens but is in the cornea, you can still correct it. Studies have found that the only people who have changes in aberrations of their cornea as they age are people who have dry eye or anterior membrane dystrophy. The rest of us still keep our good corneas. However, lenticular aberrations change with age, so you may want to put a little bit more negative spherical aberration in the cornea to anticipate the changes in the crystalline lens with aging.

The other advantage of excess negative spherical aberration is that the patient sees better up close. When the pupil gets smaller, it focuses vision though that portion of the cornea with stronger power, and the patient can read. Our goal is to make the cornea excessively prolate in the myopic treatments. You have to ablate more, but the patient will have better near vision.

Axis and centration

Jack T. Holladay, MD, MSEE, FACS [photo]
Jack T. Holladay

If you are correcting corneal aberrations, one thing you need to remember is that the visual axis and the optical axis are not the same. Most eyes are turned out 5.2°and up 1.4°, which we call the alpha angle. In laser treatment, if your treatment is tilted with respect to the visual axis, coma is induced. Every time you put an IOL in the bag, which is along the optical axis, and center it, you induce coma. That is why every IOL that is centered perfectly in the bag ends up inducing coma — because it is tilted 5.2° with respect to the visual axis of the eye.

But in laser the other thing that needs to be worked on is centration; that is, where should we put the treatment and how do we center it correctly? We need to move away from using the center of the pupil to using the visual axis.

The treatment of aberrations is not a one-size-fits-all thing. We are going to see a trend to move to the visual axis in order to get ideal correction.

Lens-based refractive surgery

More and more people are having refractive lensectomy. As a surgeon if you miss your target by more than 0.5 D or leave some residual astigmatism, the patient does not get to appreciate the new technology and performance of aspheric and presbyopia-correcting lenses.

In terms of the multifocal and accommodating lenses, the biggest factor is patient selection. You may have used patient selection questionnaires. They can help you select the patients exactly the way they should be selected. We divide vision into five zones, and then the questionnaire asks the patients which of these he would choose, if he could choose only three. The patient can choose: “I prefer no distance glasses. I don’t care about wearing reading glasses. I want to read fine print.” A standard aspheric IOL plus reading glasses is perfect for that patient.

But if that person says, “I do not want reading glasses,” and the questionnaire says, “If you get distance vision reading and you see some halos around light, would you like that option,” and the patient says, “Yes,” a multifocal lens is perfect for that patient.

What about the patient who says, “I refuse to wear distance glasses.” The person states they are a perfectionist. You are never going to do anything to make that patient happy.

The Steve Dell index is the best questionnaire that I have seen for matching the right IOL for the patient’s needs and personality.

For more information:

Jack T. Holladay, MD, MSEE, FACS, is a clinical professor of ophthalmology at Baylor College of Medicine. He can be reached at Holladay LASIK Institute, Bellaire Triangle Building, 6802 Mapleridge, Suite 200, Bellaire, TX 77401; 713-668-7337; fax: 713-668-7336; e-mail: docholladay@docholladay.com; Web site: www.docholladay.com.