Wavefront-guided LASIK can decrease aberrations and increase visual acuity

Wavefront sensing can detect spherical aberrations that need to be corrected for optimal quality of vision.

--- Theo Seiler, MD, PhD

NUREMBERG, Germany — While a majority of patients with refractive errors, whether myopic or hyperopic, may benefit from laser in situ keratomileusis (LASIK), every patient’s corneal surface differs.

“We have here a [wavefront] procedure that, in contrast to other LASIK procedures, does not lessen twilight vision,” said Theo Seiler, MD, PhD, a cataract, IOL, anterior segment specialist, at the Kongress der Deutschen Ophthalmochirurgen meeting held here. “Wavefront corrections corrected visual acuity better [than standard LASIK]. We hope a super-normal visual acuity can be achieved.”

Aberrometry

Every eye has aberrations and refractive errors, like myopia, hyperopia, astigmatism and aberrations, such as coma, spherical aberration, astigmatism of a higher order and folding of wavefronts. There are biologically limiting characteristics in the eyes with regard to visual acuity. First, the diffraction plays a role with a pupil diameter of 4.3 mm and less. Aberrations play a major role in pupils of more than 2.5 mm or 3 mm. Aberrations make visual acuity worse in patients with large pupils, especially at night or in dark settings.

It has been concluded that visual acuity can be improved if these aberrations are corrected. First, a fundus photograph is taken. Aberrations in the eye were optically corrected with so-called adaptive optics and then the photograph was taken again. By doing this, the individual photoreceptors can be clearly detected, while before one could only presume they were there. The optical quality of the optical system of the eye was improved by correcting the aberrations, which come from decentrations of the cornea and lens and from factors of the cornea.

In a study performed in Dresden, Germany, aberrations were analyzed as a function of age in patients with 5-mm and 8-mm pupils. The aberrations in these patients remained about constant until the patients turned 45 years of age, and then with presbyopia, those aberrations increase. According to Dr. Seiler, aberration depends on accommodation and accommodation can change aberrations.

Ablation profiles

Dr. Seiler explained how an aberration profile is obtained. A patient comes in, refraction is carried out and then the refraction in inputted into the computer. A standard ablation profile is established and is symmetrical. Then wavefront analysis is carried out and one will notice that the ablation profile will no longer be symmetrical and that an individual ablation profile is designed based on the total optical system. “This is the tailor-made solution we offer to the patient,” Dr. Seiler said. “It’s based on the entire refraction and not only on the corneal surface.”

A small scanning spot laser is called for, according to Dr. Seiler, and according to wavefront measurements, it is estimated that the spot size should be 1 mm or less. If a 1-mm spot size is used, Dr. Seiler recommends that the laser is more rapid, otherwise it will take too long to complete photoablation.

“Even with the largest corrections, I want it to take less than 1 minute,” Dr. Seiler said. “Therefore, 200 Hz should be utilized. Additionally, eye tracking will have to become better.”

Points to remember

Ablation must be centered in the same way as the measurement. It is no longer as easy as topography, since the information it affords is limited, Dr. Seiler said. The ablation must be centered on the same spot where the aberrometry was performed. Wavefront-guided analyzers use the same centering system, the eye tracker systems, as in the laser itself. According to Dr. Seiler, the old centering problem is not encountered with the wavefront-guided LASIK. The machine does the centering, with the middle of the pupil being the target. Then the ablation is centered accordingly. “The old centering problem we used to have, because we never knew where the visual axis was, is taken care of by wavefront-guided LASIK,” Dr. Seiler said. “You don’t have to be guided by the center of the pupil. The machine does it automatically on the visual axis.”

Dr. Seiler mentioned a new problem of taking measurements on a seated patient and performing the treatment on a patient who is lying down. Cyclotorsion must be accounted for, he said.

Prospective study

The prospective study included 31 eyes of patients with visual acuity of 0.8 or better, myopia up to 10 D and cylinder below 2.5 D. Eyes were healthy and no patient was above the age of 45. Refraction stability was to be followed for 2 years. “At that time, we didn’t know exactly whether aberration-guided LASIK would be effective enough,” Dr. Seiler said. “We, therefore, wanted to include patients where the wavefront calculated refractive error, more or less, agreed with the refractive error, ±0.5 D.”

At 3 months postoperatively, no patient had a visual acuity loss of more than one line. In 16% of eyes, visual acuity of 20/10 or better was achieved. Uncorrected vision was better than 20/30 in all patients, 93% of patients were brought to ±1 D and 68% were within ±0.5 D. According to Dr. Seiler, this procedure would at least satisfy the Food and Drug Administration guidelines. All eyes had maximum low contrast vision, which was measured with the Humphrey (Dublin, U.S.A.) 590 model. However, on average the eyes had an increase in aberrations by the factor 1.44 ±7. However, coma was corrected well, spherical aberration was not.

Regression of the aberration correction varied individually. While some patients showed no regression during the first 3 months, others showed almost complete regression.

For Your Information:

Theo Seiler, MD, PhD, can be reached at the UniversitatsSpital Zurich, Augenklinik, Nordtrakt II, Frauenklinikstrasse 24, CH-8091 Zurich, Switzerland; fax +(41) 1-12554349.