Ocular functional optical zone changes after wavefront-guided PRK

February 1, 2011

The ocular functional optical zone decreased in about one-third of eyes following wavefront-guided PRK for myopia and mixed astigmatism.

"Traditionally, the functional optical zone is analyzed based on corneal topography," said Dr. Wang, assistant professor of ophthalmology, Cullen Eye Institute, Baylor College of Medicine, Houston.

"Wavefront-guided treatments correct the ocular aberrations on the anterior cornea," she added. "Due to the effect of the internal optics, the optical quality of the anterior corneal surface may not represent the retinal image quality of the entire eye."

She and her colleague, Douglas D. Koch, MD, conducted a study to investigate the size of the ocular functional optical zone before and after wavefront-guided PRK for myopia or mixed astigmatism based on the retinal image quality.

The study included 72 eyes of 44 consecutive patients (mean age, 36 years) who underwent wavefront-guided myopic PRK (CustomVue PRK, Abbott Medical Optics [AMO]). Patients were included if they were available for follow-up 6 months after the procedure and had measurements with a pupil 6 mm or greater before and at least 6 months postoperatively.

The refractive corrections ranged from 0.63 to 7.69 D. The ocular wavefront aberrations were calculated for simulated pupils from 2 to 6 mm based on the platform measurements. The optical image quality of the higher-order aberrations were calculated, assuming full correction of the second-order aberrations.

The investigators used the Zernike tool program (AMO) to calculate the polychromatic modulation transfer function (MTF) at 9 cycles per degree with the Stiles-Crawford effect for 2- to 6-mm pupils.

Dr. Wang explained that the functional optical zone was defined as the size over which the MTF at 9 cycles per degree was 0.18 or greater. That level maintains the visual acuity of 20/20 or better.

The results showed that the mean preoperative functional optical zone was 6 mm or greater in 99% of eyes; one eye (1%) had a functional optical zone of 5.8 mm, Dr. Wang continued.

Postop functional optical zone

Interestingly, the postoperative functional optical zone in 65% of eyes was 6 mm or greater, in 80% of eyes was 5 mm or greater, and in 96% of eyes was 4 mm or greater, she said.

The smallest functional optical zone postoperatively was 3.5 mm, she added.

When the investigators looked at the changes in the functional optical zone induced by surgery, they found that 65% had a functional optical zone of 6 mm or greater; 16% had a decrease in the functional optical zone of less than 1 mm; 15% had a decrease in the functional zone of 1 to 2 mm; and 4% had a decrease that exceeded 4 mm, according to Dr. Wang.

When they evaluated the functional optical zone versus the fourth-order spherical aberrations, the investigators found that the postoperative functional optical zone decreased significantly with increasing spherical aberrations and with higher-order aberrations. Likewise, an evaluation of the change in the postoperative functional optical zone versus the change in spherical aberrations and higher-order aberrations showed the same pattern. The functional optical zone also decreased with increasing refractive correction.

"Thirty-five percent of eyes had a postoperative functional optical zone less than 6 mm after wavefront-guided PRK," Dr. Wang said. "The postoperative functional optical zone was negatively correlated with the increased spherical aberrations and higher-order aberrations. The functional optical zone decreased with increasing spherical aberrations, increasing higher-order aberrations, and increasing refraction correction.

"The study limitations were that the results were not correlated with the measures of visual function," she concluded. "These results may lead to further refinements of the treatment parameters."

FYI

Li Wang, MD, PhD
E-mail: liw@bcm.tmc.edu

Dr. Wang has no financial interest in the subject matter.