Topographically guided ablation helps with transition zone

October 1, 2006
Lynda Charters

The topographically guided algorithm in the EC-5000 CX (Nidek) excimer laser provided a good visual outcome for patients with myopia, with 95% of patients achieving 20/20 or better visual acuity and no induction of total aberrations, according to a study by Mihai Pop, MD.

George O. Waring III, MD, who is the founder of InView Vision, Atlanta, presented the study results for Dr. Pop, who is in solo private practice in Montréal, Canada.

The study was carried out using the Nidek EC-5000 CX excimer laser to perform the customized (topographically guided) aspheric transition zone (CATz) ablation profile. Dr. Waring demonstrated how the CATz profile works.

Dr. Waring explained the basic approach of the CATz profile in an eye with an eccentric flat zone undergoing this treatment. The topography is taken at baseline, a target topography is created, and then first the sphere is treated, followed by the cylinder, with the scanning slit of the Nidek laser as in the ordinary treatment. The area of irregularity is then treated, Dr. Waring explained.

In the study that Dr. Pop conducted, 60 patients (120 eyes) with myopia with and without astigmatism were enrolled. The mean patient age was 35 years. All underwent wavefront-guided LASIK. Three measurement methods were used: the OPD Scan (Nidek); the Friber visual acuity test to measure visual acuity and contrast threshold; and contrast sensitivity with Vector Vision (BrainLAB). All the variables were recorded and screened preoperatively and postoperatively during the 1-, 3-, and 6-month follow-up visits.

"The most important questions were: Was spherical aberration changed? What were the results with best spectacle-corrected visual acuity? What were the effects on contrast sensitivity in a way to try to measure what the patient is seeing? and Does this approach help the quality of the patient's vision?" he said.

The results showed that when the measurements were done repeatedly on the same eye, there was a high correlation among the measurements, indicating the high quality of the testing, according to Dr. Waring.

When the preoperative measurements were compared with the postoperative measurements, "the best news is that there is no increase from preoperatively to postoperatively in the root mean square of the higher-order aberrations, wavefront aberrations, the Strehl ratio, and the spherical aberration," Dr. Waring said. "Therefore, the problem that arises so often (induction of higher-order aberrations) did not happen, which means that night glare and halos have not increased in these patients."

The exceptions to this reduction were coma and asphericity, which increased slightly postoperatively.

"These increases were not statistically different from baseline and should not make much difference clinically," he stated.

The visual outcomes using the Nidek laser were excellent: 95% of patients had 20/20 or better uncorrected visual acuity. There was no loss of two or more lines of visual acuity, indicating that the safety of the procedure is extremely good, according to Dr. Waring.

Regarding the contrast sensitivity results with and without glare, at various time points postoperatively, the values were similar, and patients should not have glare and halos induced by the procedure, he pointed out.

"The topographically guided Nidek algorithm provided a good visual outcome and it generated less total aberrations," Dr. Waring said. "For a topographically guided system, this is a very positive result."

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