Optimal balance of wavefront aberrations in photorefractive keratectomy.

PURPOSE: To determine the ratio of defocus (Z4) to spherical aberration (Z12) in patients with optimal visual performance 10 years after multifocal aspheric photorefractive keratectomy (PRK). METHODS: The optimal ratio of defocus to spherical aberration was determined in 16 presbyopic patients (32 eyes) with 10-year follow-up after aspheric multifocal PRK with good to excellent visual acuity postoperatively and the absence of mesopic symptoms such as halos and glare. All patients were satisfied with their near and far vision. The NIDEK OPD-Scan was used to measure wavefront aberrations postoperatively for a 6-mm pupil diameter to the 6th Zernike order. Preoperative manifest refraction spherical equivalent (MRSE) was -7.61 +/- 1.04 diopters (D) (range: -6.25 to -10.00 D). RESULTS: At 10 years postoperatively, mean spherical equivalent refraction was -0.49 +/- 0.96 D. All eyes were within 1.00 D of the intended refraction. The ratio of Z4/Z12 for patients with optimal outcomes was 3.07 +/- 1.36. CONCLUSIONS: The ratio of Z4/Z12 generated in this study determines the optimum balance of defocus to spherical aberration required for excellent outcomes in patients undergoing multifocal aspheric PRK.

Confocal microscopy of corneal wound healing after Ho:YAG laser thermokeratoplasty.

PURPOSE: To investigate corneal wound healing using confocal microscopy in patients who underwent laser thermokeratoplasty (LTK) for hyperopia between +1.50 and +3.00 diopters (D). METHODS: In a prospective, nonrandomized study, 78 eyes of 42 patients were followed for 1 year using contact and non-contact confocal microscopy to evaluate corneal appearance after LTK with the Oko-1 Ho: YAG laser. The NIDEK Confoscan 4, 20x non-contact lens and 40x contact lens, was used to evaluate the morphology of the corneal layers. The Oko-1 laser was used to reduce or eliminate hyperopic refractive error. Patients underwent confocal microscopy measurement preoperatively, 1 and 2 weeks and 1, 3, 6, and 12 months after LTK. Individual examinations were reviewed and classified by the investigators. RESULTS: Preoperatively, spherical hyperopia was +2.11 +/- 0.21 D (range: +1.50 to +2.75 D). At 3 months postoperatively, the mean manifest refractive spherical equivalent (MRSE) was +0.11 +/- 0.02 D and +0.21 +/- 0.02 D at 1 year. Mean corneal pachymetry was 631.6 +/- 8.40 microm preoperatively, 637.4 +/- 12.7 microm at 3 months, and 608.4 +/- 31.7 microm at 1 year. The mean endothelial cell density was 2771.4 +/- 28.4 cells/mm2 preoperatively, 2709.4 +/- 122.3 cells/mm2 at 3 months, and 2693.2 +/- 139.1 cells/mm2 at 1 year. Activated keratocytes were present at 3 months postoperatively, which decreased over the course of 1 year. CONCLUSIONS: Laser thermokeratoplasty for low to moderate hyperopia using the Oko-1 results in formation of intrastromal fibrosis with minimal changes in corneal morphology outside the sites of treatment.

Smoothness of ablation on polymethylmethacrylate plates with four scanning excimer lasers.

PURPOSE: To perform a quantitative evaluation of smoothness of ablation on polymethylmethacrylate (PMMA) using four scanning excimer lasers available commercially for photorefractive surgery. METHODS: Ablations were done on PMMA plates with dimensions 100 x 50 x 1 mm. Four scanning excimer lasers were used, two with flying spot technology (Zeiss-Meditec MEL-70, and a Russian-made unit, Microscan) and two Nidek models with scanning slit delivery systems and an expanding iris diaphragm (EC-5000 and EC-5000 CX). Forty PMMA plates were ablated with standard -3.00-D settings using an ablation zone of 6 mm; each laser ablated 10 plates. Measurements were made in the center of each plate with the Zygo microscope, based on the principle of white light interferometry. Smoothness of ablation was characterized by three surface parameters (RMS, Ra, PV). RMS was considered the most significant parameter. RESULTS: The smoothest surface was obtained in samples produced by Zeiss-Meditec MEL-70 unit (RMS=112 +/- 23 nm), followed by the Nidek EC-5000 CX (RMS=153 +/- 12 nm), and the Microscan (RMS=181 +/- 11 nm). The smoothness of ablation produced by the Nidek EC-5000 unit (RMS=329 +/- 39 nm) was significantly less than the other three lasers (P<.01). CONCLUSIONS: Scanning excimer lasers based on flying spot technology--Zeiss-Meditec MEL-70 and Microscan, as well as the Nidek EC-5000 CX with FlexScan--created smoother ablations on PMMA plates compared to the older Nidek EC-5000 unit.