Effect of larger ablation zone and transition zone on corneal optical aberrations after photorefractive keratectomy.

OBJECTIVE: To evaluate the effects of photorefractive keratectomy on corneal optical aberrations using a 5.5-mm optical zone and a 7.0-mm transition zone. METHODS: Videokeratographs of 72 eyes from 47 patients treated for low to moderate (1-9 diopters) myopia were obtained at the preoperative and 1-, 3-, 6-, 12-, and 18-month postoperative examinations. The videokeratoscopy data files were used to calculate the wavefront variance of the corneas for small (3-mm) and large (7-mm) pupils using a previously described method. RESULTS: In general, all optical aberrations decreased postoperatively for 3-mm pupils and increased for 7-mm pupils compared with preoperative values. For 3-mm pupils, the 2 common optical aberrations (comalike [S(3)] and spherical-like [S(4)]) decreased postoperatively and never returned to preoperative values. For 7-mm pupils, however, comalike aberrations increased slightly and spherical-like aberrations increased by nearly an order of magnitude during the postoperative period. Similarly, for 3-mm pupils, the higher order S(5) and S(6) aberrations decreased throughout the postoperative period, with S(6) values showing an approximately 23-fold reduction at 12 and 18 months. For 7-mm pupils, S(5) and S(6) aberrations increased slightly, more so for S(5) (approximately 3-fold) than for S(6). Total wavefront aberrations decreased an average of 2.3 times postoperatively for 3-mm pupils, and increased significantly (P<.05) at all postoperative examinations for 7-mm pupils. Opening the pupil from 3 mm to 7 mm before surgery produced a 14-fold increase in total aberrations, whereas this same change produced an average 113-fold increase after photorefractive keratectomy. CONCLUSIONS: Corneal optical aberrations after photorefractive keratectomy with a larger ablation zone and a transition zone are less pronounced and more physiologic than those associated with first-generation (5-mm) ablations with no transition zone. CLINICAL RELEVANCE: Evaluating the postoperative corneal aberration structure will help us devise ways to minimize the wavefront aberrations of the eye through the creation of an ideal corneal first surface, thereby improving visual results for patients undergoing excimer laser ablations for refractive correction.

Change in intraocular pressure in myopic eyes measured with contact and non-contact tonometers after laser in situ keratomileusis.

PURPOSE: To study the change in the intraocular pressure (IOP) after laser in situ keratomileusis (LASIK) for correction of myopia. METHODS: One hundred twenty consecutive myopic eyes (60 patients) were included in a prospective study. All eyes received LASIK with the Nidek EC-5000 excimer laser and the Chiron Automated Corneal Shaper. Baseline refraction, keratometry, pachymetry, ablation depth, and IOP measured by Goldmann applanation tonometry and non-contact air puff tonometry were correlated with the IOP change after surgery. Sixty healthy eyes of 30 subjects served as controls. RESULTS: At 6 months, 108 eyes (90%) were examined. Compared to preoperative values, IOP decreased in 103 eyes (95.4%) when measured with applanation tonometry; it decreased in all eyes when measured with air puff tonometry. Mean change in IOP was -4.3 +/- 2.1 mmHg (range, -10.0 to +1.0 mmHg) with the applanation and -6.1 +/- 2.3 mmHg (range, -12.0 to -1.0 mmHg) with air puff tonometry. The IOP change measured with either instrument correlated significantly with the baseline IOP (P < .001) and the ablation depth (air puff, P < .001, applanation; P = .006). CONCLUSION: Intraocular pressure decreased significantly after LASIK when measured with either Goldmann (mean 4.3 mmHg) or air puff (mean 6.1 mmHg) tonometers. This decrease may delay the diagnosis or affect the management of future glaucoma that may develop in a myopic eye that received LASIK.

Diurnal fluctuations in corneal topography 10 years after radial keratotomy in the Prospective Evaluation of Radial Keratotomy Study.

PURPOSE: To correlate clinically observed fluctuations in manifest refraction, visual acuity, keratometry, and intraocular pressure (IOP) with changes in the anterior corneal surface as measured by videokeratography in patients 10 years after radial keratotomy (RK). SETTING: Four clinical centers in the United States that participated in the Prospective Evaluation of Radial Keratotomy (PERK) study. METHODS: Thirty-two eyes of 20 PERK patients who noted diurnal fluctuations in vision had clinical examination and videokeratography (TMS-1, Computed Anatomy Inc.) in the morning and evening of the same day a mean of 10.3 years (range 7.8 to 11.7 years) after RK. The videokeratographs were analyzed in terms of various indexes generated by custom-designed software. Morning-to-evening changes in the means of the various clinical and videokeratographic values were assessed using pairwise methods. RESULTS: The mean increase in myopia was 0.36 diopters (D) +/- 0.58 (SD) from morning to evening (P < .01). Analysis of the videokeratographs showed a corresponding increase in average corneal power (ACP), reflecting a steepening of 0.52 +/- 0.45 D (P < .001). The change in ACP was correlated with a change in the manifest spherical equivalent refraction (R = 0.39, P = .03) and a change in best spectacle-corrected visual acuity (R = 0.38, P = .03) over the same period. Similarly, simulated keratometry (SimK) readings correlated with the change in the manifest spherical equivalent refraction (R = 0.38, P = .03 for SimK1; R = 0.37, P = .35 for SimK2; R = 0.4, P = .02 for average SimK), although the standard clinical keratometric data did not (P = .26 for K1, P = .11 for K2, and P = .09 for the mean K). The elevation depression magnitude, a measure of the low-frequency irregularities of the cornea, showed a decrease of 0.32 +/- 1.59, which also correlated with the change in the manifest spherical equivalent refraction (R = 0.37, P = .04). Intraocular pressure tended to decrease from morning to evening (mean change of -0.97 +/- 3.29 mm Hg), but the difference was not significant. Variations in IOP in individual patients, however, were correlated with changes in the manifest spherical equivalent refraction (R = 0.37, P = .04). CONCLUSIONS: Diurnal fluctuations in corneal topographic indexes can be used to evaluate the diurnal fluctuations in refraction and visual acuity after RK. The study findings provide statistical support for the idea that IOP contributes to the diurnal fluctuation in visual acuity after RK.

Corneal topography of small-beam tracking excimer laser photorefractive keratectomy.

PURPOSE: To evaluate the topographic characteristic of photorefractive keratectomy (PRK) for low myopia performed with a small-beam (0.9 mm) tracking excimer laser. SETTING: Department of Ophthalmology, LSU Eye Center, Louisiana State University Medical Center School of Medicine in New Orleans, and the Refractive Surgery Center of the South at the Eye, Ear, Nose, & Throat Hospital, New Orleans, Louisiana, USA. METHODS: Sixty-seven eyes of 47 patients had PRK with a small-beam tracking laser. Of these, 49 eyes had data permitting evaluation of ablation centration; usable data for topographic analysis were available for 59 eyes preoperatively, 54 eyes at 1 month, 42 eyes at 3 months, and 25 eyes at 6 months, permitting measurement of various topographic parameters, including the cylinder (CYL), average corneal power (ACP), surface regularity index (SRI), surface asymmetry index (SAI), corneal eccentricity index (CEI), and coefficient of variation of corneal power (CVP). RESULTS: Preoperatively, all eyes were topographically normal. Postoperatively, no eye exhibited a "central island" by even the least-restrictive definition, and all eyes had best spectacle-corrected visual acuities (BSCVAs) of 20/20 or better at all follow-ups. Mean decentration of the ablations from the pupil centers was 0.42 mm +/- 0.28 (SD) (n = 49). There was no correlation between measured decentration and BSCVA (P = .46). The central cornea was flattened (decreased ACP; P < .001) and made oblate (decreased CEI; P < .001) as expected. There was no increase in SRI or SAI (irregular astigmatism) at 6 months compared with preoperative values (P = .91); however, CYL and CVP (varifocality) increased slightly (P = .04 and .02, respectively). CONCLUSION: The absence of significant regular or irregular astigmatism 6 months after PRK with the small-beam laser is an improvement over published results achieved with wide-beam lasers and is consistent with the excellent visual acuity results in this cohort.