Photoastigmatic Refractive Keratectomy in Myopes

Long-term results of excimer laser procedure to correct astigmatic refractive errors

BACKGROUND

The aim of this study was to evaluate long-term efficacy, safety, stability, and predictability of photorefractive keratectomy (PRK) as treatment of astigmatism.

MATERIAL/METHODS

Ninety-four eyes of 52 patients, treated with PRK for compound myopic astigmatism, compound hyperopic astigmatism, and mixed astigmatism were studied during a 36-month period. Main outcome measures were uncorrected and best-corrected visual acuity (UCVA, BCVA), refraction, and corneal transparency. Astigmatism correction was analyzed by a power vector method.

RESULTS

In myopic astigmatism group (42 eyes), postoperative UCVA was 20/40 or better in 100% of eyes, 20/25 or better in 38 eyes out of 42 (90.5%), and 20/20 or better in 25/42 eyes (59.5%). No eye lost lines of the BCVA, 40/42 (95.2%) eyes had refraction within ± 1D and 37/42 (88.1%) within ± 0.50 D. In the hyperopic astigmatism group (28 eyes), the UCVA was 20/40 or better in 100% of eyes, 20/25 or better in 26/28 eyes (92.8%), and 20/20 or better in 24/28 eyes (85.7%); 1/28 eyes (3.6%) lost 1 line of the BCVA, 23/28 eyes (82.1%) were within ± 1D, and 21/28 (75%) were within ± 0.50D. In the mixed astigmatism group (24 eyes), the UCVA was 20/40 or better in 100% of eyes, 20/25 or better in 22/24 eyes (91.7%) and 20/20 in 15/24 (62.5%) eyes. No eye lost lines of BCVA, 23/24 eyes (95.8%) were within 1.0 D, and 20/24 eyes (83.3%) were within 0.50 D of defocus refraction. Power vector analysis showed a significant reduction of blurring strength in all examined groups.

CONCLUSIONS

PRK is a safe and effective procedure for correction of all types of astigmatism, with good stability and efficacy at 3-year follow-up.

Laser eye surgery for refractive errors

Several laser and non-laser refractive surgical procedures have been used to modify the shape of the cornea and correct myopia, hyperopia, astigmatism, and presbyopia. Introduction of the excimer laser to reshape the cornea has resulted in remarkable developments in the correction of these refractive errors. Combined with other advanced ophthalmological instruments, laser refractive eye surgery has resulted in a substantial rise in the safety, efficacy, and predictability of surgical outcomes. Despite these advances, certain limitations and complications persist. In this review, we describe the history, preoperative assessment, surgical techniques, outcomes, and complications of laser refractive surgery.

Mathematical model of corneal surface smoothing after laser refractive surgery

PURPOSE

To construct a quantitative model of corneal surface smoothing after laser ablation for refractive correction.

DESIGN

Experimental study, interventional case series, and meta-analysis of literature.

METHODS

A theory of epithelial smoothing in response to corneal contour change is derived from differential equations that describe epithelial migration, growth, and loss. Computer simulations calculate the effects on postoperative epithelial thickness, topography, refraction, and spherical aberration. Model parameter is matched with laser in situ keratomileusis (LASIK) outcome in literature and in a retrospective study of primary spherical myopic (77 eyes) and hyperopic (19 eyes) corrections. Surgically induced refractive change was the main outcome measure.

RESULTS

Simulated epithelial remodeling after myopic ablation produces central epithelial thickening, reduction in achieved correction, and induction of oblate spherical aberration. Simulation of hyperopic ablation shows peripheral epithelial thickening, a larger reduction in correction, and induction of prolate spherical aberration. Simulation using a minus cylinder laser ablation pattern shows decreased astigmatism correction and increased hyperopic shift. In the LASIK series, linear regression of achieved correction vs ablation setting in hyperopic and minus cylinder corrections shows slopes of 0.97, 0.71, and 0.74, respectively. These clinical results match model predictions when the smoothing constant is set at 0.32, 0.63, and 0.55 mm, respectively.

CONCLUSIONS

Epithelial thickness modulations after ablation can be modeled mathematically to explain clinically observed regression and induction of aberration. The cornea appears to smooth over ablated features smaller than approximately 0.5 mm. The model provides an approach for designing ablation patterns that precompensate for the smoothing to improve final outcome.

Photorefractive keratectomy for compound myopic astigmatism with the MEL-70 G-Scan excimer laser

PURPOSE

To assess the safety, efficacy, predictability and stability of photorefractive keratectomy in compound myopic astigmatism with a moderate and high cylinder component.

METHODS

Photorefractive keratectomy was done in 42 eyes with compound myopic astigmatism with the spherocylindrical algorithm of the MEL-70 excimer laser, with wide ablation zones.

RESULTS

Spherical equivalent refraction changed from -4.19 +/- 1.65D to -0.05 +/- 0.31D, refractive cylinder from -2.01 +/- 0.71D to -0.09 +/- 0.20D and mean sphere from -3.22 +/- 1.76D to -0.02 +/- 0.26D. Mean uncorrected visual acuity rose from 0.12 +/- 0.17 to 0.91 +/- 0.10. No eye lost lines of spectacle-corrected visual acuity. The safety index was 1.03 and the efficacy index 0.98. Six months from the treatment all eyes were within +/- 1D, 8.9% of eyes were within 0.50D and 44% were plano of target refraction. Refractive and topographical stability were achieved between one and three months after treatment. Transient haze was observed between one and three months after PRK.

CONCLUSIONS

Photorefractive keratectomy with the MEL-70 excimer laser to correct myopic astigmatism was a safe and effective procedure with good stability at six months' follow-up. Refractive and visual outcome confirmed that excellent predictability can be expected.

Accuracy and predictability of intraocular lens power calculation after laser in situ keratomileusis

PURPOSE

To study the accuracy and predictability of intraocular lens (IOL) power calculation in eyes that had laser in situ keratomileusis (LASIK).

SETTING

Gimbel Eye Centre, Calgary, Alberta, Canada.

METHODS

Refractive outcomes in 6 cataract surgery and lensectomy eyes after previous LASIK were analyzed retrospectively. Target refractions based on measured and refraction-derived keratometric values were compared with postoperative achieved refractions. Differences between target refractions calculated using 5 IOL formulas and 2 A-constants and achieved refractions were also compared.

RESULTS

The refractive error of IOL power calculation in postoperative LASIK eyes was significantly reduced when refraction-derived keratometric values were used for IOL power calculation. Persistent residual hyperopia still occurred in some cases; this was corrected by hyperopic LASIK. Refractive results appeared more accurate and predictable when the Holladay 2 or Binkhorst 2 formula was used for IOL power calculation.

CONCLUSION

Hyperopic error after cataract surgery in post-LASIK eyes was significantly reduced by using refraction-derived keratometric values for IOL power calculation. Persistent hyperopic error was corrected by hyperopic LASIK.