Effect of lamellar flap location on corneal topography after laser in situ keratomileusis

The effect of corneal flap on optical aberrations

PURPOSE

To evaluate the changes in ocular aberrations induced by corneal flap creation.

DESIGN

Prospective interventional nonrandomized clinical trial.

METHODS

This study included 15 patients who were scheduled for laser in situ keratomileusis. A nasal hinge flap was created, using the Nidek MK-2000 microkeratome and then replaced without performing laser ablation. The ocular aberrations were measured before and after flap creation using the Nidek Optical Path Difference Scanning System ARK-10000.

RESULTS

The root mean square wavefront errors of the higher-order optical aberrations (third-, fourth-, fifth-, and sixth-order aberrations) were not significantly altered at 1 week postsurgery compared with the preoperative values (P >.35).

CONCLUSIONS

Creating a corneal flap with the Nidek MK-2000 microkeratome did not induce changes in higher-order optical aberrations as measured with the Nidek Optical Path Difference Scanning System ARK-10000 during the early postoperative period.

Laser in situ keratomileusis after conductive keratoplasty

We present a patient in whom laser in situ keratomileusis was used to treat residual hyperopia with astigmatism after conductive keratoplasty (CK). Previous CK did not affect initial flap creation, the ability to lift the flap manually 3 weeks after its initial preparation, or the refraction, topography, and refractive stability after the flap was created.

Spherical aberration after laser in situ keratomileusis and photorefractive keratectomy

PURPOSE

To assess changes in corneal asphericity after laser refractive surgery and mathematically model possible causes of the changes.

SETTING

Cornea and Laser Eye Institute, Hersh Vision Group, Teaneck, New Jersey, USA.

METHODS

The corneal topography (EyeSys 2000) of 20 eyes was measured before and after laser in situ keratomileusis, laser-assisted subepithelial keratectomy, and photorefractive keratectomy for myopia. All preoperative and postoperative maps were analyzed using the CTView 4.0, a computer software program for determining quantitative corneal spherical aberration. To define possible mechanisms of asphericity change, 2 mathematical models of corneal ablation were constructed and theoretical postoperative corneal asphericities were determined over a range of corrections from -12.0 to +6.0 diopters. Model 1 assumes homogeneous beam fluence over the ablation zone, and model 2 accounts for a theoretical ablation rate drop off peripherally as a result of the angle of incidence of the laser beam on the cornea. Postoperative clinical corneal spherical aberration was compared to the theoretically predicted asphericity values.

RESULTS

After excimer laser procedures, all corneas had positive asphericity within the ablation zone, generally changing from a prolate to an oblate optical contour. The mean asphericity (Q) was -0.17 +/- 0.14 (SD) preoperatively and +0.92 +/- 0.70 postoperatively. The mean change in spherical aberration was +1.09 +/- 0.67 of positive asphericity; the range of asphericity change was +0.40 to +2.73 in the direction of a more oblate corneal profile. A trend toward greater change in asphericity and more oblateness was observed among eyes receiving higher correction. A mathematical model taking into account theoretical beam fluence changes across the ablation zone was highly predictive of the actual postoperative asphericity measurements.

CONCLUSIONS

The cornea within the ablation zone becomes more oblate after laser refractive surgery. A mathematical model of the change in asphericity, which accounts for the angle of incidence of the laser beam across the ablation area, predicted this change in spherical aberration. If the model is correct, possible changes in laser algorithms, delivering more ablation to the peripheral optical zone, may better retain the native corneal prolate conformation. Moreover, wavefront-guided ablations may have to consider the effects of fluence variability across the optical zone to fully correct spherical as well as other aberrations.

Clinical results of laser in situ keratomileusis with superior and nasal hinges

PURPOSE

To compare the differences in the clinical results of LASIK with a nasal corneal flap and with a superior corneal flap.

SETTING

Department of Ophthalmology, Kangnam St. Mary's Hospital, Seoul, Korea.

METHODS

This randomized prospective study comprised 30 patients whose age and degrees of myopia were similar. In 1 group (25 eyes), a superior hinge was made and in the other group (25 eyes), a nasal hinge was made. Uncorrected visual acuity (UCVA), spherical equivalent (SE), astigmatism, and corneal topographical change before surgery and 1 week and 2 and 6 months after surgery were compared. The tear breakup time (BUT) and the Schirmer test before surgery and 2 months postoperatively were also compared.

RESULTS

At 6 months, there were no significant between-group differences in UCVA, SE, astigmatism, and corneal ablation decentration (P >.05). At 2 months, the results of BUT and the Schirmer tests were 6.80 seconds and 9.54 mm, respectively, in the nasal hinge group (P >.05) and 5.08 seconds and 6.05 mm, respectively, in the superior hinge group (P <.05).

CONCLUSIONS

There were no between-group differences in UCVA, SE, astigmatism, and corneal ablation decentration at 6 months, but the BUT and Schirmer test results were aggravated after LASIK in the superior hinge group.