Evaluation of corneal functional optical zone after laser in situ keratomileusis

Bilateral Comparison of Wavefront-guided Versus Conventional Laser in situ Keratomileusis With Bausch and Lomb Zyoptix

PURPOSE

One aim of corneal refractive surgery is to correct defocus and astigmatism. In the process of correcting lower order aberrations, higher order ocular aberrations increase. To evaluate the effectiveness of wavefront-guided laser in situ keratomileusis (LASIK) in reducing the increase of higher order aberration, we compared aberrational change after LASIK with conventional and wavefront-guided customized ablation.

METHODS

Our study included 48 eyes of 24 patients. We performed conventional LASIK in one eye (Group 1) and wavefront-guided customized ablation in the other eye (Group 2). Ocular aberration was measured with the Zywave, a type of Shack-Hartmann aberrometer. We then compared low and high order aberrations, contrast sensitivity, visual acuity, corneal topography, and manifest refraction preoperatively and postoperatively at 1 and 3 months.

RESULTS

Uncorrected visual acuity improved to more than 20/20 in two eyes in the conventional ablation group and in five eyes in the customized ablation group. In the conventional ablation group, Root-mean-square for higher order (RMS(H)) was 0.215 preoperatively, 0.465 (216.3%) at 1 month, and 0.418 (194.4%) at 3 months. In the customized ablation group, RMS(H) was 0.207 preoperatively, 0.380 (183.6%) at 1 month, and 0.371 (179.2%) at 3 months after LASIK. Mesopic contrast sensitivity in the customized ablation group was higher than that in the conventional ablation group, but this change was not statistically significant.

CONCLUSIONS

Wavefront-guided customized ablation reduced the increase of high order aberrations resulting from LASIK. In terms of visual acuity, patient preference, and mesopic contrast sensitivity, wavefront-guided customized ablation produced slightly-but not statistically significant-better results.

Nomogram Considerations With the Technolas 217A for Treatment of Myopia

PURPOSE

To determine whether surgeon-specific nomogram adjustments are useful when using the Technolas 217A excimer laser for treating myopia and myopic astigmatism.

METHODS

We conducted a prospective evaluation of 216 consecutive eyes with 6 months follow-up after treatment of myopia or myopic astigmatism with the Technolas 217A laser. Attempted vs. achieved change in refraction was analyzed with a statistical analysis software program. Factors such as age, corneal thickness (pachymetry), preoperative spherical equivalent refraction, preoperative cylinder, and optical zone were studied to evaluate their role in predicting refractive outcome at 6 months after LASIK.

RESULTS

The mean value of attempted spherical equivalent refraction was -5.32 +/- 2.72 D. The mean achieved refractive correction at 6 months was -5.55 +/- 2.78 D, with a mean spherical equivalent of 0.13 +/- 0.54 D. The percent achieved effect at 1 month was 105%, and at 6 months, 103%. Preoperative spherical equivalent refraction and optical zone size were strong predictors of 6-month LASIK outcome. There was a 9% difference in the percent achieved effect between a 4 and 7-mm optical zone. There was no correlation between age, preoperative cylinder, or surgeon and 6-month outcome.

CONCLUSIONS

Surgeons using the planoscan software on the Technolas 217A may experience a small initial overcorrection. There may be a benefit to reducing the treatment given with larger optical zones and smaller corrections.

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.

Pupil size and the ablation zone in laser refractive surgery: Considerations based on geometric optics

PURPOSE

To determine whether the currently accepted method of selecting a minimum ablation zone size for refractive surgery based on dark-adapted pupil diameter is substantiated by geometric optical analysis.

SETTING

Department of Ophthalmology and Visual Sciences, Texas Tech University Health Sciences Center, Lubbock, Texas, USA.

METHODS

An optical model of the anterior segment was developed to calculate the effective corneal refractive diameter (ECRD), which is the diameter of the area of cornea that refracts all incident light rays arising from objects along the line of sight though the physical pupil (PP). The concept of the entrance pupil (EP) was reexamined and developed, and the ECRD was calculated over a range of corneal curvatures (K), anterior chamber depths (ACDs), and EP sizes. The model was generalized to include oblique light rays. Calculations were performed using MatLab Optimization Toolbox software (The MathWorks).

RESULTS

For a given EP size, the ECRD was significantly influenced by K and slightly influenced by ACD.

CONCLUSIONS

For objects on the line of sight, the ECRD was smaller than the EP in all cases. Regarding rays from objects in the periphery, the ECRD expanded rapidly as the angle of oblique incidence increased. For objects on the line of sight, the ECRD is always smaller than the clinically measured pupil (EP) because the EP is substantially magnified relative to the PP. Ablation zones larger than the EP should, in theory, prevent scattered or defocused light rays from contributing to the foveal image. When considering objects in the periphery, the increase in ECRD is sufficiently rapid that current refractive procedures cannot stop scattered light from these objects from contributing to the retinal image.

Diameters of Topographic Optical Zone and Programmed Ablation Zone for Laser in situ Keratomileusis for Myopia

PURPOSE

To compare topographic optical zones with programmed ablation zone settings of eyes treated with laser in situ keratomileusis (LASIK) for myopia using the VISX S2 excimer laser.

METHODS

Two-hundred three eyes treated with LASIK using the VISX S2 excimer laser were retrospectively evaluated to determine the size of the topographic optical zone. Three to six months after LASIK, the topographic optical zone was measured at the zone of highest curvature on topography and subtraction topography. Eyes were divided into four groups (A, B, C, D) in order of increasing myopia.

RESULTS

When the topographic optical zone was compared with the programmed ablation zone, an optical zone reduction of 0.5 +/- 0.1 mm and 0.4 +/- 0.1 mm was found for the longest and shortest diameters, respectively. For eyes with spherical ablation zones, this reduction was 0.6 +/- 0.1 mm and 0.4 +/- 0.1 mm shorter than the programmed horizontal and vertical dimensions. Groups A, B, C, and D, in order of increasing myopia, all showed reductions of approximately 0.5 +/- 0.1 mm for the longest and 0.3 to 0.4 +/- 0.1 mm for the shortest diameters of the optical zone.

CONCLUSIONS

The topographic optical zone was reduced from the programmed ablation zone. This reduction was statistically significant for both elliptical and spherical ablations, and seemed to be independent of the amount of myopia.

Systematic underablation in laser in situ keratomileusis: ablation pattern identified by advanced topographical analysis

Topographical analysis based on the differential geometry of surfaces-curvature topography-was developed and applied to a patient after laser in situ keratomileusis. The patient had a minimal residual refractive error and normal best corrected visual acuity but had multiple visual aberrations, including ghosting and glare, unless the pupils were maximally constricted. The corneal loci responsible for the aberrations were difficult or impossible to identify on axial topographies but were readily identified with curvature topography. The patient's ablations appeared to be miniature versions of the intended ablation profiles, with small areas of emmetropic central cornea surrounded by annuli of rapidly increasing keratometric power; that is, systematic underablation. This may explain why some patients have visual aberrations with pupil diameters smaller than the programmed optical zones.

Use of large optical zones with the LADARVision laser for myopia and myopic astigmatism

PURPOSE

To analyze the visual acuity, contrast sensitivity, and target deviations of patients who underwent laser in situ keratomileusis (LASIK) for spherical and astigmatic myopia with up to 8.0-mm laser optical zones.

DESIGN

A retrospective noncomparative interventional case series.

PARTICIPANTS

Three hundred fifty-two eyes of 186 patients in a refractive surgery practice.

METHODS

Chart review of consecutive patients who underwent LASIK for spherical and astigmatic myopia with the Autonomous LADARVision excimer laser.

MAIN OUTCOME MEASURES

The preoperative and the 3-month postoperative visual acuity and contrast sensitivity, as well as the target deviation, were assessed for each eye. The change in best spectacle-corrected visual acuity (BSCVA), best spectacle-corrected contrast sensitivity (BSCCS), and target deviation were analyzed by size of optical zone.

RESULTS

For spherical myopes, uncorrected visual acuity (UCVA) was 20/20 or better in 55.7% of eyes. The mean deviation from target was -0.19 +/- 0.64 diopters (D), with 75.3% within +/-0.50 D of the target. None of the eyes lost more than two lines of BSCVA. A loss of three or more patches (levels) of BSCCS was seen in 2.7% of eyes, whereas a loss of four or more patches of BSCCS was seen in 1.1%. For astigmatic myopes, UCVA was 20/20 or better in 61.9% of eyes. The mean deviation from target was -0.17 +/- 0.55 D, with 67.5% within +/-0.50 D of the target. No eye lost more than two lines of BSCVA. A loss of three or more patches of BSCCS was seen in 4.3% of eyes, whereas a loss of four or more patches of BSCCS was seen in 1.2%.

CONCLUSIONS

Larger optical diameters did not adversely affect BSCVA and BSCCS. Postoperative target deviation with the nomogram used was accurate with larger optical zones.

Comparison of Videokeratographic Functional Optical Zones in Conductive Keratoplasty and Laser in situ Keratomileusis for Hyperopia

PURPOSE

To compare the videokeratographic functional optical zone of eyes treated with conductive keratoplasty to eyes treated with laser in situ keratomileusis (LASIK) for hyperopia.

METHODS

Sixteen eyes treated with conductive keratoplasty for hyperopia were retrospectively evaluated to determine the size of the videokeratographic functional optical zone. The functional optical zone of these eyes was compared to the functional optical zone of 16 eyes that underwent LASIK for hyperopia with the VISX S2 excimer laser, for comparable amounts of hyperopia. The functional optical zone was measured at the edge of central corneal steepening and paracentral flattening on videokeratography 3 to 6 months after surgery.

RESULTS

The functional optical zone after surgery measured an average of 5.6 mm horizontally and 5.6 mm vertically in the conductive keratoplasty eyes, and 4.7 mm horizontally and 5.1 mm vertically in the hyperopic LASIK eyes (P<.001 and P<.005). The mean functional optical zone area was 31.1 mm2 in the conductive keratoplasty eyes and 24.6 mm2 in the hyperopic LASIK eyes (P<.001). The functional optical zone created by conductive keratoplasty had more uniform central steepening and less peripheral blending than the functional optical zone created by hyperopic LASIK.

CONCLUSION

Conductive keratoplasty was effective at creating central steepening in the cornea. The functional optical zone resulting from conductive keratoplasty was significantly larger than that obtained with hyperopic LASIK using the VISX S2 excimer laser.

Impact of interocular differences in corneal asphericity on binocular summation

PURPOSE

To evaluate the impact that interocular differences in corneal asphericity (Q) exert on binocular summation measured as the contrast-sensitivity function.

DESIGN

Interventional case series.

METHODS

A total of 92 emmetropic subjects took part in the experiment, classified according to the interocular differences in corneal asphericity (deltaQ) measured with an EyeSys-2000 corneal topographer. Fifty-four subjects had deltaQ < 0.1; 21 subjects had 0.1 < or = deltaQ < or = 0.2; and 17 had deltaQ > 0.2. The contrast-sensitivity function (CSF) was measured monocularly (for each eye) and binocularly with a B VAT II device. The spatial frequencies used were as follows: 2.4, 3.7, 6.0, 9.2, 12, 15, 20, and 24 cycles per degree.

RESULTS

Although the binocular CSF for the three groups studied was greater than the monocular in all the spatial frequencies studied, there were significant differences in binocular summation. The average binocular summation (for all the spatial frequencies) for the group with deltaQ < 0.1 was 1.46, significantly higher than the group with 0.1 < or = deltaQ < or = 0.2, in which the average binocular summation was 1.39 (P = .035), which was also significantly higher than the group deltaQ > 0.2, for which the average binocular summation was 1.26 (P < .0001). In this last group, the summation decreased to the level of the probability summation.

CONCLUSIONS

Differences in corneal asphericity may affect the binocular visual function by diminishing the binocular contrast-sensitivity function. This result may have important implications in refractive surgery given that, although the subject becomes emmetropic, if interocular differences are induced in corneal asphericity, it could reduce binocular visual performance.