Corneal optical irregularity after excimer laser photorefractive keratectomy. The Summit Photorefractive Keratectomy Topography Study Group

Biology of keratorefractive surgery- PRK, PTK, LASIK, SMILE, inlays and other refractive procedures

The outcomes of refractive surgical procedures to improve uncorrected vision in patients-including photorefractive keratectomy (PRK), laser in-situ keratomileusis (LASIK), Small Incision Lenticule Extraction (SMILE) and corneal inlay procedures-is in large part determined by the corneal wound healing response after surgery. The wound healing response varies depending on the type of surgery, the level of intended correction of refractive error, the post-operative inflammatory response, generation of opacity producing myofibroblasts and likely poorly understood genetic factors. This article details what is known about these specific wound healing responses that include apoptosis of keratocytes and myofibroblasts, mitosis of corneal fibroblasts and myofibroblast precursors, the development of myofibroblasts from keratocyte-derived corneal fibroblasts and bone marrow-derived fibrocytes, deposition of disordered extracellular matrix by corneal fibroblasts and myofibroblasts, healing of the epithelial injury, and regeneration of the epithelial basement membrane. Problems with epithelial and stromal cellular viability and function that are altered by corneal inlays are also discussed. A better understanding of the wound healing response in refractive surgical procedures is likely to lead to better treatments to improve outcomes, limit complications of keratorefractive surgical procedures, and improve the safety and efficiency of refractive surgical procedures.

Astigmatism induced by conventional spherical ablation after PRK and LASIK in myopia with astigmatism < 1.00 D

BACKGROUND

The purpose of this study was to evaluate surgically-induced astigmatism after spherical ablation in photorefractive keratectomy (PRK) and laser-assisted in situ keratomileusis (LASIK) for myopia with astigmatism < 1.00 D.

METHODS

The charts of patients undergoing spherical PRK or LASIK for the correction of myopia with minimal astigmatism of <1.00 D from 2002 to 2012 at the John A Moran Eye Center in Salt Lake City, UT, were retrospectively reviewed. Astigmatism was measured by manifest refraction. The final astigmatic refractive outcome at 6 months postoperatively was compared with the initial refraction by Alpins vector analysis.

RESULTS

For PRK, average cylinder increased from 0.39 ± 0.25 (0.00-0.75) preoperatively to 0.55 ± 0.48 (0.00-1.75) postoperatively (P = 0.014), compared with an increase in LASIK eyes from 0.40 ± 0.27 (0.00-0.75) preoperatively to 0.52 ± 0.45 (0.00-2.00) postoperatively (P = 0.041). PRK eyes experienced an absolute value change in cylinder of 0.41 ± 0.32 (0.00-1.50) and LASIK eyes experienced a change of 0.41 ± 0.31 (0.00-1.50, P = 0.955). Mean surgically-induced astigmatism was 0.59 ± 0.35 (0.00-1.70) in PRK eyes, with an increase in surgically-induced astigmatism of 0.44 D for each additional 1.00 D of preoperative cylinder; in LASIK eyes, mean surgically-induced astigmatism was 0.55 ± 0.32 (0.00-1.80, P = 0.482), with an increase in surgically-induced astigmatism of 0.29 D for each 1.00 D of preoperative cylinder.

CONCLUSION

Spherical ablation can induce substantial astigmatism even in eyes with less than one diopter of preoperative astigmatism in both PRK and LASIK. No significant difference in the magnitude of surgically-induced astigmatism was found between eyes treated with PRK and LASIK, although surgically-induced astigmatism was found to increase with greater levels of preoperative astigmatism in both PRK and LASIK.

Wavefront aberrations in eyes with decentered ablations

PURPOSE

To compare the refractive and functional outcomes and wavefront profiles in eyes with decentered ablations and eyes with well-centered ablations.

SETTING

Medical Research Foundation, Chennai, Tamil Nadu, India.

METHODS

This retrospective analysis comprised eyes with topographically diagnosed decentered ablations after laser in situ keratomileusis (LASIK). Refraction, contrast sensitivity, and ocular wavefront aberrations were measured preoperatively and 1 month postoperatively. The induced aberrations in these eyes were compared with those in eyes with well-centered ablations.

RESULTS

Forty-six eyes (38 patients) had decentered ablations and 60 eyes (32 patients), well-centered ablations. The mean decentration in the study group was 0.86 mm +/- 0.29 (SD) (range 0.35 to 1.61 mm). There was no significant correlation between decentration and attempted refractive correction. There was, however, a statistically significant (P<.05) linear correlation between the distance of decentration and the magnitude of induced tilt (r = -0.31), coma (r = -0.41), and secondary astigmatism (r = 0.36). The induced changes in tilt, oblique astigmatism, vertical coma, and spherical aberration were statistically significantly higher in eyes with decentered ablations than in eyes with well-centered ablations. A statistically significantly higher percentage of eyes (87%) with well-centered ablations than eyes with decentered ablations (70%) had a postoperative uncorrected visual acuity (UCVA) of 20/20 or better. There was no significant difference in contrast sensitivity between groups.

CONCLUSION

Eyes with decentered ablations had a significantly higher magnitude of induced aberrations and lower UCVA than eyes with well-centered ablations.

Functional optical zone after myopic LASIK as a function of ablation diameter

PURPOSE

To analyze spherical aberrations in patients who had myopic laser in situ keratomileusis (LASIK) with different optical zones and varying degrees of attempted corrections.

SETTING

Laser Refractive Center, University of California, Los Angeles, USA.

METHODS

This retrospective analysis comprised 78 eyes of 56 consecutive patients who had LASIK for myopia with the Alcon LADARVision excimer laser. The preoperative and 3-month postoperative functional optical zones (FOZs), a measure of spherical aberrations, were assessed in each eye. The change in FOZs was analyzed by size of the primary optical zone and degree of attempted correction.

RESULTS

The mean preoperative FOZ was 6.1 mm +/- 0.6 (SD) and the mean 3-month FOZ, 5.2 +/- 1.0 mm. The mean decrease was 0.9 +/- 1.0 mm (P < .0001). The mean decreases were 0.005 +/- 0.7 mm, 0.7 +/- 0.9 mm, and 1.6 +/- 0.9 mm in the low, moderate, and high attempted correction groups, respectively (P < .05), and 1.3 +/- 0.9 mm, 1.2 +/- 1.0 mm, 0.5 +/- 1.3 mm, and 0.3 +/- 0.7 mm in optical zone groups of 6.0 to 6.5 mm, 6.6 to 7.0 mm, 7.1 to 7.5 mm, and 7.6 to 8.0 mm, respectively (P < .05).

CONCLUSIONS

Larger optical zones had fewer postoperative spherical aberrations. Higher attempted corrections had larger increases in spherical aberrations. Larger treatment diameters minimized postoperative spherical aberrations.

Simulation of Eye-tracker Latency, Spot Size, and Ablation Pulse Depth on the Correction of Higher Order Wavefront Aberrations With Scanning Spot Laser Systems

PURPOSE

The aim of this theoretical work was to investigate the robustness of scanning spot laser treatments with different laser spot diameters and peak ablation depths in case of incomplete compensation of eye movements due to eye-tracker latency.

METHODS

Scanning spot corrections of 3rd to 5th Zernike order wavefront errors were numerically simulated. Measured eye-movement data were used to calculate the positioning error of each laser shot assuming eye-tracker latencies of 0, 5, 30, and 100 ms, and for the case of no eye tracking. The single spot ablation depth ranged from 0.25 to 1.0 microm and the spot diameter from 250 to 1000 microm. The quality of the ablation was rated by the postoperative surface variance and the Strehl intensity ratio, which was calculated after a low-pass filter was applied to simulate epithelial surface smoothing.

RESULTS

Treatments performed with nearly ideal eye tracking (latency approximately 0) provide the best results with a small laser spot (0.25 mm) and a small ablation depth (250 microm). However, combinations of a large spot diameter (1000 microm) and a small ablation depth per pulse (0.25 microm) yield the better results for latencies above a certain threshold to be determined specifically. Treatments performed with tracker latencies in the order of 100 ms yield similar results as treatments done completely without eye-movement compensation. CONCWSIONS: Reduction of spot diameter was shown to make the correction more susceptible to eye movement induced error. A smaller spot size is only beneficial when eye movement is neutralized with a tracking system with a latency <5 ms.

The efficacy of multi-zone cross-cylinder method for astigmatism correction

The purpose of this study is to assess the efficacy of the multi-zone cross-cylinder method as compared with the single method for astigmatism correction using LASIK. This prospective study enrolled 40 patients (52 eyes) who underwent the cross-cylinder method using LASIK, and 52 patients (60 eyes) who underwent the single method using LASIK: all patients were given a diagnosis of complex myopic astigmatism from the department of ophthalmology of this hospital between January 2002 and July 2003. Preoperatively, the mean spherical equivalent refraction was -3.85 +/- 1.13 D in the cross-cylinder group and -4.05 +/- 1.20 D in the single method group (p = 0.23). The mean cylinder was -2.05 +/- 1.58 D in the cross-cylinder group and -1.95 +/- 1.12 D in the single method group (p = 0.31). 6 months after treatment the results were a mean spherical equivalent refraction of -0.26 +/- 0.30 D in the cross-cylinder group and -0.34 +/- 0.35 D in the single method group (p = 0.13). The mean cylinder was -0.38 +/- 0.29 D in the cross-cylinder group and -0.45 +/- 0.30 D in the single method group (p = 0.096). There were no statistically significant differences between the two groups. The mean BCVA was not different from mean preoperative BCVA in both groups (i.e., 0.98 +/- 0.10, 0.96 +/- 0.25, p = 0.86). Postoperatively, patient complications that included night halo, glare and corneal haze were not noted in either group. In conclusion, the results of cross-cylinder method are no different from the single method for the correction of a complex astigmatism. In the future, studies will have to be conducted to assess the efficacy of the cross-cylinder method in consideration of those factors that can affect the postoperative outcome.

Wavefront-optimized ablation profiles

PURPOSE

To describe a method for calculating wavefront-optimized ablation profiles to precompensate for the spherical aberration and higher-order astigmatism induced by myopic, hyperopic, and astigmatic corneal laser corrections.

SETTING

IROC-Institut für Refraktive und Ophthalmo-Chirurgie, and Institute for Biomedical Engineering, Swiss Federal Institute of Technology, Zürich, Switzerland.

METHODS

The basic ablation profile for myopic, hyperopic, and astigmatic correction is derived from the 2nd-order Zernike representation of wavefront aberrations. Including 4th-order spherical aberration and higher-order astigmatism in the theoretical calculation of the ablation profile allows precompensation for the expected amount of higher-order aberrations (HOAs). The shapes of wavefront-optimized ablation profiles are compared with the shapes of "classic" ablation profiles for myopic and astigmatic corrections.

RESULTS

The introduction of precompensating spherical aberration and higher-order astigmatism leads to a more aspheric ablation profile with a significant increase in ablation depth (up to 35%) in the midperiphery of the optical zone. The central ablation depth remains unchanged in the myopic correction but increases by 3% in cylinder correction.

CONCLUSIONS

Wavefront-optimized ablation profiles provide a simple method to precompensate for the expected 4th-order spherical aberration and higher-order astigmatism in the average eye. Further clinical studies must be performed to prove the theoretical results; demonstrate the reduction in HOAs; and predict safety, predictability, and stability of wavefront-optimized ablation profiles.

Effect of excimer laser beam delivery and beam shaping on corneal sphericity in photorefractive keratectomy

PURPOSE

To evaluate the impact of beam delivery and beam shaping on corneal profiles after myopic excimer laser photorefractive keratectomy (PRK).

SETTING

Department of Ophthalmology, Charité-Campus Virchow Hospital, Humboldt University of Berlin, Berlin, Germany.

METHODS

Standard myopic 193 nm excimer laser PRK of -3.0 diopters (D) and -6.0 D was performed in porcine eyes using 2 commercially available broad-beam lasers with band-mask and fractal-mask beam shaping, 2 flying-spot lasers, and a scanning-slit laser. A silicone replica was obtained to preserve the corneal profile and was measured with a dynamic focusing topometry system.

RESULTS

The scanning-slit and flying-spot lasers created uniform profiles comparable to those in an untreated control group. Both broad-beam lasers with band-mask and fractal-mask beam shaping created central islands and paracentral profile valleys of 15.10 microm and 17.00 microm maximum height after -3.0 D PRK and 26.45 microm and 24.31 microm after -6.0 D PRK. An anti-central-island program, which applied a series of laser pulses centrally to compensate for the central profile elevations, did not eliminate the islands. Stromal surface roughness increased with ablation depth and was significantly worse after scanning-slit ablation than after broad-beam ablation.

CONCLUSIONS

Laser-induced deviations from the intended uniform corneal profiles were associated with broad-beam ablation and increased ablation depth and therefore lessened the predictability of the refractive outcomes. Scanning-slit and flying-spot systems produced predictably uniform corneal profiles.

Maximum permissible torsional misalignment in aberration-sensing and wavefront-guided corneal ablation

PURPOSE

To determine the maximum permissible torsional misalignment in wavefront-guided refractive surgery.

SETTING

University of Zurich, Department of Ophthalmology, Zurich, Switzerland.

METHODS

The effect of torsionally misaligned ablations on the optical outcome was simulated using measured wavefront aberration patterns (2nd to 6th orders) in 130 normally aberrated eyes. The calculations were done for 3.0 mm, 5.0 mm, and 7.0 mm pupils. The optical quality of the simulated correction was rated by the root-mean-square residual wavefront error.

RESULTS

The required accuracy of torsional alignment is higher for the correction of higher-order aberrations than for cylindrical treatments only. To improve the optical performance to the level of the best 10% of a normal, untreated population, ablation would have to occur within a tolerance range of 4.0 degrees for 7.0 mm pupils.

CONCLUSIONS

The tolerance range for torisional alignment in wavefront-guided higher-order corrections depends on the amount of original optical error in each eye. Rough centration based on the surgeon's judgment may not be accurate enough to achieve significantly improved optical quality in a high percentage of treated eyes.

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.

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.

Predicted corneal visual acuity in keratoconus as determined by ray tracing

PURPOSE

To evaluate the optical quality of the central anterior corneal surface in normal eyes and in eyes with keratoconus, and to investigate the accuracy of the predicted corneal visual acuity (PCVA) index as determined by ray tracing analysis.

METHODS

Twenty keratoconus eyes with contact lens-corrected visual acuity (VA) of 20/20 or better (11 patients, group A) and 20 eyes of 15 normal subjects (group B) were evaluated. After a detailed eye examination including measurement of pupil diameter, keratometry, topography and pachymetry, each subject eye was evaluated using ray tracing analysis with the Technomed C-scan colour ellipsoid topometer, using basic software (Technomed GmbH, Baesweiler, Germany). The PCVA was determined for each patient, and the results were analysed comparatively using two-sample t-test, regression analysis and Pearson correlation analysis.

RESULTS

The average best spectacle-corrected VA was measured as 0.2 +/- 0.2 logMAR (20/32) in group A and -0.1 +/- 0.1 logMAR (20/16) in group B. The average PCVA measurements derived from ray tracing analysis for 3.0 mm, 3.5 mm and 4.0 mm pupil diameters were 0.06 +/- 0.12 logMAR, 0.14 +/- 0.13 logMAR and 0.21 +/- 0.17 logMAR, respectively, in group A, and -0.14 +/- 0.08 logMAR,-0.11 +/- 0.09 logMAR and -0.09 +/- 0.11 logMAR, respectively, in group B. There was good correlation between best corrected VA and PCVA in both groups for all pupil diameters measured (p < 0.007).

CONCLUSION

Predicted corneal visual acuity as determined by ray tracing analysis is useful for estimating best spectacle-corrected VA in normal corneas and the effect of irregular corneal astigmatism on VA in eyes with mild to moderate keratoconus. Further studies are required to evaluate the efficacy of ray tracing in evaluation of aberrations of the optical system of the eye.

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.

Maximum permissible lateral decentration in aberration-sensing and wavefront-guided corneal ablation

PURPOSE

To investigate the lateral alignment accuracy needed in wavefront-guided refractive surgery to improve the ocular optics to a desired level in a percentage of normally aberrated eyes.

SETTING

Department of Ophthalmology, University of Zurich, Zurich, Switzerland.

METHODS

The effect of laterally misaligned ablations on the optical outcome was simulated using measured wavefront aberration patterns from 130 normal eyes. The calculations were done for 3.0 mm, 5.0 mm, and 7.0 mm pupils. The optical quality of the simulated correction was rated by means of the root-mean-square residual wavefront error.

RESULTS

To achieve the diffraction limit in 95% of the normal eyes with a 7.0 mm pupil, a lateral alignment accuracy of 0.07 mm or better was required. An accuracy of 0.2 mm was sufficient to reach the same goal with a 3.0 mm pupil.

CONCLUSION

Procedures must be developed to ensure that the ablation is within a tolerance range based on each eye's original optical error. Rough centration based on the surgeon's judgment might not be accurate enough to achieve significantly improved optical quality in a high percentage of treated eyes.

Aberration-sensing and Wavefront-guided Laser in situ Keratomileusis: Management of Decentered Ablation

PURPOSE

To clarify the feasibility of aberration-sensing and wavefront-guided laser in situ keratomileusis (LASIK) to manage grossly decentered ablation and to discuss the limitations of the technology.

METHODS

Three patients with previous decentrations of the ablation zone between 1.5 to 2.0 mm were scheduled for wavefront-guided LASIK. All patients reported monocular diplopia and halos. Wavefront aberrations were measured with a Tscherning-type aberrometer. Laser ablation was done with a WaveLight Allegretto in a one-step procedure with ablation profiles calculated only from the individual wavefront map. Decentrations were determined from corneal topography.

RESULTS

Three months after surgery, patient WM and patient SU had gained uncorrected and best spectacle-corrected visual acuity. The root mean square-wavefront error decreased up to 61% and 33%, respectively, for total and higher order aberrations (Zernike modes of 3rd order and higher). There was significant enlargement of the optical zone determined by corneal topography, and both patients no longer reported diplopia and halos at 3 months postoperatively. The optical aberration of the third patient (RE), after a 5.00-D overcorrection with a 2-mm decentration, was too high for aberration-sensing; retinal images obtained from the wavefront device were too smeared and not of sufficient contrast. In addition, this patient had a residual corneal thickness of 416 microm and thus wavefront-guided LASIK was not done.

CONCLUSIONS

Wavefront-guided LASIK offers a new way of managing grossly decentered laser ablations. Unfortunately, there are still patients who have aberrations too large for wavefront sensing or with other clinical limitations such as a residual corneal thickness too thin for further treatment.

Topographic changes in corneal asphericity and effective optical zone after laser in situ keratomileusis

PURPOSE

To determine the relationship between the spherical refractive change after myopic excimer laser surgery and the effective optical zone (EOZ) and corneal asphericity determined by corneal topography.

SETTING

Baylor College of Medicine, Houston, Texas, USA.

METHODS

Preoperative and postoperative topographies along with refractions were evaluated in all patients who had had laser in situ keratomileusis since January 1999 and had at least 6 months of follow-up. The VISX Smoothscan S2 excimer laser and the Hansatome microkeratome (Bausch & Lomb) were used in all cases. Because optical zones are oval with astigmatic treatments with the VISX laser, only patients with spherical refractions and treatments were included. Thirty-nine cases met the criteria; their treatments ranged from -1.50 to -18.00 diopters (D). The preoperative and postoperative corneal asphericities (Q-values) were taken directly from the Holladay Diagnostic Summary Report on the EyeSys 2000, version 4.0. The mean diameter of the optical zone was measured on the local radius of curvature map using the outer edge of the yellow zone, which corresponds to 2 color changes or approximately 0.50 D steepening from the mean central radius (green).

RESULTS

The EOZ decreased as the amount of treatment increased. The decrease was slightly nonlinear, decreasing slightly more rapidly at higher treatments. For an "intended" 6.0 mm optical zone, the nominal EOZs from the least-squares second-order polynomial regression were 6.0 mm for -1.5 D, 5.4 mm for -5.0 D, 4.6 mm for -10.0 D, 3.8 mm for -15.0 D, and 3.2 mm for -18.0 D. The least-squares second-order polynomial regression yielded a standard error of the estimate (SEE) of +/- 0.22 mm (R(2) = 0.90). The asphericity increased nonlinearly in a positive direction (oblate) with the amount of treatment, indicating greater amounts of correction produced progressively more oblate corneal surfaces. The least-squares second-order polynomial regression yielded an SEE of +/- 0.42 (R(2) = 0.55).

CONCLUSIONS

The EOZ decreased and the Q-value increased with the amount of myopic excimer laser treatment. The optical zone was approximately 4.3 mm with a spherical treatment of -12.0 D. These findings may explain the clinical studies that indicate high myopic treatments (above -12.0 D) are associated with poor visual outcomes.

Changes in Mesopic Vision After Photorefractive Keratectomy for Myopia

PURPOSE

To evaluate the mesopic functions of excimer laser treated eyes with different treatment diameters and different laser beam delivery systems.

METHODS

In Group 1, 38 eyes were treated with the Aesculap Meditec MEL 60 ArF scanning laser beam excimer laser. The treatment diameter was 5.0 mm. In Group 2, 38 eyes were treated with the Aesculap Meditec MEL 70(G-Scan) flying spot excimer laser; the chosen treatment diameter was 6.5 mm. In Group 3, there were 38 eyes with no treatment; vision was corrected only with spectacles (control group). All eyes had 20/20 best spectacle-corrected visual acuity before surgery, as did the control group. Measurements were carried out preoperatively and at 12 months following surgery. All eyes exhibited normal corneal wound healing, and subepithelial haze was <0.5 according to Hanna's scale. Mesopic functions (mesopic vision and glare sensitivity) were tested with the Mesoptometer II.

RESULTS

The average preoperative refractive error in Group 1 was -3.40 D; in Group 2, -3.38 D; in Group 3, -3.44 D. In Group 1, 34% of the treated eyes met the night driving requirements (recognition at 1:5 contrast level), whereas in Group 2, 85%, and in Group 3, 95% of the eyes fulfilled this criteria. When contrast vision was tested under glare conditions in Group 1, 31.6%; in Group 2, 80%; and in Group 3, 94.7% of the eyes identified the target orientation (Landolt ring) at contrast level 1:5.

CONCLUSIONS

The unoperated spectacle wearers had better results in all tested functions. The larger 6.5-mm treatment diameter with the use of the flying spot laser beam delivery system resulted in better mesopic function and contrast vision under mesopic conditions than the smaller 5.0-mm diameter.

Topographic analysis of corneal regularity after penetrating keratoplasty

PURPOSE

To evaluate the accuracy of the Holladay Diagnostic Summary of the EyeSys Corneal Analysis System in predicting the corneal visual acuity potential in patients who have undergone penetrating keratoplasty (PKP).

METHODS

Astigmatism patterns, refractive and topographic astigmatism, best spectacle-corrected visual acuity (BSCVA), and hard contact lens (HCL) visual acuity of 54 patients were analyzed 3 months after PKP and compared with the Potential Corneal Acuity (PCA) value predicted by the Holladay Diagnostic Summary.

RESULTS

Qualitative patterns of astigmatism (p = 0.01) and refractive (p = 0.002) and topographic (p = 0.0002) astigmatism were significantly correlated with PCA values. Using HCL visual acuities to correct the BSCVA (HCL-corrected BSCVA) for noncorneal causes of reduced vision, we found that the PCA values of 48.1% of the patients were within one line of the HCL-corrected BSCVA; 81.5% were within two lines; and 93.0% were within three lines.

CONCLUSION

The Holladay PCA measurement may be useful in the postoperative evaluation of the optical quality of the central corneal surface in patients who have undergone PKP.

Risk factors for self-reported visual symptoms with Intacs inserts for myopia

OBJECTIVE

Potential risk factors and visual performance measures were evaluated for relationship to self-report of clinical visual symptoms after the refractive procedure for placement of Intacs microthin prescription inserts for myopia.

DESIGN

Retrospective nonrandomized comparative study.

PARTICIPANTS/INTERVENTION

Patients were participants in the U.S. Food and Drug Administration phase III KeraVision prospective clinical trials.

MAIN OUTCOME MEASURES

Study participants (n = 263) were retrospectively classified into one of three outcome groups on the basis of postoperative self-reported visual symptoms and/or request for Intacs inserts removal through month 24. Differences between outcome groups in visual acuity, refractive error, corneal geometry, corneal topography, type of preoperative corrective lens wear, and demographic variables were evaluated with multivariate logistic regression.

RESULTS

Clinical trial participants who had preoperative mean keratometry >45 diopters (D) (adjusted odds ratio [OR], 0.43; 95% confidence interval [CI], 0.21, 0.85, P = 0.02), manifest refractive astigmatism of 0.75 D or 1.00 D (adjusted OR, 0.52; 95% CI, 0.25, 1.08, P = 0.08), measured uncorrected visual acuity > or =2 lines better than that predicted by their respective cycloplegic refractive error (adjusted OR, 0.39; 95% CI, 0.14, 1.12, P = 0.08) and/or had worn soft contact lenses (adjusted OR, 0.58; 95% CI, 0.32, 1.04, P = 0.07) tended to be less likely to report postoperative clinical visual symptoms with Intacs inserts. Risk of clinical visual symptoms and request for Intacs inserts removal approximately doubled for each 0.50 D of additional postoperative defocus equivalent (crude OR, 1.86; 95% CI, 1.39, 2.48, P = 0.00). Controlling for postoperative defocus and important preoperative risk factors, subjects who reported significant clinical visual symptoms were more likely to have had preoperative uncorrected visual acuity that was worse than that predicted by their respective cycloplegic refractive error (adjusted OR, 1.84; 95% CI, 0.98, 3.42, P = 0.06). Risk of reporting clinical visual symptoms was increased with mesopic pupil diameter > or =6.5 mm (adjusted OR, 1.76; 95% CI, 0.96, 3.24, P = 0.07). Within the group of patients who reported postoperative clinical visual symptoms, 71 of 122 (58%) had ceased reporting them by month 24.

CONCLUSIONS

Adjusting for important risk factors simultaneously, this study suggested that certain preoperative characteristics may increase or decrease the likelihood, depending on the characteristic, of refractive surgery candidates to report significant clinical visual symptoms with Intacs inserts.

Prospective Randomized Study of Corneal Aberrations 1 Year After Radial Keratotomy or Photorefractive Keratectomy

PURPOSE

To evaluate the optical properties of the cornea 1 year after either radial keratotomy (RK) or photorefractive keratectomy (PRK) in a randomized group of patients with low myopia.

METHODS

Ninety-six patients with myopia between -0.75 and -5.00 D were randomized to either radial keratotomy (n = 46) or photorefractive keratectomy (n = 50). Topography maps were obtained 1 year after surgery and analyzed by computation of total corneal wavefront aberration and Zernike polynomial coefficients for pupil sizes of 2, 4, and 6 mm. The 4-mm pupil size was used for optimization of the model.

RESULTS

The total corneal wavefront aberrations after RK and PRK were similar and not statistically different. Wavefront aberrations arising from astigmatism or defocus accounted for approximately 70% of the total wavefront error at all pupil sizes in both groups. All types of aberrations, and in particular spherical aberration, increased significantly with increasing pupil size. Higher-order wavefront aberrations were almost twice as high after RK than after PRK at pupil sizes of 4 and 6 mm. Spherical aberration and coma were slightly higher after PRK than after RK.

CONCLUSIONS

Pupil size had a major effect on corneal aberrations after RK and PRK. The most important aberrations were sphero-cylindrical, in which eyes became significantly more myopic with increasing pupil size. The image forming properties of the cornea are better after PRK compared with RK due to the lesser amount of higher-order aberrations.

Poly(methyl methacrylate) model study of optical surface quality after excimer laser photorefractive keratectomy

PURPOSE

To evaluate lenses produced by excimer laser ablation of poly(methyl methacrylate) (PMMA) plates.

SETTING

University research laboratory.

METHODS

Two Nidek EC-5000 scanning-slit excimer laser systems were used to ablate plane-parallel plates of PMMA. The ablated lenses were examined by focimetry, interferometry, and mechanical surface profiling.

RESULTS

The spherical optical powers of the lenses matched the expected values, but the cylindrical powers were generally lower than intended. Interferometry revealed marked irregularity in the surface of negative corrections, which often had a positive "island" at their center. Positive corrections were generally smoother. These findings were supported by the results of mechanical profiling. Contrast sensitivity measurements carried out when observing through ablated lenses whose power had been neutralized with a suitable spectacle lens of opposite sign confirmed that the surface irregularities of the ablated lenses markedly reduced contrast sensitivity over a range of spatial frequencies.

CONCLUSION

Improvements in beam delivery systems seem desirable.

Topography-driven excimer laser for the retreatment of decentralized myopic photorefractive keratectomy

OBJECTIVE

This study evaluated the efficacy, predictability, stability, and safety of a software program (Corneal Interactive Programmed Topographic Ablation, CIPTA, LIGI, Taranto, Italy), which, by transferring programmed ablation from the corneal topograph to a flying-spot excimer laser, provides customized laser ablation for correction of postmyopic photorefractive keratectomy (PRK) decentrations.

DESIGN

Noncomparative consecutive case series.

PARTICIPANTS

Thirty-two eyes of 32 subjects with a mean age of 35 years (range, 19-49; standard deviation [SD], 7.9) had CIPTA-guided PRK at the Clinica Oculistica of Bari University (Italy). All the subjects had irregular astigmatism after postmyopic PRK because of clinically significant, decentered treatments of more than 1 mm. OPERATION: Individual topographies were acquired by a corneal topograph (Orbscan II, Orbtek, Inc., Salt Lake City, UT). These, together with refractive data, were processed to obtain a customized altimetric ablation profile that was transferred to a flying-spot laser (Laserscan 2000, Lasersight, Orlando, FL).

MAIN OUTCOME MEASURES

The topographic study of centration and data on uncorrected (UCVA) and best-corrected (BCVA) visual acuity, predictability, and stability of refraction, and any complications, were analyzed.

RESULTS

Mean follow-up was 10.3 months (range, 4-18 months; SD, 5.8). At the last postoperative examination, 29 eyes (90.6%) had a UCVA superior to 20/40. Nineteen eyes (59.4%) had a UCVA of 20/20. Twenty-two eyes (68.75%) were within 0.50 diopters (D) of attempted correction in the spherical equivalent and 28 eyes (87.5%) were within 1 D. The index of success of astigmatic correction was 0.15. No eye lost Snellen lines of BCVA, whereas 18 eyes (56.25%) gained Snellen lines. Postoperative topographies revealed well-centered treatments.

CONCLUSIONS

The combination of topographic data with computer controlled flying-spot excimer laser ablation is a suitable solution for correcting irregular astigmatism after postmyopic PRK decentrations.

Effect of preoperative pupil measurements on glare, halos, and visual function after photoastigmatic refractive keratectomy

PURPOSE

To prospectively assess the effect of preoperative variables such as pupil size on glare, halos, and visual function after photoastigmatic refractive keratectomy (PARK).

SETTING

Department of Ophthalmology, Stanford University School of Medicine, Stanford, California, USA.

METHODS

Ninety-three eyes had PARK for primary compound myopic astigmatism. Preoperative pupil diameters were measured under scotopic and photopic illuminance conditions. Postoperatively, patients were evaluated at 1, 3, 6, 9, 12, 18, and 24 months. A regression model was performed to evaluate the predictive value of assessing preoperative variables such as pupil diameter on the development of glare and halos, contrast sensitivity, and best spectacle-corrected visual acuity (BSCVA) under scotopic, photopic, and glare conditions.

RESULTS

The greater magnitude loss of BSCVA under scotopic conditions in the early postoperative period as well as the slower recovery to preoperative levels in eyes with larger scotopic pupil diameters were not statistically significant (P >.05). An increase in symptoms of glare was related more to the attempted level of spherical equivalent (SE) correction than to the pupil size during the first 12 postoperative months (P <.01). The photoablation dimensions as determined by the attempted level of astigmatic correction may result in decreases in the glare BSCVA up to 12 months after PARK (P =.03). At the 2 year follow-up, pupil diameter under both scotopic and photopic illuminance conditions was not predictive of any of the measured outcomes variables.

CONCLUSIONS

An assessment of preoperative pupil size and the attempted level of both SE and astigmatic correction may be useful in identifying patients at risk of developing symptoms or declines in visual performance after PARK. However, follow-up studies are indicated to identify variables predictive of poor visual outcomes following excimer laser refractive surgery.

Geometric Ray Tracing Analysis of Visual Acuity After Laser in situ Keratomileusis

PURPOSE

Using a geometric ray tracing model, we explain the increase in visual acuity observed in myopic patients after laser in situ keratomileusis (LASIK).

METHODS

This study included 37 eyes of 23 patients who underwent LASIK. All patients had myopia and a spectacle-corrected visual acuity of 0.95 or worse. Clinical tests included biometry, corneal topography, pachymetry, and refraction (with and without cycloplegia). Calculations were made by tracing rays through all the refractive surfaces of the eye based on a Le Grand-type theoretical model of the whole eye.

RESULTS

Comparison of spectacle-corrected visual acuity of the eye before surgery, the size of the blur circle calculated by ray tracing, and the magnification for the ocular system facilitated a numerical criterion to assess visual acuity by geometric calculation. This criterion was applied to myopic eyes that underwent LASIK, and the maximum increase in spectacle-corrected visual acuity was predicted. An actual increase in visual acuity of approximately 40% of the predicted maximum was observed in patients.

CONCLUSIONS

With geometric ray tracing, it was possible not only to obtain an estimate of the visual acuity before LASIK but also to assess the value of the maximum and probable increases in visual acuity after LASIK.

Corneal surface topography and associated visual performance with INTACS for myopia: phase III clinical trial results. The INTACS Study Group

OBJECTIVE

To characterize corneal topography with INTACS (KeraVision, Inc., Fremont, CA) an ophthalmic device designed to correct myopia, and relate findings to visual performance.

DESIGN

Prospective nonrandomized self-controlled comparative intervention study.

PARTICIPANTS/INTERVENTION

Patients were participants in the INTACS FDA phase III clinical trials.

MAIN OUTCOME MEASURES

Preoperative and postoperative month 6 videokeratographic corneal topography (EyeSys, Houston, TX) was analyzed for 165 eyes from eight clinical sites. Topographic flattening, asphericity, and surface characteristics were statistically evaluated for relationship to visual acuity, refractive data, contrast sensitivity, and subjective visual symptoms.

RESULTS

Corneal radius of curvature flattening was aspheric in nature and increased incrementally and significantly for progressively thicker INTACS (P < 0.05). Comparative stratification analyses suggest potential interactions between existing preoperative asphericity and myopia, postoperative asphericity, and visual performance outcomes. Qualitative symmetric and asymmetric toric topography patterns were related to the postoperative self-reported visual symptoms of "double images" (P < or = 0.05) and "halos" (P < or = 0.10), respectively.

CONCLUSIONS

The anterior corneal surface is aspherically flattened (prolately) with INTACS, whereas postoperative corneal asphericity is significantly more prolate than preoperative. Specific qualitative postoperative topography patterns were associated with subjective clinical visual performance.

Quantitative topographic irregularity as a predictor of spectacle-corrected visual acuity after refractive surgery

PURPOSE

To evaluate a new topographic index called topographic irregularity as a quantitative predictor of corrected vision after refractive surgery.

METHODS

We defined topographic irregularity as the summed difference at all points between a topographic refractive corneal power map and its best-fit spherocylinder. We prospectively studied 107 eyes of 107 patients 3 months after a variety of refractive procedures. Topographic irregularity was calculated from topographic maps, and the correlation between topographic irregularity and spectacle-corrected visual acuity was determined using both high-contrast and low-contrast acuity charts. This correlation was compared with correlations for the surface regularity index and the surface asymmetry index. Next, we analyzed 54 of these topographic maps to create a regression scale relating surface regularity index, surface asymmetry index, and topographic irregularity to predict spectacle-corrected visual acuity. This scale was then used to predict spectacle-corrected visual acuity on the remaining 53 postoperative patients.

RESULTS

The correlation of topographic irregularity with spectacle-corrected visual acuity (R(2) =.36) was comparable to the correlation for the surface regularity index (R(2) =.36) and stronger than for the surface asymmetry index (R(2) =.11) when spectacle-corrected visual acuity was measured with high-contrast eye charts. Topographic irregularity correlated more strongly with spectacle-corrected visual acuity (R(2) =.42) than either the surface regularity index (R(2) =.28) or the surface asymmetry index (R(2) =.14) when spectacle-corrected visual acuity was measured with low-contrast eye charts. Using the regression scale, prediction of high-contrast and low-contrast spectacle-corrected visual acuity from topographic irregularity was superior to or comparable to predictions using the surface regularity index and the surface asymmetry index.

CONCLUSIONS

Topographic irregularity has a closer correlation with spectacle-corrected visual acuity than existing topographic indexes. Topographic irregularity is also an accurate predictor of spectacle-corrected visual acuity and may be a more sensitive tool for evaluating postoperative visual performance than current topographic measures.

Prediction of letter contrast sensitivity using videokeratographic indices

PURPOSE

To analyze the relationship between corneal topography and letter contrast sensitivity.

METHOD

Experiments were conducted on 59 eyes of 51 patients who had best spectacle-corrected visual acuity of 20/20 or better and no ocular pathology except for the corneal shape. Thirty-nine eyes had an abnormal topographic pattern resulting from keratoconus, and the other 20 eyes showed a normal topographic pattern. Videokeratography was performed with the TMS-2 videokeratoscope, and the surface regularity index, surface asymmetry index, and coefficient of variation of power were obtained for each subject. Letter contrast sensitivity was measured with the CSV-1000LV with spectacle correction. The correlation between the number of correct letters and topographic indices was calculated.

RESULTS

The abnormal topography group had a significantly greater loss of letter contrast sensitivity (median = 20 letters) than the normal control (median = 23 letters; P =.0001). There were statistically significant correlations between number of correct letters and the coefficient of variation of power (r = -.77; P =. 001), number of correct letters and surface regularity index (r = -. 76, P =.001), and the number of correct letters and surface asymmetry index (r = -.64; P =.001). The linear regression equation between number of correct letters and the coefficient of variation of power was the number of correct letters = -0.05 x the coefficient of variation of power + 23.2.

CONCLUSIONS

Our results suggest that subtle visual deteriorations, which are barely detected by contrast sensitivity testing, can be predicted objectively by the corneal topographic indices.

Photorefractive keratectomy versus laser in situ keratomileusis: comparison of optical side effects. Summit PRK-LASIK Study Group

OBJECTIVE

This report presents patient-reported optical symptoms after photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK).

DESIGN

Preoperative and postoperative patient surveys in a prospective, multicenter, randomized clinical trial.

PARTICIPANTS

Two hundred twenty eyes of 220 patients entered the study; 105 were randomized to PRK and 115 were randomized to LASIK.

INTERVENTION

All patients received a one-pass, multizone excimer laser ablation as part of either a PRK or LASIK procedure. Attempted corrections ranged from 6.00 to 15.00 diopters (D).

MAIN OUTCOME MEASURES

Glare, halo, and monocular diplopia symptoms as reported by patients on questionnaires before surgery and at the 6-month follow-up. Comparison was made between symptoms when using optical correction before surgery and symptoms without correction after surgery.

RESULTS

For both the PRK and LASIK groups analyzed individually, the difference in average glare index before surgery and after surgery was not statistically significant (P = 0.54 for PRK; P = 0.15 for LASIK; t test). Twenty-four PRK patients (41.4%) reported worsening of glare symptoms from baseline compared with 11 LASIK patients (21.6%); however, the difference between the two groups was not statistically significant (P = 0.086, chi-square test). Within the PRK group, the difference in average halo index before and after surgery was statistically significant (P = 0.0003, t test); in the LASIK group, it was not statistically significant (P = 0.1 1, t test). Thirty-four PRK patients (58.6%) reported worsening of halo symptoms from baseline compared with 26 LASIK patients (50.0%); this difference was not statistically significant (P = 0.086, chi-square test). For both the PRK and LASIK groups, the difference in average diplopia index before and after surgery was statistically significant (P < 0.0001 for PRK; 0.047 for LASIK; t test). Twenty-six PRK patients (44.8%) reported a worsening of monocular diplopia symptoms from baseline compared with 19 LASIK patients (35.8%); this difference was not statistically significant (P = 0.39, chi-square test). When changes in glare and halo from before surgery to after surgery were pooled as a glare-halo index, however, the PRK group did show a significantly greater likelihood of demonstrating an increase in symptoms compared with the LASIK group (P = 0.048, chi-square test).

CONCLUSIONS

Optical sequelae of glare, halo, and monocular diplopia may occur in some patients after either both PRK or LASIK for moderate to high myopia; in contradistinction, many other patients' preoperative symptoms improve after surgery. On average, PRK patients show an increase in halo and diplopia symptoms, but not glare, after surgery, and LASIK patients show an increase in diplopia, but not glare and halo symptoms. There is a suggestion of a somewhat lesser tendency toward postoperative optical symptoms in LASIK compared with PRK treated eyes.

Comparison of topographic indices that correlate with visual acuity in videokeratography

OBJECTIVE

To evaluate the relationship between the best spectacle-corrected visual acuity (BSCVA) and two quantitative indices of the anterior corneal surface obtained by videokeratography.

DESIGN

Prospective, single center, comparative, observational study.

PARTICIPANTS

Eighty-nine normal eyes and 52 eyes with keratoconus with contact lens-corrected visual acuity of 20/20 or better.

INTERVENTION

Videokeratography was performed with the TMS-2 and the CAS system 2000.

MAIN OUTCOME MEASURES

The relationship between the BSCVA recorded in log minimal angle of resolution (logMAR) units, the surface regularity index (SRI), and the predicted corneal acuity (PCA) were assessed by linear regression analysis.

RESULTS

The BSCVAs for all eyes ranged from 0.82 to -0.30 logMAR units. BSCVA was highly correlated with the SRI (r = 0.70, P < 0.0001) and the PCA (r = -0.61, P < 0.0001). There was no statistical difference in the regression slopes and the intercepts for the estimated BSCVA using the SRI and measured BSCVA, and the estimated BSCVA using PCA and measured BSCVA.

CONCLUSIONS

Two quantitative topographic indices, SRI and PCA, are useful for estimating the effect of irregular astigmatism on visual acuity even though both indices quantify different aspects of the anterior surface of the cornea.

Evaluation of relationships among refractive and topographic parameters

PURPOSE

To examine the relationships among several refractive and topographic parameters.

SETTING

Cullen Eye Institute Department of Ophthalmology, Baylor College of Medicine, Houston, Texas, USA.

METHODS

Using computerized videokeratography (EyeSys Corneal Analysis System), 287 corneas of 150 patients were retrospectively analyzed. The Holladay Diagnostic Summary (HDS) refractive maps were used to evaluate relationships among variables of the HDS and refractive error.

RESULTS

Myopic spherical equivalent refraction (P = .0003) and more negative asphericity (Q-values) (P = .0119) were correlated with steeper corneas. The Q-values were less negative in eyes with moderate myopia (2.0 to 6.0 diopters [D]) than in those with hyperopia (1.0 D or greater). The Q-values below -0.3 were correlated with less favorable values for predicted corneal acuity and corneal uniformity index values. Mean corneal curvature measurements obtained by computerized videokeratography and standard keratometry showed a strong degree of correlation (P = .0001).

CONCLUSION

As the degree of myopia and negative asphericity increased, the corneal radius of curvature decreased. Corneal Q-values less than -0.3 were associated with reduced optical performance of the cornea.

Functional vision and corneal changes after laser in situ keratomileusis determined by contrast sensitivity, glare testing, and corneal topography

PURPOSE

To demonstrate the functional vision and corneal changes following laser in situ keratomileusis (LASIK) determined by contrast sensitivity, glare testing, and corneal topography.

SETTING

University of Texas Medical School, Houston, Texas, USA.

METHODS

Seven patients ranging in age from 20 to 61 years who had bilateral LASIK were evaluated preoperatively and 1 day, 1 week, and 1 and 6 months postoperatively. Visual acuity, using letters on the Baylor Visual Acuity Testor (BVAT) at 98% (standard acuity) and 13% contrast, and the contrast threshold were determined at 3 light levels (darkness, medium brightness acuity testor [BAT], high BAT). Pupil sizes were measured at each level, and corneal topography was performed at each visit.

RESULTS

The greatest changes were found 1 day postoperatively: The contrast threshold worsened by a mean of 0.6 lines +/- 1.0 (SD) (P = .05) in darkness, 0.4 +/- 0.7 lines (P = .05) at medium BAT, and 0.8 +/- 0.7 lines (P = .002) at high BAT. The 98% contrast acuity decreased a mean of 1.4 +/- 1.6 lines (P = .01) in darkness, 1.0 +/- 2.0 lines (P = .09) at medium BAT, and 0.8 +/- 2.3 lines (P = .22) at high BAT. The 13% contrast acuity decreased a mean of 2.2 +/- 2.6 lines (P = .01) in darkness, 1.3 +/- 1.9 lines (P = .02) at medium BAT, and 1.4 +/- 2.5 lines (P = .07) at high BAT. The predicted corneal acuity (PCA) obtained from corneal topography decreased by a mean of 3.3 +/- 3.1 lines (P = .002), and the asphericity (Q-value) increased by an average of +0.35 +/- 0.67 (P = .07). All values returned to the preoperative levels by 1 week except PCA, asphericity, visual acuity at 13%, and contrast threshold in darkness, which improved slightly but had not returned to baseline by 6 months. The 98% contrast acuity at medium BAT improved by 0.2 +/- 1.0 lines (P = .34) and 0.3 +/- 0.8 lines (P = .16) at high BAT at 1 month. The 98% contrast acuity values remained 0.3 lines over baseline through 6 months. Corneal topography showed that all corneas became oblate after LASIK to a mean Q-value of +0.47 +/- 0.40 (P = .0001) and PCA was decreased by 1.6 +/- 1.1 lines (P = .0002) at 6 months.

CONCLUSIONS

Functional vision changes do occur after LASIK. The optical quality of the cornea is reduced and the asphericity becomes oblate. Changes in functional vision worsen as the target contrast diminishes and the pupil size increases. These findings indicate that the oblate shape of the cornea following LASIK is the predominant factor in the functional vision decrease.

Natural history of corneal topography after excimer laser photorefractive keratectomy

PURPOSE

To investigate the natural history of corneal topography after excimer laser photorefractive keratectomy (PRK).

DESIGN

A prospective, single center clinical study.

PARTICIPANTS

A total of 40 eyes of 34 patients with myopia were studied.

INTERVENTION

Excimer laser photorefractive keratectomy and computer-assisted videokeratography were performed.

MAIN OUTCOME MEASURES

Preoperative, 1 week, and 1, 2, and 3 months postoperative topography patterns were compared and changes assessed by averaging defined sectors of the ablation zone in individual maps to produce composite "average" topography maps.

RESULTS

Corneal topography was generally not homogeneous at 1 week after PRK. By 3 months, there had been considerable smoothing of corneal contour. A general "central island" effect early in the postoperative period flattened over time. When right and left eyes were evaluated independently, a tendency toward a keyhole-semicircular pattern was seen on average; the maximum flattening was nasal and the least flattening was inferotemporal for both right and left eyes.

CONCLUSIONS

A tendency, on average, toward central island and keyhole-semicircular patterns is seen early in the postoperative course after PRK. Central islands tend to evolve into the keyhole-semicircular pattern, and the corneal topography in general after PRK tends to smooth considerably with time.

Photorefractive keratectomy versus laser in situ keratomileusis for moderate to high myopia. A randomized prospective study

OBJECTIVE

This report presents the results of a randomized clinical trial of photorefractive keratectomy (PRK) and laser-assisted in situ keratomileusis (LASIK).

DESIGN

A randomized, prospective multicenter clinical trial.

PARTICIPANTS

A total of 220 eyes of 220 patients entered the study cohort: 105 randomized to PRK and 115 to LASIK. The mean preoperative manifest refraction spherical equivalent was -9.23 diopters (D) in the PRK group and -9.30 D in the LASIK group.

INTERVENTION

All patients received a one-pass, multizone excimer laser ablation as part of either a PRK or LASIK procedure using the Summit Apex excimer laser. Attempted corrections ranged from 6.00 to 15.00 D.

MAIN OUTCOME MEASURES

Data on uncorrected and spectacle-corrected visual acuity, predictability,and stability of refraction, corneal haze, and flap complications were analyzed. Patients were observed for up to 6 months.

RESULTS

One day after surgery, 0 (0.0%) and 3 (4.5%) eyes in the PRK group saw 20/20 and 20/40 or better uncorrected, respectively, while 7 (10%) and 48 (68.6%) eyes in the LASIK group saw 20/20 and 20/40 or better, respectively. At 6 months after PRK, 13 (19.1%) and 45 (66.2%) eyes saw 20/20 and 20/40 or better, respectively, while after LASIK, 16 (26.2%) and 34 (55.7%) eyes saw 20/20 and 20/40 or better, respectively (odds ratio = 0.56 for likelihood of uncorrected visual acuity < 20/40 for PRK vs. LASIK, 95% confidence interval [CI] = 0.31-1.19). After PRK, 39 eyes (57.4%) were within 1.0 D of attempted correction compared with 24 eyes (40.7%) in the LASIK group (odds ratio = 0.50 for likelihood fo undercorrection 1.0 D for PRK vs. LASIK, 95% CI = 0.24-1.04); however, the standard deviation of the predictability was similar between groups: 1.01 D for PRK and 1.22 D for LASIK. From months 1 to 6, there was an average regression of 0.89 D in the PRK group and 0.55 D in the LASIK group. After PRK, eight eyes (11.8%) had a decrease in spectacle-corrected visual acuity of two Snellen lines or more; after LASIK, two eyes (3.2%) had a decrease of two lines or more (odds ratio = 3.89 for risk of loss of spectacle-corrected visual acuity for PRK vs. LASIK, 95% CI = 0.71-21.30). Only two eyes had postoperative spectacle-corrected visual acuity less than 20/32, however.

CONCLUSIONS

Although improvement in uncorrected visual acuity is more rapid in LASIK than in PRK, efficacy outcomes in the longer term generally are similar between the two procedures. There is a greater tendency toward undercorrection in LASIK eyes using the specific laser and nomogram in this study, but the scatter in achieved versus attempted correction is similar, suggesting little difference in the accuracy of the two procedures. A suggestion of decreased propensity for loss of spectacle-corrected visual acuity in LASIK eyes requires further investigation.

Topographic assessment of irregular astigmatism after photorefractive keratectomy

OBJECTIVE

To correlate new quantitative topographic indexes of corneal irregular astigmatism to best spectacle-corrected visual acuity (BSCVA) following excimer laser photorefractive keratectomy (PRK).

SETTING

Department of Ophthalmology, LSU Eye Center, and Refractive Surgery Center of the South, Ear, Nose & Throat Hospital, New Orleans, Louisiana; Manhattan Eye, Ear and Throat Hospital, New York, New York, USA.

METHODS

Videokeratography data (TMS-1) were obtained preoperatively and 1, 3, 6, 12, 18, and 24 months postoperatively from 100 eyes having PRK for low to mild myopia. Algorithms measured fine local irregularity with the surface regularity index (SRIp), varifocality with the coefficient of variation of corneal power (CVPp), and central islands with the elevation/depression magnitude (EDM).

RESULTS

The SRIp and CVPp increased after surgery and remained significantly higher than the preoperative levels throughout the 24 month follow-up (P < .05). The increase in EDM was significant from 1 to 6 months (P < .05) but not thereafter. Multiple regression analysis revealed that variables having a statistically significant relationship with postoperative BSCVA were CVPp and EDM at 1 month, CVPp at 3 months, and CVPp, haze, and age at 6 months. No statistically significant correlation between any measures of irregular astigmatism and BSCVA was found after 1 year of follow-up.

CONCLUSION

The quantitative measures used in this study are sensitive methods by which irregular astigmatism after keratorefractive procedures can be classified, evaluated, and compared.

Detection and classification of mild irregular astigmatism in patients with good visual acuity

Videokeratography has been available for a decade, and this test is essential for determining the presence and type of irregular corneal astigmatism. Three eyes diagnosed with myopic astigmatism and considered good candidates for refractive surgery with conventional examination were studied. Color-coded maps with videokeratography showed regular astigmatism in one eye and the existence of irregular astigmatism in two eyes. Videokeratography showed that one of these eyes had a keratoconus suspect pattern and the second showed a pattern consistent with pellucid marginal degeneration. Videokeratography can detect and classify irregular astigmatism in cases where routine examination shows no abnormal findings.

Corneal topography of photorefractive keratectomy versus laser in situ keratomileusis. Summit PRK-LASIK Study Group

OBJECTIVE

This study aimed to compare qualitative patterns of corneal topography early in the postoperative course after excimer laser photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK) when used for the treatment of myopia of 6.0 to 15.0 diopters.

DESIGN

The study design was a prospective, multicenter, randomized clinical trial.

PARTICIPANTS

A total of 64 eyes were treated with PRK and 54 eyes were treated with LASIK.

INTERVENTION

Using the Summit Apex excimer laser, patients received either PRK or LASIK using a single pass, multizone excimer laser ablation. Computer-assisted videokeratography was performed at designated postoperative examinations.

MAIN OUTCOME MEASURES

Videokeratography maps at 1 and 3 months after surgery were classified using a standard classification scheme. The association of topography patterns to loss of spectacle-corrected visual acuity was tested.

RESULTS

At 1 month, for the PRK (n = 60) and LASIK (n = 51) groups, respectively, 63.3% and 19.6% of eyes fell into one of the four optically irregular groups (central island, keyhole, semicircular, or irregularly irregular; P < 0.001). At 3 months, for the PRK (n = 49) and LASIK (n = 39) groups, respectively, 36.7% and 10.3% of eyes fell into one of the optically irregular groups (P = 0.004). Comparing the 1- and 3-month examination results in the PRK and LASIK groups, respectively, 19 (42%) of 45 eyes and 11 (31%) of 36 eyes had a change in topography, generally to an optically smoother pattern. The irregular groups, taken together, were associated with a greater tendency toward loss of spectacle-corrected visual acuity of two or more Snellen lines (P = 0.01). There also was greater tendency toward loss of spectacle-corrected visual acuity in the PRK group that diminished with time (P < 0.01 at 1 month, P = 0.05 at 3 months).

CONCLUSIONS

After treatment for moderate-to-high myopia, LASIK topography patterns generally are more regular than are PRK patterns. This may be a result either of masking of underlying topography perturbations by the lamellar corneal flap, thus mitigating induced topography changes, or differences in surface wound healing. This study suggests that more rapid return of spectacle-corrected visual acuity found in patients treated with LASIK may be a result of more regular topography patterns early in the postoperative course.

Results of phase III excimer laser photorefractive keratectomy for myopia. The Summit PRK Study Group

OBJECTIVE

The purpose of the study is to determine safety and efficacy outcomes of excimer laser photorefractive keratectomy (PRK) for the treatment of mild-to-moderate myopia.

DESIGN

A prospective, multicenter, phase III clinical trial.

PARTICIPANTS

A total of 701 eyes of 701 patients were entered in the study; 612 eyes were examined at 2 years after surgery.

INTERVENTION

Intervention was photorefractive keratectomy using the Summit ExciMed UV200LA excimer laser (Summit Technology, Inc., Waltham, MA). The treatment zone diameter used was 4.5 mm in 251 eyes (35.8%) and 5 mm in 450 eyes (64.2%). Attempted corrections ranged from 1.50 to 6.00 diopters (D).

MAIN OUTCOME MEASURES

Predictability and stability of refraction, uncorrected and spectacle-corrected visual acuity, refractive and keratometric astigmatism, corneal haze, contrast sensitivity, subjective reported problems of glare and halo, and patient satisfaction were the parameters measured.

RESULTS

At 2 years, 407 (66.5%) eyes achieved 20/20 or better uncorrected visual acuity and 564 (92.5%) eyes achieved 20/40 or better visual acuity. Three hundred thirty-six (54.9%) eyes were within 0.5 D and 476 (77.8%) eyes were within 1.0 D of attempted correction. Stability of refraction improved with time; 86.8% of eyes were stable within 1.0 D from 6 to 12 months, 94% were stable from 12 to 18 months, and 96.3% were stable from 18 to 24 months. There was no evidence of progressive or late myopic or hyperopic refractive shifts. One hundred fourteen (18.6%) eyes gained 2 or more lines of spectacle-corrected visual acuity, whereas 42 (6.9%) eyes lost 2 or more lines; however, of the latter, 32 (76.2%) had spectacle-corrected visual acuity of 20/25 or better and 39 (92.9%) eyes had 20/40 or better. Four hundred forty-two (72.2%) corneas were clear, 138 (22.5%) showed trace haze, 20 (3.3%) mild haze, 9 (1.5%) moderate haze, and 3 (0.5%) marked haze. On patient questionnaires, 87 (29.7%) patients reported worsening of glare from preoperative baseline; 133 (50.1%) reported worsening of halo symptoms from baseline.

CONCLUSIONS

Photorefractive keratectomy appears effective for myopic corrections of -1.50 to -6.00 D. Uncorrected visual acuity is maximized in most eyes by 3 months, although some patients require between 6 months and 1 year to attain their best postoperative uncorrected visual acuity and some may require from 1 to 2 years for stabilization of refraction. Refraction stabilizes progressively without evidence of late myopic or hyperopic refractive shifts. Optical sequelae of glare and halo occur in some patients treated with a 4.5- or 5-mm treatment zone.

Disparity between refractive error and visual acuity after photorefractive keratectomy: multifocal corneal effects

PURPOSE

To determine the relationship between postoperative refractive error and uncorrected visual acuity (UCVA) after photorefractive keratectomy (PRK) and compare the results to those in unoperated control eyes with different degrees of myopic refractive error.

SETTING

Academic cornea and refractive surgery subspecialty practice.

METHODS

Uncorrected visual acuity and manifest refraction were recorded for 52 consecutive patients who had PRK for myopia. Eight control eyes that did not have PRK and in which artificial myopia was induced were also studied to ascertain the association of UCVA with myopia in untreated eyes. Uncorrected visual acuity in postoperative eyes was compared with that in control eyes.

RESULTS

Of the 46 eyes with a myopic spherical equivalent postoperative refraction, 44 (96%) had better UCVA than control eyes with equivalent myopic refractions. Twelve of 13 (92%) eyes with refractions of -1.00 diopter or more had a UCVA of 20/40 or better.

CONCLUSION

After excimer laser PRK, patients achieved better Snellen visual acuity than might be expected from their residual refractive error, perhaps as a result of a multifocal postoperative corneal topography. Nonuniformity of the corneal surface following PRK may create "focal areas of emmetropia" that allow patients to achieve better visual acuity than the refraction may predict.

Corneal topography of excimer laser photorefractive keratectomy using a 6-mm beam diameter. Summit PRK Topography Study Group

OBJECTIVE

The purpose of the study is to define qualitative patterns of corneal topography after excimer laser photorefractive keratectomy (PRK) using a 6-mm beam diameter, investigate changes in patterns over time, and identify associations of topography patterns with clinical outcomes.

DESIGN

Multicenter, prospective cohort study.

PARTICIPANTS

Ninety-eight eyes of 90 patients with myopia who had undergone PRK using the Summit Technology, Inc., excimer laser with a 6-mm beam diameter.

INTERVENTION

Computer-assisted videokeratography data were analyzed for eyes having undergone PRK. Topography patterns at 3, 6, and 12 months after surgery were classified and associations with clinical outcomes assessed.

MAIN OUTCOMES MEASURED

Topography patterns after PRK were determined at 3, 6, and 12 months after surgery. Associations with preoperative characteristics of age and attempted correction, and postoperative outcomes of uncorrected and spectacle-corrected visual acuity, predictability, astigmatism, corneal haze, glare, halo, and patient satisfaction were analyzed.

RESULTS

At 1 year, 21.4% of corneas showed a homogeneous topography, 27.6% showed a toric-with-axis configuration, 10.2% showed a toric-against-axis configuration, 7.1% showed an irregularly irregular topography, 24.5% showed a keyhole/semicircular pattern, and 9.2% showed focal topographic variants. From 3 to 6 months, 40.1% of maps changed; from 6 to 12 months, 53.1% of maps changed, generally to optically smoother, regular patterns. Older age and higher attempted correction were associated with the development of more irregular patterns. The irregular groups showed worse predictability than did the regular groups and a tendency for slight overcorrection. The average reported glare/halo of 1.33 (scale = 0 to 5) in this study was less than in a previous study of the 4.5- to 5-mm treatment zone. However, of six patients expressing dissatisfaction with the results of surgery, three ranked their glare or halo at the maximum level.

CONCLUSIONS

Topography patterns using a 6-mm beam diameter are identifiable, improve with time, and may affect clinical outcomes after photorefractive keratectomy (PRK). The keyhole/semicircular pattern is more prevalent with a 6 mm treatment zone than with smaller treatment zones. Although optical side effects of glare and halo appear to be reduced with the 6-mm treatment, a small number of patients still report substantial glare or halo after the procedure.

Astigmatism induced by spherical photorefractive keratectomy corrections

PURPOSE

The purpose of the study is to evaluate the induced astigmatism after spherical photorefractive keratectomy on the Summit Omnimed (Summit Instruments, Waltham, MA) and the Nidek EC-5000 (Nidek Co. Ltd, Aichi, Japan) excimer lasers.

METHODS

A total of 4269 eyes of 3289 patients were treated with a 5-mm optical zone using the Summit Omnimed excimer laser and 1825 eyes of 1303 patients treated with the Nidek EC-5000 excimer laser. The final astigmatic refractive outcome was compared with the initial refraction by vector analysis (Alpin and Jaffe method).

RESULTS

Subjective astigmatic refraction for the Summit laser reduced from a mean of -0.39 diopter (D) +/- standard deviation (SD) 0.33 D (range, 0 to -2.50 D) to -0.33 D +/- SD 0.41 D (range, 0 to -3.00 D). Surgically induced astigmatism (SIA) had a mean of 0.42 +/- SD 0.34 D (range, 0 to 2.89 D). Mean SIA increased with increasing preoperative astigmatism by 0.60 D SIA for every 1.00 D of preoperative cylinder. For the Nidek laser, subjective astigmatic refraction changed from a mean of -0.18 D +/- SD 0.21 D (range, 0 to -1.25 D) to -0.30 D +/- SD 0.33 D (range, 0 to -3.00 D). Surgically induced astigmatism had a mean of -0.32 D +/- SD 0.29 (range, 0 to 3.05 D). Mean SIA increased with increasing preoperative astigmatism by 0.47 D SIA for every 1.00 D of preoperative cylinder.

CONCLUSIONS

The authors show that spherical photorefractive keratectomy corrections can induce significant astigmatic change, particularly if a large amount of preoperative astigmatism is present.

Mesopic vision in myopia corrected by photorefractive keratectomy, soft contact lenses, and spectacles

PURPOSE

To evaluate contrast vision and glare sensitivity under mesopic conditions in eyes having uncomplicated excimer laser photorefractive keratectomy (PRK) for myopia and in eyes corrected by disposable soft contact lenses, soft contact lenses, and spectacles.

SETTING

Division of Experimental Ophthalmic Surgery, University of Tübingen, Germany.

METHODS

The Mesoptometer II test was used to evaluate mesopic vision (glare sensitivity and contrast vision) in 28 eyes of 14 patients wearing disposable soft contact lenses, 20 eyes of 10 patients wearing soft contact lenses, 39 eyes of 20 patients wearing spectacles, 30 eyes of 15 emmetropic patients, and 33 eyes of 22 patients after PRK with 5.0 mm optical zone. Follow-up was between 15 and 60 months after PRK (mean 34.5 months).

RESULTS

The guidelines of the German Ophthalmologic Society state that patients must recognize Mesoptometer II contrast levels of 1:5 or better with and without glare to meet the minimum legal night-driving standards for private cars. All eyes with disposable soft contact lenses and soft contact lenses, all emmetropic eyes, and 38 eyes corrected by spectacles recognized contrast levels of 1:5 or better without glare. In contrast, 18 eyes in the PRK group were unable to recognize contrast level 1:5 without glare. With glare, 1 eye in the disposable soft contact lens group, 1 in the soft contact lens group, and 7 with spectacles were unable to recognize the 1:5 contrast level. All emmetropic eyes recognized contrast levels of 1:5 or better; 22 PRK eyes were unable to recognize contrast level 1:5 with glare.

CONCLUSION

Myopic PRK may lead to long-term impairment of mesopic vision, while soft contact lens use does not seem to markedly influence mesopic vision in eyes with low to moderate myopia.

Corneal topography using the Holladay Diagnostic Summary

PURPOSE

To describe new software developments in videokeratography and illustrate their application in patients with corneal disease or patients who have had keratorefractive surgery.

METHODS

The Holladay Diagnostic Summary provides 4 maps and 15 corneal parameters for the clinician. The maps include 2 refractive power maps on standard and auto scales, a profile difference map for determining the corneal shape relative to normal asphericity, and a distortion map to display the optical quality of the cornea. The 15 corneal parameters provide quantitative information about the cornea for a 3.0 mm pupil; e.g., effective refractive power, regular astigmatism, asphericity, and predicted corneal acuity. The maps and parameters are used to analyze a calibration ball and five clinical examples.

RESULTS

Patients with diseases such as keratoconus or patients who have had keratorefractive surgery have characteristic changes in the 4 maps and the 15 corneal parameters.

CONCLUSION

When true refractive power maps, a profile difference map, a distortion map, and 15 corneal parameters are analyzed, characteristic changes are found that can help clinicians recognize and monitor corneal disease and refractive surgery and correlate these changes with patients' visual symptoms.

Characteristics influencing outcomes of excimer laser photorefractive keratectomy. Summit Photorefractive Keratectomy Phase III Study Group

PURPOSE

To identify preoperative and intraoperative characteristics associated with outcomes of photorefractive keratectomy (PRK).

METHODS

In the phase III multicenter clinical trials of the Summit Technology excimer laser for corrections of 1.5 to 6.0 diopters (D) of myopia, three principal outcomes of PRK on 612 patients were examined: (1) uncorrected visual acuity of 20/40 or better, (2) predictability of refractive outcome within 1.0 D of attempted correction, and (3) stability of refractive result between 12 and 24 months. Multiple logistic regression was used to test for independent associations of multiple preoperative and intraoperative characteristics with each of these outcomes.

RESULTS

Older age was independently associated with lesser likelihood of achieving 20/40 or better uncorrected visual acuity (odds ratio = 1.08 per incremental year of age, 95% confidence interval [CI] = 1.04-1.12) and with decreased predictability, specifically with overcorrection (odds ratio = 1.09, 95% CI = 1.06-1.12), but age was not associated with stability of refraction. Greater attempted correction was associated independently with a decreased likelihood of 20/40 or better uncorrected visual acuity (odds ratio = 2.78 for corrections of 3.5-5.5 D, 95% CI = 1.18-6.75; odds ratio = 4.19 for corrections of > or = 5.5 D, 95% CI = 1.66-10.58), with decreased predictability (odds ratio = 1.72 for corrections of 3.5-5.5 D, 95% CI = 1.05-2.85; odds ratio = 2.95 for corrections of > or = 5.5 D, 95% CI = 1.65-5.26), and with a reduced likelihood of stability of refraction (odds ratio = 3.46 for corrections of > or = 5.0 D, 95% CI = 1.32-9.11). No intraoperative characteristics were associated with any of the outcomes assessed.

CONCLUSIONS

Using this specific excimer laser system with an optical zone of 4.5 or 5.0 mm, patient age and attempted correction are important preoperative characteristics associated with postoperative uncorrected visual acuity and predictability of PRK. Stability of refraction is strongly associated with attempted correction. Such information may help guide patient selection, determine timing of fellow eye treatment, and suggest changes in the laser treatment algorithm for individual patients. Although these findings may be representative of PRK in general, similar analyses should be performed before modifying patient treatments using either a 6.0-mm treatment zone or other laser systems.