Corneal topographic analysis after excimer photorefractive keratectomy

Central Islands and Visual Outcomes of Phototherapeutic Keratectomy Using the Photorefractive Keratectomy Mode

PURPOSE

To compare the rate of central island (CI) and visual acuity after phototherapeutic keratectomy (PTK) with and without the CI program.

METHODS

This retrospective study comprised 147 eyes of 89 consecutive patients (mean age ± SD, 71.1 ± 9.6 years) undergoing PTK with and without the anti-CI program [photorefractive keratectomy (PRK) mode and PTK mode] using the VISX Star S4 excimer laser system (Johnson & Johnson Vision, Santa Ana, US) for treatment of band keratopathy (BK) or granular corneal dystrophy (GCD). The rate of CI formation, defined as a steepening area of 3 D, 1.5 mm in diameter, on each corneal videokeratographer (ATLAS 9000; Carl Zeiss Meditec, Jena), and best spectacle-corrected visual acuity (BSCVA) were assessed 3 months after PTK.

RESULTS

We found CI formation in 17 eyes (22%) in the PRK mode group and 45 eyes (73%) in the PTK mode group at 3 months postoperatively (P < 0.001, χ test). Postoperative logarithm of the minimal angle of resolution BSCVA was 0.10 ± 0.14 in the PRK group and 0.15 ± 0.13 in the PTK group (P = 0.019, Student t test). The rate of CI was 13 eyes (33%) with BK and 6 eyes (15%) with GCD in the PRK mode group and 25 eyes (83%) with BK and 20 eyes (71%) with GCD in the PTK mode group.

CONCLUSIONS

The use of the anti-CI program for PTK significantly decreases the rate of CI formation and significantly improves BSCVA in post-PTK eyes, suggesting its viability for treatment of BK and GCD in clinical setting.

Central island formation after cataract surgery in a laser in situ keratomileusis eye: New etiology

We report a case of central corneal steepness formation after cataract surgery in a post-laser in situ keratomileusis (LASIK) eye. A 51-year-old woman with traumatic cataract and a history of myopic LASIK surgery had uneventful phacoemulsification cataract surgery. Postoperatively, the corneal topography difference map showed a central island formation of 2.1 diopters (D) compared with the preoperative level, which correlated with a clinical myopic shift of 2.00 D from the targeted emmetropia. At 4 months, most of the central island had resolved spontaneously. At 15 months, topography returned to preoperative levels. We speculate that wound hydration in the flap interface during surgery resulted in a pond-like fluid accumulation in the center of the cornea, resulting in a myopic refractive shift. We report this case because of the high probability that this phenomenon could be confounded by an erroneous intraocular lens (IOL) calculation and the potential for a serious iatrogenic complication from a premature IOL exchange or touch-up procedure.

Central islands: rate and effect on visual recovery after phototherapeutic keratectomy

PURPOSE

To retrospectively assess the proportion of patients affected by a central island (CI) and its effect on visual recovery after phototherapeutic keratectomy (PTK).

METHODS

This retrospective study evaluated 30 eyes of 21 consecutive patients (mean age ± standard deviation, 69.6 ± 6.8 years) undergoing PTK for the treatment of band keratopathy or granular corneal dystrophy. We investigated the rate of CI formation, which was defined as a steepening area of 3 D, 1.5 mm in diameter, on each corneal videokeratograph (ATLAS 9000; Carl Zeiss Meditec), and its effect on visual recovery at 3 months and at 1 year postoperatively.

RESULTS

A CI was found in 22 of 30 eyes (73%) 3 months postoperatively and in 14 of 25 eyes (56%) 1 year postoperatively. The degree of CI was significantly correlated with the change in logMAR corrected visual acuity (Spearman correlation coefficient r = 0.445, P = 0.026). The degree of CI in eyes with band keratopathy was significantly larger than that in eyes with granular dystrophy 1 year postoperatively (Mann-Whitney test, P = 0.045). The degree of CI was not significantly correlated with the ablation depth (Spearman correlation coefficient r = 0.116, P = 0.582) or the residual corneal thickness (r = -0.235, P = 0.278).

CONCLUSIONS

CI formation was found in as many as 73 and 56% of patients 3 months and 1 year after PTK, respectively, using the VISX Star S4 excimer laser system, and significantly affected the improvement of visual acuity. The anti-CI program should be applied by the manufacturer, not only for corneal refractive surgery but also for PTK in a clinical setting.

Past and present of corneal refractive surgery: a retrospective study of long-term results after photorefractive keratectomy and a prospective study of refractive lenticule extraction

Surgical correction of refractive errors is becoming increasingly popular. In the 1990s, the excimer laser revolutionized the field of corneal refractive surgery with PRK and LASIK, and lately refractive lenticule extraction (ReLEx) of intracorneal tissue, using only a femtosecond laser, has become possible. Two new procedures were developed, ReLEx flex (FLEX) and ReLEx smile (SMILE). Until this thesis, only a few long-term studies of PRK with a relatively limited number of patients had been published; therefore, this thesis intended to retrospectively evaluate long-term outcomes after PRK for all degrees of myopia for a large number of patients. Furthermore, a prospective contralateral eye study comparing FLEX and SMILE, when treating high to moderate degrees of myopia, had not been performed prior to this study. This was the second aim of this thesis. In the first study, results from 160 PRK patients (289 eyes) were presented. Preoperative spherical equivalent ranged from -1.25 to -20.25 D, with 78% having low myopia (<-6 D). Average follow-up time was 16 years (range 13-19 years), making this the longest published follow-up study on PRK patients. Outcomes from eyes with low myopia were generally superior to outcomes from eyes with high myopia, at final follow-up. Seventy-two percent were within ± 1.00 D of target refraction, as compared to 47% of eyes with high myopia. However, results from a subgroup of unilateral treated PRK patients indicated that refraction at final follow-up was affected by myopic progression. Fifty percent of eyes with low myopia had uncorrected 20/20 distance visual acuity or better, as compared to 22% of eyes with high myopia. Haze did not occur if attempted corrections were <-4 D, and only eyes with high myopia lost two lines or more of CDVA (corrected distance visual acuity). Eighty-one per cent were satisfied or very satisfied with their surgery.

CONCLUSION

The results support the continued use of the excimer laser for corneal surface ablation as a treatment option for correction of low degrees of myopia, and as the treatment of choice for subgroups of refractive patients (thin corneas, etc.). The results also highlight that treatment of higher degrees of myopia with standard PRK should only be done today under special circumstances, due to low refractive predictability, and high risk of corneal haze. Technological advances since then should be taken into account when comparing these results with contemporary techniques. In the second study, 35 patients were randomized to receive FLEX in one eye and SMILE in the other. Preoperative spherical equivalent refraction ranged from -6 to -10 D with low degrees of astigmatism. A total of 34 patients completed the 6 month follow-up period. Refractive and visual outcomes were very similar for the two methods, as well as tear film measurements and changes in corneal biomechanics. Ninety-seven percent were within ± 1.00 D of target refraction, no eyes lost two lines or more of CDVA, and contrast sensitivity was unaffected after both procedures. The changes in higher-order aberrations were also very similar. There were also no differences in tear film parameters 6 months after surgery, although less postoperative foreign body sensation was reported within the first week after surgery in SMILE eyes. Corneal sublayer pachymetry measurements demonstrated equally increased epithelial thickness 6 months after surgery. Contrary to expectations, it was not possible to measure the theoretical biomechanical advantages of a small corneal incision in SMILE as compared to a corneal flap in FLEX. The main differences between FLEX and SMILE were found when the corneal nerves and intraoperative complications were evaluated. Thus, corneal sensitivity was better preserved and corneal nerve morphology was less affected after SMILE, but intraoperative complications occurred more frequently, although without visual sequela. Finally, 97% were satisfied or very satisfied with both their surgeries.

CONCLUSION

The results support the continued use of both FLEX and SMILE for treatment of up to high degrees of myopia. Overall, refractive and visual results for both procedures were good and similar, but from a biological point of view, the less invasive SMILE technique is more attractive, as demonstrated in this study, despite being slightly more surgically demanding than FLEX.

Aspheric Optical Zones in hyperopia with the SCHWIND AMARIS

Purpose

To evaluate the corneal Functional Optical Zone (FOZ) and the Effective Optical Zone (EOZ) of the ablation, among eyes that underwent LASEK/Epi-LASEK treatments for hyperopic astigmatism.

Methods

Twenty LASEK/Epi-LASEK treatments with mean defocus +2.21 ± 1.28 D performed using the SCHWIND AMARIS were retrospectively evaluated at 6-month follow-up. In all cases pre-/post-operative Corneal-Wavefront analyses using the Keratron-Scout (OPTIKON2000) were performed. FOZ-values were evaluated from the Root-Mean-Square of High-Order Wave-Aberration (RMSho), whereas EOZ-values were evaluated from the changes of Root-Mean-Square of High-Order Wave-Aberration (ΔRMSho) and Root-Mean-Square of the change of High-Order Wave-Aberration (RMS(ΔHOAb)). Correlations of FOZ and EOZ with Planned Optical Zone (POZ) and Defocus correction (SEq) were analyzed using a bilinear function.

Results

At six-month, defocus was −0.04 ± 0.44 D, ninety percent eyes were within ± 0.50 D from emmetropia. Mean RMSho increased 0.18 ± 0.22 μm, SphAb −0.30 ± 0.18 μm, and Coma 0.07 ± 0.18 μm 6-month after treatment (6-mm diameter). Mean FOZPre was 7.40 ± 1.48 mm, mean POZ was 6.76 ± 0.22 mm, whereas mean FOZPost was 5.53 ± 1.18 mm (significantly smaller, p < 0.0001; bilinear correlation p < 0.005), mean EOZΔRMSho 6.47 ± 1.17 mm (bilinear correlation p < 0.005), EOZRMS(ΔHOAb) 5.67 ± 1.23 mm (significantly smaller, p < 0.0005; bilinear correlation p < 0.05). EOZ positively correlates with POZ and declines steadily with SEq. A treatment of +3 D in 6.50-mm POZ results in 5.75-mm EOZ (7.75-mm NPOZ), treatments in 7.00-mm POZ result in about 6.25-mm EOZ (8.25-mm nomogrammed POZ).

Conclusions

FOZPost was significantly smaller than FOZPre. EOZΔRMSho was similar to POZ, whereas EOZRMS(ΔHOAb) was significantly smaller. Differences were larger for smaller POZ or larger Defocus. SEq up to +2 D result in EOZ, at least, as large as POZ. For SEq higher than +2 D, a nomogram for OZ can be applied.

Corneal morphological changes after myopic excimer laser refractive surgery

PURPOSE

To evaluate the changes in central corneal thickness (CCT) and corneal volume (CV) in eyes that have undergone myopic photorefractive keratectomy (PRK).

METHODS

CCT and CV obtained with an Oculus Pentacam before 1, 3, and 6 months after PRK were analyzed in 84 eyes with a mean preoperative refraction of -4.93 ± 2.23 diopter. The changes were compared with the amount of refractive treatment. The differences were evaluated with the Student t test and the correlations with the Pearson index.

RESULTS

One month after PRK, CCT and CV mean differences were 73.2 ± 31.5 μm (P < 0.001) and 2.2 ± 1.7 mm (P < 0.001), respectively. Three months after PRK, CCT and CV mean differences were 66.6 ± 26.7 μm (P < 0.001) and 1.4 ± 1.3 mm (P < 0.001), respectively. Six months after PRK, CCT and CV mean differences were 65.3 ± 25.7 μm (P < 0.001) and 1.4 ± 1.3 mm (P < 0.001), respectively. The effective treatment at each follow-up point was correlated with CCT changes (R = 0.62, 0.71, and 0.73, respectively), but not with CV changes (R = 0.04, 0.04, and 0.01, respectively).

CONCLUSIONS

Our findings support the hypothesis that after myopic PRK, when a series of corneal lamellae are severed centrally, the remaining peripheral segments relax. The squeezing force on the matrix is reduced, and the distance between the lamellae expands.

Customized ablation algorithm for the treatment of steep central islands after refractive laser surgery

Steep central island (SCI) formation after photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK) represents a major drawback in the visual rehabilitation of patients after refractive laser surgery. Because of the small size of SCIs, current ablation algorithms are unable to properly calculate an ablation pattern for customized retreatment. We present the use of a new ablation algorithm for the treatment of SCIs that occurred after PRK or LASIK surgery. This algorithm uses a smaller zone of approximation and takes into account the spherical shift induced by removal of the SCI. In all 3 eyes treated, best spectacle-corrected visual acuity increased to 20/16 and remained stable at the 1- and 3-month follow-up, with disappearance of the SCI in corneal topography. This new treatment algorithm may be of benefit to patients experiencing visual side effects due to SCI formation after PRK or LASIK surgery.

Response of the Cornea for up to Four Years After Photorefractive Keratectomy for Myopia

PURPOSE

To analyze the long-term corneal topographic changes 4 years after myopic photorefractive keratectomy (PRK).

METHODS

This study comprised 15 patients (30 eyes) who had PRK surgery with a scanning-spot excimer laser (Chiron Technolas 217C; Bausch & Lomb, Dornach, Germany) and were followed up to 4 years after surgery. The eyes were subdivided into three groups according to the preoperative spherical equivalent refraction. Corneal topographic maps were obtained for all eyes with a Placido disc topographer. Preoperative and follow-up topographical data were imported into a custom software program, which computed the average composite corneal maps and difference maps for each study group to quantify the anterior corneal changes following laser ablation. The software delineated three concentric zones of the corneal surface to characterize the regional corneal remodeling following the surgery.

RESULTS

A significant central corneal steepening (approximately 0.25 D, P < .001) was calculated between the 1- and 4-year postoperative maps in all study groups. A significant steepening (P < .001) of the corneal periphery was also noted for the lower myopic ablations whereas a peripheral flattening (P < .001) was observed for the deeper ablations between 1 and 4 years after surgery.

CONCLUSIONS

The anterior corneal surface was observed to remodel for up to 4 years after surface ablation, steepening a mean of approximately 0.25 D.

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.

Correlation of Changes in Refraction and Corneal Topography After Photorefractive Keratectomy

PURPOSE

To establish which corneal power evaluation measured with corneal topography correlates best with refractive changes after photorefractive keratectomy (PRK) for myopia.

METHODS

Two hundred fifty-one consecutive eyes of 171 patients who had PRK for myopia ranging from -14.80 to -0.50 D (mean -5.43 +/- 2.978 D), calculated at the corneal plane, were included in the analysis. Data included preoperative and postoperative (1, 3, and 6-mo) subjective refraction and videokeratography with a Keratron Scout (Optikon 2000). Statistical analysis was performed to determine the correlation between the change in subjective refraction at the corneal plane and changes in six corneal power measurements: best fit sphere, simulated keratometry (Sim K), corneal apex, and center of the pupil (last two evaluated for axial and meridional curvatures).

RESULTS

The closest correlation between subjective refraction change and corneal power measurement during the three follow-up evaluations was found with Sim K (R2 = 0.904; 0.889; 0.854) and best fit sphere (R2 = 0.919; 0.909; 0.872), whereas the other measurements showed poor correlation with the different curvatures.

CONCLUSIONS

The best fit sphere corneal topography parameter correlated best with the refractive changes, primarily for low treatment amounts, whereas it showed a clear-cut underestimation in eyes that had undergone high dioptric treatments.

Ablation Centration in Laser in situ Keratomileusis for Hyperopia: Comparison of VISX S3 ActiveTrak and VISX S2

PURPOSE

To compare ablation centration and outcome measurements in laser in situ keratomileusis (LASIK) for hyperopia using the pupil-tracking VISX S3 ActiveTrak or the nontracking VISX S2 excimer laser.

METHODS

In a retrospective study, 49 consecutively treated hyperopic eyes (32 patients) that had LASIK by the VISX StarS3 ActiveTrak were compared to 49 control-matched eyes treated with the VISX StarS2 without pupil-tracking. Primary outcome variables including ablation centration, uncorrected visual acuity, best spectacle-corrected visual acuity, manifest refraction, complications, and induced cylinder analyzed by vector analysis were evaluated 3 months postoperatively.

RESULTS

Hyperopic sphere ranged between plano and +5.50 D and cylinder between 0 and +2.75 D. Ninety-five of 98 eyes (96.9%) were available for analysis at 3 months. Of these 95, 52 eyes could be used for analysis of ablation centration. Mean decentration of the ablation zone from the entrance pupil was 0.30 +/- 0.20 mm in tracked eyes (n=31) and 0.41 +/- 0.39 mm in nontracked eyes (n=21), P=.17. Two eyes (6.5%) in the tracked group were significantly decentered between 0.5 and 1 mm from the pupil center. In the nontracked group, decentration was between 0.5 and 1 mm in one eye (4.8%) and greater than 1 mm in two eyes (9.5%).

CONCLUSIONS

Comparable ablation centration in LASIK for hyperopia was achieved between actively-tracked and nontracked eyes. Decentrations (greater than 1 mm) were not seen with a tracking system in this study. Visual and refractive results were similar between the VISX StarS3 ActiveTrak and VISX StarS2 laser systems.

Diagnostic use of a rigid contact lens to show corneal topography abnormalities after laser refractive surgery

We describe 2 cases in which evaluation of rigid contact lens fluorescein patterns were used to delineate and characterize topography irregularities. Contact lens analysis confirmed and localized topography findings of an elevated central island in 1 patient and a semicircular pattern in the other patient. To determine a therapeutic strategy to correct topography irregularities after laser refractive surgery, it is critical to document a corneal elevation and delineate its location.

Anterior Corneal Surface After Nidek EC-5000 Multipass and Multizone Photorefractive Keratectomy for Myopia

PURPOSE

To evaluate the correlation between the presence of irregularities in corneal ablation and the number of ablation zones with multipass and multizone photorefractive keratectomy (PRK).

METHODS

The differential maps obtained from corneal topography performed before and 1 month after PRK in 62 eyes that had undergone PRK with the Nidek EC-5000 excimer laser were assessed for irregularities. PRK treatment ranged from -1.00 to -16.00 D (mean -5.25 +/- 2.72 D), and the number of zones ranged from one to five.

RESULTS

Of 62 differential maps, 27 had an irregular pattern according to the Hersh classification, with a non-significant correlation with the number of treatment zones (chi2 = 5.09, P >.1).

CONCLUSION

Our results suggest that corneal topography irregularities arising from multizone PRK were not related to the amount of treatment or to the number of ablation zones.

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.

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.

Monocular Diplopia Related to Asymmetric Corneal Topography After Laser in situ Keratomileusis

PURPOSE

To show a specific relationship between monocular diplopia and corneal refractive asymmetry after laser in situ keratomileusis (LASIK).

METHODS

One hundred thirty-eight eyes of 98 patients who underwent LASIK for myopia between -2.12 and -17.75 D were examined under room-lighted conditions. We examined 51 eyes at 2 weeks, 46 eyes at 3 months, 32 eyes at 6 months, and 9 eyes at 1 year after LASIK. We attempted to correlate the presence of monocular diplopia with their corneal topographical features.

RESULTS

Eight eyes of five patients (five eyes at 2 weeks, three eyes at 3 months after LASIK) produced symptoms of monocular diplopia. These symptomatic patients had a common corneal topographical feature caused by decentralized or inhomogeneous ablation. Every pupillary area in the patients' topographies contained steeper and flatter areas. The range of refractive power variation in these asymmetric areas was at least 1.50 D. The location of the secondary image correlated with the direction of the steeper area in all eight eyes. Pinhole viewing eliminated or reduced the prominence of secondary images in every case.

CONCLUSION

Monocular diplopia following LASIK appears to correlate with postoperative corneal refractive power variation inside the pupillary area, caused by decentralized or inhomogeneous ablation.

Broad Beam vs. Flying Spot Excimer Laser: Refractive and Videokeratographic Outcomes of Two Different Ablation Profiles After Photorefractive Keratectomy

PURPOSE

To assess the refractive outcomes and videokeratographic patterns in photorefractive keratectomy (PRK) of two last-generation excimer lasers: broad beam and flying spot lasers.

METHODS

Forty eyes were treated for a mean myopic correction of -5.59+/-2.17 D with a Ladarvision excimer laser (Autonomous Technology Corp). We compared the refractive outcomes and the videokeratographic patterns with those of 40 eyes treated with the Summit Apex Plus laser (Summit Technology Inc) for a mean myopic correction of -5.60+/-2.24 D.

RESULTS

During the 6-month follow-up, we found no significant difference in uncorrected and spectacle-corrected visual acuity, haze grade, regularity index, or decentration. The eyes treated by Ladarvision showed a lower refractive error at 1 month (P = .04), a lower incidence of central islands at 1 (P = .003) and 3 months (P = .04), a wider mean effective ablation area at 1 (P = .0004), 3 (P = .0000009), and 6 months (P = .0000000004), a less steep ablation edge at 1 (P = .0026), 3 (P = .015), and 6 months (P = .011).

CONCLUSIONS

The small beam excimer laser provides better videokeratographic outcomes, which lead to quicker refractive stability and better visual performance.

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.

Surgically induced topographical abnormalities after LASIK: management of central islands, corneal ectasia, decentration, and irregular astigmatism

Proper preoperative evaluation is critical for avoiding many postoperative complications associated with laser in-situ keratomileusis (LASIK). Proper nonsurgical management includes careful monitoring of patients during the recovery period using various testing procedures, including corneal topography. When surgical intervention is required, a stepwise approach often is used as a conservative treatment, allowing further treatment if necessary. Many complications after LASIK are amenable to further treatment. However, it often is advisable to monitor patients until improved instrumentation is developed. The authors review the etiology and management of several complications after LASIK by reviewing the literature and relaying their own clinical experiences.

Computerized corneal topography and its importance to wavefront technology

PURPOSE

To review the development and application of corneal topography in refractive surgery.

METHODS

Review of the literature and discussion of recent developments in corneal topography and wavefront technology.

RESULTS

Analysis of corneal topography provides critical information for the preoperative examination of patients before refractive surgery and for the evaluation and treatment of patients with complications after surgery.

CONCLUSIONS

Corneal topography will continue to be a critical diagnostic modality for refractive surgery. Even with the advent of wavefront analysis designed to detect refractive error and aberrations of the eye, it will be necessary to have detailed corneal topographic information to understand the contribution the cornea makes to vision so that custom alteration of that surface can be used to optimize vision. This will be true of the normal eye, but it will be of special importance in eyes with abnormalities that were induced by corneal surgery.

Late natural history of corneal topography after excimer laser photorefractive keratectomy

PURPOSE

To investigate the 3 month to 1 year natural history of corneal topography after excimer laser photorefractive keratectomy (PRK).

DESIGN

A prospective, multicenter, nonrandomized comparative study.

PARTICIPANTS

A total of 75 eyes of 68 patients with myopia were studied.

INTERVENTION

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

MAIN OUTCOME MEASURES

Preoperative and 3, 6, and 12 month 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 relatively smooth 3 months after PRK. By 12 months, the corneal contour in general had become even more uniform. No "central island" effect was observed. When looking at right and left eyes independently, there was a tendency toward maximum flattening nasally.

CONCLUSIONS

Corneal topography in general continues to smooth from 3 to 12 months after PRK, possibly as a result of epithelial and stromal healing and remodeling. Right and left eyes on average show mirror-image, spatially oriented topography patterns.

Corneal hydration affects ablation during laser in situ keratomileusis surgery

BACKGROUND

Studies in animal eyes indicate that the level of corneal hydration affects the ablation rate of laser surgery; the greater the hydration is, the less the ablation for a given laser pulse. Our study is an assessment in human eyes comparing the effects on ablation by blotting the corneal stromal surface under a corneal flap created for laser in situ keratomileusis (LASIK) procedures between sets of excimer laser pulses, with ablation depth in eyes not blotted between sets of laser pulses.

METHODS

We modified the surgical technique for LASIK procedures to assess the effects of the level of hydration on excimer laser ablation depth per pulse. In group 1, 40 eyes underwent LASIK surgery without any modification. Group 2 was composed of 36 eyes having LASIK procedures, but the corneal surfaces were kept relatively dry by blotting of the stromal surface between sets of laser pulses.

RESULTS

Six months after surgery, the mean spherical equivalent refractive change was from -8.38 diopters (D) to -1.44 D in group 1 and from -7.93 D to -0.09 D in group 2. For predictability, the deviation from the target refraction after surgery was assessed. Thirty-three percent (13 of 40) in group 1 and 25% (9 of 36) in group 2 were within +/-0.5 D. Forty-eight percent (19 of 40) in group 1 and 50% (18 of 36) in group 2 were within +/-1 D. Six months after surgery, 80% or more in both groups were within +/-2 D. There was myopic regression in all patients. Three months after surgery, regression averaged -0.71 D in group 1 and -1.15 D in group 2.

CONCLUSIONS

Corneal hydration levels affect the efficiency of laser ablation in LASIK procedures. With less hydrated corneas, ablation effects were greater than for corneas not blotted during the procedure, but these patients appear to undergo greater myopic regression.

The specific architecture of the anterior stroma accounts for maintenance of corneal curvature

AIM

To analyse the human corneal stroma in extreme hydration to discover if its structure is responsible for corneal stability.

METHODS

Corneas in several hydration states were used: postmortem control corneas (PM; n=3), corneas left for 1 day in phosphate buffered saline (PBS; n=4), and corneas left for 1 day (n=4), 2 days (n=4), 3 days (n=2), and 4 days (n=4) in deionised water. All corneas were fixed under standardised conditions and processed for light and electron microscopy. In addition, two fresh corneas from the operating theatre were studied which were processed 6 months after storage in sodium cacodylate buffer.

RESULTS

After 1 day in deionised water maximal stromal swelling was reached which did not change up to 4 days. The stroma of deionised water corneas (1400 microm) was much thicker than that of PBS corneas (650 microm) and PM corneas (450 microm). Deionised water treatment led to disappearance of all keratocytes leaving only remnants of nuclei and large interlamellar spaces. In these specimens the distance between the collagen fibres had increased significantly, but the diameter of the collagen fibres did not seem to be affected. A remarkable observation was that the most anterior part of the stroma (100-120 microm) in all deionised water specimens and those stored for 6 months in buffer was not swollen, indicating that the tightly interwoven anterior lamellae are resistant to extreme non-physiological hydration states.

CONCLUSIONS

The rigidity of the most anterior part of the corneal stroma in extreme hydration states points to an important role in maintenance of corneal curvature. Since a large part of this rigid anterior part of the stroma is either removed (PRK) or intersected (LASIK), it is possible that in the long run patients who underwent refractive surgery may be confronted with optical problems.

Correlation Between Refractive and Corneal Topographic Changes After Photorefractive Keratectomy for Myopia

PURPOSE

To compare videokeratographic and refractive data obtained before and after photorefractive keratectomy (PRK) for myopia.

METHODS

Seventy-four eyes underwent PRK for myopia ranging from -2.50 to -17.00 D (mean, -7.76 +/- 3.17 D). All patients had videokeratography with the EyeSys instrument before, and 1 and 6 months after PRK, and the changes in three corneal power measurements (center of the ablation, apex, and effective refractive power) were compared with refractive changes.

RESULTS

Changes obtained in the three corneal power measurements at 1 and 6 months were well correlated with manifest refraction (Pearson's coefficient ranged from 0.71 to 0.84).

CONCLUSION

Power measurements obtained with corneal topography, as described above, are a reliable and objective method for the evaluation and follow-up of PRK, provided addition of an approximate 25% correcting factor.

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.

Surgical results of photorefractive keratectomy with different operative modes

PURPOSE

To compare the predictability, efficacy, and safety of photorefractive keratectomy (PRK) using different operative modes.

SETTING

National Taiwan University Hospital, Taipei, Taiwan.

METHODS

One hundred fifty-three eyes of 80 patients who had PRK for myopia with a follow-up of at least 6 months were studied. All patients were sequentially assigned to 1 of the following surgical modes: mode 1: PRK with the Summit OmniMed excimer laser; mode 2: PRK with the Summit Apex Plus laser; mode 3: PRK with the Summit Apex Plus laser with anti-central-island pretreatment.

RESULTS

Six months after treatment, a homogeneous topographic pattern was seen in 76% of mode 1 eyes, 70% of mode 2 eyes, and 88% of mode 3 eyes. In the low myopia group (< or =-6.0 diopters [D]), the mean residual refractive error was -0.79 D +/- 0.59 (SD) in mode 1, -0.94 +/- 1.02 D in mode 2, and -0.31 +/- 0.42 D in mode 3. In the high myopia group (>-6.0 D), it was -1.93 +/- 1.51 D, -1.54 +/- 0.88 D, and -0.70 +/- 0.81 D, respectively. Uncorrected visual acuity of 20/25 or better was achieved in 81% of mode 1 eyes, 56% of mode 2 eyes, and 89% of mode 3 eyes in the low myopia group, and in 48%, 28%, and 72%, respectively, in the high myopia group.

CONCLUSIONS

Photorefractive keratectomy appears to be a predictable and effective procedure. The best results were achieved with the Summit Apex Plus laser with anti-central-island pretreatment, followed by the Summit OmniMed laser. The Summit Apex Plus laser without anti-central-island pretreatment produced less satisfactory results.

Natural history of central islands after laser in situ keratomileusis

PURPOSE

To assess the incidence and natural history of central islands following laser in situ keratomileusis (LASIK) and evaluate the association of central island characteristics with visual acuity.

SETTING

Department of Ophthalmology, China Medical College Hospital, Taichung, Taiwan.

METHODS

A consecutive series of 406 eyes of 212 patients who had LASIK was retrospectively evaluated. Uncorrected visual acuity (UCVA) was measured and corneal topography performed preoperatively and 1 week and 1, 3, 6, and 9 months postoperatively. Best spectacle-corrected visual acuity (BSCVA) was evaluated preoperatively and 1, 3, and 6 months postoperatively.

RESULTS

The topographic images obtained at 1 week demonstrated central islands in 23 eyes of 20 patients (5.7%). No new cases of central island formation were identified after 1 week. Of the 23 eyes with central islands, the 6 month post-LASIK maps were available in 20 eyes of 18 patients. There was a significant difference in the size and power of the central islands between 1 week and 6 months. However, the power and size decreased slowly. Within 6 months, only 5 of 20 central islands (25.0%) had resolved. Eight eyes were undercorrected, and 1 eye lost 2 lines of BSCVA. Central islands larger than 1.8 mm or 3.0 diopters (D) were significantly correlated with lower UCVA.

CONCLUSION

Most central islands that occur with LASIK persist more than 6 months. Large central islands (>/=1.8 mm or >/=3.0 D) are risk factors for lower UCVA. Preventive measures are necessary.

Videokeratography after excimer laser in situ keratomileusis for myopia

OBJECTIVE

The purpose of this study was to define qualitative patterns of videokeratography after excimer laser in situ keratomileusis (LASIK) for myopia and to identify associations of videokeratography patterns with clinical variables.

DESIGN

Single-center, retrospective, cohort study.

PARTICIPANTS

One hundred twenty-six eyes of 68 patients with myopia who desired surgical correction of myopia ranging from -1.50 to -8.80 diopters.

INTERVENTION

Myopia was corrected with LASIK using a Summit OmniMed/Apex excimer laser. Computer-assisted videokeratography data were acquired at 3 months after primary LASIK, and associations of videokeratography pattern with clinical variable were assessed.

MAIN OUTCOME MEASURES

Seven subgroup ablation zone patterns were characterized using tangential curvature maps; these subgroups were combined into pooled symmetric and asymmetric groups. Associations with age, attempted correction, single-zone or multizone ablation, postoperative uncorrected and spectacle-corrected visual acuity, predictability, astigmatism, corneal uniformity, glare disability, and contrast sensitivity were analyzed.

RESULTS

Thirty-two eyes (25.4%) showed a uniform pattern, 14 eyes (11.1%) showed a bow-tie pattern, 22 eyes (17.5%) showed a peninsula pattern, 22 eyes (17.5%) showed a semicircular pattern, 22 eyes (17.5%) showed an off-center blue spot pattern, 10 eyes (7.9%) showed an irregular pattern, and 4 eyes (3.2%) showed a central island pattern. Spectacle-corrected visual acuity, astigmatism, corneal uniformity, glare disability, and contrast sensitivity were significantly related to videokeratography pattern, although the difference among videokeratography groups was clinically small. Age, attempted correction, single- or multizone ablation, and postoperative uncorrected visual acuity and predictability were not predictive of videokeratography pattern.

CONCLUSIONS

Most eyes in this study had an asymmetric videokeratography pattern 3 months after LASIK. Small but statistically significant differences were identified among the videokeratography patterns that potentially may affect postoperative quality of vision.

Clinical analysis of central islands after laser in situ keratomileusis

PURPOSE

To analyze the incidence and clinical characteristics of central islands after laser in situ keratomileusis (LASIK) and to elucidate factors associated with their formation.

SETTING

Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, Korea.

METHODS

Laser in situ keratomileusis was performed in 103 eyes of 61 patients with myopia ranging from -4.0 to -13.5 diopters (D) using the Hansatome (Chiron) and SVS Apex Plus (version 3.2.1) excimer laser (Summit Technology) in which the anti-central-island program was implemented. After 1 week, corneal topography (Orbscan, Orbtek) was done and manifest refraction and visual acuity were measured.

RESULTS

Postoperatively, the mean uncorrected visual acuity (UCVA) and best corrected visual acuity (BCVA) were 0.12 and 0.06 (logMAR scale), respectively, and the mean refractive error (spherical equivalent) was 0.07 D +/- 0.76 (SD). On topographic examination, a central island was defined as an area of higher refractive power of more than 1.5 D and 2.5 mm or more in diameter. Budding or isolated central islands were observed in 12 eyes of 12 patients (11.7%). The peak, height, and area of the islands were 41.5 +/- 3.1 D, 5.6 +/- 1. 9 D, and 3.5 +/- 1.1 mm(2), respectively. In the eyes with central islands, there were statistically significant differences in the postoperative change in UCVA and BCVA (P <.05). There was no significant correlation between the occurrence of a central island and preoperative refractive error, corneal thickness, age, or in sex and correction of astigmatism (P >.05).

CONCLUSION

Despite use of the anti-central-island pretreatment program, the occurrence of central islands after LASIK was significant, as in photorefractive keratectomy. Further studies of the effect of central islands on surgical results and clinical progress and measures to prevent the occurrence are needed.

Ablation centration after active eye-tracker-assisted photorefractive keratectomy and laser in situ keratomileusis

PURPOSE

To evaluate the ablation centration after active eye-tracker-assisted photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK) and to investigate the effect of surgery, patient, and surgeon on the centration.

SETTING

Department of Ophthalmology, China Medical College Hospital, Taichung, Taiwan, Republic of China.

METHODS

This retrospective study comprised 177 eyes of 101 patients: 16 eyes had PRK and 161, LASIK. All laser treatments were performed with the aid of an eye tracker. The amount of decentration was analyzed by corneal topography. The factors influencing centration were divided into surgery related (PRK/LASIK), patient related (low/high myopia and effect of learning), and surgeon related (learning curve).

RESULTS

The mean decentration was 0.33 mm in PRK eyes and 0.35 mm in LASIK eyes. For the surgery-related factor, there was no significant difference between the PRK and LASIK eyes. For the patient-related factors, centration was better in the second eye (effect of learning) and decentration was more severe in eyes with high myopia (low/high myopia). For the surgeon-related factor, there was no significant difference between eyes that had the first 50 LASIK procedures and those that had the last 50 procedures.

CONCLUSIONS

An eye tracker, which makes the laser beam follow the eye's movements, helps to avoid severe decentration. This study showed, however, that an active eye-tracking system alone cannot ensure good centration. Patient cooperation and fixation are important.

Corneal topography of small-beam tracking excimer laser photorefractive keratectomy

PURPOSE

To evaluate the topographic characteristic of photorefractive keratectomy (PRK) for low myopia performed with a small-beam (0.9 mm) tracking excimer laser.

SETTING

Department of Ophthalmology, LSU Eye Center, Louisiana State University Medical Center School of Medicine in New Orleans, and the Refractive Surgery Center of the South at the Eye, Ear, Nose, & Throat Hospital, New Orleans, Louisiana, USA.

METHODS

Sixty-seven eyes of 47 patients had PRK with a small-beam tracking laser. Of these, 49 eyes had data permitting evaluation of ablation centration; usable data for topographic analysis were available for 59 eyes preoperatively, 54 eyes at 1 month, 42 eyes at 3 months, and 25 eyes at 6 months, permitting measurement of various topographic parameters, including the cylinder (CYL), average corneal power (ACP), surface regularity index (SRI), surface asymmetry index (SAI), corneal eccentricity index (CEI), and coefficient of variation of corneal power (CVP).

RESULTS

Preoperatively, all eyes were topographically normal. Postoperatively, no eye exhibited a "central island" by even the least-restrictive definition, and all eyes had best spectacle-corrected visual acuities (BSCVAs) of 20/20 or better at all follow-ups. Mean decentration of the ablations from the pupil centers was 0.42 mm +/- 0.28 (SD) (n = 49). There was no correlation between measured decentration and BSCVA (P = .46). The central cornea was flattened (decreased ACP; P < .001) and made oblate (decreased CEI; P < .001) as expected. There was no increase in SRI or SAI (irregular astigmatism) at 6 months compared with preoperative values (P = .91); however, CYL and CVP (varifocality) increased slightly (P = .04 and .02, respectively).

CONCLUSION

The absence of significant regular or irregular astigmatism 6 months after PRK with the small-beam laser is an improvement over published results achieved with wide-beam lasers and is consistent with the excellent visual acuity results in this cohort.

Significance of corneal topography in predicting patient complaints after photorefractive keratectomy

PURPOSE

To evaluate the sensitivity and specificity of postoperative corneal topography to predict potential patient complaints after photorefractive keratectomy (PRK).

SETTING

Doheny Eye Institute, Los Angeles, California, USA.

METHODS

Postoperative tangential corneal topographic maps, in 0.5 and 1.0 diopter (D) relative scales, were obtained from patients (n = 34) at least 4 months after PRK. Topographies of complaining (n = 18) and noncomplaining patients (n = 16) were analyzed by 6 masked examiners with 2 different experience levels in PRK (experts, n = 2; beginners, n = 4), who assigned the topographies to 1 of the 2 groups.

RESULTS

Topographies of complainers (sensitivity) and noncomplainers (specificity) were correctly classified in 53.2% overall and in 44.0% and 63.5% (P = .06) in complainers and noncomplainers, respectively. Experienced examiners were not significantly more accurate than inexperienced examiners (46.3% and 56.6%, respectively; P = .09). Images of 1.0 D scales received significantly more correct responses than those of 0.5 D scales (56.4% and 50.0% respectively; P = .03). The reproducibility between images for the same patient in both scales was significantly better for the experienced examiners than the inexperienced examiners (kappa coefficient 0.73 and 0.51, respectively; P = .05).

CONCLUSIONS

Subjective analysis of postoperative corneal topography alone is not sufficient to predict potential patient complaints after PRK. Topographic findings should be interpreted only in the context of a complete clinical examination.

Retreatment of central islands after photorefractive keratectomy

PURPOSE

To evaluate excimer laser retreatment of central islands and the refractive consequences.

SETTING

Department d'Ophtalmologie, Centre Hospitalier de l'Université Laval, Quebec City, and Sir Mortimer B. Davis Jewish General Hospital, McGill University, Montreal, Canada.

METHODS

In this study, 22 eyes retreated for central islands were retrospectively analyzed. All initial corrections and retreatments were performed with the Chiron Technolas Keracor 116 laser.

RESULTS

A marked decrease in symptoms was observed in almost all cases. Topographic analysis confirmed these results but revealed persistence of minimal islands in several cases. Retreatment of central islands also induced a small myopic correction. Using a 2.5 mm zone, the mean induced refractive effect was 0.21 diopter of myopic correction per diopter of retreatment.

CONCLUSION

Retreatment of central islands appeared safe, was relatively easy, and provided good results with a rapid recovery.

Posterior corneal surface topographic changes after laser in situ keratomileusis are related to residual corneal bed thickness

OBJECTIVE

To determine whether ectasia after laser in situ keratomileusis (LASIK) is related to residual corneal bed thickness.

DESIGN

Retrospective noncomparative case series.

PARTICIPANTS

Thirty-two eyes of 16 patients with refractive errors of -4.00 to -18.00 diopters were examined.

INTERVENTION

LASIK was performed. The topography of the posterior corneal surface was examined with the Orbscan slit scanning corneal topography/pachymetry system.

MAIN OUTCOME MEASURES

The difference in the elevation of posterior corneal surface regarding the best-fit sphere was measured.

RESULTS

After surgery, mean bulge of 17.2 +/- 7.2 microns was found in eyes with residual corneal bed of 250 microns or greater, whereas 41.0 +/- 22.1 microns was seen in eyes with bed thickness less than 250 microns (t = 4.29; P = 0.000).

CONCLUSION

Posterior corneal bulge is correlated with the residual corneal bed thickness. The risk of ectasia may be increased if the residual corneal bed is thinner than 250 microns.

Corneal hydration and central islands after excimer laser photorefractive keratectomy

PURPOSE

To determine whether uneven corneal surface hydration during excimer laser photorefractive keratectomy (PRK) is related to postoperative occurrence of central islands.

SETTING

LSU Eye Center, New Orleans, Louisiana, USA.

METHODS

A retrospective study reviewed intraoperative videotapes and postoperative videokeratography of 49 eyes of 49 patients who had excimer laser PRK for myopia. The uniformity of corneal hydration within the photoablation zone, particularly the frosty appearance of the ablated zone, was characterized. The presence or absence of a topographic central island (steepening of at least 3.0 diopters and 1.5 mm in diameter) was determined from the 1 month postoperative videokeratographs.

RESULTS

Twelve eyes (24.5%) developed central islands postoperatively. A statistically significant association was observed between the uneven surface hydration (central accumulation of fluid) within the ablation zone intraoperatively and the formation of central islands postoperatively (P < .001, Kruskal-Wallis test; Kendall tau rank correlation = 0.534; P < .001).

CONCLUSION

Nonuniform fluid distribution during photoablation was a risk factor for central island formation after PRK. Intraoperatively, the presence of excess fluid in the central cornea appeared as a shiny area. This mirror-like surface layer may reduce the rate of central ablation by reflecting and absorbing a significant amount of the incident excimer laser light.

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.

Corneal Hydration Monitored by Laser-induced Breakdown Spectroscopy

BACKGROUND

Corneal hydration is an important factor in laser corneal ablation. In photorefractive keratectomy (PRK), corneal ablation rate and final ablation surface quality are strongly dependent on corneal hydration. We used a spectroscopic technique for monitoring corneal hydration during PRK.

METHODS

Hydroxyethlymethacrylate (HEMA) was employed for corneal hydration modeling. Hydrated HEMA samples were irradiated with a pulsed Nd:YAG laser (1064 nm, 10 mJ/pulse, pulse duration 15 nsec). Successive emission spectra corresponding to different degrees of hydration were recorded on a gated optical multichannel analyzer. The weight of the sample and hence its water content was monitored during the entire procedure with a sensitive balance. One rabbit and one human cornea were used to demonstrate the spectral analogy between the model and corneal tissue.

RESULTS

The most noticeable dependence on water content of the substrate was that of atomic emission lines of Ca at 393 nm and 396 nm.

CONCLUSION

Plasma emission spectra exhibited significant dependence on sample hydration. This dependence can be used for estimation of water content of irradiated model material and real cornea.

Fourier analysis of corneal topography data after photorefractive keratectomy

PURPOSE

To evaluate the relation between measures of corneal power and refractive error after photorefractive keratectomy (PRK) by applying fast Fourier transform (FFT) to computerized corneal topography data.

SETTING

Corneal Diseases and Excimer Laser Clinic, Sunderland Eye Infirmary, Sunderland, England, and University of Dundee Department of Ophthalmology, Dundee, Scotland.

METHODS

Twenty-six left eyes of consecutive patients treated by PRK with a VISX Twenty-Twenty excimer laser were retrospectively analyzed. Preoperative and 3, 6, and 12 month postoperative data were studied. Changes in corneal parameters derived from corneal topography data using the FFT were compared with changes in refractive status vectors (spherical equivalent and astigmatic cosine and sine values).

RESULTS

Although highly correlated (r2 = 0.8839), the change in FFT-derived corneal spherical equivalent underestimated the change in refractive spherical equivalent by 25.5% over the 12 month follow-up. Decentration, measured by the 1-cycle FFT harmonic, increased significantly from a mean preoperative value of 0.12 mm +/- 0.07 (SD) to 0.51 +/- 0.35 mm 12 months postoperatively.

CONCLUSIONS

The FFT is a powerful method for extracting clinically meaningful descriptors from corneal topography data; however, care must be taken when interpreting refractive changes from corneal data.

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.