Topographical analysis of the centration of excimer laser photorefractive keratectomy

Impact of Treatment Decentration on Higher-Order Aberrations after SMILE

Purpose. To evaluate decentration following femtosecond laser small incision lenticule extraction (SMILE) and sub-Bowman keratomileusis (SBK) and its impact on higher-order aberrations (HOAs). Methods. Prospective, nonrandom, and comparison study. There were 96 eyes of 52 patients who received SMILE and 96 eyes of 49 patients who received SBK in this study. Decentration was calculated 6 months after surgery with Pentacam. HOAs and visual acuity after the surgery were examined for patients in both groups before and 6 months after surgery. Results. The mean decentration displacement in SMILE group was significantly less than SBK group (P = 0.020). 89 eyes were decentered within 0.50 mm after SMILE and SBK. The association between vertical decentration and the induced spherical aberration was insignificant in SMILE group (P = 0.035). There was an association between decentration and safety index, efficacy index, vertical coma, spherical aberration, and HOAs in root mean square (RMS, μm) after SBK (all P < 0.05). No difference was found in uncorrected and corrected distance visual acuity, safety index, efficacy index, and wavefront aberrations between the two subgroups at any delimited value after SMILE (all P > 0.05). Decentration exceeding 0.37 mm affected vertical coma and RMSh of SBK eyes (P = 0.002, 0.005). Conclusion. SMILE surgery achieved more accurate centration than SBK surgery. Vertical decentration is associated with the induced spherical aberration in SMILE.

Repeatability and Reproducibility of Quantitative Corneal Shape Analysis after Orthokeratology Treatment Using Image-Pro Plus Software

Purpose. To evaluate the repeatability and reproducibility of quantitative analysis of the morphological corneal changes after orthokeratology treatment using “Image-Pro Plus 6.0” software (IPP). Methods. Three sets of measurements were obtained: two sets by examiner 1 with 5 days apart and one set by examiner 2 on the same day. Parameters of the eccentric distance, eccentric angle, area, and roundness of the corneal treatment zone were measured using IPP. The intraclass correlation coefficient (ICC) and repetitive coefficient (COR) were used to calculate the repeatability and reproducibility of these three sets of measurements. Results. ICC analysis suggested “excellent” reliability of more than 0.885 for all variables, and COR values were less than 10% for all variables within the same examiner. ICC analysis suggested “excellent” reliability for all variables of more than 0.90, and COR values were less than 10% for all variables between different examiners. All extreme values of the eccentric distance and area of the treatment zone pointed to the same material number in three sets of measurements. Conclusions. IPP could be used to acquire the exact data of the characteristic morphological corneal changes after orthokeratology treatment with good repeatability and reproducibility. This trial is registered with trial registration number: ChiCTR-IPR-14005505.

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.

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.

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.

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.

Tracker-assisted Versus Manual Ablation Zone Centration in Laser in situ Keratomileusis for Myopia and Astigmatism

PURPOSE

Eye tracker systems have been developed concomitantly with small scanning beams to theoretically reduce ablation zone decentration and for accurate registration of all the laser pulses on the cornea. The purpose of the study was to compare the tracker-assisted with the manual centration method.

METHODS

Twenty-five patients (48 eyes) with myopia and/or astigmatism had laser in situ keratomileusis (LASIK) between August 1998 and February 1999 with the Technolas 117C laser. Twenty patients (38 eyes, 80%) were available for follow-up at 3 months after surgery. Eyes were assigned randomly to one of two ablation zone centration methods: Group 1: Tracker-assisted (20 eyes), Group 2: Manual (18 eyes).

RESULTS

Mean distance between the ablation zone center and the pupillary center in the tracker-assisted centration group was 0.55 +/- 0.30 mm (range, 0.10 to 1.4 mm), and in the manual centration group, 0.43 +/- 0.23 mm (range, 0.10 to 1.0 mm) (P = .177). There was no statistically significant difference in postoperative contrast sensitivity, glare, and Topographical Corneal Surface Regularity Index (SRI) between the two groups.

CONCLUSIONS

We obtained good results with both centration methods. We did not find superiority of the tracker-assisted over manual regarding ablation zone centration, vision quality, or regularity of the ablation.

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.

200 Hz flying-spot technology of the LaserSight LSX excimer laser in the treatment of myopic astigmatism: six and 12 month outcomes of laser in situ keratomileusis and photorefractive keratectomy

PURPOSE

To evaluate safety, efficacy, predictability, and stability in the treatment of myopic astigmatism with laser in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK) using the 200 Hz flying-spot technology of the LaserSight LSX excimer laser.

SETTING

SynsLaser Clinic, Tromsø, Norway.

METHODS

This retrospective study included 110 eyes treated with LASIK and 87 eyes treated with PRK that were available for evaluation at 6 and 12 months, respectively. The mean preoperative spherical equivalent (SE) was -5.35 diopters (D) +/- 2.50 (SD) (range -1.13 to -11.88 D) in the LASIK eyes and -4.72 +/- 2.82 D (range -1.00 to -15.50 D) in the PRK eyes. The treated cylinder was 4.00 D in both groups. Eleven (8.5%) LASIK eyes and 8 (7.4%) PRK eyes had secondary surgical procedures before 6 and 12 months, respectively, and were excluded when the 6 and 12 month outcomes were analyzed.

RESULTS

None of the eyes lost 2 or more lines of best spectacle-corrected visual acuity. Seventy-seven percent of the LASIK eyes and 78% of the PRK eyes achieved an uncorrected visual acuity of 20/20 or better; 98% in both groups achieved 20/40 or better. The SE was within +/-0.5 D of the desired refraction in 83% of the LASIK eyes and 77% of the PRK eyes; it was within +/-1.0 D in 97% and 98%, respectively. The cylinder correction had a mean magnitude of error of 0.04 +/- 0.31 D (range -0.96 to +0.85 D) in the LASIK eyes and 0.02 +/- 0.37 D (range -1.44 to +0.72 D) in the PRK eyes. Refractive stability was achieved at 1 month and beyond in the LASIK eyes and at 3 months and beyond in the PRK eyes.

CONCLUSION

The outcomes of this study are comparable to those achieved with lasers that use small-beam technology with a lower frequency, as well as with other types of delivery systems. They suggest that the 200 Hz technology used in the LaserSight LSX excimer laser is safe, effective, and predictable and that with LASIK and PRK the results are stable when treating low to moderate myopia and astigmatism up to 4.0 D.

Determining the Accuracy of an Eye Tracking System for Laser Refractive Surgery

PURPOSE

Patient eye and head movements during laser refractive surgery may result in errors between the surgical beam position and the desired location for optimum correction. This, in turn, may lead to reduced postoperative vision, including increased higher order aberrations of the eye. Active eye tracking systems are often incorporated into laser delivery systems, which aim to reduce the effect of patient eye movement.

METHODS

In this study, the accuracy of an eye tracking system designed for laser refractive surgery was determined. An enucleated porcine eye was attached to a scanning device and the movement measured using the eye tracking system. The recorded position is compared to the preprogrammed position of the scanning device.

RESULTS

The system demonstrated an accuracy of 0.06 m for an intact cornea and 0.1 mm for a cornea with a thin flap removed. This compares to an average decentration of ablation of 0.4 mm for patients relying on passive fixation, as measured by previous clinical trials.

CONCLUSION

implementation of this eye tracker would lead to improved alignment between the laser and eye during laser refractive surgery.

Eye movement during laser in situ keratomileusis

PURPOSE

To measure eye motion in patients having laser in situ keratomileusis (LASIK) using a video technique and determine centration and variance of the eye position during surgery.

SETTING

Laser refractive surgery center.

METHODS

The procedure was videotaped in 5 consecutive eyes having LASIK performed by a single surgeon with the VISX Star S2 excimer laser. Following surgery, video images of the eyes were digitized and stored in a computer for processing. Digitized images were obtained at a rate of 25 images per second during the laser procedure. The pupil margin and a visual landmark, such as a scleral blood vessel, were identified in the initial image of each eye. Custom software was used to track the location of the landmark and the pupil center in subsequent images.

RESULTS

Three of the 5 eyes were well centered on average. The remaining 2 eyes were decentered inferiorly by approximately 0.25 mm. The standard deviation in all eyes was approximately 0.10 mm.

CONCLUSIONS

With these techniques, the position of the entrance pupil center relative to the excimer laser axis could be determined. Although the system is not fast enough to be used during surgery, it does allow quantification of centration and intraoperative motion after surgery.

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 after laser in situ keratomileusis

OBJECTIVE

To evaluate the effectiveness, predictability, and safety of laser in situ keratomileusis (LASIK) retreatment for correcting residual myopia.

DESIGN

Retrospective noncomparative case series.

PARTICIPANTS AND INTERVENTION

Fifty-nine consecutive eyes (43 patients) underwent LASIK retreatment at 3 or 6 months after the primary LASIK procedure. Lifting the corneal flap and reablating the stromal bed with a VISX 20/20 excimer laser was the procedure used for LASIK enhancement.

MAIN OUTCOME MEASURES

The following parameters were studied before and after retreatment: visual acuity, refraction, videokeratography, applanation tonometry, and corneal thickness. Complications after LASIK enhancement also were evaluated. Follow-up was 12 months.

RESULTS

Before retreatment, only 3.38% of eyes (2 of 59) had an uncorrected visual acuity of 0.5 (20/40) or better, and after retreatment, this percentage increased to 60% (30 of 50) at 6 months and 61.8% (34 of 55) at 12 months. After reoperation, mean best-corrected visual acuity improved by half a line over the values before retreatment. The preretreatment refraction of -2.92 +/- 1.22 diopters (D) (mean +/- standard deviation) decreased significantly to -0.44 +/- 0.80 D at 6 months and to -0.61 +/- 0.82 D at 12 months (P < 0.001). In 82% of eyes (41 of 50) at 6 months and 81.8% (45 of 55) at 12 months, the spherical equivalent was within 1.00 D of emmetropia. There was a significant regression of effect (0.38 D) between 3 and 12 months (P < 0.01). Postretreatment refraction was related to the original refraction before the primary LASIK, the preretreatment refraction, and the ablation diameter used. Although no vision-threatening complications were found, epithelial ingrowth and flap melting were more common after than before LASIK retreatment, with 31% of eyes at 12 months with epithelial ingrowth and 10.9% with flap melting. However, LASIK enhancement improved decentration and night-vision problems.

CONCLUSIONS

LASIK retreatment was an effective and predictable procedure for correcting residual myopia. Epithelial ingrowth and flap melting were more frequent after than before LASIK retreatment, whereas decentration and night-vision symptoms improved.

Confocal microscopy reveals persisting stromal changes after myopic photorefractive keratectomy in zero haze corneas

AIMS

Micromorphological examination of the central cornea in myopic patients 8-43 months after excimer laser photorefractive keratectomy (PRK), using the slit scanning confocal microscope.

METHODS

Patients were selected from a larger cohort of individuals on the basis of full corneal clarity (haze grading 0 to +1; mean 0.3) and their willingness to participate in the study. 15 eyes of 10 patients with myopic PRK (-4 to -11 D; mean 6.7) and an uneventful postoperative interval of 8-43 months (mean 26) were examined. Contact lenses had been worn by eight of the 10 patients for 4-11 years (mean 6.7) before surgery. Controls included the five untreated fellow eyes of PRK patients, 10 healthy, age matched volunteers without a history of ocular inflammation or contact lens wear, and 20 patients who had worn rigid gas permeable (n = 10) or soft contact lenses (n = 10) for 2-11 years. Subjects were examined with a real time flying slit, scanning confocal microscope using x25 and x50 objectives.

RESULTS

In PRK treated patients and contact lens wearers, basal layer epithelial cells sporadically displayed enhanced reflectivity. The subepithelial nerve plexus was observed in all individuals, but was usually less well contrasted in the PRK group, owing to the presence of a very discrete layer of subepithelial scar tissue, which patchily enhanced background reflectivity. Within all layers of the stroma, two distinct types of abnormal reflective bodies were observed in all PRK treated eyes, but in none of the controls. One had the appearance of long (> = 50 microns), slender (2-8 microns in diameter) dimly reflective rods, which sometimes contained bright, punctate, crystal-like inclusions, arranged linearly and at irregular intervals. The other was shorter (< 25 microns), more slender in form (< 1 micron in diameter), and highly reflective; these so called needles were composed of crystal-like granules in linear array, with an individual appearance similar to the bright punctate inclusions seen in rods, but densely packed. Both of these unusual structures were confined, laterally, to the ablated area, but were otherwise distributed throughout all stromal layers, with a clear predominance in the anterior ones. These rods and needles were observed in all PRK treated corneas, irrespective of previous contact lens wear. On the basis of qualitative inspection, the incidence of rods and needles did not appear to correlate with either the volume of tissue ablated or the length of the postoperative interval. In contact lens wearing controls, highly reflective granules, reminiscent of those from which the needles were composed, were found scattered as isolated entities throughout the entire depth and lateral extent of the corneal stroma, but rods and needles were never encountered. The corneal endothelium exhibited no obvious abnormalities.

CONCLUSION

Confocal microscopy 8-43 months after PRK revealed belated changes in the corneal stroma. These were manifested as two distinct types of abnormal reflective bodies, which had persisted beyond the stage when acute wound healing would have been expected to be complete. The clinical significance of these findings in the context of contrast visual acuity and long term status of the cornea is, as yet, unknown.

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.

Corneal topographic evaluation of decentration in photorefractive keratectomy: treatment displacement vs intraoperative drift

PURPOSE

To evaluate treatment displacement and movement during treatment (drift) after excimer laser photorefractive keratectomy using tangential topographic maps.

METHODS

Forty-eight eyes of 48 patients showing axial decentration of 0.30 mm or more at 1 month posttreatment were reevaluated retrospectively to determine treatment displacement of the center of the photorefractive keratectomy ablation from the center of the pupil. A drift index was calculated to determine the relative degree of movement (drift) during treatment. We subdivided patients into four groups based on the degree of treatment displacement and drift and compared the mean axial decentration and the mean best-corrected logMAR visual acuity among the subgroups.

RESULTS

Mean treatment displacement +/- SD from the center of the entrance pupil was 0.34 +/- 0.21 mm. Thirty-eight eyes (79.2%) had ablations within 0.50 mm from the center of the entrance pupil. We observed downward displacement in 27 eyes (56.2%) and upward displacement in 21 eyes (43.8%). The drift index showed a positive, statistically significant correlation with best-corrected visual acuity (r = .58, P < .0001). Patients with low displacement and low drift had mean logMAR best-corrected visual acuity of 0.91, which was statistically significantly better than patients with high displacement and high drift (r = 0.64; P = .009).

CONCLUSIONS

In patients with gross decentration by axial topography after photorefractive keratectomy, tangential corneal topography is valuable in evaluating and differentiating photorefractive keratectomy treatment displacement from movement during treatment (drift). Patients with high drift index have worse visual outcomes after photorefractive keratectomy than those exhibiting high treatment displacement.

Topographical analysis of ablation centration after excimer laser photorefractive keratectomy and laser in situ keratomileusis for high myopia

PURPOSE

To evaluate the ablation centration after photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK) for high myopia and to assess the association between decentration and best corrected visual acuity (BCVA), glare, monocular diplopia, and halo phenomenon.

SETTING

Mater Private Hospital, Dublin, Ireland.

METHODS

Corneal topography was used to analyze centration in two groups of patients with myopia of more than 6.0 diopters: 18 had PRK and 18, LASIK. A standardized questionnaire assessed the preoperative and postoperative prevalence of glare, monocular diplopia, and halo phenomenon.

RESULTS

"Significant" ablation decentration (0.5 mm) in the LASIK group (1.33 mm) was almost twice that in the PRK group (0.75 mm). Glare increased from 27% preoperatively to 42% in the PRK group; monocular diplopia increased in the LASIK group. Halo phenomenon decreased after both procedures.

CONCLUSION

Laser in situ keratomileusis represents a step forward in the surgical correction of high myopia, but the accuracy of the corneal ablation location must be improved. Suction ring fixation of the globe or real time tracking systems may help improve centration.

Laser in situ keratomileusis to correct high myopia

PURPOSE

To evaluate the effectiveness, predictability, and safety of laser in situ keratomileusis (LASIK) in 143 eyes with myopia from 8.00 to 20.00 diopters (D).

SETTING

Alicante Institute of Ophthalmology, University of Alicante School of Medicine, Alicante, Spain.

METHODS

This prospective study comprised 143 eyes (94 patients) that had LASIK with the Chiron Automated Corneal Shaper and the VISX 20/20 excimer laser using a multizone profile and a sutureless hinged corneal flap technique.

RESULTS

Uncorrected visual acuity of 20/40 or better in 45.0% of eyes 3 months postoperatively and in 46.4% at 6 months. Best corrected visual acuity (BCVA) improved by 0.07 at 3 and 6 months and was stable after 3 months. Mean spherical equivalent was -13.19 diopters (D) +/- 2.89 (SD) preoperatively and +0.51 +/- 1.63 D at 3 months and +0.18 +/- 1.66 D at 6 months postoperatively. At 3 months, spherical equivalent was within 1.00 D of emmetropia in 57.5% of all eyes, 71.0% of eyes with a baseline refraction from -8.00 to -11.99 D (n = 59), 44.4% with a baseline refraction from -12.00 to -15.99 D (n = 54), and 53.0% of eyes with a baseline refraction from -16.00 to -20.00 D (n = 30). The respective 6 month percentages were 60.0, 72.4, 46.0, and 50.0%. The regression of effect was similar in all groups (approximately 0.50 D) between 1 and 3 months, although the high myopia group had further regression. Significant corneal steepening and an increase in corneal thickness occurred between 1 and 3 months. Flap thickness was always less than predicted with both the 130 microns plates, and achieved laser ablation was deeper than programmed. The relationships between postoperative refraction and preoperative keratometry and postoperative refraction and the difference in achieved versus programmed ablation were significant. Complications at 6 months included epithelial ingrowth, corneal flap melting, decentered ablation, and irregular astigmatism with loss of BCVA, although none was vision threatening.

CONCLUSION

In this study, LASIK was effective and predictable in the correction of high myopia but was more accurate for myopia up to 12.00 D. Current surgical algorithms must be modified to improve predictability in higher corrections. Longer follow-up is necessary to evaluate long-term incidence of vision-threatening complications.

Ablation zone centration after photorefractive keratectomy and its effect on visual outcome

PURPOSE

To determine the relation between ablation zone decentration, measured by corneal topography, and visual and refractive outcome, contrast sensitivity, glare, and subjective reports of halos after photorefractive keratectomy (PRK).

SETTING

Eye Foundation, University of Missouri-Kansas City School of Medicine.

METHODS

This study comprised 70 eyes of 70 patients enrolled into Phase III of the clinical investigation of the VISX 20/20 excimer laser for the correction by PRK of 1.00 to 6.00 diopters of pre-existing sphere. All patients were treated with 5.0 mm ablation zones. Preoperative and postoperative corneal topography in 67 eyes was done using the EyeSys system. Ablation zone decentration was measured relative to the pupillary center at all postoperative visits as determined from the difference or change map.

RESULTS

Mean distance between the ablation zone center and the pupillary center was 0.62 mm +/- 0.34 (SD) (range 0.07 to 1.67 mm). Twenty seven of 66 patients with corneal topography (40.9%) had decentrations less than 0.5 mm; decentrations in 6 eyes (9.1%) were 1.0 mm or more from the pupillary center. No correlation was observed between the magnitude of decentration and 1 year postoperative best spectacle-corrected acuity (r = -.04, P = .76), uncorrected acuity (r = -.03, P = .84), or spherical equivalent (r = -.07, P = .59); preoperative uncorrected acuity (r = .04, P = .73) or spherical equivalent (r = -.02, P = .90); or reduction in spherical equivalent (r = .02, P = .89). The magnitude of decentration was correlated with preoperative spectacle-corrected acuity (r = -.37, P = .002). There was no difference in contrast sensitivity, glare, or halos between the cases with less than 0.5 mm of decentration and those with 0.5 mm or more of decentration.

CONCLUSIONS

The degree of ablation zone decentration did not affect postoperative Snellen visual acuity or contrast sensitivity.

Assessment of Ablation Zone Centration After Photorefractive Keratectomy Using a Vector Center of Mass Formula

BACKGROUND

Adequate centration of keratorefractive surgical procedures is essential for a successful outcome. An accurate technique to mathematically describe the centration and topography of the ablation zone after photorefractive keratectomy (PRK) would be valuable in assessing the effects of these variables on subsequent visual results.

METHODS

A vector center of mass formula and computerized videokeratography were used to study the postoperative treatment zone centration and topography of 17 consecutive highly myopic patients (-6.00 to 12.00 diopters [D]). Each had undergone PRK using either a single 6.0 mm (n = 11) or three-stepped ablation zone (n = 6), with good visual results.

RESULTS

Calculations disclosed mean ablation zone decentration relative to the pupil center for all patients to be 0.20 +/- 0.16 mm using the vector center of mass formula. Areas of uniform central corneal dioptric power (mean diameter 3.4 +/- 0.8 mm) and surrounding transition zones of declining dioptric power (mean slope 1.61 +/- 0.44 D/mm) were also determined.

CONCLUSION

A new vector center of dioptric power distribution that analyzes centration and transition zone topography offers a rigorous but straightforward means to assess the effects of refractive corneal surgery procedures on central corneal topography.

Decentration of photorefractive keratectomy ablation zones after excimer laser surgery for myopia

PURPOSE

To devise a method for measuring excimer laser photorefractive keratectomy (PRK) ablation zone decentration.

SETTING

Excimer Laser Clinic, Sunderland Eye Infirmary, Sunderland Tyne and Wear, England.

METHODS

Fifty-three eyes of 53 consecutive patients having PRK for a spherical myopic refractive error were studied. Preoperative and postoperative corneal topographic maps and differential subtraction maps were produced for each cornea. Using these maps, the distance between ablation zone centers and pupil centers was measured. All measurements were repeated by a second observer.

RESULTS

Mean ablation zone decentration was 0.46 mm from the pupil center; 69% of cases decentered 0.50 mm or less. The difference between the two independent measurements was 0.05 +/- 0.06 mm (mean +/- SD). A trend toward superonasal displacement was noted.

CONCLUSION

This simple method of measuring zone decentration can be used to compare the refractive results of PRK with those of other procedures.

Centration of Excimer Laser Photorefractive Keratectomy and Changes in Astigmatism

One of the most important factors that influence the success of PRK might be corneal centration during treatment. We evaluated our centration rate in 49 eyes by using computer-assisted corneal topography. Only single zone treated eyes were chosen for the analysis and 91.5% of the eyes were centered within a 1 mm treatment zone while 51% were within 0.5 mm. At the end of the follow-up time of 10.4 months, these centration zones were found to have no influence on the attempted spherical refractive outcome, whereas increasing decentration induced an increase in postoperative astigmatism.

Corneal topography of phase III excimer laser photorefractive keratectomy. Optical zone centration analysis. Summit Photorefractive Keratectomy Topography Study Group

PURPOSE

To determine the amount of optical zone decentration in patients who have undergone excimer laser photorefractive keratectomy (PRK) and assess associations of both decentration and optical zone size with clinical outcomes.

METHODS

Optical zone centration in 185 patients after PRK was analyzed using computer-assisted videokeratography. A comparison of decentration among the five study centers was performed. Associations of clinical outcomes with procedure decentration and optical zone size were assessed.

RESULTS

Decentration from the pupil center ranged from 0.00 to 1.44 mm (mean, 0.46 mm): 21.8% were decentered 0.25 mm or less, 42.5% > 0.25 mm and < or = 0.50 mm, and 97.1% < or = 1.00 mm. In both eyes, the average decentration from the pupil center was located inferonasally. Decentrations among the five study centers were significantly different. Decentration was associated with attempted refractive correction, change in keratometric cylinder, and patient satisfaction. There was a trend toward worse postoperative uncorrected visual acuity with greater decentration but no association with best-corrected vision, predictability, or refractive astigmatism. No significant relation was found between decentration and glare/halo ranking; however, three of six patients with 1.00 mm or greater of decentration demonstrated a high glare/halo grade. Although optical zone size was not associated with glare or halo, subjective patient satisfaction was greater with a 5.0-mm optical zone than with a 4.5-mm optical zone.

CONCLUSIONS

Centration is an important surgeon-controlled variable in excimer laser PRK. Decentration of excimer laser refractive procedures was found to be nonrandom and may be influenced by preoperative pupil management. Moreover, the amount of decentration may influence clinical outcomes. Improved techniques and centering procedures on nonmiotic pupils may improve future results.

The effect of globe fixation on ablation zone centration in photorefractive keratectomy

PURPOSE

For optimal vision, the ablation zone in photorefractive keratectomy should be centered over the entrance pupil. During ablation, the globe can be immobilized by the surgeon, with a suction ring around the corneoscleral limbus. Alternatively, the globe can be immobilized by patient fixation on a target, unassisted by the surgeon. We investigated which method results in better centration of the ablation zone over the entrance pupil, by using an objective, mathematical method to determine the ablation zone center.

METHODS

Forty-eight eyes from 48 patients who underwent photorefractive keratectomy by the two techniques were studied retrospectively. The centers of the ablation zones were objectively determined by a weighted center of mass algorithm applied to the preoperative minus postoperative difference maps. The validity of the objective method was confirmed by comparison to subjective estimates of ablation zone centers made by independent human observers.

RESULTS

The 19 eyes treated by surgeon fixation had an average decentration of the ablation zone of 0.63 +/- 0.31 mm (range, 0.01 to 1.00 mm), and the 29 eyes treated by patient fixation had an average decentration of 0.41 +/- 0.23 mm (range, 0.11 to 1.18 mm) (P = .027).

CONCLUSIONS

The center of the ablation zone can be determined mathematically from the topographic map, to avoid observer bias. In this study, unassisted patient fixation during photorefractive keratectomy produced more accurate centration of the ablation zone than did surgeon fixation and has the potential for maximizing the quality of vision postoperatively.

Relation of visual symptoms to topographic ablation zone decentration after excimer laser photorefractive keratectomy

BACKGROUND

The authors used computer-assisted videokeratoscopy to analyze the relation of photorefractive keratectomy ablation zone decentration to subjective patient assessments of disturbing visual symptoms.

METHODS

Ablation zone decentration was measured 1 month postoperatively. The study population was divided into two groups: group 1, patients whose ablation zone decentrations were less than 0.50 mm; group 2, patients whose ablation zone decentrations were greater than 0.50 mm. Visual symptoms including glare, rings or halos around lights and problems with night driving were scored preoperatively and 6 months postoperatively. The Hotelling T-square and chi-square tests were used.

RESULTS

The mean decentration from the center of the ablation zone to the pupillary center was 0.30 mm and 190 degrees for group 1 compared with 0.66 mm and 198 degrees for group 2. The Hotelling T-square test showed a significant statistical preoperative/postoperative difference in group 1 (P < 0.03) for the halo symptom category. No other symptom category showed a significant statistical difference in either group for the mean scores. The Hotelling T-square test did not show a statistically significant difference between the two groups preoperatively to postoperatively regarding the mean scores of the individual patient differences for the three symptoms. The only significant statistical difference for the individual patient ratings preoperatively to postoperatively was for the halo symptom category (chi-square = 7.756; P < 0.03).

CONCLUSIONS

Multivariate analysis did not show a significant statistical difference preoperatively between the two groups or postoperatively except for group 1 with regard to the halo symptom category. It appears from this study that ablation zone decentrations less than 0.89 mm from the pupillary center do not necessarily produce unwanted visual symptoms 6 months postoperatively.

Topographical evaluation of centration of excimer laser myopic photorefractive keratectomy

We evaluated the centration of excimer laser myopic photorefractive keratectomy by analyzing the corneal topography in 60 eyes of 37 patients. Computer-assisted analysis of corneal topography was performed preoperatively and one week postoperatively. The distance and the direction from the center of the entrance pupil to the center of the ablation was calculated using a personal computer. Downward displacement of the ablation relative to the center of the entrance pupil was observed in 51 eyes (85%). The mean distance of the center of the ablation from the center of the entrance pupil was 0.51 +/- 0.31 mm (range 0.06 mm to 1.64 mm). To achieve more precise centration, we recommend applying an eye-movement tracking system to photorefractive keratectomy.

Results one year after using the 193-nm excimer laser for photorefractive keratectomy in mild to moderate myopia

As part of a clinical trial, photorefractive keratectomy using the VISX 2015 193-nm excimer laser was performed on 91 healthy eyes of 91 patients. Preoperative refractive errors (spherical equivalent) ranged from -1.00 to -7.50 diopters (mean, -4.16 +/- 1.41 diopters). No patient had more than 1 diopter of refractive astigmatism. Six months postoperatively, the average residual refractive error was +0.09 +/- 0.63 diopters (range, -2.13 to +1.63 diopters). Correction within 1 diopter of that attempted was attained in 85 eyes (93%). Uncorrected visual acuity of 20/40 or better was attained in 86 eyes (95%) and was 20/25 or better in 67 eyes (74%). At one year, follow-up information was available on 85 eyes of 85 patients. The average residual refractive error was -0.15 +/- 0.65 diopters (range, -2.50 to +1.63 diopters). Correction within 1 diopter of that attempted was attained in 85 eyes (93%). Uncorrected visual acuity was 20/40 or better in 83 eyes (98%) and was 20/25 or better in 68 eyes (80%). One patient lost three lines of best-corrected visual acuity because of corneal haze, dropping from 20/15 to 20/30, whereas all other patients returned to best-corrected visual acuity within one line of their preoperative best-corrected visual acuity. Photorefractive keratectomy with the 193-nm excimer laser appears to be a useful treatment modality for the reduction of mild to moderate myopia.

193-nm excimer photorefractive keratectomy in high myopia

PURPOSE

To evaluate the refractive results of 193-nm excimer laser photorefractive keratectomy (PRK) performed on 48 highly myopic eyes in a multicenter study.

METHODS

A Visx 2015 or 2000 argon-fluoride excimer laser and a single-zone ablation technique were used. Postoperatively, eyes were treated with topical fluoromethalone for up to 5 months. Most eyes were treated with a 6.0- to 6.2-mm beam diameter after undercorrections and increased regression were noted with a 5.5-mm beam in earlier studies. Forty-eight eyes were treated for myopia, which was between -8.0 and -15.25 diopters (D) (spherical equivalent). The mean preoperative refraction was -11.2 D. Retreatment was performed after 6 to 16 months on 11 eyes for undercorrection. All eyes not retreated were followed for at least 12 months.

RESULTS

At 6 months, follow-up was available on 47 eyes. Of these eyes, 40% and 64% achieved corrections within 1 and 2 D of attempted correction, respectively. At 1 year, 60% of eyes attained 20/40 visual acuity or better uncorrected. Eleven patients (23%) were retreated between 6 to 16 months for undercorrection and/or regression. After retreatment, 47% and 81% of eyes achieved corrections within 1 and 2 D of attempted correction, respectively. At 1 year, 15% of eyes lost two lines of best-corrected visual acuity, and no eyes lost more than two lines. There was slightly more corneal haze seen in this group compared with the haze seen in patients undergoing PRK for low and moderate myopia.

CONCLUSIONS

These data show that excimer PRK can correct high amounts of myopia with reasonable stability after 6 months. Excimer PRK is an effective surgical treatment of severe myopia, but long-term follow-up is still needed to assess the stability of its effect.

Corneal topographic analysis after excimer photorefractive keratectomy

BACKGROUND

A 2-year experience with corneal topography after photorefractive keratectomy (PRK) is reported, specifically reviewing the correlation of surface regularity index versus best spectacle-corrected visual acuity as a function of various ablation patterns. Centration and stability issues also are discussed.

METHODS

Excimer laser PRK for myopia was performed on 502 consecutive eyes. Corneal topographic analysis was performed at the 1-, 3-, 6-, and 12-month postoperative follow-up visits. In all patients, PRK was performed with an argon fluoride laser, and a topographic analysis was done.

RESULTS

Topographic analysis at the first postoperative month was useful for determining decentration of the optical zone. The mean decentration from the pupillary center for all eyes was 0.34 mm. Four main patterns of ablation were noted by subtraction analysis. At the 1-month postoperative examination, a "uniform" ablation was present in 44% of eyes, "keyhole" ablations were present in 12% of eyes, "semicircular" ablations were present in 18% of eyes, and an unusual "central island" was present in the remaining 26% of eyes. However, on subsequent follow-up the central islands tended to resolve with time. At 3 months postoperatively, 18% of eyes in that group showed a central island, 8% showed a central island at 6 months, and only 2% of eyes showed a central island at the 12-month postoperative visit. Correlation of central island topography with visual acuity and progressive hyperopia are discussed.

CONCLUSIONS

Corneal topography is essential for evaluating surface changes after excimer PRK. The surface regularity index is a good indicator of best spectacle-corrected visual acuity and is used to evaluate irregular astigmatism after PRK. Central island topographies are correlated with poor initial visual rehabilitation. Long-term stability issues are answered with continued topographic follow-up. Understanding corneal hydration changes between the central and peripheral cornea may help us understand the etiology of central islands.