Corneal topographic analysis after excimer photorefractive keratectomy

Steep central islands after myopic photorefractive keratectomy

PURPOSE

To investigate the formation of steep central islands and their reduction under modified experimental conditions.

SETTING

University Eye Hospital Münster and Schwind Co., Kleinostheim, Germany.

METHODS

Corneas of enucleated intact bovine eyes were treated with the Schwind Keratom. All experimental conditions were repeated six times in six different corneas. Eight experimental groups were looked at. Fluence was 180 to 200 mJ/cm2. Ablation mode (phototherapeutic keratectomy ([PTK] and standard myopic photorefractive keratectomy [PRK]), internal repetition rate (3 to 30 Hz), and ablation diameter (5 to 8 mm) and depth (4 to 15 diopters [D] in PRK) were varied. Modifications to reduce or avoid steep central islands included blowing nitrogen gas and aerosol over the cornea, cleaning the cornea of fluid, and using an anti-central-island software program.

RESULTS

In PTK, an increase in the internal repetition rate resulted in a decrease in the height of the steep central island. In standard PRK, increasing refractive correction up to -8.0 D and increasing the ablation diameter resulted in an increase in steep central island power. The anti-central-island program, blowing aerosol, and cleaning the cornea reduced the formation of steep central islands and blowing nitrogen gas eliminated them.

CONCLUSION

Steep central islands are created by a wide-field ablation process and are probably caused by many factors. Both software and hardware modifications can be used to reduce their formation.

Treatment of topographic central islands following refractive surgery

PURPOSE

To evaluate the safety and efficacy of using central reablation to treat topographic central islands following photorefractive keratectomy (PRK), myopic keratomileusis in situ, and laser in situ keratomileusis (LASIK).

SETTING

Department of Ophthalmology, Stanford University School of Medicine, Stanford, and Jules Stein Eye Institute, Los Angeles, California, USA.

METHODS

Central reablation was performed on eight eyes with clinically significant topographic central islands after refractive surgery. Two eyes developed central islands after PRK, five eyes after LASIK, and one eye after myopic keratomileusis in situ. A clinically significant topographic central island was defined as an area of steepening of at least 3.0 diopters by at least 1.5 mm in diameter documented by computerized videokeratography. Reablation was tailored to each eye based on the diameter and power of the topographic central island using the Munnerlyn formula.

RESULTS

All eyes experienced a reduction or elimination of the topographic central islands following central reablation. Six eyes experienced an improvement in uncorrected visual acuity, and all eyes returned to within one line of their preoperative level of best spectacle-corrected visual acuity 1 month after the procedure.

CONCLUSION

Topographic central islands following PRK, myopic keratomileusis in situ, and LASIK can be effectively treated using the excimer laser. Poor predictability of the refractive effect of central reablation may be the limitation of this treatment modality.

A Scanning and Rotating Slit ArF Excimer Laser Delivery System for Refractive Surgery

PURPOSE

This study was designed to investigate the quality of a scanning and rotating slit delivery system of an ArF excimer laser (Nidek EC-5000).

METHODS

The ablation patterns on polymethylmethacrylate (PMMA) wafers were examined by scanning electron microscopy. The influence of inhomogeneities in the beam profile was simulated on a computer and compared with a conventional large-area ablation system. The impairment of the ablation rate by radiation absorption of the ablation plume was measured as a function of the repetition rate and the application of a fixation ring.

RESULTS

The scanning and rotating slit delivery system is tolerant of small-beam non-homogeneities. The ablation rate is sensitive to the dynamics of the ablation plume.

CONCLUSIONS

Although the operating procedure takes less time with a large-area ablation system, a scanning and rotating delivery system has the advantage of reliable and homogeneous removal of corneal tissue.

Topographic detection of photorefractive keratectomy

PURPOSE

This study aimed to evaluate the sensitivity and specificity of subjective review of corneal topography to detect patients who have undergone photorefractive keratectomy (PRK).

METHODS

Topographic maps from 3 different devices were obtained from 19 patients with postoperative PRK and 9 control subjects with emmetropia and 10 control subjects with myopia. Each image was printed in an absolute and relative scale (total of 228 maps) and graded for overall shape and pattern. Fifteen masked reviewers independently rated each map as either postoperative PRK or not.

RESULTS

The overall sensitivity (ability to detect PRK) and specificity rates (ability to exclude control subjects) by reviewers were 65% and 93%, respectively. Sensitivity was influenced independently by the scale (relative, 68%; absolute, 62%; P < 0.01), experience of reviewer (experienced, 77%; inexperienced, 53%; P < 0.001), and device (Alcon, 67 +/- 29.9; Eyesys, 75 +/- 29.4%; and Tomey, 54 +/- 31.7%; P < 0.001). Low levels of preoperative myopia were consistently more difficult to detect than higher levels (low myopia -1.50 to -2.99 diopters [D] sensitivity: 53 +/- 34.5%; medium level -3.00 to -4.49 D: 67 +/- 28.9%; and high level -4.50 to -6.00 D: 77 +/- 21.1%; P < 0.0001). Differences in specificity between experienced and inexperienced reviewers were obtained when maps had a homogeneous topographic pattern (97 +/- 5.6% and 85 +/- 13.7%, respectively; P < 0.05). Several control topography patterns (e.g., homogeneous, focal, and keyhole) were disproportionately more difficult to correctly identify on the Eyesys device.

CONCLUSIONS

Topographic experience is a significant factor influencing the correct identification of PRK. Techniques also can be used to enhance detection, such as the use of different devices and scales. However, if subjective review of topography is used as the only method of detection, many patients with PRK will not be identified properly. In addition, the most prevalent preoperative myopic category in the general population (myopia < -3.00 D) also is the most difficult to detect after treatment. This reduces the usefulness of topography as a screening tool. Other techniques are needed to improve the detection of patients with postoperative PRK.

Analysis of videokeratography after penetrating keratoplasty: topographic characteristics and effects of removing running sutures

OBJECTIVE

Previous studies have shown that removal of running sutures after penetrating keratoplasty causes unpredictable changes in astigmatism. The current study was conducted to investigate whether computer-assisted videokeratography is beneficial for predicting visual outcomes after running sutures are removed.

DESIGN

The design was that of a prospective clinical study.

PARTICIPANTS

The authors prospectively studied 29 consecutive eyes undergoing a 10-0 nylon running suture removal after penetrating keratoplasty.

INTERVENTIONS

Videokeratography was performed before, 1 week, 1 month, and 3 months after removal of sutures.

MAIN OUTCOME MEASURES

Changes in refractive and topographic astigmatism after suture removal were measured. Topographic patterns and their quantitative descriptors also were analyzed.

RESULTS

An asymmetric bowtie was the most common videokeratography pattern both before and after suture removal. After suture removal, the incidence of peripheral corneal steepening increased significantly (2 vs. 21 eyes, P < 0.0001), and that of focal flattening of the midperipheral cornea decreased (13 vs. 5 eyes, P = 0.046). The mean topographic astigmatism, surface regularity index, and corrected visual acuity were improved significantly by suture removal in eyes that had localized flattening but not in eyes without this finding. Eyes having either skewed axis in astigmatism or topographic astigmatism of more than 9 diopters also showed significant decreases in astigmatism.

CONCLUSIONS

Suture removal after keratoplasty is advantageous for both reducing astigmatism and normalizing topography, especially in eyes that have localized flattening of the midperipheral cornea. Predictability of visual outcomes of a running suture removal in postkeratoplasty eyes may be improved by the use of videokeratography.

PMMA model of steep central islands induced by excimer laser photorefractive keratectomy

We designed a polymethyl methacrylate (PMMA) model of refractive steep central islands (CIs) induced by PRK. A standardized photorefractive ablation procedure was performed using seven different excimer lasers on test PMMA specimens including 70 flat plates and 98 convex contact lenses. The resulting surface was analyzed by high-resolution confocal microscopy and computerized videokeratoscopy using both TMS-1 and CAS-2000 systems. A total of 50 (54.9%) CIs were observed using computerized videokeratoscopy. The rate of occurrence of CIs was significantly reduced by shock wave absorption (P = 0.0001), aspiration of fumes (P = 0.0044), and smaller diameter ablation (P = 0.0296). The diameter of the CIIs was significantly increased for broad-beam mode ablation (P = 0.016) and for larger ablation zones, (P = 0.042). The refractive power of CIs was significantly increased in the absence of a shock wave absorption system (P = 0.001). Only 20 (40%) of the CIs detected by the TMS-1 device were identified on CAS-2000 at a 0.5 diopter (D) scale resolution level. Shock wave induced deformation and subsequent dynamic alteration of convection forces applied to emitted particles may be the primary mechanism underlying the formation of CIs after PRK, regardless of any biological response of the ablated tissue. Reported rates and characteristics of CIs may largely depend upon the specific design of videokeratoscopes.

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.

Comparison of Laser in situ Keratomileusis and Photorefractive Keratectomy to Correct Myopia from -1.25 to -6.00 Diopters

BACKGROUND

We evaluated the safety and efficacy of laser in situ keratomileusis (LASIK) for the correction of low to moderate amounts of myopia (-1.25 to -6.00 D).

METHODS

Photorefractive keratectomy (PRK) was performed on 432 eyes and LASIK on 137 eyes with a Chiron Keracor 116 excimer laser. Uncorrected and corrected visual acuity, corneal sensitivity, contrast sensitivity, and corneal topography were examined before and after surgery.

RESULTS

One-year follow-up of 307 eyes in the PRK group and 103 eyes in the LASIK group was achieved. At 1 year, 83% (85 of 103) of LASIK eyes and 72% (221 of 307) of PRK eyes had an uncorrected visual acuity of 1.0 or better. Eighty-nine percent (92 of 103) of LASIK eyes and 83% (255 of 307) of PRK eyes had a refractive error within +/- 1.00 D of emmetropia; 71% (73 of 103) of LASIK eyes and 61% (188 of 307) of PRK eyes were within +/- 0.50 D of emmetropia. Contrast sensitivity and corneal sensitivity were reduced in both groups at the early postoperative stage but gradually returned to preoperative values; their recovery took about 3 months in LASIK eyes and 6 to 12 months in PRK eyes.

CONCLUSION

LASIK is safe and more predictable than PRK to correct low to moderate amounts of myopia. Recovery from LASIK is faster than after PRK.

Factors affecting epithelial hyperplasia after photorefractive keratectomy

PURPOSE

To determine the effect of patient age, postoperative time, ablation zone diameter and depth, attempted correction, and corneal topography on postoperative corneal epithelial thickness after photorefractive keratectomy (PRK).

SETTING

Private clinic and university hospital, Stockholm, Sweden.

METHODS

This retrospective, unmasked study comprised 136 myopic patients treated unilaterally with PRK. Seventy eyes had been treated with the Summit excimer laser 27 months +/- 7 (SD) earlier using ablation zone diameters of 4.1 to 5.0 mm. Sixty-six eyes had been treated with the VISX excimer laser 6 +/- 3 months earlier using a 6.0 mm zone diameter. The untreated fellow eyes served as controls. Epithelial thickness was measured at a standardized central corneal area with a modified optical pachymeter, and corneal topography was determined using computerized videokeratoscopy.

RESULTS

In the Summit group, the epithelial layer in the PRK eyes was 12.0 microns (21%) thicker than in the control eyes (P < .001; 95% confidence interval [CI] 9.35 to 14.3 microns). This thickness differential correlated significantly with increased ablation depth and attempted correction. In the VISX group, the epithelium in the treated eyes was 7.0 microns (7%) thinner (P = .0009; 95% CI -1.9 to -6.7 microns) and thickness did not correlate with ablation depth or attempted correction. There was no correlation between epithelial hyperplasia and patient age or postoperative follow-up. With the laser groups combined, epithelial hyperplasia was greater with smaller zone sizes and a greater rate of change in power at the edge of the ablation zone.

CONCLUSION

The factors associated with an increase in epithelial thickness were small ablation zones, greater attempted corrections, and deeper ablations. Larger, smoother ablation profiles may result in less epithelial hyperplasia.

Influence of ablation plume dynamics on the formation of central islands in excimer laser photorefractive keratectomy

PURPOSE

The aim of this study was to understand the dynamics of ablation products during excimer laser photorefractive keratectomy, and their influence on the formation of central islands.

METHOD

Laser flash photography was used to investigate the dynamics of ablation products during photorefractive keratectomy. The ablation plume over polymethyl methacrylate (PMMA) and porcine cornea targets ablated with different zone diameters was imaged at various times between 10 musec and 100 msec after the ablating laser pulse. On PMMA targets, the profiles of the resulting ablation craters were measured.

RESULTS

In all cases, the ablation products formed a ring vortex. The plume velocities on cornea were found to be approximately twice as fast as on PMMA for all zone diameters. For both materials, the ablation plume evolves faster for smaller zone diameters. Central islands were observed for zone diameters between 1 and 7 mm on PMMA substrates. The islands became more pronounced with increasing zone diameter.

CONCLUSIONS

A major cause for the formation of central islands was found to be particle redeposition at the center of the ablation zone. Because of the vortex dynamics, redeposition of particles favorably occurs at the center of the ablation zone. Additionally, the dynamics of the ablation plume lead to a concentration of airborne particles over the center of the ablation zone, which also may contribute to the creation of central islands by partial absorption of the next excimer laser pulse.

Myopic photorefractive keratectomy in eyes with atypical inferior corneal steepening

PURPOSE

To evaluate the visual and refractive results of photorefractive keratectomy (PRK) in eyes with atypical inferior corneal steepening (AICS).

SETTING

Department of Ophthalmology, Hôpital Morvan, University of Breast, France.

METHODS

Using videokeratopography, we screened 310 eyes that had PRK from November 1992 through November 1993 and found that 35 eyes exhibited topographic patterns consistent with AICS with no clinical findings. The results at 6 months and 1 year were compared with those of 185 eyes with normal topography treated concurrently.

RESULTS

There were no statistically significant differences between the two groups in mean spherical equivalent, mean uncorrected visual acuity, and mean best spectacle-corrected visual acuity 6 months and 1 year after PRK.

CONCLUSION

After 1 year, PRK in eyes with AICS appeared to give results similar to those in eyes with normal topography. Further follow-up is needed.

Natural history of central topographic islands following excimer laser photorefractive keratectomy

PURPOSE

To assess the incidence and natural history of central corneal topographic islands following excimer laser photorefractive keratectomy (PRK) and photoastigmatic refractive keratectomy (PARK).

SETTING

A dedicated corneal diseases and refractive surgery unit within a National Health Service Trust ophthalmology unit in the United Kingdom.

METHODS

Corneal topographic analysis was performed in a prospective study of 100 eyes of 75 consecutive patients who had PRK. All PRK/PARK excimer laser photorefractive procedures were performed by two surgeons observing a standardized protocol using a VISX 20/20 excimer laser. Mean preoperative myopic error was 5.54 diopters (D) +/- 3.44 (SD). Corneal topographic analysis was performed on all eyes preoperatively, 1 week postoperatively, and monthly thereafter for a minimum of 6 months or until central islands, if present, resolved. All patients had a minimum 12 months follow-up.

RESULTS

Postoperatively, 29 eyes (29%) demonstrated central corneal topographic islands of greater than 3.00 D topographic power by computerized videokeratography (CVK). All central islands were identified in the first 4 weeks postoperatively. In all cases the differential dioptric power, created by the central islands within the ablation zone, decreased rapidly; within 6 months, 26 (90%) central islands had fully resolved without further treatment, and the remaining 3 (10%) resolved within 1 year of photorefractive surgery. The occurrence of central islands was related to higher preoperative myopic spherical equivalent (P = .01), greater attempted laser correction (P = .01), and greater projected depth of ablation (P = .01) (Student's two-tailed t-test).

CONCLUSIONS

Central corneal topographic islands occurred in a significantly higher proportion of eyes having excimer laser photorefractive surgery than previously believed. The islands were associated with decreased unaided vision, reduced best spectacle-corrected acuity, and other troublesome visual symptoms; however, the central islands, along with their associated visual effects, usually resolved without surgical intervention within 6 months postoperatively.

Gaussian optics for photorefractive keratectomy. Increased predictability for larger diopter corrections

PURPOSE

To evaluate clinically a formula based on spherical Gaussian optics for enhanced predictability of excimer photorefractive keratectomy (PRK) in both high and low degrees of myopia.

METHOD

Patients (24 eyes) were evaluated preoperatively with refractive measurements and corneal topography. After excimer PRK (3-24 months), postoperative remaining spectacle correction and corneal topography were measured. The attempted laser correction, calculated using algebraic equations, was compared with the achieved laser correction.

RESULTS

The absolute laser correction error (mean +/- standard deviation), defined as the difference between the achieved and attempted laser corrections, was 0.37 +/- 0.31 diopter (D) for the 24 eyes. No significant difference was found in the error for attempted corrections greater than 6 D versus those less than 6 D.

CONCLUSION

The use of a spherical Gaussian optics formulation results in accurate laser correction values for a wide range of myopia, even for large diopter corrections.

Management and Treatment of Central Steep Islands after Excimer Laser Photorefractive Keratectomy

BACKGROUND

Permanent central steep islands are an undesirable phenomenon that cause distorted images and a significant reduction in visual acuity. We describe treatment of central steep islands with repeat excimer laser photoablation in the central cornea.

METHODS

Three patients with preoperative refractions of -7.50 -2.50 x 170 degrees D (right eye), -8.00 -2.25 x 10 degrees D (right eye) and -6.00 -1.50 x 90 degrees (right eye) developed central steep islands which persisted more than 12 months. All patients lost more than two lines of spectacle-corrected visual acuity and complained of visual disturbances. We retreated the central steep islands with a VISX 20/20 excimer laser PRK ablation that matched the size of the central island measured on videokeratography.

RESULTS

Symptomatic glare and distortion were significantly reduced and the central steep islands were resolved. Several weeks after reablation, spectacle-corrected visual acuity improved to the preoperative level.

CONCLUSION

Central steep island, an infrequent complication of excimer laser photorefractive keratectomy, can be safely removed with a repeat laser ablation that matches the central circular steep area.

Excimer laser photorefractive keratectomy for patients with contact lens intolerance caused by dry eye

AIMS/BACKGROUND

To evaluate epithelial wound healing and visual outcome of excimer laser photorefractive keratectomy (PRK) performed on high myopic eyes with contact lens intolerance due to dry eye.

METHODS

PRK was performed on two groups of patients with non-Sjogren's dry eye: group A (-6 D to -9.5 D, 11 patients, 17 eyes) and group B (-11.5 D to -19.5 D, 11 patients, 16 eyes) in an attempt to eliminate the use of contact lenses (CL). The intended correction was full in group A and 10 D in group B.

RESULTS

Uncorrected visual acuity in group A was better than 20/40 in 12 (80.0%) of 15 eyes at 6 months and in 10 (90.9%) of 11 eyes at 1 year. Fourteen (92.8%) of 17 eyes in group A and four (25.0%) of 16 eyes in group B achieved refraction within plus or minus 1 D of the intended correction at 6 months. Re-epithelialisation was complete in 4 days, and epithelial cell area and permeability returned to the preoperative level within 1 month in all cases. All patients in group A were able to eliminate CL, whereas in group B, one patient needed spectacles for residual myopia and two patients resumed CL use because of regression. One eye with severe subepithelial scar formation and one eye with macular haemorrhage were observed in group B.

CONCLUSION

Our results suggest that PRK is effective for patients with high myopia (-6 D to approximately -10 D) and CL intolerance due to dry eye. Further studies are required to improve predictability and to prevent complications in PRK for very high myopia (> -10 D).

Refractive outcome and corneal topographic studies after photorefractive keratectomy with different-sized ablation zones

BACKGROUND

Discrepancies may still occur between planned and actual refractive correction in eyes undergoing photorefractive keratectomy (PRK). The authors have evaluated the use of an enlarged ablation zone.

METHOD

A computerized corneal analysis system has been used to compare the changes of the anterior surface of the cornea and the refractive changes before and 1, 6, and 12 months after PRK in 113 patients (119 eyes) treated with an excimer laser. The patients were divided into two groups: those treated with a mask with a 5-mm window (59 eyes), and those with a new mask with different window openings according to the degree of refraction at the corneal apex, starting from 5 mm in diameter for treatments less than 6.5 diopters (D) and from 7 mm in diameter for higher treatments (60 eyes). In the first group, treatment ranged from -2.5 to -16 D (mean +/- standard deviation. -8.5 +/- 3.24 D); in the second group, it ranged from -1 to -14 D (-7.8 +/- 3.06 D). Treatments were evaluated with a chi-square test.

RESULTS

In the first group of eyes, 46% were within +/-1 D at 1 month, 37% at 6 months, and 39% at 12 months. In the second group of eyes, 73% were within +/-1 D at 1 month, 60% at 6 months, and 58% at 12 months. The comparison between these data and corneal topographic changes shows that both are more stable and predictable with the new mask compared with the 5-mm mask (P = 0.002, 0.02, 0.04, at 1, 6, and 12 months, respectively).

CONCLUSIONS

The use of larger ablation zones improves the predictability and stability of refractive changes.

Transient Central Corneal Steepening After Radial Keratotomy

Three case studies are presented illustrating topographic transient central steep island following radial keratotomy. Three stages are apparent: first, a central corneal steepening; next, an overcorrection with exaggerated central flattening; and, finally, the final refractive correction. The steepening is probably caused by temporary midperipheral swelling of the cornea.

Corneal topographic changes after noncontact holmium:YAG laser thermal keratoplasty to correct hyperopia

PURPOSE

To analyze the changes in corneal curvature induced by laser thermal keratoplasty (LTK) to correct hyperopia.

SETTING

Cullen Eye Institute, Baylor College of Medicine, Houston, Texas.

METHODS

We performed LTK on nine hyperopic eyes using a noncontact holmium: YAG (Ho:YAG) laser. Five eyes received a single ring of eight spots at the 6 mm zone (Group A); four received a second ring of eight at the 7mm zone (Group B). Computerized videokeratography (CVK) was obtained preoperatively and 1, 30, 90, 180, and 360 days postoperatively. We calculated the net dioptric changes in the following CVK values: corneal curvature at the 1, 3, 5, and 7 mm zones; effective corneal refractive power (Eff RP); and spherical equivalent of subjective manifest refraction (SE SMR). We classified difference maps according to the pattern of induced change.

RESULTS

At 1 year, steepening at the 1, 3, 5, and 7 mm CVK zones was 0.5 diopter (D), 0.6 D, 0.1 D, and -0.42 D, respectively, in Group A and 1.5 D, 1.5 D, 1.1 D, and 0.54 D, respectively, in Group B. Effective corneal refractive power increased 0.6 D in Group A and 1.5 D in Group B. Mean change in SE SMR was -0.6 D in Group A and -1.4 D in Group B. Most regression occurred during the first 90 days. Difference maps showed five bow-tie, two irregularly irregular, one semicircular, and one homogeneous patterns.

CONCLUSIONS

Noncontact Ho:YAG LTK produced peripheral corneal flattening and central corneal steepening. A greater change in curvature was produced using a two-ring treatment.

Comparison of results of excimer laser correction of all degrees of myopia at 12 months postoperatively. The Melbourne Excimer Laser Group

To examine prospectively the efficacy and safety of photorefractive keratectomy and photoastigmatic refractive keratectomy.

METHODS

We treated 645 eyes (440 patients) with a VisX Twenty/Twenty excimer laser and followed them up for 12 months.

RESULTS

The percentage of eyes with myopia between -5.01 and -10.00 diopters spherical equivalent within 1 and 2 diopters of emmetropia at 12 months was 65% (123 of 189) and 90% (170 of 189), respectively, whereas the corresponding percentages for eyes with myopia greater than -10.00 diopters spherical equivalent were 39% (16 of 41) and 56% (23 of 41), and for eyes with myopia of -5.00 diopters spherical equivalent or less, 87% (238 of 273) and 99% (270 of 273), respectively. Uncorrected visual acuity of 20/20 and 20/40 or better at 12 months was attained in 47% (129 of 273) and 87% (237 of 273) of the eyes with myopia -5.00 diopters spherical equivalent or less, respectively. At 12 months, 48 (25%) of the 189 eyes with myopia between -5.01 and -10.00 diopters spherical equivalent had uncorrected visual acuity of 6/6 or better and 135 (71%), 6/12. At 12 months, one eye (2%) with myopia greater than -10.00 diopters spherical equivalent had uncorrected visual acuity of 6/6 and 11 (27%) of 41 eyes, 6/12. Ten (4%) of the 273 eyes with myopia of -5.00 diopters spherical equivalent or less, 15 (8%) of the 189 eyes with myopia between -5.01 and -10.00 diopters spherical equivalent, and nine (22%) of the 41 eyes with myopia greater than -10.00 diopters spherical equivalent had lost two or more LogMAR lines of best-corrected visual acuity at 12 months.

CONCLUSION

Excimer laser surgery is highly reliable for myopia of -5.00 diopters spherical equivalent or less and is less reliable for greater myopia.

Excimer Laser Calibration System

BACKGROUND

Excimer laser photoablation for refractive and therapeutic keratectomies has been demonstrated to be feasible and practicable. However, corneal laser ablations are not without problems, including the delivery and maintenance of a homogeneous beam. We have developed an excimer laser calibration system capable of characterizing a laser ablation profile.

METHODS

Beam homogeneity is determined by the analysis of a polymethylmethacrylate (PMMA)-based thin-film using video capture and image processing. The ablation profile is presented as a color-coded map. Interpolation of excimer calibration system analysis provides a three-dimensional representation of elevation profiles that correlates with two-dimensional scanning profilometry. Excimer calibration analysis was performed before treating a monkey undergoing phototherapeutic keratectomy and two human subjects undergoing myopic spherocylindrical photorefractive keratectomy. Excimer calibration analysis was performed before and after laser refurbishing.

RESULTS

Laser ablation profiles in PMMA are resolved by the excimer calibration system to .006 microns/pulse. Correlations with ablative patterns in a monkey cornea were demonstrated with preoperative and postoperative keratometry using corneal topography, and two human subjects using video-keratography. Excimer calibration analysis predicted a central-steep-island ablative pattern with the VISX Twenty/Twenty laser, which was confirmed by corneal topography immediately postoperatively and at 1 week after reepithelialization in the monkey. Predicted central steep islands in the two human subjects were confirmed by video-keratography at 1 week and at 1 month. Subsequent technical refurbishing of the laser resulted in a beam with an overall increased ablation rate measured as microns/pulse with a donut ablation profile. A patient treated after repair of the laser electrodes demonstrated no central island.

CONCLUSIONS

This excimer laser calibration system can precisely detect laser-beam ablation profiles. The calibration system correctly predicted central islands after excimer photoablation in a treated monkey cornea and in two treated human subjects. Detection of excimer-laser-beam ablation profiles may be useful for precise calibration of excimer lasers before human photorefractive and therapeutic surgery.

Comparison of 5-mm and 6-mm Ablation Zones in Photorefractive Keratectomy for Myopia

BACKGROUND

Variation in ablation zone diameter may alter visual acuity and/or refractive effect in photorefractive keratectomy. Despite theoretical benefits of using a smaller diameter ablation zone, clinical studies suggest that a larger ablation zone may decrease problems associated with photorefractive keratectomy.

METHODS

The results of our initial 34 consecutive eyes treated with a 5-mm diameter ablation zone using a Summit Technology ExciMed UV200LA excimer laser were compared retrospectively to our initial 34 consecutive eyes treated with a 6-mm diameter ablation zone using a Summit OmniMed excimer laser. Eyes had a spherical equivalent refraction between -1.00 and -6.00 diopters (D) and astigmatism less than 1.00 D. Patients were followed for a minimum of 6 months.

RESULTS

Eyes treated with a 6-mm ablation zone had less hyperopia and a spherical equivalent refraction closer to emmetropia at 1, 2, and 3 months (P = 0.001). Eyes treated with a 6-mm ablation zone had better uncorrected visual acuity at 1 and 2 months (P = 0.001). Less subepithelial haze was noted at 2 months (P = 0.01) and 3 months (P = 0.002) in the 6-mm group. At 6 months postoperatively, 30 of 32 eyes (94%) treated with a 6-mm ablation zone had a spherical equivalent refraction within 0.50 D of emmetropia, and all 32 eyes (100%) were within 1.00 D of emmetropia; in the 5-mm ablation zone group, 28 of 34 eyes (80%) were within 0.50 D and 29 (85%) were within 1.00 D of emmetropia. Patients treated with a 6-mm ablation zone complained less of night halos and had fewer differences between night and day vision.

CONCLUSIONS

In this study of myopia of -1.00 D to -6.00 D, eyes treated with a 6-mm ablation zone achieve a more rapid visual recovery with less variation in refractive outcome and less adverse effects than those treated with a 5-mm ablation zone.

Preliminary results of photorefractive keratectomy in active-duty United States Navy personnel

PURPOSE

To evaluate the safety, efficacy, and quality of vision after photorefractive keratectomy (PRK) in active-duty military personnel.

METHODS

Photorefractive keratectomy (6.0-mm ablation zone) was performed on 30 navy/marine personnel(-2.00 to -5.50 diopters [D]; mean, -3.35 D). Glare disability was assessed with a patient questionnaire and measurements of intraocular light scatter and near contrast acuity with glare.

RESULTS

At 1 year, all 30 patients had 20/20 or better uncorrected visual acuity with no loss of best-corrected vision. By cycloplegic refraction, 53% (16/30) of patients were within +/- 0.50 D of emmetropia and 87% (26/30) were within +/- 1.00 D. The refraction (mean +/- standard deviation) was +0.45 +/- 0.56 D (range, -1.00 to 1.63 D). Four patients (13%) had an overcorrection of more than 1 D. Glare testing in the early (1 month) postoperative period demonstrated increased intraocular light scatter (P<0.01) and reduced contrast acuity (with and without glare, (P<0.01). These glare measurements statistically returned to preoperative levels by 3 months (undilated) and 12 months (dilated) postoperatively. Two patients reported moderate to severe visual symptoms (glare, halo, night vision) worsened by PRK. One patient had a decrease in the quality of night vision severe enough to decline treatment in the fellow eye. Intraocular light scatter was increased significantly (>2S D) in this patient after the procedure.

CONCLUSIONS

Photorefractive keratectomy reduced myopia and improved the uncorrected vision acuity of all patients in this study. Refinement of the ablation algorithm is needed to decrease the incidence of hyperopia. Glare disability appears to be a transient event after PRK. However, a prolonged reduction in the quality of vision at night was observed in one patient and requires further study.

Central Islands of Corneal Steepening After Automated Lamellar Keratoplasty for Myopia

BACKGROUND

Central steep islands are a phenomena of videokeratography that demonstrate local central steepening within a larger diameter area of the cornea treated by refractive surgery that removed stromal tissue to reduce myopia. This report details four cases in which central steep islands occurred following automated lamellar keratoplasty (keratomileusis in situ).

METHODS

A total of 43 automated lamellar keratoplasty procedures were performed on 32 patients between January 1993 and September 1993. Central islands were identified using video-keratography.

RESULTS

Out of 43 cases of automated lamellar keratoplasty, central steep islands developed in six eyes of four patients. These steep areas developed within a few weeks after surgery; increases in the elevation of these islands correlated with recurring myopia. Slit-lamp microscopy revealed no epithelial surface abnormalities or difficulties with the interface of the resection. A central steep island did not prevent excellent uncorrected visual acuity. After subsequent refractive keratotomy for residual myopia, central steep islands either regressed or disappeared.

CONCLUSION

Central islands of corneal steepening may occur after automated lamellar keratoplasty for myopia. The etiology is unknown.

Effects of Ablation Diameter, Depth, and Edge Contour on the Outcome of Photorefractive Keratectomy

PURPOSE

To investigate the effects of the ablation diameter, depth, and edge contour on the outcome of excimer laser photorefractive keratectomy (PRK).

METHODS

A prospective study was conducted in which 60 patients (60 eyes) were randomly allocated to 5.00-mm, 6.00-mm, or 5.00 to 6.00-mm multizone treatment groups. All eyes underwent a -6.00 diopter (D) correction using a Summit Omnimed excimer laser.

RESULTS

In eyes treated with 6.00-mm diameter zones, the initial hyperopic shift was reduced, with significant differences at 1 and 4 weeks (p < 0.01). At 6 and 12 months, the refractive changes were closer to the intended correction with 6.00-mm diameters. The predictability of PRK was improved with 6.00-mm zones, with a significant reduction in variance of the refractive changes, at all stages postoperatively (p < 0.05 to p < 0.001). Objective measurements of haze were significantly less at 1, 3, and 6 months with 6.00-mm ablations (p < 0.05). There were no differences between the 5.00-mm and the 5.00- to 6.00-mm multizone groups. Computerized measurements of "night" halo were significantly smaller in the 6.00-mm treatment group at 1 week and 1 month (p < 0.05). At 12 months, two patients treated with 5.00-mm zones and three with the 5.00- to 6.00-mm multizone complained of severe night vision disturbances. No 6.00-mm eyes were similarly affected.

CONCLUSIONS

Treatment with a 6.00-mm spherical ablation diameter produced less initial overcorrection, improved predictability, and was associated with a reduction in postoperative halos and night vision disturbances. Creating a superficial blend zone with a 5.00- to 6.00-mm multizone treatment had no beneficial effect on the outcome.