Steep Central Islands Research Articles

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.

From Scattering to Wavefronts-What's in Between?

Some optical errors are too localized and random to be detected by commercial wavefront devices and Zernike polynomial expression. We looked beyond aberrations defined by Zernike expression to discuss implications of fine irregularities associated with highly aberrated corneal surfaces and complex surface roughness that can lead to light scattering. Most fine irregularities are related to postoperative surface roughness, complexities of corneal ablation, and the laser in situ keratomileusis (LASIK) flap. These can be characterized mathematically by a random function that includes local surface tilts, the correlation radius of irregularities (Ic), surface roughness, and other terms. The Kirchoff method of scatter analysis characterizes fine surface irregularities by replacing each point on the surface with a tangential plane, allowing it to be governed by Snellen and Fresnel laws. The joint action of the continuum of microbeams defines a complex point spread function that can be expressed by the Strehl ratio. Small, highly irregular steep central islands and flap striae may not be adequately detected by Zernike expression and may have a surface irregularity diameter of 0.1 to 2.0 mm and height of 10 to 20 microm that results in a reduced Strehl ratio below 0.8. Laser ablation inhomogeneities may have dimensions of 1 to 10 microm, resulting in a root mean square tilt value approaching 1.0 and a Strehl ratio below 0.5. Corneal surface irregularities after laser vision correction may induce significant optical aberrations and distortions apart from classical wavefront or scattering errors. As these may not be detected by commercial wavefront devices, and yet contribute to the degradation of optical performance, alternate techniques should be evaluated to detect and describe these surface irregularities.

Large optical ablation zone using the VISX S2 Smoothscan excimer laser

Purpose To prospectively evaluate the safety and efficacy of the new large-zone (6.5 mm) photoablation technology using the VISX S2 Smoothscan excimer laser. Setting University-based hospital, Stanford, California, USA. Methods Forty-two eyes of 21 patients with a mean preoperative spherical equivalent (SE) of−5.55 diopters (D)± 2.24 (SD) (range−2.13 to−10.75 D) had laser in situ keratomileusis (LASIK) using the VISX Smoothscan S2 excimer laser for simple myopia or compound myopic astigmatism. A 6.5 mm optical zone was used in all eyes. Patients were prospectively followed 1 day and 1 and 3 months postoperatively. Results At 3 months, the mean SE was reduced 94% to−0.31± 0.55 D. Ninety-one percent of eyes had an uncorrected visual acuity of 20/40 or better. Eighty-eight percent were within±1.00 D of attempted correction and 84%, within ±0.50 D. Stability within±0.50 D occurred after the first postoperative month. Vector analysis of eyes that had toric ablations demonstrated a difference vector within±1.00 D in 100% of eyes. The mean angle of error was−0.04± 6.37 degrees. Visually significant steep central islands associated with loss of best spectacle-corrected visual acuity was observed in 7.5% of eyes at 1 month. No eyes experienced significant glare or halos. Conclusions The new large-zone (6.5 mm) photoablation technology with the VISX S2 Smoothscan resulted in effective reduction of simple myopia and compound myopic astigmatism. However, with the 6.5 mm zone, there may be an increased risk of developing symptomatic steep central islands in the early post-LASIK period compared with the standard 6.0 mm treatment zone.

Selective zonal ablations with excimer laser for correction of irregular astigmatism induced by refractive surgery

Objective To demonstrate the safety and efficacy of selective zonal ablations with excimer laser for the correction of irregular astigmatism induced by refractive surgery. Design Thirty-one eyes of 26 patients. Retrospective and noncomparative case series. Methods The authors reviewed the medical results of the two series of consecutive cases of irregular astigmatism induced by refractive surgery. One group showed a topography map with a defined pattern: decentered ablation, decentered steep central island, central irregularity, and peripheral irregularity. The other group showed an irregular astigmatism without defined pattern. Selective zonal ablation was performed with a broad-beam excimer laser. Laser ablations were adjusted according to the Munnerlyn formula. A phototherapeutic keratectomy (PTK) mode was used in all cases without using viscous masking solution. Main outcome measures Uncorrected visual acuity, best spectacle-corrected visual acuity (BCVA), and Holladay diagnostic summary (Eye Sys 2.000) were considered before and after surgery to obtain the qualitative and quantitative characteristics of the visual acuity and corneal topography. The corneal uniformity index (CUI) was used to evaluate the corneal surface changes induced by the selective zonal ablations in the correction of irregular astigmatism. It proved to be a useful tool in the understanding of astigmatic changes when it is not possible to apply the vector analysis. Results In group 1, the irregular astigmatism was significantly improved in 96.4% of cases. The CUI was improved from 58 ± 0.3% to 87 ± 0.8%, P < 0.005 (Student’s t-test). The improvement of the BCVA was from 20/40 ± 20/100 to 20/25 ± 20/100, P < 0.005 (Student’s t-test). The results obtained in group 2 were not clinically significant in terms of improvement of CUI and BCVA. Conclusions Using the corneal topographic map as a guide, selective zonal ablations with excimer laser can be used to create a more regular corneal surface, when a defined pattern of topographic irregularities can be defined.

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 Munster and Schwind Co., Kleinosthelrn, Germany.Methods: Corneas of enucleated intact bovine eyes were treated with the Schwind Keratom. All experimental conditions were repeated six tunes in six different corneas. Eight expehrriental 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 anticentrai-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 polder. The anti-central-island program, blowing aerosol, and cleaning the comea 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.

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 CIs 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 ablation profilometry and steep central islands.

Photorefractive keratectomy with large diameter ablations using a uniform laser beam has produced central undercorrections, or "steep central islands" in patients, as seen with videokeratography. Using a custom optical profilometer to measure corneal ablation profiles and a VISX excimer laser system, we measured the effect of ablation algorithms, diameter, depth, and dioptric correction on enucleated porcine eyes and living rabbit eyes. Our profilometer was verified using a 43.00 diopter (D) spherical surface and a 35.00 and 43.00 D bicurve test surface as a model for the ablated cornea. The profilometer measured the test surfaces to within 3 microns of predicted values. Photorefractive keratectomies showed over-ablation peripherally and under-ablation centrally which increased with ablation diameter and dioptric correction. Fixed diameter ablations 2 to 6 mm in diameter and 10 to 80 microns deep showed stromal ablation rates vary spatially but not with ablation depth. These spatially variant ablation profiles were used to re-engineer the ablation algorithm and to produce photorefractive keratectomies with improved sphericity. Steep central islands are caused by the spatial variance of tissue ablated with a uniform laser beam irradiance. This aberration can be corrected by modifying the laser ablation algorithm to correct for the spatial variance of stromal ablation.

Clinical analysis of steep central islands after excimer laser photorefractive keratectomy.

To examine topographic irregularities known as steep central islands that may occur after excimer laser refractive surgery and affect visual acuity. We reviewed the computed corneal topographic maps of 35 eyes that had undergone excimer laser photorefractive keratectomy with an excimer laser for compound myopic astigmatism or anisometropic myopia. Steep central islands were defined as areas of steepening of at least 3 diopters and 1.5 mm in diameter. A classification system was developed based on the presence of steep central islands during the postoperative period as follows: class 0, absent; class 1, present at 1 week; class 2, present at 1 month; class 3, present at 3 months. Steep central islands were seen in 25 eyes (71%) at 1 week, 18 eyes (51%) at 1 month, seven eyes (20%) at 3 months, and four eyes (11%) at 6 months. After surgery without nitrogen gas blowing, 16 of 25 patients had class 2 or 3 steep central islands compared with two of 10 eyes when gas blowing was used. Loss of best spectacle-corrected visual acuity of 2 Snellen lines or more was seen in eight of 18 eyes with class 2 or 3 steep central islands at 1 month and three of 18 eyes at 3 months. A similar loss occurred in one of 17 eyes with class 0 or 1 steep central islands at 1 month and none of 17 eyes at 3 months. In all eyes with only class 2 steep central islands, loss of at least 1 Snellen line of best spectacle-corrected visual acuity at 1 month was associated with visual restoration at 3 months when the island was no longer present. Loss of best spectacle-correlated visual acuity is associated with steep central island formation, and may prolong visual rehabilitation after excimer laser photorefractive keratectomy.

Excimer laser calibration system.

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. 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. 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. 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.

CENTRAL STEEP ISLANDS IMMEDIATELY FOLLOWING EXCIMER PHOTOREFRACTIVE KERATECTOMY FOR MYOPIA

CENTRAL STEEP ISLANDS IMMEDIATELY FOLLOWING EXCIMER PHOTOREFRACTIVE KERATECTOMY FOR MYOPIA

Corneal Bifocal Effects by Laser Photorefractive Keratectomy-Reply

<h3>In Reply.</h3> —Dr Josephson discusses potential limitations of simultaneous vision. We discussed these potential limitations, briefly reviewed the literature on multifocal lenses in our article,¹and recommended a conservative approach to the purposeful creation of multifocal corneas in humans. At present, there is no US Food and Drug Administration-approved protocol for doing this in the United States, but we are aware of some surgeons doing multifocal corrections in Europe. As with multifocal contact lenses and intraocular lenses, we noted in our article that not all patients who undergo a refractive surgical procedure to create a multifocal cornea will tolerate the decreased contrast sensitivity and other optical limitations of this procedure. We noted that our technique of leaving a steep central island for near vision has the advantage of allowing the surgeon to subsequently convert this into a monofocal procedure by re-treating and flattening the central region.