Image quality and visual performance in the peripheral visual field following photorefractive keratectomy

PURPOSE: A theoretical and experimental study was performed to assess the possible effects of photorefractive keratectomy (PRK) on retinal image quality and thresholds in the peripheral visual field. METHODS: Simple optical calculations suggest that although the quality of the retinal image at the fovea of the postoperative PRK eye may be comparable to that in an emmetropic eye, images in the peripheral field may be markedly worse, since peripheral ray bundles may pass partly through ablated and partly through unablated cornea, giving a simultaneous-vision bifocal effect. This would be expected to create an annular zone of confusion, so that light from two different directions in object space arrives at the same point on the retina. The position of this zone and its width are a function of the ablation zone geometry, the attempted correction, and the pupil diameter, but the major effects typically occur at field angles between about 40 degrees and 60 degrees. To explore this effect, Goldmann static quantitative perimetry was carried out along the 0 degree to 180 degrees meridian in six patients who had undergone myopic PRK and eight emmetropes. RESULTS: The results showed that thresholds for the PRK group at field angles from 40 degrees to 60 degrees were significantly higher than those of the emmetropic group (P < .01). These findings arise because the retinal images in the peripheral field of originally myopic, postoperative PRK patients are, in general, significantly degraded in comparison with those of emmetropes. CONCLUSION: Although under photopic conditions, retinal image quality close to the visual axis in patients who have been corrected by PRK is similar to that in emmetropes, it may be markedly worse in the peripheral visual field.     

Iatrogenic ring scotoma after laser in situ keratomileusis

A 37-year-old ophthalmologist had bilateral simultaneous laser in situ keratomileusis (LASIK) for moderate myopia with astigmatism using the Alcon Summit LADARVision laser; an ablation zone of 5.5 mm was used. Five months after surgery, the uncorrected visual acuity was 20/20 and 20/25 but despite regular corneal topographies, the patient experienced prominent ghost images under photopic and scotopic conditions. To elucidate the nature of the problem, automated static perimetry was performed, which revealed a significant depression between 10 degrees and 30 degrees compared with a baseline study obtained 3 years earlier. The patient started brimonidine 0.2% 1 drop in both eyes every morning, which caused 1.5 to 2.0 mm of pupillary miosis (tonic pupil size 3.0 to 4.0 mm in dim light) and eradicated the ghost images. Repeat perimetry showed significant improvement in all indices. The mechanism of improvement is unclear but may be due to elimination of light scatter from the transition zone between the ablated and unablated cornea. The issue of perimetric changes after refractive surgery deserves more attention; postoperative testing may be indicated for patients in whom the ablation zone diameter is close to the mesopic or scotopic pupil size to provide an accurate lifetime baseline visual field.     

Treatment of previous decentered excimer laser ablation with combined myopic and hyperopic ablations

PURPOSE: Decentration of the ablation zone is an occasional complication of excimer laser refractive surgery. We describe a technique to recenter the ablation zone without changing the refractive status obtained by the first surgery. METHODS: Sixteen eyes of 14 patients had moderate or marked ablation decentration after previous excimer laser surgery for myopia, but with only minor residual refractive error. Five eyes had spectacle-corrected visual acuity loss and all these patients reported various symptoms such as halos, ghost images, or night driving difficulties. To recenter the ablation zone without changing the refraction, a combination myopic and hyperopic treatment was used. The hyperopic treatment was decentered toward the initial decentered myopic ablation. A myopia ablation of near equal dioptric value was then added, but decentered in the opposite direction. The Bausch & Lomb Technolas Keracor 217 laser was used. RESULTS: After the first retreatment, the centration of the ablation zone was improved in 15 of the 16 eyes. All eyes with initial spectacle-corrected visual acuity loss recovered lines of visual acuity. Subjective decrease of symptoms was described as follows: nil in one eye, mild in one eye, moderate in four eyes, and marked in ten eyes. A second retreatment was needed in five eyes: in two to improve centration and in three to correct residual ametropia. The only complication (one eye) was induced decentration 180 degrees away from the initial decentration with a 1-line spectacle-corrected visual acuity loss, and additional retreatment was required. CONCLUSION: A combination of decentered myopic and hyperopic ablation of an equivalent dioptric magnitude, each decentered 180 degrees apart, was a useful method to correct previous excimer laser treatment decentration, with minimal alteration of refractive status that was obtained by the initial surgery.     

Topography-guided ablation for treatment of patients with irregular astigmatism

PURPOSE: To evaluate the customized aspheric treatment zone (CATz) topography-guided ablation for the correction of irregular astigmatism induced by initial corneal refractive surgery or corneal injury. METHODS: CATz ablation was performed on 32 eyes of 28 patients. Each procedure was performed by photorefractive keratectomy (PRK) or LASIK using a NIDEK EC-5000 excimer laser. The eyes had decentered ablations, small optical zones, decreased best spectacle-corrected visual acuity (BSCVA), and asymmetrical astigmatism. Subjective symptoms, uncorrected visual acuity (UCVA), BSCVA, refraction, corneal topography, and higher order aberrations were measured. Mean follow-up was 161.9 +/- 129.9 days (range: 90 to 492 days). RESULTS: Data obtained at final postoperative follow-up show that UCVA and BSCVA increased by > or = 2 lines after CATz ablation in 17 and 11 eyes and decreased in 4 and 2 eyes, respectively. Higher order aberrations were decreased in 16 eyes and increased in 1 eye. Topographical maps were improved with decreased surface regularity index (20 eyes) and surface asymmetry index (22 eyes). Seven eyes required further enhancement for residual refractive errors. Thirteen patients (15 eyes) claimed they were satisfied with the outcome, 6 patients (7 eyes) stated that the outcome was lower than expected, and 4 patients (4 eyes) stated they were dissatisfied. CONCLUSIONS: CATz topographic ablation effectively improves the quality of vision and symptoms in the majority of patients with irregular corneal astigmatism from previous excimer laser refractive surgery. However, residual or induced refractive errors may need to be corrected with a second operation after CATz.     

Centration on the cornea vertex normal during hyperopic refractive photoablation using videokeratoscopy

PURPOSE: To evaluate a method for centering the ablation in standard hyperopic LASIK using an excimer laser with a video-based eye tracker system. METHODS: Results of 52 consecutive hyperopic eyes treated with the ESIRIS excimer laser were retrospectively reviewed. Ablation was shifted from the pupil center to the vertex normal of the cornea using pupillary offset measured with the Keratron Scout videokeratoscope. Outcomes were assessed 3 months postoperatively. RESULTS: All eyes preoperatively had a nasally oriented vertex normal in relation to the pupil center. Three months postoperatively a refractive outcome of < 0.50 diopters of spherical equivalent was achieved in 94% (49/52) of eyes. No eye lost more than one line of best spectacle-corrected visual acuity. CONCLUSIONS: Standard hyperopic LASIK with the ESIRIS laser system leads to good predictable efficacy and safety results when the ablation center is shifted to the cornea vertex normal based on videokeratoscopy data.     

A BASIC computer program for schematic and reduced eye construction

A simple computer program for calculating schematic and reduced eyes is presented. The user supplies distances from the front of the eye and radii of curvature of refractive surfaces (anterior and posterior cornea and lens), distance to retina, and refractive indices of ocular media (cornea, aqueous, lens, vitreous). These values are input to calculate the necessary parameters for construction of a schematic and reduced eye, using Gaussian formulae and assuming a homogenous lens. The program produces powers of optical components and of the whole eye, cardinal points of the eye, and estimates of refractive state and retinal image size. These values may be printed in tabular form or plotted to scale with a graphics plotter. The program simplifies construction of schematic eyes by allowing rapid recalculation of the eye after changing one or more input parameters.     

Stress wave amplitudes during laser surgery of the cornea

PURPOSE: To determine the stress wave amplitudes generated during photoablation of the cornea using an argon fluoride excimer laser. DESIGN: Experimental study using porcine eyes. METHODS: Profiles of the stress wave amplitudes and enucleated human eyes along the axis of symmetry of porcine eyes and enucleated human eyes were measured using a miniature piezoelectric transducer. The ablation parameters, fluence, and ablation diameters were varied within the range of clinical application. MAIN OUTCOME MEASURES: Stress wave amplitudes generated during photoablation. RESULTS: The stress waves pass through a pressure focus located in the posterior lens and anterior vitreous, where amplitudes of up to 100 atm were measured with a 6-mm or larger ablation zone. Posterior to this focus, the stress wave amplitudes rapidly decrease to less than 10 atm at the retinal site. Small diameter excimer laser spots (< or =1.5 mm) produce a declining stress wave with no pressure focus at the lens and anterior vitreous. CONCLUSIONS: Stress waves may be potentially hazardous to anterior structures of the human eye, including the corneal endothelium, lens and anterior vitreous face. They peak at the lens and vitreous with a broad beam, but not with small spot laser ablation. At posterior retinal and subretinal structures, they may be considered harmless.     

准分子激光术后超微结构变化与生物组织反应

目的 以超微结构为基础,了解准分子激光表面切削术后角膜生物组织反应特点。方法 透射电镜对38只兔眼4,8,12周角膜组织进行观察和分析,并设对照。结果 角膜基质细胞线粒体和粗面内质网损害;出现功能不良性角膜实质细胞;在各种细胞中有成堆的骶鳞体积聚,提示大量自由基的产生;免疫细胞反应和淋巴细胞的凋亡;气化性碎屑产生;角膜胶原纤维区域性溶解等。结论 准分子激光可使角膜组织产生特异性的生物组织反应,可能是临床合并症的组织病理学基础。     

Asphärische Profile für refraktiven Laserabtrag an der Hornhaut

Conventional ablation profiles for excimer lasers for myopic refractive correction of the cornea are of spheric geometry. Therefore, they induce additional imaging aberrations into the optical system of the eye, most notably spherical aberration. This is a major cause of the observed deterioration of visual quality after such corrections, especially under low illumination and ensuing larger pupil diameter. Therefore, aspheric ablation profiles compromizing the preexisting imaging/visual quality of the eye as little as possible are currently being developed and optimized for all laser platforms. Employed methods include customized correction profiles on the basis of individual wavefront data of the anterior corneal topography on the one hand, and correction profiles that minimize the induced spherical aberration in a "standardized" way on the other hand. We demonstrate for a particular laser platform how such profiles must be developed and optimized. Mathematical theoretical calculations appear to be an indispensable but insufficient prerequisite. The biological reaction of the corneal stroma and epithelium ("biodynamic response") can only be determined experimentally and must lead to adjustment of the calculated ablation algorithm.The results show that aspheric profiles developed on this basis can lead to significant reduction of induced spherical aberration. The obtainable effect is, however, limited by the biological response and the ensuing peripheral ablation depth and volume.     

Changes in corneal topography after excimer laser photorefractive keratectomy for myopia

Computer-assisted analysis of corneal topography was performed in 17 normally sighted human eyes during the first year after excimer laser photorefractive keratectomy (PRK) for myopia. Laser ablation of the central cornea produced an optical zone with a smooth power transition to the peripheral cornea. Decentration of the ablation was noted in some eyes (less than 0.5 mm in 3 eyes, 0.5 to 1.0 mm in 10 eyes, 1 to 1.5 mm in 3 eyes, and 2.1 mm in 1 eye), suggesting that careful alignment of the laser beam is critical. Improved methods to align the ablation within the center of the entrance pupil are needed. In 12 of 17 eyes, the topographic pattern appeared to stabilize between 3 and 7 months after PRK. In the remaining five eyes, central ablation power changed by more than 0.5 diopters (D) between the 6- and 12-month examinations. Regression was more common and more pronounced in eyes with intended corrections more than 5 D, whereas the majority of eyes with intended corrections of 5 D or less showed good correspondence between the final change in central ablation power and the attempted correction. Two eyes had a loss of at least two lines of best spectacle-corrected visual acuity that was attributable to irregular astigmatism, decentration of the ablation, and/or corneal opacification.     

Wavefront-guided laser in situ keratomileusis: early results in three eyes

PURPOSE: Wavefront optical aberrations induced by refractive corneal surgery correction of myopia are probably the reason for deterioration of visual performance in some eyes after surgery. Customized photoablation of the cornea to correct both the sphero-cylindrical refractive error as well as individual optical aberrations may improve postoperative visual acuity and visual performance. METHODS: In 3 eyes of 3 patients the wavefront deviations were measured by means of an aberrometer of the Tscherning-type. Based on these measurements an ablation pattern was determined and applied during a LASIK procedure using a Wavelight Allegretto scanning spot excimer laser with a spot size of 1 mm and a laser repetition rate of 200 Hz. The 3 eyes are part of a prospective study on wavefront-guided LASIK started in July 1999. RESULTS: At 1 month after LASIK, all 3 eyes had gained up to 2 lines of best spectacle-corrected visual acuity. Best spectacle-corrected visual acuity improved to 20/10 in all 3 eyes, uncorrected visual acuity was 20/10 in two eyes, and 20/12.5 in 1 eye 1 month postoperatively. The wavefront deviations were reduced by 27% on average. At 3 months, best spectacle-corrected visual acuity was 20/10 in 2 eyes and 20/12.5 in 1 eye. CONCLUSION: Wavefront-guided LASIK is a feasible approach in refractive corneal surgery. Optimized ablation patterns may further improve the visual results.     

Effects of foveal ablation on the pattern of peripheral refractive errors in normal and form-deprived infant rhesus monkeys (Macaca mulatta)

PURPOSE. The purpose of this study was to determine whether visual signals from the fovea contribute to the changes in the pattern of peripheral refractions associated with form deprivation myopia in monkeys. METHODS. Monocular form-deprivation was produced in 18 rhesus monkeys by securing diffusers in front of their treated eyes between 22 ± 2 and 155 ± 17 days of age. In eight of these form-deprived monkeys, the fovea and most of the perifovea of the treated eye were ablated by laser photocoagulation at the start of the diffuser-rearing period. Each eye's refractive status was measured by retinoscopy along the pupillary axis and at 15° intervals along the horizontal meridian to eccentricities of 45°. Control data were obtained from 12 normal monkeys and five monkeys that had monocular foveal ablations and were subsequently reared with unrestricted vision. RESULTS. Foveal ablation, by itself, did not produce systematic alterations in either the central or peripheral refractive errors of the treated eyes. In addition, foveal ablation did not alter the patterns of peripheral refractions in monkeys with form-deprivation myopia. The patterns of peripheral refractive errors in the two groups of form-deprived monkeys, either with or without foveal ablation, were qualitatively similar (treated eyes: F = 0.31, P = 0.74; anisometropia: F = 0.61, P = 0.59), but significantly different from those found in the normal monkeys (F = 8.46 and 9.38 respectively, P < 0.05). CONCLUSIONS. Central retinal signals do not contribute in an essential way to the alterations in eye shape that occur during the development of vision-induced axial myopia.     

VISX S4准分子激光不同角膜切削模式联合LASIK治疗超高度近视的研究

目的 探讨V1SX S4准分子激光不同角膜切削模式联合LASIK治疗超高度近视的临床效果。方法 临床回顾性对照分析研究。选择2005年11月至2006年12月在120例（216只眼）超高度近视LASIK矫正,运用VISX S4准分子激光,根据不同的切削模式联合分为ESB组(M Ellipse+M Sphere+M Blend)74眼、ART( Advanced Refractive Treatment)组(M Ellipse-APA+M Sphere Zone1-APA+M Sphere Zone2-APA )70眼、ESS组(M Ellipse+ Multizone M Sphere Zonel+ Multizone M Sphere Zone2)组72眼,分析各组的角膜切削、术后视力与屈光以及并发症。结果 (1)角膜平面屈光改变等效球镜ID,ESB组需切削角膜（13.32±1.42）μm,ART组切削角膜(7.66±1.10)μm,ESS模式组切削角膜(12.27±0.48)μm。(2)随访1年,术后裸眼视力均较术前提高,术后1个月内均达到术前最佳矫正视力或以上;术后3个月视力趋于稳定;术后最佳矫正视力均无丢失两行以上;术后3个月屈光等效球镜均在±0.50 D内。(3)术后眩光发生率：ESB组2只眼(2.7％),ART组4只眼(5.7％),ESS组2只眼(2.8％),均在术后6个月后缓解;术后角膜地形图分析均未发现偏中心切削;术后3个月屈光回退率：ESB组3只眼(4.1％),ART组3只眼(4.3％),ESS组2只眼(2.7％)。结论 VISX S4准分子激光能根据不同的屈光度与角膜厚度自动选择联合不同的切削模式治疗超高度近视具有可预测性、有效性、稳定性及安全性;其中ART模式节省角膜组织有利于薄角膜超高度近视的屈光矫正,但视觉质量值得商榷。     

准分子激光角膜屈光手术对人眼高阶像差的影响

目的评价不同屈光手术方式治疗屈光不正前后的高阶像差的改变。方法近视患者777例（1554眼）,按屈光度分为低、中、高三组,各206、950、398眼。根据患者自愿原则,分别给予不同的屈光手术治疗。其中,准分子激光原位角膜磨镶术（LASIK）1136眼,角膜地形图引导的LASIK术（TOGCA）404眼,波前像差引导的LASIK术（WAGCA）14眼。采用iTrace全功能视觉分析仪测量手术前后角膜及晶状体的高阶像差。采用方差分析和配对t检验对所得数据进行统计学分析。结果近视眼患者术前角膜与晶状体各像差互为补偿。角膜高阶像差：术前不同屈光度组间角膜高阶像差的差异无统计学意义,术后各组角膜高阶像差的增加随着屈光矫正度数的增加而增加,其中增加最大的是球差,TOGCA患者术后球差增加较小。晶状体高阶像差：术前高度近视组较低、中度近视组有更多的总高阶像差和球差,手术前后晶状体高阶像差的差异无统计学意义。结论角膜和晶状体高阶像差互为补偿,人眼高阶像差对视觉质量影响较小。屈光手术导致人眼高阶像差的增加,而个体化切削术能减少高阶像差的产生,提高屈光手术患者的视觉质量。     

Electron microscopy of surface smoothness of porcine corneas and acrylic plates with four brands of excimer laser

PURPOSE: This study compares ablation smoothness patterns produced on four different excimer laser devices available for photorefractive surgery. METHODS: VISX calibration plastic and porcine cornea were ablated with standard -3.00-D, -6.00-D, and -9.00-D settings using four different excimer lasers: VISX S2 Smooth Scan, Nidek EC-5000, Autonomous Ladar Vision System, and Bausch and Lomb Technolas. Electron microscopy and laser interferometry were used for qualitative evaluation of the ablated surfaces. Corneal ablation surface smoothness was graded by ten independent observers. Calibration plastic ablated surfaces were evaluated quantitatively for smoothness by laser interferometry. RESULTS: The independent observer assessment of corneal ablation surface smoothness demonstrated that the Autonomous small spot Gaussian profile laser produced the smoothest ablation surfaces, followed by the other broad beam lasers. In comparing ablation smoothness among various refractive powers (-9.00 D, -6.00 D, and -3.00 D), a trend was observed that indicated a correlation of higher refractive settings with decreasing surface smoothness. However, this trend was not statistically significant. The quantitative laser interferometry measurements supported the independent observer ranking of the Autonomous flying small spot ablation profile as the smoothest. However, there were differences between the laser interferometry smoothness rankings and independent observer smoothness rankings. CONCLUSION: There were significant differences in ablation surface smoothness among the four excimer lasers tested.     

Apparent accommodation and corneal wavefront aberration in pseudophakic eyes

PURPOSE: To assess the relationship between apparent accommodation in pseudophakic eyes, multifocal corneal effects, and wavefront aberrations of the cornea. METHODS: In 102 eyes of 86 patients who had undergone phacoemulsification and posterior chamber intraocular lens implantation, the amount of apparent accommodation was measured with an accommodometer. The degree of corneal multifocality was determined on the corneal topography by measuring the maximum and minimum corneal refractive powers within the pupillary area. Wavefront aberrations of the cornea were calculated by expanding the height data of the corneal topography into Zernike polynomials for individual pupil size. The influence of higher-order aberration on the retinal image quality was simulated by computing the point-spread function (PSF) and modulation-transfer function (MTF) from the aberration function. RESULTS: There was a significant positive correlation between the amount of apparent accommodation and corneal multifocality (Pearson correlation coefficient, r = 0.451, P < 0.001). The coma-like aberration showed a significant positive correlation with the amount of apparent accommodation (r = 0.440, P < 0.001), but the spherical-like aberration did not (r = 0.001, P = 0.993). Among the coma-like aberrations, the component representing vertically asymmetrical distribution of corneal refractive power with greater refraction located in the lower part of the eye was most relevant to apparent accommodation. Computer simulation of PSF and MTF indicated that a focus shift of 0.5 D deteriorated the retinal image significantly more in eyes without higher-order aberrations than in eyes with a moderate amount of coma-like aberrations. CONCLUSIONS: Coma-like aberration of the cornea, along with corneal multifocality, contributes to apparent accommodation in pseudophakic eyes.     

Retinal stretching limits peripheral visual acuity in myopia

Axial elongation of the myopic eye has the potential to stretch the retina, thereby reducing the sampling density of retinal neurons. Resolution acuity in the peripheral field of normal eyes is known to be sampling-limited, which suggests that retinal stretching in the myopic eye should have a direct effect on resolution acuity everywhere in the visual field except perhaps the fovea, which is usually optically limited. We tested this prediction that neural sampling density is reduced in myopic eyes by measuring resolution acuity for sinusoidal gratings in the fovea plus five peripheral locations in 60 myopic subjects exhibiting a wide range of refractive errors. Control experiments using a detection paradigm to provoke spatial aliasing verified that peripheral resolution was sampling limited. Retinal spatial frequencies of the grating stimulus were computed assuming Knapps' Law of visual optics, which ensures that retinal image size (in mm) is independent of refractive error when axial myopia is corrected by a spectacle lens located in the anterior focal plane of the eye. Results obtained at every retinal locus showed that resolution acuity declined linearly with magnitude of refractive error. Regression of the population data indicated that approximately 15 D of refractive error doubles the spacing between retinal neurons, thereby halving peripheral resolution acuity relative to the emmetropic eye. Several subjects also demonstrated sampling-limited performance in the fovea, which indicated that optical filtering by the eye's optical system failed to protect the fovea from aliasing artifacts of neural undersampling in these eyes. We conclude that stretching of the retina is a primary cause of reduced spatial resolution of the peripheral field, and occasionally of the fovea, in myopic eyes. Stretching appears to be locally uniform over the central +/-15 degrees of visual field but is globally non-uniform since the foveal region appears to stretch more than the globe itself.     

准分子激光角膜切削中心的角膜地形图分析

目的探讨准分子激光角膜切削术（ｐｈｏｔｏｒｅｆｒａｃｔｉｖｅｋｅｒａｔｅｃｔｏｍｙ，ＰＲＫ）的切削中心对视功能的影响。方法采用Ｅｙｅｓｙｓ角膜形态分析系统对随访６个月以上的９８例（１５８只眼）ＰＲＫ前、后１０天的角膜地形图进行分析，确定激光切削中心相对入射瞳孔中心的方向、距离；比较单区和多区域激光切削的偏中心差异。结果切削区中心相对入射瞳孔中心：０～０．５ｍｍ者１００只眼（６３．３％），０．５～１．０ｍｍ者５５只眼（３４．８％），＞１．０ｍｍ者３只眼（１．９％），平均０．４５ｍｍ。多区域切削３８只眼及单区域切削１２０只眼，其偏心量分别为０．５５ｍｍ和０．４４ｍｍ。差异有显著性（ｔ检验，Ｐ＜０．０５）。偏中心切削致术后最好矫正视力下降１～２行者４只眼，其偏心量＞０．５ｍｍ。结论严重的偏心切削将影响术后视功能恢复，术中眼球跟踪系统（ｐａｓｓｉｖｅｅｙｅ－ｔｒａｃｋｉｎｇ，ＰＥＴ）的应用有利于切削中心的确定，减少偏心量。     

Peripheral power errors and astigmatism of eyes corrected with intraocular lenses

We compared the theoretical peripheral power errors and oblique astigmatism of eyes corrected with commercially available intraocular lenses (IOL's) with experimental data of normal phakic eyes. The peripheral power errors and the oblique astigmatism of the pseudophakic eye are larger than those of phakic eyes. The most sensitive component of the optical system of the pseudophakic eye to the peripheral power errors is the shape of the IOL, but only if the lens is away from the iris. The corneal and the retinal surfaces do not affect the peripheral power errors and oblique astigmatism significantly. A plano-convex lens with the flat surface facing the cornea gives the least peripheral power errors and oblique astigmatism and thus the value closest to the experimental data of phakic eyes. However, this design does not give the minimum spherical aberration. Therefore, eyes corrected with IOL's are expected to have poorer peripheral retinal image quality than normal phakic eyes.     

Histological comparison of corneal ablation with Er:YAG laser, Nd:YAG optical parametric oscillator, and excimer laser

PURPOSE: To use histological techniques to assess and compare the ablation depth, local damage, and surface quality of corneal ablations by a Q-switched Er:YAG laser, an optical parametric oscillator laser at 2.94 microm, a long pulse Er:YAG laser, and a 193-nm excimer laser. METHODS: Human cadaver eyes and in vivo cat eyes were treated with a 6.0-mm diameter, 30-microm-deep phototherapeutic keratectomy ablation and a 6.0-mm diameter, -5.00-D photorefractive keratectomy ablation. Human cadaver eyes were also treated with a 5.0-mm diameter, -5.00-D laser in situ keratomileusis (LASIK) ablation. Fluences and pulse widths used were 200 mJ/cm2 and 70 ns for the Q-switched Er:YAG, 150 mJ/cm2 and 7 ns for the optical parametric oscillator laser (OPO), 500 mJ/cm2 and 50 microseconds for the long pulse Er:YAG, and 160 mj/cm2 and 20 ns for the excimer laser. In the ablation rate study, 12 porcine eyes were ablated by the OPO laser with a range of layers and at different fluences ranging from 60 to 150 mJ/cm2, all using a 1.5-mm spot on the eye. The ablation depth of these acute ablations was evaluated by light microscopy examination. RESULTS: In the acute damage study, light microscopy showed a thin surface layer in all samples with minimal thermal damage except on the long pulse Er:YAG corneas. Transmission electron microscopy revealed less than 0.3-microm surface damage for all specimens of both the optical parametric oscillator and the excimer laser samples with no evidence of collagen shrinkage. Transmission electron microscopy showed damage layers of 0.5 to 3 microm for Q-switched Er:YAG and 3 to 10 microm for long pulse Er:YAG. Scanning electron microscopy showed smooth surfaces in all eyes, although the excimer was the roughest. In the porcine eye study, ablations were produced in both PTK and PRK modes with the ablation rate per layer increasing with the fluence. At 120 mJ/cm2, the average ablation rate was 1.9 microm per layer. CONCLUSIONS: The histology from the short pulse mid-infrared optical parametric oscillator laser at 2.94 microm was comparable to the 193-nm excimer with a smooth, damage-free, ablation zone when performing PRK and LASIK.