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BACKGROUND: 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. METHODS: 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. RESULTS: 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. CONCLUSIONS: 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.

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OBJECTIVE: To evaluate histological changes in the primate cornea after excimer laser photorefractive keratectomy (PRK) and to correlate them with clinical observations. METHODS: Sixteen African green monkey (Cercopithecus aethiops sabaeus) corneas were examined by light and transmission electron microscopy 6 weeks to 18 months after 1.5- or 3-diopter PRK. RESULTS: All specimens had a smooth stromal surface surrounded by a smooth, centrally tapered Bowman's layer. Epithelial thickness appeared to peak 12 months after PRK. The trend was for the epithelium to be thickest in the central-treated area; this phenomenon was more obvious in the 3-diopter-treated corneas. The numbers of activated keratocytes beneath the treated zone peaked at 4 months and decreased thereafter, while the numbers in the untreated areas decreased in the first 2 months after surgery, increased by 4 months, and did not change thereafter. Regenerated basal lamina averaged 86% intact over ablated areas; thickness was normal and no duplications were seen. Overall, the density of hemidesmosomes was significantly less in ablated areas compared with unablated areas. CONCLUSIONS: These findings support the relationship between clinical observations of corneal haze after PRK, reestablishment of the epithelial cell layer, and the potential for stromal remodeling by active fibroblastic keratocytes beneath the ablation zone. Overall, quantification of several morphological parameters indicated that the values for the treated zone tended, with time, to approach those of the untreated cornea after PRK.

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BACKGROUND: A duplex optical image is created when the ablation zone formed by excimer photorefractive keratectomy is smaller than the entrance pupil. Visual performance and secondary effects are analyzed using a theoretical model of the optical image. METHODS: A point-spread function having a centered in-focus component surrounded by an annular out-of-focus component is calculated from pupil size, ablation size, refractive error, and photoreceptor directional sensitivity. The line-spread, edge-spread, and optical transfer functions are derived. RESULTS: In the line- and edge-spread functions, secondary maxima and curvilinear ramps are most evident with low refractive errors. The half-height widths of the point- and line-spread functions change little. The optical transfer function is reduced in proportion to the distribution of light between the image components. CONCLUSIONS: Stable point and line half-height widths explain why Snellen visual acuity is insensitive to annular blur. Contrast sensitivity correlates with symptoms of haze and fog. Halos and ghost images are associated with secondary optical maxima and curvilinear ramps. Neither visual acuity nor contrast sensitivity can predict halos or ghost images. Halos and ghost images will be most prevalent in low illumination and for low refractive corrections. High refractive errors will produce fewer visual side effects than low refractive errors.

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Excimer laser photorefractive keratectomy, developed to perform radially symmetric ablations to correct myopic or hyperopic refractive errors, was used to perform toric ablations designed to correct cylindrical errors. An expanding slit was used to flatten the cornea in the steep meridian. Four contact lens-intolerant patients underwent this procedure for correction of astigmatism (two patients after penetrating keratoplasty, one patient after corneal ulcer, and one patient with naturally occurring high astigmatism). In each patient, surgery reduced the regular component of the astigmatism; residual irregular astigmatism limited spectacle-corrected acuity in one patient. All patients experienced a shift in spherical equivalent toward hyperopia. Toric ablations with the excimer laser appear to represent a promising strategy for the correction of compound myopic astigmatism that does not rely on creation of deep corneal incisions, excisions, or compression sutures.

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Ten partially sighted and 19 normally sighted eyes underwent excimer laser photorefractive keratectomy for the correction of myopia. Nine of the partially sighted and 17 of the normally sighted eyes had 12 months of follow-up. Epithelial healing was complete in all eyes by day 6. None of the eyes had recurrent erosions, infections, or other medical complications. An increase in corneal haze after surgery was followed by a slow trend toward clearing. Average uncorrected visual acuity in the 7 normally sighted eyes with attempted corrections of 5 diopters (D) or less was 20/40 from month 2 on; the eyes with greater than 5 D attempted corrections had an average of 20/80--at month 2, which declined to 20/200--by month 6. Best spectacle-corrected visual acuity was within +/- 1 Snellen line of preoperative values in 14 of the normally sighted eyes, improved 2 or more lines in 2 eyes, and worsened two or more lines in two eyes. Hard contact lens overcorrection restored all of the two-line loss in 1 eye and 1 line of the 3-line loss in the other. Refraction and keratometry indicated corneal flattening without induced astigmatism.

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Excimer laser photorefractive keratectomy, developed to perform radially symmetric ablations to correct myopic or hyperopic refractive errors, was used to create toric ablations designed to correct cylindrical errors. An expanding slit was created, with no refractive change intended parallel to the slit opening, and central flattening was induced in the meridian in which the slit was expanded. In polymethyl methacrylate blocks, the induced cylinder (as measured with a lensometer) agreed closely (r2 = .99) with intended change, and in plastic corneas, induction of the cylinder could be demonstrated with computer-assisted topographic analysis. Adult pigmented rabbits underwent induction of toric ablations in two-diopter increments; measured keratometric change correlated with desired change (r2 = .87 at 3 weeks; r2 = .89 at 12 weeks). Toric ablations with the excimer laser appear to represent a promising strategy for the correction of cylindrical errors that do not rely on creation of deep corneal incisions, excisions, or compression sutures.

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Prior to undertaking a study in sighted human eyes, we performed photorefractive keratectomy with the 193-nm excimer laser for the correction of myopia in nine legally blind eyes to evaluate safety, efficacy, predictability, and stability. In most cases, reepithelialization was complete by 5 days after surgery; no recurrent erosions were seen. By the end of the 6-month study, all of the corneas had a 0 or 1+ clarity score, on a scale of 0 (clear) to 5+ (opaque). Keratometry and pachometry demonstrated stable flattening of the corneas. One month after surgery, changes in refraction evaluated by retinoscopy showed fair predictability, with no significant increase in refractive or keratometric astigmatism, followed by some regression of effect by the end of the study, possibly caused by anatomical remodeling. The amount of regression appeared to be directly related to the amount of correction intended, suggesting that this effect would not be clinically important in the treatment of mild to moderate myopia.

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Photorefractive keratectomy for the correction of myopia was performed in 32 eyes of 16 green monkeys. The corneas healed satisfactorily, with normal formation of basal lamina and hemidesmosomal attachments visible in 14-week histologic specimens. No recurrent erosions were observed clinically. After a transient period of faint haze, all corneas were clear at 17 weeks and remained clear through the 1-year follow-up. In terms of accuracy, all corneas demonstrated a significant flattening compared with preoperative values, but no significant difference was seen between the groups with different intended corrections (1.5 and 3 diopters). The changes in corneal shape stabilized by 17 weeks, as measured by keratometry. The clinical results suggest that mechanical removal of the epithelium is preferable to laser ablation of the epithelium. Overall, the results demonstrate that excimer laser ablation of the corneal stroma can produce a stable diptric change in the primate cornea with good healing and long-term corneal clarity.

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A 193-nm excimer laser system was used to ablate 4.5-mm optically contoured zones in the corneal stroma of rabbits to achieve optical flattening of 2, 4, 8, and 16 diopters. We studied the effect of edge profile and wound depth on reepithelialization and stromal remodeling using dichlorotriazinyl aminofluorescein, a vital dye that covalently binds to the stromal bed and delineates the boundaries of new collagen synthesis. All the corneas reepithelialized; no subsequent recurrent erosions occurred. All seven corneas that received an ablation of less than 50 microns were clear centrally at 8 weeks. At an ablation depth of approximately 100 microns, opacification and scarring were observed biomicroscopically and histopathologically in two specimens. No evidence of new collagen formation or epithelial hyperplasia was found in any of the seven corneas that remained clear. Stromal remodeling was observed in the two corneas that exhibited scarring.

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The excimer laser offers entirely new corneal surgery procedures. In this paper, we describe conditions for one such procedure, photorefractive keratectomy, the direct reshaping of the cornea's central optical zone using tissue ablation with far ultraviolet radiation. We present equations for the required tissue ablation to achieve required refractive corrections. Conditions for beam uniformity are presented and a method of achieving better beam uniformity using beam integration by rotation is demonstrated. The healing of rabbit corneas after area ablation was observed. Five days after treatment, the surface quality of deep cuts was very irregular; the shallow cuts produced more regular results, as predicted by our calculations of beam characteristics.

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A new laser iridotomy-sphincterotomy contact lens, bearing a 103-diopter optical button decentered 2.5 mm, gives the smallest iris focal spot and highest iris energy density practicably obtainable with a single optical glass refracting surface placed upon a thin Goldmann-type contact lens. By increasing iris energy density to a level 7.79 times greater than that from a plano contact lens and 2.92 times greater than that from the Abraham lens, the lens increases the iris burn temperature level above the threshold level for evaporative pyrolysis, even at exposures of 0.01 and 0.02 second, thus facilitating argon laser iridotomy and iris sphincterotomy by the linear incision method. Corneal and retinal energy exposures are greatly reduced. Used with the Nd-YAG laser, the lens increases efficiency for iridotomy and for division of vitreous bands in the anterior chamber.

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Fluorometric studies have been made with modified slit-lamp microscopes. These instruments offered considerable versatility in modifying experimental parameters, but were seldom optimized in terms of detector and illuminator apertures, filter band-pass characteristics, detection system, light-source stabilization, or optical resolution. Improper control of these parameters can result in artifacts which override useful information. A new fluorophotometer for ocular research has been designed that utilizes an optical system, which eliminates the need for a contact lens. An effort was made to optimize both the axial resolution and the minimum detectable signal. The characteristics of standard barrier filters and excitation filters were examined and compared with photomultiplier sensitivity curves. It was necessary to use double filters in order to reduce artifacts due to excitation light that was not rejected by the barrier filter. In order to optimize the signal-to-noise ratio, a photon counting detection system was used. A dark current equivalent to 3 X 10(-10) g/ml allows a useful detection range of 1 X 10(-10) g/ml.

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By careful consideration of the precision of available alignment apparatus, mode matching can be accomplished most easily. A function representing the degree of mode matching in a confocal Fabry-Perot interferometer is defined and then evaluated as a function of the angular and translational alignment errors. A method is given for optimizing the beam diameter to take advantage of the available precision of specific instruments. Experimental verification is given for a specific case.