Refractive surgery with the excimer laser

Direct Laser Interference Patterning of Diffraction Gratings in Safrofilcon-A Hydrogel: Fabrication and Hydration Assessment

Refractive index modification by laser micro-structuration of diffractive optical devices in ophthalmic polymers has recently been applied for refractive correction in the fields of optics and ophthalmology. In this work, Safrofilcon-A hydrogel, used as soft contact lenses, was processed by direct laser interference patterning (DLIP) to fabricate linear periodic patterns on the surface of the samples. Periodic modulation of the surface was attained under two-beam interference by using a Q-switched laser source with emission at 263 nm and 4 ns pulse duration. Features of processed areas were studied as a function of both the interference spatial period and the laser fluence. Optical confocal microscopy used to evaluate the topography of the processed samples showed that both structured height and surface roughness increased with laser fluence. Static water contact angle (WCA) measurements were carried out with deionized water droplets on the structured areas to evaluate the hydration properties of DLIP structures. It was observed that the laser structured areas induced a delay in the hydration process. Finally, microstructural changes induced in the structured areas were assessed by confocal micro-Raman spectroscopy showing that at low laser fluences the polymer structure remained almost unaltered. In addition, Raman spectra of hydrated samples recovered the original shape of areas structured at low laser fluence.

Very Late-Onset Corneal Scarring After Photorefractive Keratectomy Induced by Cataract Surgery

PURPOSE

To report two cases with very late-onset scarring of the cornea after photorefractive keratectomy (PRK) induced by cataract surgery.

METHODS

Case report and literature review.

RESULTS

Two patients presented with a subepithelial corneal scar more than 20 years after PRK. Scarring occurred within the first few months after cataract surgery. Scar tissue was successfully removed by laser-assisted anterior lamellar keratoplasty.

CONCLUSIONS

A few cases of late-onset corneal scarring after PRK have been described in the literature following trauma or ophthalmic surgery. Due to the need for cataract surgery in an aging population with previous PRK, this might become a more common problem.

Noninvasive intratissue refractive index shaping (IRIS) of the cornea with blue femtosecond laser light

PURPOSE

To test the feasibility of intratissue refractive index shaping (IRIS) in living corneas by using 400-nm femtosecond (fs) laser pulses (blue-IRIS). To test the hypothesis that the intrinsic two-photon absorption of the cornea allows blue-IRIS to be performed with greater efficacy than when using 800-nm femtosecond laser pulses.

METHODS

Fresh cat corneas were obtained postmortem and cut into six wedges. Blue laser pulses at 400 nm, with 100-fs pulse duration at 80 MHz were used to micromachine phase gratings into each corneal wedge at scanning speeds from 1 to 15 mm/s. Grating lines were 1 μm wide, 5 μm apart, and 150 μm below the anterior corneal surface. Refractive index (RI) changes in micromachined regions were measured immediately by recording the diffraction efficiency of inscribed gratings. Six hours later, the corneas were processed for histology, and TUNEL staining was performed to assess whether blue-IRIS causes cell death.

RESULTS

Scanning at 1 and 2 mm/s caused overt corneal damage in the form of bubbles and burns. At faster scanning speeds (5, 10, and 15 mm/s), phase gratings were created in the corneal stroma, which were shown to be pure RI changes ranging from 0.037 to 0.021 in magnitude. The magnitude of RI change was inversely related to scanning speed. TUNEL staining showed cell death only around bubbles and burns.

CONCLUSIONS

Blue-IRIS can be performed safely and effectively in living cornea. Compared with near-infrared laser pulses, blue-IRIS enhances both achievable RI change and scanning speed without the need to dope the tissue with two-photon sensitizers, increasing the clinical applicability of this technique.

Lasers in refractive surgery: history, present, and future

The history of laser refractive surgery is reviewed, followed by an overview of the current state of the field as well as a look at promising future developments.

Atomic force microscopic study of the human cornea following excimer laser keratectomy

The aim of this study was to examine the corneal surface structures with a new investigative method, the atomic force microscope following 193 nm excimer laser photoablation. Fresh human corneas were irradiated in vitro with an increasing number of impulses emitted by a 193 nm ArF laboratory excimer laser in order to produce either smooth flat surfaces or stair-like formations within the cornea. The corneas were investigated in a Topometrix(R) atomic force microscope in their native state. For comparison, three corneas were fixed with glutaraldehyde and processed for scanning electron microscopy. Atomic force microscopy and scanning electron microscopy revealed the same surface characteristics of photoablated corneas, though the preparation for scanning electron microscopy induced considerable shrinkage of the tissues. The layers of the cornea could be distinguished from each other and deeper ablations of the stroma produced a rougher surface. On the lateral walls of ablated stairs small droplets of ejected material could be seen with scanning electron microscope. Atomic force microscope produces three-dimensional images of the scanned native corneal surfaces and it could be a valuable tool to investigate the corneal smoothness. Our investigations have provided similar results as those obtained with scanning electron microscopy showing that the laser-ablated corneal surface remains relatively smooth. We suggest that the formation of condense droplets of ejected materials is based on hydrodynamic motions induced by boiling water solutions.

Corneal topography of small-beam tracking excimer laser photorefractive keratectomy

PURPOSE

To evaluate the topographic characteristic of photorefractive keratectomy (PRK) for low myopia performed with a small-beam (0.9 mm) tracking excimer laser.

SETTING

Department of Ophthalmology, LSU Eye Center, Louisiana State University Medical Center School of Medicine in New Orleans, and the Refractive Surgery Center of the South at the Eye, Ear, Nose, & Throat Hospital, New Orleans, Louisiana, USA.

METHODS

Sixty-seven eyes of 47 patients had PRK with a small-beam tracking laser. Of these, 49 eyes had data permitting evaluation of ablation centration; usable data for topographic analysis were available for 59 eyes preoperatively, 54 eyes at 1 month, 42 eyes at 3 months, and 25 eyes at 6 months, permitting measurement of various topographic parameters, including the cylinder (CYL), average corneal power (ACP), surface regularity index (SRI), surface asymmetry index (SAI), corneal eccentricity index (CEI), and coefficient of variation of corneal power (CVP).

RESULTS

Preoperatively, all eyes were topographically normal. Postoperatively, no eye exhibited a "central island" by even the least-restrictive definition, and all eyes had best spectacle-corrected visual acuities (BSCVAs) of 20/20 or better at all follow-ups. Mean decentration of the ablations from the pupil centers was 0.42 mm +/- 0.28 (SD) (n = 49). There was no correlation between measured decentration and BSCVA (P = .46). The central cornea was flattened (decreased ACP; P < .001) and made oblate (decreased CEI; P < .001) as expected. There was no increase in SRI or SAI (irregular astigmatism) at 6 months compared with preoperative values (P = .91); however, CYL and CVP (varifocality) increased slightly (P = .04 and .02, respectively).

CONCLUSION

The absence of significant regular or irregular astigmatism 6 months after PRK with the small-beam laser is an improvement over published results achieved with wide-beam lasers and is consistent with the excellent visual acuity results in this cohort.

Effect of ablation profile on wound healing and visual performance 1 year after excimer laser photorefractive keratectomy

BACKGROUND

Early photorefractive keratectomy ablations were of limited diameter and depth to maintain the integrity of the globe and to minimise postoperative haze. This study evaluated the effects of deeper, larger diameter wounds on refractive stability and corneal haze, and investigated the effects of ablation profile on wound healing and visual performance.

METHODS

One hundred patients undergoing -3.00D and -6.00D corrections were randomised to receive 5 mm, 6 mm, or multizone treatments. The multizone treatment was 6 mm in diameter, but only the depth of the 5 mm treatment. Outcome was measured by Snellen visual acuity, residual refractive error, objective techniques for haze and halos, pupil diameter, subjective night vision, and requirement for retreatment.

RESULTS

Overall, the results of 6 mm treatments were superior to those of 5 mm and multizone treatments: they had a smaller hyperopic shift (p < 0.01), a more predictable (p < 0.001) and stable refractive outcome, less haze (p < 0.05), smaller halos (p < 0.05), fewer subjective night vision problems, and fewer patients required retreatment.

CONCLUSIONS

Analysis of these data and a literature review of corneal wound healing demonstrated that the improved outcome associated with the 6 mm beam did not relate to the depth of ablation. The factor with greatest apparent influence on the development of haze and regression was the slope of the wound surface over the entire area of the ablation. Tapering the wound edge provided no additional benefit, and contributed to night vision problems. It is, therefore, recommended that small diameter or multizone treatments should not be used in low and moderate myopia.

Ultraviolet solid-state laser (213-nm) photorefractive keratectomy. In vivo study

BACKGROUND

The pulsed ultraviolet 213-nm solid-state laser has been demonstrated as an alternative to the gas argon-fluoride 193-nm excimer laser for photorefractive keratectomy (PRK). The authors studied the clinical course and histopathologic changes occurring in rabbit corneas after PRK with a 213-nm solid-state laser.

METHODS

The 213-nm output of neodymium:YAG frequency-quintupled laser was used to create 5-mm optical zone ablations in seven pigmented rabbit corneas. The radiant exposure was 250 mJ/cm2 delivered through a computer-controlled scanning delivery system with a spot size of 0.5 mm. The target ablation was 4.0 diopters with an estimated ablation depth of 40 microns. A clinical estimate of corneal epithelial healing and stromal haze was made at intervals over the 3-month study period. Animals were killed immediately after ablation, or at 10 days, 1 month, or 3 months after ablation. Corneal tissue was preserved for light microscopy and transmission electron microscopy at each study interval.

RESULTS

All corneas re-epithelialized within 10 days postoperatively. Anterior stromal haze was clinically visible at 3 days, increased until approximately 1 month, and then gradually decreased over the succeeding 2 months. Residual subepithelial haze was visible at 3 months. Results of histopathologic study documented normal epithelium healing over time; the basement membrane retained its regular thickness and hemidesmosomes were abundant at 3 months. The anterior stroma had an increased number of fibroblasts at 10 days, many of which remained until 1 month. A mild, transient, cellular reaction occurred throughout the thickness of the stroma and the endothelium.

CONCLUSION

Using the 213-nm ultraviolet solid-state laser with a scanning delivery system, PRK shows a similar clinical course and histopathologic findings to the 193-nm excimer PRK study in rabbits. It is a clinically viable procedure for refractive surgery and requires further human clinical trails to determine its efficacy.

Photorefractive keratectomy to treat low, medium, and high myopia: a multicenter study

This prospective, multicenter study investigated what influence patient age, sex, and time to re-epithelialization after myopic photorefractive keratectomy (PRK) have on stability of achieved correction and intensity of corneal haze. Seven surgeons performed PRK to correct myopia on 325 eyes using the same operative procedure, type of laser, and post-PRK treatment. Follow-up was from six to 12 months. Results indicate that corneal haze intensity is influenced by the time to re-epithelialization of the treated corneal surface. Patient age and sex may also affect haze intensity, although the results from this study were not conclusive.

ArF-excimer laser-induced secondary radiation in photoablation of biological tissue

Secondary radiation, emitted during and after the irradiation of corneal, dermal, and dental tissue by an ArF-excimer laser (193 nm), was qualitatively and quantitatively characterized. Emission of secondary radiation was found in the range of 200-800 nm. The intensity of secondary radiation in the range of 200-315 nm (UVC and UVB) is approximately 20% of the total intensity at high laser fluences (> 2 J/cm2), and approximately 50% at moderate laser fluences (< 500 mJ/cm2); 10 muJ/cm2 in the UVC and UVB were measured at the sample surface, at fluences (< 1J/cm2) which are of relevance for clinical procedures on soft tissues. In dental tissue processing, very high fluences (> 5 J/cm2) are required. As a consequence, laser-induced plasma formation can be observed. Secondary radiation can be used as a visible guide for selective removal of carious altered tissue. The data we have found might be of assistance in estimating potential hazards for future mutagenic studies in the field.

Excimer photorefractive keratectomy for myopia

PURPOSE

To study the safety, effectiveness, predictability, and stability of excimer laser photorefractive keratectomy (PRK) in 133 normally sighted eyes.

METHODS

An excimer laser was used with a fluence of 160 mJ/cm2 at a frequency of 5 Hz and an ablation zone of 5.0 mm. The effects of nitrogen purge at surgery and postoperative steroids were evaluated.

RESULTS

No significant complications occurred in any patient. After an initial over-correction, the refraction stabilized. The average results obtained at 6 months were maintained on successive examinations to 36 months. Analysis of their visual acuity in groups IIA, IIB, and III indicates that results at 6 months are predictive of final results at 1 to 3 years (P < 0.0001; r > 0.9). In myopia (1.00-6.00 diopters [D]) treated with the excimer laser, there was a trend toward improvement in results over the course of the study. In 60% in group IIA, 58% in group IIB, 71% in group III, and 88% in group III no nitrogen (no N2), +/- 1 D was obtained. For 20/40 or better uncorrected visual acuity, the outcome was 70% for group IIA, 67% for group IIB, 75% for group III, and 100% for group III no N2. Significant improvement was noted without nitrogen purge. No significant improvement was observed from steroid treatment as used.

CONCLUSION

In reducing myopia, PRK appears to be safe and effective. The results obtained are reasonably predictable and stable after 6 months. As more refinements are introduced, this procedure could become one of the most promising in refractive surgery.

Effect of contact diode laser on the cornea with and without absorbing dye

The semiconductor diode laser is a near-infrared laser; its 810-nm wavelength is maximally absorbed by melanin and has substantial transmissibility through cornea and sclera. Indocyanine green is the best photosensitive dye for the diode laser. The level of corneal damage produced by 810-nm diode laser, with and without absorbing dye (indocyanine green), and photoablative capabilities of this wavelength were studied using albino rabbits. We concluded that the contact application of this wavelength to the cornea in the presence of energy-absorbing dye causes both stromal and endothelial thermal damage. Therefore, 810-nm near-infrared semiconductor diode lasers are not suitable for photorefractive keratectomy or photoablative reprofiling.

Improved excimer laser photorefractive keratectomy system

An improved 193 nm excimer laser photorefractive keratectomy system is described here. The delivery system was optimised to produce a smooth keratectomy bed, to minimise the healing response, and to reduce the regression of the refractive result. A spatial filtering system was used to smooth the energy distribution within each pulse, and the iris was adjusted for every pulse, to eliminate steps and other irregularities from the keratectomy profile. An image rotator was used to average out any remaining non-uniformities in the energy distribution. The results from the first 39 eyes treated with this laser system with a maximum follow-up of 18 months, are summarised. No epithelial healing problems were noticed, and little or no loss of corneal clarity was noted at any stage during healing. Twenty of the keratectomy procedures were normally sighted myopic corrections, with a maximum follow-up of 5 months. Minimal regression of the refractive correction was seen during the first 8 weeks.

Corneal topography of excimer laser photorefractive keratectomy

The application of the 193 nm excimer laser for keratorefractive surgery promises to deliver a higher degree of precision and predictability than traditional procedures such as radial keratotomy. The development and evaluation of keratorefractive surgery have benefited from the parallel advances made in the field of corneal topography analysis. We used the Computed Anatomy Topography Modeling System (TMS-1) to analyze a Louisiana State University (LSU) Eye Center series of patients who had photorefractive keratectomy for the treatment of myopia with the VISX Twenty/Twenty excimer laser system. The excimer ablations were characterized by a relatively uniform distribution of surface powers within the treated zone. In the few cases that exhibited marked refractive regression, corneal topography analysis showed correlative changes. With topographical analysis, centration of the ablations relative to the center of the pupil could be evaluated. Marked improvement in centration occurred in the patients of LSU Series IIB in which the procedure to locate the point on the cornea directly over the pupil's center during surgery was refined. Corneal topographical analysis provides objective measures of keratorefractive surgical results and is able to measure the precise tissue removal effect of excimer laser ablation without the uncertainties caused by measuring visual acuity alone. Our observations forecast the need for improved aids to center the laser ablations and for the development of a course of treatment to prevent post-ablation stromal remodeling.

Corneal sensitivity after photorefractive keratectomy

Corneal anesthesia or hypesthesia can complicate refractive surgical procedures such as epikeratophakia and radial keratotomy. An esthesiometer was used to measure the corneal sensitivity in unoperated-on corneas and fellow corneas after excimer laser photorefractive keratectomy. Decrease in corneal sensitivity was noted within six postoperative weeks, with mean sensitivity being 75.2% +/- 13.3% of normal. Within the first three postoperative months, the patients operated on for correction of compound astigmatism recovered 95.7% +/- 5.3% of the corneal sensitivity, whereas the patients operated on for correction of severe myopia recovered 86.2% +/- 11.2% (P = .07). None of the patients had delayed epithelial healing or recurrent corneal erosions during the time of decreased corneal sensitivity. In otherwise normal myopic eyes, photorefractive keratectomy measurably reduced corneal sensitivity for several postoperative weeks.

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.

Controlled lens formation with unapertured excimer lasers: use with organic polymers and corneal tissues

A theoretical model is developed which allows for well-defined lenses to be formed with different dioptric properties using lasers that are known to ablate a variety of materials. With the theoretical development as a guide an experimental arrangement is defined for the ArF excimer laser and this arrangement is used to form lenses with highly smooth surfaces in three polymeric materials (polymethyl methacrylate, polycarbonate, and polyvinyl chloride) to test the validity of the theory. As part of the procedure to obtain these lenses, ablation curves (etch depth per pulse vs log of the laser fluence) were measured for each of these polymers and these curves are also reported. The experiments were extended to the rabbit cornea in which the corneal button was reshaped as part of keratomileusis surgery. This preliminary experiment resulted in a clear cornea with a smooth corneal surface and a measured change of 3 diopters in the treated eye.

A refractive and histopathologic study of excimer laser keratectomy in primates

Using a 193-nm excimer laser, we produced wide-area, refractive keratectomies on 18 cynomolgus monkey corneas and followed them up for up to 18 months. All corneas developed some subepithelial haze by one month. Electron microscopy disclosed epithelial thickening, absence of Bowman's layer, and subepithelial activated fibroblasts surrounded by disorganized collagen. By six months, the haze faded to a variable degree, the epithelium regained normal thickness, and the collagen was more organized. Persistent corneal haze at 12 months in some corneas correlated with electronlucent spaces in the subepithelial zone. Corneas were 90 microns thinner centrally two weeks after myopic ablation, but returned to preoperative thickness by six months. Myopic flattening and hyperopic steepening of 6 diopters were targeted, and over 7 diopters of each were achieved initially. Regression of induced curvature stabilized over several months. At 18 months, 4.4 diopters of myopic flattening and 5.2 diopters of hyperopic steepening remained.

Corrective measures for myopia

Many myopic people, expressing dissatisfaction with traditional methods of optical correction, are interested in a permanent correction of their refractive error which would alleviate dependence on corrective lenses. Although much effort has been put forth in the last century, there is still no method of correcting myopia which is broadly acceptable as safe and effective. The nonsurgical procedures of orthokeratology and the topical use of cycloplegics have not been well proven. Surgical measures are the current vectors of hope. Surgical procedures on parts of the eye other than the cornea have proven to be difficult. Surgery which alters the refractive power of the cornea (refractive keratoplasty) has been used frequently in the past decade. These procedures include keratomileusis, epikeratophakia and radial keratotomy. The latter is currently the most often performed method for the correction of myopia. This paper critiques the major methods, explains their historical development and basic procedures, lists major published studies and discusses their problems and promise for their future.

Corneal ablation in rabbits using an infrared (2.9-microns) erbium: YAG laser

The authors used a 2.9-microns infrared erbium:YAG (ER:YAG) laser to ablate the central corneas (a 3.5-mm wide, 180-microns deep area) of ten albino rabbits. In group 1 (10 eyes), the central epithelium was removed by mechanical scrubbing to ablate the anterior stroma. In group 2 (10 eyes), a central anterior corneal cap was removed to ablate the midstroma. In group 1, epithelial wound healing occurred in 2 to 3 days. A mild anterior haze was observed with the slit lamp, but it gradually cleared in most eyes of both groups. Histologic specimens were obtained immediately and at 2 and 6 months after ablation for light and transmission and scanning electron microscopy. This acute histopathologic study did not show any endothelial damage even after ablations at 300-microns depth, although there was thermal damage more prominent at the borders of the ablated area for an extension of 40 microns. After 2 months, in the group 1 eyes, the regenerated epithelium appeared normal except for a thickening at the margins of the ablated area. Six months postoperatively there was still epithelial thickening at the margins of the remodeled zone. The basement membrane was present with anchoring filaments and hemidesmosomes. The healing of the anterior (group 1) and midstroma (group 2) occurred with some degree of disorganization of the lamellae and an increase in the population of keratocytes. The density of keratocytes at the ablated areas at 6 months was reduced in comparison with the 2-month observation. By photokeratoscopy, the corneas were smooth and slightly flattened. The authors conclude that corneal ablation using an ER: YAG laser at 2.9 microns may meet the criteria for photorefractive keratectomy (PRK).

Evolution of excimer laser corneal surgery

Interest in the potential of the excimer laser for refractive surgery was generated by a series of experiments performed in IBM's T.J. Watson laboratories. The development of photoablative decomposition was applied to corneas with immediate implication for corneal refractive control. Scientific investigations demonstrated that submicron erosion ablation of material was possible without damage to remaining tissues. After refinement of the beam and analysis of thresholds, we subsequently demonstrated that healing of large ablated areas in rabbits and then primate eyes would occur without scar formation or loss of transparency. This prompted a number of commercial efforts to develop this device for both refractive and general corneal surgical applications. The early clinical investigations show that a predictable refractive change could be induced in a cornea and this change was stable over many months. Furthermore, the transparency of the cornea was unimpaired. These favorable early results have led to a number of clinical investigations of excimer laser corneal surgery including refractive keratectomy, a direct laser keratomileusis.

Scanning slit delivery system

We describe the structure and function of a delivery system designed for use with an argon fluoride excimer laser for corneal surgery. The basic principle of the delivery system is to shape the laser beam with a slit mask to a defined configuration. The image of the slit is moved across the cornea in a mathematically defined pattern to ablate a given amount of tissue, creating a new anterior radius of curvature for the cornea. The location of the beam on the cornea is determined by a rotating dove prism and a translating spherical lens, each of which is controlled by computer-regulated servomotors. The system is versatile and can be used for laser myopic or hyperopic keratomileusis, for correction of astigmatism by surface ablation, for creation of radial or transverse linear cuts, for excision of corneal buttons, and for uniform lamellar corneal smoothing procedures. The delivery system is currently used in the laboratory for the ablation of nonhuman primate corneas.

Human excimer laser lamellar keratectomy. A clinical study

The first ten blind human eyes in the United States to receive excimer laser (ArFl 193 nm) lamellar keratectomy (reprofiling) are presented. Seven of these patients were followed 6 to 12 months after ablation. All eyes are grossly clear in the region of ablation. Results of slit-lamp examination of all flattened ablated areas show mild superficial haze at the epithelial/stromal interface. This haze might not interfere significantly with vision in patients 7 to 10. Serial pachymetry and keratometry measurements, refraction, and digital keratoscopy show a progressive filling in of the excavated area by approximately two thirds but a loss of initial diopteric correction of only one third. Histopathologic analysis was obtained for four eyes. Transmission electron microscopy of three eyes enucleated 3 to 12 days after ablation shows 40-microns ablation depths through Bowman's layer and superficial stroma with minimal adjacent tissue damage and no inflammatory cells. The epithelium is increased in thickness by 50%, and firmly attached to the underlying stroma. A 4-month postablation specimen shows keratocyte activation with increased protein synthesis (presumed collagen and ground substance).

Effect of excimer laser radiant exposure on uniformity of ablated corneal surface

The argon fluoride (193 nm) excimer laser is being used to change the anterior corneal curvature for correction of refractive errors. Uniformity of the surface following laser ablation may play an important role in the rate of epithelial healing and amount and type of stromal scarring. To test the effect of radiant exposure (fluence) on surface smoothness, we ablated rabbit corneas with the 193 nm argon fluoride excimer laser at nine radiant exposures from 50 to 850 mJ/cm2. A total energy of 100 J/cm2 was used for each ablation at a frequency of 1 Hz. Scanning electron microscopy demonstrated progressive improvement of surface smoothness with increasing radiant exposures. Transmission electron microscopy demonstrated no consistent increase in thickness to the surface condensate (pseudomembrane) with increasing radiant exposure. Improvement in surface quality associated with increasing radiant exposures may result from a more uniform depth of ablation per pulse in the corneal lamellae that absorb laser wavelengths differently. Radiant exposures at levels where the depth of ablation is the same regardless of increasing energy densities achieve a more uniform surface because inhomogeneities in the beam and variation in energy from pulse to pulse do not affect the ablation rate.

Excimer laser trephination in penetrating keratoplasty. Morphologic features and wound healing

The imprecision of trephination of donor and recipient corneas is a major factor in post-keratoplasty astigmatism. In order to improve the quality of trephination, the authors developed a rotating slit delivery system for noncontact penetrating keratoplasty trephination using the excimer laser at 193 nm. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and light microscopy (LM) demonstrated the superior quality of excimer-cut buttons and recipient beds as compared with those obtained by free hand and suction trephines in human cadaver and rabbit eyes. The laser trephined more regularly and precisely without distortion of corneal topography and with less damage to adjacent corneal tissue. The authors morphologically examined wound healing at 6 hours, 12 hours, 3 days, 5 days, 2 weeks, 2 months, and 3 months after penetrating keratoplasty with laser and mechanical trephination in an animal autograft model. The laser did not cause any adverse alteration of wound healing processes including cellular migration, proliferation, and production of new tissue.