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

Refractive Effect of Epithelial Remodelling in Myopia after Transepithelial Photorefractive Keratectomy

(1) Introduction: We analysed epithelial changes after the treatment of moderate myopia with transepithelial photorefractive keratectomy. (2) Materials and Methods: We used optical coherence tomography data and analysed changes in the stroma and epithelium after ablation. We aimed to ascertain how much epithelium hyperplasia occurred after TransPRK; for this, we used data from 50 eyes treated with TransPRK with the AMARIS 1050 Hz, with a minimum follow-up of 4 months. (3) Results: The measured epithelial changes corresponded to a less than 0.1 ± 0.2D of spherical effect, less than 0.2 ± 0.2D of astigmatic effect, and less than 0.5 ± 0.2D of comatic effect. (4) Conclusions: The changes in epithelial thickness after aberration-neutral transepithelial photorefractive keratectomy for moderate myopia were very small, indicating a low level of epithelial hyperplasia without resembling a regression-inducing lentoid.

The effect of intraluminal contact mediated guidance signals on axonal mismatch during peripheral nerve repair

The current microsurgical gold standard for repairing long gap nerve injuries is the autograft. Autograft provides a protective environment for repair and a natural internal architecture, which is essential for regeneration. Current clinically approved hollow nerve guidance conduits allow provision of this protective environment; however they fail to provide an essential internal architecture to the regenerating nerve. In the present study both structured and unstructured intraluminal collagen fibres are investigated to assess their ability to enhance conduit mediated nerve repair. This study presents a direct comparison of both structured and unstructured fibres in vivo. The addition of intraluminal guidance structures was shown to significantly decrease axonal dispersion within the conduit and reduced axonal mismatch of distal nerve targets (p < 0.05). The intraluminal fibres were shown to be successfully incorporated into the host regenerative process, acting as a platform for Schwann cell migration and axonal regeneration. Ultimately the fibres were able to provide a platform for nerve regeneration in a long term regeneration study (16 weeks) and facilitated increased guidance of regenerating axons towards their distal nerve targets.

Development and numerical solution of a mechanistic model for corneal tissue ablation with the 193 nm argon fluoride excimer laser

We detail the development and implementation of a global ablation model that incorporates a dynamically changing tissue absorption coefficient. Detailed spectroscopic measurements rule out plasma-shielding effects during the laser-tissue interaction and thereby support a photochemical mechanism. The model predicts ablation rate behavior that agrees well with a variety of experimental ablation rate data and that substantially deviates from a static Beer-Lambert model. The dynamic model predicts an enhancement in the tissue absorption coefficient of about 25%-50% as compared with the initial, static value. In addition, the model predicts an increase in the tissue ablation rate as corneal hydration increases, which may provide additional insight into variations in refractive surgery outcome.

Corneal epithelial healing after photorefractive keratectomy: analytical study

PURPOSE

To characterize the velocity of epithelial migration after photorefractive keratectomy (PRK) with 3 different corneal ablation patterns.

SETTING

Department of Ophthalmology, Catholic University of Rome, Rome, Italy.

METHODS

Fifteen patients (30 eyes) with mild to moderate myopia and with simple to compound myopic astigmatism were enrolled for this study. The surgical procedure consisted of standardized PRK with final smoothing performed using the Technolas Keracor 217C excimer laser. The reepithelialization process was evaluated at 0 hours, 20 hours, 40 hours, and 60 hours after surgery using a digital photo camera and custom software for measurement. Digital analysis of the images was performed. Corneal topographies were taken at 1 month, 3 months, 6 months, and 12 months after PRK.

RESULTS

The mean speed of radial migration in the 10 eyes (33%) in the low spherical ablation group was 0.087 mm/h +/- 0.008 (SD). This was significantly higher than that found in the 10 eyes (33%) in the high spherical ablation group (mean speed 0.078 +/- 0.007 mm/h; P<.001) and in the 10 eyes (33%) in the cross-cylinder ablation group (mean speed 0.055 +/- 0.014 mm/h; P<.001).

CONCLUSION

Analysis of the data shows that epithelial migration along the photoablated corneal surface depends on the ablation pattern. The epithelial sliding is highly influenced by local variations in the curvature of the stromal surface. The data demonstrate that faster epithelial wound healing after PRK is predictive of optimal visual performance.

Five-Year Follow-Up of Photorefractive Keratectomy for Myopia

PURPOSE

To analyze long-term results of photorefractive keratectomy (PRK) for myopia and myopic astigmatism.

METHODS

This retrospective study included 15 eyes of 8 patients who were examined annually for 5 years after PRK. The Nidek EC-5000 laser with an ablation zone of 5.0 mm was used. Evaluations included spherical equivalent manifest refraction, uncorrected visual acuity (UCVA), best spectacle-corrected visual acuity (BSCVA), corneal haze, and corneal astigmatism calculated by Fourier analysis of videokeratography data.

RESULTS

A tendency toward myopic regression was most evident within the first postoperative year, with manifest refraction changing from +0.80 +/- 1.62 D at 1 week to -0.45 +/- 0.70 D at 1 year postoperatively (P = .007). Regression continued after the second postoperative year. There was a statistically significant difference between manifest refraction at 2 years (-0.36 +/- 0.75 D) and 5 years (-1.11 +/- 1.12 D) (P = .002). Postoperative UCVA stabilized from 3 months up to 3 years, but slightly deteriorated at 4 years and thereafter due to the myopic refractive shift. BSCVA remained stable throughout the 5-year follow-up period. Several eyes developed mild corneal haze after surgery, but haze was minimal in the majority of patients by 1 year and continued to fade over time. The asymmetry component of the cornea significantly increased after surgery, with all postoperative values significantly higher than before PRK (P < .05). Higher order irregularity increased after surgery, with a statistically significant difference between preoperative and 1 year postoperative (P < .05), but values after 2 years were not different from preoperative baseline.

CONCLUSION

Fifteen eyes with a baseline refraction of -3.00 to -9.00 D had PRK with the Nidek EC-5000 laser and a 5-mm-diameter ablation zone. Myopic regression occurred in the first year, with continued mild regression of approximately -0.75 D between 2 and 5 years. Nevertheless, the results show the procedure was relatively safe and effective in this group.

Effect of excimer laser beam delivery and beam shaping on corneal sphericity in photorefractive keratectomy

PURPOSE

To evaluate the impact of beam delivery and beam shaping on corneal profiles after myopic excimer laser photorefractive keratectomy (PRK).

SETTING

Department of Ophthalmology, Charité-Campus Virchow Hospital, Humboldt University of Berlin, Berlin, Germany.

METHODS

Standard myopic 193 nm excimer laser PRK of -3.0 diopters (D) and -6.0 D was performed in porcine eyes using 2 commercially available broad-beam lasers with band-mask and fractal-mask beam shaping, 2 flying-spot lasers, and a scanning-slit laser. A silicone replica was obtained to preserve the corneal profile and was measured with a dynamic focusing topometry system.

RESULTS

The scanning-slit and flying-spot lasers created uniform profiles comparable to those in an untreated control group. Both broad-beam lasers with band-mask and fractal-mask beam shaping created central islands and paracentral profile valleys of 15.10 microm and 17.00 microm maximum height after -3.0 D PRK and 26.45 microm and 24.31 microm after -6.0 D PRK. An anti-central-island program, which applied a series of laser pulses centrally to compensate for the central profile elevations, did not eliminate the islands. Stromal surface roughness increased with ablation depth and was significantly worse after scanning-slit ablation than after broad-beam ablation.

CONCLUSIONS

Laser-induced deviations from the intended uniform corneal profiles were associated with broad-beam ablation and increased ablation depth and therefore lessened the predictability of the refractive outcomes. Scanning-slit and flying-spot systems produced predictably uniform corneal profiles.

Smoothing of the Ablated Porcine Anterior Corneal Surface Using the Technolas Keracor 217C and Nidek EC-5000 Excimer Lasers

PURPOSE

To demonstrate efficacy of a smoothing technique to increase regularity of the anterior corneal surface after photorefractive keratectomy (PRK), using two different excimer lasers.

METHODS

Spherical ablations of -10.00 D were performed on 11 fresh porcine corneas using either the Technolas Keracor 217C scanning-spot or the Nidek EC-5000 scanning-slit beam excimer laser. Following the procedure, we performed a phototherapeutic keratectomy treatment (smoothing technique) on half of the corneal surface. The smoothing technique was performed using a viscous solution of 0.25% sodium hyaluronate, which was spread on the cornea prior to the procedure. The ablation zone was 6 mm in diameter and the transition zone extended to 3 mm. The ablation depth was set at 10 microm. Corneas were then examined with scanning electron microscopy.

RESULTS

Smoother treatment zones were apparent in porcine corneas in which smoothing was performed following PRK, with both laser systems. Results from the two lasers were not directly compared.

CONCLUSIONS

The smoothing procedure performed following PRK using a viscous 0.25% sodium hyaluronate masking solution and a scanning laser system rendered the porcine corneal surface more regular.

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.

Objective assessment of transient corneal haze and its relation to visual performance after photorefractive keratectomy

PURPOSE

Photorefractive keratectomy has the potential to cause transient corneal haze. The purpose of this study was to evaluate the relationship between transient corneal haze as measured by an objective means and high and low contrast visual performance.

METHODS

In a prospective study, 44 eyes of 28 patients were examined preoperatively and at 1, 3, 6, and 12 months after photorefractive keratectomy. Five laser in situ keratomileusis and two intrastromal corneal ring segments (Intacs [KeraVision, Fremont, CA]) were included for comparison, because these procedures are not expected to cause haze. Haze was measured using a prototype objective hazemeter, TSPC-3, a modification of the Nidek EAS-1000. Visual performance was measured using high-contrast visual acuity and the Rabin Small Letter Contrast Test.

RESULTS

Corneal haze was greatest at the 1-month examination and was consistent with a decrease in visual performance on both tests. Corneal haze resolved in 82% of eyes by 10 +/- 4 months after photorefractive keratectomy. However, visual performance had not returned to preoperative levels in 65% and 81% of these eyes on the high-contrast visual acuity test and the Small Letter Contrast Test, respectively. Eyes that underwent laser in situ keratomileusis and Intacs did not develop corneal haze; however, visual decrements were measured.

CONCLUSIONS

As a clinical tool, the TSPC-3 hazemeter objectively measures very subtle changes in haze levels. Corneal haze appears to account for only approximately 50% of visual performance changes in the early healing period after photorefractive keratectomy. Other factors, namely topographic abnormalities, are more likely to be an important cause of persistent visual disturbances.

Wound healing response in the presence of stromal irregularities after excimer laser treatment

PURPOSE

To trace the fate of stromal irregularities after excimer laser treatment and to increase our knowledge of the reasons why surface irregularities in the ablation bed cause inferior postoperative results.

METHODS

Twelve New Zealand White rabbits received a transepithelial photoablation to a preset depth of 60 microm. An electron microscopy specimen grid was then placed on the denuded stroma and another 20 microm ablation was applied in order to produce surface irregularities. Another six rabbits received a plano transepithelial photoablation to a preset depth of 80 microm. The treated corneas were harvested at various timepoints and differentially further processed for microradiography, hematoxylin-eosin -, hyaluronan (HA)- and leukocyte protein L1 staining.

RESULTS

In the grid treated corneas the subepithelial mesh pattern is clearly discernible after 1 week, and after 4 weeks it is replaced by a subepithelial layer containing HA and water. The thinning of this layer between 1 and 12 weeks is statistically significant (p<0.05). After 4 and 8 week the plano treated corneas only exhibit some subepithelial HA- and water accumulation. After 1 day the grid treated corneas show an extensive stromal infiltration of leukocytes. In the plano treated corneas the leukocytes mainly remain on the surface.

CONCLUSIONS

During the healing process stromal irregularities are flattened, leaving a homogeneous zone with increased water content. This subepithelial layer is rarefying as new subepithelial tissue is forming. Postablational irregularities induce a more pronounced healing reaction when compared to a smooth ablation surface. Leukocyte infiltration seems to play a role in this process.

Morphological changes of rat mandibular bone with ArF excimer laser in vivo

OBJECTIVE

The purpose of this study was to investigate the morphological changes of bone tissue by ArF excimer laser irradiation in vivo.

SUMMARY BACKGROUND DATA

Recently, it has been demonstrated that soft and hard tissues can be removed by excimer laser irradiation with little thermal damage.

METHODS

ArF excimer laser was irradiated on the surface of the rat mandibular bone using the following parameters: wavelength; 193 nm; output: 0.12 W; pulse repetition rate: 10 Hz; spot size: 1.0 x 4.0 mm; irradiation time: 90 and 120 seconds. The ablation depth was measured at 90 and 120 seconds. Furthermore, the irradiated surface was examined histologically using scanning electron microscopy (SEM).

RESULTS

The relationship between ablation depth and irradiation time was almost linear. Macroscopically, the ArF excimer laser beam produced a defect with clean-cut margins without carbonization in the mandibular bone. Histologically, there was minimal evidence of thermal damage to the surrounding tissue. The bottom of the defect revealed a sawtooth appearance. In SEM observation, mosaic structures corresponded to the sawtooth structures observed with light microscopy.

CONCLUSIONS

These findings suggest that it is possible to remove bone tissue using ArF excimer laser irradiation without thermal damage. The effectiveness of this laser can be attributed to the photoablation of the bone tissue.

Preliminary in vitro Study of the Histological Effects of Low Fluence 193-nm Excimer Laser Irradiation of Corneal Tissue

PURPOSE

To determine if moderate numbers of low fluence, 193-nm excimer laser pulses modify or damage the corneal stroma.

METHODS

The corneal epithelium of fresh bovine eyes was scraped off and the exposed stroma was irradiated with 200 low fluence laser pulses from an argon fluoride excimer laser. This process was performed on five eyes each at two laser fluences, 10 mJ/cm2 and 30 mJ/cm2. The ten irradiated and three control (unirradiated) corneas were sectioned and studied by electron microscopy. The maximum and minimum thickness of the anterior layer of randomly oriented collagen fibers was measured using electron microscopy.

RESULTS

The mean maximum thickness of the anterior randomly oriented layer of collagen was 1.23 +/- 0.45 microm in the control corneas, 0.67 +/- 0.32 microm in the corneas irradiated at 10 mJ/cm2, and 0.10 +/- 0.12 microm in the corneas irradiated at 30 mJ/cm2. The mean thickness of corneal stroma removed was 0.7 microm at a fluence of 10 mJ/cm2 and 1.1 microm at a fluence of 30 mJ/cm2. A thin, electron-dense pseudomembrane was present at both fluences.

CONCLUSION

We report removal of bovine corneal stroma at 10 mJ/cm2--below the previously reported ablation threshold of 20 mJ/cm2.

Advances in refractive surgery: 1975 to the present

PURPOSE

To review the major advances in the field of refractive surgery occurring over the past 25 years.

METHODS

Literature review.

RESULTS

The major developments in refractive surgery over the past 25 years are reviewed.

CONCLUSIONS

The past 25 years have witnessed great changes in refractive surgery. As a result of advancements in technology, instrumentation, and technique, we have seen improvements in the treatment of all types of ametropias. In this article, we review some of the successes and failures of the past quarter-century.

Ablation rate of PMMA and human cornea with a frequency-quintupled Nd:YAG laser (213 nm)

BACKGROUND AND OBJECTIVE

As an alternative to the standard excimer laser used for PRK, we investigated the ablation rate at 213 nm of PMMA, and human corneas under controlled hydration.

STUDY DESIGN/MATERIALS AND METHODS

The output of a frequency-quintupled Nd:YAG laser (213 nm) was transformed into a quasi-Gaussian beam. PMMA and corneal lenticules maintained under controlled hydration were ablated until perforation was detected.

RESULTS

The ablation rate of PMMA and cornea at 213 nm were similar to that at 193 nm when radiant exposure was below 200 mJ/cm2 and increased gradually between one and two times faster than that at 193 nm when radiant exposure was > 200 mJ/ cm2.

CONCLUSIONS

PMMA and cornea ablation at 213 nm are similar to that at 193 nm and are different from that at 248 nm. The difference between PMMA and cornea ablation rates should be considered when using PMMA to test ablated diopter and smoothness for photorefractive surgery.

Wound healing after photorefractive keratectomy

For more than 15 years, the excimer laser has been used as a surgical instrument on the cornea. Photorefractive keratectomy (PRK) followed radial keratotomy as researchers sought a more precise technique. In PRK, precision turned out to depend on surgical technique as well as the wound-healing process, with the 2 factors interdependent. The PRK technique has evolved toward a large diameter, flat ablation curvatures, and an even surface. The role of such factors as cytokines and interleukins has become more clear in the past 10 years. However, understanding the wound-healing process becomes more complicated with increasing know edge. Learning the contributing factors and performing trials with new drugs and antibodies to modulate wound healing have shown positive results on the experimental level. Patient selection based on the concentration of epidermal growth factor in tears may be another way to increase PRK s precision. The PRK technique has taught much about wound healing. For the technique to be competitive, increased precision, particularly in eyes with high myopia, is needed. Two other factors are imperative: controlling postoperative pain and decreasing visual rehabilitation time.

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.

Corneal Ablation Profilometry and Steep Central Islands

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.

Corneal-tissue absorption coefficients for 193- and 213-nm ultraviolet radiation

The small-signal absorption coefficients of 193- and 213-nm nanosecond laser pulses in bovine corneal tissue have been studied. The absolute reflectance of a planar quartz-cornea interface was measured at various angles of incidence for low-intensity laser irradiation (i.e., pulse fluences 3 orders of magnitude below the ablation threshold). The reflectance-versus-angle data were analyzed by the use of Fresnel theory to estimate the effective complex index of refraction of the tissue. This analysis indicated corneal absorption coefficients of 39,900 ± 9800 cm(-1) at 193 nm and 21,400 ± 4900 cm(-1) at 213 nm.

Lasers in ophthalmology

This article reviews the principle uses of ophthalmic lasers, providing historical background with an emphasis on new applications and areas of investigation. Ophthalmic photocoagulation was the first medical laser application and has restored or maintained vision in millions of people. More recently, photodisruption and, increasingly, ablation have gained prominence for treating a wide range of ocular pathology. The unique properties of lasers have also been harnessed for diagnostic purposes, with optical coherence tomography representing a significant improvement over existing imaging methods. Many ophthalmic applications of lasers have been developed, but the field is a dynamic one which continues to evolve along with laser technology itself.

Dynamics of ablation plume particles generated during excimer laser corneal ablation

BACKGROUND AND OBJECTIVE

Although the empirical characteristics of ArF excimer laser corneal ablation have been well documented, the exact ablation mechanisms are not well understood. The present paper reports a quantitative analysis of corneal ablation plumes using in situ time resolved laser light scattering and Raman spectroscopy.

STUDY DESIGN/MATERIALS AND METHODS

Bovine corneas were used as the ArF excimer laser ablation targets. Light scattering data were recorded from the ablation plume as a function of height above the tissue surface and as function of delay time with respect to the ablative ArF laser pulse.

RESULTS

Raman spectra of the ablation plume allow identification of the particles as water. Mean plume particle diameters are found to decrease with height, while the particle volume fractions are relatively constant. The total volume of plume particles correlates well with the total volume of water in the ablated corneal tissue.

CONCLUSION

The finding of a non-evolving plume composed of water spherules, combined with the excellent agreement between total volume of water in the plume and the content of water in the ablated corneal tissue, support the concept of photodecomposition or "cold ablation" for corneal tissue during ArF excimer laser ablation.

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

BACKGROUND

Replacing the gas argon fluoride (193 nm) excimer laser with a solid-state laser source in the far-ultraviolet spectrum region would eliminate the hazards of a gas laser and would reduce its size, which is desirable for photorefractive keratectomy. The authors investigated corneal reshaping using a frequency-quintupled (213 nm) pulsed (10 ns) neodymium:YAG laser coupled to a computer-controlled optical scanning delivery system.

METHODS

A 250 +/- 15-mJ/cm2 radiant exposure was used to ablate a 5-mm optical zone in human cadaver eyes and rabbit eyes. The 213-nm laser pulses were delivered through and shaped by a computer-controlled optical scanning delivery system, producing a 0.5-mm spot with a quasi-Guassian energy distribution on the cornea. Corneal surface changes were documented by computer-assisted corneal topography. Light microscopy, scanning electron microscopy, and transmission electron microscopy were performed to examine the effects on corneal surface quality and cellular components.

RESULTS

Corneal topographic measurements showed myopic corrections ranging from 2.3 to 6.1 diopters. Results of postoperative examination with the slit lamp and operating microscope demonstrated a smoothly ablated surface without corneal haze. Histologic results showed a smoothly sloping surface without recognizable steps. The surface quality and cellular effects were similar to that of previously described excimer photorefractive keratectomy.

CONCLUSION

The authors demonstrated that an ultraviolet (213-nm) solid-state laser coupled to an optical scanning delivery system is capable of reshaping the corneal surface with smooth transition. The scanning beam delivery system may offer the advantage of producing spatially resolved, customized, aspheric corrections to optimize the quality of vision after photorefractive keratectomy.

Transmission of corneal collagen during ArF excimer laser ablation

The time-resolved transmission of collagen films and 10-microns sections of bovine cornea during ArF laser ablation has been investigated. The film studies were performed on thin layers of extracted bovine corneal collagen, the principal chromophore in 193 nm photoablation. Transmission measurements were made on both dry and water-saturated films to assess the sensitivity of the ablation process to hydration. Distinct transient optical changes were observed in both fully desiccated and rehydrated films. Dehydrated films exhibit rapid reduction in film absorption over the time-course of the ablating laser pulse, presumably due to chromophore bleaching or annihilation. In contrast, rehydrated films demonstrate a short-lived enhancement of the attenuation. In either case, a single ablative laser pulse increased the long-term transmission of the film, although this increase was a factor of five greater for dehydrated films than for rehydrated samples. Results obtained from corneal tissue sections were essentially identical to those derived from hydrated collagen films.

Excimer laser smoothing of a reproducible model of anterior corneal surface irregularity

A reproducible model of an irregular corneal surface was developed to test the ability of the excimer laser to treat such surfaces. Using a 193-nm argon fluoride excimer laser set at a fluence of 160 mJ/cm2, repetition rate of 10 Hz, and 185 pulses, fresh de-epithelialized pig eyes underwent phototherapeutic ablations through a piece of stainless steel wire screen that masked the cornea. This yielded an uneven corneal surface in a grid-like pattern, with the peaks 50 microns higher than the troughs. The eyes then underwent further treatment in an attempt to smooth the center of the irregularity. Hydroxypropyl-methylcellulose 0.3% protected the valleys in 12 eyes; 2 eyes were ablated without a protecting fluid. The same laser, at the above noted settings, was used, except that both 2 Hz and 10 Hz frequencies were used. Immediately after treatment, the eyes were processed for scanning electron microscopy. The eyes treated at 2 Hz showed less surface irregularity than did those treated at 10 Hz. The eyes treated without a protecting fluid, regardless of repetition rate, had the greatest irregularities. This model is simple and reproducible, and the authors' results suggest that modifying the repetition rates of the excimer laser can influence its effectiveness in smoothing irregular corneas.

Solid state ultraviolet laser (213 nm) ablation of the cornea and synthetic collagen lenticules

We used a Q-switched Nd:YAG laser with non-linear optical crystals to produce the 5th (213 nm) and the 4th (266 nm) harmonic frequencies. Using these two wavelengths, we ablated fresh porcine corneas and type I collagen synthetic epikeratoplasty lenticules. For the 213-nm ablation, radiant exposure was 1.3 J/cm2. The ablation rate was 0.23 micron per pulse for the epikeratoplasty lenticules. We examined all tissues with light microscopy, transmission electron microscopy, and scanning electron microscopy. Histology for the 213-nm ablation showed a clean ablation crater with minimal collagen lamellae disruption and a damage zone less than 1 micron. In comparison, the 266 nm radiation showed more charring at the edges with a damage zone approximately 25 microns deep with disruption of the stromal lamella. Our results show that this solid state UV laser is a potential alternative to the excimer laser for cornea surgery.