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.

Electron paramagnetic resonance spectroscopy of free radicals in corneal tissue following excimer laser irradiation.

BACKGROUND AND OBJECTIVES: Free radicals, detected previously in corneal tissue following 193 nm laser irradiation, may be important agents in the laser/tissue interaction. Electron paramagnetic resonance spectroscopy (EPR) has been used to examine such radical formation in detail. STUDY DESIGN/MATERIALS AND METHODS: Bovine corneal strips were frozen in liquid nitrogen, irradiated with excimer laser pulses, and assayed by EPR. Exposure conditions were varied to study radical formation dependence on laser intensity and repetition. Results were measured against a quantifiable standard to calculate radical quantum yield. RESULTS: Either weak or intense laser fluences produced comparable tissue EPR signals. Radicals accumulated in frozen tissue for at least 10 initial ablation pulses. Radical quantum yield in cornea was 0.15%. CONCLUSION: Corneal radical formation is largely a photochemical process driven by the 193 nm laser radiation. Reactive radical species are produced in substantial numbers and likely have a significant clinical role.

Noninvasive monitoring of excimer laser ablation by time-resolved reflectometry.

BACKGROUND: Current excimer laser photorefractive procedures use empiric etch rates to determine specific changes in corneal shape. A real-time analytic method for monitoring the tissue ablation process may be useful in tailoring energy delivery to a specific patient and in detecting detrimental phenomena such as corneal desiccation. METHODS: We monitored excimer laser ablation by studying the amplitude and temporal characteristics of ArF laser pulses reflected from the ablation site. Two target materials were used: polymethylmethacrylate (PMMA, a synthetic polymer that undergoes an incubation phase where no ablation occurs for an initial finite number of laser pulses), and bovine cornea. Observed reflectivity changes during irradiation of PMMA were compared to profilometric ablation depth measurements. Corneal ablation was performed both with and without nitrogen gas flow at the ablation site to study the effect of tissue desiccation. RESULTS: For ablation of PMMA at 160 mJ/cm2, the incubation phase included the initial eight laser pulses. For corneal tissue ablation at a fluence of 125 mJ/cm2, flowing nitrogen gas caused significant shortening and amplitude reduction in the reflected laser signals. CONCLUSIONS: Noninvasive time-resolved reflectometry provided real-time information about target ablation. This technique may have diagnostic utility during laser corneal surgical procedures.

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.

Dynamic optical properties of collagen-based tissue during ArF excimer laser ablation.

Various attenuation mechanisms affecting the absorption of ArF excimer laser light in collagenous tissues have been studied. Temporal distortion of the laser pulse reflected from the cornea has been observed over a range of incident pulse fluences including the ablation threshold. Reflected pulse shortening begins near the ablation threshold and advances with increasing fluence. The measurement of laser light scattered 30 degrees off specular axis from collagen gel targets indicates that the reflected-pulse distortion is partially a result of scattering. Collagen film transmission studies show an increase in 193-nm light transmission in ablation conditions. These nonlinear attenuation mechanisms may impact significantly on the photoablation process.

Tangential corneal surface ablation with 193- and 308-nm excimer and 2936-nm erbium-YAG laser irradiation.

An argon-fluoride excimer laser operating at 193 nm was used to perform tangential, wide-area surface photoablation of the corneas of bovine eyes. Comparisons were made with similar tangential ablations performed with the use of 308-nm xenon chloride excimer laser and qi/gl 2936-nm erbium-YAG laser irradiation to evaluate potential photochemical and thermal effects. Light and transmission electron microscopy of the corneal tissue revealed minimal tissue architectural changes for ablation at 193 nm. The zone of ultrastructural change at the corneal surface with the 193-nm excimer laser was 120 nm thick. The clinical appearance of the corneal surfaces produced by the tangential ablation at 193 nm was the most satisfactory of those with the three wavelengths tested, being macroscopically smooth and glossy.

Excimer laser corneal ablation: absence of a significant "incubation" effect.

Pulse-to-pulse consistency of excimer laser etching of cornea has been examined via two noncontact techniques: photoacoustic probe beam deflection, and time-resolved excimer pulse reflectometry. These methods clearly document the incubation phenomenon accompanying excimer laser ablation of polymethyl-methacrylate and the absence of the effect during polyimide ablation. In comparison, results for corneal ablation indicate consistent tissue etching over a train of pulses. Consequently, incubation appears to have negligible impact on corneal ablation.

Review of the clinical literature on the use of the Nd:YAG laser for posterior capsulotomy.

An overview of the safety and effectiveness of the Nd:YAG laser for posterior capsulotomy as discussed in the current scientific literature is presented. The results of three clinical investigations in Europe and ten clinical reports in the United States are briefly summarized.