Human corneal ablation threshold using the 193-nm ArF excimer laser

PURPOSE: To determine the human corneal threshold ablation energy density for the 193-nm ArF excimer laser, approximating clinical conditions. METHODS: The VISX Star (Santa Clara, CA) 193-nm argon fluoride excimer laser was used to ablate the cornea in human eye bank eyes under clinical conditions. Corneas were exposed to energy densities of 10, 20, 30, 35, 40, 45, and 140 to 160 mJ/cm2. Corneas were fixed for light and transmission electron microscopy immediately after laser exposure. RESULTS: Different ablation thresholds for various corneal structural elements were observed. The ablation threshold for the collagen in the corneal stroma was determined to be 30 mJ/cm2. Keratocytes had ablation thresholds of 40 mJ/cm2. These different ablation thresholds accounted for the production of stromal peaks and valleys, with the keratocytes atop the peaks. CONCLUSIONS: Different corneal structural elements have different ablation threshold energy densities.     

Microsurgery of the retina with a needle-guided 193-nm excimer laser.

This article presents a method used to guide the beam from an argon fluoride excimer laser to make it suitable for microsurgical purposes and confine it to areas that can be varied in dimension from 1 micron to tens or hundreds of microns. This approach guides the excimer laser beam with an articulated mechanical arm and confines it with variable-diameter tapered tubes, possibly allowing the use of this laser in in vitro retinal surgery with endolaser techniques. Currently, because of the lack of a delivery and focusing system for the 193-nm argon fluoride beam and its absorption by biologic liquids, this laser is used exclusively in ophthalmology for topical applications, such as corneal sculpting. This new method resolves these problems in a unique way with impressive results. Specifically, it was shown that, with this needle-guided excimer laser, it is possible to remove retinal tissue accurately without detectable damage to surrounding cells. Applications of this new technique in retinal surgery are discussed.     

准分子激光屈光性角膜切削术治疗重度近视

目的 探讨准分子激光屈光性角膜切削术（ＰＲＫ）治疗重度近视的疗效。方法 使用Ｃｈｉｒｏｎ Ｖｉｓｉｏｎ的Ｋｅｒａｃｏｒ１１６准分子激光机,采用多光区切削方法治疗重度近视４９例８９眼,屈光度范围：－１０．００～－２５．００Ｄ。术后随访１ａ,并对结果进行分析。结果 ８９眼重度近视ＰＲＫ术后１ａ时,裸眼视力≥１．０者１２眼,占３．４９％,０．５～０。８者１７眼,占１９．１０％,≤０．４者６０眼,占７６７     

Therapeutic use of the 193-nm excimer laser in corneal pathologies

Purpose: To analyze the results of phototherapeutic keratectomy. Patients and methods: We performed 193-nm excimer laser phototherapeutic keratectomy (PTK) in 252 eyes of 216 patients suffering from pain and/or decrease in visual acuity. One hundred and three eyes had recurrent erosions of the cornea, 86 eyes underwent excimer laser smoothing of the cornea after pterygium surgery, 29 eyes had a bandlike keratopathy (25 rough, 4 smooth) and 34 eyes had other pathologic conditions such as amyloidosis of the cornea, anterior corneal dystrophies, scars after injuries, alkali burns, superficial stromal dystrophies and infections. Recurrent erosions and epithelial dystrophies were treated with 15–20 pulses (160–200 mJ/cm², 8 mm ablation zone) after mechanical abrasion of the epithelium. Removal of corneal opacities and scars required the use of a masking fluid (methylcellulose) in different concentrations and slit-lamp control (integrated in the delivery system of the excimer laser). Results: Some 91% of the eyes with recurrent erosions were recurrence-free. Fifty-two per cent of the eyes with pterygium had recurrences if the baresclera technique was used and 33% of the eyes if a free conjunctival graft was used. The difference was not significant. All of the patients with bandlike keratopathy were pain-free. In 88% of the eyes with special indications the treatment goal was achieved. No positive effect was seen after alkali burn, in a patient with anterior membrane dystrophy (Grayson-Wilbrandt corneal dystrophy) or in a patient with a corneal protuberance. In one patient with scleroperikeratitis a late recurrence of the opacity was observed 3 years after surgery. A loss of best corrected visual acuity was found only in one patient with bullous keratopathy in whom the treatment goal was the reduction of pain. All patients with smooth bandlike keratopathy had an improvement in best corrected visual acuity of at least one line. About 70% of patients with special indications improved by at least one line, up to nine lines. A possible hyperopic shift in all groups could be minimized using a large ablation zone. Conclusion: PTK with the 193-nm excimer laser is a safe and effective treatment for many superficial diseases of the cornea.Werner Förster is the Primary Investigator for Schwind Keratom     

Mutagenic potential of a 193-nm excimer laser on fibroblasts in tissue culture

In response to concerns regarding possible DNA damage by far ultraviolet radiation during excimer laser corneal surgery, the mutagenic potential of an argon fluoride excimer laser (193 nm) on BALB/3T3 mouse fibroblasts grown in tissue culture was investigated. The cumulative incidence of anaplastic transformation after subablative radiant exposures from 3.5 mJ/cm2/pulse to 13.4 mJ/cm2/pulse was 3.6% of all cell cultures. The incidence of anaplastic transformation in nonirradiated controls was 4.2%. Transformation after exposure to x-ray radiation (60.9 rad) was 98.8%. The difference between the incidence of transformation of nonirradiated controls or excimer-treated cultures compared with x-ray radiation-treated cells was significant P less than 0.0001 (chi square test). In this standard cell line, 193-nm laser energy does not appear to have substantial mutagenic potential.     

Reactivation of latent herpes simplex virus by excimer laser photokeratectomy.

We tested whether excimer laser photorefractive and phototherapeutic keratectomy may reactivate latent herpes simplex and cause recurrent keratitis in mice. Two of ten latently infected mice that were treated with ten excimer laser pulses to the corneal epithelium shed herpes simplex virus type 1, as did four of ten mice that were treated with 50 excimer laser pulses. Ocular shedding of herpes simplex virus was detected in four of ten mice that were treated with ethylenediamine-tetraacetic acid (EDTA) scraping of the corneal epithelium without laser keratectomy, and in six of ten mice on which combined EDTA-facilitated epithelial removal was performed followed by the application of ten excimer laser pulses. In both EDTA-treated groups, viral shedding was prolonged and 18 of 20 mice developed marked corneal opacification or neovascularization, or both. Corneal photoablation with the excimer laser may induce reactivation of latent herpes simplex virus, even in mice with clear and smooth-appearing corneas, and should be considered in the differential diagnosis of humans with persistent corneal epithelial defects after refractive or therapeutic excimer procedures.     

Long-term results of phototherapeutic keratectomy with 193-nm excimer laser for macular corneal dystrophy

PURPOSE: To investigate functional and morphologic long-term outcome of phototherapeutic keratectomy (PTK) in macular corneal dystrophy. DESIGN: Retrospective, clinical single-center study. METHODS: Between October 1990 and February 2004, 10 eyes (five oculus dexter, five oculus sinister) of 6 patients (mean age, 23 +/- 8 years [range, 15 to 37 years]) with superficial plaque-like opacities that were caused by macular corneal dystrophy were included. After epithelial debridement and pannus removal, PTK was performed with the 193 nm MEL 60 excimer laser. Intended laser ablation depth varied from 20 to 100 microm; the repetition rate was 20/s or 25/s, and the pulse number was 1774 +/- 502 (range, 976 to 2422). Mean follow-up was 4.5 +/- 3.1 years [maximum, 8.7 years]). Main outcome measures included: uncorrected visual acuity, best corrected visual acuity (BCVA), spherical equivalent, keratometric central power, keratometric astigmatism and regularity (semiquantitative classification of Zeiss keratometry; scale 0 to 3), postoperative "haze," time interval until complete epithelial closure, recurrence rate, and necessity of subsequent penetrating keratoplasty (PK). RESULTS: BCVA increased from 0.3 +/- 0.2 before the operation to 0.6 +/- 0.1 after the operation; the mean spherical equivalent increased from -0.9 +/- 1.1 diopters to -0.4 +/- 1.8 diopters. Mean keratometric central power decreased from 44.5 +/- 0.7 diopters to 42.5 +/- 0.6 diopters. During follow-up, recurrences occurred in nine eyes (90%) after 3.4 +/- 0.4 years. In six eyes, a PK was performed after an average of 5.0 years (range, 3.7 to 6.7 years). None of the transplants had a recurrence during the follow-up period. CONCLUSION: In superficial opacities that are caused by macular corneal dystrophy, PTK can increase BCVA moderately for a limited period of time. Despite possible complications, primary PK still appears to be the definite therapeutic option for patients with macular corneal dystrophy.     

An acute light and electron microscopic study of ultraviolet 193-nm excimer laser corneal incisions

The 193-nm ultraviolet beam from an argon fluoride excimer laser was focused on the corneas of rabbits to produce incisions of the type necessary for radial keratotomy. The energy densities used were in two ranges, 1.0 to 2.1 J/cm2 per pulse and 200 to 700 mJ/cm2 per pulse. The eyes were enucleated and fixed for histologic and electron microscopic examination immediately after exposure. Structural analysis of the higher energy density exposures showed ridging on the surface of the cornea, micro-pitting on the stromal surface inside the cut, and denudation of the endothelium under the ablation zone. The lower energy density incisions did not exhibit significant surface ridging or endothelial cell loss but did exhibit significant stromal swelling during the laser exposure thus making it difficult to produce incisions of a precisely controlled depth. Beam profile measurements and infrared thermal measurements of the cornea surface during laser exposure were made.     

Persistent corneal haze after excimer laser photokeratectomy in plasminogen-deficient mice

PURPOSE: Excimer laser photorefractive keratectomy creates a nonvascular wound of the cornea. Fibrin deposition and resolution after excimer laser photokeratectomy were investigated in relation to corneal repair and restoration of clarity in mice with a genetic deficiency of plasminogen. METHODS: A Summit Apex Laser (Summit, Waltham, MA) was used to perform 2-mm, 175-pulse, transepithelial photoablations that resulted in deep stromal keratectomies. Photokeratectomy was performed on the corneas of plasminogen-deficient (Plg-/-) mice and littermate control animals. Eyes were examined for re-epithelialization and clarity throughout the 21-day observational period. Histologic sections were taken during the observational period and fibrin(ogen) was detected immunohistochemically. RESULTS: Re-epithelialization was rapid and complete within 3 days in both control and Plg-/- animals. Exuberant corneal fibrin(ogen) deposition was noted in Plg-/- mice and sparse fibrin(ogen) deposition in control mice on days 1 and 3 after injury. Fibrin(ogen) deposits resolved in control mice but persisted in Plg-/- mice (74% of eyes at 21 days; P < 0.004). Corneal opacification, scarring, and the presence of anterior chamber fibrin(ogen) occurred in plasminogen-deficient mice but not in control mice. CONCLUSIONS: Fibrin(ogen) deposition occurs during corneal wound repair after photokeratectomy. Impaired fibrinolysis in Plg-/- mice caused persistent stromal fibrin deposits that correlated with the development of corneal opacity.     

"Orientation teeth" in non-mechanical laser corneal trephination for penetrating keratoplasty: 2.94 microm Er:YAG v 193 nm ArF excimer laser.

BACKGROUND/AIMS: "Orientation teeth" at the donor trephination margin and correspondent "notches" at the host margin facilitate graft orientation and avoid "horizontal torsion" induced by asymmetric suture placement. In this study the quality and reproducibility of these structures created by non-mechanical laser corneal trephination were compared using two laser emissions. METHODS: The procedure was performed in 20 enucleated pigs' eyes using open metal masks with eight "orientation teeth/notches" (0.3 x 0.15 mm, base x height), an automated globe rotation device, and either a 193 nm ArF excimer laser or a Q switched 2.94 microm Er:YAG laser. "Teeth/notches" were analysed by planimetry and scanning electron microscopy (SEM). RESULTS: Mean size was 0.30 (0.027) x 0. 16 (0.017) mm for "teeth" and 0.30 (0.035) x 0.15 (0.021) mm for "notches" (excimer), and 0.31 (0.022) x 0.16 (0.015) mm and 0.30 (0.031) x 0.14 (0.021) mm respectively (Er:YAG). Overall, variability of notches was higher than that of teeth. By SEM, comparable cut regularity and sustained ablation profile were observed with both lasers. However, the corneal surface at the cut edge appeared slightly elevated (相似文献   

Cytotoxic and mutagenic action of 193-nm and 213-nm laser radiation

PURPOSE: To compare the cytotoxic and mutagenic effect of 213-nm and 193-nm laser radiation on cultured mammalian cells. METHODS: Chinese hamster lung (V79) cells were exposed to 193-nm radiation from an argon fluorine excimer laser or 213-nm radiation from a 5th harmonic Nd:YAG laser. The cytotoxic action of the lasers was compared by determining the number of V79 cell colonies that formed 1 week after irradiating cells with different doses of 193-nm or 213-nm laser radiation or with continuous wave 254-nm radiation. The cytotoxic action of the lasers on primary cultures of human corneal fibroblasts was also compared. The mutagenic potential of the lasers was compared by measuring the number of ouabain or 6-Thioguanine(6TG)-resistant V79 mutants that formed after exposing V79 cells to 193-nm or 213-nm radiation. RESULTS: The dose of 193-nm laser radiation that resulted in 37% survival (D37) of V79 cells was estimated to be 11.3 mJ/cm2 compared to 3.2 mJ/cm2 for 213-nm laser radiation and 1.2 mJ/cm2 for 254-nm UV radiation. The mean number of ouabain-induced mutants induced at the D37 for 193-nm, 213-nm, and 254-nm laser radiation were 28, 166, and 279 mutants/10(7) cells, respectively. Continuous wave 254-nm radiation induced 6TG-resistant colonies, but there was no significant induction of 6TG-resistant mutants by either laser. CONCLUSIONS: Although the in vitro data presented herein may or may not be meaningful to humans, the 213-nm Nd:YAG laser was more cytotoxic and mutagenic than the 193-nm excimer laser on cultured mammalian cells but was less cytotoxic and mutagenic than 254-nm radiation.     

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.     

The effect of the ArF excimer laser onCandida albicans in vitro

Candida albicans colonies in Sabouraud agar plates were irradiated with the excimer laser as follows: (a) at 10 Hz, power densities of 115–300 mJ/cm² and 200–3000 pulses, and (b) power density of 115 mJ/cm² and 200 pulses at 10–50 Hz. Additional colonies were irradiated with a power density of 200 mJ/cm² at 10 Hz and 500 pulses. Each colony was cultured and the visible colony forming units were counted after 24 h. The cultures remained sterile at: 115 mJ/cm², 1500 or 3000 pulses; 200 mJ/cm², 400 or 500 pulses; and 300 mJ/cm², 300 or 400 pulses. They decreased significantly in other groups. Photoablation with a power density of 115 mJ/cm² and 200 pulses significantly decreased the number of yeast colonies in the culture plates at 30 Hz (p < 0.029) and 50 Hz (p < 0.02). Photoablation did not affect the counts in colonies located 1 or 2 mm from the treated colonies. Various energies of the excimer laser may significantly reduce or eliminate yeast in vitro.     

Complications of laser keratomileusis with the excimer laser (193 nm)]

Complications of photorefractive keratectomy (PRK) for myopia correction are presented based on 615 procedures with a follow-up of up to 2 years. Intraoperative complications with experienced surgeons are extremely rare. The used laser system worked without technical failures for the last 18 months. Gross eccentricities of the ablative zone (1.0 mm to 1.5 mm) occurred in two eyes (0.3%). Also, epithelial disorders are very rare. Recurrent erosions did not occur. Increased intraocular pressure due to the postoperative steroid medication (dexamethasone 0.1% or prednisolone 1%) was manifest in about 30% in patients with a baseline myopia of up to -9.0 D. In myopia over -9.0 D the incidence of steroid glaucoma was about 50%. Steroid responders reveal an increased risk of overcorrection. Subepithelial haze is a common symptom after PRK. Scarring interfering with vision correlates with the amount of attempted correction: up to 6.0 D the incidence of scars is about 0.5% and increases to more than 10% in corrections of more than 10 D. Also, undercorrections of more than 1 D depend strongly on the attempted correction. The most severe complication was a noninfectious corneal ulcer in a patient with systemic lupus erythematosus. Therefore, autoimmune and connective-tissue diseases represent an absolute contraindication for PRK. Corrections of more than 6.0 to 7.0 D should be considered as a relative contraindication for PRK at this time.     

Long-term follow-up of excimer laser phototherapeutic keratectomy

PURPOSE: To evaluate long-term follow-up results of excimer laser phototherapeutic keratectomy (PTK) in a Japanese population. METHODS: Twenty-six patients (31 eyes) with corneal opacity were treated with excimer laser PTK. Preoperative diagnoses included 16 eyes with band keratopathy, 10 with granular dystrophy, and 5 with corneal scar. Mean postoperative follow-up was 27 months. RESULTS: Corneal opacity was reduced in all patients. At postoperative month 12, best spectacle-corrected visual acuity (BSCVA) improved from the preoperative level in 22 eyes of 28 eyes, did not change in 3 eyes, and declined in 3 eyes. BSCVA at month 24 was better than the preoperative acuity in 17 eyes of 23 eyes, similar in 1 eye, and worse in 5 eyes. Eyes with granular dystrophy showed significantly better BSCVA improvement than those with band keratopathy. A hyperopic shift of +1.0 diopter or more occurred in 14 eyes of 28 eyes at month 12 and in 12 eyes of 23 eyes at month 24. No serious adverse effects were encountered during the 3-year follow-up period. CONCLUSIONS: Excimer laser PTK is a safe and effective procedure for the treatment of Japanese patients with superficial corneal opacity.     

Myopic photorefractive keratectomy with the excimer laser. One-year follow-up

To evaluate the efficacy, predictability, and stability of myopic photorefractive keratectomy (PRK), the authors completed a 1-year follow-up study on a consecutive series of 26 sighted eyes undergoing this procedure. The results of this follow-up are presented. In addition, side effects and complications in another 255 sighted eyes with a follow-up of 3 months to 1 year are reported to judge the safety of the procedure. Twenty-four of 26 eyes (92%) were within +/- 1.0 diopter (D) of the intended final refraction (baseline, -1.4 to -9.25 D). Fifty-eight percent of the eyes were stable within +/- 0.25 D between 6 and 12 months. One year after surgery, none of the patients lost or gained more than one line of best corrected (spectacle) visual acuity. Uncorrected visual acuity improved to 20/40 or better in 96% of the eyes and to 20/20 or better in 48% of the eyes (not including the eyes that were intentionally undercorrected). However, visual acuity with glare decreased from 20/27 preoperatively to 20/31 after 1 year. Scarring occurred in 2.8% of the treated corneas. Risk factors for scarring include noncompliance with postoperative steroid medication, high myopic corrections, and high steroid responders (3.1%). Collagen vascular and other autoimmune diseases are a contraindication for PRK.     

193 nm excimer laser sclerostomy in pseudophakic patients with advanced open angle glaucoma.

A modified open mask system incorporating an en face air jet to dry the target area during ablation and a conjunctival plication mechanism, which allows ab externo delivery of the 193 nm excimer laser without prior conjunctival dissection, has been developed to form small bore sclerostomies accurately and atraumatically. Full thickness sclerostomies, and sclerostomies guarded by a smaller internal ostium can be created. A pilot therapeutic trial was conducted in pseudophakic patients with advanced open angle glaucoma. Six full thickness sclerostomies (200 microns and 400 microns diameter) and three guarded sclerostomies were created in nine patients by 193 nm excimer laser ablation (fluence per pulse 400 mJ/cm2, pulse rate 16 Hz, air jet pressure intraocular pressure +25 mm Hg). After 6 months' follow up, intraocular pressure was controlled (< or = 16 mm Hg) in eight of the nine patients (6/9 without medication). Early postoperative complications included hyphaema (trace--2.5 mm) (6/9), temporary fibrinous sclerostomy occlusion (4/9), profound early hypotony (all patients without fibrinous occlusion), and suprachoroidal haemorrhage in one case. Conjunctival laser wounds were self sealing. Small bore laser sclerostomy procedures are functionally equivalent to conventional full thickness procedures, producing early postoperative hypotony, with an increased risk of suprachoroidal haemorrhage in association with this. Further research is required to improve control over internal guarding in excimer laser sclerostomy before clinical trials of this technique can safely proceed.     

The effect of water content on the 193 nm excimer laser ablation

Background: Water content of the corneal stroma may influence excimer laser ablation and may therefore affect residual refractive error following laser in situ keratomileusis. This study reports associations between water content of hydrogel materials and laser ablation depth.
Methods: Hydrated ( n  = 4) and dehydrated ( n  = 4) hydrogel buttons of 38%, 45%, 55% and 69% water content were ablated with the Nidek EC-5000 ArF 193 nm excimer laser, set to deliver a -6.00 DS curvature. Central curvature, optical quality and water content were measured before and after ablation. Hydrated buttons were rehydrated postablation and prior to measurement, to eliminate the effect of water removal during the procedure. The ablation depth per pulse was calculated.
Results: The average ablation rate for fully hydrated buttons was 0.51 ± 0.17 µm. The ablation rate for hydrated mat­erials (dry component ablation) reduced with increasing water content ( P  < 0.001). Dry hydrogel materials (0% water content) had an average ablation rate of 0.23 ± 0.06 µm per pulse.
Conclusions: For a constant laser energy output, lower water content materials ablated to a greater extent than higher water content materials. This model provides a simple way to assess the effect of water content and dehydration on myopic laser in situ keratomileusis.