Absorption of 193- and 213-nm laser wavelengths in sodium chloride solution and balanced salt solution.

OBJECTIVE: To determine absorption coefficients for sodium chloride solution (saline) and balanced salt solution at the 193- and 213-nm laser wavelengths. METHODS: Absorption coefficients were obtained for each of the component species found in balanced salt solution. This was achieved by measuring laser pulse transmission through solutions of varying concentration. The experiments were repeated using the 193-nm excimer and 213-nm solid-state laser wavelengths. Results for each species were then used to obtain an overall absorption coefficient and penetration depth for balanced salt solution and 0.9% sodium chloride solution. RESULTS: Absorption coefficients in balanced salt solution for the 193- and 213-nm wavelengths were found to be 140 and 6.9 cm(-1), respectively. In 0.9% sodium chloride solution, the absorption coefficient was 81 cm(-1) at 193 nm and 0.05 cm(-1) at 213 nm. At 193 nm, absorption in balanced salt solution was dominated by sodium chloride. Sodium citrate emerged as the dominant species of absorption at 213 nm. CONCLUSIONS: For the species investigated, we found reduced absorption for the longer wavelength of 213 nm. While the difference in wavelength between 193 and 213 nm is within about 10%, the respective molar absorption coefficients varied by 1 to 4 orders of magnitude. This indicates that predictions for the wavelength-dependent changes of absorption coefficients of other solutions are unreliable. CLINICAL RELEVANCE: Fluid placed on the surface of the cornea during keratorefractive surgery has proved to be a barrier to ablation for the 193-nm wavelength. The increased penetration depth through sodium chloride solution and balanced salt solution for the longer 213-nm laser wavelength may mean that these solutions cannot be used as a masking agent for keratorefractive procedures performed with this wavelength.

Investigation of corneal ablation efficiency using ultraviolet 213-nm solid state laser pulses.

PURPOSE: To determine the threshold and efficiency of corneal ablation for various values of laser fluence at the ultraviolet wave length of 213 nm. METHODS: A commercial Q-switched Nd:YAG laser was used to produce the fifth harmonic wavelength of 213 nm. Ablation trials were carried out on porcine corneas. Slit ablations of dimensions 0.5 x 2.5 mm were performed using seven values of laser fluence to obtain the most efficient fluence for ablation. The morphology of each ablation was obtained using a computer-automated confocal profiling system. These profiles were then analyzed to determine the ablation depth for the range of fluence values used. RESULTS: A fluence in the region of 200 mJ/cm2 was found to be the most efficient for ablation. The efficiency in this region was approximately 0.35 mm3/J, and the ablation rate was found to be 0.6 microm/pulse. The ablation threshold was found to occur at a fluence of 50 mJ/cm2. In the region of highest efficiency, the peak varied slightly in the fluence range between 150 and 250 mJ/cm2. CONCLUSIONS: This study confirms that the corneal ablation properties at 213 nm are comparable with those at the 193-nm excimer laser wavelength. Increased pulse energy was obtained for the fifth harmonic of Nd:YAG lasers at 213 nm through the use of new nonlinear optical crystals to perform the frequency conversion. A solid state laser is feasible to replace the excimer gas laser for performing refractive surgery procedures. For the first time, the increased energy at 213 nm allows large-beam ablations to be performed at this wavelength.

Registration of stereo and temporal images of the retina.

The registration of retinal images is required to facilitate the study of the optic nerve head and the retina. The method we propose combines the use of mutual information as the similarity measure and simulated annealing as the search technique. It is robust toward large transformations between the images and significant changes in light intensity. By using a pyramid sampling approach combined with simulated reannealing we find that registration can be achieved to predetermined precision, subject to choice of interpolation and the constraint of time. The algorithm was tested on 49 pairs of stereo images and 48 pairs of temporal images with success.

Tele-ophthalmic screening using digital imaging devices.

BACKGROUND: Digital imaging systems provide immediate, magnified images that can easily be analysed, enhanced, archived, printed on near photographic-quality paper and transferred electronically to remote computers. We have assembled a digital ophthalmic system and tested it on patients with some common causes of blindness: corneal scarring and cataract, diabetic retinopathy, macular degeneration and glaucoma. METHODS: Digital images were obtained using a variety of ophthalmic imaging devices ranging from slit-lamp, fundus camera, indirect ophthalmoscope and scanning laser ophthalmoscope. These images were compressed in order to concentrate image information (image size reduced by 90-95%) and minimize transmission time (reduced by 97-98%). Standard or mobile telephone lines were used to transmit images to remote terminals. RESULTS/DISCUSSION: Transmission time was reduced from 15-20min to 20-30s and the image size was reduced from 1.3 MB to 20-30 kB by compressing the images before transmission. Image quality is still excellent.

Computer simulation of centration effects on corneal-topography analysis of excimer laser photorefractive keratectomy ablations.

Excimer laser photorefractive keratectomy (PRK) procedures photoablate the central optical zone, potentially altering the corneal vertex normal and thus the corneal topography (CT) system reference axis. Anterior corneal surface coordinates (x, y, and z) were computer generated to simulate spherical and PRK-ablated corneal surfaces. Power maps were reconstructed by using surface tangents relative to various assumed CT reference axes, the original and displaced vertex normals. The simulation process eliminated focus, alignment, and surface-reconstruction errors. A simulated 5.00-D myopic PRK decentered 1 mm from the vertex produced a new corneal vertex 0.13 mm from the original position. Axial power-map generation by using this new reference axis produced a spherical keratectomy with astigmatic untreated periphery. This computer simulation demonstrates that reference axis changes are significant for even modest PRK decentration and can explain apparent steepening in untreated corneal areas. CT users should be aware that pre- and postoperative CT power maps may not be directly comparable.

Corneal topography bow-tie pattern: artifact of videokeratoscopy?

PURPOSE: To test the hypothesis that the bow-tie corneal topography pattern results from corneal asphericity in the presence of astigmatism. METHODS: Astigmatic color-coded power maps using different shape factors were computer generated. Each simulation was based on the calculation of dioptric power at 20 points along each of 180 hemimeridia, for a total of 3600 points. The calculations were made independent of the capture or measurement of video-keratographs. These simulations were compared to power maps taken from clinical records. RESULTS: A shape factor of 1.00 resulted in a spherocylinder color-coded map with straight-edged sectors of power. The familiar bow-tie pattern was generated using an elliptical model with a shape factor of less than 1.00. This pattern was reversed by modeling the cornea as an oblate ellipsoid using a shape factor greater than 1.00. CONCLUSIONS: By simple alteration of the amount of corneal asphericity through manipulation of the shape factor, computer simulation showed that this surface characteristic is responsible for the bow-tie pattern observed in corneal topography power maps.

The relation between corneal and total astigmatism.

Using computer-assisted videokeratoscopy we measured corneal astigmatism and compared these results over a range of corneal zone diameters with total ocular astigmatism derived by subjective refraction. Videokeratoscopes permit a more detailed analysis of the power distribution within a given corneal surface area, enabling comparison to the total astigmatism for equivalent aperture sizes. Although there were significant individual variations, the group average data supports the traditional view of a linear relation between corneal and total astigmatism. This was true across the range of apertures tested from 2 to 7 mm, with the coordinates of the relation being consistent with that of the modified Javal's rule; namely a slope of 1 and an intercept of approximately 0.50 D against-the-rule residual astigmatism.

Visual outcome of excimer laser photorefractive keratectomy for myopia. A comparison of three laser delivery systems in Australia.

PURPOSE: To compare the performance of the three excimer laser delivery systems in correcting myopia. METHODS: The first generation excimer laser systems produced by Summit, VISX and LEI were examined. The first 40 consecutive eyes with six-month refractive results were selected from the LEI centre. Forty consecutive eyes, matched for age and attempted corrections, were then selected from each of the Summit and VISX centres. The postoperative visual results were examined retrospectively and compared among the three groups. RESULTS: At six-month follow-up, the mean and standard deviation of the spherical equivalent refraction were +0.27 (SD, 0.65) dioptre in the Summit group, -0.33 (SD, 0.83) dioptre in the VISX group and -0.62 (SD, 0.89) dioptre in the LEI group (P = 0.0001, ANOVA). Thirty-five eyes (88%) in the Summit group, 33 eyes (83%) in the VISX group and 28 eyes (70%) in the LEI group achieved corrections within 1 dioptre of intended corrections (P = 0.131, chi 2 test). Four eyes (10%) in the Summit group, two eyes (5%) in the VISX group and one eye (3%) in the LEI group were overcorrected by more than 1 dioptre at six-month follow-up. Unaided visual acuity (Snellen equivalent) was 6/12 or better in 37 eyes (93%) in the Summit group, 34 eyes (85%) in the VISX group and 34 eyes (87%) in the LEI group (data missing in one eye). Loss of one or more lines of best corrected visual acuity (Snellen equivalent) was found in 6 of 39 eyes (15%) in the Summit group, 10 of 40 eyes (25%) in the VISX group and 4 of 36 eyes (11%) in the LEI group (P = 0.258, chi 2 test). Regression between one and six month follow-up differed significantly (P = 0.001, ANOVA) among the groups, with the mean of -1.25 (SD, 0.94) dioptres in the Summit group, -0.59 (SD, 1.01) dioptre in the VISX group and -0.40 (SD, 0.99) dioptre in the LEI group. CONCLUSIONS: Statistically comparable results were found in the three groups in terms of refraction and visual acuity six months after myopic photorefractive keratectomy. No statistical differences were found between the groups in terms of significant overcorrection or loss of best corrected acuity. The group treated using the Summit system demonstrated significantly greater regression than the other groups, necessitating larger initial hyperopic shifts.

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.

193-nm excimer laser sclerostomy using a modified open mask delivery system in rhesus monkeys with experimental glaucoma.

Excimer laser sclerostomy is a new glaucoma filtration procedure in which the argon fluoride excimer laser at 193 nm is delivered ab externo through a modified open mask system incorporating an en-face air jet to dry the target area and preserve hemostasis during ablation and a conjunctival plication mechanism, which allows the conjunctival and scleral wounds created by through-and-through ablation to separate once the mask is removed. No preparatory dissection of the conjunctiva is required. Five 200-microns and five 500-microns sclerostomies were formed by ablation at a pulse repetition rate of 20 Hz and a fluence per pulse of 400 mJ/cm2 in fellow eyes of five rhesus monkeys with experimental glaucoma. Overall, seven of the ten eyes attained a functional result, with intraocular pressures remaining below 21 mmHg for 6 +/- 1 days and rising to the pre-operative level after 10 +/- 3 days without adjunctive antifibroblast medication. The duration of filtration for 200-microns and 500-microns sclerostomies was similar, and parallels that previously observed for posterior lip sclerectomy in the same animal model. The three eyes with no functional result all had incorrectly positioned sclerostomies. Choroidal detachment and significant shallowing of the anterior chamber did not occur. Excimer laser sclerostomy appears to be a viable technique for filtration, provided that mask placement is accurate.

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.

Excimer laser sclerostomy: the in vitro development of a modified open mask delivery system.

The argon fluoride (ArF) excimer laser at 193 nm ablates the ocular tissues with a new order of precision and virtually no adjacent damage. A glaucoma filtration operation has been designed in which small-bore sclerostomies are created using the ArF excimer laser delivered through an open mask. The mask plicates the conjunctiva at the limbus prior to ablation. Removing the mask at the end of the procedure allows the conjunctiva to relax back to its original position, separating the conjunctival and scleral wounds. Formal conjunctival dissection is thus avoided. Feasibility studies in cadaver pig eyes, using a fluence per pulse of 400 mJ/cm2 and a pulse repetition rate of 20 Hz, indicate that sclerostomies of 300 microns diameter can be reliably formed if an en-face air jet is built into the mask to raise the pressure in the target area, preventing aqueous flooding.

Laser microsclerostomy for primary open angle glaucoma: a review of laser mechanisms and delivery systems.

A number of different lasers and delivery systems are currently used in experimental sclerostomy procedures. These are discussed with reference to their performance in terms of cutting accuracy and trauma to the adjacent tissues. Lasers emitting wavelengths close to the absorption peaks of water in the mid-infrared region and organic polymers in the far ultraviolet region have a short absorption pathlength in the sclera and produce the least adjacent thermal tissue trauma. These ablating lasers cannot be delivered gonioscopically, and contact endoscopic techniques trap hot expanding gases within the forming sclerostomy channel causing secondary thermal and mechanical damage. Optimal results should be obtained using an ablating laser delivered either through an open mask or a modified endoscopic system incorporating an adequate exhaust mechanism.

Improved method for calculation of corneal topography for any photokeratoscope geometry.

Accurate and efficient calculation of the anterior corneal surface topography by photokeratoscopy has become important for the management or surgical treatment of refractive errors. Methods have been published that describe such calculations. The algorithms described here eliminate an assumption made in some of the earlier computational methods without an excessive increase in the computation time or increased sensitivity to measurement errors. This results in an enhanced accuracy for reconstructing corneal topographies. Computer simulation results indicate an improved accuracy in measuring the topography for nonspherical corneas. The equations given here apply to any general photokeratoscope geometry and can easily be used in computer programs. In a notable previous publication concerning the more common flat faceplate keratoscope geometry, several of the equations were presented with errors. These errors are identified as an aid to anyone who had already attempted to use those equations in a computer program.

Bovine corneal stroma ablation rate with 193-nm excimer laser radiation: quantitative measurement.

Excimer lasers operating at 193 nm may become important surgical instruments in ophthalmology because of their ability to ablate tissue with excellent precision and minimal damage to adjacent tissue. However, the precision is limited by the accuracy of the measurement of the amount of tissue ablated per pulse at the fluence used. A measurement of the ablation rate of bovine corneal stroma over the range of fluences most likely to be useful for corneal surgery (50-400 mJ/cm2) is presented. The technique used produced reproducible results, the data from 47 animal eyes being averaged to further increase the precision. For this range of fluences, these results show a more precise measurement than previously published tissue ablation rate data. These results should be useful in accurately predicting the result of corneal surgery using the excimer laser if species differences are not major.

Laser-induced damage to transparent polymers: chemical effect of short-pulsed (Q-switched) Nd:YAG laser radiation on ophthalmic acrylic biomaterials. I. A review.

The use of short-pulsed lasers in ophthalmic surgery inspired and called for research on the damage inflicted by the laser radiation upon the acrylic polymers from which artificial intraocular lenses are made. The possible release of toxic monomers by laser-induced depolymerization is of great concern but past investigations of this phenomenon have been very limited. The present knowledge of various types of laser-induced damage to transparent polymers is reviewed with particular emphasis on the acrylic materials and intraocular lenses.

Laser-induced damage to transparent polymers: chemical effect of short-pulsed (Q-switched) Nd:YAG laser radiation on ophthalmic acrylic biomaterials. II. Study of monomer release from artificial intraocular lenses.

Commercial intraocular lenses and polymer specimens, both poly(methyl methacrylate) (PMMA) and poly(2-hydroxyethyl methacrylate) p(HEMA), were subjected to various levels of irradiation from a Q-switched Nd:YAG laser, and then to extraction and sample-enrichment techniques. The extracts were analysed by capillary gas chromatography. In PMMA samples, residual contents of 0.05-0.89% (wt) MMA were found and it appeared that the laser radiation did not cause a detectable increase of the monomer content. No residual HEMA could be found in p(HEMA) samples before or after laser bombardment. It is concluded that the accidental exposure of lenses to Nd:YAG laser cannot produce a significant release of monomers. Depolymerization induced by laser is a process unlikely to occur at the energy levels used in ophthalmic surgery.

Fourier analysis of the nerve fiber layer by digital imaging techniques.

We introduce a new method to examine the retinal nerve fiber layer (RNFL) that may detect early changes in its structure due to glaucoma. Viewed in the conventional manner, in a plane perpendicular to the retina, we get an estimation of the structural integrity of the RNFL. Viewed from a plane parallel to the retina, the RNFL forms a contour surface of hills and valleys. In our method red free fundus slides are digitally acquired. A waveform, representing a topographic profile cut through the resulting representation of the RNFL contour surface, was analyzed using Fourier's transform. Three cycles/mm in the Fourier power spectrum was used as an arbitrary cutoff to discriminate normal from glaucomatous RNFL. The Fourier power spectrum from normal RNFL was shifted to higher spatial frequencies, while the power spectrum from glaucomatous RNFL was shifted to lower spatial frequencies. Thus, Fourier Analysis can reveal information about such a wave form which is not obvious by visual inspection or by densitometric techniques. This method may allow for an even earlier diagnosis of injury to the optic nerve due to glaucoma.