Optical zone diameters for photorefractive corneal surgery.

PURPOSE: To examine the physiological optics of photorefractive corneal surgery and to study the effect on glare production of the optical zone diameter. METHODS: An optical analysis computer program was used to generate rays that define the edge of the optical zone for any given pupil size and glare-free field. RESULTS: The optical zone diameter must be based on the postoperative corneal curvature because the determines magnification of the pupil. The minimal optical zone diameter of uniform optical power was determined both for myopic and hyperopic surgery and for two values of anterior chamber depth. CONCLUSIONS: Optical zone diameters must be at least as large as the entrance pupil diameter to preclude glare at the fovea, and larger than the entrance pupil to preclude parafoveal glare.

Vitreous fluorophotometer data analysis by deconvolution.

The measurement process in fluorophotometry inherently involves a loss of information due to the finite sampling volume of the instrument. Mathematically, the effect is expressed as a convolution of the actual fluorescein distribution with the spread function of the instrument. Scattering by the ocular media can increase the spread function over that due to the instrument alone. A method is proposed for deconvolution of vitreous fluorophotometry data. Simulation studies and analyses of patient data demonstrate recovery of information with this method, including enhancement of retinal peaks and resolution of detail in the posterior vitreous which was not apparent from the original scans.

In vivo measurement of posterior chamber intraocular lens decentration and tilt.

We report what to our knowledge is the first extensive in vivo clinical study of intraocular lens decentration and tilt. Measurements of posterior chamber intraocular lenses, all implanted by the same surgeon in 89 eyes, showed that decentration and tilt consistently differed between right and left eyes. Lenses tended to decenter superotemporally and tilt with their superonasal edges tipped forward. Decentration magnitude relative to the cornea light reflex axis and line of sight averaged 0.64 mm, with decentration increasing slightly with increased axial length. Average tilt was 6.75 degrees, and the average tilt-induced astigmatism was 0.27 diopter. Tilt magnitude decreased with increased axial length, as did tilt-induced astigmatism and plus sphere.

Wide field specular microscopy of excised donor corneas.

Routine examination of the endothelium of eye bank excised corneas in the McCarey-Kaufman storage medium has been accomplished by using a new bottle cap with an optical quality window. A sterile system is maintained as the cornea remains in the bottle within which it is sent to the surgeon, without handling or manipulation of the tissue. In combination with the wide field specular microscope in the noncontact mode, endothelial cell measurements and qualitative information about the endothelial surface are now available.

Scanning slit confocal microscopy of fungal keratitis.

In vivo scanning slit confocal microscopy was performed in a patient with Fusarium solani keratitis. Morphologically distinctive abundant filamentous structures were observed intrastromally. Confocal microscopy of the culture plate growing F solani from the patient's corneal scraping revealed filaments morphologically similar to the filaments observed in vivo. After 1 week of medical therapy, subsequent confocal microscopy showed an increased load of filaments, supporting the decision to perform a penetrating keratoplasty. Confocal microscopy confirmed that all of the fungus was eradicated. This aided in the decision to administer corticosteroids and quickly discontinue antifungal agents.

Measurement of intraocular lens decentration and tilt in vivo.

A method for measuring the tilt and decentration of intraocular lenses (IOLs) in the static eye using the Purkinje image locations is presented. The patient fixates on a target that is coaxial with the camera or is at a predetermined angle with the camera axis. A telecentric stop is introduced in the camera so the positions of the Purkinje images on the film are independent of their distance from the camera. Measurements of the image locations on the film are used with anterior chamber depth and corneal curvature measurement to calculate the tilt and decentration of the IOL. In a group of 14 randomly selected patients with posterior chamber IOLs, 13 gave Purkinje images that could be measured. The average tilt was 7.8 degrees and the average decentration was 0.7 mm.

Intraocular lens performance in air, in water, and in situ: a computer study.

Computer analysis is used to predict performance of four intraocular lenses with assigned values of aberration. Cylinder error and asphericity error are used as examples of possible manufacturing errors. Three measures of performance are calculated: maximum optical path difference, root mean square optical path difference, and modulation transfer function. For evaluation in air these standard test conditions are assumed: collimated green light incident on the convex surface of a plano-convex lens, with a 3 mm aperture. All four lenses show substantially improved performance in water compared to air, and a further improvement in the simulated eye (i.e., in situ). However, an aberrated 30 diopter (D) lens with performance in air comparable to an aberrated 20 D lens, performs worse in situ than does the 20 D lens. This suggests that a performance test in air that is suitable for a 20 D lens (e.g., 100 line pairs per millimeter resolution) may not be adequate for a 30 D lens. A test in air at 30% resolution efficiency may be more suitable.

Intraocular lens resolution in air and water.

The resolution of 96 polymethylmethacrylate intraocular lenses with convexo-plano optics, ranging in power from 13 to 27 diopters, was measured in air and water. The resolution of each lens was expressed in linear units of resolving power, which is the maximum number of line-pairs that can be resolved per millimeter, as described in the current ANSI Standard Z80.7-1984. There was no clearly defined relationship between linear resolving power measured in air and that measured in water. Measurements on high power lenses (greater than 20 diopters) indicate that it is possible for an intraocular lens to meet the current 100 line-pair per millimeter standard for resolution and still be a limiting factor in a patient's best attainable visual acuity. An alternative method for evaluating lens resolution is to determine the resolution efficiency (the relative percentage performance of a lens compared to a diffraction-limited lens of the same dioptric power). Using these units, a consistent and predictable relationship from air to water was demonstrated. Our findings confirm that if a minimum standard of 30% resolution efficiency in air is established, in contrast to linear resolving power, the lens will perform near its diffraction limit when implanted in the eye. For intraocular lenses of materials other than polymethylmethacrylate, a minimum resolution efficiency in air other than 30% may be required.

Silicone intraocular lens resolution in air and in water.

The resolution efficiencies of 31 biconvex silicone intraocular lenses, ranging in power from 16.0 to 23.5 diopters, were tested in air and in water to see if a predictable relationship existed as previously reported with polymethylmethacrylate lenses. Resolution efficiency is defined as the percentage ratio of the actual resolving power of a lens to that of a perfect lens of the same focal length which is only limited in resolution by diffraction. The lenses ranged from 29% to 58% resolution efficiency in air. No lenses exhibiting multiple images were included. All 31 lenses achieved at least 73% resolution efficiency in water, and one lens achieved 82%. Based on these findings, a biconvex silicone lens that exceeds 30% resolution efficiency in air and does not produce multiple images can perform near its diffraction limit when implanted in the eye.

In vivo scanning slit confocal microscopy of Acanthamoeba keratitis. A case report.

A 29-year-old woman presented with clinical signs and symptoms of Acanthamoeba keratitis. Scanning slit confocal microscopy revealed a 26-mu-diameter object, resembling an Acanthamoeba cyst, in the anterior stroma. Numerous ovoid objects (possibly inflammatory cells, trophozoites, or altered keratocytes) were present. Normal keratocyte nuclei and the anterior corneal mosaic, readily imaged by scanning slit confocal microscopy of the normal cornea, were noticeably absent. Subsequent corneal biopsy confirmed the diagnosis of Acanthamoeba keratitis.

Wide field scanning slit in vivo confocal microscopy of flattening-induced corneal bands and ridges.

We used a wide field scanning slit confocal microscope to examine the response of the in vivo human cornea to flattening. Flattening-induced effects consisted of (1) anterior corneal mosaic, which appeared as a meshwork of intersecting stromal and Bowman's layer bands with overlying epithelial ridges; (2) deep and middle stromal bands, which were narrower than and unrelated in position to the anterior corneal mosaic; and (3) posterior surface ridges. The posterior surface ridges projected posteriorly into the anterior chamber consisted of endothelium, Descemet's membrane, and posterior stroma, and were unrelated in position to posterior stromal bands. Confocal microscopy is a promising modality in the examination of the cornea and its response to mechanical stress.

N-nitrosodimethylamine in drinking water using a rapid, solid-phase extraction method.

A simple, rapid method for the extraction of N-nitrosodimethylamine (NDMA) from drinking and surface waters was developed using Ambersorb 572. Development of an alternative method to classical liquid-liquid extraction techniques was necessary to handle the workload presented by implementation of a provincial guideline of 9 ppt for drinking water and a regulatory level of 200 ppt for effluents. A granular adsorbent, Ambersorb 572, was used to extract the NDMA from the water in the sample bottle. The NDMA was extracted from the Ambersorb 572 with dichloromethane in the autosampler vial. Method characteristics include a precision of 4 % for replicate analyses, an accuracy of 6 % at 10 ppt and a detection limit of 1.0 ppt NDMA in water. Comparative data between the Ambersorb 572 method and liquid-liquid extraction showed excellent agreement (average difference of 12 %). With the Ambersorb 572 method, dichloromethane use has been reduced by a factor of 1,000 and productivity has been increased by a factor of 3-4. Monitoring of a drinking water supply showed rapidly changing concentrations of NDMA from day to day.

Scanning mirror microscope with optical sectioning characteristics: applications in ophthalmology.

Optical sectioning is the simultaneous illumination and viewing of only a thin region of a specimen. An illuminated slit is imaged at the plane of interest and is swept laterally by the action of an oscillating mirror. The light returning from the specimen reflects from a second facet of the oscillating mirror and forms a stationary image of the illuminated slit. At this stationary image a second slit is placed, which passes light from the desired plane and rejects scattered light from other depths within the specimen. Light passing through the second slit is reflected from the third facet of the oscillating mirror and is focused to the final image plane. The image is reconstructed as the image of the second slit sweeps across the image plane. An important ophthalmological application is the examination of the endothelial cell layer of the cornea, either by contact or noncontact techniques. Optimization for image illuminance and resolution is discussed.

Colour wide field specular microscopic investigation of corneal surface disorders.

Colour wide field specular microscopy (WFSM) may be used to examine different phases of corneal surface pathology in vivo. A soft contact lens facilitates colour differentiation of cell surface variations by interference. Pathologic states may be characterised by modifications in apparent cell colour, brightness, morphology and configuration. Cell brightness is altered by interference phenomena, cell surface irregularities (microprojections), homogeneity of substances covering cells, intracellular components and changes in indices of refraction at the interfaces between the cell surface and the tear layers. Colour photography has been helpful in deciding which intensity variations may be due to interference effects.