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At least 775 records · Page 43Linked to original sources

PubMed2002

Arcuate keratotomy for asymmetrical steep islands after laser in situ keratomileusis and automated lamellar keratoplasty.

PURPOSE: To determine the safety and efficacy of arcuate keratotomy (AK) to reduce or eliminate steep asymmetrical islands after laser in situ keratomileusis (LASIK) and automated lamellar keratoplasty (ALK). SETTING: Rancho Bernardo Laser and Vision Center, San Diego, California, USA. METHODS: This retrospective and noncomparative interventional study comprised 8 eyes of 5 patients who received arcuate incisions to reduce topographically defined asymmetrical steep islands after LASIK (n = 7) and ALK for hyperopia (n = 1). The islands occurred unexpectedly after standard excimer laser treatment (VISX Star and S2) for myopic astigmatism that induced irregular astigmatism and unwanted optical effects. Cases occurred consecutively over 16 months, an incidence of less than 1%. Arcuate incisions extended 30 or 45 degrees in LASIK eyes based on the amount of residual refractive astigmatism and were placed at 7.0 mm on the steep axis. Radial incisions were limbal sparing to a 5.0 mm optical zone following the Casebeer nomogram. Outcome measures were uniformity of postenhancement topography, uncorrected visual acuity (UCVA), postenhancement refraction, and reduction in unwanted optical effects. RESULTS: The preenhancement mean spherical equivalent (SE) in the LASIK eyes was -0.99 diopters (D) +/- 0.48 (SD); the astigmatism ranged from 0.50 to 1.75 D. After incisional keratotomy, the mean SE was -0.21 +/- 0.12 D and the net residual astigmatism was reduced to a range of 0 to 0.75 D. All 8 eyes had AK to reduce topographically defined steep islands that were causing visual blurring and unwanted optical effects including ghosting, halos, and polyplopia. After enhancement, there was a marked reduction in the asymmetrical steep islands by topography, fewer unwanted optical effects, less refractive astigmatism, and improved UCVA. Two patients retained small inferior steep areas that may indicate forme fruste keratoconus, although this condition was not detected preoperatively by topographical indices. CONCLUSION: Uncorrected visual acuity improved and unwanted optical effects were reduced after AK on the peripheral axis of asymmetrical steep islands that occurred after uneventful LASIK and hyperopic ALK. This relatively simple incisional approach may be used until more advanced topography-linked ablations with rapid and precise eye tracking are perfected and widely available.

Adult↗

PubMed1998

Multi-capillary optical waveguides for DNA sequencing.

Cylindrical capillaries can be used as optical elements in a waveguide, where refraction will confine an appropriately focused light beam to pass through the interiors of successive capillaries in a flat parallel array. Such a capillary waveguide allows efficient illumination of samples in multiple capillaries with relatively little laser power. Analytical expressions derived under paraxial and thin-lens approximations provide guidance in selecting the capillary sizes and the refractive indices that will produce the waveguiding effect, but accurate predictions require exact ray tracing. Small reflective losses as the light passes through the capillary surfaces cause cumulative intensity decreases, but the resulting lack of uniformity can be compensated to a considerable extent by illuminating the capillary array from both sides. A 12-capillary waveguide illuminated from both sides in air has a difference of less than 10% from the strongest to the weakest illumination. By increasing the refractive index of both the external medium and the contents of the capillaries, a 96-capillary waveguide for DNA sequencing could be produced that would also provide nearly uniform illumination. A 12-capillary, bi-directionally illuminated waveguide system for DNA sequencing has been constructed. The two focused laser beams are delivered by integrated fiber optic transmitters (IFOTs), and fluorescence is collected by a set of optical fibers whose spacing exactly matches that of the capillaries in the waveguide. The system is easy to align and provides sensitive detection of fluorescence with minimal cross-talk between channels.

Electrophoresis, Capillary↗

PubMed2006

Retinal nerve fiber layer thickness in normal children measured with optical coherence tomography.

PURPOSE: To measure the peripapillary retinal nerve fiber layer (RNFL) thickness in normal children. DESIGN: Observational cross-sectional study. PARTICIPANTS: Ninety-two eyes of 92 normal children ages 4 to 17 years presenting to the Ophthalmology Clinic at the Harkness Eye Institute, Department of Ophthalmology, Columbia University. METHODS: Retinal nerve fiber layer thickness was measured with optical coherence tomography (OCT). Patient cooperation and signal strength of the OCT scans were assessed. Optic disc photographs were evaluated by a glaucoma specialist in a masked fashion. Eyes with abnormal optic discs were excluded. One eye of each subject was randomly selected for statistical analysis. The effect of several factors on RNFL thickness was investigated statistically. MAIN OUTCOME MEASURES: Retinal nerve fiber layer thickness. RESULTS: Ninety-one percent of the study subjects were Hispanic; 8%, African American; and 1%, Caucasian. Optical coherence tomography measurements were obtained in 117 of 121 (96.7%) subjects, and disc photographs were available for 92 of them. Mean age (+/-standard deviation [SD]) was 9.7+/-2.7 years. Mean global RNFL thickness (+/-SD) was 107.0+/-11.1 microm (range, 78.1-134.6). The RNFL was thickest inferiorly (136.9+/-16.9 microm) and superiorly (135.4+/-19.3 microm), thinner nasally (83.0+/-18.0 microm), and thinnest temporally (72.5+/-13.4 microm). In univariate regression analysis, age (P = 0.013) and refraction (P<0.001) had a significant effect on RNFL thickness; age had a significant effect on refraction (P<0.001). When controlling for refraction, age no longer had a significant effect. CONCLUSIONS: Optical coherence tomography can be used to measure RNFL thickness in children. Refraction had an effect on RNFL thickness. In normal children, variation in RNFL thickness is large. The normative data provided by this study may assist in identifying changes in RNFL thickness in children.

Adolescent↗

PubMed1994

Factors influencing the accuracy of the SRK formula in the intraocular less power calculation.

Several intraocular lens (IOL) power calculation formulas (either theoretical or empirical) are used to determine the emmetropic IOL power) The Sanders-Retzlaff-Kraff (SRK) linear regression formula is among the most widely recognized empirical ones. In the present study intraocular lens power calculation aiming at emmetropia was performed, using SRK formula, in 145 cataractous eyes undergoing lens implantation. The final refraction was evaluated at 8 to 12 months after surgery. The purpose of this study was the identification and quantitative evaluation of the factors which influence significantly the accuracy of SRK in the intraocular lens power calculation. The following factors were studied: (1) the error in preoperative biometry with regard to the difference between post and preoperative axial length measurements, (2) the position of the implantation of the intraocular lens (anterior versus posterior chamber), (3) the intraocular lens style, (4) the intraocular lens power level, (5) the preoperative corneal astigmatism, (6) the surgically induced corneal astigmatism, and (7) the postoperative astigmatism. Multiple regression and stepwise regression analysis showed a strong correlation (R2 = 0.65; p < 0.001) between postoperative refractive error (Rf) and error in preoperative biometry (delta AL), surgically induced corneal astigmatism (SIA) and postoperative astigmatism (Ap) only. This correlation is expressed by the following equation: Rf = 0.07 -2.55 delta AL -0.42 SIA + 0.34 Ap. This equation indicates the quantitative effect of each factor on the accuracy of the SRK formula, by defining the pattern of the fluctuations of the amount or state (myopic or hyperopic) of refractive error induced by changes of variables delta AL, SIA and Ap.

Aged↗

PubMed2006

Intraocular lens exchange due to incorrect lens power.

PURPOSE: To evaluate patients who had intraocular lens (IOL) exchange for unexpected postoperative refractive errors, determine the sources associated with the errors, and derive an empiric approach to estimating the power for IOL exchange. DESIGN: Retrospective review of interventional case series. PARTICIPANTS: Twenty-two eyes that underwent IOL exchange for correcting unexpected refractive errors after cataract surgery with IOL implantation were reviewed. INTERVENTION: All the IOLs for IOL exchange were placed in-the-bag. The same type of IOLs was used for original IOL implantation and IOL exchange in 91% (20/22) of eyes. MAIN OUTCOME MEASURES: Uncorrected visual acuity, best spectacle-corrected visual acuity (BSCVA), refraction, and reasons for IOL exchange. The equation derived from refractive change and change in IOL power was developed to calculate IOL power for exchange. RESULTS: Of the 22 cases, the identified reasons were keratometry errors in 5 (23%) and incorrect axial length (AL) determination in 3 (14%). In 3 other cases, a wrong IOL was implanted. After IOL exchange, 82% (18/22) of eyes were within +/-0.50 diopters (D) and 86% (19/22) within +/-1.00 D of emmetropia. Uncorrected visual acuity was 20/40 or better in 82% of eyes, and BSCVA was 20/40 or better in 95% (21/22) of eyes. The correlation between change of refraction and IOL power was significant (P<0.002). CONCLUSIONS: Among the identified causes, incorrect corneal power determination was the most frequent reason for incorrect IOL power implantation, followed by error in AL measurement and inserting a wrong IOL. The pre-exchange refraction can be used theoretically to calculate the IOL power for exchange.

Cataract Extraction↗

PubMedDate not supplied

Constant light affects retinal dopamine levels and blocks deprivation myopia but not lens-induced refractive errors in chickens.

Chickens were raised with either translucent occluders or lenses, both under normal light cycles (12-h light/12-h dark) and in constant light (CL). Under normal light cycles, eyes with occluders became very myopic, and eyes with lenses became either relatively hyperopic (positive lenses) or myopic (negative lenses). After the treatment, retinal dopamine (DA), DOPAC, and serotonin levels were measured by high-pressure liquid chromatography (HPLC-EC). A significant drop in daytime retinal DOPAC (-20%) was observed after 1 week of deprivation, and in both DOPAC (-40%) and DA (-30%) after 2 weeks of deprivation. No changes in retinal serotonin levels were found. Retinal DA or DOPAC content remained unchanged after 2 or 4 days of lens wearing even though the lenses had already exerted their maximal effect on axial eye growth. When the chickens were raised in CL, development of deprivation myopia was reduced (8 days CL) or entirely blocked (13 days CL). Lens-induced changes in eye growth were not different after either 6 or 11 days in CL, compared to animals raised in a normal light cycle. Thirteen days of CL resulted in a dramatic reduction of DA and DOPAC levels, but serotonin levels were also lowered. The results suggest that lens-induced changes in refraction may not be dependent on dopaminergic pathways whereas deprivation myopia requires normal diurnal DA rhythms to develop.

3,4-Dihydroxyphenylacetic Acid↗

PubMed2006

Computer-generated volume holograms fabricated by femtosecond laser micromachining.

We define computer-generated volume holograms (CGVHs) as arbitrary 3D refractive index modulations designed to perform optical functions based on diffraction, scattering, and interference phenomena. CGVHs can differ dramatically from classical volume holograms in terms of coding possibilities, and from thin computer-generated holograms in terms of efficiency and selectivity. We propose an encoding technique for designing such holograms and demonstrate the concept by scanning focused femtosecond laser pulses to produce localized refractive index modifications in glass. These CGVHs show a significant increase in efficiency with thickness. Consequently, they are attractive for photonic integration with free-space and guided-wave devices, as well as for encoding spatial and temporal information.

Journal Article↗

PubMed2003

Maximum permissible lateral decentration in aberration-sensing and wavefront-guided corneal ablation.

PURPOSE: To investigate the lateral alignment accuracy needed in wavefront-guided refractive surgery to improve the ocular optics to a desired level in a percentage of normally aberrated eyes. SETTING: Department of Ophthalmology, University of Zurich, Zurich, Switzerland. METHODS: The effect of laterally misaligned ablations on the optical outcome was simulated using measured wavefront aberration patterns from 130 normal eyes. The calculations were done for 3.0 mm, 5.0 mm, and 7.0 mm pupils. The optical quality of the simulated correction was rated by means of the root-mean-square residual wavefront error. RESULTS: To achieve the diffraction limit in 95% of the normal eyes with a 7.0 mm pupil, a lateral alignment accuracy of 0.07 mm or better was required. An accuracy of 0.2 mm was sufficient to reach the same goal with a 3.0 mm pupil. CONCLUSION: Procedures must be developed to ensure that the ablation is within a tolerance range based on each eye's original optical error. Rough centration based on the surgeon's judgment might not be accurate enough to achieve significantly improved optical quality in a high percentage of treated eyes.

Adolescent↗

PubMed1991

The optical modelling of the human lens.

The effect of varying lens shape factors and refractive index distribution on two lens performance characteristics (equivalent power and spherical aberration) are considered, using a mathematical model of the human lens which is based upon ellipsoidal iso-indical contours. Values of radii of curvature based on the work of various authors are used to determine asphericity and these inferred values compared with previous measurements. Discrepancies are found and although asphericity has no effect on equivalent power, it does affect the spherical aberration. Based on preliminary findings of the refractive index distribution in the equatorial plane of human lenses, the index distribution is described by a polynomial in the Y (the distance from the optical axis in the equatorial plane), the coefficients of which are polynomials in Z (the distance along the optical axis). The shape of the index profile was found to have a significant effect on the equivalent power and spherical aberration of the lens. The results indicate that more information on surface asphericity and accurate measurement of the index profile in the sagittal plane are required for more accurate modelling of the human lens.

Humans↗

PubMed1991

Ray tracing error correction in ophthalmic optics.

Ray tracing error correction (RTEC), a new method for various refractive calculations, is introduced. It consists of two procedures: (1) ray tracing that determines whether the ray originating from an infinite point focuses on the fovea and (2) error correction that varies the curvature of the different refractive surfaces according to an error correction formula when the ray is not focused at the fovea. These calculations are continued until the ray is focused at the fovea. The refraction of Gullstrand's schematic eye at relaxation and at accommodation, the desired postoperative corneal curvature for postoperative emmetropia after corneal ablation surgery, and the intraocular power of convex-plano, biconvex, and plano-convex intraocular lenses are presented.

Accommodation, Ocular↗

PubMedDate not supplied

[Role of cellular organoids in photoreceptor optics (studies on microwave models)].

Action of the refracting structures of the photoreceptor inner segment on the receptor directional sensitivity was studied with the microwave models. Models of photoreceptor cells enlarged by 15 000 times were made of polystyrene foam and irradiated by microwaves (5.7-8.3 mm). Focusing structures (cone ellipsoid and oil drop) increase the power propagating along the photoreceptor outer segment when illuminated axially. However, they sharply increase the directionality of the cell, so that there is no overall gain in sensitivity in the case of a large pupil. It is supposed that the main role of the focusing structures is to narrow the directional sensitivity curve of the photoreceptor, to decrease the effects of the scattered light and to increase the depth of the focus.

Animals↗

PubMed2002

Elasto-optics in double-coated optical fibers induced by axial strain and hydrostatic pressure.

Stresses, microbending loss, and refractive-index changes induced simultaneously by axial strain and hydrostatic pressure in double-coated optical fibers are analyzed. The lateral pressure and normal stresses in the optical fiber, primary coating, and secondary coating are derived. Also presented are the microbending loss and refractive-index changes in the glass fiber. The normal stresses are affected by axial strain, hydrostatic pressure, material properties, and thickness of the primary and secondary coatings. It is found that microbending loss decreases with increasing thickness, the Young's modulus, and the Poisson's ratio of the secondary coating but increases with the increasing Young's modulus and Poisson's ratio of the primary coating. Similarly, changes in refractive index in the glass fiber decrease with the increasing Young's modulus and Poisson's ratio of the secondary coating but increase with the increasing Young's modulus and Poisson's ratio of the primary coating. Therefore, to minimize microbending loss induced simultaneously by axial strain and hydrostatic pressure in the glass fiber, the polymeric coatings should be suitably selected. An optimal design procedure is also indicated.

Journal Article↗

PubMed2000

Ophthalmologic abnormalities in mentally retarded.

PURPOSE: To determine the prevalence and types of ophthalmologic abnormalities in mentally retarded subjects. METHODS: Seventy-three institutionalized mentally retarded subjects (41 boys and 32 girls) aged 5 to 19 years (mean: 11.5 years) were examined ophthalmologically during a three month period (from May to July 1999). The eye examination consisted of visual acuity testing (illiterate Snellen E-chart or acuity card procedures), pupillary reflex and motility evaluation, cover test for phoria and tropia, examination of adnexa and anterior segment (magnifying glass or slit-lamp biomicroscopy if indicated and possible), retinoscopy and direct ophthalmoscopy. All subjects were evaluated pediatrically, neurologically, psychologically and otorhinolaryngologically if indicated. RESULTS: Ophthalmologic abnormalities were found in 60.2% of the subjects. Eyelid abnormalities accounted for 21.7%, fundus abnormalities for 21.6%, refractive errors for 15% and ocular motility disorders for 13.6%. Taken separately, optic atrophy (16.4%) was the most frequent disorder, followed by refractive errors (15%), hypertelorism (12.3%), epicanthus (10.9%) and nystagmus (8.2%). Strabismus and mongoloid obliquity of lids were seen in 5.4% each other while ptosis was disclosed in 4.2% of cases. Visual acuity tested in 60 subjects revealed that 25 (41.6%) had visual impairment. Of all subjects 10.9% required corrective glasses while 9.6% needed corrective surgery for strabismus and ptosis. CONCLUSION: This study shows a high prevalence of ophthalmologic abnormalities in mentally retarded subjects. This is in agreement with the results of several other previous studies. It's therefore essential to screen mentally disabled subjects ophthalmologically.

Adolescent↗

PubMed1981

Ocular component analysis by vergence contribution to the back vertex power of the anterior segment.

Refractive errors are essentially the expression of a dysfunction in a dynamic system of light vergences. It follows that the nature of the dysfunction can be studied by reference to the light vergences. However, the usual analysis deals with surface powers and their separations within the eye. These two dimensions do not express the actual functional conditions taking place within the optical system. Assessment of the optical elements and path lengths in terms of the actual vergence they produce does so. Hence, this paper presents a method using vergence to illustrate the value of such an assessment.

Adolescent↗

PubMed1996

Effect of incision direction on refractive outcome after radial keratotomy.

PURPOSE: To determine whether the direction of radial keratotomy (RK) incisions (centripetal versus centrifugal) affects refractive outcome. SETTING: Private ophthalmology office. METHODS: The database of a single surgeon was retrospectively reviewed. Stepwise regression was used to select significant predictors of refraction change in the population. In addition to incision direction, variables evaluated were optic zone diameter, number of incisions, patient age, corneal curvature, and planned incision depth. RESULTS: All variables except planned incision depth and corneal power affected refractive outcome. After controlling for number of incisions, optic zone diameter, and patient age, centripetal incisions decreased myopia 0.87 diopters more than centrifugal incisions. CONCLUSIONS: Our results, consistent with previous investigations, found that number of incisions, optic zone diameter, and patient age were significant predictors of refractive outcome after RK. Incision direction was also a significant predictor by itself or coupled with optic zone diameter and number of incisions, with the centripetal incision decreasing myopia more.

Adult↗

PubMed2001

Accuracy and predictability of intraocular lens power calculation after laser in situ keratomileusis.

PURPOSE: To study the accuracy and predictability of intraocular lens (IOL) power calculation in eyes that had laser in situ keratomileusis (LASIK). SETTING: Gimbel Eye Centre, Calgary, Alberta, Canada. METHODS: Refractive outcomes in 6 cataract surgery and lensectomy eyes after previous LASIK were analyzed retrospectively. Target refractions based on measured and refraction-derived keratometric values were compared with postoperative achieved refractions. Differences between target refractions calculated using 5 IOL formulas and 2 A-constants and achieved refractions were also compared. RESULTS: The refractive error of IOL power calculation in postoperative LASIK eyes was significantly reduced when refraction-derived keratometric values were used for IOL power calculation. Persistent residual hyperopia still occurred in some cases; this was corrected by hyperopic LASIK. Refractive results appeared more accurate and predictable when the Holladay 2 or Binkhorst 2 formula was used for IOL power calculation. CONCLUSION: Hyperopic error after cataract surgery in post-LASIK eyes was significantly reduced by using refraction-derived keratometric values for IOL power calculation. Persistent hyperopic error was corrected by hyperopic LASIK.

Adult↗

PubMed1992

[Further development of a streak retinoscope with calibrated collimator].

Development and rationale of a new streak retinoscope have been discussed in an earlier paper. In contrast to classical retinoscopy this retinoscope determines the refraction by forming the narrowest streak in the test person's pupil with a calibrated collimator. In addition to the first prototype the extension of the lamp according to variation of the refraction is measured electronically and the data are transposed into a computer. This allows to freely choose a working distance in a range from about 30 to 120 cm and therefore it is possible to examine restless patients. Accuracy of the measurement was less than or equal to 0.25 D for Ametropia from -0.5 to 5.0 D in a working distance of 30 cm or from -4.0 to -1.5 D in 100 cm. In most persons it is not necessary to use additional glasses during refraction by choosing the adequate distance. Therefore this method can help to refract kids and other persons which could not be refracted by use of glasses. In addition this method excludes the accommodation of the patient as apposed to regular refractometers.

Calibration↗

PubMed1982

[Double refraction of crystals in the lens (spheroliths, "Christmas tree ornaments") and in the vitreous body (scintillatio nivea)].

1. The round, oval, or kidney-shaped, radially built spheroliths of the brown or morgagnian cataract show a double refraction in polarized light of about 0.160 optically positive. This is in accordance with Whewellite (calcium oxalate monohydrate). 2. In crystalline cataracts, there are crystals in polarized light with a mean double refraction of 0.046, which is in accordance with cholesterol (C27H48O). 3. In asteroid hyalinosis of the vitreous, the small bodies consisting of needle-shaped crystals arranged in a radial fashion show a small double refraction of 0.012. Therefore the crystal may be brushite (Ca HPO4 X 2 H2O).

Cataract↗