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At least 1,387 records · Page 77Linked to original sources
PubMedDate not supplied

Laser in situ keratomileusis for myopic astigmatism.

PURPOSE: To evaluate the refractive results of laser in situ keratomileusis (LASIK) for myopic astigmatic eyes, and to assess the efficacy, accuracy, stability, and safety of the procedure. METHODS: LASIK was performed on 113 eyes of 73 patients for correction of myopic astigmatism ranging from 1.00 to 5.00 D, as measured by manifest refraction, with a mean baseline refractive astigmatism of 2.09 +/- 1.12 D. The Chiron Automated Corneal Shaper was used to create a corneal flap, and laser ablation was performed using the Chiron-Technolas Keracor 116 excimer laser. Follow-up time was 12 months for all eyes. RESULTS: Refractive astigmatism was stable by 3 months after surgery. At 1 year after LASIK, refractive astigmatism was reduced to a mean of 0.25 +/- 0.31 D (range 0 to 1.00 D). Sixty-one eyes (54%) had no residual astigmatism and 98 eyes (86.7%) had 0 to 0.50 D of refractive astigmatism. The mean percent reduction of preoperative astigmatism was 87.9 +/- 14.9%. The mean axis deviation of the surgically induced astigmatism was 2.1 +/- 3.1 degrees, with 96 eyes (84.9%) within 5 degrees of the desired axis. The percent correction of preoperative astigmatism in the proper axis was 97.1 +/- 15.5%. Spectacle-corrected visual acuity improved by 2 lines in 11 eyes (9.7%), and was reduced by 1 line only in 1 eye. There were no other significant complications. CONCLUSION: LASIK with the Chiron-Technolas Keracor 116 excimer laser was effective for correction of myopic astigmatism, with good stability after 3 months. The results were predictable with an acceptable degree of accuracy. LASIK is a safe procedure with very few complications.

Astigmatism↗
PubMedDate not supplied

A model to explain the difference between changes in refraction and central ocular surface power after laser in situ keratomileusis.

PURPOSE: After refractive surgery, changes in central ocular surface power are usually less than actual changes in refraction. The aim of this paper was to examine the theoretical changes in the radius of the epithelial-stromal interface and corneal stromal refractive index which could take place to account for some of the empirical findings. METHODS: The parameters of an aspheric human eye model featuring gradient index optics was modified using data from a clinical study evaluating effects of laser in situ keratomileusis (LASIK) for correction of moderate to high myopia. Computations were performed to determine: 1) theoretical postoperative radius of the epithelial-stromal interface and hence distribution of epithelial thickness, and 2) refractive index of the stroma when the epithelium was of fixed uniform thickness. RESULTS: Within the central 2-mm diameter refractive zone of the cornea after LASIK for myopia, either of two factors could account for the difference between changes in central ocular surface power and the actual change in ocular refraction: 1) a steepening of the epithelial-stromal interface resulting from a 400% centrifugal increase in epithelial thickness, or 2) a reduction in stromal refractive index from an average of 1.376 to 1.364. CONCLUSION: The difference between the observed changes in refraction and central ocular surface power could be explained by reduction in stromal refractive index and this could be secondary to a 6.5% increase in water content of the stroma during the postoperative period. The epithelial-stromal interface after LASIK is unlikely to steepen by the extent predicted by our model. This surface is not the source of the difference between the change in refraction and change in central ocular surface power.

Corneal Stroma↗
PubMed2005

Effect of base monomer's refractive index on curing depth and polymerization conversion of photo-cured resin composites.

In this study, we examined the effect of the transmitted amount of visible light through a resin composite on the curing depth and polymerization conversion. Transmitted amount of visible light was strongly dependent on the magnitude of refractive index difference that existed between the resin and silica filler. More specifically, the differences arose from the type of base monomer used. The transmitted amount of visible light exhibited a good correlation with the curing depth and Knoop hardness ratio of the bottom surface against the top surface of the resin composite. To improve the polymerization conversion of the cavity floor, it is important to reduce the refractive index difference that exists between the base resin and silica filler.

Composite Resins↗
PubMed1990

Improvements to light transmittance in light-cured composite resins by the utilisation of low refractive index dimethacrylates.

In an effort to improve the light transmittance of light-cured composite resins while avoiding degradation of their physical properties, 4 dimethacrylates having low refractive indices and a bulky backbones such as alicyclic or fluorine-substituted bisphenol groups were synthesized. The depths of cure and physical properties of 6 experimental composite resins containing these new monomers were, then, examined and compared to those of 3 control composite resins containing UDMA, BisMEPP, or BisGMA. The depths of cure of the experimental group containing the synthesized monomers were greater than those of the control group with the exception of the composite containing UDMA. The depth of cure increased as the difference between the refractive indices of the matrix monomer and silica filler decreased. The physical properties of 2 composites in the experimental group were comparable to those of the control group. The matrix monomers of these 2 composites were a mixture of 4,8-dimethacryloxymethylene tricyclo [5.2.1.0] decane or 2,2-bis (4-methacryloxyethoxy phenyl) hexafluoropropane and TEGDMA in a molar ratio of 1/1.

Composite Resins↗
PubMed1998

Scattering properties of Bagolini lenses and their effects on spatial vision.

The effect of a Bagolini lens on spatial vision was investigated by studying its far-field diffraction pattern as produced by a coherent beam of laser light, and its effect on the contrast sensitivity function (CSF) in human subjects. For lenses of the main type studied, which were crossed by a series of slightly-irregular striated bands, each consisting of fine, parallel, etched lines of various widths and separations, the diffraction pattern consisted of undiffracted light giving a bright central spot and wide-angle, diffracted light giving a dim streak. The latter was due to the sum of the diffraction patterns associated by the irregular fine etched lines. The streak produced by a single striated band was modulated by a series of regular maxima and minima related to the width of the band. Analysis of this pattern gave the width of the band as 0.6 mm, in close agreement with direct microscopical measurements. When four bands were illuminated by a beam of about 3 mm diameter, similar to the diameter of the photopic pupil, the diffraction pattern showed no obvious maxima and minima, due to irregularity in the width and separation of the bands. The central spot contained more than 90% of the total light in the diffraction pattern. Thus the Bagolini lens, with its relatively weak far-field diffraction pattern lacking regular maxima and minima when areas > or = 3 mm in diameter were used, was expected to have only a small effect on the apparent contrast of the targets in CSF experiments. This was confirmed by the measurements: Bagolini lenses showed no significant effect on either the monocular or binocular CSF. Further similar measurements with lenses of slightly different design from another manufacturer confirmed these findings. Therefore Bagolini lenses do not disrupt vision when they are used to determine the presence of suppression and anomalous retinal correspondence.

Contrast Sensitivity↗
PubMed1998

Errors in determining the direction of the visual axis in the presence of defocus.

The visual axis is the ray path from the fixation point to the fovea by way of the nodal points. Such a ray path does not exist in unaccommodated vision when the fixation point does not coincide with the far point of the eye because of induced or natural defocus. Nevertheless an approximation to the visual axis can be obtained by the position of a small pupil close to the cornea for which a bichromatic vernier target appears correctly aligned. When the visual axis is determined, an eye rotation from the visual fixation position must occur, which is reversed upon removing the small pupil. Upon engaging in any experiments for which the visual axis is then used as a reference, there is an error in this reference position. We develop simple paraxial equations to estimate this error. We show that these equations have good accuracy. The error associated with locating the visual axis at the cornea is 0.002 mm per dioptre of defocus, which is small enough to be ignored.

Convergence, Ocular↗
PubMedDate not supplied

Effect of passive transfer status and vaccination with Escherichia coli (J5) on mortality in comingled dairy calves.

The effect of vaccination with a commercially available R-mutant coliform mastitis vaccine on the survival of comingled dairy calves on a farm with endemic salmonellosis was examined. A total of 864 calves were randomly assigned to either vaccine (n = 435) or control (n = 429) groups. Passive transfer status of each calf was determined using refractometer determination of serum total protein concentration. Logistic models were developed to determine the effects of vaccine group and passive transfer status on calf survival to 100 days of age. In a model in which serum protein concentration was treated as a categorical variable, increasing serum total protein concentrations were associated with decreased mortality until these concentrations exceeded 6.0 g/dL. Calves with serum protein concentrations > 6.0 g/dL had increased risk for mortality compared with calves with serum protein concentrations > 5.5 g/dL but < or = 6.0 g/dL. This increased risk for mortality was supported by the results of a logistic model in which serum protein concentration was treated as a continuous variable. The increased risks associated with high serum protein concentration probably reflect the effect of dehydration in calves with occult disease. Neither model demonstrated any significant association between vaccination status and survival to 100 days of age. Based on these results, the routine immunization of calves cannot be recommended as a strategy to prevent mortality on farms with endemic salmonellosis.

Animal Husbandry↗
PubMedDate not supplied

Perinephric pseudocysts in the cat: a retrospective study and review of the literature.

Perinephric pseudocysts (PNPs) are fluid-filled fibrous sacs that surround the kidney and are not lined by an epithelium. In cats, PNPs are idiopathic, but they usually occur in association with chronic renal failure (CRF). Thirteen cats with PNPs were examined. PNPs occurred in mixed breed cats of either sex with a median age of 16 years. The PNP was palpable on physical examination and usually was interpreted as renomegaly. Clinicopathologic findings reflected CRF, and urinary tract infection was common. Rarely, a primary renal disease was diagnosed concurrent with PNPs and CRF. Diagnosis of PNPs was made by ultrasound examination and fine-needle aspiration, and treatment was by surgical removal of the PNP or ultrasound-guided drainage. Compared to previous reports of PNPs, this series of cats tended to be older and no sex predilection was found, but other findings were similar to those in the literature. Cats with PNPs may have a favorable prognosis if CRF is not severe and no other concurrent diseases are present.

Animals↗
PubMed1998

Evaluation of the pinhole pupillometer.

Evaluation of pupil diameter is used in many clinical and research fields to aid diagnosis of neurological disorders and to monitor pharmacological effects upon the eye. Methods used to determine pupillary diameter have evolved from simple scale measurements to much more complex systems, supposedly improving accuracy and repeatability. However, many techniques are restricted to the consulting room or research laboratory due to the size of the equipment, its cost, or the expertise required. We have evaluated a portable pupillometer originally developed by Broca, capable of precise measurements on fixed, dilated pupils, that has the potential to be used by unsupervised patients. Luminance levels will still need to be controlled because, although luminance does not influence the mechanics of the pupillometer itself, it significantly affects the natural pupil. The pinhole size should ideally be 1.1 mm in diameter, since smaller pinholes underestimate and larger holes overestimate pupil size. The pupillometer also exhibited reasonable accuracy in fixed pupils dilated with tropicamide. Pupil measurements were overestimated by 0.5% using the 1.1 mm pinholes (insignificant if the pupil can be measured to the nearest 0.25 mm) and both intra- and inter-visit repeatabilities were relatively good. The results from this study therefore indicate that the pinhole pupillometer may be of use in pupil research, since it facilitates more frequent pupillary measurements over much longer time intervals than are currently achievable.

Humans↗
PubMedDate not supplied

Urine specific gravity measurement: reagent strip versus refractometer.

OBJECTIVE: To compare the results of urinalysis screenings for specific gravity (SG) using the reagent strip and the refractometer. SETTING: United Hospital, Grand Forks, North Dakota. PRACTICE DESCRIPTION: United Hospital is a 384-bed teaching hospital. PRODUCT COMPARISON: The Ames Multistix 10 SG reagent strip (Miles, Inc., Elkhart, IN 46515) was compared with the TS Meter (Leica, Inc., Deerfield, IL 60015). MAIN OUTCOME MEASUREMENT: The degree of correlation between the results produced by each method. RESULTS: The percentage of difference between the means of the direct strip readings and the refractometer readings was 9.68%. The percentage of difference between the means of the adjusted strip readings and the refractometer readings was 22.58%, which was significantly different. When the direct strip readings and the refractometer readings were plotted together on a graph, the points were widely scattered; this fact, and a correlation coefficient of 0.725, suggest that random error occurred in both methods. Analysis of the slope and intercept of the correlation indicated systematic error. CONCLUSION: The reagent strip method of measuring SG is accurate only in a narrow range of "average" values, and should not be used as the basis for medical diagnoses.

Humans↗
PubMedDate not supplied

Surface plasmon resonance: a technique for characterizing biomaterial surfaces.

Surface plasmon resonance is an optical technique that allows the study of surface phenomena in real time, without preliminary labelling. The technique is useful in biomaterial science to determine affinity constants, concentrations, kinetics, film thickness and dielectric properties of material surfaces. This article examines the technology and some of its applications.

Biocompatible Materials↗
PubMed1999

Tonic accommodation, age, and refractive error in children.

PURPOSE: An association between tonic accommodation, the resting accommodative position of the eye in the absence of a visually compelling stimulus, and refractive error has been reported in adults and children. In general, myopes have the lowest (or least myopic) levels of tonic accommodation. The purpose in assessing tonic accommodation was to evaluate it as a predictor of onset of myopia. METHODS: Tonic accommodation was measured in children enrolled in the Orinda Longitudinal Study of Myopia using an infrared autorefractor (model R-1; Canon, Lake Success, NY) while children viewed an empty lit field or a dark field with a fixation spot projected in Maxwellian view. Children aged 6 to 15 years were measured from 1991 through 1994 (n = 714, 766, 771, and 790 during the 4 years, successively). Autorefraction provided refractive error and tonic accommodation data, and videophakometry measured crystalline lens curvatures. RESULTS: Comparison of the two methods for measuring tonic accommodation shows a significant effect of age across all years of testing, with the lit empty-field test condition yielding higher levels of tonic accommodation compared with the dark-field test condition in children aged 6 through 11 years. For data collected in 1994, mean (+/-SD) tonic accommodation values for the lit empty-field condition were significantly lower in myopes, intermediate in emmetropes, and highest in hyperopes (1.02 +/- 1.18 D, 1.92 +/- 1.59 D, and 2.25 +/- 1.78 D, respectively; Kruskal-Wallis test, P < 0.001; between-group testing shows each group is different from the other two). Age, refractive error, and Gullstrand lens power were significant terms in a multiple regression model of tonic accommodation (R2 = 0.18 for 1994 data). Lower levels of tonic accommodation for children entering the study in the first or third grades were not associated with an increased risk of the onset of myopia, whether measured in the lit empty-field test condition (relative risk = 0.90; 95% confidence interval = 0.75, 1.08), or the dark-field test condition (relative risk = 0.83; 95% confidence interval = 0.60, 1.14). CONCLUSIONS: This is the first study to document an association between age and tonic accommodation. The known association between tonic accommodation and refractive error was confirmed and it was shown that an ocular component, Gullstrand lens power, also contributed to the tonic accommodation level. There does not seem to be an increased risk of onset of juvenile myopia associated with tonic accommodation.

Accommodation, Ocular↗
PubMed1978

Effect of anticoagulants and glucose on refractometric estimation of protein in canine and rabbit plasma.

The effect of ethylenediaminetetraacetate (EDTA) compounds on the refractometric estimation of plasma protein concentration was attributed largely to osmotic fluid shifts, as reflected in changes in hematocrit, and to addition of total solids to the plasma. With H4EDTA, these two mechanisms were additive and caused increased plasma protein readings of significant magnitude even at recommended (1--2 mg/ml) anticoagulant concentrations. For the potassium and sodium salts, the two mechanisms were partly compensatory, which ameliorated the effect at 1--2 mg/ml concentration. At higher concentrations, such as might occur if a blood collecting tube were incompletely filled, all of the EDTA compounds caused technically significant over-estimation of plasma protein. When dextrose (d-glucose) was added in-vitro to canine blood, in amounts analogous to clinical hyperglycemia, the effect upon plasma protein estimation was minimal.

Animals↗
PubMed1982

Refractometers.

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Directories as Topic↗
PubMed2000

[Phacoemulsification following radial keratotomy. Topographic and refractive analysis concerning an 18-month period (apropos of a case)].

We followed a 45-year-old man who underwent clear cornea phacoemulsification cataract extraction 3 years after radial keratotomy during an 18 month period. We report early and late refractive analysis and corneal topography during the followup period. The patient experienced an hyperopic shift which was different from the aim of 5 diopters due to an early corneal flattening of 3 diopters and an underestimation of the IOL power of 2 diopters using SRKII calculation formula. Corneal flattening regressed after 3 weeks. We recommend that these patients should be informed of an eventual early hyperopic shift and late refractive error, that Binkhorst II or Hoffer-Q intraocular lens calculation formulas be used and keratometry measurements be obtained from computerized videokeratography.

Corneal Topography↗
PubMed2000

[Effect of scleral indentation on the corneal topography and the axial length after retinal detachment surgery. A prospective study in 30 cases].

INTRODUCTION: Scleral indentation is an essential time in conventional retinal detachment surgery. It enables re-establishing retinal contact and to counterbalance vitreal tractions. It can be circular, segmental or radial. Silicone materials are often used. MATERIALS: In a prospective study, we followed up 30 eyes of patients who underwent retinal detachment surgery. We analyzed changes in corneal surface using differential map of the corneal (induced astigmatism, meridian axial deviation) and change in the axial length of the globe. RESULTS AND DISCUSSION: Follow-up examination showed corneal astigmatism: 2.62 diopter (D) at one week, 2.37D at one month and 1.80D at 3 months. Corneal axis meridians changed: 25 degrees at one week and 18.6 degrees at 2 months. Axial length was also modified. We observed a lengthening: 1.7mm during the first week which remained unchanged at 1.63 after 2 and 3 months. These changes were studied according to the different techniques used in our study and were compared with results reported in the literature. CONCLUSION: We think that conventional retinal detachment surgery causes modifications in refractis (induced astigmatism and axial lengthening).

Adolescent↗