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At least 1,063 records · Page 59Linked to original sources
PubMed1996

Corneal topography for intraocular lens power calculations.

BACKGROUND: In patients undergoing cataract extraction with PC IOL insertion, pre-op and post-op corneal power measurement is necessary to calculate induced astigmatism. In cases with irregular corneal astigmatism, measurements with traditional keratometry is inaccurate. We report the use of video keratography in three cases that required keratometry readings in order to calculate IOL power. These cases were characterized by irregular or incomplete keratometer mires, so quantitative descriptors derived from computer-assisted corneal topography (TMS-1) were used instead. CONCLUSION: The standard keratometer is useless in cases in which the mires appear irregular or incomplete. In such cases, computer-assisted corneal topography can be used successfully. Recently, a new corneal topography index has been developed, the average corneal power, which is a measure of the average corneal power within the entrance pupil. We are currently evaluating the use of average corneal power to assist in the calculation of IOL powers.

Aged↗
PubMedDate not supplied

Correction of astigmatism using positive and negative cylinder programs with the Nidek EC-5000 excimer laser.

BACKGROUND: The efficacy of a current program of positive cylinder treatment was evaluated and compared to a previous program of negative cylinder treatment. METHODS: The authors performed a comparative analysis of 123 eyes (72 patients) who underwent LASIK using the Nidek EC-5000 excimer laser with the formula developed by Chayet and the positive cylinder (71 eyes of 52 patients) and negative cylinder programs (52 eyes of 31 patients). RESULTS: A faster stabilization of correction and a marked reduction of stromal tissue ablated were obtained with the positive cylinder program than with the negative cylinder program. This was directly related to the amount of preoperative ametropia. CONCLUSION: The spectrum of ametropias that can be treated with LASIK is increased with the positive cylinder program because less tissue is ablated.

Astigmatism↗
PubMedDate not supplied

Keratometry for intraocular lens power calculation using Orbscan II in eyes with laser in situ keratomileusis.

PURPOSE: To compare the results of comeal keratometry after laser in situ keratomileusis (LASIK) obtained by the Gaussian optics formula and the clinical history method. METHODS: Sixty-one consecutive patients (121 eyes) who had undergone LASIK were recruited in this retrospective case-controlled study. The K-value obtained from the Gaussian optics formula (CalK) based on postoperative corneal topography by Orbscan II (Bausch & Lomb, Rochester, NY) and ultrasound pachymetry was compared with that obtained from the clinical history method (estK). Keratometry measured by these two methods was compared using the paired sample t test and Pearson correlation coefficient. RESULTS: A high correlation was noted between K-value obtained by the clinical history method and the Gaussian optics formula (R = 0.97, P < .001). The mean difference between the two methods is 0.13 diopters (P = .06). CONCLUSIONS: K reading derived from the Gaussian optics formula correlated closely to that obtained from the clinical history method and would be especially useful in patients with no preoperative LASIK treatment data.

Adult↗
PubMedDate not supplied

Modal reconstruction methods with Zernike polynomials.

PURPOSE: To compare the advantages and disadvantages of different techniques for fitting Zernike polynomials to surfaces. METHODS: Two different methods, Orthogonal Projection and Gram-Schmidt orthogonalization, are compared in terms of speed and performance at fitting a complex object. RESULTS: Orthogonal Projection provides an extremely rapid fitting of a surface, but leaves residual high frequency noise. The Gram-Schmidt technique provides a more accurate fit of the original object, but consumes much more computing time. Orthogonal Projection, and its associated noise, may be tolerated in classifying corneal topography. CONCLUSIONS: Orthogonal Projection has a distinct advantage in calculation time over other methods for fitting surfaces. This advantage may be exploited in cases where accurate surface fitting is not necessary, but only general features need to be extracted for classification. If fit accuracy is needed, then slower fitting techniques, such as Gram-Schmidt, should be used.

Cornea↗
PubMed1976

The optical quality of the monochromatic retinal image as a function of focus.

Changes in retinal image quality with focus for an aberration-free eye with a 5-mm pupil are discussed in terms of both geometrical and physical optics. Further modulation transfer function curves illustrate the effect of spherical aberration. These theoretical results are compared with analogous experimental data obtained at different wavelengths with quasi-monochromatic illumination, using a double-pass photo-electric, scanning instrument. The comparison shows that the approximations of geometrical optics predict ocular performance quite well, provided that the errors of focus are reasonably large (greater than or equal to 0.5 DS for a 5-mm entrance pupil). Possible sources of error in the experimental measurements are indicated.

Ocular Physiological Phenomena↗
PubMedDate not supplied

[Anomalies of optic nerve excavation: the clinical manifestations and differential diagnosis].

Clinical features, electrophysiological characteristics, neuroradiological symptoms and associations with systemic malformations have been studied in 76 children with abnormalities of the optic nerve excavation: the morning glory syndrome, optic nerve coloboma, and congenital peripapillary staphyloma. Based on the results, the criteria are determined for the differential diagnosis of these optic nerve abnormalities and the hypothesis on their different embryogenesis is confirmed.

Child↗
PubMed1999

Fresnel membrane prisms: clinical experience.

BACKGROUND: There are few published reports on the clinical application of Fresnel membrane prisms in the treatment of diplopia in adults. The authors describe the use of these prisms in patients with fourth and sixth cranial nerve palsies, restrictive motility caused by thyroid-related orbital disease, and convergence insufficiency. METHODS: Of 209 patients who had been treated with Fresnel prisms, 141 were selected. The database included patients from a private practice in Montreal, seen from 1988 to 1996, and patients seen by orthoptists in the ophthalmology department of a children's hospital in Montreal between 1992 and 1996. All the patients had diplopia associated with fourth (48 patients) or sixth (43 patients) cranial nerve palsy, thyroid-related orbitopathy (18 patients) or convergence insufficiency (32 patients). After qualitative and quantitative assessment of the ocular misalignment, a Fresnel prism was selected for power and axis and for appropriate location on the spectacle lens. Ocular dominance and side of paresis or restriction were also considered in the placement of the prism. The patient's response to treatment was documented. RESULTS: The Fresnel prisms were oriented horizontally in 72 patients (51%), vertically in 55 (39%) and obliquely in 14 (10%). They were placed on the spectacle lens before the nondominant eye in 127 cases (90%), either covering the entire lens, or on the upper or lower segment or both. The patients were followed for an average of 15 (range 2 to 96) months. Of the 141 patients 113 (80%) had a successful outcome, with relief of their diplopia. Twenty-seven patients (19%) eventually had the prismatic correction ground into the lens, 70 (50%) chose to wear the Fresnel prism on a permanent basis because incorporation into the lens was not possible or because of cost, and 17 (12%) used the Fresnel prism as a temporary device before or after surgery. Most patients who converted to incorporated prisms did so when the prismatic power became stable, usually after 6 to 8 months. Eight patients (6%) stopped using the prism because of associated side effects, such as blurred vision, persistent diplopia, torsion or optical aberrations. INTERPRETATION: The Fresnel prism is an excellent device in treating diplopia in adult patients. It is a reasonable permanent option when incorporating the prism into the spectacle lens is not possible. A fused blurred image caused by a Fresnel prism placed in front of the nondominant eye is preferable to double but clear images.

Abducens Nerve Diseases↗
PubMed2000

The effect of prism adaptation on the response AC/A ratio.

Vergence adaptation, also known as prism adaptation, is a phenomenon in which a patient's heterophoria changes after prolonged viewing through prism. The effect of prism adaptation on the accommodation-convergence relationship, quantified by the AC/A ratio, is not known. Previous studies of AC/A ratio stability and alterability have used only stimulus AC/A ratio calculations, or have measured accommodative responses to only one or two stimuli. The ideal study of AC/A ratio stability and alterability would measure accommodative responses to several accommodative stimuli, and use these along with vergence responses to calculate response AC/A ratios, rather than stimulus AC/A ratios. In addition, the gradient method should be used to avoid any effect of proximal vergence resulting from changes in target distance. This paper describes a project which investigated the effect of vergence (prism) adaptation on the gradient response AC/A ratio, using accommodative responses measured for five different accommodative stimuli. The response AC/A ratio did not significantly change following a period of adaptation to base-in prism for six of the eight subjects in this study. In addition, the response AC/A ratio did not significantly change following a period of adaptation to base-out prism for six of the eight subjects.

Accommodation, Ocular↗
PubMed2001

Biomicroscopic measurement of the optic disc with a high-power positive lens.

PURPOSE: To compare the magnification properties of four different indirect double aspheric fundus examination lenses for clinical disc biometry. METHODS: Experimental study in a model eye. The relationship between the true size of a fundus object and its image was calculated for each fundus lens for an ametropic range between -12.5 and +12.6 D using a slit lamp biomicroscope with adjustable beam length. RESULTS: Equations for determining the correction factor p (degrees per millimeter) were calculated for each fundus lens. The factor can be used in calculations to determine true optic disc size. The total change in magnification of the system from myopia to hyperopia was -21.1% to +24.0% (60-D lens; Volk Opticals, Mentor, OH), -12.9% to +16.2% (Volk super 66 stereo fundus lens), -13.2% to +13.9% (Volk 78-D lens), and -13.3% to +14.0% (Volk super-field NC lens). When the fundus lens position was altered im relation to the model eye by +/-2 mm under myopic conditions, the change in magnification of the system was -4.3% to +5.7% (60-D lens), -4.6% to +6.1% (66 stereo fundus lens), -4.9% to +6.3% (78-D lens), and -5.9% to +7.8% (super-field NC lens). In the hyperopic condition the change was -2.7% to +3.6%, -3.4% to +4.5%, -3.6% to +4.8%, and -4.5% to +6.0%. CONCLUSIONS: The study has shown that the use of a single magnification correction value for each fundus lens may not be appropriate. These findings have important implications for the way in which calculations for determining the true optic disc size and other structures of the posterior pole are performed using indirect biomicroscopy.

Diagnostic Techniques, Ophthalmological↗
PubMed2001

Verifying and evaluating progressive addition lenses in clinical practice.

BACKGROUND: Despite the fact that more than 50% of multifocal lenses dispensed in the United States are progressive addition lenses, adequate methods for clinical verification of these lenses have been lacking. Using automated lens meter techniques, the author describes a simplified method for verification of these complex lenses. METHODS: Thirty pairs of progressive lenses were measured in a modified method using a Humphrey 330 Lens Analyzer. Fifteen pairs were "premium-quality" progressive lenses: fifteen pairs were "non-premium-quality" progressives. Five criteria were assessed on each lens: Distance Zone Width (DZW). Intermediate Zone Width (IZW), Near Zone Width (NZW), Drop Distance (DD), and Maximum Astigmatic Distortion (MAD). RESULTS: "Premium-quality" progressive lenses failed to demonstrate clear-cut superiority over "non-premium-quality" progressive lenses in the five specified criteria. Individual measurements indicate considerable product inconsistency affected every brand tested. CONCLUSIONS: Premium- and non-premium-quality progressive lenses demonstrated similar performance characteristics in this study. Zone size variation in these lenses was found to be considerable, a characteristic that seemed to cut across brand lines. The AO Compact lens seemed to demonstrate a shorter drop distance than other lenses, which does enhance its suitability for use with small frames. A comparison of the Essilor Natural PAL to the Younger Image lens showed little difference in the categories measured, although peripheral distortions seemed closer to the reading zone in the image. A comparison of the MAD of lenses in this study to lenses tested in 1986 indicates a considerable improvement has been made in that important characteristic.

Eyeglasses↗
PubMedDate not supplied

[Long-term results of surgical prevention and correction of progressive myopia].

In this summing-up paper the authors discuss the results of surgical prevention and correction of progressive myopia carried out at Department of Ophthalmic Diseases of Russian University of Peoples' Friendship during two recent decades. Remote results of bandaging scleroplastic and reduced chondroplastic operations recommend scleroplastic interventions for prevention of myopia progress. Combined scleroplasty and cataract extraction stabilized the elongation of the antero-posterior axis of the eye and essentially improved visual acuity of 18-20 diopters. New variants of interlamellar operations are presented: tunnel-circular and fragmented circular, which are protected by patents of the Russian Federation. These operations are recommended for patients with stable myopia of 8-15 diopters after scleroplasty. Their advantage is low traumatism, absence of deep incisions on the cornea, wide optic center, possibility of regulating the refraction effect by replacing the implants or their complete removal without serious consequences for the cornea.

Female↗