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At least 1,243 records · Page 69Linked to original sources
PubMedDate not supplied

Inverse arcuate incision: a new approach to the correction of astigmatism.

Standard concentric arcuate transverse corneal relaxing incisions flatten the meridian incised and steepen the meridian 90 degrees away, a phenomenon designated "coupling". We have demonstrated a new method of reducing the steep corneal curvature without inducing coupling, by utilizing both transverse and radial components of an inverse arcuate incision. This technique has been found useful in cases of myopic astigmatism because it reduces the myopic spherical equivalent, which is left unchanged by standard arcuate incisions, without having to place radial incisions.

Aged↗
PubMedDate not supplied

Astigmatism following photorefractive keratectomy for myopia.

BACKGROUND: Astigmatism following photorefractive keratectomy for myopia has been reported as stable as early as 2 to 3 months. The authors report 36 out of 60 consecutive eyes with variations in the cylindrical component of their refraction at 6 months after laser treatment. METHOD: A standard photorefractive keratectomy was carried out on 60 consecutive eyes in 52 patients over a 7-month period. The manifest refraction of these eyes was followed for 6 months. RESULTS: Thirty-six eyes demonstrated a change in the cylindrical element of their refraction manifested as a change in cylinder power or axis, or both. The mean pretreatment cylinder power in the group that underwent a change in the cylindrical element was significantly higher than the mean of the group where this did not take place. The mean cylinder power change was 0.75 diopters (D) and in 9 eyes this change was 1.00 D or more. The corrected and uncorrected postoperative visual acuities were the same in the two groups. CONCLUSIONS: This observation implies meridional variability in the healing process of the anterior cornea following photorefractive keratectomy.

Adult↗
PubMed1993

Association between surgically-induced astigmatism and cataract incision size in the early postoperative period.

One hundred sixteen eyes with cataracts were prospectively randomized into four surgical groups. Silicone and polymethylmethacrylate (PMMA) intraocular lenses were implanted into the posterior chamber of these eyes through 4-, 6-, or 11-millimeter incisions by one surgeon. Surgically-induced astigmatism was evaluated in terms of: (1) the proportion of eyes achieving a refractive cylinder of 1.5 or less, without regard to axis; and (2) the induced cylinder, with regard to axis, using vector analysis. One week postoperatively, the eyes with a 4-millimeter incision had a significantly lower mean induced cylinder than those with larger incisions (P = .019); and all three phacoemulsification groups had a significantly higher proportion of eyes with a cylinder of 1.5 or less than the nuclear expression group (P < .01). One month postoperatively, the 4-millimeter-incision group had a significantly lower mean induced cylinder (P = .044) as well as a significantly higher proportion of eyes with a cylinder of 1.5 or less, than the larger-incision groups (P = .05). Three months postoperatively, no value of any parameter in any group differed significantly from that of that parameter in any other group.

Aged↗
PubMedDate not supplied

Astigmatic and refractive stabilization after cataract surgery.

We assessed the timing of astigmatic and refractive stabilization following six cataract surgery procedures with intraocular lens implantation in 229 eyes divided into six groups in the following incision sizes and methods of wound closure: 11-mm incision with running suture closure (26 eyes) 6.5-mm incision with running suture closure (29 eyes) 6.5-mm incision with single horizontal suture closure (25 eyes) 6.5-mm incision without suture closure (46 eyes) 5.5-mm incision without suture closure (51 eyes) 3.2-mm incision without suture closure (52 eyes) Analyzed up to 6 months postoperatively were: the mean and standard deviation of axis-based keratometric cylinders the absolute value of the induced-cylinder vector the spherical equivalent of the refractive power. In the 11- and 6.5-mm incision running suture groups, these parameters did not stabilize during the study period. In the 6.5-mm incision horizontal suture and sutureless groups, the values stabilized at 3 months postoperatively; in the 5.5-mm incision group, at 1 month; and in the 3.2-mm incision group, at 2 weeks. These results indicate that the appropriate point at which to prescribe postoperative correction spectacles differs significantly depending on the procedure, and that smaller incisions with wound-closure methods that do not exert vertical force are associated with fewer postoperative refractive changes.

Aged↗
PubMed1996

Spectacle lenses which correct oblique astigmatism at all ocular rotations: investigation of "Merté's' conicoidal surfaces.

Merté (1950) described lens forms that correct oblique astigmatism at all ocular rotations. The front surfaces of all lenses were flat, and the back surfaces were made aspherical as conicoids. Merté did not provide a derivation of his formulae for the appropriate conicoidal surfaces. We show that Merté's formula is correct and describe the nature of appropriate conicoidal back surfaces of spectacle lenses.

Astigmatism↗
PubMed1999

The challenge of fitting astigmatic eyes: rigid gas-permeable toric lenses.

It is generally acknowledged that rigid gas permeable lenses (RGP) afford superior visual correction of the ametropic eye compared with hydrophilic lenses. However, there are occasions when a spherical rigid lens will not provide an acceptable result in terms of fitting or vision on astigmatic eyes. In these instances a toric RGP lens will be required. This paper deals with the definitions, indications, fitting technique, calculations and designs relating to this modality of fitting. Whilst often perceived as a complex area within contact lens practice, the basic optics and fitting of toric RGP lenses is not overly difficult. A simple empirical means of fitting the lenses will be described which has a high 'First-fi' success rate.

Journal Article↗
PubMed1991

Effects of spherical and astigmatic defocus on acuity and contrast sensitivity: a comparison of three clinical charts.

Two contrast sensitivity charts (Vistech and Pelli-Robson) have become available to the eye care practitioner. Their value as clinical tools for the assessment of visual function may be enhanced because of either an insensitivity to the effects of optical focus or a hypersensitivity to defocus. We compared the sensitivity to defocus of these charts to the traditional Snellen chart by examining the effect of up to +/- 5.00 D of spherical and astigmatic defocus on performance with each chart. In order to simulate the two types of clinical examination scenarios, tests were performed both with and without mydriatic/cycloplegic agents. The Pelli-Robson contrast sensitivity chart was very resistant to the effects of all types of optical defocus. As predicted, the high spatial frequencies on the Vistech chart were sensitive to defocus. However, although contrast sensitivities for the low frequencies were affected less, optical defocus produced significant decreases in low frequency Vistech contrast sensitivity. In addition, the Vistech chart was very insensitive to axis 180 blurring lenses. There was no indication that either contrast sensitivity chart was more sensitive to defocus than the standard Snellen chart.

Adult↗
PubMed1999

Dynamics of a misaligned astigmatic twisted Gaussian beam in a Kerr-nonlinear parabolic waveguide.

Using the Galerkin criterion in the basis of flexible generalized Gaussian modes, the equations of motion are derived for the parameters of a misaligned astigmatic twisted Gaussian beam in an axially symmetric nonlinear medium. Nontrivial features of the beam dynamics (e.g., phase locking, cycle generation, nonlinear symmetry change) in a parabolic waveguide with Kerr nonlinearity are revealed.

Journal Article↗
PubMed2003

Interaction between genetic and inductive factors controlling the expression of dispersal and dormancy morphs in dimorphic astigmatic mites.

Some astigmatic mites display dimorphic deutonymphs (hypopus) which are facultatively intercalated in their development cycle between protonymph and tritonymph. Such species, among them Glycyphagus privatus and Glycyphagus ornatus show three potential developmental pathways: (1) to bypass the hypopus stage and develop directly from the protonymph to the tritonymph and the subsequent reproductive stage when conditions are favorable; (2) to leave the original site and disperse by means of a phoretic hypopus morph; or (3) to survive inimical life conditions in the natal environment by means of a sedentary hypopus morph. By producing both dispersing (and afterwards at the arrival site reproducing) and sedentary (drought-hardy and dormancy-prone) progeny each single parent attains a selective advantage through a risk-reducing insurance against irregularly fluctuating and often fatal life conditions of their temporary patch habitats. Both genetic heterogeneity and ecological plasticity for hypopus production adapt the Glycyphagus species to cope with variation in the environment. Both traits (for dispersal and survival) are extremely polymorphic with genotypes ranging from low to high propensities for production of each hypopus type. There is a substantial environmental effect on genetic expression such that expression of both morphs depends on the quality of food. This ecological response allows a fast reaction of the mite to the current trophic environment. Phoretic morphs are predominantly expressed at favorable trophic conditions and sedentary morphs at poor trophic conditions. Ecological influences may override genetic propensities and vice versa. Although selection imposed by changing environmental patterns adjusts the frequencies of genotypes over generations and provides for long-term adaptation, the short-term process of environmental induction adapts the population within a generation to transient-habitat disturbances. The interaction of genetic and ecological determinants explains the varying proportions of directly developing mites, phoretic hypopodes, and sedentary hypopodes, in a population at any moment.

Animals↗
PubMed1993

Ocular rotation factor and orientational demand with astigmatic lenses.

Ocular movements required in most visual functions are affected by lenses in front of the eye. Fundamental to an understanding of these movements are concepts such as the ocular rotation factor and the ocular orientational demand. In this paper the ocular rotation factor for thin spherical lenses is generalized for astigmatic lenses. A compact matrix equation is obtained for the orientational demand in the case of such lenses. The equations are satisfactory in ordinary situations but they break down under certain circumstances. Equations are derived for these special circumstances in particular and also for all circumstances in general.

Eye Movements↗
PubMed2002

Digital refraction distortion correction with an astigmatic coherence sensor.

We demonstrate the sensing and correction of an isoplanatic refractive distortion (not lens aberrations), using the complete measurement of the partially coherent field in an aperture that the previously described astigmatic coherence sensor provides. Isoplanatic distortions, and in general distortions that do not cause energy loss, maintain the orthogonality of the coherent modes. We use the fact that a common distortion will occur to all coherent modes to separate the distortion from the source behind it, rather than requiring a reference source at a different wavelength. Digital deconvolution was performed on the full four-dimensional partially coherent field for simultaneously computing the distortion and the source intensity distribution.

Journal Article↗
PubMed2006

Propagation of partially coherent twisted anisotropic Gaussian Schell-model beams through an apertured astigmatic optical system.

By expanding the hard-aperture function into a finite sum of complex Gaussian functions, we derived an approximate analytical formula for a partially coherent twisted anisotropic Gaussian Schell-model (AGSM) beam propagating through an apertured paraxial general astigmatic (GA) optical system by use of a tensor method. The results obtained by using the approximate analytical formula are in good agreement with those obtained by using the numerical integral calculation. Our formulas avoid time-consuming numerical integration and provide a convenient and effective way for studying the propagation and transformation of a partially coherent twisted AGSM beam through an apertured paraxial GA optical system.

Journal Article↗