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At least 1,027 records · Page 57Linked to original sources
PubMed2003

Fourier analysis of corneal astigmatic changes following photorefractive keratectomy.

The purpose of this study was to evaluate the corneal irregular astigmatism following photorefractive keratectomy (PRK) for myopia. The corneal topography of 30 eyes of 26 patients was measured with the TMS-1 videokeratoscope before and 1 month after PRK. Axial dioptric data were decomposed into four components; A0 (Sphericity), C1 x 2 (Asymmetry), C2 x 2 (Regular astigmatism), and C3 (higher-order irregularity) for the central 3 and 6 mm zone by Fourier series harmonic analysis. Post-operative topographies were divided into those with an irregular and those with a homogeneous pattern, and the Fourier components were compared. In the 6 mm zone, A0 was significantly decreased (P < 0.001), and C1 x 2, C2 x 2, and C3 were significantly increased (P = 0.001, 0.005, 0.002, respectively). In the 3 mm zone, A0 decreased (P < 0.001) and C1 x 2 increased (P < 0.001) significantly. C1 x 2 was correlated with the post-operative corrected visual acuity (P < 0.001, r = 0.647). The irregular pattern group had a larger C1 x 2 component (P < 0.001). The treatment displacement was not correlated with any component. In conclusion, irregular topography due to intraoperative drift or asymmetrical wound healing may play a more important role in the post-operative corneal optical property than mild treatment displacement.

Adult↗
PubMed2000

Confocal microscopy of a patient with irregular astigmatism after LASIK reoperations and relaxation incisions.

PURPOSE: Laser-assisted in situ keratomileusis (LASIK) is widely used for correcting refractive errors. If the predicted refractive result is not achieved after the first operation, a re-operation can be performed by ablating more stromal tissue after reopening the flap. The goal of this study was to analyze, by using in vivo confocal microscopy, the morphologic changes associated with repeated LASIKs. METHODS: Clinical examination, computed corneal topography, and real-time in vivo confocal microscopy were performed on both eyes of a 50-year-old patient with induced irregular astigmatism leading to decreased best-corrected vision in the left eye after LASIK. The left cornea had been operated on 5 times (LASIK with two reoperations followed by two relaxing incisions), and the right cornea twice (LASIK with one reoperation). RESULTS: Microfolds at the level of the Bowman's layer and highly reflective particles at the flap interface were observed in both corneas. The subbasal nerve plexus was severed in the left eye. In addition, we identified epithelial material in the flap margin and inside one of the two relaxing incisions placed inferotemporally. CONCLUSION: Repeated LASIKs may stretch the flap and result in microfolding at the Bowman's layer. This and deposition of particles in the flap interface may increase with the number of reoperations, challenging the healing response. Microfolding and occurrence of foreign material in the interface may add to the irregular astigmatism and poor visual outcome after LASIK. Clinical in vivo confocal microscopy offers new possibilities for the assessment of ultrastructural changes after corneal refractive surgery.

Astigmatism↗
PubMed2001

Introduction of corneal astigmatism through placement of a scleral fixation ring in eye bank eyes.

PURPOSE: To determine the effect of scleral fixation ring placement in induction of astigmatism by topographic analysis. METHODS: Baseline measurements were used in human eyes from the Utah Lions Eye Bank. In two control eyes, the effect of placing and replacing the eye in the holder was determined. In nine eyes, the effect of placing a 14-mm ring 2.0-mm from the limbus and an 18-mm ring 4.0-mm from the limbus, both in line with and between the rectus muscles, was determined. Induced astigmatism (IA) was determined by Alpins' vector analysis method. RESULTS: Control eyes had a mean variation of 0.26 +/- 0.18 diopters (D) (range, 0.07-0.65). The study eyes had a mean IA of 1.55 D with statistically significant differences noted when comparing 14-mm rings (0.92 +/- 2.00 D) versus 18-mm rings in line with the rectus (3.02 +/- 1.49 D; p = 0.02), with 18-mm rings in line with the rectus muscles (3.02 +/- 1.46 D) versus between the muscles (1.37 +/- 1.14 D; p = 0.03), and all measurements with the 14-mm ring (1.0 +/- 1.49 D) versus the 18-mm ring (2.14 +/- 1.51 D; p = 0.04.). CONCLUSIONS: Significant IA can occur with placement of fixation rings, especially when suture placement is near the insertions of the rectus muscles.

Astigmatism↗
PubMed2002

Correction of myopia and astigmatism after penetrating keratoplasty with laser in situ keratomileusis.

PURPOSE: We evaluated whether laser in situ keratomileusis (LASIK) was a safe and effective treatment for myopia and astigmatism after penetrating keratoplasty (PK). METHODS: We performed a retrospective review of medical records of all the patients who underwent LASIK following PK at the University of Minnesota between January 1999 and March 2000. RESULTS: Seventeen patients (20 eyes) underwent LASIK following PK between January 1999 and March 2000. Mean age of the patients at the time of LASIK was 37 years (range, 20-62). Keratoconus was the indication for PK in the majority of the eyes (73.7%). Anisometropia and/or contact lens intolerance was the indication for LASIK following PK. No intraoperative complications occurred. Following LASIK, the best spectacle-corrected visual acuity remained within 1 line of preoperative visual acuity in 94.7% of the eyes. The mean sphere was reduced by 3.93 diopters (80.0%) and the mean cylinder was reduced by 2.83 diopters (69.9%) from the preoperative values at the last follow-up visit. Uncorrected visual acuity became 20/40 or better in 73.7% of the eyes after LASIK. CONCLUSIONS: LASIK is a safe procedure in eyes in which PK has previously been performed. LASIK is effective in the treatment of myopia and astigmatism following PK.

Adult↗
PubMed2004

In vivo confocal microscopy of epithelial inclusions from aberrant wound healing after astigmatic keratotomy.

OBJECTIVE: To report confocal microscopic findings in vivo of delayed prominent epithelial inclusions at a gaped incision groove after astigmatic keratotomy (AK). METHODS: Astigmatic keratotomy using paired arcuate incisions was performed on the right eye of a 59-year-old man who had a preoperative refraction of +2.50 DS, -7.00 DC x 80. The procedure and initial postoperative course were uneventful, and his refraction was OD +1.00 DS, -2.50 DC x 60 at 16 months. However, at 17 months postsurgery, tiny pearl-like lesions appeared along one of the incision grooves. In vivo confocal microscopy was performed to investigate these lesions. RESULTS: Under confocal microscopy, clusters of epithelial inclusions inside the gaped incision groove corresponded to the pearl-like lesions observed clinically. A confluent layer of flat, regular and polygonal epithelial cells covered the wall of the groove. Activated keratocytes were observed adjacently. The number of keratocytes around the groove, however, did not appear to increase in comparison to normal corneal wound healing and scar formation. No foreign body, infective, or inflammatory signs were observed. CONCLUSIONS: Aberrant wound healing was identified in post-AK incisions, similar to post-radial keratotomy cases. Confocal microscopy is a useful tool to study the wound healing of AK incisions and to rule out foreign bodies or infective elements as illustrated by this case. To the best of our knowledge, this is the first reported in vivo confocal study of AK wound healing in humans.

Astigmatism↗
PubMed1977

Astigmatism after contact lens wear.

With-the-rule astigmatism increased by more than 3 D in 2 habitual long-term wearers of contact lenses after they abruptly stopped wearing their lenses. Reducing wearing time gradually over a 2- to 3-month period caused less than 1 D increase in with-the-rule astigmatism in 2 other long-term wearers. All 4 persons had worn polymethylmethacrylate contact lenses for 12 or more years.

Adult↗
PubMed1984

Effect of target configuration on the measurement of astigmatism.

In this study, five dissimilar crossed cylinder targets were used to determine the cylindrical component of the refractive error of 30 subjects. Our purpose is to see if variations in target configuration affect the results of the test. Although there are statistical differences when targets with different configurations are used in determining astigmatism subjectively, these differences are not clinically significant if we assume that the least amount of error required for clinical significance is +/- 0.25 D. We conclude that dissimilar crossed cylinder targets yield similar astigmatic findings clinically.

Adult↗
PubMed1986

Influence of lid position on astigmatism.

The influence of 3 different lid positions on the astigmatism of 50 young healthy eyes was investigated with an autorefractor. The results showed that a significant amount of astigmatism may be induced when the palpebral aperture is voluntarily narrowed.

Adolescent↗
PubMed1989

Correcting astigmatism.

Measurement of refraction in the principal meridians and off-axis meridians in both directions from the visual axis was undertaken. The extent of disagreement between the sine-squared law and the actual astigmatism of living astigmatic eyes was calculated. The corrected visual acuity correlates with the residual ametropia. The conoid of Sturm is not the same for each eye.

Astigmatism↗
PubMed1994

Paraxial ray tracing through noncoaxial astigmatic optical systems, and a 5 x 5 augmented system matrix.

The matrix method of tracing a paraxial ray through a coaxial optical system that contains spherical refracting elements requires a 2 x 2 system matrix. This paper shows that a suitable 5 x 5 system matrix enables one to apply the same method in the general case of systems that may be noncoaxial and that may contain astigmatic elements. The systems may contain prisms and decentered astigmatic lenses, for example.

Astigmatism↗
PubMed2000

Accommodation causes with-the-rule astigmatism in emmetropes.

PURPOSE: To measure the changes in astigmatism when bilateral emmetropes accommodate. METHODS: Bilateral emmetropes accommodative responses were measured with an improved photorefractometer PR-1100, which measured binocular refraction in all meridians simultaneously as a fixation target was shown in natural space. RESULTS: The accommodative responses in the vertical meridian are greater than those in the horizontal meridian. In the horizontal meridian, the accommodative responses are less than the accommodative stimulus. CONCLUSIONS: When accommodating the majority of bilateral emmetropes show with-the-rule astigmatism and lag of accommodation in the horizontal meridian.

Accommodation, Ocular↗
PubMed2001

Astigmatism management: ablation patterns.

Treatment of astigmatism is possible with the excimer laser. Results with this technology are more predictable and accurate than with incisional techniques. Ablation profiles differ for the various types of astigmatism, and the best approach is the one in which the least amount of tissue is removed from the cornea. Surgical success requires appropriate preoperative counseling, careful preoperative measurements, and meticulous intraoperative technique. Reported outcomes are excellent and future developments will lead to even better results.

Astigmatism↗
PubMed1994

Bitoric rigid contact lens with prism fitting in rare cases of moderate corneal and residual astigmatism.

We reviewed all records of bitoric rigid gas permeable (RGP) contact lenses with prism ballast fittings performed at Tulane University Hospital and affiliated clinics from 1985 to 1993. We found eight eyes of six patients fit were fit with this lens, which is indicated for the patient with a moderately astigmatic cornea in addition to residual refractive astigmatism. In this design, a small amount of prism is added to a standard bitoric design to achieve rotational stability. All patients achieved 20/25 or better vision. We provide an outline for fitting bitoric RGP contact lenses with prism ballast.

Adult↗
PubMed2005

Corneal irregular astigmatism after hyperopic laser in situ keratomileusis.

PURPOSE: To evaluate the changes in irregular astigmatism after hyperopic laser in situ keratomileusis (LASIK). METHODS: In a prospective case series, 15 eyes of 12 patients who had hyperopic LASIK were evaluated. Corneal topography was obtained before and after hyperopic LASIK. Corneal irregular astigmatism (asymmetry and higher-order irregularity) was calculated using Fourier harmonic analysis of the topography data. RESULTS: Hyperopic LASIK significantly increased the asymmetry component at 3 (P = 0.0085) and 6 months (P = 0.0307) postoperatively. The mean higher-order irregularity was not significantly changed at any postoperative time-point (P = 0.222). The achieved change in the spherical equivalent was significantly correlated with the post/preoperative ratio of asymmetry (R = 0.695, P = 0.0030). CONCLUSION: Hyperopic LASIK significantly increases the asymmetry component of the cornea, which is correlated with the achieved change in the spherical equivalent.

Adult↗
PubMed1986

Two simple calculating schemes for use in ophthalmic optics--II. Tracing axial pencils through systems including astigmatic surfaces at random axes.

A paraxial computing scheme is described for tracing an axial pencil of rays through any system containing astigmatic surfaces with their axes at random. In conjunction with a scheme for tracing oblique rays (Bennett, 1986) it is applied to a problem in visual optics posed by Keating (1982). This involves determining the spectacle magnification for a schematic eye with three strongly astigmatic surfaces, corrected by a spectacle lens of bi-toroidal form, the axes of all five surfaces being mutually oblique. It is shown that the suggested computing schemes yield very similar results to those found by the matrix methods of Keating. Although only two rays are needed to determine the principal dimensions of the basic retinal image of a circle, further insight into its true shape is obtained from a multiple trace, using a computerized version of the ray tracing scheme.

Astigmatism↗
PubMed1989

Alternatives to toric contact lens fitting--for regular and irregular astigmatism.

Although toric contact lenses have improved in design and development and their use is increasing, further consideration should be given to the alternative methods of astigmatism correction by contact lenses. Much can still be achieved by non-toric lens, both for a wide range of astigmatism and for ease of supply. A number of these alternatives are examined, their advantages and disadvantages reappraised, and the merits of lens correction compared with surgical methods considered.

Astigmatism↗
PubMed2005

Compensation of aniseikonia in astigmatic pseudophakic eyes.

PURPOSE: A recently published manuscript addressed the problem of compensating for aniseikonia between pseudophakic astigmatic eyes using a least-squares calculation scheme. The purpose of this paper is to revisit this topic with the specific aim of providing explicit formulae for the determination of the intra-ocular lens required to produce a specified transverse image size at the plane of the retina and the characteristics of the contact or spectacle lens required to realize some desired refractive outcome. METHODS: The 4 x 4 ray transference is central to the development of all formulae presented in this paper. Specifically, the formula for the determination of the power of the intra-ocular lens required to achieve some transverse image size at the retina is derived directly from the disjugacy of the pseudophakic eye. RESULTS: The formula is applicable to both stigmatic and astigmatic systems without restriction. A detailed numerical example for an unusual eye is provided. CONCLUSION: A formula for the determination of the intra-ocular lens required to produce any given transverse image size at the retina is derived. This approach does not require the application of the Moore-Penrose pseudo-inverse and one is able to work rather with the properties of the optical system directly without further modification.

Algorithms↗
PubMed1978

Astigmatism in infants.

Compared with children of school age, infants show ten times the incidence and considerably greater amounts of clinically significant astigmatism. The amount begins to decrease in the second semester of life, and the incidence declines during the third year. The unanticipated results bear on both the etiology and the neural sequelae of astigmatism.

Age Factors↗