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PubMed1997

Vector analysis of astigmatism changes by flattening, steepening, and torque.

PURPOSE: To understand the effect of astigmatism surgery by analyzing astigmatic changes according to their component parts. SETTING: Cheltenham Eye Centre, Melbourne, Australia. METHODS: The component parts of the astigmatic changes considered were flattening, steepening, clockwise torque, and counterclockwise torque. Calculations to determine the astigmatic change were performed by vector analysis using rectangular coordinates after doubling the astigmatism and surgical vector axes. A reference axis was used for the resolution of astigmatic change to ascertain its effect along a selected meridian. RESULTS: When correcting astigmatism, the orientations of incisional (tissue addition) or nonincisional (tissue ablation) techniques in any cornea are at right angles to each other. Since differences exist in the measured astigmatism depending on whether it is measured topographically or by manifest refraction, an on-axis correction in one will occur with an off-axis effect in the other. The net result is a reduced flattening effect and a proportionately increased torque effect for the off-axis component. CONCLUSION: When treatment is applied off one of the four primary axes, the treating vector can be resolved into its component parts of flattening, steepening, and torque. Analyzing changes in this way provides a uniform means of assessing astigmatic changes for all types of cataract and refractive surgery and quantifies the flattening effect when treatment is applied off the intended meridian.

Astigmatism↗
PubMed1998

Treatment of irregular astigmatism.

PURPOSE: To treat irregular astigmatism by applying separate appropriate treatments in each of the two distinct hemidivisions of the cornea. SETTING: Cheltenham Eye Centre, Melbourne, Australia. METHODS: Two general surgical strategies are presented. The first applies the principles of optimization separately to each corneal hemidivision to achieve the maximum reduction in astigmatism when measured topographically and refractively. The second is for targeting symmetrical orthogonal topographic goals for each semimeridian to create the regular state in differing ways. These are performed in one of the following ways: without changing refractive astigmatism; by reducing the associated ocular residual astigmatism; by shifting the less favorably placed topography semimeridian to the other more favorably located one; by shifting both topographic semimeridians to more favorably located sites. This is an alternative when a potential improvement in the best corrected visual acuity is sought and the maximum reduction of astigmatism is not the priority. RESULTS: The calculated treatments necessary to achieve various improved astigmatic states, together with each of their respective separate refractive astigmatism targets, are presented. A single refractive astigmatism value for the entire cornea is also calculated by vector summation. CONCLUSION: Consideration of each of the two distinct hemidivisions of the eye enables improved treatment of irregular astigmatism, potentially resulting in improved visual outcomes.

Astigmatism↗
PubMed1999

Induced astigmatism after photorefractive keratectomy.

PURPOSE: To retrospectively analyze a group of patients to determine whether their induced astigmatism was caused by asymmetry in the laser beam, asymmetry in ablation rates, or wound healing in different corneal meridians. SETTING: Single-center physician office. METHODS: In this study, 146 eyes of 116 patients who had photorefractive keratectomy (PRK) for myopia with the Apex laser (Summit Technology) were retrospectively identified. In 28 eyes, the patient's chair had been rotated 90 degrees from its usual position under the laser. The vector-summated mean change in astigmatism in eyes with the chair rotated 90 degrees was compared with that in a group of control eyes in which the chair was in the usual position. RESULTS: The vector-summated mean change in the control eyes was 0.30 diopter (D) at 83 degrees. Forty-eight of 113 eyes (42.5%) had induced with-the-rule (WTR) astigmatism, and 14 of 113 eyes (12.4%) had induced against-the-rule (ATR) astigmatism. In the eyes in which the chair was rotated 90 degrees, vector-summated mean change was 0.10 D at 13 degrees (P < .0005). One of 27 eyes (3.7%) had induced WTR astigmatism, and 13 of 27 eyes (48.1%) had induced ATR astigmatism (P < .001, chi-square). CONCLUSION: Astigmatism induced by myopic PRK with the Apex laser was small. The axis of induced astigmatism rotated 90 degrees when the patient's chair was rotated, implying that it is inhomogeneities in the beam rather than meridional asymmetry in ablation rates or wound healing that are responsible for induced astigmatism.

Adult↗
PubMed2005

[Asphericity of the cornea and astigmatism].

BACKGROUND: Regarding astigmatism of the cornea, curvature as well as asphericity depend on the meridional axis. Their functional dependence and relation to the quality of the retinal image are still unclear. METHODS: The astigmatic eye was modelled using a biconoid anterior corneal surface by means of a commercially available optical designer programme (Zemax EE, Zemax). The influence of asphericity and astigmatism on the quality of the retinal image was determined by means of ray tracing. Thirty eyes with astigmatism of up to 5.3 D underwent corneal topography (Keratograph C, Oculus) which allowed a numerical evaluation of the cylindric power as well as the asphericity in the main meridians. RESULTS: The quality of the retinal image of an eye with corneal astigmatism can be improved by a factor of 2.28 if the asphericity is optimised. Correction of the central astigmatism only (without considering asphericity) yields only a rather marginal improvement. The average difference of the asphericity in the main meridians is close to zero, however, the individual difference ranges from - 0.372 to + 0.444 which is definitely clinically relevant. CONCLUSIONS: Anisotropic asphericity of the cornea may significantly enhance or compensate central corneal astigmatism. Clinically manifest astigmatism is an individually variable combination of asphericity and curvature difference in the two main meridians and is dependent on the pupil size. Laser correction of corneal astigmatism must take meridional asphericity into account.

Astigmatism↗
PubMedDate not supplied

[Effect of corneal astigmatism on contrast sensitivity in mono- and multifocal pseudophakia--a theoretical study of the physical eye].

BACKGROUND: Several clinical studies indicate, that the visual function of multifocal IOLs (MIOL) is impaired by corneal astigmatism. To assess the influence of uncorrected corneal astigmatism on the contrast sensitivity function (CSF) of mono- and multifocal IOLs, an "optical implantation" of physical eyes with astigmatic corneas and IOLs was performed in younger subjects. METHODS: The virtual image of physical eyes with a 40 dpt achromate as artificial cornea and the (M)IOL in a water bath was projected on the retina of the observer by means of an exactly adjusted video objective. Silicone lenses with defined astigmatisms (+1; +2; +4; +6 dpt) were put in front of the achromate to produce an artificial corneal astigmatism. We compared results of a standard monofocal IOL (Pharmacia 811B), a multizone progressive MIOL (AMO Array SSM-26NB) and a diffractive MIOL (Pharmacia 811E). CSF through these IOLs in the physical eyes was measured in ten healthy subjects (mean age: 27.4 y.) with the B-VAT II-SG Video Acuity Tester (Mentor O&O), which uses sine wave gratings of five different spatial frequencies (1.5; 3; 6; 12; 20 cpd). RESULTS: Without astigmatic lenses, all IOLs showed a mean CSF within the age-related norm, but the monofocal IOL yielded significantly better results than both MIOLs at three spatial frequencies (3; 6; 12 cpd). With additional astigmatic lenses of 2 dpt and more, mean CSF of all IOLs was below normal range, but there was no difference in the performance of the three lens styles. CONCLUSION: CSF of MIOLs seems to be less sensitive to uncorrected corneal astigmatism than CSF of the monofocal IOL. This suggests, that a higher preoperative astigmatism does not severely affect the image quality through a multifocal IOL.

Adult↗
PubMed1995

[Induced astigmatism in extracapsular cataract extraction with tunnel incision and various wound closures].

BACKGROUND: For planned extracapsular cataract extraction the no-stitch technique with 11-mm tunnel width has been well established. Four modifications of wound closure were performed to further reduce surgically induced astigmatism. PATIENTS AND METHODS: In this prospective study we controlled 250 eyes of 250 consecutive patients 4 months after surgery with four different wound constructions: sutureless wound closure (n = 70), singular perpendicular suture (n = 100), cross suture (n = 40) in 12 o'clock position or sutureless wound closure in temporal position (n = 40). RESULTS: Surgically 'Induced Astigmatism' was for eyes with preoperative 'With the Rule Astigmatism' (vs Against the Rule astigmatism), operation in 12 o'clock position and sutureless wound closure 2.22 +/- 0.77 D (1.66 +/- 0.94 D), with perpendicular suture 1.66 +/- 0.93 D (1.24 +/- 0.82 D), and with cross suture 1.47 +/- 0.96 D (0.9 +/- 1.13 D). Temporal incision was only performed in preoperative 'Against-the-Rule-Astigmatism' eyes and resulted in 0.6 D of 'Induced Astigmatism'. Preoperative average astigmatism was 0.86 +/- 0.68 D (1.01 +/- 0.95 D). CONCLUSIONS: For preoperative 'With the Rule Astigmatism', operation in 12 o'clock position and singular perpendicular suture and for 'Against the Rule Astigmatism' (especially > 1.5 D) temporal incision is recommended.

Astigmatism↗
PubMed1998

Corneal astigmatism after scleral buckling surgery assessed by Fourier analysis of videokeratography data.

PURPOSE: To evaluate quantitatively the changes in corneal curvature, including irregular astigmatism, after scleral buckling surgery for retinal detachment. METHODS: In 29 eyes of 29 patients undergoing scleral explant surgery, videokeratographic measurements were carried out before and 1 week, 1 month, and 3 months after surgery. Using Fourier harmonic analysis, dioptric data on mire rings were decomposed into spherical, regular astigmatic, and irregular astigmatic (decentration and higher order irregularity) components. RESULTS: The irregular astigmatic component significantly increased at 1 week postoperatively but returned to the preoperative level 1 month after surgery. The regular astigmatism also displayed a transient increase up to 1 month after surgery. The increases in regular astigmatism were significant in eyes that had scleral buckling of < or =180 degrees but not in eyes with buckles extending >180 degrees. CONCLUSION: Changes in irregular astigmatism after retinal detachment surgery were quantitatively evaluated. The scleral buckling surgery causes a transient increase in irregular astigmatism as well as regular astigmatism.

Adolescent↗
PubMed2003

Correction of astigmatism after penetrating keratoplasty by relaxing incision with compression suture: a comparison between the guiding effect of photokeratoscope and of computer-assisted videokeratography.

PURPOSE: To report the results of astigmatism correction after penetrating keratoplasty by relaxing incision with compression suture and to compare the guiding ability of a photokeratoscope with that of computer-assisted videokeratography. METHODS: In the two independent retrospective series, 11 eyes received a relaxing incision with compression suture guided by photokeratoscope or computer-assisted videokeratography. RESULTS: In 22 eyes, the relaxing incision with compression suture showed a mean 56% reduction in astigmatism and 78% reduction in vector-calculated astigmatism. The mean postoperative visual acuity was improved 2.92 Landolt lines. The 11 eyes in which treatment was guided by photokeratoscope demonstrated a mean reduction of 50% and 71% in astigmatism and vector-calculated astigmatism, respectively, and visual acuity increased 2.44 Landolt lines. Another 11 eyes in which treatment was guided by computer-assisted videokeratography achieved a mean reduction of 67% and 90% in astigmatism and vector-calculated astigmatism, respectively, together with 3.41 Landolt lines improvement in visual acuity. There were no significant differences in astigmatism correction and visual acuity improvement between the two groups. CONCLUSION: The results demonstrate that the relaxing incision with compression suture is an effective and safe procedure for correcting high astigmatism after penetrating keratoplasty. The two instruments have no significant difference in their guiding capacities for this procedure.

Adolescent↗
PubMed1995

Corneal topography and irregular astigmatism.

BACKGROUND: The term irregular astigmatism has been used to refer to a variety of types of optical irregularity, although there are no widely recognized quantitative definitions of irregular astigmatism. This paper proposes a particular approach to defining and quantifying irregular astigmatism, and relates this model to blur disc formation. METHODS: A model of irregular astigmatism is developed and illustrated using corneal topography data. This method quantifies the variation in dioptric power with meridian by expressing that profile as a series of sinusoidal components. The relation between these components of irregular astigmatism and defocus blur discs is described, as well as the relation between irregular astigmatism and other models used to quantify aberrations. RESULTS: Irregular astigmatism is expressed in diopters, and may predict blur of the retinal image in the same way that spherocylindrical defocus causes blur. CONCLUSIONS: This model of irregular astigmatism is an efficient, quantitative means of describing irregular astigmatism.

Astigmatism↗
PubMed2004

Refractive astigmatism and the toricity of ocular components in human infants.

PURPOSE: Many studies have characterized astigmatism in infancy, but few have been longitudinal or contained ocular component data. This study characterized the frequency, orientation, and longitudinal change with age of infant astigmatism. Additional factors investigated were the influence of early astigmatism on emmetropization and its relation to corneal and lenticular toricity. METHODS: Three hundred two infants were enrolled in the study. Of these, 298 provided data for at least one visit at 3 +/- 1 months, 9 +/- 1 months, 18 +/- 2 months, and 36 +/- 3 months. Testing included cycloplegic retinoscopy (cyclopentolate 1%), video-based keratophakometry, and ultrasonography over the closed eyelid. RESULTS: Astigmatism > or =1.00 DC was common at 3 months of age (41.6%) but decreased in prevalence to 4.1% by 36 months (p < 0.0001). The most common orientation was with-the-rule at 3 months (37.0% compared with 2.7% for against-the-rule) but against-the-rule at 36 months (3.2% compared with 0.9% for with-the-rule). Most of the change in the average value of the horizontal/vertical component of astigmatism (J0) occurred between 3 and 9 months (-0.26 +/- 0.36 D; p < 0.0001) with no significant change between 9 and 36 months (-0.05 +/- 0.36 D; p=0.09). Spherical equivalent refractive error was not correlated with J0 at 3 and 9 months (R=0.002, p=0.48 and R=0.001, p=0.56, respectively). The two were only weakly correlated at 18 and 36 months (R=0.06 for each age, p <0.0001, p=0.0002, respectively). Changes in spherical equivalent between 3 and 9 months were unrelated to either the initial value of J0 (partial R for J0=0.0001; p=0.85) or the change in J0 (partial R for change in J0=0.0031; p=0.31). Across all the ages, corneal toricity was with-the-rule, and lenticular toricity was against-the-rule (produced by the toricity of the posterior lens surface). The cornea and anterior lens surface became more spherical with age, contributing to the shift away from with-the-rule refractive astigmatism. Toricity of all the refractive surfaces became less variable with age. CONCLUSIONS: Consistent with many reports, astigmatism was common in early infancy but decreased in prevalence with age, particularly when with-the-rule in orientation. The reduction in percentage of infants with astigmatism appeared to be caused by decreases in the toricity of the cornea and the anterior lens combined with decreases in the variability of corneal and lenticular surfaces. Astigmatism in infancy appeared to be unrelated to emmetropization of spherical equivalent refractive error.

Age Distribution↗
PubMed2005

Intraoperative correction of induced astigmatism after spherical correction of hyperopia with conductive keratoplasty.

PURPOSE: To evaluate the treatment of surgically induced astigmatism intraoperatively during conductive keratoplasty (CK) for correcting hyperopia. METHODS: Conductive keratoplasty uses radiofrequency energy applied to the peripheral corneal stroma to shrink the collagen and alter the central cornea to correct hyperopia. Nineteen consecutive patients (27 eyes) who underwent CK for hyperopia and were treated intraoperatively for induced astigmatism were examined. By using automated keratometric readings taken during the procedure, additional spots were applied at the minus cylinder or flat axis at the 7-mm zone until the intraoperative astigmatism was 2 diopters (D) or less. RESULTS: The intraoperative treatment reduced the astigmatism by an average of 2.30 +/- 1.32 D (P=0.00001). The mean induced astigmatism was 3.33 +/- 0.14 D for eyes that received eight spots, 4.12 +/- 1.13 D for eyes that received 16 spots, 4.43 +/- 0.82 D for eyes that received 24 spots, and 4.60 +/- 1.08 D for eyes that received 32 spots. Additional spots reduced astigmatism in most patients to less than 2 D. CONCLUSIONS: Intraoperative treatment of astigmatism through the addition of more spots at the minus cylinder or flat axis reduced the degree of induced astigmatism. Surgically induced astigmatism was observed more frequently in patients who received 32 treatment spots and 6-mm treatment zone application.

Astigmatism↗
PubMed1994

Corneal astigmatism induced by superior versus temporal corneal incisions for extracapsular cataract extraction.

PURPOSE: We tested the hypothesis that superior corneal sections induce a shift to 'against-the-rule' astigmatism and temporal corneal sections lead to the preferred 'with-the-rule' astigmatism. METHODS: We conducted a prospective randomised trial of superior versus temporal corneal incisions in extracapsular cataract surgery in a hospital practice. Thirty nine eyes of 37 patients were included. The induced astigmatism was analysed by three methods. RESULTS: When analysed by Cravy's method of induced astigmatic cylinders, the superior incision induced a greater degree of 'with-the-rule' astigmatic cylinder prior to sutures removal on day 77 (P < 0.049). Long term, the temporal incision produced 1.44 dioptre of 'with-the-rule' astigmatism, while the superior section produced 0.08 dioptre of 'against-the-rule' astigmatism (P < 0.001). CONCLUSION: The results of this small trial indicates that the superior corneal incision produces significantly less astigmatism than the temporal incision. The temporal incision induces a moderate degree of 'with-the-rule' astigmatism.

Aged↗
PubMed2007

A review of astigmatism and its possible genesis.

Astigmatism is a refractive condition encountered commonly in clinical practice. This review presents an overview of research that has been carried out examining various aspects of this refractive error. We examine the components of astigmatism and the research into the prevalence and natural course of astigmatic refractive errors throughout life. The prevalence of astigmatism in various ethnic groups and diseases and syndromes is also discussed. We highlight the extensive investigations that have been conducted into the possible aetiology of astigmatism, however, no single model or theory of the development of astigmatism has been proven conclusively. Theories of the development of astigmatism based on genetics, extraocular muscle tension, visual feedback and eyelid pressure are considered. Observations and evidence from the literature supporting and contradicting these hypotheses are presented. Recent advances in technology such as wavefront sensors and videokeratoscopes have led to an increased understanding of ocular astigmatism and with continued improvements in technology, our knowledge of astigmatism and its genesis should continue to grow.

Age Factors↗
PubMed1997

Cataract patients in a defined Swedish population 1986-1990. VIII. Postoperative astigmatism, intraocular pressure and visual acuity.

PURPOSE: Cataract surgery is often followed by a certain amount of astigmatism that changes in the postoperative period. However, there are large variations in both size and changes of the postoperative astigmatism. I have analysed the variations and the influence of different explanatory variables on the postoperative astigmatism. METHODS: Data for all patients undergoing cataract surgery from 1986 up to and including 1990 in the Lund Health Care District were prospectively recorded. Except for cases operated on with both keratoplasty and cataract, all cases were included in the study. The different explanatory variables considered included sex, age, preoperative axial length, preoperative average keratometry, preoperative intraocular pressure, glaucoma history, diabetes history, uveitis history (including both anterior and posterior uveitis), and a history of rheumatoid arthritis. RESULTS: Besides age and sex, several variables significantly influenced the development of size and/or change in the postoperative astigmatism. They were preoperative astigmatism (polar value), preoperative intraocular pressure, and whether or not the cataract patient had an insulin dependent diabetes. Phacoemulsification as extraction mode, the location of the incision, complications at surgery, and whether or not a sphincterotomy was performed also influenced the two parameters of astigmatism. It was also important whether or not the operation was performed by a high volume surgeon. CONCLUSION: In this material, the most important predisposing factors for rapid changes in the postoperative astigmatism were large preoperative astigmatism (polar value), young age, low preoperative intraocular pressure, if an ECCE were chosen as the extraction type, and the surgeon. The same variables and, in addition, if the location of the incision was anterior to the limbus were the most important explanatory variables in generating an early, large with-the-rule astigmatism.

Astigmatism↗
PubMed1991

Astigmatism and school myopia.

Changes in astigmatism were followed for a 3-year period among 238 myopic children as part of a clinical trial of myopia treatment. Children with mild myopia and no previous myopic spectacle correction and astigmatism less than or equal to 2 D were included in the study. The prevalence of astigmatism of at least 0.25 D was 55% at the beginning of the follow-up at a mean age of 10.9 years, increasing to 76% during the 3-year period. At the same time the mean astigmatism increased from -0.26 D to -0.45 D. Most of the astigmatism was against the rule; with the rule astigmatism represented 18% of the astigmatism at the beginning and 24% at the end of the study. There was a weak correlation between the spherical equivalent and astigmatism at the beginning of the follow-up (r = 0.122, n = 240, P = 0.029) but not at the end of the follow-up. Myopic progression controlled for the spherical equivalent, was not related to degree of astigmatism at the beginning of the study.

Astigmatism↗
PubMed1999

LASIK for post penetrating keratoplasty astigmatism and myopia.

AIMS: To report the results of a series of patients who were treated with LASIK to correct post penetrating keratoplasty ametropia. METHODS: 26 eyes of 24 patients underwent LASIK to correct astigmatism and myopia after corneal transplantation; 14 eyes also received arcuate cuts in the stromal bed at the time of surgery. The mean preoperative spherical equivalent was -5.20D and the mean preoperative astigmatism was 8.67D. RESULTS: The results of 25 eyes are reported. The mean 1 month values for spherical equivalent and astigmatism were -0.24D and 2.48D respectively. 18 eyes have been followed up for 6 months or more. The final follow up results for these eyes are -1.91D and 2.92D for spherical equivalent and astigmatism. The patients undergoing arcuate cuts were less myopic but had greater astigmatism than those not. The patients receiving arcuate cuts had a greater target induced astigmatism, surgically induced astigmatism, and astigmatism correction index than those eyes that did not. One eye suffered a surgical complication. No eyes lost more than one line of BSCVA and all eyes gained between 0 and 6 lines UCVA. CONCLUSIONS: LASIK after penetrating keratoplasty is a relatively safe and effective procedure. It reduces both the spherical error and the cylindrical component of the ametropia. Correction of high astigmatism may be augmented by performing arcuate cuts in the stromal bed.

Adult↗
PubMed1997

[Clinical analysis of photorefractive keratectomy for correction of myopic astigmatism].

OBJECTIVE: To evaluate the efficacy and accuracy of photorefractive keratectomy (PRK) for correction of myopic astigmatism. METHODS: Keracor 116 excimer laser was used. According to the preoperative astigmatism, 387 eyes were divided into group A (no astigmatism), B[astigmatism lower than 2 diopters(D)] and C (astigmatism greater than 2 D), and treated with PRK. Vectorial analysis was used to calculate the surgical effects. RESULTS: The mean preoperative astigmatism was 1.29 +/- 0.48 D and 3.07 +/- 0.72 D, the surgical correction was 1.10 +/- 0.67 D and 2.42 +/- 0.93 D, and the angle between pre- and post-operative astigmatic axes was 5.8 degrees +/- 2.1 degrees and 4.9 degrees +/- 2.4 degrees in group B and C, respectively. CONCLUSION: The correction of astigmatism with PRK is accurate, but some regression has been observed, especially in those with high preoperative astigmatism.

Astigmatism↗
PubMed2000

[Analysis of postoperative astigmatism after phacoemulsification with intraocular lens implantation].

OBJECTIVE: To study the corneal astigmatism and the changes of surgically induced astigmatism after phacoemulsification with intraocular lens (IOL) implantation. METHODS: Thirty-eight eyes of 38 patients treated with phacoemulsification through a superior inversed frown shaped scleral incision were examined by corneal topography and auto-ref-keratometer preoperatively, one day, one month and three months postoperatively. RESULTS: The postoperative corneal astigmatism at one day, one month and three months were (1.10 +/- 0.71), (1.08 +/- 0.66) and (0.87 +/- 0.55) D respectively. There was no statistic significance between the postoperative astigmatism at three months and at preoperative one day (P > 0.05). The mean surgically-induced astigmatism at postoperative one day, one month and three months were (1.37 +/- 1.03), (1.24 +/- 0.93) and (1.04 +/- 0.75) D respectively. There was significant difference between the astigmatism at three months and at one day postoperatively (P < 0.05). CONCLUSIONS: The superior inversed frown shaped scleral small incision with no suture has little effect on corneal astigmatism. Vector analysis can systemically evaluate surgically induced astigmatism. The results of corneal topography are more reliable to reflect changes in corneal curvature than that of auto-ref-keratometer, and corneal topography has more important clinical value in evaluating surgically induced astigmatism.

Astigmatism↗