Spherocylindrical Refractive Errors Research Articles

Does the Accuracy and Repeatability of Refractive Error Estimates Depend on the Measurement Principle of Autorefractors?

PurposeThe purpose of this study was to determine the accuracy and repeatability of refractive errors obtained using three autorefractors based on different measurement principles, vis-à-vis, gold-standard retinoscopy.MethodologyAccuracy of noncycloplegic, sphero-cylindrical refractive error of 234 eyes was obtained using the rotary prism-based RM-8900 closed-field autorefractor, photorefraction based Spot vision screener, wavefront aberrometry based E-see, and streak retinoscopy by four different examiners, masked to the results of each other. Intersession repeatability of autorefractors was determined by repeat measurements in a subset of 40 subjects.ResultsRetinoscopy values of M, J0, and J45 power vectors for the cohort ranged from −10.2 to 8 D, −1.4 to 1.8 D, and −0.9 to 1.2 D, respectively. Across autorefractors, the interequipment bias of M and J0 power vectors were statistically insignificant (< ±0.5 D; P > 0.05) but the corresponding limits of agreement were ±2.5 and ±1 D, respectively, without any trend across instruments or the patient's age (P > 0.5). Repeatability of M and J0 power vectors were ±0.75 D and ±0.40 D, respectively, across autorefractors. The range of J45 power vector was too narrow for any meaningful analysis.ConclusionsRefractive errors measured using autorefractors operating on different principles show minimal bias and good short-term repeatability but relatively large agreement limits, vis-à-vis, retinoscopy. Among them, the wavefront aberrometry based E-see autorefractor performs relatively better in all measurement parameters evaluated here.Translational RelevanceAlthough autorefractor estimates of noncycloplegic refractive error appears independent of their measurement principle, their relatively poor agreement with gold-standard retinoscopy warrants caution while used for screening and quantification of refractive errors.

Uncorrected Refractive Error and Distance Visual Acuity in Children Aged 6 to 14 Years.

This study presents the relationship between distance visual acuity and a range of uncorrected refractive errors, a complex association that is fundamental to clinical eye care and the identification of children needing refractive correction. This study aimed to analyze data from the Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error Study to describe the relationship between distance uncorrected refractive error and visual acuity in children. Subjects were 2212 children (51.2% female) 6 to 14 years of age (mean ± standard deviation, 10.2 ± 2.1 years) participating in the Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error Study between 2000 and 2010. Uncorrected distance visual acuity was measured using a high-contrast projected logMAR chart. Cycloplegic refractive error was measured using the Grand Seiko WR-5100K autorefractor. The ability of logMAR acuity to detect various categories of refractive error was examined using receiver operating characteristic curves. Isoacuity curves show that increasing myopic spherical refractive errors, increasing astigmatic refractive errors, or a combination of both reduces distance visual acuity. Visual acuity was reduced by approximately 0.5 minutes of MAR per 0.30 to 0.40 D of spherical refractive error and by approximately 0.5 minutes of MAR per 0.60 to 0.90 D of astigmatism. Higher uncorrected hyperopic refractive error had little effect on distance visual acuity. Receiver operating characteristic curve analysis suggests that a logMAR distance acuity of 0.20 to 0.32 provides the best balance between sensitivity and specificity for detecting refractive errors other than hyperopia. Distance acuity alone was ineffective for detecting hyperopic refractive errors. Higher myopic and/or astigmatic refractive errors were associated with predictable reductions in uncorrected distance visual acuity. The reduction in acuity per diopter of cylindrical error was about half that for spherical myopic error. Although distance acuity may be a useful adjunct to the detection of myopic spherocylindrical refractive errors, accommodation presumably prevents acuity from assisting in the detection of hyperopia. Alternate procedures need to be used to detect hyperopia.

Visian® ICLV4c™ and Artiflex®: Comparative Analysis with the HD AnalyserTM and Integration with Subjective Performance and Anatomical Parameters.

PurposeTo analyze and compare the mid-term objective and subjective performance of two phakic intraocular lenses (PIOLs) – Visian® ICL™ V4c and Artiflex® and to correlate those results with anatomical parameters.Patients and MethodsObservational retrospective study including thirty patients (15 for each PIOL) randomly selected from those who underwent bilateral implantation of one of the PIOLs (60 eyes were analysed). Uncorrected and corrected distance visual acuities (UDVA and CDVA), anterior chamber structural parameters, and corneal endothelial cells density and morphology were evaluated. Quality of vision was assessed both subjectively with a questionnaire, and objectively using a double-pass technology optical quality analysis system.ResultsAt the end of follow-up the Artiflex® group showed better UDVA (0.95±0.11 vs 0.99±0.05, p=0.027) and higher proportion of eyes reaching at least 20/20 (28/30 vs 18/30, p=0.005). Excellent and similar objective scatter index (1.79±0.9 vs 2.14±1.6, p=0.306) and modular transfer function cutoff frequency (26.91±9.2 vs 26.51±11.9, p=0.784) results were achieved in the HD Analyzer® and the questionnaire showed comparable high Satisfaction (55.7±3.5 vs 54.3±5.4, p=0.386) and low Dysphotopsia Scores (3.1±1.8 vs 2.1±2.2, p=0.213). The ICL group showed inferior anterior chamber volume (p<0.001), and angle (p<0.001) and superior pupillary diameter (p=0.007). Minimum follow-up was 24 months.ConclusionBoth the Visian® ICLV4c™ and the Artiflex® showed excellent optical performance through the HD AnalyzerTM technology, translated in high patient satisfaction. Overall, both PIOLs showed similar safety, predictability, stability and effectiveness of for the reduction of high sphero-cylindrical refractive errors, despite better UDVA results with the Artiflex®.

Advancing Digital Workflows for Refractive Error Measurements.

Advancements in clinical measurement of refractive errors should lead to faster and more reliable measurements of such errors. The study investigated different aspects of advancements and the agreement of the spherocylindrical prescriptions obtained with an objective method of measurement (“Aberrometry” (AR)) and two methods of subjective refinements (“Wavefront Refraction” (WR) and “Standard Refraction” (StdR)). One hundred adults aged 20–78 years participated in the course of the study. Bland–Altman analysis of the right eye measurement of the spherocylindrical refractive error (M) identified mean differences (±95% limits of agreement) between the different types of measurements of +0.36 D (±0.76 D) for WR vs. AR (t-test: p < 0.001), +0.35 D (± 0.84 D) for StdR vs. AR (t-test: p < 0.001), and 0.0 D (± 0.65 D) for StdR vs. WR (t-test: p < 0.001). Monocular visual acuity was 0.0 logMAR in 96% of the tested eyes, when refractive errors were corrected with measurements from AR, indicating that only small differences between the different types of prescriptions are present.

The relationship between patient age and residual refractive error after uneventful laser in situ keratomileusis for moderate-to-high hyperopia.

To determine if the manifest sphero-cylindrical residual refractive error, at various time points over a 12-month postop period after laser in situ keratomileusis (LASIK) was associated with patient age at time of surgery. Patients with moderate to high hyperopia (3.00-7.00 DS) and astigmatism ⩽2 DC underwent LASIK using Wavelight Allegretto Eye Q (400 Hz). Treatments were centered on corneal vertex, flaps were made with Moria M2 mechanical microkeratome. Pre-and postoperative uncorrected and corrected distant visual acuity, best corrected spherical equivalent (SEQ) were measured. Measurements were taken at 1 week, 1, 3, 6, and 12 months after the surgery. Target refraction was emmetropia. Total of 161 patients were treated. In binocular cases, data from the right eyes were included for analysis. In this article, we report on refraction data only. Raw data were subjected to several permutations to elicit any links between refractive outcomes and patient age. The key findings were as follows y = postop SEQ (diopters), x = patient age (years), ln(x) = natural logarithm of patient age: At 1 month, y = x[0.049 -0.011.ln(x)] (R = -0.205, p = 0.001, n = 161). At 3 months, y = x[0.077 -0.017.ln(x)] (R = -0.355, p < 0.001, n = 161). At 6 months, y = x[0.088 -0.020.ln(x)] (R = -0.382, p < 0.001, n = 161). At 12 months, y = x[0.093 -0.021.ln(x)] (R = -0.409, p < 0.001, n = 161). There was no significant association between x and y at 1 week (p > 0.05). Residual postop refractive error after LASIK for hyperopia has a logarithmic association with patient age at time of surgery. In younger patients there is tendency toward undercorrection, the opposite occurs in older patients and this persists 1 year after treatment.

Central corneal regularization (CCR): an alternative approach in keratoconus treatment

BackgroundTo evaluate the safety and efficacy of an approach that combines corneal customized transepithelial therapeutic ablation to treat irregular corneal optics and accelerated corneal collagen cross-linking (CXL) to strengthen the corneal tissue and stop the progression of keratoconus. The transepithelial therapeutic ablation applied a novel concept named central corneal regularization (CCR) which could correct the corneal morphological irregularities and the eye’s spherocylindrical refractive error with minimal stromal tissue removal.MethodsRetrospective study. Eyes that underwent CCR combined with CXL were evaluated preoperatively and up to 12 months postoperatively for visual acuity, subjective refraction, corneal haze, pachymetry and maximum keratometry (Kmax).ResultsTwenty four eyes of 24 patients with a mean age of 28.92 ± 9.88 years were treated. The mean spherical equivalent (SE) refractive error changed from − 0.74 ± 1.17 D preoperatively to − 1.05 ± 1.52 D at 12 months postoperatively. The mean uncorrected distance visual acuity (UDVA) and corrected distance visual acuity (CDVA) improved. No eye lost lines of CDVA, 21 had a mean improvement of 3.21 lines. The mean cylinder error and Kmax value dropped from − 3.06 ± 1.83 D and 51.38 ± 3.29 D to − 1.04 ± 0.80 D and 48.70 ± 2.58 D, respectively. The mean haze score at 3, 6 and 12 months was 0.56, 0.19 and 0.06, respectively.ConclusionsCCR combined with CXL offers promising results as a safe and effective treatment in keratoconic patients.

Unaided visual acuity and blur: a simple model.

This article proposes a simple model that describes the quantitative relationship between unaided visual acuity and blur attributed to refractive errors. The standard model for describing the relationship between visual acuity and blur, as published by Raasch, is used as a starting point to develop a simpler model based on heuristic arguments. The basis of Raasch's data is augmented by published findings in the range of low-level refractive errors. Sphero-cylindrical refractive errors are transformed into a single blur quantity b, also termed dioptric distance, which serves as an input in both models. The possible influence of the cylinder axis and the pupil size is not included. The quite simple model for the unaided minimum angle of resolution, MAR ∝ 1+ b(2), nicely matches available data and improves the SE of the regression by a factor of 2 in comparison to Raasch's model. Both models considered in this article describe measurement data equally well. They differ in terms of complexity and functional form. The simple model provides a valid description for low-level refractive errors, where Raasch's model fails. Actual uncertainties in experimental data on unaided visual acuity, especially the frequent lack of information on pupil diameter, prevent meaningful numerical comparison and the refinement of both models. However, theoretical arguments are provided in support of the simple model.

The Light Adjustable Lens – A Review

The purpose of this manuscript is to review the design, development and clinical results of the light adjustable lens (LAL). Evolution in techniques, biometry and intraocular lens (IOL) calculation have improved outcomes and increased expectations after cataract surgery. Nevertheless, Imprecise IOL power determinations, pre-existing and surgical-induced astigmatism, and previous corneal refractive surgery continue to limit post-operative uncorrected vision. The LAL was designed to provide a stable, precise correction of refractive errors with a safe, non-invasive post-operative procedure. The concept behind the LAL is based on photochemistry and diffusion. The adjustment occurs when all non-attached solutes equally distribute themselves throughout the optic after irradiation with ultraviolet light that causes the photosensitive macromers to polymerise in the irradiated region. Clinical results have been positive, with the largest series (122 eyes) showing 97 % of patients within 0.25 D of attempted spherical equivalent and 100 % uncorrected vision 20/25 or better. In conclusion, the LAL has demonstrated to be a safe, accurate and reliable method of post-operative, non-surgical correction of residual sphero-cylindrical refractive error.

Adaptation to astigmatic blur.

Adapting to blurred or sharpened images alters the perceived focus of subsequently viewed images. We examined whether these adaptation effects could arise from actual sphero-cylindrical refractive errors, by testing aftereffects in images simulating second-order astigmatism. Image blur was varied from negative (vertical) through isotropic to positive (horizontal) astigmatism while maintaining constant blur strength. A 2AFC staircase was used to estimate the stimulus that appeared isotropically blurred before or after adapting to images with astigmatism. Adaptation to horizontal blur caused isotropically blurred images to appear vertically biased and vice versa, shifting the perceived isotropic point toward the adapting level. Aftereffects were similar for different types of images and showed partial selectivity so that strongest effects generally occurred when testing and adapting images were the same. Further experiments explored whether the adaptation depended more strongly on the blurring or "fuzziness" in the images vs. the apparent "figural" changes introduced by the blur, by comparing how the aftereffects transfer across changes in size or orientation. Our results suggest that strong selective adaptation can occur for different lower order aberrations of the eye and that these may be at least partly driven by the apparent figural changes that blurring introduces into the retinal image.

Ocular Biometry and Open-Angle Glaucoma: The Los Angeles Latino Eye Study

To examine the associations among myopic refractive error (MRE), corneal power (CP), axial length (AL), and the prevalence of open-angle glaucoma (OAG) in an adult Latino population. Population-based, cross-sectional epidemiologic study. A total of 5927 Latinos aged 40 years and older in the Los Angeles Latino Eye Study (LALES). Myopic refractive error was defined as a non-cycloplegic sphero-cylindrical refractive error of < or =-1 diopter (D). Axial length was measured by A-scan ultrasound. Corneal power was measured using a Humphrey auto-refractor. Open-angle glaucoma was diagnosed by a combination of optic nerve and visual field changes. Pseudophakic and aphakic eyes were excluded from the analysis. The associations among MRE, AL, CP, and the prevalence of OAG were calculated using a logistic regression model, adjusting for age, gender, intraocular pressure, central corneal thickness, diabetes mellitus, family history of glaucoma, and lens nuclear opacification (NO). Odds ratios (OR) for the prevalence of OAG. After adjusting for covariates, persons with greater MRE (OR 1.82; confidence interval [CI], 1.20-2.77; P = 0.005), longer AL (OR 1.25; CI, 1.03-1.50; P = 0.02), and flatter corneas (OR 1.21; CI, 1.08-1.35; P = 0.0007) were associated with a higher prevalence of OAG when compared with those with emmetropic refractive error, shorter AL, and steeper corneas. Persons with an MRE, flatter corneas, and longer ALs should be considered to be at higher risk of having OAG.

The basics of wavefront aberrometry

AbstractWavefront technology has been first used by astronomers for years in order to reduce higher order aberrations induced by the earth’s atmosphere, it has been introduced into clinical eye care recently.&lt;br /&gt; Ophthalmological practice involves performing a full subjective refraction. The sphere, cylinder and axis of astigmatism are measured. We are only correcting two components of a whole host of refractive components of the optics of an eye. These two components (sphere and cylinder) constitute by far the majority of the optical aberration of an eye. &lt;br /&gt; Even a basic exam yields important information about optical quality. However, all ophthalmologists have been faced with patients reporting visual acuity (or contrast sensitivity, glare, etc.) at levels much lower than would be expected from an eye exam.&lt;br /&gt; Wavefront aberrometry measures aberrations over the entire eye taking into account not only spherocylindrical refractive error, but also spherical aberration, trefoil, coma, secondary astigmatism as well as other “higher order” aberrations described by Zernike polynomials. Higher order aberrations are thought to contribute to more than 20% of the total number of aberrations in a normal eye an increase with pupil size. In the majority of normal patients, these high order aberrations play a minor role, however, in cases of refractive surgery, keratoconus and orthokeratology, they can induce a number of visual disturbances. &lt;br /&gt; We will define higher‐order aberrations and show how to measure them, giving you a basic working knowledge of wavefront sensing (also known as aberrometry). &lt;br /&gt; We will show how wavefront sensors work, what are Zernike polynomials, what they tell us and we will show present and future clinical applications of wavefront aberrometry.

Evaluation of Internal Refraction with the Optical Path Difference Scan

To evaluate internal refraction and its relation to other optical properties of the eye across a large range of refractive errors, which can provide useful information for the assessment and design of intraocular lenses and corneal ablation patterns. Cohort study. Three hundred ninety-four eyes of 197 healthy subjects with a mean age of 27+/-7 years (range, 18-42). All eyes underwent optical path difference scans to evaluate corneal topography, whole and internal ocular refraction (determined by the subtraction of corneal refraction from whole ocular refraction), and total and higher-order aberrations (HOAs) were assessed. After the conversion of any spherocylindrical refractive errors to vectorial data (sphere equivalent, blurring strength; cylinder, J(0) [power of Jackson cross cylinder at 90 degrees and 180 degrees] and J45 [power of Jackson cross cylinder at 45 degrees and 135 degrees]), the distribution of internal refraction among refraction groups (high myopes, low to moderate myopes, hyperopes, mixed astigmats, and emmetropes) and relationships between internal refraction, corneal refraction, and wavefront aberrations were analyzed. The compensation relation and its rate between corneal and internal astigmatism was assessed by the compensation factor (CF). Whole ocular power and astigmatism, corneal power and astigmatism, internal power and astigmatism, CF for astigmatic data, location of the highest internal refraction zone, and wavefront aberrations. The highest refraction zone was mostly (90%) located in the center in myopes, whereas it was located at the nasal side (71%) in hyperopes. There was a significant correlation between whole ocular and internal powers (P<0.01), but no correlation was observed between corneal and internal powers (P>0.05). Internal astigmatism was mostly against the rule. The mean CF J(0) was 0.63+/-3.78 and CF J(45) was 0.57+/-2.47. The magnitude of the internal astigmatism under the 3-mm zone was correlated with the magnitude of corneal astigmatism (P<0.05). The distribution of astigmatic CF differed among refraction groups. There were significant correlations between internal power and spherical aberration (P<0.05) and internal cylinder under the 5-mm zone and HOAs (P<0.001). There is a remarkable tilt in internal refraction in hyperopes. Although there is a tendency of undercompensation of the corneal astigmatism by internal astigmatism in the entire group of eyes, the compensation differs among refraction groups.

The basics of wavefront aberrometry

AbstractWavefront technology has been first used by astronomers for years in order to reduce higher order aberrations induced by the earth’s atmosphere, it has been introduced into clinical eye care recently.Ophthalmological practice involves performing a full subjective refraction. The sphere, cylinder and axis of astigmatism are measured. We are only correcting two components of a whole host of refractive components of the optics of an eye. These two components (sphere and cylinder) constitute by far the majority of the optical aberration of an eye.Even a basic exam yields important information about optical quality. However, all ophthalmologists have been faced with patients reporting visual acuity (or contrast sensitivity, glare, etc.) at levels much lower than would be expected from an eye exam.Wavefront aberrometry measures aberrations over the entire eye taking into account not only spherocylindrical refractive error, but also spherical aberration, trefoil, coma, secondary astigmatism as well as other “higher order” aberrations described by Zernike polynomials. Higher order aberrations are thought to contribute to more than 20% of the total number of aberrations in a normal eye an increase with pupil size. In the majority of normal patients, these high order aberrations play a minor role, however, in cases of refractive surgery, keratoconus and orthokeratology, they can induce a number of visual disturbances.We will define higher‐order aberrations and show how to measure them, giving you a basic working knowledge of wavefront sensing (also known as aberrometry).We will show how wavefront sensors work, what are Zernike polynomials, what they tell us and we will show present and future clinical applications of wavefront aberrometry.

Objective refraction from monochromatic wavefront aberrations via Zernike power polynomials

With the increased popularity of wavefront sensor devices there has been significant interest in relating the subjectively measured sphero-cylindrical refractive error to the objectively measured wavefront aberrations. We investigate the applicability of four different measures for objective refraction assessment that are derived from a set of estimated Zernike wavefront coefficients. Two of the measures are based on the second- and higher-order Zernike polynomials while the other two measures are based on the estimated dioptric power representations. For the latter, closed-form expressions for dioptric powers based on the estimated focal length and the wavefront curvature are derived. We call these two new representations the 'refractive Zernike power polynomials' and 'curvature Zernike power polynomials'. Data from 120 eyes from 60 normal subjects were used to assess the correlation between the subjective and objective refractions. The results indicate that the objective sphero-cylindrical refraction calculated from the estimated refractive power map via the Zernike power polynomials is superior to the other considered representations and achieves best correlation with subjective sphero-cylindrical refraction.

Potential Higher-Order Aberration Cues for Sphero-Cylindrical Refractive Error Development

To investigate analytically whether higher-order wavefront errors comprising combinations of trefoil along 30 degrees (trefoil30), vertical coma, and spherical aberration could provide cues to sphero-cylindrical refractive error development. A total of 25 test wavefronts, subdivided into five different types and five levels of higher-order root mean square errors (HO-RMS), were created for the study. One type contained spherical aberration only, producing HO-RMS levels between 0.1 and 0.5 microm. Four wavefront types contained coma, trefoil, and spherical aberration of various sign combinations also producing HO-RMS levels between 0.1 and 0.5 microm. From the 25 wavefronts, refractive power maps were created and 2025 different sphero-cylindrical combinations were added to each refractive power map. For each sphero-cylinder combination, the visual Strehl ratio based on the modulation transfer function (VSMTF) was calculated. Retinal images and refractive power histograms were calculated for the refractive power maps corresponding to the peak of the VSMTF. Spherical aberration affected the best focal plane thereby inducing spherical or defocus cues. The VSMTF produced by vertical coma and trefoil30, in combination with spherical aberration, could be improved with sphero-cylinders of various magnitudes and directions (i.e., with-the-rule, against-the rule, myopic astigmatism, or hyperopic astigmatism). Clinical significance of sphero-cylinders (i.e., >or=0.25 D) was reached at HO-RMS levels between 0.2 and 0.3 microm for a 5-mm pupil zone. In the context of compensatory blur driven eye growth, commonly occurring combinations of the three considered higher-order aberrations have the potential to produce cues to eye growth resulting in myopia and with-the-rule astigmatism.

Quality of vision following clinically successful penetrating keratoplasty

Purpose: To evaluate visual function following clinically successful penetrating keratoplasty (PKP). Setting: Department of Ophthalmology, Ege University, School of Medicine, Izmir, Turkey. Methods: Patient group (PG) included 9 patients (12 eyes) who had clinically successful PKP in our department. The control group (CG) included 12 people (18 eyes) who had no ocular disease other than refractive errors. Those with a visual acuity level less than 20/25 were not included in the study. Contrast sensitivity levels and light threshold values of the central retina were measured; scanning-slit corneal topography–pachymetry and aberrometric analysis were performed. Results: There were no statistical difference in terms of age (32.55 years ± 9.25 (SD) in PG, 36.75 ± 5.85 years in CG; P = .53), cylinder power in plus form (2.60 ± 1.25 diopter (D) in PG, 2.79 D ± 2.51 D in CG; P = .88), and spherical equivalent of refractive errors (−3.66 ± 3.57 D in PG, −5.52 ± 3.37 D in CG; P = .29) between the PG and CG. Cambridge low-contrast grating scores were 96.5 ± 41.1 in grafted eyes and 148 ± 27.7 in CG ( P = .004). Central retinal light sensitivity was measured as 29.91 ± 2.39 db in PG and 33.08 ± 1.56 db in CG ( P = .001). In corneal topographic analysis, mean kappa intercept was 0.69 ± 0.37 mm in PG and 0.55 ± 0.24 mm in CG ( P = .20). Lower-order Zernike root mean squares (RMS) were 7.30 ± 3.89 μm for PG and 8.58 ± 3.46 μm for CG ( P = .37). However, higher-order Zernike RMS were 2.15 ± 0.78 in PG and 0.38 ± 0.10 in CG, which is a statistically significant difference ( P<.001). Conclusions: Even though the clinically successful PKP patients have correctable amount of spherocylindrical refractive errors with spectacle lenses, they still have reduced visual quality because of the significantly high amount of higher- order aberrations when compared with naturally occurring refractive errors.

Update on customized excimer ablations: recent developments reported in 2002.

Customized corneal ablation is an exciting frontier in refractive surgery that incorporates wavefront technology to detect and correct higher order aberrations in addition to spherocylindrical refractive errors. The goal is to achieve super normal vision in terms of acuity and contrast. As the concept of wavefront customized ablations is still new, there are a number of aspects of its clinical application that need analysis and understanding. Numerous reports have appeared in the literature during the past year that address the developments, concerns, and limitations of wavefront technology and custom ablation. We have attempted to summarize and discuss the significant reports in this current review. Our focus is on the optical and physiologic limits of wavefront customized correction, including the effect of accommodation, aging, and flap creation on the aberration profile. In addition, we also present the laser technology requirements, and clinical outcomes of customized excimer ablations that are reported to date.

Statistical variation of aberration structure and image quality in a normal population of healthy eyes.

A Shack-Hartmann aberrometer was used to measure the monochromatic aberration structure along the primary line of sight of 200 cyclopleged, normal, healthy eyes from 100 individuals. Sphero-cylindrical refractive errors were corrected with ophthalmic spectacle lenses based on the results of a subjective refraction performed immediately prior to experimentation. Zernike expansions of the experimental wave-front aberration functions were used to determine aberration coefficients for a series of pupil diameters. The residual Zernike coefficients for defocus were not zero but varied systematically with pupil diameter and with the Zernike coefficient for spherical aberration in a way that maximizes visual acuity. We infer from these results that subjective best focus occurs when the area of the central, aberration-free region of the pupil is maximized. We found that the population averages of Zernike coefficients were nearly zero for all of the higher-order modes except spherical aberration. This result indicates that a hypothetical average eye representing the central tendency of the population is nearly free of aberrations, suggesting the possible influence of an emmetropization process or evolutionary pressure. However, for any individual eye the aberration coefficients were rarely zero for any Zernike mode. To first approximation, wave-front error fell exponentially with Zernike order and increased linearly with pupil area. On average, the total wave-front variance produced by higher-order aberrations was less than the wave-front variance of residual defocus and astigmatism. For example, the average amount of higher-order aberrations present for a 7.5-mm pupil was equivalent to the wave-front error produced by less than 1/4 diopter (D) of defocus. The largest pupil for which an eye may be considered diffraction-limited was 1.22 mm on average. Correlation of aberrations from the left and right eyes indicated the presence of significant bilateral symmetry. No evidence was found of a universal anatomical feature responsible for third-order optical aberrations. Using the Marechal criterion, we conclude that correction of the 12 largest principal components, or 14 largest Zernike modes, would be required to achieve diffraction-limited performance on average for a 6-mm pupil. Different methods of computing population averages provided upper and lower limits to the mean optical transfer function and mean point-spread function for our population of eyes.

Three-dimensional representation and qualitative comparisons of the amount of tissue ablation to treat mixed and compound astigmatism

Purpose To compare the shape and volume of the lenticules of corneal tissue ablated for the correction of spherical, cylindrical, and spherocylindrical refractive errors using Boolean operations of theoretical 3-dimensional (3-D) surfaces. Setting Department of Ophthalmology, Rothschild Foundation, Paris, France. Methods Digital modeling software was used to perform graphic representations of ablated lenticules on 3-D virtual surfaces. Various Boolean operations were performed between different preoperative and postoperative surfaces, and the additional and subtractive properties of ablated theoretical lenticules were analyzed to determine profiles of ablated lenticules for mixed and compound myopic and hyperopic astigmatism. Results Negative-cylindrical treatment, used to treat simple myopic astigmatism, was equivalent to the combination of a positive-cylindrical and a negative-spherical treatment of the same magnitude. Combining a pure negative-cylindrical and a positive-spherical treatment in a sequential strategy when treating compound astigmatism resulted in redundant ablation (plano lenticule), leading to an unnecessary increase in the amount of tissue ablation. Conclusions Negative-cylindrical treatments result in greater tissue ablation than corresponding positive-cylindrical treatments. For any given compound astigmatic error, the strategy using the greater magnitude of positive cylinder incurs the minimal amount of tissue ablation.

Refractive change in response to acute hyperbaric stress in refractive surgery patients

Purpose To determine whether acute hyperbaric stress affects visual acuity or refractive power after keratorefractive surgery. Setting Clinical multiplace hyperbaric chamber and ophthalmology clinic, University of California, San Diego, California, USA. Methods This prospective convenience sample study included 3 groups: 3 patients who had had bilateral myopic radial keratotomy (RK); 2 who had had bilateral myopic laser in situ keratomileusis (LASIK); and 4 control subjects who had no previous corneal refractive surgery or ocular pathology but had a myopic refractive error (−1.25 to −8.38 diopters [D]) similar to that in the treated patients before refractive surgery. One additional patient had had unilateral LASIK only and was included in the LASIK and control groups. Best spectacle-corrected visual acuity (BSCVA), manifest spherocylindrical refractive error, and intraocular pressure were measured at baseline, at 4 atmospheres absolute (atm abs), and on return to ambient pressure. Corneal pachymetry and keratometry were measured at baseline and on return to ambient pressure. Results The mean BSCVA changed from 0.06 logMAR (20/25 Snellen equivalent) at baseline to 0.10 logMAR (20/25) at 4 atm abs in the RK group and from 0.00 logMAR (20/20) to −0.06 logMAR (20/15) in the LASIK group; it did not change in the control group. The mean refractive error changed from 0.25 D at baseline to 0.50 D at 4 atm abs in the RK group, from −0.90 to −1.02 D in the LASIK group, and from −4.58 to −4.53 D in the control group. Conclusion Acute hyperbaric stress did not appear to alter refractive power after corneal surgery.