인공고관절의 세라믹 볼 헤드의 기계적 안정성 평가를 위한 3 차원 유한요소 해석

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.

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

We determined the accuracy and precision of 33 objective methods for predicting the results of conventional, sphero-cylindrical refraction from wavefront aberrations in a large population of 200 eyes. Accuracy for predicting defocus (as specified by the population mean error of prediction) varied from -0.50 D to +0.25 D across methods. Precision of these estimates (as specified by 95% limits of agreement) ranged from 0.5 to 1.0 D. All methods except one accurately predicted astigmatism to within +/-1/8D. Precision of astigmatism predictions was typically better than precision for predicting defocus and many methods were better than 0.5D. Paraxial curvature matching of the wavefront aberration map was the most accurate method for determining the spherical equivalent error whereas least-squares fitting of the wavefront was one of the least accurate methods. We argue that this result was obtained because curvature matching is a biased method that successfully predicts the biased endpoint stipulated by conventional refractions. Five methods emerged as reasonably accurate and among the most precise. Three of these were based on pupil plane metrics and two were based on image plane metrics. We argue that the accuracy of all methods might be improved by correcting for the systematic bias reported in this study. However, caution is advised because some tasks, including conventional refraction of defocus, require a biased metric whereas other tasks, such as refraction of astigmatism, are unbiased. We conclude that objective methods of refraction based on wavefront aberration maps can accurately predict the results of subjective refraction and may be more precise. If objective refractions are more precise than subjective refractions, then wavefront methods may become the new gold standard for specifying conventional and/or optimal corrections of refractive errors.

Accuracy and precision of objective refraction from wavefront aberrations

Purpose The purpose of the Correction of Myopia Evaluation Trial (COMET) was to evaluate the effect of progressive addition lenses (PALs) compared with single vision lenses (SVLs) on the progression of juvenile-onset myopia. Methods COMET enrolled 469 children (ages 6-11 years) with myopia between -1.25 and -4.50 D spherical equivalent. The children were recruited at four colleges of optometry in the United States and were ethnically diverse. They were randomly assigned to receive either PALs with a +2.00 addition (n = 235) or SVLs (n = 234), the conventional spectacle treatment for myopia, and were followed for 3 years. The primary outcome measure was progression of myopia, as determined by autorefraction after cycloplegia with 2 drops of 1% tropicamide at each annual visit. The secondary outcome measure was change in axial length of the eyes, as assessed by A-scan ultrasonography. Child-based analyses (i.e., the mean of the two eyes) were used. Results were adjusted for important covariates, by using multiple linear regression. Results Of the 469 children (mean age at baseline, 9.3 +/- 1.3 years), 462 (98.5%) completed the 3-year visit. Mean (+/-SE) 3-year increases in myopia (spherical equivalent) were -1.28 +/- 0.06 D in the PAL group and -1.48 +/- 0.06 D in the SVL group. The 3-year difference in progression of 0.20 +/- 0.08 D between the two groups was statistically significant (P = 0.004). The treatment effect was observed primarily in the first year. The number of prescription changes differed significantly by treatment group only in the first year. At 6 months, 17% of the PAL group versus 30% of the SVL group needed a prescription change (P = 0.0007), and, at 1 year, 43% of the PAL group versus 59% of the SVL group required a prescription change (P = 0.002). Interaction analyses identified a significantly larger treatment effect of PALs in children with lower versus higher baseline accommodative response at near (P = 0.03) and with lower versus higher baseline myopia (P = 0.04). Mean (+/- SE) increases in the axial length of eyes of children in the PAL and SVL groups, respectively, were: 0.64 +/- 0.02 mm and 0.75 +/- 0.02 mm, with a statistically significant 3-year mean difference of 0.11 +/- 0.03 mm (P = 0.0002). Mean changes in axial length correlated with those in refractive error (r = 0.86 for PAL and 0.89 for SVL). Conclusions Use of PALs compared with SVLs slowed the progression of myopia in COMET children by a small, statistically significant amount only during the first year. The size of the treatment effect remained similar and significant for the next 2 years. The results provide some support for the COMET rationale-that is, a role for defocus in progression of myopia. The small magnitude of the effect does not warrant a change in clinical practice.

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A Randomized Clinical Trial of Progressive Addition Lenses versus Single Vision Lenses on the Progression of Myopia in Children

A method of and apparatus for improving vision and the resolution of retinal images is described in which a point source produced on the retina of a living eye by a laser beam is reflected from the retina and received at a lenslet array of a Hartmann-Shack wavefront sensor such that each of the lenslets in the lenslet array forms an aerial image of the retinal point source on a CCD camera located adjacent to the lenslet array. The output signal from the CCD camera is acquired by a computer which processes the signal and produces a correction signal which may be used to control a compensating optical or wavefront compensation device such as a deformable mirror. It may also be used to fabricate a contact lens or intraocular lens, or to guide a surgical procedure to correct the aberrations of the eye. Any of these methods could correct aberrations beyond defocus and astigmatism, allowing improved vision and improved imaging of the inside of the eye.

Method and apparatus for improving vision and the resolution of retinal images

The description of sphero-cylinder lenses is approached from the viewpoint of Fourier analysis of the power profile. It is shown that the familiar sine-squared law leads naturally to a Fourier series representation with exactly three Fourier coefficients, representing the natural parameters of a thin lens. The constant term corresponds to the mean spherical equivalent (MSE) power, whereas the amplitude and phase of the harmonic correspond to the power and axis of a Jackson cross-cylinder (JCC) lens, respectively. Expressing the Fourier series in rectangular form leads to the representation of an arbitrary sphero-cylinder lens as the sum of a spherical lens and two cross-cylinders, one at axis 0 degree and the other at axis 45 degrees. The power of these three component lenses may be interpreted as (x,y,z) coordinates of a vector representation of the power profile. Advantages of this power vector representation of a sphero-cylinder lens for numerical and graphical analysis of optometric data are described for problems involving lens combinations, comparison of different lenses, and the statistical distribution of refractive errors.

Power vectors: An application of Fourier analysis to the description and statistical analysis of refractive error

Purpose To demonstrate the power vector method of representing and analyzing spherocylindrical refractive errors. Setting School of Optometry, Indiana University, Bloomington, Indiana, USA. Methods Manifest and keratometric refractive errors were expressed as power vectors suitable for plotting as points in a 3-dimensional dioptric space. The 3 Cartesian coordinates ( x , y , z ) of each power vector correspond to the powers of 3 lenses that, in combination, fulfill a refractive prescription: a spherical lens of power M , a Jackson crossed cylinder of power J 0 with axes at 90 degrees and 180 degrees, and a Jackson crossed cylinder of power J 45 with axes at 45 degrees and 135 degrees. The Pythagorean length of the power vector, B , is a measure of overall blurring strength of a spherocylindrical lens or refractive error. Changes in refractive error due to surgery were computed by the ordinary rules of vector subtraction. Results Frequency distributions of blur strength ( B ) clearly demonstrate the effectiveness of refractive surgery in reducing the overall blurring effect of uncorrected refractive error. Power vector analysis also revealed a reduction in the astigmatic component of these refractive errors. Paired comparisons revealed that the change in manifest astigmatism due to surgery was well correlated with the change in keratometric astigmatism. Conclusions Power vectors aid the visualization of complex changes in refractive error by tracing a trajectory in a uniform dioptric space. The Cartesian components of a power vector are mutually independent, which simplifies mathematical and statistical analysis of refractive errors. Power vectors also provide a natural link to a more comprehensive optical description of ocular refractive imperfections in terms of wavefront aberration functions and their description by Zernike polynomials.

Power vector analysis of the optical outcome of refractive surgery

The purpose of this 3-year, randomized clinical trial was to determine the difference in myopia progression in adolescents wearing soft contact lenses over a control group wearing spectacles. A total of 175 adolescents between the ages of 11 and 14 years were randomized into 2 groups, spectacle wearers and soft contact lens wearers. The main result was that the spherical equivalent change between the groups showed no clinical or statistically significant difference. However, when a power vector analysis was used, which uses all the refractive error data, a small but statistically significant (F test = 4.24, T2 = 17.35, p < 0.01) difference between the groups was found (i.e., the refractive error of the spectacle wearers had a slight increase in astigmatism). It can be concluded that soft contact lens wear does not lead to additional myopia progression in adolescents.

Myopia progression in adolescent wearers of soft contact lenses and spectacles.

Standardized sensory, perceptual, linguistic, intellectual, and cognitive tests were administered to 470 children, approximately 96% of the students entering the first grade in the four elementary schools of Benton County, Indiana, over a 3-year period (1995--1997). The results of 36 tests and subtests administered to entering first graders were well described by a 4-factor solution. These factors and the tests that loaded most heavily on them were reading-related skills (phonological awareness, letter and word identification); visual cognition (visual perceptual abilities, spatial perception, visual memory); verbal cognition (language development, vocabulary, verbal concepts); and speech processing (the ability to understand speech under difficult listening conditions). A cluster analysis identified 9 groups of children, each with a different profile of scores on the 4 factors. Within these groups, the proportion of students with unsatisfactory reading achievement in the first 2 years of elementary school (as reflected in teacher-assigned grades) varied from 3% to 40%. The profiles of factor scores demonstrated the primary influence of the reading-related skills factor on reading achievement and also on other areas of academic performance. The second strongest predictor of reading and mathematics grades was the visual cognition factor, followed by the verbal cognition factor. The speech processing factor was the weakest predictor of academic achievement, accounting for less than 1% of the variance in reading achievement. This project was a collaborative effort of the Benton Community School Corporation and a multidisciplinary group of investigators from Indiana University.

Sensory, Cognitive, and Linguistic Factors in the Early Academic Performance of Elementary School Children The Benton-IU Project

Purpose To report the change in shape of the peripheral cornea (asphericity, Q) as it relates to myopia progression in adolescence. Methods Forty-eight subjects with initial ages between 11 and 13 years of age were observed for 5 years. Each subject had participated in a variety of soft contact lens studies and all had worn daily wear soft contact lenses successfully for the 2 years before collection of the second data set. Results Myopia increased by an average of 1.46 D and was strongly correlated with a 0.56-mm increase in axial length. There was no contribution to the change in myopia from the change in central radius of the cornea. A significant (p Conclusion These data show a shift to a more positive Q (in the oblate direction) with increased myopia.

Douglas G. Horner

Papers

Power vectors: An application of Fourier analysis to the description and statistical analysis of refractive error

Power vector analysis of the optical outcome of refractive surgery

Myopia progression in adolescent wearers of soft contact lenses and spectacles.

Sensory, Cognitive, and Linguistic Factors in the Early Academic Performance of Elementary School Children The Benton-IU Project

Longitudinal changes in corneal asphericity in myopia.