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Lens Power (lens + power)
Selected AbstractsOptical Fourier filtering for whole lens assessment of progressive power lensesOPHTHALMIC AND PHYSIOLOGICAL OPTICS, Issue 4 2000T. Spiers Summary Four binary filter designs for use in an optical Fourier filtering set-up were evaluated when taking quantitative measurements and when qualitatively mapping the power variation of progressive power lenses (PPLs). The binary filters tested were concentric ring, linear grating, grid and "chevron" designs. The chevron filter was considered best for quantitative measurements since it permitted a vernier acuity task to be used for measuring the fringe spacing, significantly reducing errors, and it also gave information on the polarity of the lens power. The linear grating filter was considered best for qualitatively evaluating the power variation. Optical Fourier filtering and a Nidek automatic focimeter were then used to measure the powers in the distance and near portions of five PPLs of differing design. Mean measurement error was 0.04 D with a maximum value of 0.13 D. Good qualitative agreement was found between the iso-cylinder plots provided by the manufacturer and the Fourier filter fringe patterns for the PPLs indicating that optical Fourier filtering provides the ability to map the power distribution across the entire lens aperture without the need for multiple point measurements. Arguments are presented that demonstrate that it should be possible to derive both iso-sphere and iso-cylinder plots from the binary filter patterns. [source] Oculometry findings in high myopia at adult age: considerations based on oculometric follow-up data over 28 years in a cohort-based Danish high-myopia seriesACTA OPHTHALMOLOGICA, Issue 4 2010Hans C. Fledelius Abstract. Purpose:, To present and discuss oculometry data in a series of adults with high myopia followed between the ages of 26 and 54 years. Emphasis is on axial length (AL) findings and corneal curvature radius (Crad). Methods:, Thirty-four out of the 39 individuals recruited as teenagers from a Copenhagen 1948 birth cohort with myopia of at least 6 D have had current follow-up exams, to include AL measurements (by ultrasound, 1974,2002; the latter year also with the Zeiss IOLMaster) and keratometry. The cross-sectional and longitudinal analyses are based primarily on the eyes with high myopia; however, the fellow eye is also assessed in unilateral cases. Results:, At age 54 years, the maximum myopia in the series was ,26 D; the highest AL value was 35.4 mm. The myopia had increased in most, with an increase from the 26-year oculometry baseline averaging 1.0 D [standard deviation (SD) 1.84]. Ultrasound measurements over the 28 years gave a significant correlation between axial eye elongation and myopia progression of adult age (r = 0.65). The regression line was y = 0.43 + 0.36x, with myopia increase on the x-axis. Throughout sessions, the association between AL and refraction was given by correlation coefficients numerically above 0.8, whereas AL and Crad had r -values of 0.3,0.5. However, a mean Crad in the sample of 7.66 (SD 0.28) mm meant that the more general expectancy of rather flat corneas in high myopia was not fulfilled. Our data further suggested a reduction in lens power over the study period. Conclusion:, In relation to refraction, AL and Crad remain the two main oculometry parameters. Apparently the correlation patterns regarding the cornea that are broadly valid for axial ametropia in the population cannot be extended to the marginal high myopia tail of the distribution. A significant proportion of eyes with high myopia thus had steeper corneas than expected, as a so-called index contribution (albeit a small one) to the marginal refractive error. [source] Biometric calculation of intraocular lens power for cataract surgery following pupil dilatationCLINICAL & EXPERIMENTAL OPHTHALMOLOGY, Issue 2 2008Shveta Bansal MRCSEd Abstract Background:, The ability to perform biometry accurately on a dilated pupil can greatly facilitate the efficiency of a cataract service as it can be done on the day of surgery. The purpose of this study was to assess the repeatability of axial length (AL) calculations in undilated pupils and measure the difference in predicted and actual refractive outcomes in dilated pupils compared with undilated pupils. Methods:, First, intraobserver repeatability was assessed by taking two consecutive recordings of AL using applanation A-scan ultrasonography in undilated pupils in 21 eyes. The mean AL for each eye was compared with a measurement made following pupil dilatation. Second, we audited the mean spherical equivalent refractive errors following routine cataract surgery in 38 patients with undilated pupils and 36 patients with dilated pupils. Results:, The mean difference in intraobserver measurements was ,0.05 mm (standard deviation [SD] 0.15) with pupils undilated. Following pupil dilatation, the mean dilated AL differed from the mean undilated AL reading by only 0.03 mm (P > 0.05). The mean differences between planned and actual refractive error were 0.71D (SD 0.54) and 0.55D (SD 0.41) in dilated and undilated patients, respectively. This was not statistically significant (P > 0.05). The range of differences between target and actual refraction was ,1.45D to 2.70D for undilated patients and ,1.88D to 1.18D in dilated patients. Conclusion:, Although there was a greater spread of postoperative refractive errors in the dilated group, there were no statistically or clinically significant differences in postoperative refractive errors between the two categories of patients. Our study shows that applanation biometry may be safely performed for the purpose of cataract surgery after pupil dilatation. [source] Paediatric pseudophakia: analysis of intraocular lens power and myopic shiftCLINICAL & EXPERIMENTAL OPHTHALMOLOGY, Issue 3 2007William F Astle MD Abstract Background: At the Alberta Children's Hospital, the authors have been performing paediatric cataract extraction with intraocular lens (IOL) implant for over 10 years. The authors examined the amount of myopic shift that occurs in various age groups and cataract types, in order to evaluate the success of predicting the appropriate power of IOL to implant. Methods: This study is a retrospective review children undergoing small incision posterior chamber foldable IOL implantation between age 1 month and 18 years, from 1995 to 2005. 163 eyes of 126 patients underwent surgery. All patients were followed for a minimum of 6 months postoperatively. The children were divided into four groups at time of surgery: Group A: 1,24 months, Group B: 25,48 months, Group C: 49,84 months, Group D: 85 months,18 years. Results: The mean target refraction for the groups were: Group A: +6.37 D, Group B: +4.66 D, Group C: +1.95 D, and Group D: +0.97 D. Children under 4 years experienced the most myopic shift and the largest mean rate of refractive change per year. Mean change Group A: ,5.43 D, Group B: ,4.16 D, Group C: ,1.58 D, Group D: ,0.71 D. Eighty-nine per cent of patients with unilateral cataracts had a postoperative refraction within 3.00 D of the fellow eye at last follow-up visit (mean = 3.16 years). Conclusions: The rate of myopic shift is high in children under age 4 years at time of surgery, shifting as much as ,12.00 D. The mean postoperative target refraction should probably be increased from previous literature recommendations. The patient's age at time of cataract surgery and the refractive power of fellow eye are all factors to consider when deciding what power IOL to surgically implant in a paediatric patient. [source] Contact lens fitting for post-radial-keratotomy residual myopiaCLINICAL & EXPERIMENTAL OPHTHALMOLOGY, Issue 1 2003Jeewan S Titiyal MD Abstract Purpose:,To formulate guidelines for contact lens fitting following radial keratotomy and to achieve a relationship between contact lens fitting parameters and refractive and keratometric parameters. Methods:,Thirty-five eyes of 30 patients who had post-radial-keratotomy residual myopia were evaluated for contact lens fitting. The base curve was selected by ,hit and trial' method until a proper fit was achieved, following which an over-refraction was done and final power of the contact lens obtained. Results:,The mean contact lens base curve was 43.04 ± 1.62 D, which was steeper than the postoperative keratometry by 2.35 ± 0.74 D. The base curve and the postoperative keratometry were correlated by regression analysis (r = 0.895, P < 0.001). The mean overall diameter of the contact lenses was 9.64 ± 0.08 mm. The mean contact lens power was 5.81 ± 3.07 D, which was more myopic than the residual myopia by 2.79 ± 1.67 D. Regression analysis showed definite correlation between the contact lens power and postoperative myopia (r = 0.854, P < 0.001). Conclusion:,For correction of post-radial-keratotomy residual myopia, contact lens fitting parameters can be determined by a regression formula using keratometric and refractive values. [source] | ||||||||||