Gene/Protein Disease Symptom Drug Enzyme Compound
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The predictability of a theoretical, computerized (PC-assisted) intraocular lens (IOL) power calculation method and of the Sanders-Retzlaff-Kraff [SRK] I and II methods was evaluated from preoperative and postoperative biometry in 202 cataractous patients who had extracapsular cataract extraction (ECCE). The theoretical method resulted in the lowest range and standard deviation of the error, and the highest correlation coefficient between the observed and the predicted refraction (P less than .05). The superiority of the theoretical approach was most clearly demonstrated when the postoperative measurements were used in the predictions (P less than .001). This demonstrated the potential accuracy of the formula used and the importance of incorporating methods to predict the IOL position after surgery. If the prediction of the IOL chamber depth was properly corrected for the axial length dependence, a high prediction accuracy could be obtained in short as well as in long eyes.
J Cataract Refract Surg 1990 Mar
PMID:Theoretical versus SRK I and SRK II calculation of intraocular lens power. 232 81

A new implant power calculation formula (SRK/T) was developed using the nonlinear terms of the theoretical formulas as its foundation but empirical regression methodology for optimization. Postoperative anterior chamber depth prediction, retinal thickness axial length correction, and corneal refractive index were systematically and interactively optimized using an iterative process on five data sets consisting of 1,677 posterior chamber lens cases. The new SRK/T formula performed slightly better than the Holladay, SRK II, Binkhorst, and Hoffer formulas, which was the expected result as any formula performs superiorly with the data from which it was derived. Comparative accuracy of this formula upon independent data sets is addressed in a follow-up report. The formula derived provides a primarily theoretical approach under the SRK umbrella of formulas and has the added advantage of being calculable using either SRK A-constants that have been empirically derived over the last nine years or using anterior chamber depth estimates.
J Cataract Refract Surg 1990 May
PMID:Development of the SRK/T intraocular lens implant power calculation formula. 850 49

We compared the predictive accuracy of the SRK/T formula to the SRK II, Binkhorst II, Hoffer, and Holladay formulas in seven series of cases totaling 1,050 eyes. In the combined group, the SRK/T and Holladay formulas performed only slightly better than the other formulas. In short eyes (less than 22 mm), all formulas performed well, with the SRK/T, SRK II, and Holladay formulas performing marginally better. In moderately long eyes (greater than 24.5 mm, less than or equal to 27 mm), the Hoffer and Binkhorst II formulas had a greater proportion of cases with greater than 2 diopters (D) of error and the SRK/T and Holladay were again marginally better. In the very long eyes (greater than 27 mm and less than or equal to 28.4 mm), there were only 11 cases and all formulas performed well since none had greater than 2 D of prediction error. In an extremely long eye data set (greater than 28.4 mm), the SRK II formula clearly gave the poorest result. Eyes of this length occurred in only 0.1% of cases in our unselected series. Results support the contention that the present second and third generation IOL power formulas give fairly equivalent accuracy. Other factors, such as availability, ease of use, and ability to tailor or individualize, become major considerations.
J Cataract Refract Surg 1990 May
PMID:Comparison of the SRK/T formula and other theoretical and regression formulas. 235 22

Many second generation intraocular lens power calculation formulas have recently been introduced. This study explores the performance of these formulas while controlling for a potential source of variation--the lens type. For this study, all 1,157 cases studied used the Cilco CPLU posterior chamber lens. All surgeries were performed using similar phacoemulsification techniques by only two physicians (R.M.C. and S.C.G.). The SRK, SRK II, Holladay, and Binkhorst formulas were compared among themselves and also with a piece-wise nonlinear regression formula ("best fit") developed specifically from these data by the authors. Performance of the SRK II, Holladay, and best fit were better than the older SRK and Binkhorst for most axial length ranges. For these data, the Holladay and best fit formulas performed marginally better overall than the other formulas. It was also found that manipulation of specific surgeon constants significantly affected the performance of the Binkhorst formula, but had little effect on the other formulas.
J Cataract Refract Surg 1990 Jul
PMID:Intraocular lens implant power calculations: investigations controlling for lens type. 238 Sep 26

Twenty-one consecutive triple procedures (keratoplasty, cataract extraction, lens implantation) performed by one surgeon using identical suturing technique, donor size, and donor/recipient size disparity were analyzed for visual outcome and refractive error. Ninety-five percent of all grafts were clear with an average follow-up of 11.8 months. Of patients with good preoperative visual potential, 84% achieved 20/40 or better visual acuity, and the majority of these patients obtained 20/40 acuity within 6 months of surgery. Sixty-three percent of eyes with 20/40 or better acuity had refractions within +/- 2 diopters of the predicted post-operative refraction. The most recent 14 eyes in this series had IOL power calculations performed utilizing the SRK regression formula and 43.00 K's (surgeon's average post-keratoplasty keratometry values). Within this group, 79% achieved 20/40 or better vision. Eighty-two percent of these eyes had refractions within +/- 2 diopters, and 100% were within +/- 3 diopters of the predicted value. These findings demonstrate that a single surgeon using standardized keratoplasty can achieve good refractive results in the triple procedure.
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PMID:The effect of standardized keratoplasty technique on IOL power calculation for the triple procedure. 255 54

Four patients underwent cataract extraction with posterior chamber lens implantation several years after radial keratotomy. All four patients experienced an initial hyperopic shift caused by an early postoperative corneal flattening of greater than or equal to 1 diopter. This flattening partially regressed, leaving the patients with a mean of 0.42 diopter of persistent corneal flattening. We found the Binkhorst and the Holladay intraocular lens calculation formulas to be more accurate than the SRK II for these patients. Corneal curvature measured with the keratometer was less accurate for intraocular lens calculations than was a value derived by subtracting the refractive change induced by the radial keratotomy from the patients' keratometric measurements obtained before radial keratotomy.
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PMID:Refractive complications of cataract surgery after radial keratotomy. 259 47

The error in prediction of emmetropic intraocular lens power or postoperative refractive error after lens implantation was analyzed in three groups of eyes after posterior chamber lens implantation. Regression line calculation with the SRK equation or with a group-specific regression was compared with theoretical calculations in unselected, long myopic and short hyperopic eyes. The cut-off length was below 22.0 mm for the short eyes and above 25.9 mm for the long eyes. In the unselected and hyperopic group, there was only a small difference in mean error and error variance when the three calculation methods were compared. In the high myopic group, the range of error increased in all methods. The worst results were obtained with the standard SRK equation because the slope of the regression line in myopic eyes differs from the classical regression line calculated on an average population of implants. Lens calculation in high myopic eyes should therefore be performed with a specific regression line or by theoretical calculation.
J Cataract Refract Surg 1989 Nov
PMID:Effectiveness of intraocular lens calculation in high ametropia. 235 33

In order to determine the power of intraocular lenses more easily prior to cataract surgery, we compared the results obtained by automatic measurement of the corneal refractive power by the Humphrey Autokeratometer with manually obtained measurements using the Zeiss ophthalmometer. Similarly, the axial length was determined by automatic measurements using the Digital B System IV (Cooper Vision) and manual measurements using the Ophthascan S (Biophysic Medical). Statistical evaluation showed that there were no significant differences in the corneal refractive power determined by the two methods or in the IOL power calculated using the SRK formula. Therefore, automatic measurement of both corneal refractive power and axial length following the above method can be a useful alternative to the preoperative determination of the IOL power.
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PMID:[Automatic biometry and keratometry in comparison with the manual technic]. 266 73

The contact technique for ultrasound biometry was compared to the immersion technique. One hundred eyes were measured by both methods. Two groups were created based upon the axial length measurement: a group of 46 short eyes (axial length less than 23.3 mm) and a group of 54 long eyes (axial length greater than 23.3 mm). In each group the different types of measurement influenced the results obtained for the anterior chamber depth and the axial length of these eyes such that the contact technique yielded shorter measuring values than the immersion technique. In considering both methods, the difference in the anterior chamber depth and the axial length was smaller in the first group. Shorter measurements produced stronger intraocular lens power, which is equivalent to the axial length shortening by using the contact technique. The effect on the implant power calculation and the postoperative deviation of the pseudophakos refraction is shown by comparing the theoretical formula with the SRK formula. This prospective study describes the greater range of refractive pseudophakos deviation when using the contact method.
J Cataract Refract Surg 1989 Jul
PMID:Comparison of contact and immersion techniques for axial length measurement and implant power calculation. 267 12

Assessment of intraocular lens (IOL) power may rely, when biometric measurements of the eye are not available, on refraction previous to cataract development. Such an assessment, however, is generally considered unreliable. To improve the predictability of this method, we correlated refraction measurements with predictions of IOL power. In contrast to previous studies which used retrospective assessment of precataract refraction, this study included only noncataractous eyes and correlated the direct refractive measurements obtained with hypothetical IOL power values. These values were calculated by placing axial length and corneal power measurements of the same eyes in the SRK formula. We conclude that the regression formula obtained may improve the clinical judgment required for predicting lens implant power in cataractous eyes, on the basis of precataract refractive measurements.
J Cataract Refract Surg 1989 Jul
PMID:Assessment of intraocular lens power on the basis of precataract refractive measurements. 267 13


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