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Eva Nong, John C. Merriam, Lei Zhang, Malka Schlesinger; Optimization of the A Constant for the SRK/T Formula. Invest. Ophthalmol. Vis. Sci. 2012;53(14):6711.
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The SRK/T formula for the calculation of the power of an intraocular lens (IOL) remains popular with cataract surgeons. This study examines the effect of axial length (AXL) and the average preoperative keratometry value (Kavg) on the value of the A constant and compares models to optimize the A constant with the manufacturer’s standard A constant.
This is a retrospective study of 509 eyes from 361 patients, all of whom were operated by a single surgeon using a temporal incision of 2.2 or 2.6 mm and the Acysof SN60WF IOL (Alcon Laboratories). Our study includes 143 (39.6%) males and 218 (60.4%) females, with mean ages of 76 and 75 years, respectively. With the biometry data from the IOLMaster and the postoperative manifest refraction, we used the SRK/T formula to calculate the precise A constant for the spherical equivalent of the postoperative refraction of each eye. Mixed linear models were used to determine the association between the A-constant and age, gender, AXL, and Kavg. The four models to determine the A constant use 1) AXL as a continuous variable, 2) AXL as a categorical value with 7 axial length groups ranging from less than 22 mm to greater than 27 mm, 3) AXL as a categorical value, adjusted for Kavg, and 4) AXL as a continuous variable, adjusted for Kavg and including a quadratic AXL term.
There is a statistically significant relationship between AL (P<0.001), Kavg (P<0.001), and the A-constant. The relationship between gender and the A-constant also is significant (P=0.005); but as axial length is highly correlated with gender, gender does not provide any additional information to the models. There does not appear to be any relationship between age and the A-constant. The difference between the A constant predicted by our models and the manufacturer’s suggested A constant (118.7) ranges from 0 at short axial lengths to 1.5 at long axial lengths. The mean difference between achieved postoperative spherical equivalent and the target refraction predicted by each of the models is similar and is less than ± 0.3 diopters. The best fit model includes a quadratic axial length term.
Considering AXL and Kavg to optimize the A constant may improve the accuracy of the predicted postoperative refraction.
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