Jornal de Oftalmologia Clínica e Experimental
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A Method to Improve the Accuracy of Optimized A-Constant for IOL Calculation Formulas

Lei Zheng and John C Merriam

Objective: To achieve more accurate IOL calculation results using multiple A-constants for SRK/T formula.
Methods: This is a retrospective clinical study. The study includes 650 cases of phacoemulsification with an Acrysof SN60WF IOL (Alcon, TX). The study excluded eyes that had corneal refractive surgery, eyes with any condition that precluded accurate and consistent keratometry. The study used the average of 5 valid measurements of axial length (AL) and corneal curvature using IOLMaster (Carl Zeiss Meditec, Inc. Germany) prior to surgery and manifest refraction measured at least 3 months after surgery. Patients were divided into 7 groups based on AL from less than 22 mm to greater than 27 mm. The average A-constant with standard deviation was calculated in each group, and these groups were compared to each other using independent two-sample t-test (p<0.05).
Results: The A-constants changed with AL. The A-constant of the four groups with AL less than 25 mm did not differ significantly each other (P<0.05). The two groups with AL less than 26 mm and less than 27 mm also did not differ statistically. Both are different than the first four groups. The A-constant for the group of AL 27 mm and more were different from all other groups. Second order polynomial described the relationship of the A-constant to AL.
Conclusion: Our results suggest that using different A-constants for eyes with different ALs, especially longer than 26 mm, may improve the accuracy of post-operative refraction results.