Optimization of the A Constant for the SRK/T Formula

Purpose: To evaluate the effect of axial length (AL) and the average preoperative keratometry (K) on the A constant in the SRK/T formula. Methods: The retrospective, comparative case series includes 635 eyes from 407 cataract patients from Columbia University Medical Center from January 2006 to August 2010, operated by a single surgeon using a temporal incision and the Acrysof SN60WF IOL (Alcon Laboratories, TX). Using the postoperative manifest refraction and biometry data, we calculated the precise A constant (Ap) necessary to yield the postoperative spherical equivalent for each eye. To optimize the A constant, we developed three regression models (linear, quadratic, and categorical in 7 AL groups) to relate these precise A constants to AL and K. We verified our method with another series of 45 eyes for which we calculated mean errors (defined as the difference between the spherical equivalent of the postoperative refraction and the predicted postoperative refraction) using the optimized and manufacturer’s suggested A constants. Results: There is a statistically significant relationship between AL (P < 0.001), K (P < 0.001) and the A constant. Ap increased as AL increased and as K decreased. In the validation data set, optimizing the A constant reduced mean errors from 0.50 D to 0.25 D and also reduced hyperopic refractive outcomes. Conclusions: The A constant for longer eyes with flatter corneas is larger than the A constant for shorter eyes with steeper corneas. Optimizing A constants using both AL and K improved the predictability of refractive outcomes without modification to the SRK/T formula.