Intraocular lens geometry makes a difference: author's reply

Abstract

Editor, I want to thank Wolfgang Haigis for his comments on the geometry of the intraocular lens (IOL) (Haigis 2004). This issue is important in every method of IOL power calculation. It is true that optic configuration makes a difference in the effective refractive power of the IOL, as it affects the position of the effective lens plane. When Alcon (Fort Worth, TX, USA) changed the optic design of the MA60BM (biconvex 2 : 1 configuration) to that of the MA60AC (biconvex 1 : 1), the effective lens plane was shifted anteriorly, thereby increasing the refractive effect of the IOL and decreasing the A-constant. So much for the A-constant and other fudge factors. However, this does not mean that the physical position of the IOL shifts by a comparable amount. In fact, it is our experience, based on hundreds of ultrasound measurements of postoperative anterior chamber depth (ACD), that the average (measurable) ACD changes surprisingly little in response to optic configuration. By contrast, the ACD does change according to haptics design, and according to whether the IOL is placed within or outside the capsular bag, the type and size of capsulotomy, etc. However, thanks to the standardized capsulorhexis technique, time is running out for ‘personalizing’ IOL power calculations according to surgical technique. Accordingly, my statement that postoperative ACD was 4.74 mm for both IOL types was based on these measurements, assuming that no major error was made (Olsen 2007). Of course, the Olsen formula takes the different optic configurations of the two lenses into account in a separate algorithm, ensuring that the correction of principal planes is carried out according to ‘thick lens’ optics. I agree with Wolfgang Haigis that in theory the ACD of the IOL with the steepest anterior curvature is expected to be the smallest, other things being equal. However, if we had been able to improve our predictions by this small correction, all comparison groups would have been changed by an equal amount, which would not have altered our conclusions. We therefore still believe that our 5-variable method of predicting ACD is more accurate than the Haigis 2-variable method. As we demonstrated in Olsen (2006), lens thickness was ranked third after axial length and preoperative chamber depth in the statistical association of postoperative ACD with the IOL. Unfortunately, lens thickness is not measured with the current version of the Zeiss IOLMaster (Carl Zeiss Meditec AG, Jena, Germany) and therefore must continue to be measured with ultrasound.