Going beyond the A-constant for IOL calculations

The idea seems reasonable: introduce a variable into lens power calculations in order to fine-tune the results and improve accuracy. The A-constant, which is a bit of a misnomer because it is a variable and not actually a constant, can help provide a way to adjust based on the IOL type — including refractive index and lens geometry — IOL position, calibration of biometry devices and even surgeon technique. Once this is honed for a particular surgeon and IOL, it can be used to adjust all IOL calculations for that surgeon so that, as a whole, all of the eyes undergoing cataract surgery can achieve a more myopic or more hyperopic outcome.Adjusting the A-constant will change the IOL power with a 1:1 ratio, so that increasing the A-constant by 0.5 D means that the calculated IOL power will be increased by 0.5 D across the board for all eyes. For a particular model of IOL, many factors will affect the A-constant. If the lens geometry is such that the haptics have a posterior angulation, then the optic will sit deeper within the eye, and the A-constant will be higher than for an IOL that sits more anterior. IOLs are labeled for their optical power, which is measured outside of the eye, and the variability of how it sits within the eye, the refractive index and the lens design (plano-convex vs. bi-convex, for example) will change the effective refractive power in the eye. This is why two different IOL models will have different A-constants and thus different IOL calculations even though both of them measure the same optical power outside of the eye. If you know that a particular model of IOL has an A-constant of 119.2 but then you switch to a different IOL with an A-constant of 118.7, you must then also drop the IOL power by 0.5, which is the difference between 119.2 and 118.7 in order to maintain the same refractive outcome.