New frontiers in IOL power calculations

April 1, 2011

Following a premium IOL implantation it is now possible to maintain the target refractive error of 0.5 D but errors can still arise in the calculation of the desired IOL power. Modernizing formulae may help to reduce prediction error in IOL power calculations.

Lens surgery is refractive surgery. This has always been the case, you may say, but as we continue to improve our refractive results, patient expectations also increase and you don't want to be off the target by a 0.5 D refractive error following a premium IOL implant. The good news is that with current technology we are able to deliver this goal. The bad news is that surgeons themselves do not always understand how an error may arise in the calculation of the desired IOL power.

In the old days of ultrasound biometry, the most common cause of error was axial length measurement. However, after the introduction of optical biometry (Zeiss IOLMaster), 10 years ago, this source of error has decreased dramatically. The precision of laser biometry is typically within 0.01 mm between repeated readings corresponding to an error in the spectacle plane of only 0.025 D(!). As the axial length determination is no longer a major problem, other sources of error such as K-readings errors or formula-dependent inaccuracy in the prediction of the postoperative anterior chamber depth (ACD) are coming into the light.

IOL calculation formulae

Time and surgical technique have changed since the development of these old IOL power calculation formulae. We now use small incisions with virtually no change in the corneal shape, we use capsulorhexis which guarantees a standardized placement of the IOL in-the-bag, and we use IOLs of high optical quality ensuring that the labelled power equals the true power. ("This is currently assumed to be the case, however, it has not yet been qualified through research.") In the future we may be facing an even more controlled surgical environment. Imagine the decrease in surgical variability when femtosecond laser surgery becomes the standard!

What now represents the bottleneck of IOL power calculation is neither the error associated with the axial length measurement nor the lack of a precise A-constant nor other magic 'surgeon factors' used in the past to hold some of the unknowns. No, the truth is that much of the error arises from the use of an out-dated method for the prediction of the postoperative ACD in the individual case.

With the introduction of the new Haag-Streit Lenstar LS900 optical biometer, you can have all the intraocular distances measured with the accuracy of the laser interferometry technique. Using my improved ACD prediction algorithm we have found significant higher accuracy than the older IOL formulae (Figure 1). This high-definition optical biometry holds great promises for a new era in IOL power calculation.