To further improve on post-op outcomes
In addition to IOL misalignment, which obviously leads to refractive surprises, two additional factors deserve our attention when planning toric IOL implantation: 1) the influence of posterior corneal astigmatism on total corneal astigmatism and 2) the influence of anterior chamber depth (ACD) on the conversion of astigmatism from the IOL to the corneal plane.
The influence of posterior corneal astigmatism
The magnitude of posterior corneal astigmatism is directly proportional to that of the anterior one. In general, if Scheimpflug measurements are not available, about 0.25–0.50 D of ATR astigmatism should be added to the keratometric astigmatism to get closer to the total corneal astigmatism. However, since posterior astigmatism is not a fixed value and large discrepancies can be found among patients, direct measurements are preferable.
The total corneal astigmatism, which can be measured by the Galilei (Ziemer, Port, Switzerland), Pentacam (Oculus, Wetzlar, Germany) or Sirius (CSO, Florence, Italy), has to be entered into one of the several available calculators to obtain the required IOL toricity.
The influence of anterior chamber depth
The distance between the anterior surface of the cornea and the IOL [i.e., the anterior chamber depth (ACD)] influences the conversion of the IOL toricity from the IOL plane to the corneal plane. Not all calculators take this effect into account and some of them adopt a fixed ratio. This approach provides good results in average eyes, but can lead to some inaccuracies in eyes with shallow or deep ACDs (i.e., in hyperopic and myopic eyes).
In short hyperopic eyes, the above-mentioned ratio can be as low as 1.3, whereas in long myopic eyes it can be higher than 1.8 (in medium eyes it is 1.46). As a consequence, using a fixed ratio can lead to cylinder overcorrection in hyperopic eyes and cylinder undercorrection in myopic eyes. The solution to this problem has been described by Dr Han Bor Fam a few years ago and is known as meridional analysis.1 In this, the IOL power has to be calculated first for the steep corneal meridian and then again for the flat one, on condition that the predicted ELP is the same for both of them and corresponds to the one calculated for the average corneal power. This method allows surgeons to accurately assess the conversion of the corneal cylinder to the IOL plane and helps avoiding over or undercorrections of the corneal astigmatism.
When, selecting the appropriate IOL toricity, we recommend relying on calculators that keep this effect into account, such as the ASSORT Calculator by Dr Noel Alpins, FACS, the Holladay IOL Consultant Software by Dr Jack T. Holladay, MSEE, FACS, or the on-line calculators provided by manufacturers such as AMO, Rayner or Zeiss.2–4
In conclusion, toric IOLs represent an excellent tool to get highly satisfied patients. Optimizing our calculations will further improve our outcomes.
1. H.B. Fam and K.L. Lim, J. Cataract Refract. Surg., 2007; 33:2072–2076.