SEP 28, 2012
This large study examined refractive outcomes after cataract surgery in Sweden and found that while emmetropia (spherical equivalent -0.5 to +0.5 D and < 1.0 D astigmatism) is the goal in most cataract cases, it was reached in only two-thirds of eyes planned for emmetropia. Factors precluding emmetropia included remaining corneal astigmatism and biometry prediction errors in astigmatic and ametropic eyes.
The authors analyzed planned and actual postoperative refractions and preoperative and postoperative corneal astigmatism for cataract procedures performed in 2008 through 2010 at cataract surgery units participating in Sweden's National Cataract Register (NCR). I am always impressed by the amount of information we can get from Swedish NCR studies.
The results included postoperative refraction for 17,056 procedures and corneal astigmatism for 7,448 procedures. Emmetropia was targeted in 78.1 percent of eyes and achieved in 52.7 percent. Forty-three percent had less than 1 D of astigmatism. "Reading myopia" of -3.5 to -1.6 D was targeted in 7 percent of eyes and achieved in 7.8 percent. Planned hyperopia greater than 1 D or myopia greater than -3.5 D was rare.
The mean absolute biometry prediction error was 0.402 D in all eyes. However, astigmatic eyes and eyes planned for myopia or hyperopia had higher biometry prediction errors. Younger patients were more often astigmatic and planned for a more myopic outcome. Preoperatively, one third of eyes had more than 1 D of corneal astigmatism, a figure that was largely unaltered postoperatively. The mean induced astigmatism was 0.525 ± 0.804 D in all eyes.
The authors note that biometry prediction in this study is slightly less accurate than that reported in controlled studies that used tools, such as optimized formulas. However, they say the outcomes are reasonably good considering that this was a study of a nonselected population. Nonetheless, they conclude that the fact that only two-thirds of eyes planned for emmetropia achieved this reflects the need for refining biometry and IOL calculations, especially for astigmatic and ametropic eyes.