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To take full advantage of new diagnostic technologies, what steps should IOL manufacturers take to improve IOL power and sizing?
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Need tighter tolerance on IOL sizes
 William B. Trattler |
Our ability to more precisely hit our refractive target with cataract surgery could be helped with tighter manufacturing tolerances when we select a particular power IOL. Currently, when we order a specific lens power such as an 18 D lens or an 18.5 D lens, lens manufacturers are allowed to provide a range of powers. For example, if a surgeon orders an 18 D lens, the power could range from 18.3 D to 17.7 D.
Even though surgeons are focusing on optimizing the refractive results of patients who undergo cataract surgery, especially those who receive presbyopia-correcting IOLs, the fact that surgeons may be implanting an IOL that is off by 0.3 D or 0.4 D can potentially negatively affect the postoperative uncorrected visual acuity. To meet surgeon and patient demands, lens manufacturers will need to tighten up the IOL powers they are providing for surgeons. In other words, if an 18 D lens is ordered, the IOL that is labeled 18 D should be as close to 18 D as possible. Current and future technologies, including intraoperative wavefront aberrometry and femtosecond laser cataract surgery, will continue to improve our ability to optimize our postoperative visual results. With the help of lens manufacturers potentially increasing their lens tolerances, we may be able to improve our ability to end up on target, which will help enhance our patients’ postoperative visual results.
William B. Trattler, MD, is an OSN SuperSite Board Member and specializes in refractive, corneal and cataract eye surgery at the Center For Excellence In Eye Care, Miami. Dr. Trattler is a consultant, on the speakers bureau and/or receives research support from Abbott Medical Optics, Bausch + Lomb, WaveTec, Lenstec and LensAR.
IOLs are accurate enough
 Jack T. Holladay |
Manufacturers do not need to change anything. The accuracy of IOLs is in 0.5 D steps for almost all lenses. A 0.5 D increment in IOLs is 0.37 D in the spectacle plane, so the question is: Do we need IOLs labeled more than 0.37 D in the spectacle plane? If the increment is 0.37 D, then the farthest away from that the diopter should be is half of that amount. If you are right in between, the most error you can make is 0.18 D. That is accurate enough.
The tolerance is in our axial length measurements, keratometry measurements and the prediction of the effective lens position. They all must be improved before reducing the step size of IOLs will have any effect. Sverker Norrby has shown this very clearly. The standard deviation for IOL calculations is 0.5 D, which means 67% of surgical cases are within 0.5 D at the spectacle plane. However, it has nothing to do with making the lens power a step size smaller. The most significant problem in accuracy is the IOL calculation after refractive surgery, in which the cornea is no longer normal. Standard keratometry and topography measurements on these abnormal corneas are imprecise. Making the lens in 0.25 D steps is not going to make any difference because the real problem is the measure of the corneal power.
Norrby demonstrated about 2 years ago in the Journal of Cataract and Refractive Surgery what is called “error analysis in engineering.” There is no reason to make the IOL step size smaller, when the error in our prediction is not from the lens power, but from the axial length, keratometry and the estimate of the effective lens position. People who recommend going to a smaller step size for the IOL have not really looked at it from an engineering standpoint in terms of the accuracy of all the variables that go into lens calculations.
Jack T. Holladay MD, MSEE, FACS, is OSN Optics Section Editor and clinical professor of ophthalmology at Baylor College of Medicine, Houston. Dr. Holladay is a consultant to WaveTec, Abbott Medical Optics, Nidek, Oculus, AcuFocus, Allergan and Carl Zeiss Meditec.
Reference:
- Norrby S. Sources of error in intraocular lens power calculation. J Cataract Refract Surg. 2008;34(3):368-376.