MAY 2013

EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.

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4 Supplement to EyeWorld May 2013 Toric IOLs and the ANSI guidelines by Daniel H. Chang, MD The group has published guidelines recommending that toric IOLs not exceed 5 degrees of rotation postoperatively. Here's what that means for clinicians Daniel H. Chang, MD " The toric lenses currently under FDA review will have much better optical properties than those available now. " to 0.5 D of sphere and cylindrical power for most labeled dioptric power ranges directly affect our ability to provide accurate outcomes for our patients. Effectiveness analyses, including the percentage of eyes achieving specified ranges of MRSE, UCVA, and cylindrical correction, are specified as well. s ophthalmic surgeons, we have enjoyed increasingly predictable outcomes with cataract surgery. The intraocular lenses (IOLs) that we use have become so precise that patients are now demanding excellent uncorrected visual function, which we provide with more advanced and complex IOLs. The American National Standards Institute (ANSI), a private not-for-profit organization that creates standards for diverse fields of science and engineering, has released guidelines on IOL manufacturing standards, including a specific set of guidelines for toric lenses. This standard (ANSI Z80.302010) "applies to any monofocal [IOL] whose primary indication is the reduction of astigmatism either with the correction of aphakia or the modification of the refractive power of a phakic eye."1 Issues covered include optical properties, mechanical properties, labeling, clinical investigations, and the like. A key point with these ANSI standards is that they do not stand alone—they are built upon other standards and guidelines, including the international ISO standards. While some of the details may be more relevant to IOL manufacturers than to surgeons, many specifications do affect our routine clinical use of toric IOLs. For example, the recommended tolerance limits of 0.3 Toric IOLs are unique in their requirement for proper axis orientation. From a manufacturing standpoint, ANSI Z80.30-2010 specifies axis orientation marks to be within 5 degrees of the cylindrical axis. This standard is important in giving surgeons the means to align a toric IOL accurately. It is well known that every degree of rotation in a toric lens will result in a 3% loss of power, so IOLs that rotate as little as 5 degrees will lose upward of 15% of their astigmatic corrective power. Ultimately, the accurate alignment of a toric IOL requires good surgical skills in conjunction with tight manufacturing tolerances. An IOL manufacturer cannot ensure that a surgeon places a toric IOL accurately. However, assuming a toric IOL is implanted on-axis, good materials, design, and manufacturing should allow that IOL to stay onaxis. The ANSI guidelines for rotational stability are defined by the consistency of the IOL axis on two consecutive visits at least three months apart. Stability of the toric IOL axis is considered achieved if 90% of IOLs rotate no more than 5 degrees. It's the design of the IOL, including its material and haptic configuration, that will determine rotational stability. Toric IOLs that are too slick or perhaps with plate haptics may not be able to conform to the ANSI's rotational guidelines.2 In my personal experience, I've moved away from IOLs with postop- A Toric lens rotational stability Degree rotation Percentage power loss 1-2 degrees 3-6% 5 degrees 15% 10 degrees 30% 15 degrees 45% 20 degrees 60% erative rotation issues. In my hands, some of the commercially available toric lenses are reasonably stable in the postop period, but they can be difficult to manipulate intraoperatively when I need to rotate them to the proper axis. The new hydrophobic acrylic toric IOL from Abbott Medical Optics (Santa Ana, Calif.) that has recently received FDA approval demonstrated 90+% rotational stability postoperatively with an average rotation of only 2.74 degrees in the recent clinical trial. Based on material and design similarities to the same family of one-piece monofocal and multifocal IOLs, this new toric IOL should be easy to work with intraoperatively and stay well within the rotational guidelines outlined in the ANSI standard. In addition to rotational stability, the ANSI standard gives detailed specifications describing the wavefront sensor test methods for determining the optical quality of lenses. This is important because I find that many patients with 1.5 D or less of astigmatism do better with a lowdispersion (low chromatic aberration) monofocal hydrophobic acrylic aspheric non-toric lens than the commercially available toric lenses with higher chromatic aberration. I believe that this observation is a function of the overall optical quality of the IOL being more important than the toricity component alone. References 1. American National Standards Institute. American National Standards for Ophthalmics —Toric Intraocular Lenses. The Vision Council; Alexandria, Va. Approved March 24, 2010. 2. Buckhurst PJ, Wolffsohn JS, Naroo SA, Davies LN. Rotational and centration stability of an aspheric intraocular lens with a simulated toric design. J Cataract Refract Surg. 2010;36:1523-1528. Dr. Chang is in private practice, Empire Eye and Laser. He is also a volunteer surgeon and founding member on the board of directors for Advanced Center for Eyecare, a non-profit clinic providing medical/surgical eyecare to uninsured and underinsured people of Kern County, Calif. He can be contacted at 661-325-3937.

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