Eyeworld

SEP 2022

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

Issue link: https://digital.eyeworld.org/i/1475139

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54 | EYEWORLD | SEPTEMBER 2022 R EFRACTIVE by Liz Hillman Editorial Co-Director About the physicians Daniel Chang, MD Cataract and Refractive Surgeon Empire Eye and Laser Center Bakersfield, California Douglas Koch, MD Department of Ophthalmology Cullen Eye Institute Baylor College of Medicine Houston, Texas Relevant disclosures Chang: Carl Zeiss Meditec Koch: None A ngle kappa—"People don't fully understand it, but most don't even realize they don't," Daniel Chang, MD, told EyeWorld. Dr. Chang is the first to admit that he didn't. The subject of centration is 1) poorly defined in the literature, 2) inconsistent in its application, and 3) probably not as im- portant for IOL surgery as most people suspect. Dr. Chang and George Waring IV, MD, wrote a perspective in 2014 describing what they found to be inconsistencies with definitions, applications, and use of various ocular reference axes and angles. 1 They also proposed a new, practical, and reproducible coordinate system for centration of refractive treatments. "When someone asks me about angle kap- pa, I ask how much effort they want to make to fully understand it." Dr. Chang said. "The con- cepts are actually quite elegant, but the tough part is to unlearn what was previously partially understood." Angle kappa background Angle kappa, Dr. Chang said, is based on 100-year-old concepts and terminology that were originally created for the management of strabismus, which he said requires a different type of function and precision than intraocular surgery. "Angle kappa is defined as the angle that subtends the visual axis and the pupillary axis. Angle lambda (previously angle kappa) sub- tends the line of sight and the pupillary axis. If this sounds confusing, add the fact that the visu- al axis has at least three different definitions in- volving nodal points; line of sight and pupillary axis depend on pupil location and thus change with lighting and accommodation; and throw in the question of whether these lines actually intersect to form an angle," Dr. Chang said. With these concepts being used for lens cen- tration, Dr. Chang said they fall short of what physicians need them to do. "When you try to apply these to intraocular surgery, they don't work. We've been stuck applying old concepts and terminology to a new problem of a very different nature," he said. When and how did physicians start using angle kappa with lens surgery? Dr. Chang said it started with presbyopia-correcting IOLs. The rings on these IOLs made any decentration rela- tive to the pupil margin an obvious discrepancy. "There was an assumption that the 'mis- alignment' was causing some visual dissatisfac- tion," he said. "I think that's where it started. When we observe a discrepancy, we associate that with an outcome, and we start doing something about it. Terminology is simply the way we communicate our observations." A 2011 study suggested that "there may be a role of misalignment between the visual and pupillary axis (angle kappa)." 2 Dr. Chang said "people took preoperative 'angle kappa' measurements and suggested that higher angle kappas would result in problematic outcomes. Without clear evidence, centration became a convenient scapegoat for undesirable surgical outcomes. Unfortunately, other import- ant factors, like IOL material and design, were not given as much consideration." Nomenclature changes Dr. Chang and Dr. Waring attempted to address the ambiguity with the current nomenclature and proposed a new, reproducible definition and technique for centering IOLs on what they called the subject-fixated coaxially sighted cor- neal light reflex (SF-CSCLR). They also called the chord distance between the SF-CSCLR and the pupil center "chord mu." Since putting forth this concept, scientists at Carl Zeiss Meditec suggested calling it the "Chang-Waring reflex" and "Chang-Waring chord." Dr. Chang noted that "angle kappa" is actually a chord distance, not an angle. The naming has since moved toward chord kappa, and Dr. Chang hopes that the new terminology would help to clarify. Dr. Chang said that the pupil center defines the pupillary axis. That point in space is a sim- ple concept and probably more important than defining the whole axis. Positioning an IOL at the pupil center can be tricky because the pupil is dilated intraoperatively. The visual axis, actually defined by nodal points, is even trickier. "How do I know where the nodal point is? Many assume the corneal light reflex (CLR) defines the visual axis. We Clearing up angle kappa continued on page 57

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