REFRACTIVE 18 EWAP DECEMBER 2022 Contact information Chang: dchang@empireeyeandlaser.com; www.empireeyeandlaser.com Koch: dkoch@bcm.edu 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 important 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 kappa, I ask how much effort they want to make to fully understand it.” Dr. Chang said. “The concepts 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 Clearing up angle kappa by Liz Hillman Editorial Co-Director the angle that subtends the visual axis and the pupillary axis. Angle lambda (previously angle kappa) subtends the line of sight and the pupillary axis. If this sounds confusing, add the fact that the visual axis has at least three different definitions involving 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 centration, 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 relative to the pupil margin an obvious discrepancy. “There was an assumption that the ‘misalignment’ was causing some visual dissatisfaction,” 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 important 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 corneal 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. This article originally appeared in the September 2022 issue of EyeWorld. It has been slightly modified and appears here with permission from the ASCRS Ophthalmic Services Corp.
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