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How lens position, capsulotomy size/shape affect centration, visual outcomes
IOL centration is affected by both the lens position at the end of surgery and possibly by the effect of the capsulotomy size and shape in the late postoperative period.
By Lynda Charters; Reviewed by Daniel H. Chang, MD
Bakersfield, CA-The factors that determine IOL centration are paramount to visual outcomes.
Daniel H. Chang, MD, explained how the effect of the IOL position at the end of surgery and how the capsulotomy size and shape may affect changes in the IOL position in the late postoperative period.
“Good centration is important to maximize the visual quality and minimize the visual side effects of surgery,” said Dr. Chang, who is in private practice in Bakersfield, CA.
“For example, with aspheric IOLs, the advantage of the [lens] design is lost when the IOL decentrates by more than 0.8 mm,” he said. “The effect of decentration of diffractive multifocal IOLs has not been quantified. Decentration frequently becomes the ‘scapegoat’ when patients with good Snellen visual acuity are unhappy with the surgical outcome.”
IOL centration has traditionally been defined differently in different situations. In laboratory studies, the optical center or the center of a dilated pupil is considered the center. In clinical settings, monofocal IOLs are considered centered when the edge of the IOL is not observed. For multifocal IOLs, the center of the undilated pupil is considered the center, Dr. Chang explained.
However, none of these definitions takes the visual axis into account. The visual axis (the line that connects the fovea to the nodal point and to the fixation) is related to the pupillary axis (the line from the center of the pupil that runs orthogonal to the cornea) by angle kappa, the angle between the pupillary axis and the visual axis. The corneal vertex, as represented by the coaxially sighted corneal light reflex, is the point where the visual axis intersects the cornea.
Knowing the specific definitions of these axes may help, but “applying this in a clinical setting is the challenge,” Dr. Chang said. “There are many ways that IOLs can be aligned, sometimes intentionally and something otherwise.”
Though surgeons have a few different options, Dr. Chang explained that the best place to center the IOL is on the visual axis. This is evidenced by the results of hyperopic LASIK studies and eye-model studies. Cosmetically, centering the IOL on the pupillary axis looks better because the pupils can be easily seen both during surgery and postoperative slit lamp evaluations, he noted.
“Centration on the visual axis provides the best visual outcomes,” he said.
Adjustments in IOL position can be made primarily by moving the lens perpendicular to the primary region of the peripheral bag-haptic contact, which is determined by the design. Dr. Chang provided a simulation of an implanted multifocal, one-piece lens (Tecnis, Abbott Medical Optics), which is a 13-mm hydrophobic, acrylic lens with modified C-loop haptics.
When compressed in an 11-mm bag, the maximum point of contact is about 45° to 60° counterclockwise from the haptic insertions. Therefore, it is easiest to shift the IOL in a direction corresponding to points about 30° to 45° clockwise from the haptic insertion. Once in the bag, the IOL can be nudged in this direction, and the bag can accommodate this manipulation.
A study performed by Dr. Chang showed that the intraoperative and postoperative day 1 positions of IOLs in 18 eyes were “very consistent.” The stability, however, of the centration over time is probably more important.
The capsular edge may become important in the late postoperative period because scarring and fusion of the peripheral capsule occurs, and asymmetric capsular fusion could shift the IOL, he explained. The IOL can shift anteriorly to posteriorly. In cases in which there is asymmetric fusion, asymmetric forces could move the IOL horizontally.
Capsular healing can be dependent on IOL design, Dr. Chang said. He provided an example of an eye with a Tecnis IOL implanted in which the capsular sides were fused nicely. However, in the area central to the location of the haptics, there was no fusion and the capsule was open 4 months postoperatively. Additionally, the capsulorhexis edge remained free of fibrosis and was not touching the IOL.
In another study, Dr. Chang showed that from postoperative day 1 to 12 months, the IOLs did not move significantly (about 0.1 mm) compared relative with the visual axis. There was slightly more movement (about 0.13 mm) of the IOL in relation to the pupil center at month 12.
Factors pertinent to IOL centration are both the IOL position at the end of the surgery and possibly the capsulotomy size and shape. The latter may affect changes in the IOL position in the late postoperative period, but this would depend on the exact shape of the capsulotomy and the design of the IOL, Dr. Chang summarized.
Further study is required to learn how the capsular and IOL position change over time and the effect of the capsulotomy characteristics and IOL design on the changes. Finally, the visual effects of IOL decentration must be determined.
Daniel H. Chang, MD
Dr. Chang is a consultant to Abbott Medical Optics. This article is adapted from Dr. Chang’s presentation during Refractive Surgery 2012 at the annual meeting of the American Academy of Ophthalmology.