Eyeworld

DECEMBER 2023 | EYEWORLD | 49 R LESSONS LEARNED by Ellen Stodola Editorial Co-Director About the doctors Brandon Ayres, MD Co-Director of the Cornea Fellowship Program Wills Eye Hospital Philadelphia, Pennsylvania Susana Marcos, PhD Professor Department of Ophthalmology University of Rochester Medical Center Rochester, New York L ens tilt or decentration can occur with intraocular surgery, and while not al- ways something that physicians need to address, this may require attention with more complicated cases or with larger decentrations. A physicist and a physician dis- cussed what they look for and techniques that might be used. The eye is a very non-aligned system, said Susana Marcos, PhD. The fovea does not align with the optical axis of the eye. Even the optical components of the eye (cornea, crystalline lens, and pupil) are not aligned with respect to each other. "In the majority of cases, the tilt or decen- tration of the lens does not result from surgery but from the natural misalignment of the cap- sular bag," she said, adding that preserving the natural misalignment is generally a good thing. Dr. Marcos said that she and colleagues generated computer models of pseudophakic eyes using anatomical information and the intraocular lens geometry, including tilt and decentration of the lens, obtained using ful- ly quantitative 3D anterior segment imaging. The wavefront aberrations measured matched measurements from aberrometry when tilt and decentration were included. "We found that in 75% of the cases, having the small amounts (and orientation) of the tilt and decentration produced slightly better quality than having zero tilt and decentration. In a sense, the tilt and decentration of the lens tend to compensate [for] the effects of the off-axis position of the fovea." There are a number of techniques to mea- sure tilt and decentration of the lens (before and after surgery) and estimate the effect on retinal image quality, Dr. Marcos said. Her lab has developed techniques based on Purkinje imaging, 1,2 Scheimpflug imaging, 1 and most re- cently, on OCT to measure tilt and decentration of IOLs in vivo. 3,4 "With 3D quantitative OCT, we can measure corneal topography (anterior and posterior), lens and fovea alignment, and construct eye models," she said. "Those tech- niques do not prevent tilt and decentration from happening but certainly can quantify the effect on image degradation." When deciding whether action is needed to address lens tilt or decentration, Dr. Marcos said this will likely depend on the optical design of the lens. Other factors to consider are corneal aberrations. Using patient-specific computer eye models (based on measured topography and biometry), it is possible to test the effect of tilt and decentration on the retinal image quality and predict the impact on vision, she said. Lens tilt and decentration can be a broad spectrum, according to Brandon Ayres, MD. It runs the gamut of a little bit of decentration due to an asymmetric capsulorhexis all the way to a tilted suture-fixated lens that induces 4 or 5 D of astigmatism. When thinking about tilt and decentration with standard monofocal lenses, Dr. Ayres said this is something that can be seen but is not usually a major concern. With some of the more advanced technology lenses, the aspheric IOLs especially, if they begin to decenter, there may be a decrease in quality of vision. If there's a capsule complication and you can't get the lens in perfectly centered or if there's asymmetric phimosis or scarring of the capsule, it can cause a decrease in visual quality. There's less of an is- sue optically if you're using a non-aspheric or a zero asphericity lens, Dr. Ayres said. If there are small amounts of decentration due to patient healing, it's not going to impact quality of vision as much. Assessing IOL tilt and decentration continued on page 50 Late subluxation of a posterior chamber IOL in the capsular bag due to pseudoexfoliation. In this case, the entire lens/capsule will have to be repositioned or removed and replaced in order to improve vision. Source: Brandon Ayres, MD

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