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Phakic IOLs induce fewer optical aberrations than LASIK
Phakic IOLs hold optical advantages over LASIK, studies by a physicist show.
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Phakic IOLs induce fewer postoperative higher-order aberrations than LASIK, according to a physicist who has investigated the optical qualities of both modalities.
“Phakic IOLs currently enjoy an advantage over LASIK in terms of lower levels of postoperative higher-order aberrations,” said Edwin J. Sarver PhD. “These lower levels appear to provide noticeably improved patient vision.”
Dr. Sarver discussed the optical quality of phakic IOLs and LASIK in a presentation at the Refractive Surgery Subspecialty Day at the American Academy of Ophthalmology meeting in Chicago.
A clinical study showed lower levels of postoperative higher-order aberrations in eyes with phakic IOLs than in eyes that underwent LASIK, Dr. Sarver said. And ray tracing studies in his laboratory suggest that decentration of phakic IOLs induces less spherical aberration and coma than decentration of LASIK treatment, he said.
“Postsurgical higher-order aberrations, third order and above, are generally higher for LASIK patients than for phakic IOL patients,” he said.
Clinical results
Dr. Sarver said the higher-order aberrations chiefly affected by phakic IOL implantation or LASIK are coma and spherical aberration.
In a study of 19 eyes after LASIK and 20 eyes after phakic IOL implantation, wavefront analysis showed spherical aberration of 0.39 µm in the LASIK eyes and 0.13 µm in the phakic IOL eyes, he said.
The induction of spherical aberration by LASIK can be attributed to variable ablation depth per laser pulse and to biological response, Dr. Sarver said. Coma, on the other hand, can be induced by tilted or offset optical elements. Optical modeling studies show that eyes after LASIK are more susceptible to induction of coma than eyes with phakic IOLs, he said.
“Spherical aberrations are not very sensitive to phakic IOL misalignments of tilt, decentration and translation,” Dr. Sarver said. “Coma, however, is much more sensitive to the phakic IOL tilt and decentration.
Better vision quality
Dr. Sarver discussed laboratory tests he has done to evaluate the difference in optical quality between LASIK treatment and phakic IOL implantation.
Dr. Sarver and colleagues used a model of a 10 D myopic eye with a phakic IOL in the anterior chamber to evaluate the effect of lens tilt, decentration and axial defocus on spherical aberration and coma.
They found that spherical aberration in the eye model was relatively insensitive to tilt of the phakic IOL over a range of 10·. Coma was more sensitive to tilt, he said, but up to about 5· the magnitude of coma is small.
For decentration, spherical aberration was also relatively insensitive, the researchers found, over a range of 0.8 mm, Dr. Sarver said.
“But as we saw for tilt, coma is more sensitive than spherical aberration to the phakic IOL decentration over the range of 0.8 mm, up to a decentration of about .5 mm,” he said. “But again, the coma is small.”
Axial shift, or defocus, has no effect on coma, Dr. Sarver pointed out. But spherical aberration varied with axial translation of a phakic IOL more than it did with tilt or decentration.
“But still we’re only talking about 0.02 µm,” he said. “So for moderate amounts of phakic IOL misalignment, that is tilt up to 4·, decentration to 0.5 mm and axial translation up to 1 mm, the higher-order aberrations are not significantly affected.”
Point spread functions
To evaluate the difference in the magnitude of aberrations between LASIK and phakic IOL implantation, Dr. Sarver and colleagues used comparisons of point spread functions and image simulations with a 5.5 mm pupil and monochromatic light.
They evaluated phakic IOL eyes and LASIK eyes for spherical aberration only, coma only, and combined spherical aberration and coma.
For increasing magnitudes of coma only and spherical aberration only, “it’s pretty clear that the increased magnitude … leads to a greatly spread out point spread function indicating a lower quality of the simulated image, or the retinal image,” Dr. Sarver said.
“When we look at the combined coma plus spherical aberration [for LASIK], we can see the point spread function is much more spread out than the coma and spherical aberration for the phakic IOL,” he said.
Simulated eye chart images generated using the same information as the point spread functions showed a similar “significant difference” between phakic IOL and LASIK eyes, Dr. Sarver said.
He noted that wavefront aberrations such as spherical aberration and coma can be compared quantitatively, but that not all aberrations affect image quality equally.
“The image simulations help provide a qualitative comparison,” Dr. Sarver said. “In studies in which the LASIK was performed in one eye and the phakic IOL in the other, most patients preferred the vision in the phakic IOL eye. The image simulations help explain why they had this preference.”
Although phakic IOLs currently hold the advantage over LASIK, this may change as LASIK continues to improve, Dr. Sarver said. But phakic IOLs may continue to improve as well, he noted.
“For example, phakic IOLs could reduce spherical aberrations through the use of aspheric surfaces,” he said.
For Your Information:
- Edwin J. Sarver, PhD, can be reached at Sarver and Associates Inc., 131 Phillips Road, Carbondale, IL 62902; 618-529-4225; fax: 618-457-5600; e-mail: ejsarver@aol.com.
- Daniele Cruz is an OSN Staff Writer who covers all aspects of ophthalmology.