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Nomogram accounts for aberration interactions
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SAN FRANCISCO — A nomogram for laser refractive surgery that accounts for the interactions of certain higher-order aberrations may improve results in PRK and LASIK, according to a speaker here.
Manoj Subbaram, MD, said the interaction of preoperative higher-order aberrations can have significant effects on sphere and cylinder after laser refractive surgery.
“Compensating for these interactions can definitely improve results of LASIK and PRK,” he said, in a presentation at the American Society of Cataract and Refractive Surgery meeting.
Dr. Subbaram noted that in the clinical trial data submitted to the Food and Drug Administration for premarket approval for the Bausch & Lomb Zyoptix laser system, “there was only moderate predictability of postoperative refractive error,” with 76% of eyes within ±0.5 D of target.
In an effort to improve this result, Dr. Subbaram and his co-author Scott M. MacRae, MD, analyzed the study results and found that overcorrection was five times more likely in the study than undercorrection. In addition, they found that the eyes with overcorrections had greater amounts of defocus, spherical aberration and third-order root mean square aberrations.
“This gave us a clue that there might be something going on in terms of aberration interaction that affects our postoperative results,” he said.
Eyes in the study with a lot of postoperative positive spherical aberration tended to be hyperopic, Dr. Subbaram said, leading to a hypothesis that defocus is related to spherical aberration. The researchers also noted a relationship between fourth-order secondary astigmatism and second order astigmatism.
“And this is just the tip of the iceberg,” he said. “There are a lot more interactions going on.”
The nomogram they devised uses the patient’s preoperative manifest refraction and compensates for the aberration interactions that the researchers “decoded,” Dr. Subbaram said.
The new nomogram was evaluated in a series of 508 eyes, of which 445 underwent LASIK and 63 underwent PRK. The LASIK eyes had a mean preoperative refractive error of –4.5 D, and the PRK eyes had a mean preop refractive error of –6.6 D. All eyes had a high degree of preoperative higher-order RMS, at least 0.5 µm.
At 1 month postoperative, the sphere and spherical equivalent were “pretty much at zero,” Dr. Subbaram said, and all eyes were within ±1 D of plano.
In addition, 94.6% of the LASIK eyes and 90.5% of the PRK eyes had uncorrected visual acuity of 20/20 or better.