Rochester nomogram helps improve custom LASIK results for myopia

Rochester, NY-The Rochester nomogram for custom LASIK provided significantly better postoperative visual acuity, better predictability, and reduced range of postoperative spherical equivalent compared with the previous FDA study in eyes with greater degrees of myopia, according to Manoj Subbaram, PhD.

The nomogram accomplished this by taking into account the numerous interactions between aberrations in the eye, Dr. Subbaram said.

Dr. Subbaram recounted the rationale for the formulation of the Rochester nomogram. The results of the Zyoptix (Bausch & Lomb) FDA trial for custom myopia correction were good, but there was only moderate predictability of postoperative refractive error.

"The eyes with postoperative hyperopia had greater amounts of preoperative positive spherical aberration and third-order root mean square (RMS)," Dr. Subbaram said. "This provided a clue that there might be aberration interaction that was affecting our postoperative results."

First, Dr. Subbaram and his colleagues evaluated defocus and spherical aberration, which are the two major components.

"When treating a high degree of preoperative myopia, the result tends to be a high degree of postoperative positive spherical aberration and the eyes tend to be hyperopic postoperatively," he said. "So there is a two-way interaction between defocus and spherical aberration.

"We also found that eyes with postoperative hyperopia tend to have a high degree of postoperative third-order RMS-the interaction between third-order term and defocus," he said. "They also found a good correlation between the amount of coma and the risk of developing postoperative astigmatism."

Overall, the second-order terms are affected by the third-order terms; there is a two-way interaction between defocus and spherical aberration, he said. In addition, there is interaction between secondary fourth-order astigmatism and the corresponding second-order cylinder term. Finally, there is a correlation between spherical aberration and coma.

"This is just the tip of the iceberg," he said. "There are many more interactions occurring. This all happens at the retinal image plane, and it is being interpreted by the brain. The best sphero-cylindrical correction is the manifest refraction, which compensates for most of these interactions."

Nomogram features

The Rochester nomogram incorporates some unique features, according to Dr. Subbaram.

"It uses the manifest preoperative refraction, it compensates for the interactions of the aberrations, and it is based on individual eye parameters and not on a constant surgeon offset," he said.

The study included 508 eyes treated with the Rochester nomogram. Of those, 445 eyes had been treated with LASIK, and 63 eyes had been treated with PRK. All eyes had high degrees of myopia preoperatively. The eyes that underwent LASIK had an average of –4.5 D and the eyes that underwent PRK an average of –6.5 D of myopia. The preoperative higher-order RMS was at least 0.35 μm.

"This is significantly greater than any study that has been reported thus far," he said.

"The 1-month results show that the mean postoperative sphere and the spherical equivalent were about 0 with a range of ±1 D," Dr. Subbaram said. "The standard deviation was ±0.25 D. It is difficult to obtain better than that."

The percentages of uncorrected visual acuity were impressive, with 95% of patients who underwent LASIK and 90% of those who underwent PRK achieving 20/20 or better. The predictability of the refractive error was similar, with 96.4% of the LASIK patients and 90% of the PRK patients achieving within 0.5 D of the intended spherical equivalent, according to Dr. Subbaram.

The percentage of eyes that underwent LASIK that were overcorrected was 1.3%, which is a substantial reduction from the FDA trial in which 21.8% of eyes were overcorrected.