Higher-order aberration interactions with customized ablations needto be neutralized

Aberration interactions with customized refractive surgery is a new concept that evolved after Dr. MacRae and his colleague, Manoj V. Subbaram, BS Optom, PhD, studied the data from the FDA custom Zyoptix trial (Bausch & Lomb). The study results showed that 22% of eyes were overcorrected by 0.5 D or more in terms of their spherical value, he said.

Refractive surgeons are already familiar with the idea that treating defocus induces positive spherical aberration with either conventional or custom laser treatments. With this new data analysis, Dr. MacRae showed that the aberration interaction also goes in the opposite direction with preoperative spherical aberration as well as third-order aberrations affecting sphere.

"If a patient has a lot of coma preoperatively, we actually pull back on our treatment sphere to avoid a spherical overcorrection. If someone has a lot of positive spherical aberration preoperatively, we decrease the magnitude of the spherical correction to avoid inducing hyperopia," he explained.

The Rochester nomogram

A year and a half ago, Drs. MacRae and Subbaram developed the Rochester nomogram to neutralize these aberration interactions. After completing an extensive statistical evaluation of the FDA custom Zyoptix trial data, they determined that preoperative manifest refraction affects the postoperative results. So instead of using the wavefront sphere refraction, they use the preoperative manifest refraction, including preoperative spherical equivalent and J0 and J45 astigmatism terms, to determine the amount of spherical correction. The Rochester nomogram also compensates for any aberration interactions, he said.

"The treatment of sphere is based on individual eye parameters and not a constant surgeon offset," Dr. MacRae said.

So far, Dr. MacRae has treated more than 560 eyes using the Zyoptix platform with the Rochester nomogram and found the results to be very effective. In a study of 175 eyes dilated with 2.5% neosynephrine to determine the manifest refraction and treated with customized ablation with the Zyoptix laser system and Rochester nomogram, the average postoperative standard deviation was 0.34 D. In the group of 90 eyes that were not dilated with neosynephrine and treated with the Zyoptix system and Rochester nomogram, the standard deviation was 0.23 D. Compared with the standard deviation of manifest refraction repeatability with no treatment in the Bullimore study (0.2 D), the results with the Rochester nomogram treating about 5 D of myopia are "pretty amazing," Dr. MacRae said.