Risk of significant postoperative cylinder examined

"An unresolved issue with myopic custom LASIK is the incidence of postoperative cylinder. The goal of our study was to try to determine the risk factors for astigmatism after custom LASIK to treat myopia," Dr. Subbaram explained. He is a postdoctoral researcher at the Center for Visual Science and Department of Ophthalmology, University of Rochester, Roch-ester, NY.

He explained that the first factor the investigators found was a discrepancy in the preoperative manifest subjective refraction in the wavefront-measured cylinder. Currently, the laser setting for cylinder correction is equal to the magnitude of astigmatism measured by the wavefront sensor. For example, if a patient has –1.0 D of with-the-rule astigmatism and the wavefront measures about –0.5 D of astigmatism, the wavefront measurement is programmed into the laser treatment.

In another example of a patient with –1.0 D of with-the-rule astigmatism and a wavefront measurement of –1.5 D of astigmatism, an overcorrection would be expected in the vertical meridian, and therefore development of against-the-rule astigmatism postoperatively, Dr. Subbaram explained.

"Our clinical results show that the overcorrection exists, but the amount of axial rotation is about 49° and less than anticipated. We don't know the reason for this, but we do know that there is a significantly greater amount of rotation than in the example of the first patient," he said.

Among 175 patients treated using the customized Zyoptix platform (Bausch & Lomb), about 40% of patients had about 0.5 D of postoperative cylinder after LASIK; very few eyes, according to Dr. Subbaram, exceeded that level of cylinder.

An interesting finding is the degree of preoperative coma.

"In these eyes, the degree of preoperative coma was significantly greater compared with the eyes that did not develop significant cylinder postoperatively," he said. "Eyes with preoperative coma have a risk factor for the development of postoperative cylinder, which is the aberration interaction from the third-order coma to the second-order cylinder."

The investigators conducted an optical analysis to determine the optical relationship between coma and astigmatism. With good centration, he pointed out, there is no relationship between coma and cylinder. However, if there is constant reduction in aperture diameter where the aberrations are measured, there are higher amounts of vertical coma that translate as vertical tilt but no induced cylinder.

"With aperture decentration of different amounts, the coma is now interpreted as some degree of astigmatism that in turn also affects the defocus, which is essentially a spherical equivalent," he said.

When the investigators determined the optical relationship between coma and second-order cylinder, they revisited the clinical data to determine the risk factors.

"With a low discrepancy between the manifest and the wavefront cylinder values and low coma, there is low or no postoperative astigmatism. When there is a high discrepancy and high coma, the preoperative coma is the reason for the high discrepancy. When treated, the coma treatment also provides the extra cylinder correction and the patient ends up with low or no postoperative astigmatism," Dr. Subbaram said.