Cornea’s biomechanical response affects result of LASIK

PARIS — The biomechanical response of the cornea to tissue ablation affects the refractive outcome of LASIK perhaps more than most surgeons realize, according to a physicist speaking here. She said nomograms that take into account the cornea’s biomechanical changes after ablation may help to achieve more satisfying results from LASIK.

“We think we are removing a lenticule from the cornea during laser ablation, but actually we are changing the structure of the cornea, and its shape changes as a result of that,” said Cynthia Roberts, PhD. She spoke here during a clinical research symposium on night vision problems after refractive surgery at the European Society of Cataract and Refractive Surgeons meeting.

Dr. Roberts noted that excimer lasers are not programmed to achieve a certain visual acuity result; they are programmed to change the shape of the cornea. Correcting the shape of the central cornea to correct spherical and cylindrical error is relatively straightforward, she said, but when correction of higher-order aberrations is the target, the critical area becomes larger.

“Problems arise when the pupil expands,” Dr. Roberts said. She said with myopic corrections, spherical aberration increases toward the periphery of the cornea.

“With increasing amounts of correction, there is increasing corneal steeping outside the central optical zone, leading to spherical aberration,” she said.

The biomechanical response of the cornea to excimer laser ablation modifies the clinical result, she said, but it is possible to use “biomechanical-friendly” algorithms to take this change into account.

During discussion after her presentation, Dr. Roberts said the WaveLight algorithm, which “addresses corneal spherical aberration rather than total optical spherical aberration,” is an example of a more biomechanical-friendly approach.