Second-harmonic signaling used to study corneal structure, function

Second-harmonic signaling with the femtosecond laser (IntraLase, Advanced Medical Optics) can be used to probe corneal structure and function. In combination with laser-induced optical breakdown, it also can help create three-dimensional measures of elasticity as well as provide details on the biomechanical properties of the eye, according to James V. Jester, PhD, professor of ophthalmology, University of California, Irvine.

Various groups have studied second-harmonic signaling with the femtosecond laser, mostly in animal models, he said. Studies of this technology in humans have begun to appear, however, including two recent papers that looked at keratoconus.

Both back- and forward-scattered views of the collagen structure can be obtained, although the back-scattered mode is more clinically significant, Dr. Jester said. He and others have examined sutural collagen lamellae and have found that short filaments extend deeply into the anterior portion of the cornea. In a study in six normal patients, investigators found a high amount of sutural lamellae within the cornea. In all but one of 13 eyes with keratoconus, however, the sutural lamellae did not extend and insert into the Bowman's membrane as in the eyes of normal patients; one eye had no sutural lamellae.

"We think that the lamella are lost, either developmentally or prior to progress of the disease, and this may alter the biomechanical properties of the anterior part of the cornea, which leads to the development of keratoconus," Dr. Jester said.

Femtosecond laser second-harmonic signaling also is being used to investigate corneal fibrosis and evaluate the optic nerve head and lamina cribrosa.