The effect of tear film on higher order corrections applied to the corneal surface during wavefront-guided refractive surgery.

PURPOSE: To investigate whether the property of the surface of a fluid film to smooth irregularities in the underlying surface will cause the tear film to partially erase the effect of higher order corrections ablated onto a corneal surface during wavefront-guided refractive surgical correction.

METHODS: Coating flow theory shows that the film surface initially replicates the underlying surface. An iteration method is given to allow calculations to be made of the tear fluid flow and resulting surface change during the time between blinks. A new experimental technique is introduced using a corneal topographer and live video imaging to observe changes in tear surface curvature as the tear film flows.

RESULTS: Tear flow changes in an eye with a high amount of aberration due to an unsuccessful LASIK procedure are simulated to show that while tear fluid occurs and causes changes in the tear/air surface, the change when expressed in terms of unwanted aberrations is below that which would cause any image degradation. Results of the live video technique on a cornea with an irregular surface 4 days after photorefractive keratectomy show that irregularities in the tear film surface immediately after a blink are not smoothed to any extent in the time before the next blink.

CONCLUSIONS: Tear film flow following initial film formation by a blink can be demonstrated by computer simulation and observed with live corneal topography video. Tear film dynamics will not erase the effects of ablative corrections for higher order aberrations.