Abstract
Purpose: :
The surface of the pre–corneal tear film is the most powerful refractive surface in the eye. If the thickness of the tear film is essentially constant, measurement of this surface is equal to measurement of the anterior corneal surface. However, if the anterior corneal surface is irregular, the tear film may fill in and partially or fully erase the irregularities. This study analyzed a corneal surface known to be highly irregular to determine whether the flow of the tear film following a blink could be detected in changes in the mire pattern of corneal topography; the time needed for the tear film to stabilize following a blink as a function of time since the PRK; and whether the irregularities seen just after a blink are subsequently erased by tear flow.
Methods: :
Live video of the photokeratoscope mire pattern of an eye was captured on days 4, 5, 8 and 11 following PRK refractive surgery. The mire patterns were analyzed to measure the amount of change and the time taken by the tear film to stabilize, and to estimate change in the measured anterior corneal surface caused by tear flow.
Results: :
Subtle, short term changes in the pre–corneal tear film surface were readily detected in a frame–by–frame analysis of the video. The greatest amount of flow–change occurred in the first 1 to 2 seconds after the upper lid fully opened following a blink. Small pattern changes occurred after the first 2 seconds, but mire irregularities were in general not erased. The tear film instability reduced quickly so that post–blink changes resolved rapidly on day 8 and were barely noticeable on day 11.
Conclusions: :
Analysis of live video of photokeratoscope mire patterns is effective for the analysis of subtle, short term changes in the pre–corneal tear film surface. Although some localized irregularities in the anterior corneal surface are somewhat smoothed by the flow of tear film, the basic underlying anterior corneal surface is well represented by a measurement of the pre–corneal tear film surface.
Keywords: refractive surgery: PRK • cornea: epithelium • cornea: tears/tear film/dry eye