PURPOSE: We developed a system for analyzing corneal pachymetric and keratometric topography simultaneously, and encompassing the 6 mm PRK treatment zone. METHOD: The system employed a broad-band ultrasound probe (10-70 MHz) mounted via a stack of five DC precision-motors to a vibration isolating support structure. Three rectilinear motor stages controlled horizontal (x,y) and vertical (z) positioning with 1 micron precision. Two spherical arc cradles controlled transduce angulation with 0.005 deg. precision. Standard immersion scanning technique with vertical alignment of the visual axis by a fixation target was used. Each scan set consisted of four hemi-meridional arc scans at 45 deg. intervals. Echo signals were digitized and stored. I-scan digital analysis (± 1 micron (SD) point localization) produced a 3-D coordinate matrix describing the epithelial, stromal and endothelial surfaces. Software we developed was used to produce color maps depicting (1) surface height, (2) topographic pachymetry and (3) local instantaneous radius of curvature for epithelial and stromal layers individually. Normal, pre- and post-PRK corneal examples were scanned. RESULTS: The optimal scanning arc radius for maintaining the full cornea within the focal range of the transducer was 8 mm. A 30-deg. arc allowed imaging of the 4.5 mm central region of the cornea, while a 55 deg.s allowed 8.3 mm and 60 degees allowed 11 mm. In normals, epithelial an I stromal maps depicted progressive thickening and flattening centrifugally. Epithelial paachymetric micro-irregularity (2.8 microns) was evident throughout, while stromal pachymetric profiles showed a smoother concentric regularity. PRK treated cornea showed increased stromal surface irregularity within the treatment zone. Regional irregularities in the local radius of the corneal surface resulted from both epithelial and/or stromal thickness profile variations. Regions showing a more regular local radius could be seen when stromal surface irregularity was compensated for by epithelium. CONCLUSION: This system will provide a unique method of studying physiological gradient optics of the normal cornea, and provide an analytical tool for evaluating and monitoring the functional anatomy of the healing cornea after PRK.
|Original language||English (US)|
|Journal||Investigative Ophthalmology and Visual Science|
|State||Published - Feb 15 1996|
ASJC Scopus subject areas
- Sensory Systems
- Cellular and Molecular Neuroscience