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W.J. Dupps, M.V. Netto, R.R. Krueger, S. Herekar; Surface Wave Elastometry of the Cornea: Effects of Epithelial Debridement and Central Keratotomy . Invest. Ophthalmol. Vis. Sci. 2005;46(13):2758.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To determine the effect of the epithelium and keratotomy depth on corneal elastometry using a prototype surface wave elastometer in porcine tissue. Methods: The elastometer (Sonic Eye, PriaVision, Menlo Park CA) consists of a handheld probe with a resonant element and receiver spaced 4.5 mm apart. The system reports real–time ultrasonic time–of–flight data, an indicator of corneal stiffness, in units of meters per second. Intravitreal pressure was maintained at 15 mmHg by direct infusion for experiments in 7 porcine globes. Ten corneal regions of interest (ROI) were defined, incorporating central vertical and horizontal measurements and 8 additional vectors in the corneal periphery. Replicate measurements at each ROI were performed serially 1) on dry epithelium, 2) after debridement with a #64 blade, 3) after creation of a 250–um–deep, 3–mm long vertical incision with a diamond keratotomy knife, and 4) after extension of the incision depth to 750 um. Differences in velocity were compared by paired students t–tests. Results: Across all regions of interest, mean velocities ranged from 116 to 153 m/s with a standard deviation between eyes of 40 m/s. Intra–measurement repeatability averaged 3 m/s and 4 m/s in intact and debrided corneas, respectively (sd). Epithelial debridement produced variable changes in velocity that were not statistically significant (p > .05 in all regions). The standard deviation of elastometer measurements decreased to 25 m/s after epithelial debridement. Horizontal measurements across the incision decreased from a baseline of 130 ± 33 m/s to 79 ± 17 m/s (p = .001) and 66 ± 31 m/s (p = .24) with 250 um and 750 um incisions, respectively; no significant change in velocity occurred in the vertical meridian overlying the incision axis (p =.72 and .95) Conclusions: In preliminary experiments in a porcine model, the epithelium did not contribute systematically to wave velocity measurements but did contribute to measurement variability between eyes. Central keratotomy reduced wave velocity only across severed lamellae and did not affect wave speed parallel to the incision (where lamellar disruption is minimal). Tripling of the incision depth did not significantly reduce wave velocity, suggesting that the velocity is predominantly a function of anterior corneal properties. A more sensitive analysis of sampling depth and regional elastometric differences is underway. Ultrasonic corneal elastometry may provide a tool for measuring constitutive biomechanical properties of the cornea and investigating correlations to surgical response.
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