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Ronald H. Silverman, Raksha Urs, Harriet O. Lloyd; Effect of Corneal Excision on Speed-of-Sound. Invest. Ophthalmol. Vis. Sci. 2012;53(14):3625.
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A speed-of-sound constant, c, of 1636-1640 m/s has long been used to calculate corneal thickness in ultrasound pachymetry. In contrast, most laboratory studies have reported c-values well below 1600 m/s. These studies were largely based on excised ex vivo tissue due to the requirement of placing a reflective planar surface behind the cornea for this measurement. We developed a technique for corneal c determination in whole globes, and our aim was to determine if excision affected the measurement.
An 18 gauge needle was inserted across the anterior chamber of nine ex vivo pig eyes parallel and just anterior to the iris plane with the needle extended beyond the globe at both ends. The eyes were submerged in normal saline such that the needle was normal to the beam axis of a 20 MHz focused ultrasound transducer. We scanned the eye in a plane parallel to the needle, digitizing echo data at a 400 MHz sample rate (12 bits/sample). We interpolated the expected needle echo position behind the cornea from the needle echoes obtained on either side of the globe, where there was no intervening cornea. c was then determined from the temporal shift of the echoes behind the cornea and the delays to each corneal surface. We then excised the cornea retaining 1 mm of sclera and measured range to a glass plate with and without intervening cornea present, from which c was determined. This was repeated after 1 hour in normal saline. All experiments were conducted at room temperature.
For intact corneas, c averaged 1543±11.4 m/s. Corneas averaged 1.35±0.15 mm in thickness. Immediately and one-hour after excision the respective velocities were 1545±8.9 and 1542±5.6 m/s and thicknesses were 1.44±0.19 and 1.48±0.17 mm. Paired t-tests did not demonstrate a statistically significant change associated with excision.
These findings do not support the supposition that the process of excision alone would account for the discrepancy between laboratory and clinical corneal c-values. The eyes used in this experiment were processed a day after enucleation, and corneal thicknesses appear indicative of swelling, suggesting that possible tissue changes occurring over this time period might complicate interpretation of results. Application of this technique to eyes immediately following enucleation will be required to verify these findings.
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