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J. Liu, X. He, K. Richdale, M. Bullimore, K. Zadnik; In vivo Measurements of Corneal Speed of Sound and Stiffness. Invest. Ophthalmol. Vis. Sci. 2008;49(13):647. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Commercial ultrasound pachymetry assumes a single speed of sound (1640 m/s) for the cornea. We have developed an ultrasonic system that is capable of non-invasive characterization of corneal biomechanical properties in vivo. The goal of this study is to investigate corneal stiffness and the speed of sound in the cornea in normal subjects, and the potential influence of these variables on the accuracy of ultrasound pachymeter.
The study was approved by the Institutional Review Board at the Ohio State University. Twenty-five normal volunteers (16 female and 9 male) were recruited. The mean age was 36 ± 10 years (mean ± SD; range, 23-56 years). The subjects were measured in a supine position with both corneas anesthetized by the application of one drop of topical proparacaine. An eye cup filled with buffered saline was placed on the eye as in A-scan ultrasound. An ultrasound transducer (10 MHz, XMS, Olympus-NDT) was placed in the saline at 1 cm from the cornea. The ultrasound system consisted of a Pulser/Receiver (5900PR, Olympus-NDT), a digitizer (Acqiris, DP105) and a PC. A linear elastic wave propagation model was used to characterize the corneal biomechanical properties. The reflection spectral curves predicted by the model were fitted to the experimental measured ultrasonic spectra using the Levernberg-Marquardt nonlinear, least squares algorithm, which uniquely estimates corneal thickness, density, and stiffness. The corneal speed of sound was calculated using the established formulas: speed-of-sound = sqrt(stiffness/density). The potential errors in ultrasound pachymetry were estimated by comparing the corneal thickness based on our measured speed of sound and the assumed speed of sound used in clinical pachymetry.
The mean (± SD) speed of sound was 1636 ± 49 m/s (range: 1476-1718 m/s). The mean (± SD) corneal stiffness was 3.14 ± 0.21 GPa (range: 2.44-3.40 GPa). There was a strong correlation between corneal stiffness and speed of sound (R^2 = 0.77). For a cornea with an actual thickness of 530 micron, the lower and upper bounds of the corneal speed of sound found in this study corresponded to a 59-micron overestimation or a 24-micron underestimation of the corneal thickness in ultrasonic pachymetry readings.
The mean value of the corneal speed of sound was consistent with those reported in the literature. Importantly, we found a large range of corneal speed of sound, which was highly correlated with corneal stiffness in normal subjects. Ultrasound pachymetry may significantly overestimate or underestimate corneal thickness owing to the assumptions about the speed of sound in the cornea.
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