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Ashutosh Richhariya, Virender S. Sangwan, Sunil Punjabi, Geunyoung Yoon; Quantification of Changes in Optical Properties of Cornea with Stress In Vitro. Invest. Ophthalmol. Vis. Sci. 2012;53(14):6897.
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© ARVO (1962-2015); The Authors (2016-present)
To quantify changes in stress distribution and optical properties of the cornea with respect to intraocular pressure (IOP) in vitro.
A polariscope equipped with a Shack-Hartmann wavefront sensor was developed for measuring stress and wavefront aberrations. To cancel out topographic changes a custom designed wet test cell was used to immerse the cornea in water. IOP levels were controlled by connecting the wet cell to an adjustable water column. While maintaining the orientation four porcine corneo-scleral rims were mounted in the test cell and were subjected to button inflation test. Polariscope and wavefront sensor images were recorded simultaneously at different IOP levels of 0, 10, 15 and 20 mmHg. These images were processed to quantify the maximum shear stress, direction of principal stress and Zernike coefficients. Two-dimensional normalized cross correlation was calculated between shear stress distribution and the measured wavefront aberration. The statistical significance of the change in average stress in nasal-temporal and inferior-superior directions, and Zernike coefficients was evaluated using the t-test.
Stress levels at each pressure step were significantly different from each other (p<0.001). The average stress along nasal-temporal and inferior-superior directions was significantly different (p<0.001). This difference was less in the central cornea and increased towards the limbus region. Magnitudes of Zernike coefficients were decreased with increasing IOP and there was statistically significant difference in diagonal astigmatism, defocus, vertical coma and diagonal secondary astigmatism between 0, 15 and 20 mmHg. The average correlation coefficient between the stress magnitude distribution and corresponding wavefront pattern was 0.12, 0.32, 0.31 and 0.35 at 0, 10, 15 and 20 mmHg, respectively.
The results suggest that changes in the magnitude of stress distribution due to IOP have a significant impact on the optical quality of the cornea. The ability to quantify two-dimensional stress and the aberrations of the cornea simultaneously can improve our understanding of sources of postoperative penetrating keratoplasty astigmatism.
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