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L. O. Lee, A. M. Mahmoud, D. Z. Reinstein, T. J. Archer, C. J. Roberts; Corneal Biomechanics of Hyperopia and Myopia. Invest. Ophthalmol. Vis. Sci. 2009;50(13):1754.
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The purpose of this study was to compare the differences in corneal biomechanical response between hyperopic and myopic eyes using the Reichert Ocular Response Analyzer (ORA).
The medical records of one thousand sixty-six eyes from 842 myopic patients and eight hundred five eyes from 438 hyperopic patients seeking refractive surgery at the London Vision Clinic between June 2006 and May 2008 were retrospectively reviewed. The pre-op data in the hyperopic and myopic groups were matched based on age (±2 years) and pachymetry (±20µm) on a 1-to-1 basis, resulting in enrollment of nine hundred sixty-six eyes with four hundred eighty-three hyperopic eyes from 274 subjects and four hundred eighty-three myopic eyes from 275 subjects. The ORA had been utilized to measure corneal hysteresis (CH) and corneal compensated intraocular pressure (IOPcc). Both the infrared and pressure signals were subsequently analyzed with custom software to extract multiple signal parameters, including the amplitude of the first infrared peak, Peak 1, and it corresponding full width half max (FWHM1).
CH, IOPcc, Peak1, and FWHM1 were all significantly different between hyperopes and myopes. CH was significantly higher (p<0.0001) in hyperopes (11.07 ± 1.60 mmHg) than in myopes (10.47 ± 1.64 mmHg), indicating that hyperopic eyes can dissipate more energy than myopic eyes. IOPcc was significantly lower (p<0.0001) in hyperopes (15.05 ± 3.51 mmHg) than in myopes (16.50 ± 3.31 mmHg), reflecting lower intraocular pressure in hyperopic eyes. Peak 1 was significantly greater (p=0.008) in hyperopes (832.87 ± 89.24 mmHg) than in myopes (816.96 ± 95.85 mmHg), suggesting a stiffer cornea in hyperopes. FWHM1 was significantly wider (p<0.0001) in hyperopes (12.15 ± 2.33 ms) than in myopes (11.53 ± 2.14 ms), showing that hyperopic corneas deformed at a slower rate of speed.
Differences in corneal biomechanical response to an air puff between hyperopic and myopic eyes have been demonstrated. Hyperopic eyes have greater intraocular pressure, are stiffer, and deform more slowly than myopic eyes. This may indicate that the development of hyperopia and myopia may be influenced by corneal biomechanics, with the stiffer hyperopic eyes resisting biomechanical deformation over time, and the more compliant myopic eyes having greater susceptibility to expansion.
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