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A. Elsheikh, D. Wang, M. Brown, P. Rama, M. Campanelli, D. Pye; Variation of Corneal Biomechanical Properties With Age. Invest. Ophthalmol. Vis. Sci. 2007;48(13):4024.
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To determine the stress-strain behaviour of corneal tissue under loading and unloading conditions and how the behaviour was affected by age.
Thirty-six human corneal specimens ranging in age between 50 and 95 years were tested under inflation conditions to determine their stress-strain behaviour. The test rig enabled tight control of temperature, hydration and pressure application rate and this resulted in an improved consistency of results. Posterior pressures up to 55, 110 and 165 mmHg were applied in cycles of loading and unloading while monitoring corneal behaviour. The pressure-deformation results were analysed using shell theory to derive the stress-strain behaviour.
The corneas demonstrated clear nonlinear behaviour under loading, with an initial low stiffness stage and a final high stiffness stage. The transition between the two stages coincided with intraocular pressures between 12 and 20 mmHg. The behaviour fitted an exponential power function of the form: stress (MPa) = a [e(b.strain)-1], where a and b were (0.0070 and 50), (0.0077 and 59) and (0.0061 and 83) for corneas within age ranges 50-64, 65-79 and 80-95 years, respectively. During pressure unloading, the corneas demonstrated an initial high stiffness, reducing gradually with lower pressure levels. The average stress strain relationships during unloading were again exponential of the form: stress (MPa) = c [e(d.strain)-1], where c and d were (0.00004 and 359), (0.00005 and 427) and (0.00025 and 510) for corneas within the three age ranges considered, respectively. The results show a considerable age-related increase in stiffness under both loading and unloading conditions.
The cornea demonstrates considerable stiffening with age with the behaviour under both loading and unloading conditions closely fitting an exponential power function typical of collagenous tissue. These results may have significant implications for the performance of corneal surgery on older patients, but more particularly on the measurement of intraocular pressure in clinical practice.
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