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N. E. Knox Cartwright, J. R. Tyrer, J. Marshall; Change in Corneal Stiffness With Age and Following Cross-Linking. Invest. Ophthalmol. Vis. Sci. 2009;50(13):5474.
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The biomechanical properties of the cornea affect measurement of intraocular pressure, determine keratectasia susceptibility and influence surgical outcomes. Using riboflavin-ultraviolet A (R-UVA) corneal cross linking (CXL) these properties can be modified clinically; other agents such as glutaraldehyde can be used in the laboratory. Very few studies have attempted to determine the normal range of biomechanical variation in the population of the modifying effect of surgical intervention and those that have have mostly used nonphysiological techniques. The purpose of this study was to quantify the change in human biomechanical properties that occur with age and following CXL treatment.
Strain and apical displacement change in 50 human corneas was measured using a radial-shearing speckle pattern interferometer following increase in intraocular pressure from 15.0 to 15.5 mmHg. Measurements were taken from a further 9 corneas before and after epithelial debridement combined with either no, R-UVA or 10% glutaraldehyde treatment. Donors aged between 24 and 102 years. Corneal Young’s modulus was calculated by mathematical back analysis.
Between ages of 25 and 100 corneal stiffness increased linearly from 0.29 to 0.52 MPa (R2=0.74). R-UVA treatment increased corneal Young’s modulus by 430% (p<0.05) and glutaraldehyde CXL by 730 % (p<0.05). If it were valid to extrapolate the linear relationship between stiffness and age indefinitely, this was equivalent to ageing the cornea to nearly 600 years with R-UVA and just over 1000 years with glutaraldehyde CXL.
The mechanical changes characterised in this study could account for the halt of keratoconus progression with age and the arrest of ectasia development following R-UVA treatment. The changes following CXL are so substantial that they raise concern that CXL treatment could have as yet unforeseen consequences.
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