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Riccardo Vinciguerra, Cynthia Roberts, Ashraf Mahmoud, Claudio Azzolini, Paolo Vinciguerra; Corneal geometric stress factor to evaluate response to corneal collagen cross-linking in keratoconus. Invest. Ophthalmol. Vis. Sci. 2013;54(15):1621. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To evaluate corneal stress distribution based purely on geometry without consideration of loading via intraocular pressure, pre and post corneal collagen cross-linking (CXL) with a new tomographic parameter, “Corneal Geometric Stress Factor”.
Tomographic data from four hundred and eighty subjects (323 right eyes and 340 left eyes) were collected retrospectively from Istituto Clinico Humanitas (Rozzano, Italy) with up to 70 months pre-CXL and 60 months post-CXL. Pentacam U12 files (Oculus Optikgerate GmbH, Wetzlar, Germany) were transferred to The Ohio State University and processed independently using custom software. Corneal Geometric Stress Factor (CGSF) was calculated at corresponding points from curvature and pachymetric maps to create a CGSF map. CGSF evaluates the cornea’s contribution to Hoop stress without considering the applied load (intraocular pressure) and it can be expressed as the radius of curvature over twice of the thickness (CGSF=R/2t). Cone Location and Magnitude Index (CLMI) and Flat zone Location and Magnitude Index (FLMI) were applied to the CGSF map to obtain maximum stress and minimum stress and to calculate the level of asymmetry in the stress pattern. Pre and post CXL regression analyses were performed.
Regression analysis showed a significant (p<0.0001) positive correlation of asymmetry stress distribution before CXL demonstrating increasing asymmetry in the stress pattern, and a significant negative correlation after CXL (p=0.0001) demonstrating a pattern of reducing asymmetry over time. Maximum and minimum stress factors similarly had positive correlation before CXL (p<0.0001), indicating increasing stress over time and negative correlation after CXL (p<0.0001), indicating decreasing stress over time.
Biomechanical analysis shows CXL is able not only to stop the progression of the disease, but even to reverse the cycle of biomechanical decompensation of keratoconic corneas. These findings can be explained by the fact that the decrease of thickness and curvature associated with CXL treatment induces a consequent reduction in both minimum and maximum stress as well as a reduction of asymmetry in the stress distribution. IOP can modify the stress magnitude, but not change the pattern demonstrated.
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