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R Glenn Hepfer, Changcheng Shi, George O Waring, Hai Yao; The Effect of Corneal Collagen Crosslinking on Stromal Transport, Electrical, and Mechanical Properties. Invest. Ophthalmol. Vis. Sci. 2014;55(13):2989.
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© ARVO (1962-2015); The Authors (2016-present)
This study investigates the effect of crosslinking (CXL) on transport, electrical, and mechanical properties of the stroma. We hypothesize that regional changes in transport properties, observed with fluorescence recovery after photobleaching (FRAP), will be a sensitive measurement of crosslinking effect. We predict that conductivity and compression tests will detect changes in electrical and mechanical properties due to CXL.
Standard, epithelium-off corneal CXL was performed on fresh, intact porcine eyes while controls received riboflavin without irradiation. FRAP was performed at different stromal depths, and nasal-temporal (X) and superior-inferior (Y) diffusivities were calculated with a custom program. To evaluate the effect of crosslinking on diffusivity in the anterior-posterior (Z) direction, an electrical method was employed to calculate overall conductivity, dependent on small ion diffusivity. For confined compression tests, an additional untreated control group was added. Swelling pressure of buttons were measured in saline followed by a creep test to calculate hydraulic permeability and aggregate modulus using the biphasic theory. Dynamic tests measured the storage and loss modulus.
Normalized XY diffusivities of crosslinked and control porcine corneas were significantly increased (P=0.005, ANOVA) in the anterior cornea. This difference decreased as stromal depth increased (R=-0.87, Pearson’s). However, in the Z direction, conductivity decreased (P<0.0001, ANOVA) in porcine corneas, indicating a corresponding decrease in diffusivity in the Z-direction. Aggregate modulus increased with CXL while this increase was not statistically significant. Hydraulic permeability decreased and swelling pressure increased in both CXL and riboflavin groups when compared to untreated. Storage and loss modulus both increased in CXL when compared to riboflavin and untreated groups.
The effect of CXL is more pronounced in the anterior stroma. We present a non-destructive ex vivo method for quantifying and comparing crosslinking effect with respect to stromal depth. The observed anisotropic change in diffusivity has implications for the CXL mechanism. The changes detected by static confined compression tests were due to effects of the dextran in the riboflavin solution while those detected by dynamic tests were due to crosslinking.
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