Exogenous collagen cross-linking reduces scleral permeability: modeling the effects of age-related cross-link accumulation.

PURPOSE: To investigate the relationship between scleral permeability and nonenzymatic cross-link density. METHODS: Scleral discs 18 mm in diameter were dissected from the medial and lateral equatorial regions of 60 cadaveric porcine eyes. Samples were incubated for 24 hours with control solution or methylglyoxal at concentrations of 0.001%, 0.01%, 0.10%, and 1.00%. Nonenzymatic cross-link density in treated and control groups was quantified with the use of papain digest and fluorescence spectrophotometry. Treated scleral discs were mounted in a customized Ussing-type chamber connected to vertical tubing, and specific hydraulic conductivity was determined according to the descent of a column of degassed saline at room temperature. Permeability to diffusion of fluorescein in a static chamber was determined for another set of treated scleral samples. RESULTS: Methylglyoxal treatment effectively increased nonenzymatic cross-link content, as indicated by the average fluorescence for each group. Specific hydraulic conductivity (m(2)) was reduced with increasing cross-link density. Similarly, the permeability coefficient for the fluorescein solute consistently decreased with increasing methylglyoxal concentration, indicating diffusion impedance from the treatment. CONCLUSIONS: Nonenzymatic cross-link density can be significantly increased by treatment with methylglyoxal. Porcine sclera showed a nonlinear reduction in solute permeability and specific hydraulic conductivity with increasing cross-link density. This model indicates that age-related nonenzymatic cross-link accumulation can have a substantial impact on scleral permeability.

Collagen cross-links reduce corneal permeability.

PURPOSE: To investigate the relationship between corneal permeability and nonenzymatic cross-link density. METHODS: Corneas were dissected from 90 cadaveric porcine eyes. Samples were incubated for 24 hours with control solution or methylglyoxal at concentrations of 0.01%, 0.10%, and 1.00%. Nonenzymatic cross-link density in treated and control groups was quantified by papain digest and fluorescence spectrophotometry. Control and treated corneas were mounted in a customized Ussing-type chamber connected to vertical tubing, and specific hydraulic conductivity was determined according to the descent of a column of degassed saline at room temperature. Permeability to diffusion of fluorescein in a static chamber was determined for a similar set of corneal samples. RESULTS: Methylglyoxal treatment effectively increased nonenzymatic cross-link content, as indicated by the average fluorescence for each group. Specific hydraulic conductivity (m(2)) was reduced with increasing cross-link density. Similarly, the permeability coefficient for the fluorescein solute consistently decreased with increasing methylglyoxal concentration, indicating diffusion impedance resulting from the treatment. CONCLUSIONS: Nonenzymatic cross-link density in the cornea can be significantly increased by treatment with methylglyoxal. Porcine cornea showed a nonlinear reduction in solute permeability and specific hydraulic conductivity with increasing cross-link density. This model suggests that age-related nonenzymatic cross-link accumulation can have a substantial impact on corneal permeability.

Structural factors that mediate scleral stiffness.

PURPOSE: The intent of this study was to correlate measures of structurally relevant biochemical constituents with tensile mechanical behavior in porcine and human posterior sclera. METHODS: Posterior scleral strips 6 x 25 mm were harvested from 13 young porcine and 10 aged human eyes and stored frozen at -20 degrees C. Mechanical hysteresis from 10 consecutive load cycles to a peak stress of 1.0 MPa was recorded via a custom-built soft tissue tester. In a parallel study, tissue adjacent to the mechanical test specimens was apportioned for each of five assays measuring: total collagen content, nonenzymatic cross-link density, elastin content, glycosaminoglycan content, and water content. RESULTS: The average porcine scleral modulus at 1% strain was 75% less than that measured for human tissue (0.65 +/- 0.53 MPa versus 2.60 +/- 2.13 MPa, respectively; P < 0.05). However, the average strain energy absorbed per loading cycle was similar (6.09 +/- 2.54 kJ/m(3) vs. 5.96 +/- 2.69 kJ/m(3) for porcine and human sclera respectively; P > 0.05). Aged human sclera had relatively high fluorescence due to nonenzymatic cross-link density (2200 +/- 368 vs. 842 +/- 342; P < 0.05) and low hydroxyproline content (0.79 +/- 0.17 microL/mL/g versus 1.21 +/- 0.09 microL/mL/g; P < 0.05) while other measured biochemical factors were statistically similar (P > 0.05). CONCLUSIONS: Aged human tissue had superior mechanical stiffness despite reduced collagen content, partially because of the accumulation of nonenzymatic cross-links. Differences in collagen content and cross-link density either had no effect or offsetting effects on the ability of the tissues to absorb strain energy.