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Structural Factors That Mediate Scleral Stiffness

¹From the Department of Mechanical Engineering, University of California, Berkeley, Berkeley, California; and the ²Departments of Orthopaedic Surgery and ³Ophthalmology, University of California, San Francisco, San Francisco, California.

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°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/m³ vs. 5.96 ± 2.69 kJ/m³ 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 µL/mL/g versus 1.21 ± 0.09 µL/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.

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