Structural factors mediating scleral stiffness

¹ Mechanical Engineering, University of California, Berkeley, Berkeley, California, United States; Orthopaedic Surgery, University of California, San Francisco, San Francisco, California, United States
² Orthopaedic Surgery, University of California, San Francisco, San Francisco, California, United States
³ Mechanical Engineering, University of California, Berkeley, Berkeley, California, United States
⁴ Ophthalmology, University of California, San Francisco, San Francisco, California, United States
⁵ Ophthalmology, University of California, San Francisco, 10 Koret Way, K301, San Francisco, California, 94143-0730, United States

^* To whom correspondence should be addressed. E-mail: stewartj{at}vision.ucsf.edu.

	Abstract

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 mm by 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, non-enzymatic crosslink density, elastin content, glycosaminoglycan content and water content. Results: 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 versus 5.96 +/- 2.69 kJ/m3 for porcine and human sclera respectively; p>0.05). Aged human sclera had relatively high florescence due to non-enzymatic crosslink density (2200 +/- 368 versus 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 has superior mechanical stiffness despite reduced collagen content partially because of the accumulation of non-enzymatic crosslinks. Differences in collagen content and crosslink density either had no effect or offsetting effects on the ability of the tissues to absorb strain energy.

Key Words: myopia, collagen, sclera, biomechanics, crosslinks

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