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A. Elsheikh, B. Geraghty, D. Alhasso, P. Rama; Regional Biomechanical Behavior of the Human Sclera and Its Variation With Age. Invest. Ophthalmol. Vis. Sci. 2009;50(13):3948.
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Experimental tools were developed to determine the stress-strain behavior of the anterior, equatorial and posterior regions of the human sclera and how the behavior varied with age.
Specimens from thirty-six human scleras ranging in age between 6 and 96 years (62.3±19.4) were tested under uniaxial tension to determine their stress-strain behavior. Each sclera produced five 4mm-wide strip specimens including two in the anterior region, two in the equatorial region and one in the posterior region. The thickness in the three regions was 653±107, 701±96 and 1056±44 microns, respectively. The specimens were subjected to four cycles of uniaxial tension up to a max stress of 3 MPa and with a strain rate of 1% per minute. The load-deformation results from the last loading cycle were analyzed to derive the stress-strain behavior.
The scleras demonstrated clear nonlinear behavior under loading, with an initial low stiffness and a final high stiffness. The transition between the two stages coincided with stresses (0.05-0.25 MPa) equivalent to those expected under intraocular pressures of 35-175 mmHg. Up to 0.25 MPa stress, the behavior patterns in the three scleral regions were similar and fitted the stress-strain equation: stress (MPa) = 0.20 [e^(28.5 strain) -1]. The similarity in behavior continued up to 2 MPa stress, beyond which the stiffness in posterior then equatorial specimens began to degrade (P<0.01). There was also a slow increase in Young’s modulus (a stiffness measure) with age of the order of 0.04 MPa per year, but this trend was not significant (P=0.10).
The human sclera demonstrates slow stiffening with age with the behavior closely fitting an exponential power function typical of collagenous tissue. Although scleral thickness varies between the anterior, equatorial and posterior regions, all three regions exhibit almost the same stress-strain behavior. The results improve our understanding of scleral biomechanics and provide scleral material models suitable for use in ocular numerical simulations.
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