Structural Factors That Mediate Scleral Stiffness

Department of Mechanical Engineering, University of California, Berkeley, Berkeley, California 94143-0730, USA.
Investigative ophthalmology & visual science (Impact Factor: 3.4). 07/2008; 49(10):4232-6. DOI: 10.1167/iovs.08-1970
Source: PubMed


The intent of this study was to correlate measures of structurally relevant biochemical constituents with tensile mechanical behavior in porcine and human posterior sclera.
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.
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).
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.

Download full-text


Available from: David S Schultz, Jun 01, 2015
  • Source
    • "Decreases in aggrecan concentration would significantly reduce the glycosaminoglycan concentration in the posterior sclera and may lead to increased scleral rigidity, which has been associated with hyperopia and high myopia (Friedman, 1994). Additionally, with increasing age, the sclera increases in stiffness, due to the accumulation of nonenzymatic glycation-type cross-links of collagen fibrils with age (Coudrillier et al., 2012; Schultz et al., 2008). This age-related increase in stiffness is greatest in the anterior sclera, followed by the equatorial and then posterior sclera (Geraghty et al., 2012). "
    [Show abstract] [Hide abstract]
    ABSTRACT: Myopia is a common ocular condition, characterized by excessive elongation of the ocular globe. The prevalence of myopia continues to increase, particularly among highly educated groups, now exceeding 80% in some groups. In parallel with the increased prevalence of myopia, are increases in associated blinding ocular conditions including glaucoma, retinal detachment and macular degeneration, making myopia a significant global health concern. The elongation of the eye is closely related to the biomechanical properties of the sclera, which in turn are largely dependent on the composition of the scleral extracellular matrix. Therefore an understanding of the cellular and extracellular events involved in the regulation of scleral growth and remodeling during childhood and young adulthood will provide future avenues for the treatment of myopia and its associated ocular complications. Copyright © 2014 Elsevier Ltd. All rights reserved.
    Full-text · Article · Apr 2015 · Experimental Eye Research
  • Source
    • "Aging causes physiological, functional and structural changes in the human eye. It is manifested in variations in elasticity of the eye tissues (cornea and sclera) [1]−[6], vascular ocular hemodynamics [7]−[9] biometric parameters of the eye globe (e.g., smaller anterior chamber volume) [10], decreased aqueous production [11], as well as an increased latency of saccadic eye movements [12]. Majority of these factors are linked to the ocular rigidity phenomenon, which was reported to increase with age [13]. "
    [Show abstract] [Hide abstract]
    ABSTRACT: Purpose To ascertain whether the incidence of ocular dicrotic pulse (ODP) increases with age, it is more pronounced in glaucomatous than in normal eyes and whether it is related to cardiovascular activity. Methods 261 subjects aged 47 to 78 years were included in the study and classified into four groups: primary open angle glaucoma (POAG), primary angle-closure glaucoma (PACG), glaucoma suspects with glaucomatous optic disc appearance (GODA) and the controls (CG). Additionally, in each group, subjects with ODP were divided into two age subgroups around the median age. A non-contact ultrasonic method was used to measure corneal indentation pulse (CIP) synchronically with the acquisition of electrocardiography (ECG) and blood pulse signals. ODP was assessed from the acquired signals that were numerically processed in a custom written program. Results ODP incidence was about 78%, 66%, 66% and 84% for CG, GODA, POAG, and PACG group, respectively. With advancing age, the ODP incidence increased for all subjects (Δ = 12%), the highest being for the PACG and POAG groups (Δ = 30%). GODA group did not show an age-related increase in the incidence of ODP. Conclusions The ocular dicrotism, measured with non-contact ultrasonic method, was found to be a common phenomenon in elderly subjects. The increased ODP incidence in PACG and POAG group may correspond to either higher stiffness of glaucoma eyes, biochemical abnormalities in eye tissues, changes in ocular hemodynamics, may reflect the effect of medications or be a combination of all those factors. The results of GODA group suggest different mechanisms governing their ocular pulse that makes them less susceptible to generating ODP and having decreased predisposition to glaucoma.
    Full-text · Article · Jul 2014 · PLoS ONE
  • Source
    • "It is thus an important goal to characterize the mechanical properties of sclera in order to better understand their involvement in the pathogenesis of glaucoma. Uniaxial mechanical tests (Downs et al., 2005; Elsheikh, 2010; Girard et al., 2007; Palko et al., 2011; Schultz et al., 2008) and inflation tests (Coudrillier et al., 2012; Fazio et al., 2012; Girard et al., 2009a; Tang and Liu, 2012) have been used to study the mechanical responses of the sclera. Biaxial mechanical testing has been used to mimic inflation tests (Gere, 2004; Sacks, 2000) by loading a planar square specimen along two orthogonal axes to characterize the mechanical properties of an anisotropic tissue. "
    [Show abstract] [Hide abstract]
    ABSTRACT: This study aimed to characterize the mechanical responses of the sclera, the white outer coat of the eye, under equal-biaxial loading with unrestricted shear. An ultrasound speckle tracking technique was used to measure tissue deformation through sample thickness, expanding the capabilities of surface strain techniques. Eight porcine scleral samples were tested within 72h postmortem. High resolution ultrasound scans of scleral cross-sections along the two loading axes were acquired at 25 consecutive biaxial load levels. An additional repeat of the biaxial loading cycle was performed to measure a third normal strain emulating a strain gage rosette for calculating the in-plane shear. The repeatability of the strain measurements during identical biaxial ramps was evaluated. A correlation-based ultrasound speckle tracking algorithm was used to compute the displacement field and determine the distributive strains in the sample cross-sections. A Fung type constitutive model including a shear term was used to determine the material constants of each individual specimen by fitting the model parameters to the experimental stress-strain data. A non-linear stress-strain response was observed in all samples. The meridian direction had significantly larger strains than that of the circumferential direction during equal-biaxial loadings (P's<0.05). The stiffness along the two directions was also significantly different (P=0.02) but highly correlated (R(2)=0.8). These results showed that the mechanical properties of the porcine sclera were nonlinear and anisotropic under biaxial loading. This work has also demonstrated the feasibility of using ultrasound speckle tracking for strain measurements during mechanical testing.
    Full-text · Article · Dec 2013 · Journal of Biomechanics
Show more