Scleral mechanics: Comparing whole globe inflation and uniaxial testing

Abstract

The purpose of this study was to assess fundamental differences between the mechanics of the posterior sclera in paired eyes using uniaxial and whole globe inflation testing, with an emphasis on the relationship between testing conditions and observed tissue behavior. Twenty porcine eyes, consisting of matched pairs from 10 pigs, were used in this study. Within pairs, one eye was tested with 10 cycles of globe pressurization to 150 mmHg (~10x normal IOP) while biaxial strains were tracked via an optical system at the posterior sclera. An excised posterior strip from the second eye was subjected to traditional uniaxial testing in which mechanical hysteresis was recorded from 10 cycles to a peak stress of 0.13 MPa (roughly equivalent to the circumferential wall stress produced by an IOP of 150 mmHg under the thin-walled pressure vessel assumption). For approximately equivalent loads, peak strains were more than twice as high in uniaxial tests than in inflation tests. Different trends in the load-deformation plots were seen between the tests, including an extended “toe” region in the uniaxial test, a generally steeper curve in the inflation tests, and reduced variability in the inflation tests. The unique opportunity of being able to mechanically load a whole globe under near physiologic conditions alongside a standard uniaxially tested specimen reveals the effects of testing artifacts relevant to most uniaxially tested soft tissues. Whole globe inflation offers testing conditions that significantly alter load-deformation behavior relative to uniaxial testing; consequently, laboratory studies of interventions or conditions that alter scleral mechanics may greatly benefit from these findings.

Numerical Simulation and Experimental Validation of the Inflation Test of Latex Balloons

“…D and D are the initial and instantaneous diameters of the eyeball at the sclera near the optic nerve zone, respectively. As a qualitative confirmation of the predictions, the experimental data reported by Lari et al(2012) during the inflation test of pig eyes is also plotted. The stress-strain computed curves are shown in Figure 16a together with the results reported by Lari et al (2012) for comparison in Figure 16b.…”

“…As a qualitative confirmation of the predictions, the experimental data reported by Lari et al(2012) during the inflation test of pig eyes is also plotted. The stress-strain computed curves are shown in Figure 16a together with the results reported by Lari et al (2012) for comparison in Figure 16b. Similar trends can be appreciated where in Figure 16a it is seen that the sclera presents a lower degree of deformation (λ 1.34) with respect to the cornea (λ 1.9) under the same stress load, so it is inferred that the sclera shows a more rigid behavior than the cornea.…”

“…Due to the harmful effect of the high intraocular pressure that occurs in the disease called glaucoma (Ruiz -Ederra et al, 2005;Norman et al, 2010), different experimental (e.g., Schultz et al, 2008;Elsheikh et al, 2010;Elsheikh et al, 2008) as well as numerical (e.g., Belleza et al, 2000;Lari et al, 2012;Asejczyk et al, 2011) studies have been proposed to determine the mechanical characteristics of the sclera Elsheikh et al, 2010;Schultz et al, 2008) and the cornea (e.g., Elsheikh et al, 2008) and, in addition, to establish the state of the stress on the walls of the eyeballs subjected to an internal pressure (Belleza et al, 2000).…”

Stiffening effects of riboflavin/UVA corneal collagen cross-linkingis hydration dependent

“…Then, the submersion chamber of the machine was filled with mineral oil (Hatami-Marbini, 2014a;Hatami-Marbini and Rahimi, 2014a). The mineral oil (Fisher Science Education, Illinois) was used as a bathing solution to prevent corneal swelling and keep thickness constant during the experiments (Elsheikh and Anderson, 2005;Lari et al, 2012;Woo et al, 1972), As discussed in our previous publications (Hatami-Marbini and Rahimi, 2014b), a preconditioning protocol (composed of five ramp loadings upto 10% strain and three stress relaxation steps) was used to have a recoverable state and remove any inelastic strain state.…”

A wide-angle X-ray fibre diffraction method for quantifying collagen orientation across large tissue areas: application to the human eyeball coat

“…Experimental and numerical studies of the cornea and sclera in isolation have identified both tissues as nonlinear viscoelastic materials with significant anisotropy deriving from their preferential collagen alignment (Boote et al, 2005;Pinsky et al, 2005;Burgoyne et al, 2005;Nguyen et al, 2008;Elsheikh & Alhasso, 2009;Girard et al, 2009;Elsheikh et al, 2010;Lari et al, 2012;Coudrillier, Boote et al, 2012;Coudrillier, Tian et al, 2012). However, corneal and scleral biomechanical behaviour is tightly coupled (Asejczyk- Widlicka et al, 2011), and accurate prediction of corneal, scleral and whole eye behaviour will therefore rely on detailed quantitative data on collagen orientation over the entire ocular coat.…”