Viscoelastic Characterization of Peripapillary Sclera: Material Properties by Quadrant in Rabbit and Monkey Eyes

Abstract

In this report we characterize the viscoelastic material properties of peripapillary sclera from the four quadrants surrounding the optic nerve head in both rabbit and monkey eyes. Scleral tensile specimens harvested from each quadrant were subjected to uniaxial stress relaxation and tensile ramp to failure tests. Linear viscoelastic theory, coupled with a spectral reduced relaxation function, was employed to characterize the viscoelastic properties of the tissues. We detected no differences in the stressstrain curves of specimens from the four quadrants surrounding the optic nerve head (ONH) below a strain of 4 percent in either the rabbit or monkey. While the peripapillary sclera from monkey eyes is significantly stiffer (both instantaneously and in equilibrium) and relaxes more slowly than that from rabbits, we detected no differences in the viscoelastic material properties (tested at strains of 0-1 percent) of sclera from the four quadrants surrounding the ONH within either species group.Glaucomatous optic neuropathy is one of the three leading causes of blindness in the US [1,2] and is the result of damage to the neural tissues of the optic nerve head (ONH). Its clinical hallmarks are posterior cupping (bowing back) of the surface of the ONH, and a characteristic pattern of visual field loss.Corresponding Author: J. Crawford Downs, PhD, Department of Biomedical Engineering, Tulane University, Lindy Boggs Center, Suite 500, New Orleans, LA 70118; Tel: (504) 865-5897; fax: (504) NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author ManuscriptWhile IOP-lowering is central to the treatment of the disease [2,3], there is wide disagreement over the role of IOP in the development and progression of glaucoma [3][4][5]. Much of the confusion over the role of IOP in glaucoma stems from two groups of patients: those exhibiting ocular hypertension (elevated IOP) that never develop the disease, and those that develop glaucoma with no measurable increase in IOP (normal tension glaucoma).The sclera is the principal load-bearing tissue of the eye and consists primarily of collagen. The primary function of the sclera is to resist intraocular pressure, provide attachment sites for the ocular musculature and support the retina in the spheroid shape necessary for focused vision. The thickness and predominant collagen fibril orientation of this tissue are dependent on location in the globe [6][7][8][9]. However, whether the fibril orientation has a large effect on scleral material properties remains controversial [6,[10][11][12][13].The scleral shell is pierced by the scleral canal, an elliptical hole (1-2.5 mm in diameter) in the posterior portion of the globe, through which the retinal ganglion cell axons pass on their path to form the orbital optic nerve (Fig. 1). The scleral canal is spanned by a series of thin, fenestrated connective tissue sheets known as the lamina cribrosa (Fig. 1B, 1C). The lamina cribrosa provides structural and nutritional support to the retinal ganglion cell axons as they pass ...

Modeling individual-specific human optic nerve head biomechanics. Part I: IOP-induced deformations and influence of geometry

“…Overall the levels of strain predicted for the pre and post-laminar and laminar tissue were potentially biologically significant, but as discussed in Sigal et al (2007Sigal et al ( , 2008, to the best of our knowledge, the levels and modes of strain that have physiologic relevance in each tissue are still unknown, as is whether the physiologically relevant measure is the median or the peak. Downs et al (2003) proposed that levels of strain above 3.5% could be pathophysiologic. Slightly higher strain levels (5-6%) were sufficient to induce a wide range Fig.…”

Material Properties of the Human Lumbar Facet Joint Capsule

“…Six cycles were sufficient to reduce the differences between subsequent load-displacement curves to less than 5%, and were on the same order (number and duration) as has been used for other spinal ligaments [24]. The 3-min interval was selected based on published experiments testing the biomechanics of intact lumbar spine specimens [21,22] and is on the same order of magnitude as has been used in other soft tissue experiments [25,26].…”

Ischemic model of optic nerve injury

“…Recent biomechanical modeling of posterior segment of the eye and optic nerve head suggests that disproportionate stress and strain forces occur within regions of the optic nerve and may contribute to regional patterns of axon loss. 103 These physical forces may cause the axons passing through specific regions of the lamina cribrosa to be more vulnerable to mechanical stress from elevated IOP.…”

“…103 These anatomic features may cause the axons passing through the lamina cribrosa within these regions to be more vulnerable to mechanical stress, such as an increase in IOP.…”