Mechanical Behavior of the Sclera

Abstract

Induced deformations, similar to the posterior pole deformations due to advanced myopia, were studied in a series of 30 rabbits. The induced deformations were produced by means of a specially designed cup. An analysis of the applicability of the shell theory is made, and it is concluded that the thin shell theory is applicable to the mechanical study of the eyes. The basic experiment consisted of introducing a controlled deformation at different pressure levels; a wide range of types of increments and times of substained deformations were studied. It is concluded that the scleral tissue is an elastic material and has a fairly high ‘Creep’ rate. The material properties indicate that the eye can reaccommodate induced pressure, and thus glaucoma and myopia could be present without external indications of ‘high pressure’ in the eye.

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

“…The average equilibrium stress observed at the termination of the stress relaxation test (t = 1000 s) is equivalent to IOPs of approximately 26 and 12 mm Hg in the monkey and rabbit, respectively [30]. Sclera is constantly under a pressure load and must have properties that allow the tissues to maintain that stress in equilibrium, and this finding is consistent with the other studies on the properties of rabbit sclera [31,32].…”

“…The average equilibrium stress observed at the termination of the stress relaxation test (t = 1000 s) is equivalent to IOPs of approximately 26 and 12 mm Hg in the monkey and rabbit, respectively [30]. Sclera is constantly under a pressure load and must have properties that allow the tissues to maintain that stress in equilibrium, and this finding is consistent with the other studies on the properties of rabbit sclera [31,32].All previously reported testing of sclera, as well as that in this study, has been performed in vitro where the sclera was not supported by the surrounding extraocular tissues. There is evidence to suggest that extraorbital pressure in the rabbit is substantial (about 20 percent of IOP) and follows fluctuations in intraocular pressure [33].…”

“…5,6). This may also explain the fact that the instantaneous elastic modulus we report for rabbit sclera (Table 1) is higher than the elastic modulus reported previously [32,36]. However, this may also be due to the fact that we calculate the true instantaneous modulus, and all previously reported values likely underestimate the true modulus due to stress relaxation that occurs during any finite loading ramp.…”

Mathematical modeling of Maklakoff’s method for measuring the intraocular pressure

“…Its elastic properties depend on the Young's modulus E and Poisson's ratio ν which may be different for the cornea and the sclera. 1 -eye socket, 2 -eyeball, 3 -eyelids, 4 -optic nerve, 5 -connective, adipose and muscular tissues, 6 -sclera, 7 -cornea, 8 -membrane, 9 -anterior chamber, 10 -lens The data on the Young's modulus obtained from tensile tests on samples and described in the literature have a considerable scatter: 1 < E < 100 MPa [2][3][4][5]. Based on the data of [6], we will assume the Poisson's ratio to be equal to ν = 0.45.…”

Is Keratoconus a True Ectasia?<subtitle>An Evaluation of Corneal Surface Area</subtitle>

“…[27][28][29] Similarly, the increasing amount of flattening (ie, a larger radius of curvature) in thicker corneal regions distant from the cone should allow higher stresses to be tolerated without requiring any concomitant increase in surface area from stressinduced stretching.…”