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Research Papers

Shear Behavior of Bovine Scleral Tissue

[+] Author and Article Information
Alan Argento

Professor
Department of Mechanical Engineering,
The University of Michigan-Dearborn,
4901 Evergreen Road,
Dearborn, MI 48128
e-mail: aargento@umich.edu

Wonsuk Kim

Department of Mechanical Engineering,
The University of Michigan-Dearborn,
4901 Evergreen Road,
Dearborn, MI 48128
e-mail: wskim@umich.edu

Frank W. Rozsa

Department of Ophthalmology and
Visual Sciences,
Kellogg Eye Center,
University of Michigan,
Ann Arbor, MI 48105
e-mail: rozsa@med.umich.edu

Kori L. DeBolt

Department of Mechanical Engineering,
The University of Michigan-Dearborn,
4901 Evergreen Road,
Dearborn, MI 48128
e-mail: deboltk@mail.gvsu.edu

Sophia Zikanova

Department of Mechanical Engineering,
The University of Michigan-Dearborn,
4901 Evergreen Road,
Dearborn, MI 48128
e-mail: szikanova@gmail.com

Julia R. Richards

Professor
Department of Ophthalmology and
Visual Sciences,
Kellogg Eye Center,
University of Michigan,
Ann Arbor, MI 48105
e-mail: richj@med.umich.edu

1Corresponding author.

Manuscript received December 21, 2013; final manuscript received April 24, 2014; accepted manuscript posted May 8, 2014; published online May 22, 2014. Assoc. Editor: Jonathan Vande Geest.

J Biomech Eng 136(7), 071011 (May 22, 2014) (12 pages) Paper No: BIO-13-1586; doi: 10.1115/1.4027615 History: Received December 21, 2013; Revised April 24, 2014; Accepted May 08, 2014

Ocular tissue properties have been widely studied in tension and compression for humans and a variety of animals. However, direct shear testing of the tissues of the sclera appear to be absent from the literature even though modeling, analyses, and anatomical studies have indicated that shear may play a role in the etiology of primary open angle glaucoma (POAG). In this work, the mechanical behavior of bovine scleral tissue in shear has been studied in both out-of-plane and in-plane modes of deformation. Stress–strain and relaxation tests were conducted on tissue specimens at controlled temperature and hydration focusing on trends related to specimen location and orientation. There was generally found to be no significant effect of specimen orientation and angular location in the globe on shear stiffness in both modes. The in-plane response, which is the primary load carrying mode, was found to be substantially stiffer than the out-of-plane mode. Also, within the in-plane studies, tissue further from the optic nerve was stiffer than the near tissue. The viscosity coefficient of the tissue varied insignificantly with distance from the optic nerve, but overall was much higher in-plane than out-of-plane.

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Figures

Grahic Jump Location
Fig. 1

(a) Posterior view of a bovine left eye. The dashed lines drawn on the tissue are cutting lines separating the sclera into four quadrants; the circles indicate sample extraction locations relative to the edge of the optic nerve head. In (b) the analogous features are shown in a posterior view of the right eye.

Grahic Jump Location
Fig. 2

(a) flexible plastic template used to draw lines on the sclera to separate it into four 90 deg quadrants and to define test sample orientation when cutting from the sclera (see Fig. 3); (b) right bovine eye sclera prepared for specimen extraction along with a global cylindrical coordinate system defined by r and θ.

Grahic Jump Location
Fig. 3

Examples of specimen locations and orientations. (a) For the left eye the black rectangle in Quadrant II indicates a τrθ specimen 10 mm from the ONH and oriented at 0 deg. The corresponding coordinate angle θ is also shown. The black square in Quadrant III indicates a τrθ specimen 3 mm from the ONH and oriented at 45 deg. (b) For the right eye the black square in Quadrant II indicates a τrz specimen 3 mm from the ONH and oriented at 0 deg. The corresponding coordinate angle θ is also shown. The black square in Quadrant III indicates a τrz specimen 10 mm from the ONH and oriented at 45 deg.

Grahic Jump Location
Fig. 4

(a) Photograph of a mounted specimen loaded in the testing apparatus within the environmental testing chamber. (b) Detail showing a 6 mm × 6 mm specimen mounted onto acrylic bars inserted in grips for a test to measure τrz.

Grahic Jump Location
Fig. 5

Specimen geometry. In both cases the shaded area denotes the sheared surface and t denotes the anatomical thickness of the scleral wall. (a) Specimen used to determine τrz and γrz; (b) specimen used to determine τrθ and γrθ.

Grahic Jump Location
Fig. 6

(a) An rz specimen at the start of a test. The acrylic mounting bars can be seen at the top and bottom of the photo; (b) specimen after 1 min of testing; (c) specimen at the end of the test. Here shear force, displacement, and shear angle are indicated; and (d) typical stress–strain curves. Stress–strain curves up to 2% strain: (e) in-plane near case and (f) out-of-plane near case. The mean values of slopes are denoted by dashed lines.

Grahic Jump Location
Fig. 7

A typical shear stress relaxation curve with the definitions of relaxation-model parameters

Grahic Jump Location
Fig. 8

Peak stress and corresponding strain of each in-plane shear test: (a) far and (b) near groups. The hollow markers denote individual test values, and the solid markers denote the locations of the mean values. The mean values are 0.17 MPa and 65% in (a) and 0.17 MPa and 86% in (b).

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