In parallel, a 5-mm diameter cylindrical flat-punch indenter was mounted on a 10-N load cell (MTS 100-090-795; S-Beam type; load resolution, 0.0001 N). The indenter assembly was then screw mounted onto the crosshead of a universal testing machine (UTM; MTS Alliance RT/5; MTS Systems Corp., Eden Prairie, MN, USA). The prepared eye was placed underneath the indenter assembly such that the indenter was positioned approximately 10 mm away from the limbus at 12 o'clock along the superior sclera. After alignment, the UTM moves the crosshead-mounted indenter downward at a rate of 20 mm/min to indent the sclera to a depth of 1 mm. The load (
F) − displacement (
δ) data from the indentation were recorded during the indentation. The indentation can be divided into an initial partial contact regime and a second full contact regime when the indentation depth is greater than
Display Formula
/2
R (
Fig. 2). To simplify analysis, the full contact regime with constant contact area is used for determination of the scleral properties in this study (see Appendix for details). The load-displacement curve in the full contact regime is linear, and the tangent modulus,
20 a scleral property that describes the stress–strain curve at a specific stress for the sclera, can be used to characterize its elastic behavior. The sclera is a structure consisting of multiple tissue layers. The scleral tangent modulus describes the structural response of the sclera as a whole, and is not the properties of the stroma or individual layers within. In this study, a scleral tangent modulus,
E|
IOP, defined as the slope of the stress-strain curve at fixed IOP,
21 is used to describe the scleral indentation behavior. The details of the IOP independence in
Equation 1 can be found in the
Supplementary Material. In
Equation 1,
S|
IOP is the scleral stiffness derived from the slope of the load-displacement curve for an eye held at constant IOP,
a is a geometric constant of the indentation, and
ν is the Poisson's ratio of the sclera. The sclera was assumed to be incompressible such that
v = 0.5. The value of
a depends on
μ (see
Table), which, in turn, depends on the thickness and radius of curvature of the sclera as shown in
Equation 2 (See Appendix for more details).