Substantial Preconditioning Effect Observed In the Inflation Tests of Juvenile Tree Shrew Sclera

Abstract

Purpose : The tree shrew (TS) is increasingly used as animal model for myopia and glaucoma, where scleral biomechanics is known to play a key role. We present the mechanical response of 6 juvenile TS eyes to cyclic IOP.

Methods : Scleral shells were inflation tested within 48h after enucleation. The loading protocol consisted of 20 preconditioning cycles (5-50 mmHg at 1 mmHg/s) followed by a 30min rest at 5mmHg and a final load-unload cycle. 3D digital image correlation was used to track the surface displacements and calculate the strains. The maximum principal strain was averaged across the scleral surface. Mechanical stress was estimated assuming that the sclera is a thin pressure vessel.

Results : With increasing preconditioning cycles the stress-strain curves (Fig.1a) shifted to higher strains and showed a decreasing of the area between the loading-unloading curve (hysteresis). The peak strain decreased as the tissue softened with each cycle (Fig.1b), where the 13 cycles were required to adequately precondition the sclera. The cycle-dependent changes were recovered in the final cycle after 30min (Fig.1a). All 6 TSs showed a nonlinear stiffening with IOP (Fig.2). The scleral stiffness at high IOP and the locking strain at which collagen fibers straighten were estimated to (0.339±0.231)MPa and (0.76±0.093)%, respectively.

Conclusions : We have characterized the nonlinear and cycle-dependent mechanical response of the juvenile TS sclera to IOP. In contrast to other species (DOI:10.1115/1.4025105), TS sclera showed a substantial preconditioning effect (softening) during cyclic inflation testing. This effect is tissue dependent and cannot be assumed from tests performed on tissues of other species.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.

 

Fig.1: a) Representative stress-strain curves of the 20-cycles preconditioning and the final load-unload cycle. The final load cycle reveals a recovery of both the cyclic softening and hysteresis, after 30min rest. b) The incremental change in peak strain Δε_max shows a substantial softening of the sclera during the 20 cycles. The best fitting function suggests that 13 cycles are required to reach repeatable mechanical response (Δε_max<0.1%).

 

Fig.2: Stress-strain curves of the final loading cycle showing the nonlinear stiffening of the sclera with IOP. High-IOP stiffness (tangent of the dotted line) and locking strain (intercept of dotted line with strain-axis) at which the scleral collagen fibers straighten.