Compressive mechanical properties of rat and pig optic nerve head

Abstract

Glaucoma is the leading cause of irreversible blindness worldwide. Elevated intraocular pressure (IOP), the primary risk factor for glaucoma, is thought to induce abnormally high strains in optic nerve head (ONH) tissues, which ultimately result in retinal ganglion cell damage and vision loss. The mechanisms by which excessive deformations result in vision loss remain incompletely understood. The ability of computational and in vitro models of the ONH to provide insight into these mechanisms, in many cases, depends on our ability to replicate the physiological environment, which in turn requires knowledge of tissue biomechanical properties. The majority of mechanical data published to date regarding the ONH has been obtained from tensile testing, yet compression has been shown to be the main mode of deformation in the ONH under elevated IOP. We have thus tested pig and rat ONH tissue using unconfined cyclic compression. The material constants C₁, obtained from fitting the stress vs. strain data with a neo-Hookean material model, were 428 [367, 488] Pa and 64 [53, 76] Pa (mean [95% Confidence Interval]) for pig and rat optic nerve head, respectively. Additionally, we investigated the effects of strain rate and tissue storage on C1 values. These data will inform future efforts to understand and replicate the in vivo biomechanical environment of the ONH.

Keywords: Compression; Mechanical properties; Neo-hookean; Optic nerve head.

Figure 1:

Schematic of sample preparation process for rat (top) and pig (bottom) ONH samples

Figure 2:

C₁ values for pig and rat optic nerve head samples subjected to cyclic unconfined compression at a 5%/minute strain rate. Colors correspond to individual samples. (A) shows 11 samples. Pig eyes were obtained from a slaughterhouse as a mixed lot on three separate days, thus we can only be certain these samples came from ≥ 7 pigs. (B) shows 9 samples from 5 rats. Different marker shapes correspond to different animals. The central line in each box marks the median, and the box edges mark the 25th and 75th percentiles. Whiskers cover the range of all data points not considered outliers. One sample in the pig data set (panel A) had a consistently high C₁ value and was identified as an outlier in cycles 2–5; this is denoted with a red plus sign over the green point markers.

Figure 3:

C₁ values for a subset of pig (A) and rat (B) samples that were tested at both 5%/min and 40%/min strain rates. The sample identified as an outlier in the larger porcine 5%/min data set (Figure 1) is indicated in orange.