Optical Rheology Of Porcine Sclera Using Noninvasive, Noncontact Polarization-sensitive Optical Coherence Tomography

Abstract

Purpose: : The biomechanics of the sclera has been investigated mainly with ex vivo experiments and computer simulations because of the difficulty of in vivo measurements. Since conventional imaging methods are not sensitive to elastic properties of tissue, a new method is required for in vivo elasticity measurements. We hypothesize that the elasticity correlates with birefringence, because both are related to the microstructure of tissue. The purpose of this study is to investigate the relationship using a tension tester and polarization-sensitive optical coherence tomography (PS-OCT).

Methods: : The Young’s modulus and birefringence of 11 porcine eyes were measured within 24 hours of sacrifice. Four pieces of scleral stripes which were parallel to the limbus, with a width of 4 mm, were dissected from each porcine eye. The dissected positions were from 0-deg latitude at the equator of the eyeball to 75-deg latitude near the optic nerve head. Birefringence of the sclera was measured with a PS-OCT developed in our laboratory. The strain and force were measured with a uniaxial material tester as it was stretched with a speed of 1.8 mm/min after preconditioning. A derivative of the exponentially-fitted stress-strain curve at 0% strain was extracted as the Young’s modulus. The correlation between Young’s modulus and birefringence was examined by using Pearson’s correlation coefficient.

Results: : Statistically significant positive correlation between Young’s modulus and birefringence was found at a position of 25 deg from the equator (p = 0.039, r = 0.626). Other positions did not have statistically significant correlation. Kruskal-Wallis tests showed statistically significant difference of birefringence among the piece positions of latitude (p = 0.013) and that of Young’s moduli (p = 0.020). Succeeded F-tests showed that the birefringence of sclera at 75 deg and Young’s moduli at 0 deg had statistically significant smaller and larger variances compared to all other positions (p < 0.05 and 0.001, F-statistic > 4.394 and 14.102), respectively.

Conclusions: : The evidence of a correlation between elasticity and birefringence of the sclera was demonstrated for the first time. This work may become a basis for glaucoma research with a perspective on scleral biomechanics, using PS-OCT in vivo.

Keywords: sclera • stress response • myopia