Abstract
Purpose :
The equatorial sclera is central to overall globe mechanics. However, compared to the front or back of the eye, little is known about equatorial sclera mechanics and its underlying microstructure. Our goal was to quantify equatorial sclera mechanical behavior and collagen fiber architecture, focusing on anisotropy and fiber preferred orientation.
Methods :
Five porcine eyes were obtained from a slaughterhouse within 24 h postmortem. Equatorial sclera samples (11 mm × 11 mm) were excised from each of four anatomical quadrants: nasal, superior, temporal, and inferior. Each sample was mounted on a custom-built biaxial mechanical testing system, with the loading axes aligned with the equatorial and meridional directions of the sample. Four submillimeter glass markers were attached to the surface of the sample for optical tracking of tissue deformation (strain). The sample was then loaded under equal-biaxial stress control, with the maximum stress of 120 kPa. Each test consisted of ten loading/unloading cycles, and only data from the tenth cycle was used in our analysis. The sample was immersed in PBS during the test. After testing, the samples were fixed overnight in 10% formalin. Four samples from three eyes were cryosectioned into 30-µm-thick sections. Sections were imaged using polarized light microscopy, and collagen fiber orientations determined using a previously reported technique. Orientation maps were registered and quantified to determine fiber orientation distributions and the preferred orientation of each sample.
Results :
The superior and inferior quadrants had the highest anisotropy, with the stiffness along the equatorial direction much larger than that along the meridional direction. The preferred fiber orientation in all four quadrants was in the meridional direction.
Conclusions :
A sclera that is softer in the meridional direction may steer the eye to respond to loads by elongating longitudinally, without increasing equatorial diameter. This may reduce optical distortions and insult to the delicate neural and vascular tissues. Surprisingly, the higher equatorial sclera mechanical stiffness did not concur with the preferred meridional collagen fiber orientations. The origin of this discrepancy remains unknown. It is possible that not all fibers are equally stiff. Ongoing efforts are to analyze the collagen architecture of more samples.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.