Peripapillary Sclera (PPS) Collagen Fiber Crimp Period Increases with Distance from the Optic Nerve Head (ONH)

Abstract

 

Purpose

 

The biomechanical sensitivity of the ONH to intraocular pressure (IOP) depends strongly on the properties of the sclera, especially the PPS. A primary cause of the nonlinear properties of the sclera is the waviness, or crimp, of the constitutive collagen fibers. Our goal was to measure the dependence of PPS collagen fiber crimp period on distance from the ONH.

 

Methods

 

Six eyes of three sheep (<2 years) were fixed at 5mmHg overnight using gravity perfusion fixation (cannula in the anterior chamber) and immersion fixation (10% formalin). Following fixation, the ONH and surrounding sclera were excised and cryosectioned coronally (30µm). Sections through the lamina cribrosa were selected and image mosaics of the whole section acquired using light microscopy (Nikon, 10x objective, 0.5 NA, 0.803 µm/pixel). The crimp period (averaged over 3 consecutive periods) was then measured manually (Fiji) in PPS regions proximal (<900µm) and distal (1400-3000µm) to the ONH (>10 measurements per region, 30-48 measurements per eye). A linear mixed-effect model was fit to the data accounting for multiple measurements in each eye and correlation between contralateral eyes.

 

Results

 

Distal crimp period was significantly larger than the proximal crimp period (p < 0.001, Table and Figure). Over the 6 eyes crimp periods were (mean ± SD): 39.4±6.1 µm for proximal sclera and 65.1±18.8 µm for distal sclera.

 

Conclusions

 

The 50% difference in crimp period between the distal and proximal PPS regions denotes potentially different biomechanical properties between these regions, although the consequences of this are not yet known. The proximal region encompasses a ring of fibers circumferential to the ONH, and a smaller crimp period therein may provide biomechanical stability and a more uniform distribution of deformations in response to loading, protecting the ONH from insult.

 

 

TABLE 1. Mean (SD) crimp periods per eye in distal and proximal regions to ONH

 

 

FIG 1. Close-ups of proximal (B) and distal (C) PPS of eye 1A. The colors represent the local collagen orientation relative to the fibers in the surrounding area, such that collagen fibers curved in one direction are yellow and purple in the other (A). Fiber orientations were determined as described previously (IOVS 2013 54:65). The larger crimp period in the distal region (C) is visible in the thicker yellow-purple bands as compared with the proximal region (B).

 

Keywords: 708 sclera • 577 lamina cribrosa • 599 microscopy: light/fluorescence/immunohistochemistry