Microstructural evidence that lamina cribrosa beams (LC), peripapillary sclera (PPS) and central retinal artery (CRA) have distinct mechanical behaviors when stretched

Abstract

Purpose : To characterize the microstructural response of LC, PPS and CRA to stretch.

Methods : A novel custom multi-axial micro-stretcher system compatible with an IX83 microscope was developed. Coronal sections from sheep optic nerve head 16µm thick were mounted and tested while imaging with instant polarized light microscopy (IPOL, Fig 1). Collagen fiber orientation was analyzed using color-orientation mapping and circular statistics. We tracked the collagen waviness (SD of orientation) and stretch (change in length) at multiple locations in the PPS (N=7), LC (7), and CRA (3). Stretch-waviness curves were fitted by a sigmoid function and characterized by the stretch at half-waviness (Fig 2). Unequal variance t-tests were used to compare tissues. For fair comparison across tissues, all data shown was obtained from the same section and test.

Results : Before stretch, LC, PPS, and CRA exhibited undulations in color along the fibers, indicating crimp (Fig 1). After stretch, most of the color undulations were gone, indicating decreased crimp. All stretch-waviness curves followed a sigmoidal decreasing pattern, but the rates of decrease varied substantially (Fig 2). The waviness in the PPS decreased the fastest with stretch, followed by that in the LC and CRA. The stretch at half-waviness in the PPS (1.77±0.39%) was significantly lower (p < 0.002) than that in the LC (5.91±2.30%) and CRA (13.1±1.10%).

Conclusions : Collagen fibers in the PPS, LC, and CRA do not uncrimp simultaneously during stretching, indicating tissue-dependent rates of stiffening with load. The PPS stiffens at a lower level of stretch, which could help protect the LC and CRA from excessive deformations. The heterogeneity in LC responses suggests a wide range of beam mechanical properties, stemming from the anisotropy and nonuniformity of the LC structure. High stretch at half-waviness in CRA may relate to the lack of blood pressure during tests.

This is a 2020 ARVO Annual Meeting abstract.

 

IPOL images in initial and stretched states. Colors indicate local collagen fiber orientation. Rectangles indicate locations where we tracked the collagen waviness and stretch, with colors corresponding to the curves in Fig. 2.

 

Stretch-waviness curves. The waviness was normalized for easy comparison. The bottom-right boxplot shows the distribution of the stretch at half-waviness in each tissue.