Don’t judge a beam by its width; thin lamina cribrosa beams have different collagen microstructure than thick beams

Abstract

Purpose : The collagenous beams of the lamina cribrosa (LC) provide mechanical support to RGC axons. Traditional thought is that thin beams are structurally soft, and therefore stretch more than thick beams under high IOP, potentially endangering nearby axons. We hypothesize that thin beams have different mechanical properties than thick beams, allowing them to compensate for their size. We test if thin and thick beams have different collagen microstructure, which determines the stiffness of the tissue

Methods : 7 eyes from 4 sheep were pressure fixed at 5 mmHg in 10% formalin overnight. The ONHs were sectioned coronally (30µm). For each eye 1-3 sections at the level of the LC were imaged with polarized light microscopy and analyzed to determine collagen fiber orientation at a resolution of 0.73 µm/pixel (Jan et al, IOVS 2017; Fig 1). For every beam, we measured the waviness, tortuosity and width. Linear mixed effects models were used to test whether waviness and tortuosity were associated with LC beam width, directly or after nonlinear transforms

Results : In total we measured the tortuosity, waviness and width of 1216 LC beams. Crimp waviness and tortuosity were both positively correlated with beam width (p<0.0001; Fig 2). The best fitting statistical models were found using the square of the waviness and the square root of the tortuosity

Conclusions : Thin beams were less wavy than their thicker counterparts, implying that thin beams will stiffen first as IOP increases. This difference may allow thin beams to support similar amounts of IOP-induced loading as thicker beams, thus providing a similar level of support to the axons. Our results suggest a strong heterogeneity in LC beam mechanical properties, and that it may not be appropriate to assume that LC regions with thin beams are more at risk of damage. However, measurements of beam-level properties are needed to confirm these findings

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.

 

Fig 1 LC beams of a sheep eye fixed at 5mmHg shown with wide field of view (A) and close up colored by local fiber orientation (B) or enhanced to show collagen fiber crimp (C).

 

Fig 2 Both tortuosity and waviness increased with beam width (p<0.0001).