IOP Elevation Reduces the Waviness of the Load Bearing Collagen Fibers in the Lamina Cribrosa

Abstract

 

Purpose

 

Collagen fiber waviness, or crimp, is the primary cause of the nonlinear response of the load-bearing tissues of the eye, and is therefore central to determine the effects of IOP and sensitivity to elevated IOP. Despite the critical importance of fiber crimp in biomechanics, it has not been measured in ocular tissues. Our goal was to measure the crimp of the lamina cribrosa trabeculae and determine how it changes as IOP increases.

 

Methods

 

Sixteen eyes of eight young sheep (<2 years) were obtained from the local abattoir within two hours of sacrifice and kept at 4°C until fixation. Eyes were fixed at 0 (no cannula), 5, 15, 20, 30 or 50 mmHg by infusing 10% formalin through a cannula inserted into the anterior chamber connected to a reservoir. Eyes were then immersed in 10% formalin and IOP maintained for 24 hours. Following fixation the ONH and surrounding sclera were excised and cryosectioned coronally (30µm). Digital photographs were obtained (20x objective, 0.5 NA, 0.5 reducer, Olympus DP26-CU camera, 8 bit greyscale, 0.345um/pix), stitched into mosaics and were analyzed using custom routines to determine local variations in collagen fiber orientation longitudinal to lamina cribrosa trabeculae as a measure of waviness (Figure 1). A minimum of 14 lamina trabeculae were selected for measurement in each eye, with two of these intentionally of trabeculae suspected to have particularly high crimp.

 

Results

 

There was substantial waviness at low IOPs, decreasing with IOP (Figure 2). Very low waviness (angle spread below 1°) first appeared at an IOP of 15 mmHg. Even at an elevated of 30 mmHg some trabeculae still retained moderate waviness (3-4°). An exponential model fit to the log-transformed waviness as a function of IOP demonstrated that as IOP increases there was a significant decrease in waviness (p<0.0001) and in the spread of the waviness itself.

 

Conclusions

 

Our results show that the collagen fibers of the lamina trabeculae are substantially crimped at low IOPs. Increases in IOP stretch the fibers, reducing crimp, first in some trabeculae, and eventually in all of them. To the best of our knowledge this is the first report of experimental measurements of crimp in ocular tissues.

 

 

Collagen fibers with and without crimp

 

 

Waviness as a function of IOP. Each point is a measurement in a trabecula. Deliberate measurements of high crimp are triangles.Dotted line shows the exponential model fit

 

Keywords: 577 lamina cribrosa • 568 intraocular pressure • 708 sclera