Cerebral blood vessels are critical to maintaining the health of the brain. The function of these vessels gets disrupted during traumatic brain injury (TBI). Even when cerebral vessels do not bleed or rupture during TBI, they may be subjected to excessive deformation beyond their physiological or in-vivo (IV) length. This alters their mechanical properties.

The earlier investigation by Bell reported softening of the cerebral blood vessels, when stretched beyond their IV length, beginning at an overstretch level of 1.2. The recent investigation in our lab using Collagen Hybridizing Peptide (CHP) to detect collagen damage reported that collagen fibers begin to rupture when the vessels are stretched axially to approximately 1.3 times their IV length. It can thus be predicted that rearrangements in the microstructure of the vessels might be responsible for softening below this threshold, rather than ruptures or breaking of the fibers. The earlier investigation using CHP also found that the fibers oriented in the direction of loading tend to get damaged with overstretch.

Given that collagen plays a significant role in the mechanical properties of blood vessels at large deformations, we hypothesized that changes in collagen orientation were the microstructural mechanism responsible for the sub-rupture softening observed. Based on the previous work, we expected to see axial fibers in the adventitia become less uniform in their orientation following axial overstretch. Medial fibers are not expected to have a significant change in their orientation since they are circumferentially oriented. We investigated these questions using sheep middle cerebral arteries (MCAs).

We tested three MCAs from three different ewes, using a horizontal tester previously developed in our lab. Collagen fibers were imaged using nonlinear optical microscopy (NLOM). The captured data were then processed using Orientation J, Fibril Tool, and Line Tool and was finally analyzed statistically to test the hypothesis.

It was found that collagen retains its mean orientation post-overstretch. Based on image processing tools used here, it was found that axially oriented, adventitial fibers did not become more disorganized, as hypothesized, post-overstretch. We need advanced methods of data acquisition and image processing to determine if there is any subtle change in collagen orientation, which could not be detected in this investigation. To the best of our knowledge, this is the first study investigating the effect of mild axial overstretch on collagen orientation in cerebral arteries.