Subfailure damage mechanics in cerebrovascular injury

The ability of a cerebral vessel to deliver blood to the brain may become impaired through disease or trauma. Even in the absence of obvious structural disruption, the mechanical properties of a vessel may be changed as a result of trauma. A more complete characterization of the mechanical properties of blood vessels will allow for better prevention and treatment of traumatic brain injury (TBI). Accordingly, two types of mechanical tests were performed in vitro on sections of ovine middle cerebral artery (MCA).;The influence of subfailure damage on mechanical properties has been explored in soft tissues such as ligaments but remains unexplored in cerebral blood vessels. Eighteen vessels from eight different ewes were tested to determine the occurrence of subfailure injury. Injury was defined as a stretch level that produces an unrecoverable change in the passive mechanical response. Vessels were preconditioned around in vivo loads and then subjected to a baseline response test consisting of an axial stretch from the buckled state to in vivo length while pressurized at 13 kPa (100 mmHg). Each specimen was then subjected to a different level of axial overstretch (above the in vivo length but below ultimate strain) while similarly pressurized, simulating loading conditions potentially associated with TBI. Following injury, baseline response tests were repeated at various times to investigate any time-dependent recovery of vessel response.;A linear relationship was found between the level of axial overstretch and the percent change in maximum baseline force and stiffness. For each increase of .1 in overstretch, the maximum baseline force and stiffness were reduced about 16 and 14%, respectively. This postinjury laxity matches similar findings on ligaments. It was also found that there was no significant recovery after up to 6.5 hours in the maximum baseline force and stiffness. This indicates that within the range studied, any level of axial overstretch will permanently change the passive mechanical properties of a vessel.;Eighteen vessels were also subjected to biaxial tests to characterize the mechanical properties of uninjured ewe MCA. Tests were coordinated with concurrent lamb MCA testing so that experiments would offer a valid comparison between lamb and ewe. The mechanical properties of vessels may be further related to individual vessel wall constituents through microscopy imaging.