Significance of source and size in the mechanical response of human cerebral blood vessels
Introduction
Traumatic brain injury is the leading cause of death and disability in children and young adults in the United States and Europe (Kraus et al., 1994; Sosin et al., 1995). Cerebral blood vessels are damaged in 94% of all fatalities associated with head trauma (Graham, 1996). These vessels additionally form part of the composite structure of the brain and, because of their relative stiffness in comparison to brain tissue, play an important role in the overall response of the brain to loading (Zhang et al., 2002). Nevertheless, computational models of traumatic brain injury do not yet account for the vasculature, in part because detailed mechanical properties of this tissue have not been available (Bandak and Eppinger, 1994; Vossoughi and Bandak, 1996). Incorporation of the cerebral vasculature into finite element models of the head is vital to understanding brain injury tolerance and designing effective head protection systems.
A recent study of the axial mechanical properties of small human cerebral blood vessels obtained from the surface of the brain during surgeries demonstrated that the arteries are dramatically stiffer than the veins and fail at approximately twice the stress and half the stretch (Monson et al., 2003). The investigation further indicated that arterial response is not affected by strain rate and that the influence of rate on vein behavior is small, if significant at all. Although no prior studies had been conducted on cortical vessels, results of sub-failure tests on large cerebral arteries (Chalupnik et al., 1971) compare well with new findings. In agreement with recent results on cortical veins, the most referenced study of bridging veins (Lee and Haut, 1989) reported a lack of rate dependence, but bridging veins failed at much lower stretch magnitudes than cortical veins. Because both of the earlier studies examined large vessels taken from autopsy, it is difficult to know if similarities and differences are meaningful and clinically relevant, since both vessel source (surgery or autopsy) and size, or position in the circulation, potentially influence structure and mechanical properties.
In order to more fully describe cerebral vessel behavior, the potential effects of vessel source and size were evaluated through mechanical tests on both large (middle cerebral artery, bridging vein) and small (cortical) cerebral vessels obtained through autopsy. In concert with the belief that axial stretching is the dominant loading mode for these vessels, they were extended longitudinally to failure. Properties of cortical vessels from autopsy and those from surgery were compared to study the importance of tissue source. To elucidate the role of vessel size, properties of cortical and large vessels from autopsy were examined. The relative significance of a number of other variables was also considered.
Section snippets
Procedure
Specimens were obtained, according to an approved human subjects protocol, from the cerebral hemispheres of nine cadavers of both sexes (6 male, 3 female) undergoing autopsy, ranging in age from 38 to 80 (mean of 59). Cases were limited to those where no cerebral vessel pathology was previously diagnosed. Once the brain was removed, cortical vessels were resected from the surface of the temporal cortex to provide a direct comparison to fresh samples obtained in surgeries. Thirty-five and 19
Results
Cortical arteries from autopsy typically had outer diameters 2–3 times smaller than resected middle cerebral arteries (Table 1). In contrast, average bridging vein outer diameter was just 40% larger than that of cortical veins. Cortical vessels obtained from autopsies tended to be slightly larger than the comparison group from surgery, most likely because larger vessels are easier to resect and there were no restrictions for dissection in autopsy as there were in surgery. No gross visual
Discussion
While the mechanical properties of fresh human cerebral blood vessels obtained from surgeries have recently been reported (Monson et al., 2003), specimens from this source are necessarily limited both in quantity and size. The main objectives of this work were to define the influence of vessel source and size on the properties of human cerebral vessels in order to evaluate the cadaver tissue model and to determine what data may be accurately utilized to define a complete properties set for the
Acknowledgements
This work was funded by the Centers for Disease Control and Prevention under Grant R49/CCR919722-01. Additional support was provided by the UCSF Brain and Spinal Injury Center and by the CDC-funded San Francisco Injury Center. We sincerely appreciate the help of Mel Abulencia in the acquisition of specimens from autopsy and the expertise of Peter Bacchetti and Alan Bostrom in the statistical analysis.
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