Significance of source and size in the mechanical response of human cerebral blood vessels

Abstract

Cerebral blood vessels are frequently damaged in traumatic brain injury. Mechanical properties of fresh human cerebral vessels obtained through surgeries have been reported. Because surgical sources of human specimens are rare and produce a limited amount of material, we sought to compare the properties of more readily available cerebral arteries and veins obtained from cadavers to fresh vessel data. Additionally, because the previous study was limited to small vessels available in surgery, it was unknown how generally applicable the results were to larger cerebral arteries and veins. In the current study, large and small cerebral vessels from autopsy were stretched axially. Data from these and similar tests on fresh vessels were combined to determine the significance of source and size on mechanical properties. Structural comparisons of histological samples were additionally utilized to characterize differences. Results indicate that specimens from autopsy and surgery behave similarly except that vessels from autopsy tend to be less extensible. While tests on large vessels were limited, small arteries obtained from autopsy tended to be slightly stiffer than large arteries. In contrast, bridging veins from cadavers were typically stiffer and stretched less before structural failure than cortical veins from the same source. These effects are, however, secondary to differences identified between arteries and veins in the previous study.

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.