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Subfailure overstretch induces persistent changes in the passive mechanical response of cerebral arteries.

Bell ED, Sullivan JW, Monson KL - Front Bioeng Biotechnol (2015)

Bottom Line: This subfailure deformation could result in altered mechanical behavior.The observed softening also generally resulted in increased non-linearity of the stress-stretch curve, with toe region slope decreasing and large deformation slope increasing.These changes may have significant implications in repeated TBI events and in increased susceptibility to stroke post-TBI.

View Article: PubMed Central - PubMed

Affiliation: Department of Bioengineering, University of Utah , Salt Lake City, UT , USA ; Laboratory of Head Injury and Vessel Biomechanics, Department of Mechanical Engineering, University of Utah , Salt Lake City, UT , USA.

ABSTRACT
Cerebral blood vessels are critical in maintaining the health of the brain, but their function can be disrupted by traumatic brain injury (TBI). Even in cases without hemorrhage, vessels are deformed with the surrounding brain tissue. This subfailure deformation could result in altered mechanical behavior. This study investigates the effect of overstretch on the passive behavior of isolated middle cerebral arteries (MCAs), with the hypothesis that axial stretch beyond the in vivo length alters this response. Twenty nine MCA sections from 11 ewes were tested. Vessels were subjected to a baseline test consisting of an axial stretch from a buckled state to 1.05* in vivo stretch (λIV) while pressurized at 13.3 kPa. Specimens were then subjected to a target level of axial overstretch between 1.05*λIV (λz = 1.15) and 1.52*λIV (λz = 1.63). Following overstretch, baseline tests were repeated immediately and then every 10 min, for 60 min, to investigate viscoelastic recovery. Injury was defined as an unrecoverable change in the passive mechanical response following overstretch. Finally, pressurized MCAs were pulled axially to failure. Post-overstretch response exhibited softening such that stress values at a given level of stretch were lower after injury. The observed softening also generally resulted in increased non-linearity of the stress-stretch curve, with toe region slope decreasing and large deformation slope increasing. There was no detectable change in reference configuration or failure values. As hypothesized, the magnitude of these alterations increased with overstretch severity, but only once overstretch exceeded 1.2*λIV (p < 0.001). These changes were persistent over 60 min. These changes may have significant implications in repeated TBI events and in increased susceptibility to stroke post-TBI.

No MeSH data available.


Related in: MedlinePlus

Percent decrease in strain energy under the axial stress-stretch curves, as calculated from initial overstretch test data, and post-overstretch failure test data (data truncated to stop at the previous overstretch level). Red error bars indicate SD for each group. Blue line connects group means to clarify trends. (○) indicates individual data points. (*) indicates statistical difference from the control (non-overstretched) group. (x) indicates statistical difference from the adjacent group subjected to a lower overstretch level.
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Figure 6: Percent decrease in strain energy under the axial stress-stretch curves, as calculated from initial overstretch test data, and post-overstretch failure test data (data truncated to stop at the previous overstretch level). Red error bars indicate SD for each group. Blue line connects group means to clarify trends. (○) indicates individual data points. (*) indicates statistical difference from the control (non-overstretched) group. (x) indicates statistical difference from the adjacent group subjected to a lower overstretch level.

Mentions: The strain energy was also affected by overstretch, with increasing levels of overstretch leading to larger reductions in strain energy, similar to the pattern of change observed with in vivo stiffness. As shown in Figure 6, the percent decrease, relative to the control group, was not significantly different in the 1.1*λIV group but was significant for all higher overstretch levels (Table 1). Further, the overstretch groups 1.2*λIV (p = 0.001) and 1.3*λIV (p = 0.012) were different from the adjacent lower overstretch group. However, once overstretch exceeded 1.3*λIV, differences between adjacent groups were no longer significant, similar to the pattern observed with in vivo stiffness. It should be noted (Figure 6) that there is a particular data point in the control group that is far lower than all the others, indicating a physically unreasonable increase in strain energy in the post-injury (sham) axial stretch test. Due to small axial forces at low stretch levels, the luminal pressure has a relatively large influence on the total axial stress (Eq. 5). Thus, it is likely that this unexpected data point is due to a slight variation in luminal pressure between the first and second stretch tests used to calculate %ΔU. This effect from pressure, and subsequent larger deviation in the control data, could also be the reason for the lack of significance in the differences between the control and 1.1*λIV overstretch groups.


Subfailure overstretch induces persistent changes in the passive mechanical response of cerebral arteries.

Bell ED, Sullivan JW, Monson KL - Front Bioeng Biotechnol (2015)

Percent decrease in strain energy under the axial stress-stretch curves, as calculated from initial overstretch test data, and post-overstretch failure test data (data truncated to stop at the previous overstretch level). Red error bars indicate SD for each group. Blue line connects group means to clarify trends. (○) indicates individual data points. (*) indicates statistical difference from the control (non-overstretched) group. (x) indicates statistical difference from the adjacent group subjected to a lower overstretch level.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4309201&req=5

Figure 6: Percent decrease in strain energy under the axial stress-stretch curves, as calculated from initial overstretch test data, and post-overstretch failure test data (data truncated to stop at the previous overstretch level). Red error bars indicate SD for each group. Blue line connects group means to clarify trends. (○) indicates individual data points. (*) indicates statistical difference from the control (non-overstretched) group. (x) indicates statistical difference from the adjacent group subjected to a lower overstretch level.
Mentions: The strain energy was also affected by overstretch, with increasing levels of overstretch leading to larger reductions in strain energy, similar to the pattern of change observed with in vivo stiffness. As shown in Figure 6, the percent decrease, relative to the control group, was not significantly different in the 1.1*λIV group but was significant for all higher overstretch levels (Table 1). Further, the overstretch groups 1.2*λIV (p = 0.001) and 1.3*λIV (p = 0.012) were different from the adjacent lower overstretch group. However, once overstretch exceeded 1.3*λIV, differences between adjacent groups were no longer significant, similar to the pattern observed with in vivo stiffness. It should be noted (Figure 6) that there is a particular data point in the control group that is far lower than all the others, indicating a physically unreasonable increase in strain energy in the post-injury (sham) axial stretch test. Due to small axial forces at low stretch levels, the luminal pressure has a relatively large influence on the total axial stress (Eq. 5). Thus, it is likely that this unexpected data point is due to a slight variation in luminal pressure between the first and second stretch tests used to calculate %ΔU. This effect from pressure, and subsequent larger deviation in the control data, could also be the reason for the lack of significance in the differences between the control and 1.1*λIV overstretch groups.

Bottom Line: This subfailure deformation could result in altered mechanical behavior.The observed softening also generally resulted in increased non-linearity of the stress-stretch curve, with toe region slope decreasing and large deformation slope increasing.These changes may have significant implications in repeated TBI events and in increased susceptibility to stroke post-TBI.

View Article: PubMed Central - PubMed

Affiliation: Department of Bioengineering, University of Utah , Salt Lake City, UT , USA ; Laboratory of Head Injury and Vessel Biomechanics, Department of Mechanical Engineering, University of Utah , Salt Lake City, UT , USA.

ABSTRACT
Cerebral blood vessels are critical in maintaining the health of the brain, but their function can be disrupted by traumatic brain injury (TBI). Even in cases without hemorrhage, vessels are deformed with the surrounding brain tissue. This subfailure deformation could result in altered mechanical behavior. This study investigates the effect of overstretch on the passive behavior of isolated middle cerebral arteries (MCAs), with the hypothesis that axial stretch beyond the in vivo length alters this response. Twenty nine MCA sections from 11 ewes were tested. Vessels were subjected to a baseline test consisting of an axial stretch from a buckled state to 1.05* in vivo stretch (λIV) while pressurized at 13.3 kPa. Specimens were then subjected to a target level of axial overstretch between 1.05*λIV (λz = 1.15) and 1.52*λIV (λz = 1.63). Following overstretch, baseline tests were repeated immediately and then every 10 min, for 60 min, to investigate viscoelastic recovery. Injury was defined as an unrecoverable change in the passive mechanical response following overstretch. Finally, pressurized MCAs were pulled axially to failure. Post-overstretch response exhibited softening such that stress values at a given level of stretch were lower after injury. The observed softening also generally resulted in increased non-linearity of the stress-stretch curve, with toe region slope decreasing and large deformation slope increasing. There was no detectable change in reference configuration or failure values. As hypothesized, the magnitude of these alterations increased with overstretch severity, but only once overstretch exceeded 1.2*λIV (p < 0.001). These changes were persistent over 60 min. These changes may have significant implications in repeated TBI events and in increased susceptibility to stroke post-TBI.

No MeSH data available.


Related in: MedlinePlus