Biomechanical properties of porcine cerebral bridging veins with reference to the zero-stress state

Passive mechanical and morphometric properties of porcine cerebral bridging veins were studied. Fifteen cerebral bridging veins were obtained from 7 pigs. The superior sagittal sinus, bridging veins and the meninges were excised and placed in aerated calcium-free Krebs solution. The outflow cuff segment is a narrow region at the junction of the cerebral bridging veins and superior sagittal sinus. The principal direction of collagen fibres was longitudinal in the bridging vein and circumferential in the cuff region. The diameter was smaller in the outflow cuff segment than in the cerebral bridging veins in the pressure range studied (0-23 mm Hg) whereas the thickness was highest in the outflow cuff segment (p < 0.01). The circumferential stress-strain analysis showed that the outflow cuff segment was extensible up to a strain of 0.25. At higher strains the outflow cuff segment was progressively stiffer than the cerebral bridging vein (p < 0.05). The longitudinal stress-strain relation for the cerebral bridging vein was shifted to the left compared to the outflow cuff segment (p < 0.05). When compared to the stress-strain properties in the circumferential direction, the outflow cuff segment was more extensible and the cerebral bridging vein stiffer in longitudinal direction (p < 0.05). The opening angle of the outflow cuff segment and the cerebral bridging vein was 115 +/- 4 and 120 +/- 4 (means +/- SE) without statistical difference between the two regions. In conclusion the difference in biomechanical properties between the outflow cuff segment and the cerebral bridging vein was associated to their difference in histology and fibre arrangement. This indicates that the function of the outflow cuff segment is to act as a flow-limiting resistance to the outflow from the cerebral circulation.