Distribution of lamellar deformations: implications for properties of the arterial media.

Most computations of arterial mechanics treat the wall as a mechanically homogeneous body, but there are no data to support or refute this. To evaluate this assumption, experiments were performed that measured the deformation of 4 elastic lamellae located at 4 equidistant points across the thickness of the media. Data were obtained at 25-mm Hg pressure steps between 0 and 200 mm Hg. To satisfy the constraints of incompressibility in an isovolumetric cylinder, the innermost structures must undergo larger deformations than the outermost structures. This manifests as thinning of the wall. Therefore, each experiment was performed twice: once with a vessel segment in its normal cylindrical configuration, and again with a contiguous vessel segment turned inside-out to form an inverted cylinder. The deformations of individual lamellae obtained in normal and inverted vessel segments were averaged to determine their extensions independent of location. Results showed that the extensibilities of the lamellae were equal at all 4 anatomic locations across the media, suggesting equal stiffnesses of the lamellae. Other studies were performed to examine the distribution of the circumferential retractions of the lamellae that occurs when a vessel is extended longitudinally. Results showed that circumferential retraction also was distributed uniformly across the wall. These findings demonstrate that the elastic lamellae behave uniformly in both the circumferential and longitudinal directions at different locations across the wall thickness. Because of the interlocked structure of the elastin, muscle, and collagen in the media, these findings suggest that although the media is histologically heterogeneous, it acts mechanically as a homogeneous material.