The role of radial elastic properties in the development of aortic dissections

Purpose: The response of the upper and lower thoracic aorta to radial tensi le stresses was compared with the response to circumferential and longitudi nal stresses to understand the role of tensile stress in the tearing phase of an aortic dissection. Methods: Square tissue samples (1.6 by 1.6 cm) were cut from the upper and lower segments of six porcine thoracic aortas and were elongated in the rad ial direction with a tensile testing machine. The radial extensibility of t he thoracic aorta was compared with adjacent tissue samples that were teste d in tension in the circumferential and longitudinal directions based on Yo ung's modulus (ie, the ratio of tensile stress to strain). Results: The elastic properties of the thoracic aorta in the radial directi on were markedly different from both the circumferential and longitudinal p roperties. The average Young's modulus (calculated immediately before faili ng) was significantly lower in the radial direction for both the upper and lower thoracic segments (61.4 +/- 4.3 kPa, SEM) than the Young's modulus of corresponding segments in the circumferential and longitudinal directions that were not tested to failure (151.1 +/- 8.6 kPa and 112.7 +/- 9.2 kPa, r espectively; P < .05). Sections 7 mu m thick were collected from four sampl es obtained from one upper thoracic aorta that were strained at 0, 1.0, 2.5 , and 4.0 and then stained either with Movat's pentachrome or with hematoxy lin and eosin. Histological analysis of the samples stressed in the radial direction revealed that smooth muscle cells were torn loose from their atta chments to each other and to adjacent elastin. Conclusion: Although the aorta normally functions under radial compressive stresses associated with lumen blood pressure, these results show that the aorta tears radially at a much lower value of stress than would have been p redicted from previous studies that have reported longitudinal and circumfe rential Young's modulus. This could explain why dissections propagate readi ly once the initial tear occurs.