PhD Candidate McGill University Montréal, Quebec, Canada
Background: In acute type A aortic dissections (ATAD), the intimal entry tear is located between the aortic root and the aortic arch. Limited data is available on the local predilection of the initial tear and associated local aortic diameters. Given that the ascending aorta is subjected to large hemodynamic as well as external stresses, understanding anatomical and biomechanical significance of entry tear locations is critical. Our study investigates the relationship between the site of the intimal entry tear and the local aortic diameter in ATAD and evaluates the role of local stresses.
METHODS AND RESULTS: Since 2015, 130 patients underwent emergency surgery for ATAD repair at the McGill University Health Centre. Of those, 88 had a clear entry tear noted at the time of surgery. Entry tear location as well as diameter measurements throughout the aorta were collected. Diameters were obtained through intraoperative TEE or preoperative CT scans. Clinical information such as height, weight, sex and genetics was also collected. Aortic diameters and entry tear location were compared and helped to formulate a simplified mechanical analysis of the stresses involved.
Tears occurred in all sections of the aorta, with the majority occurring at the ascending aorta (46.9%), followed by the arch (28.4%), the root (16.0%) and the STJ (8.64%). Although the average ascending diameter of the entire cohort is near the surgical threshold (49.8 ±9.20mm), aortic diameter at the tear location is on average below the threshold (44.4±9.24mm). Diameters at specific tear sites are even smaller (39.5±9.01mm for tears at the root, 39±6.52mm at the STJ and 35.6±2.74mm at the arch). Only 42% of entry tears occurred at the maximal aortic diameter. Torsional stress on aortic tissue to aortic motion causes stress concentration zones throughout the aorta. These zones are located at diameter transition zones rather than peak diameter values.
Conclusion: This analysis is a reminder that diameter alone is not a good indicator of dissection risk. In ATAD patients, the entry tear often occurs in a segment of the aorta that does not meet the surgical threshold away from the maximal aortic diameter. From an engineering standpoint, aortic tissue faces increased stress in areas where diameter changes abruptly. Dissections likely originate from very small tissue structural defects in the aortic wall that create stress concentration factors, compounded by torsional and hemodynamics loading.
