Ascending Thoracic Aortic Aneurysm Wall Stress Analysis Based in Surgical vs. Nonsurgical Patients
Nick Flores1, Justin Inman1, Matthew Lum1, Zhongjie Wang1, Yue Xuan1, Michael D Hope2, Liang Ge1, *Elaine E Tseng1
1Department of Surgery, University of California San Francisco and San Francisco Veterans Affairs Medical Centers, San Francisco, CA;2Department of Radiology, University of California San Francisco and San Francisco Veterans Affairs Medical Centers, San Francisco, CA
OBJECTIVES: Current guidelines for elective surgery of ascending thoracic aortic aneurysms (ATAAs) are based upon aneurysm size, growth and symptoms, with aneurysm size being the primary determinant for risk stratification of adverse events. Since the risk of rupture is proportional to the diameter of the aneurysm, aneurysmal size of ≥5.5cm is the recommended criterion for elective surgical repair for patients. Biomechanically, dissection or rupture occurs when wall stress exceeds wall strength. Therefore, patient-specific in vivo wall stress measured by finite element analysis (FEA) could potentially be a better predictor of ATAA dissection or rupture. The aim of this study was to compare the peak ATAA wall stresses in patients with an ATAA ≥5.0cm and <5.0cm and determine correlation between diameter and wall stress.
METHODS: Patients with ATAA ≥5.0cm ATAA (n=42) and <5.0cm (n=50) were recruited for the study. Patient-specific aneurysm geometry was obtained from ECG-gated computed tomography angiography and meshed, and pre-stress geometry was determined. ATAA walls were assumed to be hyper-elastic material with fiber orientation incorporated. Wall stress distribution and its magnitude were determined under a pressure of 120 mmHg using FEA software (LS-DYNA). Both circumferential and longitudinal peak wall stresses were compared in this study.
RESULTS: The 99% peak circumferential wall stresses of ATAA ≥5.0cm were 516.98±101.49kPa and ranged between 310.42 and 803.92kPa, while the 99% peak longitudinal wall stresses were lower at 289.70±63.26kPa and ranged between 202.05 and 519.13kPa. The 99% peak circumferential wall stress of ATAA <5.0cm were 441.48±105.41 kPa and ranged between 277.91 and 712.43kPa, while the 99% longitudinal wall stresses were lower at 247.97±68.81kPa and ranged between 158.86 and 478.66kPa. The 99% peak wall stresses were significantly different between ATAA ≥5.0cm and <5.0cm in both circumferential and longitudinal directions (p=.0008 and p=.0032, respectively). The 99% peak wall stresses for ATAAs were poorly correlated with diameter with R2 of 0.2107 for circumferential stress and 0.1582 for longitudinal stress.
CONCLUSIONS: Finite element analyses comparing peak wall stresses in patients with ≥5.0cm vs <5.0cm ATAA diameter showed significantly greater wall stresses in the larger vs. smaller aneurysms in both circumferential and longitudinal directions. However, peak wall stresses in both circumferential and longitudinal directions was poorly correlated with ATAA diameter. These data suggest that determination of patient-specific peak ATAA wall stresses may be more helpful as a biomechanical marker for the risk of dissection.
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