Material properties for modeling traumatic aortic rupture

Traumatic aortic rupture is a significant cause of fatalities in frontal automobile crashes. However, such ruptures are difficult to reproduce experimentally in cadaveric surrogates, and it is difficult to observe dynamic aortic response in situ. So, the aortic injury mechanism or mechanisms remains in dispute. This study is a staged investigation of the physical parameters and mechanisms of human aortic rupture. The investigation includes both experimental study of local and global viscoelastic properties and failure properties of aortas using aortic tissue samples, excised aortas in vitro, and whole human aortas in situ in cadaver thoraxes. This study is the first phase in a staged programme to develop a finite element computer model of aorta injury to examine the mechanisms of aorta injury in automobile crashes. The high-rate local biaxial properties of porcine aorta tissue are determined from samples taken from the isthmus region, the most common area of failure in traumatic aorta injury. Using porcine aortas, similar in structure and physical characteristics to human aortic tissue, biaxial oscillatory response is determined at large strains and high strain rates. From this data, a hyperelastic material model with a failure threshold is developed that is in good agreement with local property data determined from oscillatory tests at 20 Hz and 65 Hz. Further, whole aorta tests are performed using pressure application with aortic pressure time histories similar in onset rate to those seen in cadaveric sled testing. These tests establish the ultimate stretch ratio and strain to failure for human aorta specimens. The specimens show no significant difference in response between the in situ tests and the in vitro tests. This indicates either that the internal thoracic boundary conditions may not be important in the stress and strain level of aorta failure or that the number of in situ tests (3) was too small to establish a difference. A Weibull survival analysis of the whole aorta failure tests shows significant dependence of aortic ultimate stretch ratio on age. A 50% risk of failure is 852 kPa in the circumferential direction and 426 kPa in the longitudinal direction. For pressure, the 50% risk of failure for all the tests is approximately 101 kPa. This increases to greater than 120 kPa for subjects below 68 years.