Biomechanical properties of the porcine thoracic aorta: An in- depth analysis of elastic modulus, stress-strain behavior and tissue toughness with regard to anatomical location and loading orientation

Technical Summary: Biomechanical Properties of the Porcine Thoracic Aorta

Problem Statement
The mechanical integrity of the aortic wall is fundamental to its physiological function and its susceptibility to pathologies such as aortic dissection and abdominal aortic aneurysms (AAA). Computational models, particularly those based on the finite element method (FEM), rely heavily on accurate material properties to predict wall stress and rupture risk. However, the aorta is mechanically heterogeneous and anisotropic, with properties varying by anatomical location, loading direction, and tissue microstructure. While porcine aortas are widely used as experimental models due to their structural and biomechanical similarity to humans, a systematic characterization of mechanical properties along the entire thoracic aorta remains limited. Previous studies often lacked comprehensive data on how elastic modulus, ultimate stress, strain at failure, and tissue toughness vary with longitudinal anatomical location, circumferential position, and loading orientation, or were complicated by methodological differences and incomplete failure analysis.

Methodology
This study provided an in-depth biomechanical characterization of the porcine thoracic aorta using uniaxial tensile testing.

• Specimen Acquisition: Ten porcine thoracic aortas were obtained from a local slaughterhouse. Specimens were stored at -20°C in physiological buffer and thawed prior to testing.
• Sampling Strategy: Each aorta was divided into six 5 cm segments (S1–S6) starting from the ascending aorta near the coronary ostia. From each segment, eight tissue patches were harvested. These were further subdivided into circumferential and longitudinal specimens, and categorized by anatomical position (Posterior, Anterior, Left, Right). This resulted in a total of 480 specimens (48 per aorta).
• Testing Protocol: Specimens were subjected to uniaxial tensile testing in Ringer's solution at 37°C using an ADMET system with a 10-lb load cell. Custom 3D-printed clamps were used to minimize slippage and damage.
• Preconditioning: To ensure reproducibility, specimens underwent 10 preconditioning cycles. Force thresholds were individually calculated based on the original aortic diameter and Laplace's law to correspond to target pressures of 70 and 140 mmHg.
• Failure Testing: Following preconditioning, specimens were monotonically elongated at a constant rate of 0.5 mm/s until rupture.
• Data Analysis: Raw force-displacement data were converted to engineering and true stress-strain curves. Key parameters extracted included low-strain (10–30%) and high-strain (70–90%) moduli, rupture stress, rupture strain, and tissue toughness (energy absorbed until rupture). Statistical analysis was performed using ANOVA with Tukey's post hoc test.

Key Results
The study revealed distinct regional, orientation-, and strain-dependent biomechanical behaviors:

• Circumferential Homogeneity: No significant differences were found in mechanical properties based on the circumferential position (anterior, posterior, left, right) within any given segment. This suggests a uniform circumferential build-up of the wall in response to symmetric pressure fields.
• Longitudinal Heterogeneity: Biomechanical properties varied significantly along the cranial–caudal axis (proximal to distal).
  • Stiffness: The high-strain modulus in longitudinal specimens remained largely stable across segments S1–S5, with a marked increase confined to the most distal segment (S6). Conversely, circumferential specimens showed a progressive increase in high-strain modulus from proximal to distal segments.
  • Anisotropy: At high strains, circumferential specimens exhibited significantly higher stiffness, strength, and toughness than longitudinal specimens, a difference that increased towards the distal aorta before converging in the most distal segment (S6). At low strains, longitudinal specimens tended to be stiffer, though this was only statistically significant in segment S4.
  • Toughness: Toughness decreased distally in circumferential specimens but increased in longitudinal specimens. Consequently, the pronounced difference in toughness between orientations observed in proximal segments (S1–S4) disappeared in the distal segments (S5–S6).
• Failure Modes: Rupture behavior differed by orientation. In longitudinal specimens, the intima consistently ruptured first, followed by the media and adventitia at higher strains. In circumferential specimens, all three layers ruptured simultaneously.

Significance and Claims
The authors claim that these findings demonstrate that the porcine thoracic aorta cannot be treated as a mechanically uniform structure. The study emphasizes that anatomical location and loading direction are critical factors in experimental aortic biomechanics, biomechanical modeling, and the development of vascular devices.

• Modeling Implications: The data suggest that computational models of aortic disease and rupture risk should utilize region-specific and anisotropic material properties rather than homogeneous assumptions. Furthermore, the sequential failure pattern observed in longitudinal specimens (intima first) supports the use of layer-specific models, particularly for simulating dissection-prone conditions.
• Pathophysiological Insight: The observed increase in stiffness and decrease in compliance towards the distal aorta may help explain the preferential development of aneurysms in the abdominal aorta. If this trend of increased stiffness continues into the abdominal region, the tissue may be less able to buffer pulsatile loads, leading to higher local wall stress and progressive dilation.
• Methodological Contribution: By providing a high-resolution characterization of regional heterogeneity, the study aims to refine biomechanical models and support the development of more accurate predictive tools for vascular disease progression.

The authors acknowledge limitations, specifically the use of frozen tissue (which may affect mechanical properties) and uniaxial testing (which does not fully replicate the biaxial in vivo stress state). They suggest that future work should aim to implement biaxial testing and fresh tissue protocols to further validate these findings.