Regional variations in canine descending aortic tissue mechanical properties change with formalin fixation

Diseases of the aorta can alter the local mechanical properties of the tissue, leading to aneurysms and plaque instability. Local tissue properties are best evaluated from surgical samples or autopsy tissue using mechanical testing ex vivo. We examined whether formalin-fixed tissues preserve regional and local variations in tissue properties when compared to fresh tissues in order to determine if fixed tissue can be used to infer mechanical changes associated with tissue remodeling.

Equibiaxial mechanical tests were performed on canine descending thoracic aorta to quantify the regional and local tissue stiffness. Samples were taken from four locations along the aorta and from the lateral and medial side at each location. Half of the samples were randomly formalin fixed and used to measure the effect of fixation on local thickness, stiffness, and anisotropy.

In fresh tissue, regional differences in tissue stiffness and thickness are present. Aortic tissue stiffens and thins along the aorta. Fixation did not alter thickness, significantly increased tissue stiffness, and altered the directional dependency of the mechanical properties (anisotropy) at low strain. Formalin fixation altered local stiffness of the aorta near the aortic arch.

The changes in mechanical properties along the aorta were preserved in formalin-fixed samples. However, our results show that formalin fixation can change the variation in tissue stiffness and significantly affects the anisotropic properties of vascular tissues. Formalin fixation introduces spurious changes in mechanical properties, and we therefore strongly recommend the use of fresh aortic tissues for biomechanical analysis.