Publisher’s version / Version de l'éditeur:
Proceedings. The 20th Canadian Congress of Applied Mechanics, 2005
READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE. https://nrc-publications.canada.ca/eng/copyright
Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la
première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.
Questions? Contact the NRC Publications Archive team at
PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information.
NRC Publications Archive
Archives des publications du CNRC
This publication could be one of several versions: author’s original, accepted manuscript or the publisher’s version. / La version de cette publication peut être l’une des suivantes : la version prépublication de l’auteur, la version acceptée du manuscrit ou la version de l’éditeur.
Access and use of this website and the material on it are subject to the Terms and Conditions set forth at
Effect of Formalin Fixation on the Local Mechanical Properties of Aortic
Tissue
Rouleau, Léonie; Delorme, Sébastien; Thibault, Francis; Mongrain, Rosaire;
Leask, Richard L.
https://publications-cnrc.canada.ca/fra/droits
L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.
NRC Publications Record / Notice d'Archives des publications de CNRC:
https://nrc-publications.canada.ca/eng/view/object/?id=b05acdaf-730a-41d2-9724-05458683f14a https://publications-cnrc.canada.ca/fra/voir/objet/?id=b05acdaf-730a-41d2-9724-05458683f14a
Effect of Formalin Fixation on the Local Mechanical Properties of Aortic Tissue
Léonie Rouleau a, Sebastien Delorme c, Francis Thibault c, Rosaire Mongrain b,d, Richard L. Leask a,d Departments of Chemicala and Mechanicalb Engineering, McGill University
Industrial Materials Institutec , NRC Institut de Cardiologie de Montréald
ABSTRACT
Objective: Heterogeneous tissue changes occurring with CVD are poorly understood. The aim of this study was to test if the heterogeneity of aortic tissue mechanical properties is preserved in formalin fixed tissue.
Methods: To test the regional variation in tissue properties, uniaxial tensile tests were preformed under displacement control (strain rate 5 mm/s, gauge length 5 mm) on the descending thoracic aorta from dogs (n=8) and pigs (n=6). Paired samples were taken from various regions along the aorta, with one sample stored in Krebs-ringer bicarbonate buffer solution and the other fixed in 10% formalin, at 4ºC. All samples were
mechanical tested after 48 hrs of storage.
Results: The data from the stress-strain curve was used to compare local differences in mechanical properties by comparing model parameters (elastic, neo-Hookean, Ogden, Mooney-Rivlin). The tissue thickness tends to decrease from the proximal to distal regions. The magnitude of the elastic modulus of formalin fixed tissue is significantly higher than fresh tissue. Formalin fixed tissue preserves the heterogeneity in local tissue elastic modulus in the longitudinal direction and circumferential direction.
Conclusions: Formalin fixed tissue shows similar local variation in mechanical properties as seen in fresh tissue. Tissue property
heterogeneity is preserved longitudinally and circumferentially.
INTRODUCTION
Cardiovascular diseases occur in different locations throughout the arterial tree. The regional variation of the mechanical properties is very important to the pathology of CVD. A number of studies have attempted to simulate the biomechanics of the aorta, however these studies have been limited by the assumptions made about the tissue properties (isotropic). In order to model the response of blood vessels and to better test endovascular devices, regional material properties have to be assessed experimentally. Fixed tissue is more readily available for
biomechanical analysis. Studies have been conducted on the shrinkage and the mechanical properties of fixed tissue (1,2). Only a few studies have investigated the regional variation along the aortic tree (3). However, no regional variation studies have been performed yet for formalin fixed tissue.
OBJECTIVES
The aim of this study was to evaluate the possibility of using formalin fixed tissue to detect the local differences in tissue properties using porcine and canine samples stretched in the longitudinal and circumferential direction.
METHODS
Canine (n=8) and porcine (n=6) specimens were obtained from animals used in other studies. Uniaxial tensile tests were performed using the EnduraTEC ELectroForce® 3200 soft tissue tester. Experiments were designed to allow the characterization of non-linear behaviour of descending thoracic aorta in the near physiological region.
Loosely adherent connective tissue was dissected from the samples. The orientation was identified using a permanent marker and the tissue
conserved at 4ºC either in 10% formalin or in krebs-ringer bicarbonate buffer solution. After 24 hrs, fixed samples were re-hydrated using krebs-ringer solution. The aorta was sectioned so that samples from one region were
randomized for the fresh and fixed criteria as well as the circumferential and longitudinal direction. Experiments were performed after 48 hrs of storage under displacement control at 5mm/sec (gauge length: 5mm). Samples were kept wet during the test which included preconditioning cycles (n=8) for engineering strains of 50%, 100% and 150% of the relaxed length. Dimensions were measured using dial calipers (0.001 in precision). The data from the uni-axial stress-strain curves were used to compare local differences in mechanical properties by comparing model parameters (elastic, neo-Hookean, Ogden, Mooney-Rivlin).
RESULTS
The difference in thickness was not significant between fixed and fresh samples or between medial (inner) and lateral (outer) samples. The tissue thickness tends to decrease along the aortic tree. There is a significant difference between the thickness of tissue from the aortic arch versus the low thoracic region. Fixation did not affect the ability to detect regional difference in tissue thickness.
Nonlinear model parameters and a linear
regression of the elastic region of the engineering stress-strain curves were computed for all samples to estimate the local elastic modulus. The results show that in a few cases (all porcine) significant regional difference in the tissue properties were seen in fixed tissue that was not detected in the fresh samples. In the remaining cases, the regional and directional variations in model parameters were comparable in fixed and fresh samples.
CONCLUSIONS
Preserving aortic tissue in formalin significantly alters the tissue stress-strain relationship. The magnitude of the local elastic modulus of formalin fixed tissue is significantly higher than fresh tissue. Despite the difference in model parameter magnitude, the local variation of tissue properties were preserved in formalin fixed tissue. Conclusions based on the heterogeneity of tissue properties would not be different from the fresh and formalin fixed tissue groups.
LIMITATIONS
Inherent experimental error in the dimension measurements as well as the affect of
temperature and the viscoelastic properties such as creep and stress relaxation were minimized by testing all samples under the same conditions. In addition, it has been assumed the in vitro uniaxial mechanical properties are representative of the in vivo properties.
ACKNOWLEDGEMENTS
The authors would like to thank Chantal Maltaisb, Nathalie L’Heureuxb, Marie-Pierre Mathieub, Jean Laurierb and Aaron Bestermann.
REFERENCES
Dobrin, P. B. "Effect of histologic preparation on the cross-sectional area of arterial rings." J
Surg.Res. 61.2 (1996): 413-15.
Gratzer, P. F. and J. M. Lee. "Altered mechanical properties in aortic elastic tissue using
glutaraldehyde/solvent solutions of various dielectric constant." J Biomed.Mater.Res. 37.4 (1997): 497-507.
Tanaka, T. T. and Y. C. Fung. "Elastic and inelastic properties of the canine aorta and their variation along the aortic tree." J Biomech. 7.4 (1974): 357-70.
Figure 1: Canine Descending Thoracic
Aorta
Figure 2: Uniaxial tensile test on aortic sample
Figure 3: Representative Engineering Stress-Strain curve for fixed and fresh tissue
