Investigation of the Heterogeneous Mechanical Properties of the Intact and GAG-depleted Thoracic Aortic Tree using Indentation

Abstract

1.INTRODUCTION

To assess the risk of lethal aortic complications such as those caused by aneurysms and dissections [1], current clinical evaluation tools are essentially based on anatomical dimensions. As such, they ignore the heterogeneity of the aorta, which may play an important role, and they have inherent limitations that negatively impact the quality of patient care. Recent efforts have focused on deciphering the influence of the aortic composition and its mechanical functions. The extracellular matrix (ECM) of the aorta is obviously of prime interest, as a mixture of elastin, collagen and ground substance such as glycosaminoglycans (GAG) [2]. The purpose of this study was to evaluate the regional mechanical properties of the aorta along its tree using indentation, and most importantly, to evaluate the effect of glycosaminoglycans (GAG) on the tissue’s mechanical response.

2. METHODS

Five porcine thoracic aortas were acquired from a local abattoir and cleaned from surrounding fatty tissue. Three aortic strip samples were extracted from each of the ascending, aortic arch, and descending thoracic regions. One sample was tested under fresh conditions, the second sample served as a control, and the third sample underwent enzymatic GAG depletion. A 100mM ammonium acetate buffer, pH 7.0, was used for control and GAG-depleted samples. GAG depletion was ensured using 15U/mL hyaluronidase, 0.075U/mL chondroitinase ABC, 0.75U/mL heparinase for 48 hours at 37^oC.

The compressive properties were evaluated using a commercially available mechanical tester: Mach-1 Biomomentum (Biomentum Inc., Laval, QC, Canada). Indentation mechanical testing was performed using a 150-gf load cell along with a 1-mm diameter indenter. The sample was placed in a sample chamber filled with phosphate buffered saline at room temperature and was strained to 20% of its thickness from the intimal layer, after 40 preconditioning cycles. Analyses were conducted at 10% strain.

The efficiency of GAG removal treatment was evaluated by quantifying GAG levels after weighing samples and digesting them in papain digestion buffer. A dimethylmethylene blue spectrophotometric assay was used to quantify GAG.

3. RESULTS AND CONCLUSION

We first confirmed that the treatments did not alter the tissue response by comparing the properties of fresh and control samples, as no significant difference was found. The stress at 10% indentation (from intima) was significantly higher in the ascending region compared to the arch and descending thoracic regions for all fresh, control and GAG-depleted tissue. These findings confirm that the aorta is biomechanically heterogeneous along its tree, and more specifically, that the ascending region exhibited higher compressive stiffness compared to the arch and descending thoracic regions. In addition, GAG-depleted samples in the ascending region exhibited a significantly stiffer response compared to the fresh and control samples. However, this was not the case in the arch and in the descending thoracic regions. These findings suggest that the compressive stiffness of the aorta is influenced by the presence of GAG, and that more work is needed to decipher the mechanism of interaction between GAG and other ECM constituents, in order to better understand their influence on compressive stiffness.

ACKNOWLEDGEMENTs

This work was supported by the Natural Sciences and Engineering Research Council of Canada.

REFERENCES

1. Martufi, Giampaolo, et al. "Is there a role for biomechanical engineering in helping to elucidate the risk profile of the thoracic aorta?." The Annals of thoracic surgery 101.1 (2016): 390-398.
2. Ghadie, Noor M., Jean-Philippe St-Pierre, and Michel R. Labrosse. "Intramural Distributions of GAGs and Collagen vs. Opening Angle of the Intact Porcine Aortic Wall." Annals of Biomedical Engineering 50.2 (2022): 157-168.