O-001 - MICROMECHANICAL CHARACTERISATION OF WALL FAILURE IN ABDOMINAL AORTIC ANEURYSM (AAA) TISSUE

TOPIC:
Abdominal Aortic Aneurysms
AUTHORS:
Hossack M. (Liverpool Vascular and Endovascular Service ~ Liverpool ~ United Kingdom) , Fisher R. (Liverpool Vascular and Endovascular Service ~ Liverpool ~ United Kingdom) , Torella F. (Liverpool Vascular and Endovascular Service ~ Liverpool ~ United Kingdom) , Madine J. (Institute of Systems, Molecular and Integrative Biology, University of Liverpool ~ Liverpool ~ United Kingdom) , Akhtar R. (Mechanical, materials and Aerospace Engineering, university of Liverpool ~ Liverpool ~ United Kingdom)
Introduction:
Use of a maximum diameter threshold as the sole indicator for aneurysm repair risks rupture during surveillance in higher-risk cases, and unnecessary repair in others. Efforts have been made to improve and personalise risk prediction. Here, we utilise nanoindentation, a high-resolution technique capable of measuring the material properties of vascular tissue non-destructively at an appropriate length-scale in order to characterise the micromechanical properties of aneurysmal aortic tissues with the aim of identifying high-risk cases and directing specific management.
Methods:
Full thickness anterior aortic wall tissue samples were harvested from 16 patients undergoing repair of degenerative AAA. We probed the micromechanical properties using nanoindentation with a 100 micrometer flat punch tip, determining the shear storage modulus (G′) and loss modulus (G''). We performed 4-5 indentations in axial orientation on cross-sectional wall samples in 3 layers (inner, middle, outer) where possible. At least 4 samples were tested from each patient. In total, there were 102 samples (1269 indentations). We stratified micromechanical findings according to maximum transverse diameter (MTD), established through interrogation of pre-operative contrast-enhanced computerised tomography scans.
Results:
Aortic wall tissue demonstrated a pattern of significantly reducing stiffness from the inner to middle (median 31.5 kPa vs 24.4 kPa, P<0.05) and middle to outer layers (24.2 kPa vs 13.1 kPa, P<0.05). Wall stiffness increased as MTD increased from 50-59mm to 60-69mm (median 20.7 kPa vs 29.5 kPa, P<0.05). At 70-79mm, wall stiffness reduced (median 22 kPa, NS), and reduced further as MTD exceeded 80mm (median 19.6 kPa, P<0.05) (Figure 1).
Conclusion:
The mechanical properties of vascular tissues depend largely on the extracellular matrix. AAA is characterised by elastolysis and compensatory collagen synthesis. This micromechanical approach has demonstrated that at higher MTD, AAA wall becomes less stiff. This may indicate a failure of the compensatory collagen network resulting in a higher risk of rupture. Further studies are needed to correlate aortic micro-stiffness with vessel wall biochemistry, histology and clinical presentation.
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