The Society for Vascular Surgery practice guidelines on the care of patients with an abdominal aortic aneurysm

Nothing in biology makes sense except in the light of evolution, quipped Theodosius Dobzhansky. The theory of evolution argues that each biological species was not suddenly and independently created but that all life forms are interrelated by virtue of having descended from common ancestors through the accumulation of modifications. Indeed, nothing we know about living organisms would make any sense if they were not so interrelated. And the theory that biological species are descended from common ancestors provides an indispensable heuristic to understand why living organisms are what they are and do what they do.

Nothing in Biology Makes Sense Except in the Light of Evolution

Conduit arteries become stiffer with age due to alterations in their morphology and the composition of the their major structural proteins, elastin and collagen. The elastic lamellae undergo fragmentation and thinning, leading to ectasia and a gradual transfer of mechanical load to collagen, which is 100–1000 times stiffer than elastin. Possible causes of this fragmentation are mechanical (fatigue failure) or enzymatic (driven by matrix metallo proteinases (MMP) activity), both of which may have genetic or environmental origins (fetal programming). Furthermore, the remaining elastin itself becomes stiffer, owing to calcification and the formation of cross-links due to advanced glycation end-products (AGEs), a process that affects collagen even more strongly. These changes are accelerated in the presence of disease such as hypertension, diabetes and uraemia and may be exacerbated locally by atherosclerosis. Raised MMP activity, calcification and impaired endothelial function are also associated with a high level of plasma homocysteine, which itself increases with age. Impaired endothelial function leads to increased resting vascular smooth muscle tone and further increases in vascular stiffness and mean and/or pulse pressure. The effect of increased stiffness, whatever its underlying causes, is to reduce the reservoir/buffering function of the conduit arteries near the heart and to increase pulse wave velocity, both of which increase systolic and pulse pressure. These determine the peak load on the heart and the vascular system as a whole, the breakdown of which, like that of any machine, depends more on the maximum loads they must bear than on their average. Reversing or stabilising the increased arterial stiffness associated with age and disease by targeting any or all of its causes provides a number of promising new approaches to the treatment of systolic hypertension and its sequelae, the main causes of mortality and morbidity in the developed world. Copyright  2007 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/path.2101

Ageing of the conduit arteries.

The mechanical properties of skin are important for a number of applications including surgery, dermatology, impact biomechanics and forensic science. In this study, we have investigated the influence of location and orientation on the deformation characteristics of 56 samples of excised human skin. Uniaxial tensile tests were carried out at a strain rate of 0.012 s(-1) on skin from the back. Digital Image Correlation was used for 2D strain measurement and a histological examination of the dermis was also performed. The mean ultimate tensile strength (UTS) was 21.6±8.4 MPa, the mean failure strain 54%±17%, the mean initial slope 1.18±0.88 MPa, the mean elastic modulus 83.3±34.9 MPa and the mean strain energy was 3.6±1.6 MJ/m(3). A multivariate analysis of variance has shown that these mechanical properties of skin are dependent upon the orientation of the Langer lines (P<0.0001-P=0.046). The location of specimens on the back was also found to have a significant effect on the UTS (P=0.0002), the elastic modulus (P=0.001) and the strain energy (P=0.0052). The histological investigation concluded that there is a definite correlation between the orientation of the Langer lines and the preferred orientation of collagen fibres in the dermis (P<0.001). The data obtained in this study will provide essential information for those wishing to model the skin using a structural constitutive model.

https://researchrepository.ucd.ie/bitstream/10197/4616/2/Repos%20-%20not-%20commented2%20done.pdf

Characterization of the anisotropic mechanical properties of excised human skin

During the resorbable-polymer-boom of the 1970s and 1980s, polycaprolactone (PCL) was used in the biomaterials field and a number of drug-delivery devices. Its popularity was soon superseded by faster resorbable polymers which had fewer perceived disadvantages associated with long term degradation (up to 3-4 years) and intracellular resorption pathways; consequently, PCL was almost forgotten for most of two decades. Recently, a resurgence of interest has propelled PCL back into the biomaterials-arena. The superior rheological and viscoelastic properties over many of its aliphatic polyester counterparts renders PCL easy to manufacture and manipulate into a large range of implants and devices. Coupled with relatively inexpensive production routes and FDA approval, this provides a promising platform for the production of longer-term degradable implants which may be manipulated physically, chemically and biologically to possess tailorable degradation kinetics to suit a specific anatomical site. This review will discuss the application of PCL as a biomaterial over the last two decades focusing on the advantages which have propagated its return into the spotlight with a particular focus on medical devices, drug delivery and tissue engineering.

The return of a forgotten polymer : Polycaprolactone in the 21st century

Coronary heart disease is a major and global health care challenge, and its biomechanical analysis has been shown to be a useful complement in answering related clinical questions. Compelling evidence has been accumulated showing that two primary biomechanical factors predispose to the disease: low or oscillating wall shear stress and high wall mechanical stress (or strain) of plaque tissue components. While the level of sophistication with which vascular biomechanical analysis can be made has increased dramatically, the utility of biomechanical models in clinical practice remains very limited and their capability is by far not fully exploited. This chapter aims at reviewing the principles of the biomechanical analysis of coronary arteries in relation to the interplay of biomechanical, biochemical, and clinical factors. Techniques, methods, and modeling assumptions are discussed in relation to their clinical relevance, and thus toward the feasibility of translating research results and developing new treatment techniques and diagnostic tools.

The interaction of biochemical, biomechanical, and clinical factors of coronary disease

An abdominal aortic aneurysm (AAA) is a focal dilation of the abdominal aorta and is associated with a risk of fatal rupture. Experimental studies suggest that myo-inositol may exert beneficial effects on AAAs through favourable changes to biological pathways implicated in AAA pathology. The aim of the Inositol in the MAnaGemENt of abdominal aortic aneurysm (IMAGEN) trial is to assess if myo-inositol will reduce AAA growth. IMAGEN is a multi-centre, prospective, parallel-group, randomised, double-blind, placebo-controlled trial. A total of 164 participants with an AAA measuring ≥ 30 mm will be randomised to either 2 g of myo-inositol or identical placebo twice daily for 12 months. The primary outcome measure will be AAA growth estimated by increase in total infrarenal aortic volume measured on computed tomographic scans. Secondary outcome measures will include AAA diameter assessed by computed tomography and ultrasound, AAA peak wall stress and peak wall rupture index, serum lipids, circulating AAA biomarkers, circulating RNAs and health-related quality of life. All analysis will be based on the intention-to-treat principle at the time of randomisation. All patients who meet the eligibility criteria, provide written informed consent and are enrolled in the study will be included in the primary analysis, regardless of adherence to dietary allocation. Currently, there is no known medical therapy to limit AAA progression. The IMAGEN trial will be the first randomised trial, to our knowledge, to assess the value of myo-inositol in limiting AAA growth. Australian New Zealand Clinical Trials Registry, ACTRN12615001209583
 . Registered on 6 November 2015.

/pdf/inositol-in-the-management-of-abdominal-aortic-aneurysm-1pfv5hy01a.pdf

T. Christian Gasser

Papers

The interaction of biochemical, biomechanical, and clinical factors of coronary disease

Inositol in the MAnaGemENt of abdominal aortic aneurysm (IMAGEN): study protocol for a randomised controlled trial

Morphological and Biomechanical Evolution of Abdominal Aortic Aneurysms During Surveillance

Fracture of the porcine aorta. Part 1: symconCT fracture testing and DIC.

Abdominal Aortic Aneurysm Rupture Risk Assessment Using Machine Learning to Integrate Biomechanical, Geometrical, and Patient Characteristics