ReviewBiomechanics of abdominal aortic aneurysm
Introduction
Abdominal aortic aneurysm (AAA) is a focal, balloon-like dilation of the terminal aortic segment that occurs gradually over a span of years. This condition is growing in prevalence in the elderly population, with approximately 150,000 new cases being diagnosed every year (Ouriel et al., 1992; Bengtsson et al., 1996). An AAA may rupture if it is not treated, and this is ranked as the 13th most common cause of death in the US (Patel et al., 1995). Current AAA repair procedures are expensive and carry significant morbidity and mortality risks (Darling et al., 1977; Wain et al., 1998; Turnipseed et al., 2001; Velazquez et al., 2001; Gabrielli et al., 2004; Ghansah and Murphy, 2004; Blankensteijn et al., 2005; EVAR trial participants, 2005a, EVAR trial participants, 2005b; Goueffic et al., 2005; Schouten et al., 2005; van Marrewijk et al., 2005; Dillavou et al., 2006).
Because most patients with AAA are elderly, and/or have co-morbid conditions, and because current repair techniques are not without complications (Moore and Rutherford, 1996; Blum et al., 1997; Wain et al., 1998; Cuypers et al., 1999; Zarins et al., 2000; Brewster, 2001; Hallin et al., 2001; Sicard et al., 2001), the clinician is faced with a dilemma: deciding when the risk of AAA rupture justifies the risks associated with repair. However, there is no currently accepted technique available to quantify the risk of rupture for individual AAA. The decision to electively repair an AAA is widely based on the “maximum diameter criterion”; i.e., when the aneurysm reaches a certain size (typically 5 or 5.5 cm), it is thought that the risk of rupture warrants repair (Dryjski et al., 1994). However, this criterion is only a general rule-of-thumb and is unreliable (Darling et al., 1977; Geroulakos and Nicolaides, 1992). Autopsy studies have shown that small AAAs can rupture (Choksy et al., 1999; Hall et al., 2000), while some of those considered large will not rupture, given the life expectancy of the patient (Darling et al., 1977). Indeed, intervention based on the “maximum diameter criterion” may be offered too late or not be necessary at all for a particular patient. Therefore, there is a need for a new method that will reliably predict the likelihood of AAA rupture on a patient-specific basis as opposed to a “one-criterion-fits-all” approach.
Through increasingly complex degrees, beginning with the simplified Law of Laplace, biomechanical considerations have been used to consider AAA rupture potential, and these are reviewed below. The basic premise of this consideration is that AAA rupture follows the basic principles of material failure; i.e., an aneurysm ruptures when the mural stresses or deformation meet an appropriate failure criterion.
In this review, the inability of various empirical criteria to accurately assess AAA rupture potential is discussed. Following this, the history and the state-of-the-art of AAA biomechanics are presented, including a summary of investigations into the biomechanical behavior of AAA tissues, modeling AAA wall stress and tensile strength distribution and factors which influence them, and the potential clinical utility of these estimates in predicting AAA rupture. It should be noted that this review will not consider the growing body of literature pertaining to the biomechanics of endovascular aneurysm repair or grafting (Marston et al., 1996; Li and Kleinstreuer, 2005a), but rather will be focused on the native aneurysm itself.
Section snippets
Fallacy of empirical criteria to assess AAA rupture potential
Following the pioneering work of DuBost and Vorhees in the middle of the 20th Century (Dubost et al., 1952; Blakemore and Voorhees, 1954), AAA repair became the mainstay of the vascular surgeon's practice. Ever since, clinicians have attempted to develop means to accurately predict the risk of aneurysm rupture. Nearly all of the criteria that have been proposed to date have been based on empirical data as opposed to sound physical principles. The most commonly used criterion is the “maximum
Biomechanical behavior of AAA tissues
An AAA is typically comprised of two primary structures—the diseased and dilated aortic wall and an ILT, which is a large, stationary blood clot incorporated with blood cells, platelets, blood proteins, and cellular debris (Adolph et al., 1997). Since ILT is contained in most AAAs (Harter et al., 1982), it is prudent to consider the biomechanical behavior of both this material and the AAA wall, as well as in vivo studies that evaluated the biomechanical behavior of the entire AAA structure in
AAA wall stress
Given that the Law of Laplace suggests that the wall tension in AAA is elevated compared to the undilated aorta, the earliest investigations of AAA biomechanics were focused on estimating AAA wall stress. In 1986, Stringfellow et al. (1987) used the Law of Laplace to determine the wall stresses in a hypothetical AAA by idealizing its geometry as cylindrical or spherical. A simplified two-dimensional (2D) stress analysis was also performed to evaluate the effect of aorta-aneurysm geometry.
AAA wall strength
Our 1996 reports on the uniaxial tensile testing of freshly excised specimens appears to be the first to provide measures of AAA wall strength (Raghavan et al., 1996; Vorp et al., 1996b). We found that AAA wall tissue was approximately 50% weaker than control (non-aneurysmal) abdominal aorta. We also noted in this work that there were no significant differences in strength between circumferentially-oriented and longitudinally-oriented AAA tissue specimens. We have since performed tensile
Clinical application of AAA biomechanics
There have been only a few reported studies where a biomechanical approach was applied to clinical data to determine whether a correlation exists between biomechanical parameters and AAA rupture. Sonesson et al. (1999) investigated the beta stiffness of ruptured and non-ruptured aneurysms and concluded that this parameter could not be used as an indicator of eventual AAA rupture. Hall et al. (2000) described the relationship between AAA wall stress derived from the Law of Laplace (i.e., based
Conclusion
Current clinical assessment methods to evaluate AAA rupture potential are unreliable. In general, an enlarging AAA is accompanied by both an increase in wall stress and a decrease in wall strength, and both of these parameters are critical and need to taken into account as the instant of AAA rupture occurs when the former exceeds the latter. For these reasons, much attention has been focused over the years on the biomechanics of AAA, particularly with regards to wall stress assessment. The Law
Acknowledgements
The author would like to acknowledge the financial support of AAA biomechanics research from The Whitaker Foundation, The Pittsburgh Foundation, the Competitive Medical Research Fund of the University of Pittsburgh Medical Center, and the NHLBI (Grants R01-HL-060670 and R01-HL-079313). The valuable support and clinical insights of the Division of Vascular Surgery at the University of Pittsburgh Medical Center, particularly Drs. Marshall Webster, Michel Makaroun, and David Steed, is gratefully
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