Jarin Kratzberg

Biomechanical failure properties and microstructural content of ruptured and unruptured abdominal aortic aneurysms.

PURPOSE To test the hypothesis that ruptured abdominal aortic aneurysms (AAA) are globally weaker than unruptured ones. METHODS Four ruptured and seven unruptured AAA specimens were harvested whole from fresh cadavers during autopsies performed over an 18-month period. Multiple regionally distributed longitudinally oriented rectangular strips were cut from each AAA specimen for a total of 77 specimen strips. Strips were subjected to uniaxial extension until failure. Sections from approximately the strongest and weakest specimen strips were studied histologically and histochemically. From the load-extension data, failure tension, failure stress and failure strain were calculated. Rupture site characteristics such as location, arc length of rupture and orientation of rupture were also documented. RESULTS The failure tension, a measure of the tissue mechanical caliber was remarkably similar between ruptured and unruptured AAA (group mean ± standard deviation of within-subject means: 11.2±2.3 versus 11.6±3.6N/cm; p=0.866 by mixed model ANOVA). In post-hoc analysis, there was little difference between the groups in other measures of tissue mechanical caliber as well such as failure stress (95±28 versus 98±23 N/cm(2); p=0.870), failure strain (0.39±0.09 versus 0.36±0.09; p=0.705), wall thickness (1.7±0.4 versus 1.5±0.4mm; p=0.470) , and % coverage of collagen within tissue cross section (49.6±12.9% versus 60.8±9.6%; p=0.133). In the four ruptured AAA, primary rupture sites were on the lateral quadrants (two on left; one on left-posterior; one on right). Remarkably, all rupture lines had a longitudinal orientation and ranged from 1 to 6 cm in length. CONCLUSION The findings are not consistent with the hypothesis that ruptured aortic aneurysms are globally weaker than unruptured ones.

Role of graft oversizing in the fixation strength of barbed endovascular grafts

PURPOSE The role of endovascular graft oversizing on risk of distal graft migration following endovascular aneurysm repair for abdominal aortic aneurysm is poorly understood. A controlled in vitro investigation of the role of oversizing in graft-aorta attachment strength for endovascular grafts (EVGs) with barbs was performed. METHODS Barbed stent grafts (N = 20) with controlled graft oversizing varying from 4-45% were fabricated while maintaining other design variables unchanged. A flow loop with physiological flow characteristics and a biosynthetic aortic aneurysm phantom (synthetic aneurysm model with a bovine aortic neck) were developed. The stent grafts were deployed into the aortic neck of the bio-synthetic aortic aneurysm phantom under realistic flow conditions. Computed tomography imaging of the graft-aorta complex was used to document attachment characteristics such as graft apposition, number of barbs penetrated, and penetration depth and angle. The strength of graft attachment to the aortic neck was assessed using mechanical pullout testing. Stent grafts were categorized into four groups based on oversizing: 4-10%; 11-20%; 21-30%; and greater than 30% oversizing. RESULTS Pullout force, a measure of post-deployment fixation strength was not different between 4-10% (6.23 +/- 1.90 N), 11-20% (6.25 +/- 1.84 N) and 20-30% (5.85 +/- 1.89 N) groups, but significantly lower for the group with greater than 30% oversizing (3.67 +/- 1.41 N). Increasing oversizing caused a proportional decrease in the number of barbs penetrating the aortic wall (correlation = -0.83). Of the 14 barbs available in the stent graft, 89% of the barbs (12.5 of 14 on average) penetrated the aortic wall in the 4-10% oversizing group while only 38% (5.25 of 14) did for the greater than 30% group (P < .001). Also, the stent grafts with greater than 30% oversizing showed significantly poorer apposition characteristics such as eccentric compression or folding of the graft perimeter. The number and depth of barb penetration were found to be positively correlated to pullout force. CONCLUSION Greater than 30% graft oversizing affects both barb penetration and graft apposition adversely resulting in a low pullout force in this in vitro model. Barbed stent grafts with excessive oversizing are likely to result in poor fixation and increased risk of migration.

The effect of proteolytic treatment on plastic deformation of porcine aortic tissue.

The objective of this work was to assess whether selective proteolysis of elastin and/or collagen in a porcine aorta followed by mechanical creep loading would result in an aneurysm-like permanent tissue stretch. The underlying motivations were to (1) test the feasibility of developing an in vitro abdominal aortic aneurysm (AAA) model, and (2) understand what role, if any, that passive creep-induced stretching plays in aneurysmal dilation. Multiple circumferentially oriented flat specimen strips were cut from the porcine thoracic aorta of ten adult pigs. Specimens were subjected to one of six treatment protocols: Untreated controls (UC;N=23), complete elastin degradation (E;N=10), partial elastin degradation (E(p);N=10), partial collagen degradation (C(p);N=22), and partial degradation of both elastin and collagen (E(p)+C(p);N=3). All specimens were then subjected to cyclic creep (10 min/cycle) with increasing load amplitude until failure. The zero-load strain prior to the creep cycle where failure occurred was defined as load-induced plastic strain. The plastic strain induced by treatment alone, creep loading alone and the total was determined for all specimens. The total plastic strain was significantly greater for E (mean +/- SD = 48.2 +/-17.6,p<0.005), E(p)(41.6+/-11.1,p<0.0005), but not for E(p)+C(p)(48.9+/-21.6,p=0.17) and C(p)(22.2+/-12.8,p=0.14) compared to UC (17.7+/-6.1). Of the total plastic strain, treatment-induced plastic strain was high for those specimens subjected to partial or total elastin degradation (E,E(p),E(p)+C(p)), but not for those where elastin was intact (C(p)). However, load-induced plastic strain in the treated specimens was not different in any of the treated groups compared to controls. Maximum total plastic strain of 78.6% was induced in one porcine aortic tissue from the E group. Even this is far lower than what would be needed for creating a realistic in vitro AAA model. Our findings do not support the feasibility of developing an in vitro AAA by enzymolysis followed by passive stretching of the aorta. The findings also suggest that AAA formation is unlikely to be a passive creep-induced stretching of a proteolytically degraded aortic wall as conventional thinking may suggest, but rather may be predominantly due to growth and remodeling of the aortic wall.

Heterogeneous, Variable Wall-Thickness Modeling of a Ruptured Abdominal Aortic Aneurysm

Pressure-induced mechanical stress in ruptured abdominal aortic aneurysms (AAA) was investigated using a finite element model with measured variations in wall thickness and material properties. We harvested an 8-cm ruptured AAA from a cadaver and recorded its geometry on bi-plane photographs and three-dimensionally reconstructed. The wall thickness was measured at over 100 sites on the aneurysm surface using digital calipers. Regional mechanical property variation (failure strengths and hyperelastic material parameters) was determined using data from uniaxial extension tests of 19 test strips cut from various regions of the AAA surface. The measured data for wall thickness and mechanical properties were transferred to the 3D AAA surface model and nonlinearly interpolated on the surface to obtain the point-to-point regional distribution for thickness and material parameters. Modeling the AAA as a thick shell, finite element stress analysis was performed. The peak stress was found to be exactly at the rupture site and was substantially higher than population averages. The measured low wall thickness at the rupture site appeared to play a major role in elevating stresses indicating that localized thin wall may be an important risk factor for AAA rupture.Copyright

Role of aortic stent graft oversizing and barb characteristics on folding

OBJECTIVE To evaluate folding in infrarenal stent grafts in relation to oversizing, barb angle, and barb length using computed tomography images of stent grafts deployed in explanted bovine aortas. METHODS Computed tomography data from an in vitro investigation on the effect of oversizing of 4% to 45% (n = 19), barb length of 2 to 7 mm (n = 11), and barb angle of 10° to 90° (n = 7) on device fixation were examined for instances of folding. Folding was classified as circumferential or longitudinal and quantified on an ordinal scale based on codified criteria. Cumulative fold ranking from 0 (no fold) to 6 (two severe folds) for each deployment was used as the measure of folding observed. RESULTS Of the 37 cases, cumulative mean ± standard deviation fold ranking for stent grafts oversized >30% (n = 5) was significantly greater than the rest (3.4 ± 1.7 vs 0.5 ± 1.2, respectively; Mann-Whitney U test; P < .005). When barb length was varied from 2 to 7 mm (oversizing held at 10%-20%), folding was noted in one of 11 cases. Similarly, when barb angle was varied from 0° (vertical) to 90° (horizontal), folding was not noted in any of the seven cases. The pullout force was not significantly different between stent grafts with and without folding (5.4 ± 1.95 vs 5.12 ± 1.89 N, respectively; P > .5). At least one instance of folding was noted in the seven of seven (100%) stent grafts with oversizing >23.5% and in only five of 30 (14%) stent grafts with oversizing <23.5%. CONCLUSIONS Stent graft folding was prevalent when oversized >30%. Large variations in barb length and angle did not aggravate folding risk when oversized within the recommended range of 10% to 20%.

A Dynamic Arterial Tree Phantom for studies of bolus chasing CT Angiography

Computed Tomography (CT) and Magnetic Resonance (MR) imaging techniques have become an important role in diagnosing the vascular diseases. Before a clinical trial of the newly developed techniques, it is essential to evaluate them on a phantom. However, there is no existing arterial tree phantom that serves for flood flows. To that end, we design and develop a Dynamic Arterial Tree Phantom (DATP), which is able to produce many kinds of blood flow pattern through using a programmable pump functions as a heart. Contrast enhanced CT scan is also performed on this phantom for adaptive bolus chasing techniques.

Failure Properties of Ruptured and Unruptured Abdominal Aortic Aneurysms

Rupture of abdominal aortic aneurysm (AAA) is a poorly understood phenomenon. Some aneurysms rupture as they grow larger, while many very large ones do not. There have been numerous reports on the failure properties of unruptured AAA [1], but similar data on ruptured AAA is scarce. Some reports suggest that greater pressure-induced tension in the AAA wall may predispose some to rupture [1]. But what of failure properties? It is conceivable that aneurysms which rupture are globally weaker than ones that do not or perhaps have very localized weak spots. Such key issues have not been explored because of lack of specimens from ruptured AAA. In this exploratory study between clinical and engineering investigators, unruptured and ruptured AAA were wholly harvested from fresh cadavers and the regional variations in wall thickness, mechanical properties and cellular content were compared.Copyright

The Cook Zenith Devices

The Cook Zenith family of devices includes a wide range of custom and off-the-shelf endovascular grafts and devices capable of treating patients with complex anatomy and pathology, including those previously considered unsuitable for endovascular repair. Available devices include the Zenith AAA, Zenith TX2 TAA, Zenith Fenestrated Graft, Iliac Branch Graft, and Helical Branch Graft, and the Zenith Dissection Stent. In addition, low-profile versions of the AAA (abdominal aortic aneurysm) and TAA (thoracic aortic aneurysm) grafts (designed based on a combination of nitinol stent technology and thin-wall graft material) are now available as investigational devices, allowing for endovascular treatment of patients with smaller access vessels. This chapter includes a review of device evolution and a brief review of the literature, which indicates that all commercially available devices are associated with positive clinical outcomes.

The Effect of Aortic Endovascular Graft Oversizing on Barb Penetration and Fixation Strength

Endovascular repair of abdominal aortic aneurysm (AAA), where an endovascular graft (EVG) — a stented vascular graft — is implanted intraluminally into the AAA has shown excellent short term outcome. However, long term outcome of implanted EVGs is fraught with new complications, the most severe of which is endoleak from graft migration, which can lead to re-pressurization of the AAA and potentially rupture. Graft migration is defined as the distal drift of an implanted EVG of 5mm or more from its initial anchor site (Figure 1). There have been many design changes to help decrease the rate of EVG migration including the addition of proximal attachment barbs to grafts to help secure them to the aortic wall. However, studies show that freedom from migration rates have not significantly increased for those grafts containing barbs compared to grafts without barbs [1]. We believe that controlled studies of endovascular graft parameters can lead to improvements in its design that increase graft attachment strength and hence decrease the risk of migration. The aim of the current study was to assess a key design variable in barbed grafts namely, graft oversizing (GO), defined as the ratio of expanded graft dia to aorta dia. We sought to assess the relationship between GO and attachment strength in barbed EVGs. Specifically, we hypothesized that a high GO will impede the ability of the EVG barbs to effectively penetrate the aortic wall.© 2008 ASME

Phenomenological Test Method to Assess of Material Symmetry in Thick Soft Tissues

Characterizing the material symmetry of a biological soft tissue can aid in understanding and modeling its mechanics. Optical methods have been reported for identification of structural fiber orientations in thin tissues such as heart valve leaflets [1], but not in thick tissues such as arteries because optical methods are not as effective with thick tissues. Besides fiber architecture, effective transmural material symmetry of a planar tissue needs to be known a priori in order to perform and interpret common testing methods such as planar biaxial testing or inflation testing (for cylindrical specimens). Nielsen et al. [2] reported on planar radial testing to study inhomogeneous properties of elastic membranes. We submit that planar radial extension testing (PRET) of a thick circular soft tissue specimen will reveal the underlying material symmetry. We performed numerical simulations of planar radial extension testing and assessed its feasibility using a simple custom-fabricated device.Copyright