Tantalum-Dacron coknit stent for endovascular treatment of aortic aneurysms: a preliminary experimental study

A comprehensive review of biological and materials properties of Tantalum and its alloys

Tantalum (Ta) and its alloys have been used for various cardiovascular, orthopedic, fracture fixation, dental, and spinal fusion implants. This review evaluates the biological and material properties of Ta and its alloys. Specifically, the biological properties including hemocompatibility and osseointegration, and material properties including radiopacity, MRI compatibility, corrosion resistance, surface characteristics, semiconductivity, and mechanical properties are covered. This review highlights how the material properties of Ta and its alloys contribute to its excellent biological properties for use in implants and medical devices.

Vascular Smooth Muscle Cells in Aortic Aneurysm: From Genetics to Mechanisms

Aortic aneurysm, including thoracic aortic aneurysm and abdominal aortic aneurysm, is the second most prevalent aortic disease following atherosclerosis, representing the ninth-leading cause of death globally. Open surgery and endovascular procedures are the major treatments for aortic aneurysm. Typically, thoracic aortic aneurysm has a more robust genetic background than abdominal aortic aneurysm. Abdominal aortic aneurysm shares many features with thoracic aortic aneurysm, including loss of vascular smooth muscle cells (VSMCs), extracellular matrix degradation and inflammation. Although there are limitations to perfectly recapitulating all features of human aortic aneurysm, experimental models provide valuable tools to understand the molecular mechanisms and test novel therapies before human clinical trials. Among the cell types involved in aortic aneurysm development, VSMC dysfunction correlates with loss of aortic wall structural integrity. Here, we discuss the role of VSMCs in aortic aneurysm development. The loss of VSMCs, VSMC phenotypic switching, secretion of inflammatory cytokines, increased matrix metalloproteinase activity, elevated reactive oxygen species, defective autophagy, and increased senescence contribute to aortic aneurysm development. Further studies on aortic aneurysm pathogenesis and elucidation of the underlying signaling pathways are necessary to identify more novel targets for treating this prevalent and clinical impactful disease.

Use of electron microscopy to study platelets and thrombi

Electron microscopy has been a valuable tool for the study of platelet biology and thrombosis for more than 70 years. Early studies using conventional transmission and scanning electron microscopy (EM) provided a foundation for our initial understanding of platelet structure and how it changes upon platelet activation. EM approaches have since been utilized to study platelets and thrombi in the context of basic, translational and clinical research, and they are instrumental in the diagnosis of multiple platelet function disorders. In this brief review, we provide a sampling of the many contributions EM based studies have made to the field, including both historical highlights and contemporary applications. We will also discuss exciting new imaging modalities based on EM and their utility for the study of platelets, hemostasis and thrombosis into the future.

Translational Relevance and Recent Advances of Animal Models of Abdominal Aortic Aneurysm

Human abdominal aortic aneurysm (AAA) pathophysiology is not yet completely understood. In conductance arteries, the insoluble extracellular matrix, synthesized by vascular smooth muscle cells, assumes the function of withstanding the intraluminal arterial blood pressure. Progressive loss of this function through extracellular matrix proteolysis is a main feature of AAAs. As most patients are now treated via endovascular approaches, surgical AAA specimens have become rare. Animal models provide valuable complementary insights into AAA pathophysiology. Current experimental AAA models involve induction of intraluminal dilation (nondissecting AAAs) or a contained intramural rupture (dissecting models). Although the ideal model should reproduce the histological characteristics and natural history of the human disease, none of the currently available animal models perfectly do so. Experimental models try to represent the main pathophysiological determinants of AAAs: genetic or acquired defects in extracellular matrix, loss of vascular smooth muscle cells, and innate or adaptive immune response. Nevertheless, most models are characterized by aneurysmal stabilization and healing after a few weeks because of cessation of the initial stimulus. Recent studies have focused on ways to optimize existing models to allow continuous aneurysmal growth. This review aims to discuss the relevance and recent advances of current animal AAA models.

Visual Overview—

An online visual overview is available for this article.

Management of Abdominal Aortic Aneurysm: A Decade of Progress

Since the world was first introduced to the concept of endovascular aneurysm repair by Parodi's landmark procedures in 1990, stent-grafts have assumed a prominent role in the management of abdominal aortic aneurysm. Most modern systems are trackable, accurate, and secure. The resulting endovascular procedure is safe, durable, effective, and versatile. Perhaps the most significant increment in the applicability of the endovascular technique was achieved by the development of bifurcated stent-grafts, which dispensed with inadequate distal aortic implantation sites. Additional branches and fenestrations now permit endovascular repair in cases of thoracoabdominal, pararenal, juxtarenal, and bilateral iliac aneurysms. These advances in device performance have been accompanied by a rapid dissemination of necessary skills, leading to the development of a new superspecialty of vascular therapy, with elements of vascular surgery, interventional radiology, and interventional cardiology.

Endovascular Stent-Anchored Aortic Grafts: A Comparison between Self-Expanding and Balloon-Expandable Stents in Minipigs

Purpose:

To study endovascular graft attachment with self-expanding Gianturco Z-stents and balloon-expanded Palmaz stents and the effect of these devices on the renal ostia.

Methods:

Ten stent-grafts were constructed, 5 with Gianturco Z-stents and 5 with Palmaz stents. The endografts were implanted under fluoroscopic guidance into the abdominal aorta of 10 pigs so that the uncovered portion of the proximal stent extended over the renal artery orifices. Distal aortic blood pressure and flow were measured before and after graft placement and 1 hour postprocedure. The aorta was then exposed surgically, and the central portion of the stent-graft was inspected through an aortotomy to assess perigraft leakage.

Results:

Stent-graft implantation was accurate and hemostatic in all cases, despite longitudinal folding of the graft due to oversizing. However, transverse folds produced pressure gradients (> 15 mmHg) between the ends of the graft in two cases. In another case, a pressure gradient resulted from partial thrombosis of the graft. In two cases, renal artery occlusion and thrombosis occurred due to coverage by the graft material. In two other animals, one of the renal arteries was entirely uncovered by a stent. The remaining 16 renal arteries were covered by the proximal stent but not the graft, as intended. One (6.25%) of these arteries thrombosed, but the remainder were grossly patent when the animals were sacrificed at 1 hour.

Conclusions:

Both Palmaz and Gianturco Z-stents produced hemostatic endovascular graft attachment, even in the presence of moderate graft oversizing. The risk of acute renal artery occlusion from juxtarenal stenting does not appear to be prohibitive, but longer term observations are needed.

Closure of a Popliteal Arteriovenous Fistula Using an Autologous Vein-Covered Palmaz Stent

Purpose:

To report the use of autologous vein to cover a stainless steel stent designated for repair of a traumatic popliteal arteriovenous (AV) fistula.

Methods and Results:

Autologous cephalic vein was harvested to cover a Palmaz biliary stent selected to close a traumatic popliteal AV fistula that persisted despite reparative attempts with balloon occlusion and coil embolization. The vein-covered stent was delivered percutaneously and deployed, successfully obliterating the vascular communication. Patency of the popliteal artery was documented arteriographically at 5 months, and symptomatic improvement continues at 10 months.

Conclusion:

The simplicity of this percutaneous approach and the use of autologous vein to cover endovascular prostheses create the possibility for evaluating this technique in myriad anatomical situations.

An Experimental Model for the Acute and Chronic Evaluation of Intra-Aneurysmal Pressure

Purpose:

To develop an animal model for the acute and chronic monitoring of pressure within abdominal aortic aneurysms (AAAs) to be treated with endovascular grafts.

Methods:

A strain-gauge pressure transducer was placed within an AAA created from a prosthetic vascular graft. Prosthetic aneurysms were implanted into 17 canine infrarenal aortas. The intra-aneurysmal pressure was monitored and correlated with noninvasive forelimb sphygmomanometry for 2 weeks. After this time, an intravascular manometer catheter was passed into the aneurysm. Simultaneous pressure measurements were obtained using the implanted strain-gauge pressure transducer, the manometer catheter, and the forelimb sphygmomanometer. Angiography was performed to assess intraluminal morphology, aneurysm anastomoses, and adjoining aortic vessels. In addition, two control animals underwent intra-aneurysmal pressure monitoring after standard surgical aneurysm repair.

Results:

There was excellent correlation (r = 0.97) between the pressure measurements obtained with the implanted strain-gauge pressure transducer and the intravascular manometer. Close correlation was also observed between the implanted strain-gauge transducer and the forelimb sphygmomanometer (r = 0.88) during postprocedural monitoring. Intra-aneurysmal pressure was lowered dramatically by surgical exclusion (aneurysm: 15/5 ± 7/4 mmHg; systemic: 124/66 ± 34/17 mmHg; p < 0.001). The prosthetic aneurysms were successfully imaged with angiography.

Conclusions:

This animal model provides an accurate and reproducible means for measuring intra-aneurysmal pressure on an acute and chronic basis. It may be possible to use this model in the assessment of endovascular devices to determine their efficacy in reducing intra-aneurysmal pressure. Evaluation of complications associated with their use, such as patent aneurysm side branches, perigraft channels, and perianastomotic reflux, may also be possible.

Mechanical characteristics of composite knitted stents

We used metal wires and fibers to fabricate a composite knitted stent and then compare the mechanical characteristics of this stent with those of a pure metallic stent of the same construction in order to develop a stent that offers a comparable degree of expandability as metallic stents but can be used for highly curved lesions that cannot be treated using metallic stents. We fabricated two types of composite knitted stent (N-Z stents), using nitinol wire with a diameter of 0.12 mm and polypara-phenylene-benzobisoxazole (PBO) multifilament fiber (Zyron AS; Toyobo, Osaka, Japan). Stents were knitted into a cylindrical shape using the same textile pattern as a Strecker stent. Two loop lengths (L) of nitinol wire were used in the N-Z stents: L = 1.84 mm (N-Z stent L = 1.84) and L = 2.08 mm (N-Z stent L = 2.08). For the sake of comparison, we fabricated a metallic stent of nitinol using the same textile pattern (N-N stent L = 1.92). We applied a radial compression force diametrically to each stent and applied a bending force diametrically at the free end of a stent with one end fixed in order to evaluate the relationship between stent elasticity and load values. In addition, we macroscopically evaluated the generation of kinks when the stent was bent 180 degrees . The radial compressive force when the stent diameter was reduced by 53% was 6.44 N in the case of N-Z stent L = 1.84, 6.14 N in the case of N-Z stent L = 2.08, and 4.96 N in the case of N-N stent L = 1.92 mm. The composite stent had a radial compressive force higher than that of a metallic stent. The restoring force to longitudinal direction at a 90 degrees bending angle was 0.005 N for N-Z stent L = 1.84, 0.003 N for N-Z stent L = 2.08, and 0.034 N for N-N stent L = 1.92. The restoring force of the composite stent was significantly lower. Finally, the composite stent generated no definitive kinks at a bending angle of 180 degrees , regardless of loop length. However, the N-N stent clearly produced kinks, causing blockage of the inner cavity. In conclusion, the use of a metal and fiber composite in the construction of a knitted stent ensures an expansion performance comparable to that of metallic stents, while providing better kink resistance.

Phasic hemodynamics and reverse blood flows in the aortic isthmus and pulmonary arteries of preterm lambs with pulmonary vascular dysfunction

Time-domain representations of the fetal aortopulmonary circulation were carried out in lamb fetuses to study hemodynamic consequences of congenital diaphragmatic hernia (CDH) and the effects of endothelin-receptor antagonist tezosentan (3 mg/45 min). From the isthmic aortic and left pulmonary artery (PA) flows (Q) and isthmic aortic, PA, and left auricle pressures (P) on day 135 in 10 controls and 7 CDH fetuses (28 ewes), discrete-triggered P and Q waveforms were modelized as Pt and Qt functions to obtain basic hemodynamic profiles, pulsatile waves [P, Q, and entry impedance (Ze)], and P and Q hysteresis loops. In the controls, blood propelling energy was accounted for by biventricular ejection flow waves (kinetic energy) with low Ze and by flow-driven pressure waves (potential energy) with low Ze. Weak fetal pulmonary perfusion was ensured by reflux (reverse flows) from PA branches to the ductus anteriosus and aortic isthmus as reverse flows. Endothelin-receptor antagonist blockade using tezosentan slightly increased the forward flow but largely increased diastolic backward flow with a diminished left auricle pre- and postloading. In CHD fetuses, the static component overrode phasic flows that were detrimental to reverse flows and the direction of the diastolic isthmic flow changed to forward during the diastole period. Decreased cardiac output, flattened pressure waves, and increased forward Ze promoted backward flow to the detriment of forward flow (especially during diastole). Additionally, the intrapulmonary arteriovenous shunting was ineffective. The slowing of cardiac output, the dampening of energetic pressure waves and pulsatility, and the heightening of phasic impedances contributed to the lowering of aortopulmonary blood flows. We speculate that reverse pulmonary flow is a physiological requirement to protect the fetal pulmonary circulation from the prominent right ventricular stream and to enhance blood flow to the fetal heart and brain.

An animal model for type II endoleaks with use of a tsuzumi drum-shaped stent-graft

PURPOSE

To create an animal model of type II endoleaks after endoluminal deployment of a specially designed stent-graft (SG).

MATERIALS AND METHODS

Five swine were used. A tsuzumi drum-shaped SG consisting of a covered Z stent with its diameter narrowed at the center was deployed in the midthoracic aorta. In this way, a residual space (RS) was created between the aortic wall and the graft to simulate an aneurysm sac. A 5-F catheter was placed into the RS, and then aortography, RS angiography, and pressure measurements were performed. Follow-up was performed at 3 and 10 days after the procedure. Mean pressure indexes (MPIs) were calculated as the ratio of the mean RS pressure to the aortic pressure. Histologic examination was also performed.

RESULTS

RSs with two or three pairs of intercostal arteries were successfully created in all cases. Aortography showed two type II endoleaks in five swine just after SG deployment and four type II endoleaks at 10 days. RS angiography showed circulation between the RS and the intercostal arteries in all cases. The mean MPI was 69.4% +/- 10.4% just after SG deployment and increased to 87.8% +/- 5.2% at 10 days. By gross examination, RS patency was retained.

CONCLUSIONS

A swine model of type II endoleaks was successfully created endoluminally. This model does not require direct surgery to the aorta and its side branches and promises to be useful to study the mechanism of and therapy for type II endoleaks.

Embolization-driven Occlusion of the Abdominal Aortic Aneurysmal Sac as the Basis of Prevention of Endoleaks in a New Swine Model

OBJECTIVES

To assess the effect of a new polymer in embolization of endoleaks using an animal model.

METHODS

A modified aortic stent-graft was placed in 20 pigs. Embolization was performed at the time of graft insertion with non-cytotoxic n-butyl-2-cyanoacrylate-metacryloxysulpholane and lipiodol (0.2:0.8ratio, 2 ml). Angiography, scanning electron microscopy and immuno-histochemistry were obtained at day 0, 1 week and 3 months.

RESULTS

In control animals both type I and II endoleaks were demonstrated. In treated animals, neither type-I nor type-II endoleaks were observed and a fibro-proliferative response was demonstrated within the aneurysm thrombus.

CONCLUSIONS

Host vascular responses govern the fate of the excluded aneurysm. Embolization of the sac and feeding arteries with non-cytotoxic glue sealed all occlusions by stimulating a massive restenosis-like process.

Compliance matching stent placement in the carotid artery of the swine promotes optimal blood flow and attenuates restenosis

OBJECTIVES

We assessed the value of a gradient-compliant stent in an animal model.

METHODS

Bilateral carotid arteries were stented with nitinol stents having variable-oversizing, variable-stiffness, and with (CMS, 10 animals) and without (SMART, four animals) compliance-matching endings. Angiography, hemodynamic, scanning-electron-microscopic and histological analyses were performed at 3-month. The protocol was completed in 14 among 19 swines.

RESULTS

Transient (1-month) exaggerated recoil, attributable to stress-induced phasic inhibition of vasorelaxation, developed at CMS endings. At mid-term, all stents were endothelialized; CMS-stents, but not SMART-stents, were incorporated into walls (one-strut-thickness). Restenosis developed outside SMART-stents (cell migration+wall-compensatory enlargement) whereas CMS-stents elicited no or focalized cell-accumulations at endings that bulged vascular walls radially outward. SMART-stents were blood-flow neutral, whereas CMS-stents favored (higher-stiffness, higher-oversizing) or opposed (lower-stiffness, less-oversizing) carotid blood flow.

CONCLUSIONS

Direct carotid stenting with stents having compliance-matched endings and specific requirements of stiffness and oversizing can optimize blood flow to the brain and restrict local restenosis.

A Canine Model for Studying Endoleak after Endovascular Aneurysm Repair

PURPOSE

The aim of this study was to create an animal model of endoleak after stent-graft placement for abdominal aortic aneurysm (AAA) in which a large aneurysmal sac would be preserved for the testing of techniques for its percutaneous occlusion.

MATERIALS AND METHODS

Infrarenal AAAs were created in nine dogs by anastomosis of an isolated segment of the inferior vena cava to the right side of the abdominal aorta in combination with a large anterior patch from the external jugular vein. One hour later, animals underwent percutaneous implantation of polytetrafluoroethylene-covered Z stent endografts with three 3-mm-diameter holes through the fabric. Aortograms were obtained before and after surgery, after endograft placement, and at the time of animal sacrifice at 1 week or 1, 2, 3, or 6 months. Pressures within the aorta and the aneurysm sac were recorded before animal sacrifice. Gross and histologic evaluations of the specimens were then carried out.

RESULTS

Immediately after endograft placement, all nine animals had artificial type III endoleaks with angiographic filling of lumbar arteries and veins. One animal died of surgical complications within 2 days of surgery and is not included in our data analysis. One aneurysm ruptured at 1 week. At completion of the study, six endografts were patent and two were occluded. The aneurysm sac had enlarged by approximately 50% in seven animals. At follow-up, type I endoleak was present in three animals, type II endoleak was present in three, and the artificial type III endoleak was present in all six animals with patent endografts. The pressure differential between aorta and aneurysm sac was 36 mm Hg, with a mean aortic pressure of 87 mm Hg +/- 13.3 and a mean aneurysmal sac pressure of 51 mm Hg +/- 28.1. The aneurysmal sac exhibited early thrombus formation at 1 week, which progressed to complete thrombosis in 1-6 months.

CONCLUSIONS

The model is technically feasible but would be useful in testing occlusive techniques for residual aneurysm sacs only in the acute phase after endograft placement. It would be not reliable for chronic evaluation because of rapidly progressive thrombosis in most aneurysm sacs and occasional complete thrombosis of the AAA and endograft.

Endovascular aneurysm repair: Magnetic resonance monitoring of histological organization processes in the excluded aneurysm

BACKGROUND

The purpose of the present study was to systematically analyze the histopathologic organization processes in excluded aneurysms after endovascular stenting and to develop a noninvasive monitoring method for these processes using MRI.

METHODS AND RESULTS

In 36 mongrel dogs, autologous aortic aneurysms were created. Endovascular treatment was performed using covered stents. Follow-up was after 1 week, 6 weeks, and 6 months. MRI was performed with T2-weighted turbo-spin-echo sequences and T1-weighted spin-echo sequences and was repeated after contrast bolus with gadolinium. Histopathologic findings were correlated to signal intensities (SIs) of MRI images. SIs of distinct areas were analyzed and related to the SI of the reference tissue (SI ratio). The histological organization process was gradated in the following 4 classes: class 0, detritus without organization; classes I and II, connective tissue proliferation with increasing fiber synthesis; and class III, dense fibrous connective tissue. The SI ratios of T2-weighted images were significantly reduced from 4.76 in detritus (0) to 1.70 in dense fibrous connective tissue (III) as a function of histopathologic classes. SI ratios of T1-weighted images were reduced from 1.84 (0) to 1.12 (III). Contrast bolus with gadolinium-DTPA showed no change of SI ratio in detritus (0.99) but an increase from 1.12 (I) to 1.70 (III) as organization increased.

CONCLUSIONS

The histological organization of excluded aneurysms can be monitored by MRI. Progressive organization is indicated by decreasing SIs in T2- and an increasing signal increase in T1-weighted images after gadolinium bolus.

Macroporous polyester-covered stent in an experimental abdominal aortic aneurysm model

PURPOSE

To validate a recently described animal model of abdominal aortic aneurysm (AAA) and to assess a new macroporous polyester-covered stent for endovascular AAA exclusion.

METHODS

Twenty adult sheep had AAAs surgically created by replacing a segment of the infrarenal aorta with an autologous jugular venous graft. Three months later, surviving animals underwent percutaneous implantation of macroporous polyester-covered nitinol stents; 3 animals with untreated AAAs served as controls. Follow-up surveillance included spiral computed tomography at 1 month and digital subtraction angiography at 3 and 6 months. Endografted animals were sacrificed at 1, 3, and 6 months after implantation; specimens from all animals were examined grossly and microscopically.

RESULTS

Seven (35%) animals died within 24 hours of causes related to the technique; 1 animal developed paraplegia and was sacrificed on day 1. Three (25%) animals died of spontaneous aneurysm rupture at <10 days, and 6 received the stent-graft at 3 months. The macroporous cover did not prevent continued perfusion of the sac early after stent-graft deployment, but all aneurysms were excluded on the 1-month CT.

CONCLUSIONS

Spontaneous AAA rupture occurred earlier and was not as frequent as previously described for this model. Implantation of the covered stent was feasible, but aneurysm exclusion was not immediate.

Animal models for atherosclerosis, restenosis, and endovascular graft research

Animal models have significantly advanced our understanding of the mechanisms of atherosclerosis and restenosis formation and the evaluation of therapeutic options. The current focus of research is on preventive strategies against restenosis and includes pharmacologic and biologic interventions directed primarily against smooth muscle cell proliferation, endovascular devices for recanalization and/or drug delivery, and an integrated approach using both devices and pharmacobiologic agents. Devices aimed at the percutaneous endoluminal exclusion of aortic aneurysms have also generated interest recently. The experience over many decades with animal models in vascular research has established that a single, ideal, naturally available model for atherosclerosis, restenosis, or for that matter aneurysm formation, does not exist. Presently, rabbits and pigs are favored for the former two areas of study, and dogs and sheep appear to provide suitable models for testing devices for endoluminal repair of aneurysms. The development of transgenic variants of currently available models may widen our options in the future. Nevertheless, an appreciation of the individual features of natural or stimulated disease in each species is of the utmost importance for the proper design and execution of relevant experiments.

Hemodynamics and wall mechanics after stent placement in swine iliac arteries: comparative results from six stent designs

PURPOSE

To compare the hemodynamics and wall mechanics of swine iliac arteries after placement of six types of stent.

MATERIALS AND METHODS

Stents were placed in the iliac artery of 18 pigs (three pigs each underwent placement with one of six types of stent); 16 untreated pigs served as control animals. Iliac arterial hemodynamics and wall mechanics were measured 4 days after placement.

RESULTS

Four stents (Palmaz-Schatz, Cordis, Warren, NJ; and Strecker, Cragg, and Symphony, Boston Scientific/Vascular, Natick, Mass) caused decreased pulsatile flow rate in the treated and contralateral iliac arteries; one (Memotherm; Bard, Covington, Ga) caused increased flow pulsatility; and one (Wallstent; Schneider, Plymouth, Minn) had no effect. No compliance mismatching was noted for the Cragg, Symphony, and Memotherm stents, whereas a decrease in compliance was noted for the Palmaz-Schatz, Strecker, and Wallstent designs. The Palmaz-Schatz and Strecker stents caused increased arterial wall rigidity, the Symphony and Wallstent designs had no effect, and the Memotherm and Cragg stents caused decreased wall rigidity. Stents made of stiff metal yielded different early results than did stents made of the less rigid nitinol.

CONCLUSION

Soon after implantation, the six stent designs elicited varying changes in blood flow, arterial compliance, and arterial wall mechanics. Contralateral arterial flow also was affected.

Twin-tube endografts for aortic aneurysms: an experimental feasibility study

PURPOSE

To explore feasibility of twin-tube endografts (TTEGs) for treatment of abdominal aortic aneurysm (AAA).

MATERIALS AND METHODS

AAAs were created in six dogs by overdilation of Palmaz stents. TTEGs made of seven 9.5-mm-diameter Gianturco Rösch Z (GRZ) stents and covered with PTFE were deployed simultaneously from both femoral arteries through 10-F sheaths to exclude the AAA. Follow-up aortography was performed immediately and at 2, 4, and 8 weeks. Two dogs were killed at 4 weeks and four were killed at 8 weeks. Gross and histologic examinations were performed.

RESULTS

AAA creation and TTEG placement were successful in all six dogs. TTEGs completely excluded AAAs in five dogs. Minor temporary perigraft leak was seen in one dog immediately after stent placement. Four dogs had TTEGs patent to termination at 8 weeks. In two dogs with oversized TTEGs, one of the twin endografts occluded at 2 weeks. At autopsy, TTEGs were formed in semicircular shape and histology did not show any significant inflammatory reaction.

CONCLUSION

TTEGs show promise for treatment of AAA, with the potential of reducing the introductory sheath size. Further experimental studies, particularly with TTEGs extended into iliac arteries, are warranted.

Effects of folate supplementation in hyperhomocysteinemic pigs

OBJECTIVES

The aim of this study was to evaluate the therapeutic effects of folic acid in the pig model of hyperhomocysteinemia.

BACKGROUND

We have previously shown that pigs fed a methionine-rich diet develop hyperhomocysteinemia, arterial lesions and thrombotic events. Elevated homocysteine level is an independent risk factor for atherosclerosis that can be markedly lowered with daily folic acid administration. However, it is not known whether this treatment can prevent arterial lesions.

METHODS

Three groups of pigs were studied: 8 control subjects received a standard diet; 8 received a methionine-rich diet for four months; 8 received a methionine-rich diet for 1 month and then the methionine-rich diet + 5 mg/day folic acid for 3 months. At month 4 after hemodynamic investigation, all the pigs were sacrificed.

RESULTS

Control animals developed few usual vascular streaks. All the pigs fed a methionine-rich diet without folic acid treatment developed hyperhomocysteinemia (10.3+/-1.3 micromol/liter at basal state, 18.2+/-2.5 micromol/liter at one month and 14.6+/-3.8 micromol/liter at four months), hemodynamic abnormalities and diffuse arterial lesions with smooth muscle cell hyperplasia, endothelial alterations and elastic lamina dislocation. In this group, one pig died of venous thromboembolism and one of myocardial infarction. The pigs fed a methionine-rich diet + folic acid displayed similar arterial lesions and two had thrombotic events (one myocardial infarction and one pulmonary embolism), despite normalization of homocysteine levels (10.9+/-1.3 micromol/liter at basal state, 19.5+/-2.5 micromol/liter at one month and 11.4+/-3.8 micromol/liter at four months).

CONCLUSIONS

In the pig model of hyperhomocysteinemia, 5 mg/day folic acid did not prevent arterial lesions or thrombotic events.

Experimental techniques and models in the study of the development and treatment of abdominal aortic aneurysm

BACKGROUND

It is still unclear what initiates aneurysmal dilatation and what determines whether or not an aneurysm will continue to expand and rupture. Early detection and operative repair of an abdominal aortic aneurysm (AAA) still remains the only effective means of reducing the high mortality rate associated with the condition. Endovascular techniques are being developed in an attempt to reduce the mortality rate associated with elective repair. A variety of animal models and experimental techniques have been described in the investigation of the pathophysiology of AAA and in the development of improved endovascular surgical and pharmacological therapies. This article discusses these models and techniques, their advantages and some of the problems encountered in extrapolating experimental findings to the human condition.

METHODS

This review is based on a search of the Medline database from 1966 to March 1998 using recognized key words and text words. A further search was then conducted on references quoted within selected relevant publications.

RESULTS AND CONCLUSION

Treatment of rodent aortas with intraluminal elastase or periaortic calcium chloride creates reproducible aneurysms that have certain similarities to the human pathology; such aneurysms have been favoured in the investigation of the pathophysiology of aneurysm expansion. However, these models lack several of the prominent features of the human lesion, such as atherosclerosis and intraluminal thrombosis. The development of gene knockout mice may lead to a more analogous aneurysm formation, with associated atherosclerosis. Many large animal models have been used in the development of endovascular techniques but, in general, these do not mimic the human pathophysiology and fail to predict medium- and long-term complications.

Healing response associated with balloon-dilated ePTFE

Deployment of endovascular grafts composed of a metallic stent surrounded by expanded polytetrafluoroethylene (ePTFE) stretches the polymer beyond its original dimensions, altering the structural characteristics of the ePTFE. We hypothesized this structural modification would alter the healing response associated with the implant. In this study, 4 mm i.d. of ePTFE (30 microns internodal distance) vascular grafts were balloon dilated using angioplasty balloons having final diameters of 6 (1.5X), 8 (2X), 10 (2.5X), 12 (3X), and 18 (4.5X) mm. Following balloon dilatation of the ePTFE, a circular punch (6 mm in diameter) was used to prepare polymer samples for implantation. The ePTFE circular patches were implanted within subcutaneous tissue and epididymal fat pads of male Sprague-Dawley rats. After 5 weeks, the implants were removed and analyzed for fibrous capsule formation, inflammation, and neovascularization associated with the material. Histological analysis revealed the formation of fibrous capsules only with control subcutaneous implants. The inflammatory response associated with subcutaneously implanted ePTFE was decreased significantly following balloon dilatation to at least 2.5 times the original diameter of the graft. In contrast, ePTFE implanted within adipose tissue demonstrated a significantly greater inflammatory response following balloon dilatation when compared to control implants. Only ePTFE balloons dilated to 6 mm and implanted within adipose tissue demonstrated neovascularization to any extent. These data suggest the structural modifications incurred by ePTFE following balloon dilatation dramatically affect the inflammatory response associated with an implant. Therefore, polymeric materials used for endovascular graft technology require designs that consider changes in polymer healing inherent to device design.

An experimental model for the acute and chronic evaluation of intra-aneurysmal pressure

PURPOSE

To develop an animal model for the acute and chronic monitoring of pressure within abdominal aortic aneurysms (AAAs) to be treated with endovascular grafts.

METHODS

A strain-gauge pressure transducer was placed within an AAA created from a prosthetic vascular graft. Prosthetic aneurysms were implanted into 17 canine infrarenal aortas. The intra-aneurysmal pressure was monitored and correlated with noninvasive forelimb sphygmomanometry for 2 weeks. After this time, an intravascular manometer catheter was passed into the aneurysm. Simultaneous pressure measurements were obtained using the implanted strain-gauge pressure transducer, the manometer catheter, and the forelimb sphygmomanometer. Angiography was performed to assess intraluminal morphology, aneurysm anastomoses, and adjoining aortic vessels. In addition, two control animals underwent intra-aneurysmal pressure monitoring after standard surgical aneurysm repair.

RESULTS

There was excellent correlation (r = 0.97) between the pressure measurements obtained with the implanted strain-gauge pressure transducer and the intravascular manometer. Close correlation was also observed between the implanted strain-gauge transducer and the forelimb sphygmomanometer (r = 0.88) during postprocedural monitoring. Intra-aneurysmal pressure was lowered dramatically by surgical exclusion (aneurysm: 15/5 +/- 7/4 mmHg; systemic: 124/66 +/- 34/17 mmHg; p < 0.001). The prosthetic aneurysms were successfully imaged with angiography.

CONCLUSIONS

This animal model provides an accurate and reproducible means for measuring intra-aneurysmal pressure on an acute and chronic basis. It may be possible to use this model in the assessment of endovascular devices to determine their efficacy in reducing intra-aneurysmal pressure. Evaluation of complications associated with their use, such as patent aneurysm side branches, perigraft channels, and perianastomotic reflux, may also be possible.

Chronic intraaneurysmal pressure measurement: an experimental method for evaluating the effectiveness of endovascular aortic aneurysm exclusion

PURPOSE

To evaluate and compare the intraaneurysmal pressure (IAP) after exclusion using two different endovascular grafts.

METHODS

Eight mongrel dogs had a 3 x 3 cm polytetrafluoroethylene (PTFE) aneurysm sewn as an interposition graft of the infrarenal aorta. A pressure transducer implanted into the aneurysm wall permitted continuous electronic IAP monitoring. Four aneurysms were excluded with a transluminally placed endovascular graft made of a PTFE graft and two Palmaz stents (PTFE-EG), three were excluded with a tantalum-Dacron endovascular graft (TD-EG), and one was surgically treated with a standard PTFE graft (PTFE-Surg). The dogs were observed for 18 to 50 days (mean, 37.5 days) and were evaluated after surgery with duplex and spiral computed tomographic scans.

RESULTS

All grafts successfully excluded the aneurysms without perigraft channels or leaks as documented by arteriogram and duplex and computed tomographic scans. The mean IAPs after repair with all PTFE-EGs were significantly lower (p < 0.001) than the mean systemic pressures. In addition, the mean IAP reduction was significantly greater (p < 0.005) in the PTFE-EG group than in the TD-EG group.

CONCLUSIONS

Aneurysm exclusion with PTFE-EG significantly lowered IAP, did so significantly better than the TD-EG, and approached the IAP reduction obtained by standard repair. Such pressure reduction is necessary for effective protection against aneurysm rupture.

Endoluminal stent-grafts for infrarenal abdominal aortic aneurysms

BACKGROUND

The treatment of aortic aneurysms with endovascular stents or stent-graft prostheses is receiving increasing attention as an alternative to major abdominal surgery. To define the clinical value of this technique, we prospectively studied the use of stent-graft endoprostheses made of nitinol and covered with polyester fabric for the treatment of infrarenal abdominal aortic aneurysms.

METHODS

We treated a total of 154 patients at three academic hospitals. Twenty-one patients with aortic aneurysms not involving the aortic bifurcation received straight stent-grafts, and 133 patients with aortic aneurysms involving the bifurcation and the common iliac arteries received bifurcated stent-grafts. After a unilateral surgical arteriotomy, the endoprostheses were advanced through the femoral arteries and placed under fluoroscopic guidance. Computed tomography and intraarterial angiography were performed during an average follow-up of 12.5 months.

RESULTS

The primary success rate, defined as complete exclusion of the abdominal aortic aneurysm from the circulation, was 86 percent in the group receiving straight grafts and 87 percent in the group receiving bifurcated grafts. In three patients the procedure had to be converted to an open surgical operation. Minor (n=13) or major (n=3) complications associated with the procedure (including 1 death) occurred in 10 percent of the patients. All patients had a postimplantation syndrome, with leukocytosis and elevated C-reactive protein levels.

CONCLUSIONS

Our results suggest that endovascular treatment of infrarenal abdominal aortic aneurysms is technically feasible and can effectively exclude abdominal aortic aneurysms from the circulation. With further refinement, endoluminal repair may emerge as an interventional strategy to treat infrarenal aortic aneurysms, especially in patients at high surgical risk.

Mechanical characteristics of dilated polytetraflouroethylene used for transluminally placed endovascular grafts

This study was conducted to assess the mechanical characteristics of dilated polytetraflouroethylene (PTFE) for use in transluminally placed endovascular grafts (TPEGs). Ten-centimeter lengths of 3- and 4-mm thinwalled PTFE were dilated to 8, 10, 12, and 15 mm diameters (3 mm) and 10-, 14-, 16-, and 20-mm diameters (4 mm), respectively (n = 6 for each size). The dilated PTFE segments were evaluated for leakage, further dilation, structural changes (with electron microscopy), and changes in wall thickness occurring after 24 hours of perfusion at pressures of 300-350 mmHg. Both 3- and 4-mm thinwalled PTFE could be dilated to five times their initial diameter before rupture occurred. Three-millimeter grafts dilated to 12- and 4-mm grafts dilated to 14 mm remained resistant to leakage at perfusion pressures up to 350 mmHg. When 3-mm grafts were dilated to 15 mm, the PTFE leaked saline at a rate of 20.3 +/- 9.3 cc per hour at 300 mmHg. pressure. Four-millimeter grafts dilated to 16- and 20-mm diameters leaked saline at 8.4 +/- 7.8 and 52.8 +/- 22 cc per minute, respectively, at the same pressure. No grafts were found to increase in diameter after 24 hours of pressure perfusion. Electron microscopy revealed that PTFE node size was significantly smaller in dilated grafts than in undilated grafts, but there was no significant change in internodal distance. This data suggests that thinwalled PTFE can be dilated to large diameters and retain sufficient strength to resist supraphysiologic pressures. Long-term studies are needed to determine the late structural integrity of dilated PTFE.

Aortic stents covering the renal arteries ostia: an animal study

PURPOSE

To evaluate the consequences of placing an aortic stent over the renal arteries.

MATERIALS AND METHODS

The renal ostia of 11 pigs were covered by Strecker stents placed in the aorta. At 1 month, the degree of renal ostial stenosis was determined by means of angiography and gross pathologic and histologic examination. Any reduction in area of the renal ostia was considered significant. Preprocedure and 1-month serum creatinine levels were also examined.

RESULTS

One stent migrated and was excluded from the study. There was one angiographic failure and, among the remaining 18 renal arteries evaluated, one was occluded, six were stenosed, and 11 were patent. Of the 10 samples available for pathologic examination, one was excluded from study because one stent was not fully deployed. A neointima was covering the struts crossing or encircling the renal arteries ostia with a mean area coverage of 43% +/- 30% (range, 0-84%). Serum creatinine levels rose from 71.1 mumol/L +/- 7.1 preoperatively to 94.2 mumol/L +/- 6.7 postoperatively (P < .01).

CONCLUSION

An aortic stent placed over the renal arteries in pigs may compromise renal perfusion in the long-term because neointima tends to fill the spaces between the struts.

Comparison of magnetic resonance imaging measurements of abdominal aortic aneurysms with measurements obtained by other imaging techniques and intraoperative measurements: possible implications for endovascular grafting

PURPOSE

Abdominal aortic aneurysm morphologic evaluation with conventional imaging techniques is inadequate when endovascular repair is being contemplated. This study has addressed the problem with magnetic resonance imaging (MRI).

METHODS

Twenty patients (14 men, 6 women) with a median age of 73 years were recruited and assessed according to current endovascular graft selection criteria. Thirteen patients subsequently underwent open aneurysmorrhaphy, and the intraoperative parameters have been compared with those of duplex ultrasonography and MRI.

RESULTS

No significant difference was demonstrated in the diameter of the infrarenal neck among ultrasonography, MRI, and intraoperative findings (p > 0.05, Mann Whitney U Test) and also during assessment of infrarenal neck length; however, duplex sonography accurately defined the renal ostia in only five cases. MRI visualized 38 of 40 renal arteries. Distal aortic involvement (cuff diameter and length) and the length and diameter of the common iliac arteries were accurately determined by MRI in all cases, and no significant difference was demonstrated with the intraoperative findings. Comparison of the intraoperative and MRI aneurysm lengths suggested a slight trend of overestimation by MRI resulting from angulation of the aneurysm, but this figure did not reach statistical significance. Only two patients met the current criteria for endoluminal straight grafting.

CONCLUSIONS

Both MRI and duplex sonography accurately predicted aortic morphologic characteristics; however, MRI provided the most comprehensive anatomic picture for patient selection and should be considered the nonionizing imaging modality of choice when an endovascular repair is being contemplated.

Endovascular stent-anchored aortic grafts: a comparison between self-expanding and balloon-expandable stents in minipigs

PURPOSE

To study endovascular graft attachment with self-expanding Gianturco Z-stents and balloon-expanded Palmaz stents and the effect of these devices on the renal ostia.

METHODS

Ten stent-grafts were constructed, 5 with Gianturco Z-stents and 5 with Palmaz stents. The endografts were implanted under fluoroscopic guidance into the abdominal aorta of 10 pigs so that the uncovered portion of the proximal stent extended over the renal artery orifices. Distal aortic blood pressure and flow were measured before and after graft placement and 1 hour postprocedure. The aorta was then exposed surgically, and the central portion of the stent-graft was inspected through an aortotomy to assess perigraft leakage.

RESULTS

Stent-graft implantation was accurate and hemostatic in all cases, despite longitudinal folding of the graft due to oversizing. However, transverse folds produced pressure gradients (> 15 mmHg) between the ends of the graft in two cases. In another case, a pressure gradient resulted from partial thrombosis of the graft. In two cases, renal artery occlusion and thrombosis occurred due to coverage by the graft material. In two other animals, one of the renal arteries was entirely uncovered by a stent. The remaining 16 renal arteries were covered by the proximal stent but not the graft, as intended. One (6.25%) of these arteries thrombosed, but the remainder were grossly patent when the animals were sacrificed at 1 hour.

CONCLUSIONS

Both Palmaz and Gianturco Z-stents produced hemostatic endovascular graft attachment, even in the presence of moderate graft oversizing. The risk of acute renal artery occlusion from juxtarenal stenting does not appear to be prohibitive, but longer term observations are needed.

Comparison of the deployment and healing of thin-walled expanded PTFE stented grafts and covered stents

This study evaluated the deployment and short-term healing of thin-walled expanded polytetrafluoroethylene (ePTFE) stented grafts and covered stents as endoluminal prostheses in normal canine aortas and in an abdominal aortic aneurysm (AAA) model. Stented grafts consisted of a 7 cm length of 3 mm internal diameter ePTFE graft (Impra, Inc., Tempe, Ariz.) with two P-188 Palmaz stents (Johnson & Johnson Interventional Systems, Warren, N.J.) deployed along the inner surface of the ends of the graft to secure the prosthesis to the arterial wall. Covered stents were fabricated by placing a 3.7 cm length of 3 mm internal diameter thin-walled ePTFE graft over a P-394 Palmaz stent. Four covered stents and four stented graft prostheses (two of each prototype in the normal canine aorta and AAA model) were implanted in eight animals. One prosthesis of each type in each model was removed at 30 days and one at 60 days. Prior to removal, prostheses were evaluated by CT scan, arteriography, and intravascular ultrasound imaging with values compared to those obtained when the prostheses were deployed. Gross inspection and microscopic evaluation were performed at scheduled explantation. In general, the stented grafts were more difficult to accurately deploy. Healing and maintenance of long-term patency without significant luminal obstruction or occlusion occurred in only one 30-day sample in a normal canine aorta. The 30-day stented graft specimen that had been implanted in an AAA and required the addition of a covered stent to seal a maldeployment of the distal segment was also patent. The 60-day stented graft in the normal canine aorta was occluded with narrowing of the graft between the stents. The 60-day stented graft in the AAA was patent with one central fold and thrombus occupying approximately 20% to 30% of the lumen at this site. In contrast, the covered stent devices were less difficult to accurately deploy. All of the covered stent devices were patent with well-incorporated surfaces. Deployment of covered stents was more accurate and less complicated compared to stented grafts. All patent endoluminal prostheses and stent surfaces were well incorporated into aortic tissues. Problems with graft narrowing, folding, and subsequent thrombosis occurred in the unstented segments of the stented grafts. These preliminary findings support the further development and use of completely supported (stented) devices as endoluminal prostheses.

Closure of a popliteal arteriovenous fistula using an autologous vein-covered Palmaz stent

PURPOSE

To report the use of autologous vein to cover a stainless steel stent designated for repair of a traumatic popliteal arteriovenous (AV) fistula.

METHODS AND RESULTS

Autologous cephalic vein was harvested to cover a Palmaz biliary stent selected to close a traumatic popliteal AV fistula that persisted despite reparative attempts with balloon occlusion and coil embolization. The vein-covered stent was delivered percutaneously and deployed, successfully obliterating the vascular communication. Patency of the popliteal artery was documented arteriographically at 5 months, and symptomatic improvement continues at 10 months.

CONCLUSION

The simplicity of this percutaneous approach and the use of autologous vein to cover endovascular prostheses create the possibility for evaluating this technique in myriad anatomical situations.

Classical and endovascular surgery: indications and outcomes

For more than 40 years, endarterectomy and bypass grafting have been the primary means of surgically revascularizing peripheral vessels threatened by atherosclerotic disease. However, with today's endovascular technology, stenoses and occlusions in nearly every circulatory system can be approached intraluminally with a wide variety of techniques: thrombolysis, laser angioplasty, atherectomy, balloon dilation, and intravascular stents. Just as exciting is the newer technique of endoluminal grafting, which has extended percutaneous therapy to aneurysmal disease in the thoracic and abdominal aorta and distal arteries, as well as to long-segment occlusive disease. Today's vascular surgeon is in a unique position to combine his or her classical surgical training with these catheter-based interventions. Certainly, the potential advantages of percutaneous therapy as compared to surgical reconstruction are significant: no general anesthesia or lengthy incisions, shorter hospitalization, lower morbidity and mortality, earlier intervention in the course of the disease, and less complicated reapplication in the event of disease recurrence. Undoubtedly, endovascular techniques will become a major component of the vascular surgeon's armamentarium, and as we approach the year 2000, they will be the treatment of choice in nearly every vascular pathology and circulatory system.