Abdominal aortic aneurysm (AAA) is one of the clinically important diseases in vascular surgery, with an incidence of approximately 20-40 cases per 100,000 population per year [1]. The mortality rate due to AAA is about 1.7% in people aged 65-74 years. As AAA tumors increase in size, their growth rate increases, the risk of rupture increases, and the mortality rate of ruptured AAA is as high as 80-90% [2].The 30-day mortality rate of traditional open surgery for AAA is 4-12%, and most patients are not suitable for open surgery due to advanced age or other serious medical conditions.In 1991, Dr. Parodi, an Argentinean physician, first reported the use of stented artificial vessels (later referred to as “stent”) to successfully cure AAA, ushering in a new era of endovascular aneurysm repair (EVAR) treatment for AAA. The following will discuss the issues related to EVAR for AAA.
1. Indications and contraindications of EVAR
Asymptomatic AAA with a diameter greater than 5~5.5 cm, AAA with a diameter of 4~5 cm but with a rapidly increasing tumor (an increase in tumor diameter greater than 0.5 cm within six months) and all AAA with obvious symptoms or ruptured should be treated surgically; EVAR should be considered for high-risk patients of advanced age or with severe medical diseases that cannot tolerate traditional surgery. for AAA that do not meet the above requirements for small AAA, it is currently surgical treatment is not considered to improve the prognosis [3]. EVAR is more appropriate for AAAs that are difficult to operate with conventional surgery, such as inflammatory AAAs, AAAs with combined horseshoe kidneys, and AAAs with a history of abdominal surgery that makes reoperation difficult. in addition, ruptured abdominal aortic aneurysm (rAAA) has a lower mortality rate with emergency EVAR than with conventional open surgery [4].
The main determinants of whether EVAR is feasible and what type of stenting to adopt are the length of the proximal aneurysm neck and the extent of distal aneurysm involvement. Currently, there are Schumacher and Ahn stents based on the length of the proximal aneurysm neck and the extent of distal aneurysm involvement, respectively, in abdominal aortic aneurysms. However, from the perspective of endoluminal treatment, the Schumacher type may be more relevant; straight stents can be used for type I AAA and bifurcated stents for type II AAA, of which type IIC requires reconstruction of one internal iliac artery at the same time, and in addition, if this type is accompanied by occlusion or severe tortuosity of one iliac artery, the common iliac artery can be blocked and an aortic unilateral iliac stent (AUI stent) can be performed. Type III AAA is a contraindication to EVAR because it may affect renal artery flow if stenting is performed. However, with the recent development of new stents with larger proximal bare stent mesh, which have less impact on renal blood flow, and even stents with branches, the indication of proximal aneurysm neck can be relaxed, and even proximal short aneurysm neck is no longer a contraindication to EVAR.
Since AAA expands in the long-axis direction and the proximal and distal positions of the tumor are relatively fixed, it causes distortion of the neck, tumor and iliac artery into an angle, and the distortion into an angle makes it difficult to pass the guiding wire, guiding sheath and catheter, making the procedure difficult. The maximum angulation of the proximal neck of the AAA suitable for EVAR should not exceed 120 o, and the maximum angulation of the iliac artery should not exceed 90 o. The restriction of the angulation of the aneurysm can be relatively relaxed.
In addition, proximal aneurysm neck diameter >30 mm, severe calcification (calcification area ≥25% proximal aneurysm neck area), appendicular thrombus area ≥50% proximal aneurysm neck area, and irregular proximal aneurysm neck morphology (aneurysm neck diameter expansion of more than 4 mm per centimeter under the renal artery, i.e., conical or trapezoidal shape, and other irregular morphology) [5]; excessive thinness of the distal importing artery (e.g., external iliac artery internal diameter <7 mm), a severe stenosis or occlusion and external iliac artery internal diameter >18 mm are relative contraindications to EVAR. The presence of collateral arteries connected to the AAA and iliac artery aneurysms, such as the collateral renal artery and the inferior mesenteric artery, may lead to organ ischemia or regurgitation of blood in the aneurysmal cavity after EVAR and is a contraindication to EVAR.
2. Preoperative imaging evaluation
Appropriate imaging is necessary for successful EVAR to determine whether EVAR is feasible and the correct choice of stent type and size. The examination methods are spiral CT angiography (SCTA), magnetic resonance angiography (MRA), digital subtraction arteriography (DSA), color Doppler ultrasound and intravascular ultrasound (IVUS), and SCTA, MRA and DSA are commonly used in clinical practice.
(1) SCTA
SCTA is the first choice, which can accurately measure various parameters of AAA, such as the diameter of the neck of the aneurysm, the maximum diameter, etc. It can also clearly show all the branch arteries of the abdominal aorta, such as the superior mesenteric artery, the renal artery and the paranephric artery. The contrast enhancement can clearly distinguish between the attached thrombus and the residual lumen of the aneurysm, and can correctly distinguish the calcified arterial wall. In addition, the 3D images formed by computer reconstruction processing can accurately measure the diameter and length of the aneurysm, proximal and distal aneurysmal necks and iliac arteries, which can more visually display the morphology of the aneurysm and iliac arteries.
(2) MRA
The results of the examination are similar to those of SCTA, and the measured AAA parameters are not significantly different from those of SCTA. In recent years, with the use of rolled contrast enhancement, the accuracy of MRA has been significantly improved compared with traditional MRA, while its observation range is larger, which is especially suitable for the examination of AAA with suspected involvement of the iliac artery.
(3) DSA
It has the characteristics of immediacy and high resolution, so it is mostly used for intraoperative measurement and observation of EVAR. It should be noted that DSA cannot correctly distinguish between the arterial wall and the attached wall thrombus, so its measured AAA diameter is often smaller than the actual value, and also smaller than the measured value of CTA and MRA. A graduated contrast catheter should be used mainly to measure during imaging, especially the length of the entire aneurysm, so that the magnification caused by contrast can be avoided and the accuracy of measurement can be improved.
3.EVAR of AAA
(1) Technical points
① Accurate measurement of AAA parameters is the basis of successful EVAR, which is closely related to the correct selection of stent size and type as well as the occurrence of postoperative endoleaks. Since SCTA can clearly show the wall thrombus attached to the tumor cavity, the measurement of proximal and distal tumor neck diameter, iliac artery diameter and tumor body of infrarenal AAA is superior to DSA by SCTA, but the actual length of infrarenal artery to the bifurcation of iliac artery is more accurately measured intraoperatively with a graduated contrast catheter.
② Correct selection of stent According to AAA type and various parameters, straight, bifurcated and AUI type stents should be selected, requiring a size 10-15% larger than the proximal and distal aneurysm neck diameters; too large or too small may accelerate proximal and distal aneurysm neck expansion or stent displacement.
(iii) Accurate positioning of the low renal artery opening, proximal and distal aneurysm necks, and the aortic and iliac artery bifurcations is an essential step in the procedure. One of the keys to a successful EVAR is to ensure the flow of blood through both renal arteries and at least one internal iliac artery. Therefore, the stent should be released with its proximal metal marker placed below the opening of the low renal artery (usually the right renal artery) to ensure proper blood supply to both renal arteries. For abdominal aortic aneurysm involving the iliac artery, blood flow to the internal iliac artery on one side should be ensured to maintain blood supply to the pelvic organs and gluteal muscles; otherwise, reconstruction of the internal iliac artery on one side must be added after completion of EVAR.
(2) Key points in the management of complex AAA
①Complex proximal aneurysm neck
Aneurysm neck is too short
(1) proximal bare stent transrenal artery technique: release the proximal bare stent across the renal artery; (2) proximal bare stent internal support technique: add a large dilated diameter Palmaz stent to the proximal neck of the aneurysm; (3) cover part of the renal artery and perform renal artery stenting: cover 4/5 of the renal artery and then perform balloon dilatation and stenting (Genesis stent, etc.) on the renal artery (4) trimming the artificial vessel on the upper edge of the main stent into a notch shape and aligning the notch with the renal artery during release so that the renal artery blood flow can be preserved; (5) choosing a stent with branch vessels.
Excessive torsion
(1) Select a stent with good flexibility (Zenith stent, etc.); (2) Place the connection part of the two stents at the angle so that the stent can conform more easily to the change of the angle of the neck of the tumor; (3) Add a Palmaz stent with large expansion diameter proximally; (4) Place the connecting rod side of the stent as close as possible to the large bend side of the angle.
Severe calcification and wall thrombosis
(1) use a large expansion diameter Palmaz stent for internal support; (2) add an extension type Cuff (stent).
Irregular shape
(1) For conical or trapezoidal tumor neck, determine the proximal neck diameter according to the following principles: if the neck length is ≤15mm, calculate the diameter at the lower edge of the neck; if the neck length is >15mm, calculate the diameter at 15mm below the proximal anchorage point of the intended stent; (2) Calculate the diameter of irregular tumor neck according to the diameter at the widest point.
(3) Complex tumor
Aneurysm angle <120°: implantation of AUI type stent + contralateral common iliac artery occlusion + femoral-femoral artery artificial vessel bypass.
A large number of attached thrombi in the aneurysm cavity, and the maximum diameter of the residual aneurysm cavity is <16 mm: implantation of AUI-type stent + contralateral common iliac artery occlusion + femoral-femoral artery artificial vessel bypass.
AAA involving the common iliac artery (type IIb, type IIc) or combined with internal iliac aneurysm: in principle, at least one internal iliac artery should be preserved. (1) Type IIb AAA generally does not require treatment of the internal iliac artery; (2) Type IIc AAA should be evaluated in detail preoperatively to determine how to preserve one internal iliac artery and to avoid postoperative internal iliac artery regurgitation leading to type II endoleaks. Generally, if the internal iliac artery on the side to be covered is not occluded, it should be embolized preoperatively; if the internal iliac artery on the side to be preserved has stenosis, it should be endovascularly formed or surgically reconstructed preoperatively; if both internal iliac arteries cannot be preserved, one internal iliac artery needs to be embolized or reconstructed in stages; (3) AAA with combined internal iliac aneurysm should be embolized first before EVAR.
(4) Complex access artery (femoral or iliac artery)
Severe tortuosity, angulation
Straightening of the access artery by ultra rigid guidewire or double guidewire techniques or introduction of a large dilated diameter palmaz stent to correct the access artery torsion before introduction of a stent-type prosthesis.
Exposing the external iliac artery by the extraperitoneal route and straightening it downward or removing the excessively twisted external iliac artery before introducing a stent-type artificial vessel.
Select a stent-type artificial vessel with good flexibility (Zenith stent, etc.); (4) Introduce an AUI-type stent via the contralateral access artery + ipsilateral common iliac artery occlusion + femoral-femoral artery artificial vessel bypass.
Severe stenosis or occlusion
(a) Balloon dilatation and angioplasty or bare stent implantation of the stenosed access artery followed by introduction of a stent-type artificial vessel.
Introduction of AUI-type stent via the contralateral access artery + femoro-femoral artery artificial vessel bypass (with ipsilateral common iliac artery blockage if necessary).
4. Management of special postoperative complications
(1) Endoleak
Type I and type III endoleaks can gradually increase in size until rupture if left untreated, whereas type II and type IV endoleaks can close on their own due to thrombosis and are less likely to cause AAA rupture. The prevention of endoleaks should be carried out throughout the preoperative evaluation, intraoperative operation and postoperative follow-up. The current indications for the management of endoleaks after EVAR are: type I endoleaks, type III endoleaks, type II endoleaks with endotension, and endotension without significant endoleaks on imaging.
Prevention and management of type I endoleaks.
Accurate release of stented prosthetic vessels.
repeated balloon dilation of the proximal and distal anchorage areas where endoleaks occur.
Addition of an extended cuff or a bare stent with a larger dilation diameter to the proximal and distal anchorage areas where endoleaks occur.
selective intubation of spring-ring embolization, thrombin injection or particulate embolization via endoleaks; (5) surgical cervical annuloplasty of the proximal and distal anchorage areas where endoleaks occur.
Management of type II endoleaks
Type II endoleaks with persistent branch artery regurgitation need to be managed by (1) superselective cannulation of the lumbar artery via the iliac lumbar artery microcatheter and particulate embolization of the lumbar artery; (2) superselective cannulation of the superior mesenteric artery, Riolan arch, and submesenteric artery microcatheter and particulate embolization of the submesenteric artery; (3) lumbar-guided puncture and filling of the lumen with embolic material; (4) laparoscopic or conventional open surgery to ligate (3) laparoscopic or conventional open surgery to ligate the regurgitant branch arteries.
Management of type III endoleaks
Balloon dilation or addition of a bare stent with larger dilation diameter to the site of stent-type artificial vessel connection.
Addition of another short segment of clad stent to the site of rupture of the clad material of the stent
conversion to introduction of an AUI-shaped stent via one iliac artery + sealing of the contralateral common iliac artery + femoro-femoral artificial vessel bypass.
conversion to conventional open surgery.
Management of internal tension
Follow-up observation and management accordingly in case of endoleaks.
Those without endoleaks but with persistent AAA enlargement are referred to conventional open surgery.
(2) Treatment of stent-type artificial vessel displacement
For distal displacement, introduce an extended cuff proximal to the stent and, if necessary, add a bare stent with a large expansion diameter to increase the internal support.
For complete coverage of the renal artery by proximal displacement, the balloon was expanded at the proximal end of the stent and gently tugged downward, driving the stent to move slightly downward to reveal the opening of the renal artery. For partial coverage of the renal artery affecting its perfusion, a bare stent may also be introduced to widen the renal artery opening.
Conversion to conventional open surgery.
(3) Management of continued AAA enlargement
Treatment of the primary etiology: e.g., endoleak or stent displacement.
Transfer to conventional open surgery if the cause is unknown and there is a risk of rupture.
(4) Management of stent-type artificial vessel infection [7
Removal of the stent by conventional open surgery and in situ or extra-anatomic prosthetic vessel grafting
Thorough debridement and drainage, local irrigation with sensitive antibiotic solution plus systemic anti-infection treatment.
5. Outlook
EVAR has gradually become the main treatment for AAA due to its advantages of minimally invasive, less intraoperative bleeding, shorter hospital stay, faster recovery and low operative mortality, and two large multicenter clinical trials in the United States reported [8,9] that the perioperative mortality rate of Talent stent endoluminal repair for AAA was 0.8% and 1.9%, respectively. However, not all patients with AAA are suitable for EVAR, in which the morphological index of AAA is the main factor to determine whether EVAR is feasible. However, with the improvement and enrichment of vascular interventional devices, the continuous optimization of stent-type artificial vessels and their introduction systems, and the accumulation and advancement of endoluminal techniques, the indications for EVAR will be further expanded, and some of those considered as contraindications in the past will be addressed by new endoluminal techniques and devices.