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Internal Iliac Artery Embolization as an Adjunct to Endovascular Aneurysm Repair
An important requirement for endovascular aortic or iliac aneurysm repair is the presence of an adequate distal landing zone in the common iliac artery. Aneurysmal aortas are associated with dilated common iliac arteries in about 20% of patients, making patients ineligible for standard endovascular repair. Because of the limit of a maximum diameter of commercially available iliac limbs, patients with large-diameter common iliac arteries do not have a sufficient landing zone and therefore require extension of the stent graft into the external iliac artery and embolization of the internal iliac artery (IIA) to prevent retrograde flow into the aneurysm from the IIA and thus a type II endoleak. Although larger (aortic) cuffs can be used (bell-bottom technique), a common iliac artery larger than 22 mm in diameter is considered a diseased landing zone, prone to further dilation and a potential source of subsequent type I endoleak. Furthermore, the presence of mural thrombus within the dilated common iliac artery can increase the risk of distal thromboembolism.
Preventing a type II endoleak in this setting can be achieved by embolization of the IIA or by surgical ligation. When a short segment of the common iliac artery proximal to the iliac bifurcation has a diameter small enough to allow sealing but not to allow endograft fixation, extension of the iliac limb into the external iliac artery (with an oversizing of 10%–15% in the distal common iliac artery) may be performed without prior embolization of the IIA. Alternatively, surgical reimplantation may be performed or bifurcated stent grafts or covered stents can be used to preserve flow into the IIA. This leads to less common occurrence of buttock claudication and may therefore be considered a primary choice in younger patients with suitable anatomy.
Pretreatment embolization usually involves occluding one or both internal iliac arteries in order to be able to expand the anatomic inclusion criteria to perform endovascular treatment of aortoiliac aneurysms extending up to or beyond the iliac bifurcation. Indications for IIA occlusion are extension of the AAA into the common iliac artery with an insufficient distal neck, aneurysms involving the IIA, and insufficient distal landing zone.
Relevant Anatomy
The IIA is the major blood supply to the gluteal area through its posterior division. The anterior division supplies the rectosigmoid and genital areas. It provides a communication between the systemic and visceral circulation through collateral vessels to the superior rectal branch of the inferior mesenteric artery (IMA). The IMA provides the main blood supply to the left colon by way of the left colic artery, sigmoidal branches, and superior rectal artery. Ileocolic and right and middle colic arteries from the superior mesenteric artery communicate with the IMA branches through the marginal artery of Drummond and the arcade of Riolan. Prolonged occlusion of one or two of these systems is usually without any clinical sequelae caused by existence of collaterals.
In addition, there are collateral pathways through the lumbar arteries, deep iliac circumflex artery, lateral and medial femoral circumflex artery, and iliolumbar and obturator arteries toward the IIA. The risk of pelvic ischemia increases when both internal iliac arteries and the IMA are occluded simultaneously. Such sudden interruption of flow can lead to severe complications.
Embolization Tools
Embolization can be performed by using coils or a vascular occlusion plug. Coils are available in various sizes (length and diameter) and shapes (spiral/helical versus complex) and with or without polyester (Dacron) fibers. Hydrocoils are coated with a gel that, once placed in a buffered solution (e.g., blood), takes up water into the polymer matrix after about 5 minutes, resulting in volumetric expansion. This results in a better filling of the vascular space and use of fewer coils. Coils can be placed by means of various delivery systems, including placement by using a coil pusher or guidewire versus use of detachable coil systems.
There is no place for the use of glue (such as cyanoacrylate, Ethibloc, and Onyx), or microspheres (polyvinyl alcohol or trisacryl gelatin microspheres) in preoperative embolization.
Pushable Coils
Since the original stainless steel coils were introduced, a lot of modifications have been made. Currently, most of the commercially available coils are made of platinum and come in standard sizes of 0.035 inch and 0.038 inch. Microcoils are available in a coil thickness of 0.018 inch. An advantage of the platinum coils is their compatibility with magnetic resonance imaging (MRI) in patients in whom follow-up of the stent graft will be by MRI.
Coils are advanced using a coil-pusher wire or a guidewire. When using a guidewire (which has a slightly tapered tip) in combination with large-lumen microcatheters (>0.018 inch) there is a risk of the wire becoming trapped between the coil and the inner side of the microcatheter, thus locking the coil and guidewire within the catheter. Alternatively, the squirt technique can be used: After the coil is loaded into the (micro)catheter, a saline-filled Luer-lock syringe (2 or 3 mL) is connected to the (micro)catheter, and the (micro)coil is advanced under fluoroscopic guidance through the catheter by administering small boluses of saline. This technique can only be used in cases where an extremely stable catheter position has been obtained, and it should never be the delivery technique for placing the first coil in a large target vessel.
Detachable Coils
Detachable coils developed for applications in neurointerventional radiology are also used in peripheral interventions. The availability of coils in long lengths and in various complex shapes, combined with the possibility to reposition them before final release, makes them extremely useful in procedures where accurate embolization of high-flow, large-diameter vessels is needed.
Release of detachable coils can be accomplished using various techniques that vary with type and manufacturer of the coils. Release mechanisms commonly used are electrolytic release, pressure-controlled release, and mechanical detachment, using either an interlocking release mechanism or a torsion-controlled detachment.
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