Embolization Materials

Siobhan M. Flanagan

Olga Duran-Castro

Jafar Golzarian

Therapeutic embolization is defined as the deliberate introduction of occluding material into a blood vessel in order to reduce or obstruct blood flow. Several factors determine the selection of an embolic material. One of the most important factors is the degree of permanence desired; therefore, agents are often classified as temporary or permanent agents (Table e-93.1). After a traumatic injury, a temporary agent is often used because it allows healing of an otherwise normal vessel to occur before blood flow is reestablished. Conversely, in a patient with an arteriovenous fistula, permanent vascular occlusion is required in order to achieve the therapeutic endpoint. Another consideration is the desired location or level of occlusion. When treating an arteriovenous malformation, agents which occlude flow at the level of the nidus are needed. A more proximal occlusive agent would be needed for procedures such as gastroduodenal artery occlusion prior to radioembolization. Finally, there are important characteristics of each embolic agent with which the interventionist should be familiar. These include size, radiopacity, material composition, mechanism of occlusion, and biologic behavior.

Indications for Embolization (1)

1. Occlusion of vascular abnormalities which have potential to cause adverse health effects (e.g., congenital or acquired aneurysm, pseudoaneurysm, vascular malformation)

2. Treatment of acute or recurrent hemorrhage

3. Devascularization of benign or malignant tumors for palliation or to reduce operative blood loss

4. Ablation of non-neoplastic tissue causing adverse health effects (e.g., hypersplenism, varicocele)

5. Flow redistribution to protect normal tissue (e.g., gastroduodenal artery and right gastric artery embolization in hepatic artery chemoembolization or radioembolization) or to facilitate subsequent treatment (e.g., right portal vein embolization to induce left lobe hypertrophy prior to surgical resection)

6. Management of endoleak

7. Targeted delivery of drug or other agents (e.g., chemotherapy, beta emitting spheres)

Mechanical Occlusive Devices

1. Coils

Coils are made from either stainless steel or platinum and are available in a wide variety of sizes. They may have fibers placed at right angles to the long axis of the coil, or coatings such as hydrogel, to increase the surface area and thereby increase the speed and permanence of thrombosis. Coils rely on mechanical obstruction, platelet activation, and the patient’s own clotting cascade to fully occlude vessels. Therefore, in the setting of thrombocytopenia and coagulopathy, the efficacy of coil embolization is compromised. Tight coil packing is therefore important to achievement of arterial occlusion and reduction of early recanalization. All coils are permanent devices and should be used when permanent occlusion is desired. When larger nonterminal vessels are occluded with coils, there is relatively rapid formation of collateral arteries. These collateral vessels bypass the point of occlusion and perfuse the distal vascular bed, although at a lower pressure than that prior to embolization.

Table e-93.1 Characteristics of Embolic Agents

Embolic Agent	Duration of Action	Vessel Size	Mechanism of Action	Technical Notes
Gelfoam	Temporary	Small to medium	Flow obstruction	Shapeable, occlusive effect lasts for several weeks
Avitene	Temporary	—	Flow obstruction	Vessel recanalizes within 8 weeks
Coils	Permanent	Small to large	Flow obstruction	Can size to vessel
Vascular plugs	Permanent	Medium to large	Flow obstruction	Can size to vessel
Detachable balloons	Permanent	Medium to large	Flow obstruction	May deflate over time
PVA	Permanent	Small	Flow obstruction	May clog catheter compared to spheres
Embolic spheres	Permanent	Small	Flow obstruction	Uniform, less catheter clogging issues compared to PVA
Drug eluting particles	Permanent	Small	Flow obstruction	Tumor treated via ischemic effect and local drug deposition
Glue	Permanent	Capillary to large, conforms to vessel lumen	Flow obstruction	Forms to vessels and can penetrate distally
Onyx	Permanent	Capillary to large, conforms to vessel lumen	Flow obstruction	Forms to vessels Requires occlusion balloon to form pushable cast and prevent reflux
Alcohol	Permanent	Capillary to large	Thrombosis	Only requires small aliquots to be effective
Ethanolamine	Permanent	Capillary to large	Thrombosis	Less penetrating effect compared to alcohol
Sclerosants (Sotradecol, etc.)	Permanent	Capillary to medium	Sclerosant	Best for use in venous structures
PVA, polyvinyl alcohol.

Coil stability is essential to prevent nontarget embolization. Stability can be aided by the following:

a. Use of a guiding catheter

b. Coil oversizing. Oversizing is essential to minimize the risk of dislodgment. However, this should be weighed against the negative effect that an elongated and incompletely formed coil has on hemostasis. Oversizing by approximately 15% has been suggested in arteries (1). A greater degree of oversizing is required in veins. Certain coils such as the AZUR Peripheral HydroCoil (Terumo Medical Corp, Somerset, NJ) do not require oversizing.

c. Detachable coils. Detachable coil designs allow the operator to test the stability of the coil before detaching, which is preferred in high-risk situations or those in which the embolization target is in close proximity to an arterial branch whose patency is desired. Detachable coils can be released by electrolytic or mechanical detachment, or by degradation of a polymer adhesive.

In a high-flow situation such as arteriovenous fistula, embolization can be performed using detachable coils with the double microcatheter technique. After placing the first microcatheter at the desired level of deployment in the target vessel, a coil can be delivered but not detached. It is used to prevent migration of more proximal coils delivered through the second microcatheter. The distal coil can be detached or retracted at the end of the procedure (2).

d. Coil anchoring technique. The use of anchoring techniques using coils or other devices can help achieve the stable deployment of coils into a large vessel with high flow or high wall compliance. Coil anchoring devices include both purpose-built commercial devices and modifications of existing designs (e.g., coil cages) (3). Coil anchoring devices are particularly useful for occlusion of large arteriovenous fistulae in the lungs and large portosystemic collaterals.

(1) The Amplatz spider (Cook Medical Inc, Bloomington, IN) is a stainless-steel, self-expanding metallic device that can be introduced through a guiding catheter or vascular sheath. It prevents the movement of coils and allows rapid occlusion of the vessel. One modification allows the spider to be screwed onto a threaded guidewire before loading into the catheter, which allows it to be retrieved and repositioned to ensure accurate placement.

(2) Retrievable coil anchors offer the advantage of improved safety due to the ability to retrieve and redeploy suboptimally placed devices. They are also intended to enhance occlusive efficacy by allowing a high density of occlusive material without compromising the self-anchoring capability of the nested coils.

As new coil products become available, each product boasts desirable features compared with its predecessors. For example, the Ruby coil (Penumbra Inc, Alameda, CA) offers a high volume coil that can be easily delivered through a high flow microcatheter. It can be used in a variety of situations, most notably when aneurysmal packing is needed (4). Many other coils are available in different sizes and shapes that can be used in smaller microcatheters to reach more distal vessels in cases such as gastrointestinal (GI) bleeding. The amount of fibers, volume of the metal, and radial force can affect tractability, packing, and delivery. Those qualities can affect coil selection based on the clinical scenario.

2. Vascular plugs

Vascular plugs are permanent occlusive devices which consist of self-expanding nitinol mesh in a three-dimensional (3D) disc geometry. The Amplatzer Vascular Plug (AVP) (St. Jude Medical, St. Paul, Minnesota, MN) is available in four versions (AVP I, II, III, and IV). These plugs vary by the number of mesh layers, the shape and number of lobes, diameter range, unconstrained length, and the length of time to achieve vessel occlusion. Each plug has radiopaque platinum
marking bands, is attached to a delivery wire by a microscrew, and must be deployed through either a guide catheter or sheath (5 to 9 Fr.). The maximal length of the delivery system is 100 cm for plugs I and II and 120 cm for plug III. AVP I has a single mesh layer, single lobe design which is well suited for a short landing zone due to its short unconstrained length (7 to 8 mm). The AVP IV is available in 4- to 8-mm sizes and can be deployed using a 0.038-in. guidewirecompatible braided diagnostic catheter no longer than 125 cm. Due to the low profile and flexible design of the AVP IV

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