Design and fixation
The basic design of a covered stent-graft includes a fabric tube and a metal framework attached to it. The framework consists of annular or zigzag ring stents made from shape-memory alloys (nitinol or Elgiloy). The framework is attached to the fabric with sutures or adhesive. The ends of some models feature uncovered stents for better fixation.
There are two main mechanisms for anchoring the graft in the aorta:
- Passive fixation: the stent is held in place by radial force, creating friction against the vessel wall.
- Active fixation: the device features small hooks that embed into the aortic wall, reliably preventing distal migration.
Types of stent-grafts
Depending on the aneurysm location and the patient's anatomical features, various designs are used:
- Bifurcated grafts: used for the abdominal aorta. They consist of a main body, a long limb, and a short limb (contralateral gate). The second limb is inserted separately, mimicking a normal bifurcation.
- Complex grafts (fenestrated and branched): required for aneurysms involving major branches of the aorta (e.g., Crawford type IV or in the thoracic aorta near the carotid/subclavian arteries).
- Parallel grafts: chimney (chimney), snorkel, and sandwich techniques. They preserve blood flow in target branches by running parallel to the main stent-graft.
- EVAS system: endovascular aneurysm sealing, where tubular stents are surrounded by PTFE polymer bags.
Preoperative planning and requirements
Successful implantation requires precise imaging (the gold standard is arterial-phase CTA with thin slices). Multiplanar reconstruction is used to measure vessel diameters and lengths.
An essential condition is the presence of suitable sealing zones (proximal "neck" and distal landing zone):
- Neck length: minimum 8–15 mm for the abdominal aorta and 20 mm for the thoracic aorta.
- Risk factors: short neck, severe atheromatosis, thrombosis, severe angulation (bending), and barrel-shaped necks.
The diameter of the selected stent-graft must exceed the aortic diameter by 15–20% (oversizing) to ensure a secure seal. If anatomy is unfavorable, complex custom-made grafts are required.
Deployment procedure
Stent-grafts are delivered in a compressed state inside an introducer sheath. Upon withdrawal of the sheath, the stent expands via its own radial force. A molding balloon may be used to improve device apposition to the walls in the sealing zones.
During deployment, it is important to account for the Windsock effect: systolic blood pressure can displace a partially deployed graft. Positioning and the absence of blood leakage (endoleaks) are monitored directly in the OR using cone-beam CTA.
Briefly: answers to topical questions
What are the types of endoleaks in endovascular aneurysm repair?
There are 5 types of endoleaks during endovascular treatment of aneurysms.
- Type I (Type I) — secondary leakage due to inadequate sealing at the stent-graft fixation site.
- Type II (Type II) — retrograde filling of the aneurysm sac via branch arteries.
- Type III (Type III) — mechanical damage to the graft or separation of its modular components.
- Type IV (Type IV) — secondary leakage due to the porosity of the graft material.
- Type V (Type V) — Endotension, expansion of the aneurysm sac without visible abnormalities on CTA.
Class and exam questions
What is oversizing during stent-graft placement?
This refers to selecting a stent-graft whose diameter is 15–20% larger than the true aortic diameter. This is necessary to ensure tight apposition and a secure seal.
What is the CHEVAR technique?
CHEVAR is an endovascular aneurysm repair technique using parallel stent-grafts (the chimney technique). It preserves blood flow in vital aortic branches (e.g., renal or mesenteric arteries).
Why are uncovered stent struts needed at the ends of the graft?
Bare metal is used for proximal fixation above the renal arteries. This anchors the graft without obstructing blood flow to vital organs.
What else to study on this topic
- Classification of endoleaks and their management
- Detailed analysis of EVAS systems and their differences from standard stent-grafts
- Use of Endoanchors for the prevention of type I endoleaks
- Features of bifurcated system deployment and modular assembly
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