The Ovation abdominal stent graft system is a unique polymer-based trimodular endoprosthesis that has demonstrated safety and effectiveness in treatment of abdominal aortic aneurysms, especially in patients with challenging neck anatomy and narrow iliac vessels ( Fig. 7.1 ). The stent graft is made of a 32 mm long nitinol suprarenal stent, with radiopaque (RO) markers near the proximal edge of the graft. The main stent graft body is made up of low-permeability PTFE (polytetrafluoroethylene) and lacks a metallic frame. There are two O-rings proximally with polymer channels and smaller rings along the main body and limbs, providing support to the stent graft. By replacing the metallic stent with polymer channels, the stent graft profile is reduced and is deliverable through 14 or 15 French sheaths. While other aortic stent grafts achieve fixation and sealing with an outward radial force of a self-expanding nitinol stent, the Ovation stent graft uncouples fixation and sealing ( Fig. 7.2 ). Fixation is achieved by the nitinol suprarenal stent and sealing is achieved by filling the graft’s proximal most conformable O-ring with a polymer during graft deployment ( Fig. 7.2 ). The sealing rings conform to vessel wall and create a circumferential seal without exerting a chronic radial force. Lack of radial force removes pressure from the neck and is thought to prevent neck dilatation, migration and type IA endoleaks. The rings inflate gradually, allowing them to conform to neck irregularities such as thrombus and calcifications. The polymer fills the stent graft bottom-to-top and then the contralateral limb. The polymer filling is diverted away from the sealing ring when it comes across an aortic wall irregularity, and excess inflation is diverted inwards into the aortic lumen, avoiding any significant stress on the aortic wall.
Deployment
After delivery of the stent graft, it is unsheathed and the proximal nitinol stent is deployed in two stages with the RO markers ensuring proper positioning of the graft ( Fig. 7.3B ). After the release of the first knob, the mid-crown is deployed. The stent graft can still be repositioned at this time. It is prudent to start deployment higher than the intended final position, since it is easier to pull the stent graft down than to push it up. After the second knob is released, the suprarenal stent is completely deployed, anchoring the stent graft. The polymer is mixed in the two syringes of the polymer kit and infused using the autoinjector ( Fig. 7.3A ). The polymer begins to fill the stent graft, inflating the sealing ring against the aortic wall. The autoinjector exerts a constant pressure against the aortic wall allowing the seal to conform to the aortic wall. Holding the stent delivery system or applying forward pressure on it is not recommended. The stiff wire is withdrawn into the body of the graft to facilitate conformance of the stent to native aortic anatomy, especially when acute angulation is present. The deployment of the main body is completed by turning the third knob to disconnect the filling tube from the stent graft.
Instructions for use (IFU)
According to the IFU, the anatomic criteria for the use of the Ovation stent graft include proximal aortic diameter between 16 and 30 mm at 13 mm below the inferior most renal artery, with an aortic angle of ≤60 degrees if proximal neck is ≥10 mm and ≤45 degrees if proximal neck is <10 mm. There should be at least 10 mm iliac landing zone with diameters between 8 and 20 mm. Patients with known allergy to the device materials (including PTFE, polyethylene glycol (PEG)-based polymers, fluorinated ethylene propylene [FEP] or nitinol) should not be treated with the Ovation stent graft.
Polymer-related complications
As mentioned above, the Ovation stent graft is contraindicated in patients who are allergic to device materials including PTFE, PEG, or nitinol. There have been reports of inadvertent leakage of polymer into the systemic circulation with resulting allergic reactions including anaphylaxis. In the majority of cases, the exact cause of the leakage was not identified and was attributed to defects in the polymer kit system.
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Polymer should be injected using the autoinjector only
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DO NOT detach delivery system prematurely unless there has been an allergic reaction
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If there is leakage, detach autoinjector and polymer syringe immediately
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In case of allergic reaction/anaphylaxis, use antihistamines, steroids, vasopressors, and airway protection as necessary
To prevent polymer-related complications, the IFU should be followed when deploying the stent. The following steps and key points should be considered to prevent polymer-related issues:
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After deployment of the suprarenal stent, the delivery system should not be pulled as it may cause inadvertent detachment of the polymer fill connecter from the main body leading to leakage of the polymer into the blood.
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The delivery system should not be detached prematurely (time to polymerize is 20 minutes for the TriVascular Fill Polymer Kit or 14 minutes for the CustomSeal Kit) to prevent leakage of the polymer.
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The patient’s body temperature should also be considered. The polymer is designed to cure at core body temperature of 37°C and might take longer at lower temperatures. If the core body temperature is lower than 35°C, it may require at least one additional minute per degree below 35°C prior to disconnection.
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If there is an error in mixing or transferring the polymer or delay in infusing after mixing (3 or more minutes after mixing if using the TriVascular Fill Polymer Kit or injection has been delayed 2 or more minutes using the CustomSeal Kit), the polymer should be discarded and a new kit opened.
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The polymer should be infused using the autoinjector only; hand injections can damage the stent causing delamination of the polymer channels.
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The filling of the polymer should be initially observed under fluoroscopy. Lack of filling of the polymer channels or rapid emptying of the syringe may indicate that the polymer is not being infused into the stent and may be leaking into the blood.
If there is suspicion or evidence of leakage, the autoinjector and the polymer fill syringe should be immediately detached from the delivery system to ensure patient safety.
In case of allergic reactions, standard treatment (antihistamines, steroids, vasopressors, airway protection) should be administered.
Type IA Endoleak
Type IA can result from incomplete or lack of filling of the sealing rings or secondary to in-folding of the sealing ring. The IFU and the key points described in the previous section should be followed to prevent and minimize the chances of a type IA endoleak.
Incomplete filling of chambers
In case of incomplete or no filling of the sealing ring, proximal seal can be achieved by deployment of a Palmaz stent (Cordis, Miami Lakes, Florida).
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The Palmaz stent can be mounted on a compliant or non-compliant balloon, which is sized to match the native aorta. We routinely use the Z-Med balloon (B. Braun Medical Inc, Bethlehem, Pennsylvania) to deploy the Palmaz stent.
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The main body as well as the bilateral iliac limbs are completely deployed.
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A sheath is advanced to the level of the sealing rings, protecting the Palmaz stent as it is delivered to the desired deployment area, to prevent dislodgement of the stent during transit.
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The proximal edge of the Palmaz is then aligned with the RO markers of the Ovation stent.
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Before deployment of the Palmaz, all wires and catheters should be withdrawn to avoid entrapment behind the stent and the sheath pulled back to just below the stent.
In-flow stenosis/Sealing ring in-folding
The sealing rings inflate slowly and any excess inflation is directed inward into the aortic lumen to limit stress on the aortic neck. This can lead to circumferential aortic lumen narrowing, especially in patients with heavily calcified, irregular neck ( Fig. 7.4 ). While this can lead to up to 60% in-flow stenosis, there is no evidence that this is hemodynamically significant. In-folding can be prevented by avoiding excessive oversizing. The O-rings are designed to inflate to a specific diameter and if cramped within a small aortic neck, the rings will expand inward when outward expansion is limited by the aortic wall. While other stent grafts rely on oversizing and outward radial force for seal, the Ovation stent graft, with its unique sealing mechanism, does not need to be unduly oversized. The manufacturer-recommended sizing incorporates adequate oversizing for different aortic diameters and should be followed. In case of excessive stenosis or in-folding, the following steps can be taken:
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A compliant or non-compliant balloon can be inflated 14–25 minutes after polymer mix for Custom Seal and 20–40 minutes for the Ovation Polymer. The balloon is centered over the middle of the sealing rings and inflated to conform to the rings or resolve any in-folding.
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If stenosis/in-folding persists, it can be treated by placement of a Palmaz stent as described earlier ( Fig. 7.4 ).
