The Neuroform Stent, the First Microcatheter-delivered Stent for Use in the Intracranial Circulation

Several years ago, in an editorial comment for Neurosurgery, our group discussed the use of coronary stents as an adjunct to the coil embolization of intracranial aneurysms (11). At the time, we pointed out the potential pitfalls associated with the widespread use of stents in the intracranial circulation. Balloon-mounted stents intended for the coronary circulation lack the suppleness necessary to navigate the tortuosities inherent to the intracranial circulation to reach lesions beyond the carotid siphon. These stents are ideal for sidewall aneurysms on straight vessels, but because most intracranial aneurysms arise at branch points, the efficacy of coronary stents in the treatment of these lesions is greatly reduced. Also, the balloon inflation needed to deploy the stent can be associated with a considerable risk of vessel dissection or rupture and can lead to delayed stenosis (1, 5, 9, 10). To minimize these factors, efforts were made to develop a self-expanding stent pliable enough to navigate the turns of intracranial vessels and effectively span the neck of a branch-point aneurysm, yet with enough radial force to contain an intra-aneurysmal coil mass. The first available such stent was the Neuroform stent (Boston Scientific Target, Fremont, CA). The Neuroform stent is constructed of a nickel-titanium alloy known as nitinol, which possesses a high degree of elasticity and deformability. This self-expanding stent has an ultrathin, open-cell mesh design and exerts a lower radial force than stents used in the treatment of atherosclerotic disease. Instead of being mounted on a balloon, the stent comes preloaded in a coaxial, over-the-wire, 3-French microcatheter delivery system. The delivery catheter has a braided shaft with a hydrophilic coating to facilitate vascular access. Because the stent is fully enclosed within the microcatheter, it can be passed through vessels without abrading the vessel wall. The stents come in a variety of diameters and lengths, ranging from 2.5 to 4.5 mm and 10 to 20 mm, respectively. The ultrathin property of the stent struts makes the stent essentially radiolucent, and this is offset by four radiopaque platinum marker bands at each end (Fig. 1). The entire system consists of the 3-French microdelivery catheter with the stent preloaded and a second 2-French stabilizer catheter. Both catheters are equipped with a Luer-lock hub to allow continuous flushing during the procedure (Fig. 2). FIGURE 1.: Diagram of Neuroform stent in its deployed state. Four platinum marker bands at each end of the stent aid in the fluoroscopic visualization. The ultrathin struts and open-cell design (inset) increase the flexibility of the stent and facilitate passage through the carotid siphon and into the distal intracranial circulation.FIGURE 2.: Diagram of 2-French stabilizer catheter, which fits inside the 3-French microdelivery catheter and stabilizes the stent when it is unsheathed by pulling back the microdelivery catheter. Both the stabilizer and microdelivery catheter are connected to continuous heparinized saline flush.The Neuroform stent is intended for aneurysms with a low (<2) dome-to-neck ratio, and once the decision to use the stent has been made, a series of steps must be followed. The steps that precede routine coil embolization, including adequate placement of a guide catheter and systemic heparinization (some practitioners prefer to avoid heparinization in ruptured aneurysms), are followed in the use of a Neuroform stent. With the guide catheter in place, a digital subtraction roadmap is made, and a microwire (0.014 inch) that is loaded coaxially inside a microcatheter is navigated distal to the aneurysm. The microcatheter is then advanced over the wire, and the wire is removed and replaced with an exchange-length (300-cm) microwire. The microcatheter is then removed, and the microdelivery catheter is advanced over the exchange wire to the point at which the stent spans the neck of the aneurysm. The stent manufacturer recommends that a minimum 4-mm margin be maintained between the stent and each side of the aneurysm neck along the parent vessel to avoid vessel damage or stent migration. Because the maximum stent length is 20 mm, the stent is not designed for use in the treatment of any aneurysm with a neck greater than 12 mm in length. Henkes et al. (4) suggested that the combination of two stents can be used to resolve this issue, but we do not use such a technique. Once the stent is in position, the stabilizer catheter is advanced over the wire until it abuts the proximal end of the stent. At this point, the microdelivery catheter is pulled back over the stabilizer catheter and microwire so that the stent is unsheathed. As the stent expands, the platinum marker bands (four at each end) can be seen to spread, and the stent itself becomes radiolucent. With optimal deployment, the marker bands should be seen both proximal and distal to the neck of the aneurysm. The stabilizer and microdelivery catheters are then removed over the exchange-length wire. A standard-length microcatheter used for coiling is advanced over the wire until it is beyond the stent, then the exchange wire is removed and replaced with a standard-length microwire. The reason for this maneuver is to ensure that the microcatheter is within the stent instead of between the stent and the vessel wall. If the exchange-length wire is removed before the coiling microcatheter is advanced, there is a small chance that the stent position could shift as the standard-length microwire and catheter are advanced to the aneurysm. The lack of significant radial force of the stent poses a problem should the interventionist attempt to access the aneurysm directly without the appropriate use of an exchange wire, because the stent can be shifted inadvertently by a microwire or microcatheter. If the properties of the exchange wire used for stent navigation are compatible with a safe aneurysm manipulation, this can be done, saving the cost of using a new microwire. Once expanded, the interstices of the stent are large enough to accommodate the coiling microcatheter. The microcatheter is pulled back within the stent, and the microwire is directed into the aneurysm. The catheter is then advanced over the wire between the stent interstices and into the aneurysm. Coil embolization of the aneurysm is then performed in the standard manner. The above steps work extremely well in patients in whom the aneurysm arises from the side of a relatively straight vessel. Unfortunately, most intracranial aneurysms occur at branch points, and the vessels involved are rarely straight. Although the ultrathin struts and open-cell design make the stent pliable, the tortuosity inherent to the intracranial circulation sometimes makes passage and deployment of the Neuroform stent difficult. We and others (2, 3) have found an unacceptable amount of friction when attempting to deploy the stent as it was intended over a microexchange wire with the 2-French stabilizer. The stabilizer catheter lacks the strength sufficient for the stent to be moved over the wire through the microdelivery catheter when the target area of the parent vessel assumes a bend. As a result, some centers have ceased using the stabilizer catheter over the exchange wire. In an article in this issue of Neurosurgery, Fiorella et al. (3) review their series of 22 aneurysms (17 unruptured, 5 ruptured) in 19 patients treated with the Neuroform stent with or without coil embolization. They report that they had difficulty in using the stabilizer catheter and replaced it with a 0.017-inch coil pusher. To perform such a switch, the exchange wire needs to be removed, and then the coil pusher is introduced and advanced to deploy the stent. The coil pusher is slightly more robust, and without the added friction of the exchange wire, the stent can be stabilized more effectively for deployment. However, the coil pusher is not the panacea for difficult Neuroform stent deployments. The greatest contributor to friction in the system is the fact that the microdelivery catheter changes shape when positioned in a curve. Because the microdelivery catheter is unbraided, it tends to assume an oval configuration when passed through a curve in a vessel. As the radius of that curve becomes tighter, the microdelivery catheter becomes more oval. Such distortion can cause the microcatheter to lose up to 60% of its luminal diameter (K Canady, Product Manager, Boston Scientific/Neurovascular, personal communication, July 2003). This reduction in luminal diameter effectively locks the stent in place within the catheter; thus, a significant amount of force is needed to deploy the stent. Unfortunately, the lack of braiding in the delivery catheter not only increases the potential for "ovalization" but also makes the catheter susceptible to stretching and possibly even to rupture. We have observed significant stretching of the microdelivery catheter after procedures in which deployment was difficult, and we have even observed one catheter rupture. Fortunately, we were able to use a snare and retrieve the broken segment of catheter with no untoward sequelae. The difficulty often encountered in manipulating the stent within the microdelivery catheter obviously creates problems in stent deployment. Fiorella et al. (3) also report difficulties with stent deployment in six patients, with the final stenting result varying from successful deployment (in three patients) and suboptimal positioning of the stent (in two patients) to a missed target (in one patient). In two other patients, the stent was displaced during microcatheter manipulation after deployment. In one of these patients, the proximal portion of the stent was displaced into a giant cavernous internal carotid artery aneurysm. Boston Scientific/Target has made several modifications to both the microdelivery catheter and the stent that will serve to improve the operator's ability to deploy the stent. The four platinum marker bands at either end of the stent have been rounded at their edges in an effort to decrease any friction that might be encountered as the stent is moved within the catheter. The microdelivery catheter has been modified so that the end is now braided, and bench testing of the catheter has demonstrated a significant decrease in the ovalization of the catheter when curved tightly. The continuous braid also increases the structural integrity of the catheter, thereby reducing its susceptibility to stretch and rupture. The two modifications have led to a 26% reduction in the force necessary to deploy the stent (K Canady, personal communication, July 2003). We have had several opportunities to use the Neuroform2 stent and have encountered none of the difficulties described above. The stabilizer catheter works nicely in the new delivery catheter, obviating the need to use a coil pusher. Because of these changes, we expect to see this stent being used more often in the treatment of aneurysms in which the dome-to-neck ratio is not ideal. In addition to being aware of pitfalls encountered with the deployment of the first-generation device, operators should realize that the Neuroform stent is extremely thrombogenic. Like all intravascular stents, the Neuroform stent is a foreign body and, as such, stimulates platelet aggregation as soon as it comes into contact with the patient's blood. A platelet monolayer quickly forms on the struts of the stent, and eventually a thrombus will form if the aggregation is left unchecked (7). In their report, Fiorella et al. (3) describe four cases of stent-related thromboembolic complications, two with clinical consequences and two subclinical, found during postoperative magnetic resonance imaging. Six patients presented with significant groin or retroperitoneal hematomas. In their series, there were two deaths, one of which was directly related to a thromboembolic event treated with thrombolytic agents. The second death was caused by a contralateral intracranial hemorrhage 6 weeks after the aneurysm was treated. We strongly recommend that patients receive dual antiplatelet therapy for several days before stent placement. We currently prefer to use clopidogrel (75 mg/d) in combination with aspirin (325 mg/d) for 7 to 10 days before the procedure and then continue this regimen for at least 1 month after the procedure (6, 8). Alternatively, if a patient needs urgent treatment and has not received pretreatment with the above regimen, a loading dose of clopidogrel (375–600 mg) is given. This dose provides a platelet inhibition of 55% within 1 hour and 80% within 5 hours of administration (6). Clopidogrel, because of its ability to impede the mitogenic effect of platelet secretion that leads to fibrin production and cellular proliferation, acts synergistically with aspirin to significantly reduce the occurrence of postprocedure ischemic events. Unfortunately, the aspirin-clopidogrel regimen is contraindicated in patients with an acutely ruptured aneurysm that might benefit from stent-assisted coiling. Although it is possible to administer the loading dose of clopidogrel in these patients and then deploy the stent, such an approach could be complicated by a repeat hemorrhage. Also, should thrombus form during the procedure, the intra-arterial thrombolysis needed to combat the thrombus would increase the risk of repeat hemorrhage significantly. Therefore, we think that the ideal aneurysm for the Neuroform stent is one that has not ruptured and is treated electively after appropriate antiplatelet therapy has been initiated. Also, since the stent can shift because of its lack of radial force, we will often consider performing the stent-assisted embolization in a staged manner, which allows the process of endothelialization to heal the stent to the vessel wall. Staging the procedure is by no means necessary, but it also might theoretically decrease any thromboembolic risk associated with the stent. The Neuroform stent represents a significant advance in the endovascular treatment of intracranial aneurysms, but use of the first-generation device has led to several technical problems. The manufacturer has addressed these problem areas, and the Neuroform2 stent should be easier to deploy. However, the thrombogenic properties inherent to the stent will continue to limit its effectiveness in the treatment of acutely ruptured aneurysms. We look forward to further advances that will benefit patients who sustain these lesions. Acknowledgment We thank Paul H. Dressel for preparation of the illustrations.