Published online before print February 28, 2013, doi: 10.3174/ajnr.A3451
AJNR 2013 34: E31-E32
K. Wanga and S. Yuana
aDepartment of Neurosurgery
General Hospital of Jinan Military Command
Jinan, China
We read the interesting article by Schneiders et al,1 entitled “A Flow-Diverting Stent Is Not a Pressure-Diverting Stent.” The authors measured intra-aneurysmal pressure before, during, and after placement of a flow-diverting (FD) stent and found that the pressure inside the aneurysm momentarily decreased during placement but was restored to baseline values within minutes. They agreed with the argument that the use of an FD stent for treatment of intracranial aneurysms should be combined with insertion of coils in the aneurysmal sac. After carefully observing the conventional angiogram in their article, we found that the included angle of the radiopaque longitudinal markers of the stent at the neck was approximately 60°. Therefore, the stent was partly elongated at the neck of the aneurysm.
The metal coverage rate (MCR) of an FD stent can change as a result of either compression or stretching. The MCR (ζ) and pore density of the stent can be calculated according to the following formula:
where N represents the wire number, d stands for the wire diameter of the stent, Brepresents the length of the wire, which spirals a complete turn, and α represents the included angle between wires along the long axis of the stent. All measured units are in millimeters. The tendency of the MCR corresponding to the included angle of the stent wires is shown in Fig 1 when an FD stent is compressed or stretched.
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- Fig. 1. Correlation of the MCR with the included angle between wires along the long axis of an FD stent. This stent was made of 48 wires (N) with a wire diameter of 0.04 mm (d) and a wire spiraling a complete turn length of 14.2 mm (B). When the included angle (α) is 90°, the MCR decreases to a minimum of approximately 20%. When the included angle is <30° (eg, the stent is stretched in the microcatheter) or >150° (eg, the stent is compressed at the neck), the MCR sharply increases.
An FD stent with an MCR of 30% significantly affects the hemodynamics in the aneurysm. An FD stent with a 35% actual MCR at the neck can predict >95% of angiographic aneurysm occlusions in rabbits.2 The Pipeline Embolization Device (PED; Chestnut Medical Technologies, Menlo Park, California) provides a 30%–35% MCR with approximately a 142°–150° nominal included angle between wires when it is fully expanded.3 The Silk flow diverter (SFD; Balt Extrusion, Montmorency, France) provides 35%–55% MCR of the internal diameter of the target vessel at a nominal diameter, and the included angle between wires is >120°, as shown by a previous study.4 In an in vitro study of SFD morphology, the local actual MCR of the FD stent at different curvatures in different vessel models can change from 19% to 63%.5 The MCR decreases to a minimum, which is only approximately 20%, when the included angle decreases to 90° in an FD stent as shown in Fig 1. This actual MCR at the neck may not significantly change hemodynamics. Even though the included angle between wires can enhance the MCR when decreased to <90°, the stent is severely stretched; this change may cause the stent not to open.
The proper morphology of an FD stent at the neck is when the included angle between wires is equal to or higher than its nominal angle, where the local actual MCR can achieve or exceed 35%. In vivo and in vitro studies have also indicated that properly pushing the microguidewire or microcatheter can gain more MCR at the neck for stasis and thrombosis of aneurysms.2,3 The included angle between wires of an FD stent at the aneurysm neck can be easily displayed by angiographic CT.
References
- Schneiders JJ, Vanbavel E, Majoie CB, et al. A flow-diverting stent is not a pressure-diverting stent. AJNR Am J Neuroradiol 2013;34:E1–4 » Abstract/FREE Full Text
- Wang K, Huang Q, Hong B, et al. Correlation of aneurysm occlusion with actual metal coverage at neck after flow-diverting stent implanted in rabbit models. Neuroradiology 2012;54:607–13 » CrossRef » Medline
- Ma D, Dargush GF, Natarajan SK, et al. Computer modeling of deployment and mechanical expansion of neurovascular flow diverter in patient-specific intracranial aneurysms. J Biomech 2012;45:2256–63 » CrossRef » Medline
- Kulcsár Z, Ernemann U, Wetzel SG, et al. High-profile flow diverter (Silk) implantation in the basilar artery: efficacy in the treatment of aneurysms and the role of the perforators. Stroke 2010;41:1690–96 » Abstract/FREE Full Text
- Aurboonyawat T, Blanc R, Schmidt P, et al. An in vitro study of Silk stent morphology. Neuroradiology 2011;53:659–67 » CrossRef » Medline
Reply
Published online before print February 28, 2013, doi: 10.3174/ajnr.A3514
AJNR 2013 34: E33
R. van den Berga, J.J. Schneidersa, S.P. Fernsa and C.B. Majoiea
aDepartment of Radiology
E. Vanbavelb
bDepartment of Biomechanical Engineering and Physics
Academic Medical Center, University of Amsterdam
Amsterdam, the Netherlands
We thank Drs Wang and Yuan for their detailed mathematic analysis of our Technical Note entitled “A Flow-Diverting Stent Is Not a Pressure-Diverting Stent.” The authors indeed made a good observation that the SILK stent (Balt Extrusion, Montmorency, France) was not optimally deployed at the initial stage of the procedure. The cause of the suboptimal initial deployment was either due to the mass effect by the aneurysm or to focal vasospasm. The latter is less likely because treatment had already been performed at day 3 after presentation, but more important, the patient did not show signs of a subarachnoid hemorrhage.
The illustration (Fig 1 in the original article) indeed shows that the struts of the Silk stent are arranged at an angle indicating deployment below its nominal diameter. In such circumstances, the flow-diversion effect was probably less than at optimal (nominal) deployment. This difference might have impacted the flow-dynamic effect in that measurements with the ComboWire (Volcano Corporation, Rancho Cordova, California) were affected, and the pressure and flow reduction might have been better when optimal deployment was achieved. However, despite the drawback, we did see changes in the pressure inside the aneurysm with time (Fig 2 in the original article), with a temporary reduction in pressure inside the aneurysm but a subsequent return to baseline values 5 minutes after deployment (from t = 5 minutes to t = 10 minutes). The deployment of the stent remained stable during this interval; therefore, the alteration in the pressure curve occurred when the stent was suboptimally deployed. One might question whether intra-aneurysmal pressure would have dropped more in a situation of optimal deployment. However, this change could only have occurred if substantial leakage had existed through the aneurysm wall or via other exit pathways. In this case, simple physics dictates intra-aneurysmal pressure to be Pintra =Part·Rleak/(Raccess + Rleak), with Part, the local arterial pressure; Raccess, the access resistance through the stent; and Rleak, the resistance of the leakage pathways. A better deployment raises Raccess, but there seems very little reason to assume that it would approach the high resistance of any tiny leakage pathways, if these exist at all. Hence, we expect intra-aneurysmal pressure to be nearly equal to intra-arterial pressure, even when stent deployment is more optimal. Of relevance, on the basis of similar physics, pressure pulsations in the aneurysm are expected to be damped better if access resistance is higher or if aneurysmal compliance is higher. Alas, we did not perform any repeat measurements in other patients to confirm such physical analysis.
In addition to our own response, we also asked for the manufacturer of the Silk stent, Mr N. Plowiecki, to respond. He agreed with the analysis of the stent struts and deployment by Drs Wang and Yuan and supports their statements that the analysis of the flow-dynamic effect should be seen in relation to and as a function of the angle of the struts of the stent.
At present, we have a 3-year follow-up on the occlusion rate of the aneurysm. On a recent contrast-enhanced MR angiography study, complete occlusion of the aneurysm is seen. Moreover, the mass effect of the aneurysm has diminished. The suboptimal deployment of the Silk stent has at least not hindered the progressive occlusion of the aneurysm. The patient is in stable neurologic condition, and no hemorrhage has occurred since treatment.
