American Journal of Neuroradiology 31:E41, April 2010
© 2010 American Society of Neuroradiology

L.-D. Jou^a
aDepartment of Radiology Baylor College of Medicine Houston, Texas

The softness of endovascular coils enables the packing of an intracranial aneurysm at a higher density so complete embolization is possible. While softness is an important concept, it is rarely discussed in a quantitative fashion. This information is often regarded as proprietary by manufacturers and is not available to the public. Thus, we do not know what makes a coil soft and what makes a coil stiff. White et al reviewed the physics of coils in an article in the American Journal of Neuroradiology,¹ but we think that there is a need to clarify the definition of coil stiffness (or softness) further. In their article, stiffness of a coil is defined as the spring constant of the secondary structure. This might be the easiest way to analyze a coil because coil structure is very similar to a spring in appearance. However, the coil is not compressed to fill the aneurysm during an endovascular procedure (as in Fig 1A). Instead, a coil is often bent to fit the available space as in Fig 1B. It is much easier to bend a coil than to compress it, and for this reason, one can hardly compress a spring (say, taken from a retractable pen) without buckling it first. On the other hand, the pitch of the secondary structure is usually very small, so a coil has little capacity for compression.

View larger version (21K): [in this window] [in a new window] Fig 1. Filling an aneurysm by compressing (A) and bending (B) a coil.

Bending of a coil actually twists the wire of a coil (the primary coil structure). Bending stiffness of a coil is defined as 

where G is the shear modulus,  the Poisson ratio, p the pitch,  the pitch angle, and D₁ and D₂ are the diameters of the primary and secondary structures, respectively.² The pitch angle is related to the pitch in the following way: tan() = p/D₂. The equation itself may appear to be complicated, but bending stiffness is proportional to GD₁⁴ sin() after simplification. The basic principle has been explored in Marks et al,³ when they described how the coil responded to bending; the stiffness is the slope in Fig 3 of their article.

Both equation 1 and the formula in White et al¹ show that the stiffness is proportional to the fourth power of the primarystructure diameter (D₁). The major difference between bending and compression is that bending stiffness is not inversely proportional to D₂³. Therefore, the diameter of the secondary structure is less important in determining the stiffness; the pitch angle, on the contrary, is quite critical in the stiffness of a coil. Last, the pitch affects the bending stiffness, not the spring constant. Fine-tuning the compression stiffness of a coil by changing the pitch simply cannot be achieved by the way described in White et al¹ because the pitch did not appear in their definition of softness.

Coils used for endovascular therapy have multiple structures at various scales, and the mechanics of coils are actually quitecomplex. The first and last few coils used in a procedure may require completely different considerations. It is critical to fully examine all possible deformations before we can determine the softness of a coil.

References

1. White JB, Ken CGM, Cloft HJ, et al. Coils in a nutshell: a review of coil physical properties. AJNR Am J Neuroradiol2008;29: 1242–46[Abstract/Free Full Text]
2. Meagher JM, Altman P. Stresses from flexure in composite helical implantable leads. Med Eng Phys 1997;19: 668–73[CrossRef][Medline]
3. Marks MP, Tsai C, Chase H. In vitro evaluation of coils for endovascular therapy. AJNR Am J Neuroradiol 1996;17: 29–34[Abstract]

Reply

Published ahead of print on February 25, 2010
doi: 10.3174/ajnr.A2030

American Journal of Neuroradiology 31:E42, April 2010
© 2010 American Society of Neuroradiology

D.F. Kallmes^a and H.J. Cloft^a
aMayo Clinic Rochester, Minnesota

J.B. White^b
bTexas A&M Health Science Center College Station, Texas

C.G.M. Ken^c
cSan Mateo, California

We greatly appreciate Dr Jou’s interest in our work. His approach is innovative. We are gratified to see that the major determinant of coil softness, the diameter of the primary coil structure, is the same in his analysis and ours. We concur that empiric testing of coil softness remains problematic, but both his calculations and ours appear in line with our own clinical experience.

Gary Duckwiler, MD, Director of Clinical Affairs and Fellowship Director, Division of Interventional Neuroradiology, Ronald Reagan UCLA Medical Center

Flow diversion stents and endoluminal flow-disrupting devices are new therapeutic approaches to treat challenging intracranial aneurysms [1,2]. The first generation Pipeline Embolization Device (PED) has already been used in clinical practice. The reported results of PED treatment of intracranial aneurysms appear to be promising and very encouraging for the neuro-endovascular field [1,2]. However, there has always been a lingering concern associated with any intracranial stent that they may compromise the blood flow in small side branches coming from the stented segment of artery. After the introduction of low profile, self-expandable intracranial stents such as Neuroform, Wingspan, and Enterprise, we started to realize that eloquent perforators could sustain their patency with an approximately 10% area-coverage stent [3]. Unfortunately, with a low coverage stent, the likelihood of aneurysm occlusion without adjunctive treatment is very low. Finding the “sweet spot” of neck coverage versus side branch occlusion is the ultimate target to prevent aneurysm recurrence. The introduction of PED has expanded this envelope in that we may be able to put a stent that has 30% area coverage while maintaining the patency of perforating arteries, although the long-term patency of stented artery including perforating arteries still needs to be evaluated carefully.

Masuo et al introduced a very unique experimental method to access the patency of perforating arteries coming off a major intracranial artery after the stent placement [4,5]. They used a rabbit abdominal aorta and lumbar artery to simulate the relationship between a major intracranial artery and its perforating arteries. Their studies published in AJRN 2002 (healthy aorta) and 2005 (atherosclerotic aorta) suggest that risks of perforator occlusion may increase when a stent is placed in an atherosclerotic artery [4,5]. The safety and biocompatibility of PED and PED2 were tested utilizing the same methodology except that the abdominal aortae were as healthy as a human pediatric case. Thus, the indication of PED placement for a challenging intracranial aneurysm associated with atherosclerotic change must be planned with extra caution.

Another open question is the interim effect of flow diversion on the stability of cerebral aneurysms.  Intra-aneurysmal flow and its relationship to growth and rupture of aneurysms is a highly debated topic [6].  From the landmark ISAT trial, it was proven that intra-aneurysmal coiling reduced the rerupture rate compared to historical rates [7].  Whether or not high-coverage stents will provide the same benefits predictably, without causing a negative impact of intra-aneurysmal flow dynamics remains to be seen, especially given the need for coagulation management in the acute phase.  In this study, both side branch preservation and aneurysm occlusion were obtained, which is a great first step.

The PED seems to be a promising potential solution in wide neck and possibly fusiform aneurysms that are not treatable by conventional surgery or endovascular embolization. Certainly, any efforts to further delineate the safety profile and outcome predictability of high coverage stents is valuable and necessary.

References
1. Fiorella D, Woo HH, Albuquerque FC, Nelson PK. Definitive reconstruction of circumferential, fusiform intracranial aneurysms with the pipeline embolization device. Neurosurgery 62: 1115-1121: 2008

2. Lylyk P, Miranda C, Ceratto R, Ferrario A, Scrivano E, Luna HR, Berez AL, Tran Q, Nelson PK, Fiorella D. Curative endovascular reconstruction of cerebral aneurysms with the pipeline embolization device: the Buenos Aires experience. Neurosurgery 64: 632-643: 2009

3. Biondi A, Janardhan V, Katz JM, Salvaggio K, Riina HA, Gobin YP. Neuroform stent-assisted coil embolization of wide-neck intracranial aneurysms: strategies in stent deployment and midterm follow-up. Neurosurgery 61: 460-469: 2007.

4. Masuo O, Terada T, Walker G, Tsuura M, Matsumoto H, Tohya K, Kimura M, Nakai K, Itakura T.  Study of the patency of small arterial branches after stent placement with an experimental in vivo model. AJNR Am J Neuroradiol 23: 706-710: 2002

5. Masuo O, Terada T, Walker G, Tsuura M, Nakai K, Itakura T. Patency of perforating arteries after stent placement? A study using an in vivo experimental atherosclerosis-induced model. AJNR Am J Neuroradiol 26:543-548: 2005

6. Tateshima S, Tanishita K, Omura H, Villablanca JP, Vinuela F. Intra-aneurysmal hemodynamics during the growth of an unruptured aneurysm: in vitro study using longitudinal CT angiogram database. AJNR Am J Neuroradiol. 28: 622-627: 2007

7. Molyneux AJ, Kerr RS, Yu LM, Clarke M, Sneade M, Yarnold JA, Sandercock P; International Subarachnoid Aneurysm Trial (ISAT) Collaborative Group. International subarachnoid aneurysm trial (ISAT) of neurosurgical clipping versus endovascular coiling in 2143 patients with ruptured intracranial aneurysms: a randomised comparison of effects on survival, dependency, seizures, rebleeding, subgroups, and aneurysm occlusion. Lancet. 366: 809-817: 2005

A recent article published by Dr Robert Willinsky and colleagues from Toronto is noteworthy and an excellent addition to the literature on the subject of endovascular treatment of ruptured aneurysms. This is one of the largest series on this subject and addresses, amongst other things, the important issue of rebleeding after endovascular repair.

In this large series, the rehemorrhage rate after embolization was 2.1%. The risk of rebleeding was 1.6% in the first 30 days, decreasing to 0.7% after this time. The authors use a protocol of imaging the ruptured aneurysms with enhanced MRA at the time of discharge, followed by another MRA relatively early at 2-3 months to detect early recanalization/regrowth of aneurysms. Interestingly, the rebleeding rate from initially completely coiled aneurysms was similar to those with neck and body remnants. Therefore, their approach is not to strive for a “perfect” angiographic result that tends to increase the complication rates. They did not specifically comment on the issue of use of stents (and associated antiplatelet therapy) in the setting of ruptured aneurysms and whether or not any of the rebleeds occurred in the context of stent assisted coil embolization.

The complication, recanalization and retreatment rates presented are similar to other large series. As would be expected, their complication rates have steadily decreased with time concurrent with improvements in endovascular catheter and coil technology as well as increasing experience with this technique. There are many excellent suggestions throughout the manuscript relating to optimal treatment approach towards ruptured aneurysms as well as follow-up after therapy. It is very interesting to read how these approaches have evolved over the last 14 years at the authors’ institution. This article is vitally important in furthering our understanding of endovascular therapy of ruptured aneurysms.

Reference
Willinsky RA, Peltz J, da Costa L, et al.  Clinical and Angiographic Follow-up of Ruptured Intracranial Aneurysms Treated with Endovascular Embolization.  AJNR Am J Neuroradiol, first published March 25, 2009 as doi:10.3174/ajnr.A1488.