Background The development of detachable coils is one of the most pivotal developments in neurointervention, providing a tool that could be used to treat a wide variety of hemorrhagic stroke. From the original Guglielmi detachable coil, a number of different coil designs and delivery designs have evolved. This article reviews the history of commercially available detachable coils.
Methods A timeline of detachable coils was constructed and coil design philosophies were reviewed.
Results A complete list of commercially available coils is presented in a timeline format.
Conclusions Detachable coil technology continues to evolve. Advances in construction and design have yielded products which may benefit patients in terms of safety, radiation dose reduction and cost of treatment. Continued evolution is expected, irrespective of competing disruptive technologies.
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There are many developments that have shaped and formed the practice and discipline of neurointervention. The development of cerebral angiography by Egaz Moniz,1 the experiments of Serbinenko2 with balloon embolization of cerebral aneurysms and the development of digital subtraction angiography are all important inflection points in the development of neurointervention. Initially, aneurysms were treated with detachable balloons for vessel deconstruction.3 ,4 The development of detachable coils5 revolutionized the field of neurointervention, providing an endovascular approach that could be safely and consistently used to treat aneurysms in a minimally invasive fashion.
Guglielmi wrote about the thought processes and events that resulted in the development of his eponymous coil which would become the primary key product of Target Therapeutics, would later be acquired by Guidant, and then Boston Scientific and later still Stryker.6
The International Subarachnoid Aneurysm Trial7 catalyzed a wider scale acceptance of coil embolization as a treatment for ruptured aneurysms presenting with subarachnoid hemorrhage, leading to increasing use in ruptured anterior circulation aneurysms. A review by Andaluz et al8 of endovascular and microsurgical treatment of aneurysms spanning 1993–2003 using the National Inpatient Sample demonstrated that endovascular procedures doubled, with steady numbers of clipping procedures performed during that epoch. A similar review by Brinjikji et al9 spanning 2001–2008 using the National Inpatient Sample concluded that, by 2008, the proportion of unruptured aneurysms with endovascular treatment had risen to 63%, with lower rates of long-term hospitalization and mortality.
Figure 1 is a timeline of commercially available coils in the North American market and figure 2 is a timeline of the development and release of the detachment mechanisms employed by the various coil vendors. A detailed discussion of the individual mechanisms is beyond the scope of this article. Figure 3 summarizes the major landmark events in the proliferation of coil technologies. Editorial liberty has been exercised with full apologies to any actors that were not listed.
Coil histopathology and occlusion mechanism
While it is likely that we do not yet fully understand the mechanism of coil function in aneurysms, case series histology reports have shown thrombus formation in the early phases after coil insertion,10 with later ingrowth of angiogenesis and scar formation, differing at the fundus, within and around the coil mass11 with demonstrable endothelialization. Surgical experience with coiled aneurysms has anecdotally shown that coils are not irreversibly adherent to the aneurysm fundus during removal.12–14 Histopathologic analysis of bioactive and gel coils has also been performed.15 ,16 However, widespread histopathologic analysis of aneurysms has not been performed, and further insight into aneurysm pathophysiology and mechanisms of growth, recurrence and progression is desirable.
The first coil
The first commercially available detachable coil5 was relatively stiff by today's standards, helical and, most importantly, detachable. While the duration required for detachment ranged up to 45 min, the detachment mechanism allowed for controlled deployment, repositioning and removal.
The helical morphology of the coil allowed the loops to approximate the periphery of the aneurysm at a given size. However, distribution of the two-dimensional design did not consistently allow for good neck coverage. Moreover, the initial cross-sectional diameter was small, and thus neck coverage in larger aneurysms was not reliable. In the ensuing years, the initial helical design would be modified.
Target introduced a three-dimensional coil with an intrinsic shape that would seek to retain a spherical overall configuration when fully deployed. These would become known as framing coils, which marked a major advance in the philosophy of aneurysm embolization. With a relatively robust frame bounding the neck and dome of an aneurysm, filling the frame with softer coils allowed the operator a greater ‘safety’ margin in filling coil behavior as the frame would bound the outward movement of subsequent coils towards both the aneurysm dome and the parent vessel.
While it is difficult to quantify, it is likely that this three-dimensional framing coil resulted in fewer attempts at repositioning and more reliable retention within the aneurysm after detachment, leading to a significant reduction in the time and fluoroscopy needed to obliterate an aneurysm. Other three-dimensional coils would later be added, such as the Micrusphere (Micrus (now Codman), Raynham, Massachusetts, USA), Compass and Cosmos (Microvention, Tustin, California, USA) and Axium 3D (ev3 (now Covidien), Irvine, California, USA). The commonality of these coils was a relatively robust coil strength and intentional tendency towards three-dimensional peripheralization. Interestingly, the Micrusphere was a relatively short coil compared with the lengthier alternatives. The relatively short length allowed for very consistent spherical three-dimensional morphology, perhaps related to the fact that there were fewer subsequent coil loops after the initial frame.
With both framing coils and two-dimensional helical filling coils it was possible to fill much of most aneurysms. However, the irregular residual spaces in the coil mass were sometimes not spacious enough to allow for easy deployment of even small-sized helical coils. By introducing even softer coils (0.010 inch system) based around more filamentous core wires and less tightly wound secondary winds, coils better suited for filling in the small irregular residual space became available.
The development of the Orbitz Galaxy coil included a small core wire ‘finishing coil’ produced by compressing the coil between a ball bearing and a larger metallic shell, producing a coil with multiple irregular breakpoints, theoretically allowing a different behavior from small thin helices. The relatively material-free hydraulic detachment mechanism of the Orbitz Galaxy may have contributed to its perceived softness in small aneurysms.
The withdrawal of coils from the aneurysm sac would occasionally be complicated by the trapping or locking of the distal end of the coil within the coil mass. This effect would result in stretching and unwinding of the coil—and even fracture. Once the platinum coil unwinds, it is difficult or impossible to either advance the coil back into the aneurysm or withdraw the coil completely back into the microcatheter. The addition of a polymer filament in the center of the coil wind would resist stretching of the metallic coil, helping to maintain the desired physical characteristics of the implant. This feature became an essentially standard facet of coil design.
While platinum is now essentially the only primary material in clinical use, efforts were made to use other materials such as nitinol17 and tungsten.18 The relative stiffness of these materials may have been an impediment to widespread acceptance and use. Platinum is an inert metal with material characteristics favorable for coil manufacturing. It is possible that new materials for coil embolization will be found in the future with benefits over existing technologies.
Some coils such as the VortX Coil (Stryker) or, more recently, the Axium and polyglycolic acid (PGA) fibred and nylon coils (ev3/Covidien) were designed with coils of polymer attached to the primary coil. The thinking behind the fibered coils was that the presence of the fibers might be useful in impeding blood flow and encouraging stasis and thrombosis. While these coil designs never became widely used or extensively studied, they remain an interesting concept.
Since optimal aneurysm repair would result in endothelialization of the coil mass at the neck, the idea of a proinflammatory bioactive coil arose, ostensibly to accelerate the wound healing cascade and cellular ingrowth into the coil mass and at the neck.18 ,19 Early formulations of these coils included the original Matrix coil which was nearly 50% polylactic acid (PGLA) by volume, arranged at the outer surface of the coils.
PGLA was reintroduced as a filament inside the secondary wind of the coil, thus limiting the degree of coil volume loss after PGLA dissolution. The Matrix 2 coil (Stryker) is of this design, and was shown to be not inferior to bare platinum coils in the Matrix and Platinum Science trial.20 PGA21 is present in the Cerecyte22–24 coil series (Codman) and some of the fibred coils available from ev3/Covidien.
Packing density and hydrocoil
Packing density has been associated with a lower rate of recurrence by multiple authors, spurring the development of softer coils that can be placed to increase the volumetric fill from a coil during aneurysm embolization, with the most recent data from randomized controlled HELPS trial.25
Gel-platinum hybrid coils were introduced by Microvention, starting with nominal 0.018 inch system coils that swell to four times the volume once introduced into an aqueous milieu. While stiffer than equivalently sized coils during delivery, after delivery the coils would swell, increasing the volume of inserted coils, thus increasing the packing density of the coil mass. The initial acrylamide-based formulation was then extended to 0.012 inch system sizes and a 0.010 inch system. The hydrogel was wrapped by the secondary wind and provided a marked increase in coil volume.
The introduction of HydroSoft and HydroFrame coils reduced the amount of gel as well as changing the gel formulation. The later coils were constructed with gel inserted within the primary wind. With a lesser volume of gel, it softened the coils relative to earlier designs, also improving working time. For instance, the original Hydrocoil design had a limited working time and required heating in either steam or warm solution, whereas the new coils did not have such a limited working time.
Recent histopathologic data showed a reduced inflammatory response of gel-coated coils in comparison with bare platinum,16 which is of uncertain effect. The HELPS trial showed a 7% reduction in recurrence rates in patients treated with hydrogel, but no statistically significant difference in outcomes compared with bare platinum coils. The role of gel in aneurysm coiling remains uncertain.
Clinical trials on modified coils
Randomized trials have not demonstrated clear benefit of modified coated coils over bare platinum coils,20 ,22 with an excess (albeit small) of adverse outcomes in the Cerecyte arm of the Cerecyte Coil Trial compared with bare platinum coils (6 (5.4%) vs 1 (0.8%), p=0.025) in ruptured aneurysms.26 In the Matrix and Platinum Coil Trial there was no statistical significant difference between the two arms in ‘target aneurysm recurrence’ at 12 months, with similar rates of re-intervention of about 10%.20 The Hydrocoil Endovascular Aneurysm Occlusion and Packing Study studied major angiographic occurrences at 18 months as well as morbidity and mortality, with a primary outcome of 36% in the control group and 29% in the study group (p=0.13). However, the rate of ‘major angiographic recurrence’ was slightly lower in the Hydrocoil group than in the control group (24% vs 34%, p=0.049).25 Thus, to date, there has been no dramatic benefit with modified coils when tested in randomized trials, although there may be a small benefit in the Hydrocoil trial.
Longer coil lengths
The introduction of Cashmere and Presidio coils by Micrus Corporation (later acquired by Codman) centered on longer coil lengths than had been previously available for relatively small sizes, improving the volume filled per coil, simultaneously increasing the cross-sectional area of a coil with the introduction of 0.014 inch Cashmere coils and 0.010 inch and 0.016 inch Presidio coils. Both bare platinum and PGA versions were available.
Different primary winds
The Deltapaq Micrus (now Codman) series included a novel precessing triangular primary wind, intended to allow for a greater number of points where the coil can bend and deflect during filling and finishing. In vitro data by Mehra et al27 showed superior regularity in distribution within a volume relative to helical primary wind designs from the same manufacturer.
Helical two-dimensional coils have remained in the role of a filling coil up until the present. Ostensibly, the role of a filling coil is to fill the bulk of the aneurysm, respecting the frame and margin at the inflow zone or ‘neck’ of the aneurysm. The helical coils have an inherent helical shape which may or may not be ideal for filling irregular residual volumes, and new approaches to filling the bulk of the aneurysm volume were conceived and products developed.
The Penumbra 400 (Penumbra) has a much larger 0.020 inch cross-sectional area which is four times the cross-sectional area of a 0.010 inch coil system. By using a nitinol core wire and an outer shell of platinum, the approach was to use a very high volume to length coil to rapidly fill aneurysms.
The Variable Fill Coil by Microvention-Terumo introduced a new approach to filling by a wave and loop morphology aimed at allowing a smaller number of coil units to fill a larger variety of spaces, using the loop portion of the coil to serve as the minimum size and the wave portion to peripheralize the coil around the aneurysm body.
The DeltaMaxx coil is a nominal 0.018 inch coil with a triangular primary wind, with the expectation that the coil will combine the distributive characteristics of the thinner versions of the primary wind with a larger volume per length.
Stents and flow diverters
Wide-necked large aneurysms remain challenging with high recurrence rates, posing anatomic difficulties in endovascular reconstruction relating to neck morphology. Balloon remodeling remains an important tool in the neurointerventional armamentarium.
However, the introduction of cerebrovascular stents such as the Neuroform (Stryker) or Enterprise (Codman) stents allowed for neck remodeling while preserving flow through the parent vessel during the intervention. Both jailing and trans-stent microcatheter techniques have been described.
Low porosity stents and/or flow diverters would later be introduced, based on the idea that low porosity at the aneurysm neck might allow for endothelial cell growth to cover and reconstruct the parent vessel. Early data from the Pipeline experience (Covidien/ev3)28 ,29 and the Silk experience30 are promising. However, these technologies have uncertain long-term complication rates and may not yet be appropriate for the widespread treatment of small aneurysms.
With increasing scrutiny over the price of devices, it is important for physicians to be aware of the cost dimensions and pricing strategies of selected coils while balancing implant characteristics. A recent editorial by Cloft31 illustrates this point well. At present, the current remuneration structure leaves device manufacturers with different financial incentives than procedurists. Generally, the greater the number of small volume coils implanted, the more financially lucrative a case is for a device manufacturer. Conversely, the greater the number of coils normally implies longer procedure and fluoroscopy times. Price structures that align device construction with procedural efficiency may help align patient interests with device companies and physicians.
The delivery wire and detachment zone
While much attention has been given to coil design and construction, an inextricable component of coil delivery is the delivery wire and the detachment mechanism. While a detailed discussion of pusher wire and detachment technology is beyond the scope of this paper, given that operator haptic experience of coil behavior is mediated largely through the coil pusher, the design of the pusher has a profound effect on the perception of the coil in the aneurysm.
The detachment mechanism is necessarily different in stiffness than both the coil and the distal delivery wire, which may have effects on the behavior of the distal microcatheter while pushing the last several millimeters of a coil out of the delivery microcatheter. Stiffer detachment zones relative to the coil can result in straightening of the distal microcatheter. Thus, improvements in both delivery wires and detachment mechanisms in terms of physical properties and detachment times have been important aspects of coil evolution (figure 2).
Over the years, vendors have sought to find quantitative methods to describe the design benefits of their coils, measuring spring constants, angle straightening, catheter deflection as well as the forces necessary to bend the coil, wire and detachment zones. There is wide variance in the testing methods and choice of quantitative measures, such that there is no accepted gold standard single metric to show coil ‘softness’. At present, the mechanisms of recurrence remain debated, invoking aneurysm growth and coil compaction as well as a host of other perhaps unforeseen factors. As such, it cannot even be concluded that the softest coil is the most advantageous coil.
With the vast array of coils available to the neurointerventionalist, the number of individual products that may be stocked in an angiography suite exceeds any reasonable stocking space and the addition of ischemic stroke devices introduces an even larger number of devices. Careful selection of devices and device portfolios thus becomes an unexpected yet critical aspect of managing a neurointerventional service.
This article discusses primarily the history of coils, neglecting detailed discussion of coil adjuncts such as neurovascular stents and balloons and their impact on aneurysm embolization techniques. A detailed discussion of the mechanical properties and construction of the coils has been neglected. White et al32 provide an excellent overview of coil construction and dimensions.
The development and proliferation of coil technology has ushered in a dizzying array of coils with different design strategies, manufacturing processes and detachment mechanisms. Despite the development of new flow diverters, coil technology appears to have a persistent role in the endovascular care of vascular disorders, particularly intracranial aneurysms. The development of new coil designs, redesigns and perhaps new materials may yet surprise us as to the resilience of this family of technologies.
This article seeks to chronicle in brief the history of coils in the North American market, but it can only be a summary as the real story lies in the lives of the patients treated.
The authors thank Christine Moore for her excellent editorial assistance and Ross Papalardo and his expert ‘pen’. They also thank the management (in alphabetical order) of Blockade, Codman, Covidien, Microvention, Penumbra and Stryker for their input with regard to product lists and approval dates.
Contributors All authors contributed to this manuscript.
Competing interests None.
Provenance and peer review Not commissioned; internally peer reviewed.
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