Coil pliability matters: Novel metrics for assessment of in vitro coil softness

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The deployment of an embolisation coil is “much more art than science” according to Daniel Sahlein (Goodman Campbell Brain and Spine, Indianapolis, USA). In this article, Sahlein considers the importance of embolisation coil pliability, describes a novel test for the assessment of coil softness, and considers how this may influence clinical practice.

Embolisation coils have been widely used for intracranial aneurysm treatment since the introduction of Guglielmi detachable coils in the 1990s. Platinum coils have typically been used in these endovascular procedures and are commercially available in various shapes and sizes. Technological advancements in coils include the improvement of pliability or softness, addition of shapes, modification of the mechanical detachment systems, addition of coatings, and increased softness of the distal delivery wire or mandrel.⁵ Coiling remains the preferred treatment for ruptured aneurysms. Flow diversion is currently not indicated for treatment of ruptured aneurysms due to the potential risk associated with use of antiplatelet therapy and questions over the degree of aneurysm protection following flow diversion and prior to stent construct endothelialisation. While braided endosaccular devices such as the Woven EndoBridge (WEB, Sequent Medical) can be used to treat ruptured aneurysms, the device stiffness during initial deployment can result in an aneurysm perforation, particularly in small and/or off-angle aneurysms.¹²

Typically, wide neck and medium‒large intracranial aneurysms treated with endovascular coiling sequentially utilise framing, filling and finishing coils. One of the major risks associated with intracranial aneurysm coil embolisation is intraprocedural aneurysm rupture (IAR) or perforation.² This risk is associated with the mechanical characteristics of a thin and heterogeneous aneurysm wall that is difficult to predict and impossible to measure in a clinical setting.¹ Tactile feedback during coil deployment is often subjective and the perception may vary widely between neurointerventionalists and is likely to change and become more refined with experience. Furthermore, visual feedback—watching for coil “accordioning” in the catheter—is used concomitantly by neurointerventionalists and factored variably depending on the size of the aneurysm, the degree of coil occlusion, and the catheter tip position.  Adding a layer of complexity to this paradigm, the literature has consistently shown that aneurysm remnants have a risk of rupture¹¹ and that the degree of incomplete treatment is inversely correlated with recurrent rupture risk: i.e. the less the aneurysm is treated, the greater the risk of recurrent haemorrhage.⁹ The neurointerventionalist, therefore, is left to balance two discrete risks; pack coils more and risk aneurysm perforation, or, embolise less and potentially undertreat—thereby exposing the aneurysm to recurrent haemorrhage. Taken as a whole, the practice is much more art than science.

Nevertheless, the treatment and its associated hazards can be studied scientifically, and effective measures can be taken to mitigate risk. Coils have the potential to introduce undesirable point or localised forces on the aneurysm wall which can lead to an IAR.^4,14 The incidence rate of IAR is low but the mortality rate associated with such complications remains high.¹⁰ Causes of IAR are likely multifactorial and may be related to characteristics of the aneurysm such as wall thickness or integrity as well as properties of the procedure such as catheter tip position. For example, it is possible that a very soft, small coil can perforate an aneurysm if deployed through a catheter tip that is pinned against an aneurysm wall (immediately apposed to the dome). IAR rates as reported in the literature have found no specific correlation related to coil type (framing, filling, finishing) and sequence.^11,8,16  IAR rates are somewhat dependent on aneurysm location¹⁰, and size.¹³

Nevertheless, a coil that is more pliable (soft), with resultant decreased force against the aneurysm wall and less amplitude of that force (more consistent low force), is likely to result in a decreased risk of perforation.

Finding space

Another important coil characteristic is the ability to fill in the available volume within the aneurysm sac, colloquially referred to as “finding space”. This is particularly relevant during multi-coil embolisations but is important in the treatment of small ruptured single-coil cases as well when trying to achieve higher packing density.^3,15 Mechanical analysis of coils has been traditionally considered using first order Hookean principles¹⁷ and these calculations do not often account for higher order effects associated with coil rearrangement in an aneurysm sac.

A recent report in the Journal of Neurological Sciences¹⁸ uses a model of force against an aneurysm wall—throughout the length of a coil deployment—to study comparable, commercially available coils. The authors have developed an in vitro system to isolate differences in coil characteristics by using a model internal carotid artery (ICA) and a synthetic aneurysm, a fixed position of a catheter tip at the level of the neck, a consistent velocity of coil deployment using a motor, and a digital pressure sensor (Futek Advanced Sensor Technology) affixed to one side of the outer surface of the aneurysm to measure the perpendicular component of force as a coil is being deployed. The authors integrated a commercially available automated tracking and measurement system (Interventional Device Testing Equipment) with their customised aneurysm acrylic model. A schematic of the setup is shown in Figure 1.

In addition to the computer-controlled motor for coil delivery, leveled test surface, and synthetic ICA and aneurysm model, the setup also included a camera to monitor the deployment. Briefly, a computer-controlled motor was used to push the coil through a simulated synthetic ICA segment into the model aneurysm. The perpendicular component of force experienced by the aneurysm wall was tracked over the deployment of the single coil and a force displacement curve was obtained for each case. An energy metric (effective work done to deploy the coil) was calculated and reported as the average work number (AWN)—equal to the area under the force displacement curve for each deployment. Specifically the authors compared framing and finishing coils from two manufacturers. Specifically, the authors studied the following coil groups: (a) soft coils: Axium Prime Detachable Coil (Extra Soft) (Medtronic, 2.5mm x 4cm, n=20) and Target 360 Nano Coil (Stryker, 2.5mm x 4cm, n=20), and (b) frame coils: Axium Prime Frame Detachable Coil (Medtronic, 5mm x 15cm, n=20) and Target 360 Standard Coil (Stryker, 5mm x 15cm, n=20).

Overall, the force component curves had three phases. Firstly, an initial low force phase—during the initial introduction of the coil when there is enough volume inside the aneurysm sac for the coil to deploy and bend unimpeded by other coil loops. Then there is a mid-deployment phase when the peaks and valleys in the force-displacement curve begin to develop; peaks occur when the coil is pushed against the aneurysm wall and valleys when the coil bends or loops into the open space in the aneurysm. This is followed by the final phase of deployment—as the coil continues to fill the aneurysm model, there is progressively reduced available volume which translates into higher force being detected by the distal force sensor. The peaks and valleys become most apparent in this end phase as a longer coil is deployed within the aneurysm. These characteristics are evident in the deployment of the soft category of coils, the extra soft (ES) Coil and a Nano Coil (Figure 2), which as expected impart less force on the aneurysm wall than the Axium Prime FC and Target 360 Standard framing coils. In addition, there were significant differences between the Axium Prime and Target 360 Standard coils (Figure 3a), in the total perpendicular component of force on the aneurysm wall during deployment—quantified with the AWN. There were also differences in the force gradient between the peaks and valleys, particularly between the framing coils—Axium Prime FC and Target 360 Standard coils (Figure 3a). The increased magnitude of the force gradient shows that the Target 360 Standard coil offered a higher resistance to rearrangement within the aneurysm sac. The lower peaks for the Axium FC coils indicate that the coils rotate more smoothly to facilitate rearrangement within the available aneurysm sac volume.

Although finishing coils are structurally different from the framing coils, similar behaviour is also observed with progressive filling of these coils. In this experiment, the ES coil exhibits lower aneurysm wall forces than the Nano coil and appears to offer less resistance to rearrangement within the aneurysm sac (Figure 3b). These measurements could be somewhat correlated to coil design. For example, the ES coil has a smaller wire diameter (0.0013 inch primary wire and 0.0011inch stretch resistant strand diameters) that may allow for smoother transitions and adjustments within the aneurysm sac.

The authors compared the AWN for the coil deployments (Figure 4) using analysis of variance (ANOVA). Not surprisingly, the AWN for the finishing coils was significantly lower than for the framing coils. In addition, the AWN for Axium ES coils was significantly lower than Target Nano coils, and the AWN for Axium FC coils was significantly lower than Target Standard 360 coils (p<0.001, Figure 4).

They also calculated theoretical coil stiffness using an established method.¹⁷ Reference is made to a report on estimating the stiffness of a coil from material properties and structural dimensions.

A comparison of this stiffness factor between the coils seems to correlate with the measurement trends for pliability reported by the authors. Also noted in the report is the contribution of other factors beyond the basic stiffness factor calculation such as the thickness of the polypropylene filament and the secondary shape characteristics. These factors together with stiffness calculations are summarised in Table 1. This doesn’t take into account what the authors deem high-order effects such as coil behaviour and its engagement with the aneurysm wall during deployment—a parameter measured by the in vitro model. Nevertheless, results are consistent with the results of the wall force model.

Another important factor to consider clinically is the force imparted with finishing coils towards the end of the coiling procedure. As observed in the force-displacement curve, the force (peaks) increase with progressive coil deployment. At this point, the available volumetric space in the aneurysm is fairly limited to allow for extensive coil rearrangements. In this instance, a more pliable coil could possibly conform better to the available volumetric space in the aneurysm and potentially reduce the risk against coil loop herniation into the parent artery, a factor likely to be associated with thromboembolic risk.

This in vitro model is able to study coil characteristics specifically by holding fixed variables such as catheter tip position and rate of coil deployment. Some elements of the in vitro model depart from clinical practice. However, the in vitro setting allows for more controlled direct comparison between coil types and better understanding of the dynamics involved in coil volumetric filling and the forces applied on the aneurysm wall (two characteristics that are clinically relevant).¹⁸ The differences between the coils are statistically significant, confirming that: (a) Axium ES coils are more pliable than Nano coils, and (b) Axium Prime FC coils are more pliable than Target Standard 360 coils.¹⁸ Although benchtop performance data may not always reflect clinical performance, metrics assessed here are likely to have an impact on procedural efficacy and safety.

Daniel Sahlein is the fellowship director of the Goodman Campbell Brain and Spine neuroendovascular fellowship training programme, and a consultant, speaker and proctor for Medtronic, and a consultant for Stryker, Microvention and Phenox.

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The views expressed in this article are based on the experiences of the author and do not represent the views of Medtronic.