We thank Dr. Guglielmi for the enlightening historical perspective on the inception of the three-dimensional-shape Guglielmi detachable coil (GDC-3D) (3). Guglielmi has clarified the impetus for the design and his seminal experimental work, which eventually led to the development of the GDC-3D. Regarding compaction and return to one-dimensional coil stack observed with conventional GDC coils under certain loose-packing situations, one must keep in mind that, unlike benchtop models, vessel wall aneurysms are biologically active viscoelastic structures that have the potential for structural remodeling in response to altered mechanical forces (1, 2). This feature makes comparison of in vitro results with long-term in vivo clinical efficacy difficult; therefore, it is presently unclear how the benchtop behavior of the GDC-3D will translate into long-term clinical recanalization or compaction.
We have found that, in addition to being associated with increased friction during deployment, the GDC-3D has inherent mechanical characteristics unique to its design, which need to be taken into account by the operator when deciding which type of coil to use for a particular type of aneurysm.
First, the tendency of the GDC-3D is to form a coil mass that is akin to a sphere having the smallest common diameter of an oblong or elliptical aneurysm (Fig. 2 A). A conventional GDC (one-dimensional or two-dimensional variant) tends to conform better to the inner surface of the aneurysm wall, even when it is irregular or oblong in shape (Fig. 2B). Oversizing the GDC-3D in an attempt to overcome this issue may lead to high transmitted stress to the aneurysm wall.
A second feature of the GDC-3D concerns its packing density when compared with the conventional GDC variants. The same omega loop design that provides three-dimensional character to the GDC-3D also interferes (Fig. 3 A) with the smooth concentric “onion-skin” embolization pattern that has become expected from the consecutive deployment of coils with sequentially decreasing diameter (Fig. 3B). This interference makes it difficult to achieve as high a packing density of the coil mass when using serial GDC-3D compared with serial one-dimensional or two-dimensional GDCs.
The three-dimensional behavior of the GDC-3D coil and its innate tendency to form a three-dimensional structure on deployment is attenuated at larger coil diameters. Conversely, smaller-diameter GDC-3D coils tend to be stiffer and therefore less well suited for acutely ruptured small-size aneurysms, as mentioned previously (3) and confirmed by Guglielmi.
The fourth feature, and perhaps the one with the most acute clinical relevance, is the phenomenon of compartmentalization, which has been observed when using a GDC-3D as a first coil in smaller aneurysms (<8 mm) (Fig. 4). Subsequent coils, which, when using a conventional GDC as the first coil, would deploy concentrically in the center of the coil mass created by the first coil, can instead deploy exclusively into one of a number of compartments created by the omega loops between the GDC-3D and the aneurysm wall. When the deploying second coil becomes confined in such a compartment, further coil advancement can exert very high focal stress at the aneurysm wall. This phenomenon of compartmentalization, which is rarely observed when a conventional GDC is used as the first coil, must be carefully excluded to avoid inadvertent intraprocedural aneurysmal rupture.
We agree with Guglielmi that care must be taken when using the GDC-3D coil because of its unique mechanical characteristics which set it apart from the conventional one-dimensional and two-dimensional variants. These features and Guglielmi’s concerns make it clear that the GDC-3D is best used by operators with significant clinical experience. It is quite possible that the lower packing density observed in aneurysms treated exclusively with GDC-3D coils may have little or no detrimental effect on the long-term occlusion rate. Ultimately, the indication for use of a GDC-3D or a conventional GDC variant will have to await long-term angiographic follow-up, including careful analysis of coil compaction and aneurysm recanalization.
Adel M. Malek
Randall T. Higashida
1. Dzau VJ, Gibbons GH: The emerging concept of vascular remodeling. N Engl J Med 330: 1431–1438, 1994.
2. Malek AM, Izumo S: Control of endothelial cell gene expression by flow. J Biomech 28: 1515–1528, 1995.
3. Malek AM, Higashida RT, Phatouros CC, Dowd CF, Halbach VV: Treatment of an intracranial aneurysm using a new three-dimensional-shape Guglielmi detachable coil: Technical case report. Neurosurgery 44: 1142–1145, 1999.