The mechanism of catheter kickback in the final stage of coil embolization for aneurysms: the straightening phenomenon

Abstract

The catheter kickback phenomenon often occurs in the last stage of coil packing for cerebral aneurysms. This behavior is considered the result of either a lack of space in the sac or a stiff detaching zone. In order to clarify its mechanism, focused stretch-resistance (SR) coil simulation models were tested. Various commercially available SR coils were inserted into a cylinder or an aneurismal sac made from a silicone tube with a smaller than prescribed diameter. A magnified SR coil model (straight type) of fishing line was created for simulation. Numerical analysis for the changes in coil behavior were verified. All SR coils showed hardening and straightening at the last few millimeters, resulting in catheter kickback. In a magnified coil experiment, straightening was also realized when folding into a narrow cylinder. The SR line coursed in the canal of the first loops and shifted to the outside in the middle portion. Gaps among first coil pitches were enlarged on after insertion into the narrower space. Shortage of the SR line was calculated to reach a maximum of 32%. The straightening phenomenon is due to SR line shortening and subsequent condensation of pitches of the first loops at the coil end. Coil tail flexibility was lost, and the coil behaved as a stiff wire. Straightening is an important factor in the kickback phenomenon. Shorter final SR coils should be selected, and coil designs should be improved.

Figure 1

The influence of the enlargement of gaps in pitches. A) Normal pitch at the prescribed loop diameter. B) Enlarged gap between pitches on inserting the coil into the narrow space. Note the relative shortage of SR line (arrow). SR: stretch resistance.

Figure 2

Kickback phenomenon in saccular aneurysm model showing various types of coils inserted into an inadequately small diameter model. Here 4mm diameter x 8 mm long coils are inserted into the 3 mm diameter aneurysm model. A) ED extrasoft coil non SR type. B) ED extrasoft SR type. C) GDC ultrasoft. D) Cerecyte ultipaque. E) Fluoloscopic image inserting ED extrasoft SR type. Abbreviations: ED: electrically detachable, GDC: Guglielmi detachable coil.

Figure 3

Various brands of 4 mm diameter × 8 mm long coils and the cylinder model of the kickback phenomenon. Cylinders have inadequately small diameters. Here 4 × 8 mm coils are inserted into 2 mm diameter cylinders. A) ED extrasoft coil non SR type. B) ED extrasoft SR type. C) GDC ultrasoft. D) Cerecyte ultipaque.

Figure 4

SR coil model made of fishing line. A) Magnification of 16X. The SR line (arrow) fixed at the coil tip and the lengths of the tail and the coil were the same. B) The coil model is found to bend smoothly in the wide rectangular space (55 mm width) (2X). C) The coil model is found to straighten in the narrow one (asterisk) (35 mm width). The straightening part is boxed. D) Schema of the boxed part of straightening (arrow) in (C).