Technical Note on VDS

Summary

A new system was deviced that allows the electrolytical detachment of platinum microcoils at variable lengths for the endovascular treatment of intracranial aneurysms.

The detachment element consists of two short platinum coil segments, which are connected by a threat of stainless steel. The steel threat is interrupted by electrolysis, using a continuous current with 1-2 mA at a voltage of 4-8 Volt. The average detachment time in heparinized blood is about 30-40 sec. The detachment elements can be used to connect either an insertion wire with a platinum coil or to connect several segments of platinum coils with variable helices and lengths. If several detachment elements are integrated in a coil, electrolysis interrupts only the element, which is next beyond the distal tip of the catheter.

The electrolytical process does not affect the detachment elements proximal and distal to the element adjacent to the tip of the catheter. Single or several coil segments can be pulled back into the microcatheter if necessary. The system is expected to allow a faster and more complete endovascular coil occlusion of intracranial aneurysms. The principles described in this paper are claimed by the German pending patent DE 100 10840 A1.

Introduction

Endovascular occlusion of intracranial aneurysms using electrolytically detachable coils has gained a wide acceptance. Until now single coils are introduced through a microcatheter into the aneurysmal sac. If fitting to the actual site and space, one coil after the other is detached electrolytically. The procedure is time consuming and expensive since several coils are mostly required. The goal of the treatment is the dense filling of the aneurysm. The selection of the last coil(-s), however, may be difficult. The last coil should fill the remaining space, without leaving a coil loop out of the aneurysm inside the parent artery.

It was the aim of our efforts to develop a system, which allows the electrolytical detachment of platinum coils at variable lengths within a short time. The process of electrolysis should use continuous current within the range of similar, already existing systems. The coil or coil segments should be retrievable into the microcatheter before detachment, if not fitting satisfactorily.

The system was realized using a principle not described before. The principle and the physical properties of the system, which is called Variable Detachable System (VDS)¹, are described below.

The Detachment Element

The system is based on the use of detachment elements. One detachment element consists of three different parts. A wire of stainless steel is embraced by two platinum coil segments. The stainless steel wire is welded to the platinum coil wires. The platinum coil segments leave about 3 mm of the steel wire uncovered in the middle. This part of the steel wire is subject to the electrolytical process, which finally interrupts the connection between the two coil segments and thus the continuity to either the endovascular implant or the insertion wire. Figure 1 shows a schematic drawing of the detachment element.

Figure 1.

Description of the VDS Principle

The above described detachment element allows for connecting multiple implants in only one system, as shown in figure 2.

Figure 2.

A variety of coil segments with different secondary helices, lengths and stiffnesses can be connected to each other and to an insertion wire by welding them to the platinum coils of the detachment element. The shape of the secondary helices and all the other physical properties can be chosen similar to the existing single coil systems. In difference to all existing single coil systems the VDS offers the option to choose the detachment element at the desired length of the coil. This might require an anticipation of all the processes of coil occlusion well in advance. To take best advantage of this principle the operator should select a combination of combined coil segments that will allow complete occlusion of the aneurysm without compromise of the parent vessel.

The physical characteristics for the stainless steel wire of the detachment element and the insertion wire follow different requirements. The insertion wire should for obvious reasons be most flexible. Electrolytical detachment within short time is best achieved through highly corrosive stainless steel. Chromium for instance is considered a crucial element. Being always an element of stainless steel, a lower amount of comprised Chromium decreases the corrosive behaviour and thus lowers the detachment times, while the strength of the steel will be increased with increasing amount of Chromium through variable mechanisms.

The VDS is advantageous since it largely avoids the drawbacks of this compromise. The detachment elements are made of a special steel, while the insertion wire can be kept more flexible.

In vitro experimentation showed that during the ongoing process of electrolytical detachment this specific element is affected, which is distal to but closest to the tip of the micro catheter. Detachment elements inside the micro catheter are sufficiently insulated from the flowing blood. The slow flow of the saline solution within the micro catheter does not allow for sufficient ion transport to support electrolytic disruption. It is part of the principle that there might be detachment elements distally to the one selected for separation. Those elements are hardly affected by the subsequent electrolysis. The applied continuous current hits initially all exposed detachment elements. The element closest distally to the tip of the micro catheter is subject to a rapid thinning of the connecting wire. Decreasing cross area and increasing current density lead to increasing of the electrical resistance at this position. This prevents further electrolysis at more distal elements. The further integrity of these elements, however, is not considered crucial, since all the connected coil segments are located inside the aneurysmal sac. Animal studies ² and design verification efforts revealed that a disruption of the distal elements might occur within a period of time after the electrolytical detachment.

The fact that the electrolytical process in this system mainly affects only one detachment element and average detachment times of less than 1 min explain the in vivo observation of missing electro-thrombotic effects.

Periprocedural Handling

The VDS principle allows a combination of different coil designs and stiffnesses. One might envision that a first coil segment with a three-dimensional shape (e.g. 6 mm) is followed by a sequence of tension safe and very soft, short segments. In order to exploit the common principle of radiopaque micro catheter markers increments of 3 cm appear advantageous. For the final segments shorter increments of 1 cm will probably allow a more complete filling of the aneurysmal sac without increasing the risk of coil loop displacement into the parent artery.

Increasing friction inside the micro catheter is the limiting factor for the total length of a single Variable Detachable System, being around 30 cm.

Retrievability of several coil segments into the micro catheter is supported by a conical shape of the distal coil part of each of the detachment elements. Stretching of the coil segments during withdrawal can be avoided by using Tension Safe technology (i.e. inserting a nitinol filament within the primary helix of the platinum coil).

Discussion

Several principles for the controlled insertion and detachment of platinum micro coils for endovascular treatment of intracranial aneurysms have been established. The most popular ones are those of Guglielmi and Sepetka (EP 0484 468 B1, EP 0800 790 B1, EP 0804 905 B1), which are primarily based on the concept of detaching a platinum coil directly from an insertion wire. This principle until now proved more successful than all its mechanical competitors. The associated restrictions, however, are varied. The material used for manufacturing of the insertion wire will unavoidably become an endovascular implant due to the detachment process. This restricts the selection of potential metals and alloys. Until now a length-variable detachment based on this technology has not been realized. Even with the latest modification of this system (with electrically isolated embolization coils or coil segments), which allowed reduced detachment times, elec-trothrombosis might be induced within the aneurysmal sac³.

Electrothrombosis is no longer considered a significant factor for the successful endovascular exclusion of an intracranial aneurysm from blood circulation ⁵. Electrothrombosis, however, might be one of the many possible factors related to the relatively high incidence of thromboembolic complications during and after GDC procedures ³. The most convincing argument against the concept of electrothrombotic treatment of aneurysms is the proven fact that coils, which are detached by mechanical or electrothermal means, are equally efficient ⁴. The deliberate use of intravenous heparine and antiplatelet agents during GDC procedures is now considered mandatory for the avoidance of thromboembolic events. Medical anticoagulation and antiagreggation will unavoidably interfere with or offset the process of electrothrombosis.

The key factor of endovascular aneurysm occlusion, however, is mechanical by nature. The goal is to achieve a homogeneous and dense packing of coils within the aneurysmal sac.

We consider the VDS principle described above as a first step towards the idea of treating an aneurysm by using a single system of varied coils, well adapted to the changing physical requirements during the process of coil occlusion.