Garnette R. Sutherland, M.D.
Department of Clinical Neurosciences,
University of Calgary,
John J.P. Kelly, M.D.
Department of Clinical Neurosciences,
University of Calgary,
David W. Boehm
Concept Solutions, Inc.,
James B. Klassen
Concept Solutions, Inc.,
Garnette R. Sutherland, M.D.,
Department of Clinical Neurosciences,
University of Calgary,
Foothills Medical Centre,
1403 29th Street NW,
Calgary, Alberta, T2N 2T9, Canada.
Received, November 8, 2007.
Accepted, December 26, 2007.
There have been a number of advances in both design and material selection for aneurysm clips since Walter Dandy (3) used a V-shaped malleable silver clip to isolate an intracranial carotid aneurysm from the cerebral circulation. Over the past decade, the most significant advancement has been the development of magnetic resonance (MR)-compatible clips composed of a titanium alloy, Titanium-6Al-4V (1, 13, 28), or pure titanium (15). These aneurysm clips allow patients to be investigated with MR imaging (MRI), but the titanium alloy results in susceptibility artifact, making subsequent assessment of the aneurysm site problematic (9, 21, 24, 25, 27, 30). We describe the development of MR-compatible aneurysm clips that should not obscure the image of the aneurysm neck or associated parent and daughter vessels.
Currently, MR-compatible aneurysm clips are constructed using Titanium-6Al-4V because pure titanium has been thought to lack the necessary strength (2). This alloy also contains the elements aluminum and vanadium, together with trace amounts of carbon, iron, oxygen, and hydrogen (2), which results in mechanical properties superior to those of pure titanium. Even pure titanium produces MR susceptibility artifact, which is similar to that observed with titanium alloys (17).
Alternative, biocompatible materials have been used in the production of various medical devices, including joint prostheses and surgical instruments. One such group of materials is ceramics (17, 22). Compared with titanium and other metals, certain ceramics have substantially less MR susceptibility artifact but equal or greater strength (17, 18).
This report describes the development of an MR-compatible aneurysm clip composed of ceramic jaws and a small titanium alloy spring (14). The use of ceramics greatly reduced susceptibility artifact at the region of interest. The titanium alloy spring maintained the necessary closing force. Anovel applicator was developed for use with the ceramic clips. The design of the clip-applicator interface increased functionality by improving visibility during clip placement and removal.
MATERIALS AND METHODS
Before the design of the novel aneurysm clip, material selection was performed based on the goal of MR compatibility with minimal suscep-tibility artifact and in keeping with the American Society for Testing and Materials Committee F-4.05 guidelines for aneurysm clip biocompatibility and closing forces (16). Aneurysm clip components include the spring, pivot, and jaws. Ti 6Al-4V ELI titanium (Ti, 89.4%; Al, 6.08%; V, 4.06%; C, 0.14%; Fe, 0.20%; O, 0.12%; N, 0.006%; Y, 0.005%) was chosen for the spring and pivot for its biocompatibility and extremely high modulus of elasticity, or ability to flex without permanent deformation (2). The extra low interstitial grade provides improved fracture toughness and ductility over the standard alloy (2).
For the jaws, both silicon nitride ceramic and yttria-stabilized zirconia ceramic were tested. Silicon nitride is a nonreflective gray color, making it suitable for intense surgical lighting. Because of its high strength and hardness, it is widely used in applications such as highspeed industrial bearings and seals (11). Zirconia is characterized by extraordinary strength and resistance to crack propagation (8). Among other applications, yttria-stabilized zirconia is used in the manufacture of knife blades. It does not have the brittleness characteristic of many ceramics (6). Silicon nitride approaches the strength of high-grade titanium, but yttria-stabilized zirconia can even exceed that strength. Interestingly, although zirconia is stronger and tougher than silicon nitride, it is more easily prototyped and manufactured. Most varieties are white in color, but a densification process during fabrication results in a final beige color, which reduces reflectivity making it suitable for the microsurgical environment. These materials are biocompatible, which makes them well suited to medical implant applications (22).
After material selection, computer-aided design (CAD) of the aneurysm clip was completed using the Solidworks CAD program (SolidWorks Corp., Concord, MA). Material strength was tested virtually, using the COSMOSWorks Advanced Professional Finite Element Analysis program (SolidWorks Corp.), to ensure that the safety factor was approximately 10 times the maximum force that could be exerted on the clip during use. Once the design was finalized, clips in an array of shapes and sizes were machined using a high-precision computer numerically controlled mill (Model OM2; HAAS Automation, Inc., Oxnard, CA) with a high-speed spindle.
In addition to the ceramic clips, a series of Titanium-6Al-4V ELI temporary clips was constructed. Titanium alloy was chosen for the temporary clips because they are not imaged, and the alloy is less expensive than ceramic to prototype and manufacture. The temporary clips were fitted with a gold-anodized titanium spring that provided a reduced closing force.
Assorted applicator configurations were also designed and constructed based on ergonomics, balance, applicator-clip interface, and maximization of line of sight. Various applicator latching mechanisms were considered, with the objective of providing the surgeon with certainty that the latch has become disengaged and, once disengaged, can not become re-engaged during clip placement. These features were important in overcoming current limitations in applicator design.
The closing force was measured for both the permanent and temporary clips using an Imada ZPS-DPU-1 force gauge (IMADA, Inc., Northbroook, IL) to measure the closing force. In addition, the effect of opening and closing the clips for 50 cycles, from fully opened to fully closed, was determined.
Magnetic resonance imaging was conducted at both 1.5 and 3.0 T on two silicon nitride clips (one short straight and one short aperture) and two yttria-stabilized zirconia clips (one long straight and one long aperture) and Yas¸argil MR-compatible titanium clips (Aesculap, AG & Co., Tuttlingen, Germany). Initial MRI was performed at 3.0 T using several different phantom substrates. Akiwi fruit provided the clearest resolution of the clip jaws and surrounding area. Fast spin echo T1 (repetition time [RT], 500 ms; echo time [TE], 15 ms; flip angle, 90 degrees; bandwidth, 15.6 kHz; slice thickness, 4 mm; field of view, 26 cm; matrix, 256 192; number of excitations, 2) and T2 (TR, 4575 ms; TE, 102 ms; flip angle, 90 degrees; bandwidth, 31.25 kHz; slice thickness, 4 mm; field of view, 26 cm; matrix, 256 256; number of excitations, 2; echo train length, 12 ms; refocus flip angle, 160 degrees) imaging sequences were performed at various planes and angles to assess the artifact characteristics of ceramic jaw clips and Yas¸argil MRcompatible clips.
A second MRI test was performed at 1.5 Tesla using a cadaveric head. Before imaging, a right frontal temporal craniotomy was performed. The right Sylvian fissure was opened, together with the right carotid cistern. A long apertured ceramic clip was placed across the right middle cerebral artery, and a long straight clip was positioned across the right carotid artery. Fast spin echo T1 (TR, 420 ms; TE, 12.1 ms; flip angle, 90 degrees; bandwidth, 16 kHz; slice thickness, 4 mm; field of view, 26 cm; matrix, 256 192; number of excitations, 3) and T2 (TR, 4000 ms; TE, 90 ms; flip angle, 90 degrees; bandwidth, 25 kHz; slice thickness, 4 mm; field of view, 26 cm; matrix, 256 256; number of excitations, 2; echo train length, 8 ms; refocus flip angle, 180 degrees) imaging sequences were acquired in both the coronal and axial planes.
CAD drawings were developed based on sizes and shapes of commercially available aneurysm clips (Fig. 1A). To provide maximum opening distance and necessary closing force, a novel spring mechanism was developed based on ceramic jaws, a titanium spring, and a titanium pivot (Fig. 1A). The imaging requirement called for positioning the titanium components away from the section of the jaws used to secure the aneurysm neck. The configuration allows a compact design that combines the rigid properties of ceramic with the elastic properties of titanium. Finite element analysis of the ceramic jaws showed the highest stress to be located in the midportion of the jaws, and to be well below the flexural strength of silicon nitride (Fig. 1B). Finite element analysis of the titanium spring showed an even distribution of stress, which minimized issues related to fatigue. Special consideration was given to preventing jaw crossing. This was accomplished by the geometry of the ceramic’s contact surface with the pivot and spring. The design also allowed for component manufacture within an acceptable range of manufacturing tolerances.
Initially, short straight and short aperture clip jaws were manufactured from silicon nitride. Because yttria-stabilized zirconia was more easily prototyped and readily available, all subsequent clips were manufactured from this ceramic (Fig. 1C). Short straight, long straight, short aperture, and apertured-T temporary clip jaws were manufactured from titanium (Fig. 1D). As a result of the hardness of the ceramic and the minute detail, the prototyping process required several weeks per clip. Jaw crossing has not been observed in any of the clips manufactured, despite their being intentionally applied in a manner that would cause jaw crossing in conventional clips.
The applicator was constructed from titanium and 301 stainless steel and includes a novel latching mechanism (Fig. 2). The latch is constructed from titanium and is designed to withstand up to 20 pounds of side load without damage. It is spring-loaded to remain seated against one arm of the applica-tor (Fig. 2, B–E). The latch may be engaged using one hand, and it provides a tactile feedback click to the surgeon when it disengages (Fig. 2, B–E). Once disengaged, the latch cannot become unintentionally re-engaged (Fig. 2E, inset).
The new clip design, which incorporates a spring, pivot, and ceramic jaws, allows the applicator-clip interface to be considerably reduced in size (Fig. 3). Aball-and-socket interface incorporated into the design, allowed the applicator to be actuated on the ceramic jaws rather than encompass the spring, which provided a clear line of sight along the jaws with no obstruction (Fig. 3A). In addition, the ball-and-socket interface allows the applicator to engage the clip at various angles, so that the clip can be removed from the aneurysm or repositioned without applying torque to the vascular structures (Fig. 3B).
Asilicon nitride short straight clip achieved an effective closing force of 151 g. With 50 cycles of opening and closing, the closing force stayed within 150 5 g (Fig. 4). The short straight temporary clip had a closing force of 100 g. Over the course of 50 cycles of opening and closing, there was more variability in closing force as compared with the permanent clip, ranging from 98 g to 109 g.
Images obtained from kiwi fruit with implanted aneurysm clips at 3.0 T showed a susceptibility artifact with the MRcompatible Yas¸argil clip was several times the diameter of the cross-section of the jaws (Fig. 5, left). The artifact extending beyond the end of the titanium jaws did not intersect the jaws themselves. The titanium spring of the ceramic clip had a sus-ceptibility artifact similar to that of the spring end of the Yas¸argil clip (not shown). The ceramic jaws were only detectable in the MRI scan as a void that did not extend outside the width of the clip (Fig. 5, right). The phantom material directly contacting the ceramic jaws was completely free of artifact or image distortion.
A right frontotemporal craniotomy was performed, the right sylvian fissure and carotid cistern opened, and two yttriastabilized zirconia clips placed across vascular structures (Fig. 6A). The 1.5-T imaging studies obtained from the cadaver shown in Figure 6A produced minimal susceptibility artifact within the sylvian or carotid cisterns (Fig. 6, B and C).
The goal of this development project was to create an aneurysm clip with a significantly reduced magnetic susceptibility artifact. This has been accomplished through the use of ceramic jaws. The imaging benefit was shown in both phantom and cadaveric models.
There is a compelling need to assess patients radiologically after clipping of cerebral aneurysms, both perioperatively and over the long term. Acutely, it is important to assess whether the aneurysm has been obliterated completely, and often whether vasospasm involves the parent and daughter branches in the vicinity of the clip. Currently, this assessment requires contrast angiography. In several reported series, residual aneurysm has been observed in up to 14% of cases (29). Although the natural history of residual aneurysm has not been established, aneurysm regrowth and rupture has been reported (4, 5). Long-term assessment would also provide a comparative data set for coiled aneurysms. Ceramic aneurysm clips offer the ability to evaluate the aneurysm complex, either intra- or postoperatively, with MRI (12, 26). This would decrease the need for x-ray evaluation and its associated risks (7).
Compared with either pure titanium or the titanium alloy used for the construction of most MR-compatible aneurysm clips, ceramics are known to cause significantly less susceptibility artifact (17). Furthermore, artifact increases with magnet field strength for most biomaterials other than ceramics (18). For example, spin echo T1 susceptibility artifact for a 2.0 mmdiameter sample increases from 8.2 to 12.1 to 19.6 mm as field strength increases from 0.5 to 1.5 to 3.0 T, respectively, whereas zirconia values were 2.3, 2.3, and 2.5 mm for the same three field strengths (18). Other investigators have shown that at 1.5 T, ceramics are not associated with any local temperature change (23).
After considering numerous configurations in the CAD modeling process, it became apparent that a number of secondary benefits could also be achieved. The pivoting design, which was necessitated by the use of the rigid ceramic material, also eliminated the need for the applicators to encompass the spring. In contrast to conventional clip systems, this design allowed the applicator tips to be the same width as the aneurysm clip jaws, providing a significant advantage in visualization of the clip during placement and removal. This also eliminated the need for a second set of appliers for removing the clip.
The clip design also reduces or eliminates the possibility of jaw crossing observed with other MR-compatible titanium alloy clips, particularly those with longer jaws (10, 19). The pivoting design of the ceramic clip uses a four-point contact surface interface between the ceramic jaws and the titanium pivot and spring. This geometry automatically aligns the jaws such that jaw crossing requires significant force; it has not been observed using the clips that have been manufactured to date. Moving the spring as far from the clipping area of the jaws as possible decreases the potential for susceptibility artifact at the aneurysm neck. In addition, it requires a very high spring con-tact force as a result of the low mechanical advantage of the spring, which was located close to the pivot. This high spring contact force had the additional benefit of holding the jaws together securely, preventing the possibility of jaw crossing or disassembly. The asymmetric side spring, which was necessary to allow the clip-applicator interface, provided improved positioning of the applicator tips.
The ball-and-socket design for the clip-applicator interface allows the clip to stay at the initial grasping angle and allows application, repositioning, or removal of the clip at various angles. Conventional applicator latching systems are prone to damage and/or unintentional re-engagement. This is a problem because it may prevent the applicator from disengaging from the clip after placement across the aneurysm. The latch on the new applicator is designed to give the surgeon a tactile feedback click when it disengages and it offers a significant safety advantage in eliminating the possibility of accidental re-engagement.
Prototyping of the ceramic components was extremely challenging and time-consuming. It is expected, however, that the assembled nature of this design will provide advantages in manufacturing. Conventional clips require significant time and skill to fabricate. In contrast, the titanium and ceramic components of this new design are well suited to high-volume production using industry-standard molding and casting processes. Consistent tolerances from part to part should lead to consistent closing pressure from clip to clip, something that has not been observed with other MRcompatible aneurysm clips (20). Consistency of closing force has also been proven for 50 cycles. Given its many design advantages, it is concluded that this new clip will provide improved performance in addition to its magnetic resonance imaging advantages.
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This study was supported by a grant from the Canada Foundation for Innovation
The design of aneurysm clips has varied minimally over the past 30 years, except for introduction of titanium clips for magnetic resonance imaging (MRI) compatibility. Although titanium clips are MRI safe, they still cause imaging artifact and obscure critical potential MRI and magnetic resonance angiography (MRA) information around the neck of the clipped aneurysms. In this report, Sutherland et al. propose an important radical redesign of aneurysm clips. The blades of the clip are ceramic with only the spring mechanism remaining titanium. Theoretically, postoperative MRI and MRA images could be obtained with preserved visualization of the aneurysm neck region. The design of the clip allowing for a low profile applier is also very appealing. The clip can apparently be rotated within the applier to achieve the perfect angle of application. Previous designs along these lines have always resulted in a more bulky applier.
It remains to be established that the closing forces are comparable to existing clips in the clinical setting. Also, the lifespan of the clips once implanted and the shelf-life of the clips previous to implantation are issues that will be necessary to resolve before widespread clinical use of the ceramic clips. The design is intriguing, and I look forward to future clinical trials.
Robert A. Solomon
New York, New York
The golden days of postoperative conventional angiography are soon expected to be over as noninvasive, safe, and fast methods such as computed tomographic angiography and MRA are replacing it in most cases. Complex and giant aneurysms may differ as they often necessitate even intraoperative angiography to confirm complete neck occlusion and at the same time the patency of the surrounding arteries. In less complex ones, direct visualization in combination with intraoperative indocyanine green angiography and Doppler ultrasound will enable us to close the wound. However, unexpected neck remnants and branch occlusions may occur as these methods are not 100% reliable addressing the importance of postoperative angiographic control. Dynamic computed tomographic angiography as a postoperative means necessitates using of titanium clips as otherwise its value is just to see the patency of adjacent branches. MRA is still slower to perform and often more difficult in poor condition patients who may need a respirator and extensive monitoring. Still, any step towards developing clips with less artifact in MRA and computed tomographic angiography is welcome so that we may control the quality of clipping with modern methods that may not even have harmful radiation side effects.
Juha A. Hernesniemi
In modern neurosurgery, the titanium clip is a worthy development for improving image quality as well as biocompatibility in MRI. However, the problem remains that titanium clips do not allow us to inspect clipped aneurysms in detail because of their susceptibility artifact. In their article, Sutherland et al. introduce an aneurysm clip with ceramic jaws and a titanium spring to resolve this problem. Their new clip has decreased susceptibility artifact in comparison with the recent ordinary titanium-body clip. The authors show beautiful results demonstrating that their ceramic clip has the potential to improve MR visualization of the clipped aneurysm. Their clips need to be developed for clinical use, but we believe that their approach is on the right track.