Prototypical metal/polymer hybrid cerebral aneurysm clip: in vitro testing for closing force, slippage, and computed tomography artifact

Laboratory investigation

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Object

The aim of this study was to explore the possibility that a hybrid aneurysm clip with polymeric jaws bonded to a metal spring could provide mechanical properties comparable to those of an all-metal clip as well as diminished artifacts on computed tomography (CT) scanning.

Methods

Three clips were created, and Clips 1 and 2 were tested for mechanical properties. Clip 1 consisted of an Elgiloy spring (a cobalt-chromium-nickel alloy) bonded to carbon fiber limbs; Clip 2 consisted of an Elgiloy spring with polymethylmethacrylate (PMMA) jaws; and Clip 3 consisted of PMMA limbs identical to those in Clip 2 but bonded to a titanium spring. Custom testing equipment was set up to measure the aneurysm clip clamping forces and slippage. Clips 2 and 3 were visualized in vivo using a 64-slice CT unit, and the slices were reformatted into 3D images.

Results

According to the testing apparatus, Clip 2 had a similar closing force but less slippage than three similar commercial aneurysm clips. The artifact from the cobalt alloy spring on CT scanning largely offset the advantage of the nonmetal PMMA limbs, which created no artifact. The hybrid titanium/PMMA clip (Clip 3) created very little artifact on CT and allowed visualization of the phantom through the limbs.

Conclusions

It is feasible to build a potentially biocompatible hybrid cerebral aneurysm clip with mechanical properties that closely resemble those of conventional metallic clips. Further testing should be directed toward establishing the reliability and biocompatibility of such a clip and optimizing the contour and surface treatments of the polymer limbs.

Abbreviations used in this paper:CCD = charge-coupled device; CT = computed tomography; FE = finite element; MR = magnetic resonance; PMMA = polymethylmethacrylate.

Article Information

Address correspondence to: Alexander C. Mamourian, M.D., Division of Neuroradiology, University of Pennsylvania, Dulles Building, 3400 Spruce Street, Philadelphia, Pennsylvania 19104. email: alexander.mamourian@uphs.upenn.edu.

© AANS, except where prohibited by US copyright law.

Headings

Figures

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    Photographs illustrating aneurysm clips held in a clip applier. A: Clip 1, a carbon/cobalt alloy hybrid. B: Clip 2, a PMMA/cobalt alloy hybrid. C: Clip 3, a titanium/PMMA hybrid.

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    Schematic demonstrating FE model of PMMA clip limb. A = distance from the blade tip to the location of the prescribed force; B = length of the original metal blade inserted into the PMMA blade; ID = inner diameter; L = length of the PMMA blade; OD = outer diameter.

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    Schematic revealing the distribution of the von Mises stress computed from the FE model of the PMMA hybrid clip jaw.

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    Schematic demonstrating the clip clamping force testing apparatus.

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    Schematic revealing the clip slipping force testing apparatus.

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    Photograph of the phantom showing the clear lines that support the four clips. From the upper left (in a clockwise direction): Clip 3, a titanium/PMMA hybrid; Clip 2, a cobalt alloy/PMMA hybrid; a cobalt alloy clip; and a titanium clip.

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    Graphs revealing the opening and closing clamping forces plotted against the opening distance (measured at 2 mm from the tip of the jaws). Solid dots connected with a line indicate the opening force; hollow dots with a line, the closing force.

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    Graph representing the force pulling on a clip plotted against time.

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    Coronal T1-weighted MR image of a water phantom demonstrating the relative artifacts from four aneurysm clips. From upper left (clockwise): Clip 3, Clip 2, a cobalt alloy clip, and a titanium clip. Note the sharp margin of the limbs of Clip 3 (arrow) compared with the artifact around the limbs of the titanium clip below, a reflection of the diminished artifact arising from the PMMA limbs of Clip 3.

  • View in gallery

    Three-dimensional image showing three clips on a contrast-filled aneurysm phantom. The cobalt alloy/PMMA clip is at the top, the titanium/PMMA clip in the middle, and a commercial titanium clip at the bottom. Note the transition to nearly invisible PMMA limbs in the middle clip (arrow).

  • View in gallery

    Images reformatted from 0.625-mm CT scans using a Vitrea workstation revealing the limbs of a titanium clip (left) and the PMMA limbs of the hybrid clip (right, arrows) attached to the silicone aneurysm phantom. Slices were oriented perpendicular to the limbs. The PMMA limbs are nearly invisible.

  • View in gallery

    Three-dimensional image of the cobalt alloy/PMMA hybrid clip showing the considerable CT artifact common to cobalt alloy clips. This artifact largely negates any advantage of the PMMA limbs on this clip design.

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