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September 2007
Wire-Interwoven Nitinol
A new class of stents may expand treatment options for patients with peripheral arterial disease.
Peripheral arterial disease affects more than 12 million people in the US. The superficial femoral artery (SFA) is commonly involved in this disease process, and stenosis or occlusion of the SFA is a common cause of claudication and is often part of critical limb ischemia. Interventional therapy of SFA disease using laser-cut nitinol tube stents is increasing as several reports show improved patency when compared with balloon angioplasty alone.1-3 However, the SFA poses particular problems for stent placement because it elongates and foreshortens with movement, can be externally compressed, is subject to flexion, and often is affected with profound dystrophic calcification. Limitations of present stent technology include insufficient radial force to withstand elastic recoil and external compression (in some cases), stent kinking, and stent fracture (Figure 1). Several generations of laser-cut nitinol tube stent technology are available, with newer generations showing improved flexibility and more fracture resistance, albeit with some decrease in radial force.4
A new class of stents made of wire-interwoven nitinol (WIN) has now been developed by IDev Technologies, Inc. (Houston, TX) and may address some of these issues. Bench testing of the WIN stents is showing increased radial force, increased flexibility, and ex vivo fracture resistance. Results from a series of three cyclical fatigue tests comparing the SUPERA™ wire-interwoven nitinol stent with several other commercially available laser-cut nitinol tube stent technologies show that the SUPERA self-expanding stent is likely to have superior radial force, flexibility, and durability.
TESTING THE FATIGUE LIFE OF STENTS
A series of three fatigue tests was designed to compare
the structural integrity and long-term durability of the
new SUPERA WIN stent with several of the second-generation
laser-cut nitinol tube stents. The fatigue failure
modes were decoupled and isolated to assess the affect
of each mode on stent performance. The tests were
designed to simulate 10 years of use (approximately 1
million cycles/y) under forces associated with the peripheral
vasculature: torsional fatigue, bending/extension
fatigue, and compression fatigue. Four laser-cut nitinol
tube stent designs currently commercially available were
tested with the SUPERA wire-interwoven nitinol stent. The laser-cut nitinol tube stents evaluated represent the
most current stent technologies available.
During each of the three fatigue tests, six SUPERA wire-interwoven nitinol stents were compared to two laser-cut nitinol tube stent products per brand of similar size and shape under the same test conditions. When an exact size could not be matched, the commercially available size that most closely matched the tested size of the SUPERA stent was used. All of the stents were visually inspected twice daily for possible failure during fatigue cycling. Testing continued and was stopped at daily inspection points until the stents either failed or had been subjected to at least 10 million cycles during each of the three tests. All tests were conducted at an independent testing lab.
Torsion Fatigue Test
The torsion fatigue test applied a uniform angular displacement to 6-
mm X 80-mm stents
subjected to 45° of
angular displacement.
Initially, all
stents were mounted
without angular
displacement, noted
as 0° (Figure 2).
Angular displacement
was then
applied to the lower
end of the stent by
turning the lower
fixation point 45°
while the upper end
of the stent
remained stationary
(Figure 3). All five
brands of stents survived
the initial 10
million cycles. As a
result, a more
aggressive torsion
fatigue test using
the same test samples was initiated whereby the angle
of displacement was changed from 0° to 45° to an
angle of -90° to 90°. As shown in Table 1, only Product
A-1 failed shortly after the 90° angular displacement
was initiated (approximately 300,000 cycles). The
remaining stents survived an additional 10 million
cycles, thereby successfully completing 20.3 million
cycles, at which time the test was stopped.
Flexion/Extension Fatigue Test
The flexion/extension fatigue test simulated the
anatomical geometry and displacement associated with
the knee. Stents were mounted on the bending test fixture
so that each stent was loaded with an initial 25-g
tensile load when straight (0° of deflection) (Figure 4),
and then each stent was bent to a 120° angle (Figure 5).
As the fixture rotated, the tension was relieved, and the
stent was allowed to bend freely at a radius that most
naturally fit the particular stent design. The test was
designed to induce large bending forces near the center
of the stents without subjecting the ends of the stents to
the bending forces. The tension that was applied to the
stent was distributed along the entire length of the stent.
Figure 5 shows that the laser-cut nitinol tube stents that
were evaluated buckled when bent to a 120° deflection.
As shown in Figure 6, no laser-cut nitinol tube stents
remained unfractured at 100,000 cycles in the test. All laser-cut nitinol tube stents failed by an evaluation point
21,087 cycles, with the exception of one Product B stent,
which subsequently failed by the 92,415-cycle-count evaluation
point (Table 1). The SUPERA wire-interwoven nitinol
stents, on the other hand, were unaffected at the
completion of 10 million cycles.
Compression Fatigue Test
The compression fatigue test comprised two different
test conditions used to determine the radial strength of the
stents at their center. Similar to the flexion/extension
fatigue test, the length of the tested stents was not expected
to affect the results of this test provided the stents evaluated were longer than the compression mandrel. The
compression test fixture is shown in Figure 7. Before testing
began, all of the stents were first subjected to a common
force, measured in pounds, and measured for displacement.
This first test better characterized the compressive
stress/strain curves for each of the devices tested. As shown
in Figure 8, approximately 4 lbs of compressive force were required to produce a .2-inch displacement for the laser-cut
nitinol tube stents, whereas the same 4 lbs of loading
resulted in only a .03-inch displacement for the SUPERA
wire-interwoven nitinol stent. Similarly, at a compression of
.2 inches (5 mm), the SUPERA wire-interwoven nitinol stent
had 360% greater radial strength (>18 lbs) than all of the
other laser-cut nitinol tube stents tested. From these data, a
common force (4 lbs) was translated into displacements for
both the laser-cut nitinol tube stents and the SUPERA wireinterwoven
nitinol stents. After determining this common
force, all stents were then subjected to a fixed displacement
to simulate an approximate 4 lbs of external/compressive
force. Although all laser-cut nitinol tube stents failed by 1
million cycles, the SUPERA wire-interwoven nitinol stents
completed the full 10 million cycles without failure.
CONCLUSION
The complexity of motion and the variable forces
experienced by stents placed in SFA or femoropopliteal
arteries have limited the ability to treat many common
vascular conditions effectively with stents. The SUPERA
wire-interwoven nitinol stent is a new generation of selfexpanding
(WIN) design stents that may fill a need for
patients with SFA disease, as bench testing has demonstrated
improved flexibility, radial force, and fracture
resistance. Additional tests of the SUPERA stent in the
clinical setting are needed to determine if these bench
tests will correspond with improved clinical outcomes.
Craig M. Walker, MD, is Medical Director and Founder, Cardiovascular Institute of the South in Houma, Louisiana. He has disclosed that he is a stockholder in Cardiva, CV Therapeutics, IDev, and Spectranetics; a consultant to Boston Scientific, Cardiva, CV Therapeutics, ev3, FlowMedica, IDev, Possis, and Spectranetics; serves on the speaker's bureau for Abbott, AstraZeneca, Boston Scientific, Cardiva, Cordis, CV Therapeutics, FlowMedica, Possis, Spectranetics, and Toshiba; and is on the medical/scientific advisory board for Cardiva, Edwards Lifesciences, FlowMedica, Philips, Spectranetics, and The Medicine Company. Dr. Walker may be reached at (800) 445-9676; craig.walker@cardio.com.
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