Barbs on Vascular Stents: A Three-Part Protocol to Evaluate Durability
Conti, J.C., Strope, E.R., Rohde, D.R.
Dynatek Dalta Scientific Instruments, Fourth and Main, Galena, MO 65656
Dozens of new stent designs are under evaluation to address the problems of occlusion, reocclusion, and aneurysm formation. A common design element utilizes small barbs or projections from the stent that pierce the adjacent vessel wall and anchor the stent or graft/stent combination.
Recently, a barb-containing stent that had supposedly passed durability testing when evaluated using currently accepted accelerated protocols failed in vivo due to barb breakage. These failures have resulted in the need for protocols to specifically address the fatigue properties of the barb itself.
Methods: A three-part protocol has been generated for the isolated durability testing of the stent barbs. These include: 1) determination of the loading per barb in a cardiovascular simulator, 2) determination of frequency response characteristics of the bending point where the barb attaches to the stent, and 3) a high speed bending test at a frequency determined in step 2.
Flow Studies – In this example, a stent/Dacron graft combination designed to treat descending aortic/iliac bifurcation aneurysms was mounted into a natural rubber synthetic artery with a radial compliance of 6%. This segment was attached to the outlet of the ascending aortic portion of a computer-controlled cardiovascular simulator. The interface of the ascending aorta and stent-mounted synthetic descending aorta was a segment of a second synthetic artery that possessed high longitudinal compliance.
The length of the interface segment was monitored during flow studies that ranged from 5 to 30 L/min cardiac output. This changing length was related, via calibration, to device loading. Testing was done at a pulse rate of 70bpm with a systolic interval of 35%, blood pressure of 120 over 80mm Hg, at 37°C.
Frequency Response – Isolated barb/stent segments were mounted on a dynamic micromechanical tester. Stress/strain curves were generated to determine the strain necessary to elicit the loading per barb that was found in the flow studies. This testing was initially done at 70 bpm. Testing frequency was then increased while monitoring peak stress, strain, and modulus. In each case a testing frequency was reached in which the stent/barb segment could no longer keep up with the loading rate. This frequency was used in the next section of the testing.
Durability Testing – The above data were used to determine the parameters for the high speed flex testing of the stent/barb segment. This testing was accomplished by loading multiple samples onto a vibration plate that allowed for the precise bending of the segment.
Results – The total load on the device was a function of the geometry and graft material as well as the flow rate. Load per barb was a function of total load as well as the number of barbs used on the device.
Dynamic mechanical testing was carried out under stroke control using a deflection of 0.1mm representing a peak load on the barb of 16g. Testing was possible up to a frequency of 50Hz.
Long term durability studies are in progress and results for these investigations as well as a complete review of all experimental results will be presented.
Conclusions – The durability of isolated vascular stent barbs is being evaluated using protocols that consider both the loading on individual barbs in a cardiovascular simulator as well as the inherent frequency response of the barb. These considerations should result in more reliable long term high speed durability testing carried out at frequencies much higher than if the barbs were part of the final implant morphology.