Radial Fatigue and Pulsatile Durability
Determine the fatigue resistance of a stent using an in-vitro accelerated test.
ViVitro’s proprietary fatigue tester measures dynamic stent displacement directly using an ultrasound measurement system. Others measure the compliant tube displacement which may different from the stent displacement. The ViVitro system does not require deriving any specific tube compliance relationship between tube outside diameters and inside diameter.
Estimated test period assumes test frequencies of 50 Hz for standard stent. Test frequency and test duration may vary upon device behaviour and performance. Test frequency and strain amplitude may vary upon device behaviour if different stent diameters are used within the same fatigue tester.
Silicone tubes are simulating arteries in the stent fatigue tester. The silicone tubes are filled with a test fluid as specified by the sponsor. The fatigue tester is brought to specified test temperature. The stents samples are deployed in mock arteries (silicone tubes). They are deployed to sponsor defined diameter. The initial position of each stent is identified within the silicone tube. Anomalies during the deployment are recorded.
Using our proprietary thin tube fatigue tester design, a pressure difference (cycling dynamic or static) can be applied between the external and internal sides of the silicone tubes. Since the tubes are much more flexible than the stents, the stents are directly subjected to the pressure difference between the internal and the external sides of silicone tubes. Thus, our proprietary thin tube fatigue tester design does not require deriving any specific tube compliance relationship between tube OD and ID.
Before starting cycling, the stent diameter (in 1 plane) is measured both at rest and under the peak hydrostatic pressure difference amplitude as defined below using a ultrasound (US) system. The mean difference between the stent diameters at rest and under peak static pressure measured by the US system defines the target strain.
An initialization phase (cycles not counted) is performed to set the cyclic pressure difference magnitude and the frequency acting on the silicone tubes containing the stents. The strain at test frequency is measured using US diameter measuring system in order to ensure that the dynamic strain amplitude is equivalent to the target strain. This step is also used to set up the test frequency by verifying that the frequency of deformations is similar to the test frequency measured by the counter.
Cycle counting is initiated once the setup is completed.
The applied pressure difference, the frequency, the number of cycles and the stent position within the mock artery are verified once a day (working days only). A visual inspection is also performed once a day (working days only) to detect any stent failure. At regular intervals of time as defined by the sponsor, stent diameters and strains are measured at the test frequency. Cyclic pressure difference (input variable) is maintained at a constant value throughout the entire duration of the fatigue test allowing mean stents strain (output variable) to vary.
The test stops when number of cycles defined by the sponsor is reached. At the end of the test, the stent diameter and strain are measured at the test frequency.
Note on the fatigue test principle
Cyclic pressure is applied on external surface of the silicone tube (artery model) which generates a force perpendicular to the external surface of the stent.
- In displacement controlled tests, as used in common fatigue testers, stents are subjected to “uniform” diameter decrease, and the stresses inside the stent are evenly distributed to reduce local stress: each part of the stent contributes at its best to support the total stress imposed by the displacement. In this case, the stresses are approximately equivalent in all parts of the stents.
- In pressure controlled tests, as used in our proprietary thin tube fatigue tester, the external diameter of the stent is subjected to uniform pressure, and each part of the external surface of the stent is subjected to the same pressure or load. Any stent that has been subjected to a manufacturing process, always contains weak and strong parts. The weakest parts are subjected to the same pressure than the other parts which implies higher stresses on the weakest parts. This implies that the weakest part of the stent is going to fail first causing strut rupture, or complete stent crush and/or collapse.
Stent collapse failure mode cannot be generated in testers using a displacement controlled mode. Stent collapse has been identified as a cause of stent failure as documented in several publications (see “IFU Medtronic Vascular Endeavor Zotarolimus-Eluting Coronary Stent” and “Focal Stent Collapse in a Patient With Systemic Sclerosis”, Lucia Di Francesco, PhD, Leo Finci, MD, Bernhard Reimers, MD, Carlo Di Mario, MD, and Antonio Colombo, MD, Catheterization and Cardiovascular Diagnosis 44:57–60 – 1998).
In most in-vitro studies, it is assumed that the displacement of the arterial wall in a healthy vessel is perfectly radial and can be simulated using displacement guided test. However, stents are used to treat pathological arteries, which have arterial wall with uneven rigidity. Therefore displacements due to pulsatile blood pressure are not uniform. In these instances, it can be assumed that inside a stent, the pressure applied by the blood on the arterial wall is uniform, and that the arterial wall exerts in reaction an equivalent pressure to reach equilibrium.
Pressure controlled fatigue testers (pressure is applied to the external stent diameter) are more revealing of stent flaws than displacement controlled testers.
The intent of the preconditioning is to simulate access to the lesion site to induce stresses (cold working effect) on stent before the expansion in the artery during the clinical use. The pre-conditioning step is performed using IDTE1000 from Machine Solutions Inc. A pump is used to produce a constant flow (about 0.2l/min) in a simulated tortuous path. The test fluid is water regulated at 37+/-2°C. Accessories such as haemostatichemostatic introducer sheath, guiding catheter and guidewire are used according to the IFU. Each stent delivery system is automatically advanced (on guidewire inside guiding catheter) for a distance of 800 mm through tortuous path at a constant rate 15 mm/s. Forces are not measured during this test. Then the stent system is manually withdrawn.
Scanning Electron Microscopy (SEM) Inspection
SEM inspection is conducted on both the stent sample that have been subjected to the fatigue test and the controls. assess the possible alterations produced by fatigue. The stents structure is inspected to identify potential fracture zones, surface topography detects, inter or intra-granular defects, cracks, crevices, and shear planes. The most significant and the most representative observations are recorded (photographs) and their dimensional characteristics are estimated. The inherent characteristics of the controls are used to determine if observations are related to mechanical fatigue deformations and defects
Stent that are permanently implanted in a patient are subjected to the loading and unloading cycles of the artery. In order to ensure that the structure of the stent does not undergo alterations over time and continues to fulfill its role of mechanical support of the artery, a fatigue test of the stent is necessary. As stent need to maintain their structural integrity for a period of minimum 10 years, the fatigue-to-life of a stent is assessed under accelerated simulated physiological conditions.
ViVitro Labs’ proprietary thin tube fatigue tester design does not require deriving any specific tube compliance relationship between tube OD and ID. Up to 21 stents can tested simultaneously in one machine. Stent are positioned in transparent silicon tubes with thin walls. Tubes are filled with relevant test solutions depending on stent type and test requirements. A cyclic or a static differential pressure can be directly applied on the silicone tubes. Stent dynamic displacement is directly measured in-line by ultrasound (us) system in compliance with standard requirement and in order to ensure that dynamic amplitude levels are meeting specifications limits. Applied pressure difference, test frequency, number of cycles and stent positions within the mock artery models are monitored daily. A visual inspection is also performed daily to detect visible stent failure.
Dynamic stent diameters and displacement are measured directly on the device at periodical intervals and at both the beginning and the end of the test. In order to detect early onset of failure and fracture, periodical diameter and displacement monitoring and measurement can be performed to assess stent behavior. Stent preparation, cycle counting, test frequency measurements, calibration of the pressure transducers are performed in compliance with and controlled by proper operating procedures.
- Stent fatigue test bench, including:
- Control unit including the cycle counter, the temperature control system and the control switches.
- Temperature sensor
- Temperature heater
- Air flow valve, Burkert
- Fluid level switch
- Constant flow pump
- Pressure relief valve
- Activation system
- Validyne pressure probe and control unit with membrane, measurement uncertainty ±2 mmHg
- Dynamic US diameter measuring system measurement uncertainty ±5%
- Silicone tubes used as mock arteries
- For preconditioning, IDTE 1000 including:
- Constant flow pump
- Temperature control system (37+/-2°C)
- Flow gauge
- Strain: a periodic measurement of the strain is performed using an US dynamic diameter measuring system.
- Stent diameter: a periodic measurement of the stent diameter is performed using an US dynamic diameter measuring system
- Test frequency
- Pressure difference
- Coronary and peripheral stent structures up to 10 mm
- CrCo, SS, NiTi, Bioresorbable, Bioabsorbable, Covered, Coated stent
Cardiovascular implants — Endovascular devices — Part 2: Vascular stents
ASTM F2477 – 19
Standard Test Methods for in vitro Pulsatile Durability Testing of Vascular Stents
ASTM F3211 – 17
Standard Guide for Fatigue-to-Fracture (FtF) Methodology for Cardiovascular Medical Devices
ASTM E178 – 16a
Standard Practice for Dealing With Outlying Observations
ASTM E456 – 13A(2017)e5
Standard Terminology Relating to Quality and Statistics
ASTM E468 – 18
Standard Practice for Presentation of Constant Amplitude Fatigue Test Results for Metallic Materials
ASTM E739 – 10(2015)
Standard Practice for Statistical Analysis of Linear or Linearized Stress-Life (S-N) and Strain-Life (ε-N) Fatigue Data
ASTM E1823 – 20b
Standard Terminology Relating to Fatigue and Fracture Testing
ASTM F2942 – 19
Standard Guide for in vitro Axial, Bending, and Torsional Durability Testing of Vascular Stents
- Crush Resistance – Pressure Control
- Crush Resistance – Constant Displacement
- Recoil, Foreshortening
- Scanning Electron Microscopy (SEM) and Optical Inspection
- Simulated Use Pre-conditioning
Stent Fatigue Tester
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