Assessing Radial Tests for Endovascular Implants

TESTING

Radial strength and radial stiffness are critical functional attributes in the design of endovascular devices such as stents, stent grafts, collagen plugs, embolic filters, and vena cava filters. Testing methods for determining radial strength and stiffness must provide comparative and quantitative values for modeling the device interaction with the target treatment site. A standardized method for radial stiffness and strength assessment may help improve understanding of the causes of late stent thrombosis. Some evidence indicates that this coronary event is associated with stent malapposition, which is often the result of insufficient radial stiffness or strength.¹

Accurate characterization and testing of these forces is imperative to the design verification of endovascular devices. The radial force of endovascular devices must be adequate to prevent migration of the device and maintain patency of the lumen, but not so great that it overexpands the vessel or damages it in any other way. Many factors influence radial strength and stiffness; a minor change to any of these factors can affect a device's functionality.

Manufacturers must understand the causes and consequences of inappropriate radial stiffness and strength. In addition, they must understand the various testing methods and be able to evaluate those methods. This article discusses the factors of radial strength and radial stiffness and offers insight into benchtop testing methods.

FDA, Regulatory Bodies, and Device Manufacturers

FDA's Guidance for Industry and FDA Staff—Non-Clinical Tests and Recommended Labeling for Intravascular Stents and Associated Delivery Systems recommends that radial strength and stiffness be part of the tests performed on intravascular stents.² In that document, FDA defines radial stiffness as “the change in stent diameter as a function of uniformly applied external radial pressure.” Radial strength is defined as “the pressure at which a stent experiences irrecoverable deformation.” FDA does not recommend one specific test method. However, ASTM International's F04.30.06 committee is working on a guidance document for radial stiffness and strength testing that should go through initial ballot this year.

Radial strength and stiffness are significant values because they help characterize how well a device will perform its intended function. For example, an embolic filter is intended to temporarily expand into a vessel lumen downstream of an area needing intervention, such as an occluded carotid artery. Once it is expanded, the embolic filter must be seated securely against the vessel wall to ensure that all blood moves through it and not around it.

The filter's job is to capture any particles large enough to cause a blockage or a stroke. If an embolic filter does not have enough radial strength, then blood will flow around it and it may not capture all of the large-sized particles.

Similar to an embolic filter, a stent must have enough radial strength to hold open a vessel and maintain its deployed position. A stent, however, is a permanent implant. It must maintain its radial strength over time to prevent stent migration, which could leave the patient vulnerable to restenosis or an aneurism. Conversely, a device with too much radial strength or stiffness may overexpand the target vessel, causing the smooth muscle to thin and leave the patient vulnerable to an aneurism.

Factors That Affect Radial Strength and Stiffness

The overall structural design of the device plays the largest role in a device's radial strength and stiffness. In general, devices with very open structures (or low material-per-square-inch ratios) have less radial stiffness and strength than more-closed structures (or high material density). Strut thickness also affects radial strength and stiffness. A thicker strut provides greater radial strength and stiffness compared with a device that has a thinner strut.

The material of the device also plays a large role in its radial strength and stiffness; material selection can make up for an open structure of the device by providing increased radial stiffness. For example, some cobalt chromium alloys provide greater radial strength than stainless steel. Therefore, an open cobalt-chromium stent may provide greater radial stiffness and strength than a closed stainless-steel stent.

Secondary processes of a device material also influence radial strength and radial stiffness. Heat treating, annealing, and polishing are common secondary processes. Heat treating of the raw material prior to cutting the stent hardens the material and affects strength and stiffness.

The annealing process softens the material and allows the stent to expand without breaking. If the material is over­annealed, the stent will be too soft and will have insufficient radial strength and stiffness. The common parameters in annealing are temperature, length of cooling time, and position of the material. The length and temperature affect how soft the material is after annealing, and the position of the material affects the uniformity of the material properties.

The method of polishing devices, such as electropolishing, also affects the radial strength and stiffness of the device by disrupting the material surface.

For shape-memory alloys such as nitinol, these secondary processes can be varied to give the same material different final properties. The elastic range or softness of a stent can be increased to impart improved flexibility, which improves deliverability while maintaining adequate radial strength and stiffness. The secondary processing of shape memory alloys gives the material its transition temperature (the temperature at which a stent exhibits its optimal radial strength and stiffness). This temperature can be set at normal body temperature (37°C).

When looking at other stent materials under development, such as polymers, more of the radial strength and stiffness is dependent on the material blend and how quickly the materials bio­absorb. Having a strong understanding of how each of these factors affect radial strength and stiffness enables faster device development tailored to specific vessel types.

Benchtop Test Methods

For manufacturers to appreciate the radial strength and stiffness of their devices, they need to understand what information they can obtain from benchtop test methods. They must also understand the strengths and limitations of the benchtop test method they choose.

When evaluating benchtop test methods for radial strength and stiffness measurement, a key factor is finding a method with adequate force resolution to measure small differences in individual devices. The resolution must be high enough to delineate differences in fabrication and to identify minor material differences. For most devices, 0.05-N resolution for radial force is adequate.

Comparing device performance is useful for manufacturing, R&D, quality control, and quality assurance. For example, engineers must be able to compare current designs against previous designs (or competitive designs) during research, development, and feasibility work. It provides valuable information to move the design forward or to change direction. Data must be easy to share with management and marketing to ensure that new designs meet clinical needs. Such data are quality control tools.

A benchtop test method for the radial strength and stiffness should enable a user to measure a known good product and set an acceptance window for all manufactured devices. By doing so, the same nondestructive benchtop test method can be used to sample product from each manufacturing lot in a reliable way. Because each data set is compared with the acceptance window, the method increases efficiency.

For comparison purposes, the benchtop method must pass a gauge repeatability and reproducibility study as well as be easy for operators and technicians to set up and use. The segmental compression method uses 12 segment points of contact to uniformly and radially compress the device. This method meets all gauge requirements, as opposed to other methods that meet only a portion of the requirements.

Quantitative Values of Benchtop Testing

There is ongoing effort to prove that radial strength and stiffness values that come from the benchtop method can be used in modeling device interaction with the target vessel treatment site. Each benchtop method is used to measure radial forces, but each method loads the specimen differently.

Whether thin-film loop, flat plate, v-block, or segmental compression is used to measure radial strength and stiffness, values coming out of those methods should be both comparative and quantitative. Figure 1 shows a finite-element analysis model of several different loading conditions.

The thin-film loop method uses a strip of thin film that is wrapped around the stent and then pulled with a tensile testing system. It is considered to have a single point of contact. The thin-film loop has a pinch point where the film overlaps and connects to the tensioning device. This pinch point can cause nonuniform compression of the device. Variability from site to site or even test to test is also increased with the thin-film loop method due to variability in film thickness, flexibility, and age.

The flat plate method is a two-point contact method that uses two flat plates attached to a tensile testing system. The stent is placed between the two plates and compressed.

The v-block method is a four-point contact method that utilizes two v-shaped blocks on a tensile testing system that come together to radially compress the stent. The v-block has unique loading conditions compared with the thin-film loop and segmental compression methods.

To assist with quantifying radial strength and stiffness performance, an outsourced testing provider devised a test using segmental compression to prove that benchtop test output can be quantitative and qualitative. The method applies specifically to segmental compression but may also be used with other methods by using the appropriate modulus of elasticity equation for that loading condition. Due to the loading method variability previously described, the equation on page 100 (for calculating the modulus of elasticity) applies only to the segmental compression method.

The segmental compression method is sensitive and repeatable compared with incumbent methods or standards. In order to prove quantitative results using segmental compression, an experiment was created in which the elastic modulus of a standard material is measured in two ways. The first is a standard compression test with a tensile test bed and a solid cylinder test specimen. The second uses a segmental compression system and a cylindrical shell test specimen. All samples were molded out of urethane to create isotropic samples.

The segmental compression system was used to compress the cylindrical shells and measure the diameter change and hoop force. Those values were then converted to stress versus strain by using the equation for modulus of elasticity of a thick-walled pressure vessel under external pressure, as shown here:

This equation depicts the modulus of elasticity of a thick-walled pressure vessel under external pressure. OD1 equals unstressed outside diameter, OD2 is stressed diameter, HF is the measured hoop force, T is unstressed wall thickness, and equals Poisson's ratio.

Solid cylinder specimens were tested on the tensile test bed, and stress-strain curves were calculated. The average percentage difference between the two methods measuring the modulus of elasticity of urethane was 0.4%, indicating that the segmental compression method is accurate in its ability to measure force and deformation of a sample. Therefore, the radial stiffness and strength values of devices tested using this method can be used in modeling the device's interaction with the target vessel (see Figure 2).

Conclusion

Understanding the radial strength and stiffness of endovascular devices, as well as the benchtop test method used to determine these values, is vital to the design process for device manufacturers. When the benchtop test method has the appropriate resolution, manufacturers can quickly and effectively measure the effect of small changes in structure, material, and secondary processes. With improved knowledge of a device's radial strength and stiffness, manufacturers can improve device functionality at the intended treatment site and improve overall patient outcomes.

Multiple benchtop test methods are available, and they often have drastically different loading conditions. Manufacturers need to identify the best system for testing their devices. Once a benchtop test method is chosen and has passed a gauge repeatability and reproducibility study, the data can then also be used for manufacturing quality assurance and regulatory body submissions. With this greater understanding of the different methods, manufacturers will also be able to better evaluate contract testing groups.

Melissa Lachowitzer is a product manager and the contract testing lab manager for Machine Solutions Inc. in Flagstaff, AZ. She can be reached at [email protected].

References

1. ASTM WK15227, “New Standard Test Method for Vascular Stent Radial Stiffness and Strength Proposed by ASTM Medical Devices Committee,” (West Conshohocken, PA: ASTM International, 2004).

2. Guidance for Industry and FDA Staff—Non-Clinical Tests and Recommended Labeling for Intravascular Stents and Associated Delivery Systems [online] (Rockville, MD: FDA, 13 January 2005 [cited 17 April 2008]); available from Internet: www.fda.gov/cdrh/ode/guidance/1545.html.

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