Abstract:
Apparatus tests the durability of a peripheral artery medical device based upon anatomical loading conditions. A peripheral artery medical device is mounted to a support element, typically a hollow tube, having first and second end portions. End holding elements are mounted to a base and are secured to the first and second end portions. The apparatus further comprises means for applying a plurality of cycles of at least one of the following loading conditions to the medical device support element at the location of the peripheral artery medical device: torsion, tension/compression, bending and pinching. In some embodiments the apparatus comprises an environmental chamber housing at least the support element so to mimic the service temperature environment of the medical device. A method for testing the durability of a peripheral artery medical device based upon anatomical loading conditions is also disclosed.

Description:
CROSS-REFERENCE TO OTHER APPLICATIONS  
       [0001]     This application claims priority from U.S. Provisional Application No. 60/657,504 filed Mar. 1, 2005, titled “Apparatus and Methods for Durability Testing of Peripheral Artery Medical Devices”. 
     
    
     FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     None.  
       BACKGROUND OF THE INVENTION  
       [0003]     Medical devices, such as stents and covered stents, used in the peripheral arteries to treat a number of arterial diseases (including atherosclerosis, aneurysm, injury with pseudoaneurysm, etc.) are subject to forces not seen in coronary artery implants. These forces have come to light in a number of different forums, including peer reviewed journal articles, implant clinical trials, and changes in federal guidelines for design validation of stents. The arteries of the periphery, such as the superficial femoral and the popliteal are long arteries with a relatively small number of side branches. This lack of tethering allows the arteries to flex and deform with the movements of the muscles and tendons (for example during knee and hip flexion). The forces that the artery can encounter include: torsion, axial tension/compression, pinching or kinking (radial compression), and bending. These forces can work in unison or individually.  
       BRIEF SUMMARY OF THE INVENTION  
       [0004]     Previous requirements for product release in this field, that is medical devices used in the peripheral arteries, typically included a theoretical analysis for the implants&#39; delivery and the forces an implant would encounter from the pulsatile artery movement for blood flow. While these tests are important, it is believed that these tests not sufficient to ensure adequate durability for implants that are placed in the highly mobile peripheral arteries. It is believed that for proper testing, the forces these implants are expected to encounter need to be replicated by mechanical testing.  
         [0005]     First aspect of the present invention is directed to apparatus for testing the durability of a peripheral artery medical device based upon anatomical loading conditions. A support element has first and second end portions and a body therebetween. A peripheral artery medical device is mounted to the support element. The body defines a centerline. The apparatus also includes means for engaging the support element. The apparatus further comprises means for applying a plurality of cycles of at least one of the following loading conditions to the support element at the location of the medical device: torsion, tension/compression, bending and pinching. A cycle counter is used to count the cycles of the at least one loading condition. In some embodiments an environmental chamber is used to house at least the support element so to mimic the service temperature environment of the medical device. The support element may comprise hollow tubing housing the peripheral artery medical device.  
         [0006]     A second aspect of the invention is directed to apparatus for testing the durability of a peripheral artery medical device based upon anatomical loading conditions. A support element has first and second end portions and a body therebetween. A peripheral artery medical device is mounted to the support element. The body defines a centerline. End holding elements are mounted to the base and are secured to the first and second end portions of the medical device support element. The apparatus further comprises means for applying a plurality of cycles of at least one of the following loading conditions to the medical device support element at the location of the peripheral artery medical device: torsion, tension/compression, bending and pinching. In some embodiments the apparatus comprises an environmental chamber housing at least the support element so to mimic the service temperature environment of the medical device. The loading conditions applying means may act through the end holding elements to apply at least one of torsion and tension/compression loading conditions. The loading conditions applying means may also contact the medical device support element at a position between the end holding elements to apply at least one of bending and pinching loading conditions.  
         [0007]     A third aspect of the invention is drifted to a method for testing the durability of a peripheral artery medical device based upon anatomical loading conditions. A peripheral artery medical device is loaded to a support element, the support element having first and second end portions and a body therebetween, the body defining a centerline. The support element is engaged by testing apparatus. The durability of the peripheral artery medical device is tested by applying a plurality of cycles of at least one of the following loading conditions to the support element by the testing apparatus: torsion, tension/compression, bending and pinching. The testing is monitored. In some embodiments the testing is carried out in an environmental chamber housing at least the support element so to mimic the service temperature environment of the medical device.  
         [0008]     The present invention is particularly useful for the testing of stents for use in the peripheral arteries (e.g., superficial femoral, popliteal, carotid). The invention provides anatomically relevant physical testing platforms for the accelerated development of stent implants as well as other peripheral artery medical devices. The inventions also allow for side-by side comparison of the durability and performance of specific implant designs.  
         [0009]     The inventions disclosed have a number of varying components that when combined allow for anatomically relevant physical testing platforms. Aspects of the invention include the individual force testers, the body temperature chamber to house the testers and the support system for the test articles.  
         [0010]     Various features and advantages of the invention will appear from the following description in which the preferred embodiments have been set forth in detail in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is an overall view of test article capture tubing used with the present invention.  
         [0012]      FIG. 2  is an overall view of an atherosclerotic model capture tubing used with the present invention.  
         [0013]      FIG. 3  is an overall view of a test environment heat chamber used with the present invention.  
         [0014]      FIG. 4  is an overall view of a compression/elongation tester made according to the invention.  
         [0015]      FIG. 5  is an overall view of a torsion tester made according to the invention.  
         [0016]      FIG. 6  is an overall view of a pinch tester made according to the invention.  
         [0017]      FIG. 7  is an alternative embodiment of the pinch tester of  FIG. 6  using a rack and pinion arrangement.  
         [0018]      FIG. 8  is an alternative embodiment of the pinch tester of  FIG. 7  using an air piston powered arrangement.  
         [0019]      FIG. 9  is an overall view of a bend tester made according to the invention.  
         [0020]      FIG. 10  is an alternative embodiment of testing system comprising at a self-contained environment and tester.  
         [0021]      FIG. 11  is an alternative embodiment of the pinch tester of  FIG. 8  using a high speed wheel arrangement.  
         [0022]      FIG. 12  is an alternative embodiment of the torsion tester of  FIG. 5  using a cam operation arrangement. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]     The following description of the invention will typically be with reference to specific structural embodiments and methods. It is to be understood that there is no intention to limit the invention to the specifically disclosed embodiments and methods but that the invention may be practiced using other features, elements, methods and embodiments. Like elements in various embodiments are commonly referred to with like reference numerals.  
         [0024]     The support system for testing peripheral artery medical devices, sometimes referred to as implants, is typically in the form of tubing  13 , such as Dow Corning Pharma-80 silicone tubing ( FIG. 1 ). Tubing  13  is preferably chosen to resemble the stiffness of an aged or diseased artery. Tubing  13  should be available in different internal diameters for use with different implant sizes. It is also advantageous to allow for the stent, or other peripheral artery medical device, to be loaded as intended during clinical use, i.e., allow for use in a 37 C environment (water or air) and allow for balloon dilatation. Similarly tubing  13  must remain stable during force application.  
         [0025]     An alternative embodiment of tubing  13  of  FIG. 1  is shown in  FIG. 2  used as an artherosclerotic disease modeled vessel. As shown in  FIG. 2 , tubing  13  includes one or both of a narrowed section  14  and a localized stiffening section  15  in the tubing wall to simulate diseased sections.  
         [0026]     An environmental test chamber  10 , shown in  FIG. 3 , can be important to the testing of implants due to the heat dependence of the materials utilized for implants. Self-expanding stents are often made of a nickel titanium alloy, which utilizes transition temperatures to aid in their flexibility. Testing of these implants in a room-temperature environment is often not appropriate for an implant that is used at body temperature. A heat chamber that consistently maintains body temperature (37 C) for multiple testing apparatus is particularly useful for physical testing. The use of large slotted shelves  19  allow for the heat to distribute evenly. Test chamber  10  uses a heating element  21 , thermocouples  20 , feedback electronics, circulation fan  17  for air movement, and sealable doors  18  to maintain temperature during testing. A controller  16  uses feedback electronics to control circulation fan  17  and heating element  21  to control the temperature within chamber  10 . An alternative embodiment of this air heat chamber  10 , not shown, is an individual chamber just around the test article that utilizes heated (37 C) fluid to maintain test article temperature.  
         [0027]     Testers  11 , discussed below with reference to  FIGS. 4-12 , preferably include the following: a comparable tubing holding device (pins sized to the tubing ID work well), ease of viewing the sample during testing—clear or open test articles, and testing parameter adjustment (e.g. speed, force, displacement adjustments). Magnet-actuated or photo electronic counters allow for accurate measurement of cycles to failure or test completion. Testers  11  are designed to simulate a specific force and/or a specific range of motion that the medical device is likely to encounter when used in the body. All test indenters are preferably consistently in contact with the tubing surface so that they do not impart unwanted force applications (ramming or loaded forces).  
         [0028]     The various embodiments of testers  11  include a base  29  supporting a motor assembly  23 . Motor assembly  23  could include various types of drives including servo motors, worm gears, compressed air systems, etc. Speed and travel controllers  22  are also mounted on based  29  and allow for modifications of the speed (cycles/min) or the travel (percentage of force application or distance of force mechanism movement).  
         [0029]     The torsion tester  11  of  FIG. 5  uses two rubber mounting grommets  28  and tubing collars  25  to act as holding arms to suspend a flexible tube  13 , which in turn, holds the test specimen. Tubing collars  25  are used for holding the test article tubing  13 . A pin, not shown, is typically placed into the center of tubing  13  and then a grommet  28  is squeezed around the pin/tubing assembly by the tubing collars. The mounting grommets  28  are typically made of rubber and are sized for the specific tubing  13  being used. Grommets  28  tighten around tubing  13  thus allowing force to be transferred efficiently. Grommets  28  also prevent unwanted movement of tubing  13 .  
         [0030]     An actuation device, not shown, within motor assembly  23 , such as a stepper motor, optical encoder, compressed air driver, etc., rotates torsion tester rotating arm  27 , typically at rotation angles of between 0° and 90° in each direction, as indicated by an arrow  27 A. The rotation of arm  27  creates a twist on tubing  13 , placing the specimen in a torsion loading condition. The degree of twist or rotation imparted to the specimen should be adjustable to be consistent with measured or estimated rotation data from the clinical environment. Cycles are recorded on a cycle counter  26 , which are used to document the cycle life of a given specimen for a particular test.  
         [0031]     The axial compression/elongation tester  11  (see  FIG. 4 ) also uses a grommet based holding system for the tubing containing the test specimen. The tester  11  of  FIG. 4  includes a compression/elongation tester actuation arm  24 . The distances arm  24  moves in and out, indicated by arrow  24 A, create the compression and elongation of the test article. This travel is controlled by controller  22 . The amount of compression/elongation is typically between 1% and 50% of the length of tubing  13 . The position of support  50  on base  29  may be laterally adjusted as indicated by arrow  52  to permit off-axis testing. One end of tubing  13  is adjustable for varying specimen lengths and also for varying the degree of off-axis compression/elongation desired. The percentage of compression or elongation can be modified, either dependently or independently.  
         [0032]     A bend tester  11  (see  FIG. 9 ) allows for test articles to be bent over a consistent specified radius  40 . Radius  40  should be changeable, typically from a minimum radius of 5 mm to as large as 12 cm. A fully adjustable bend fixture  40 A allows a stent-loaded tubing  13  to bend around a specific curve (though a specific arc). A cam-driven arm or other movement device implements the bend action. The tubing should not whip and therefore a stiffening strip may be included with the tubing to support the return action. In the embodiment of  FIG. 9 , a rotation arm  30 ,  31  and a cam arm  32  are used to drive a force indenter  33 . The amount of travel, indicated by arrow  33 A, imparted to indenter  33  can be adjusted through different attachment points  32 A on cam arm  30 ,  31 . Indenter  33  is housed within and is guided by a cam groove  41  to allow straight, consistent motion of the cam arm  32  and indenter  33 .  
         [0033]     Force indenter  33  used to impart the force and area of force onto tubing  13  that contains the test specimen (typically a stent). Indenter  33  is always in contact with tubing  13  to ensure that there is no ramming force imparted on the test sample. Indenter  33  can be modified for height, width and tip radius to impart varying amounts of force onto the stent.  
         [0034]     The bend tester  11  of  FIG. 9  also includes a bend indenter guide plate  38  used to ensure that tubing  13  remains in contact with bend indenter  33  at all times during the travel. It also prevents tubing  13  from coming out of the bend path. A bend tubing holder  39  allows for tubing  13  to remain in the bend travel path. It also facilitates review and adjustment of the stent loaded tubing during the durability testing.  
         [0035]     A pinch tester  11 , see  FIG. 6 , allows for compression of the test article perpendicular to the axis or centerline. This pinch force replicates what may be the most important force applied to stents during movement. Adjustable travel parameters allow for testing long-term accelerated durability as well as shorter-term design modification durability. The indenter (force applicator)  33  applies force over a relatively uniform area. This area should be scalable to allow for varying force application. Motor assembly  23  can include a motor, cam, rack and pinion, air, or other mechanical power system to apply the force. A back plate can be used to support the tubing/stent during force application, which can be hinged to allow for freer movement than a rigid back plate would allow. In the embodiment of  FIG. 6 , a moveable back plate  36  is used to allow the stent tubing assembly  13  to shift as it would anatomically. Movable back plate  36  holds the stent/tubing assembly  13  correctly aligned with force indenter  33 . Tubing holders  34  are used for holding the stent to the moveable back plate  36 . Movable back plate  36  also allows for easy viewing and access to the stent/tubing assembly  13  for review during testing. This additional movement may better replicate the movement of an artery in its surrounding tissue bed. Hinge points  35  are located on either side of the center of tubing  13 , thus allowing a smooth bending of the tubing. Hinge points  35  allow back plate  36  to move with the application of force indenter  33  to allow tubing/stent assembly  13  to shift as it would anatomically.  
         [0036]     The tester allows for accurate cycle counting. The machine also allows for varying the test article sizes (lengths and diameters).  
         [0037]     A number of alternative embodiments of pinch tester  11  of  FIG. 6  are shown in  FIGS. 7, 8 , and  11 . Tester  11  of  FIG. 7  uses a small base  29  and a rack and pinion movement system  37 . System  37  uses a gear and toothed rod for moving the force applicator  33 . This embodiment aides in the efficiency of the tester  11  by allowing the system in be run at a higher speed. Tester  11  of  FIG. 7  is adjustable for stroke and contact time. The  FIG. 8  embodiment of tester  11  is similar to the  FIG. 7  embodiment but uses a compressed air motor assembly  23  to move force indenter  33 .  
         [0038]     The tester  11  of  FIG. 11  uses a rotating wheel  44  with indents to cause force indenter  33  to impinge on the test article. Wheel  44  contains raised and lower section onto which an indenter shaft  45  rides. Wheel  44  can spin at a much higher speed and allow for consistent travel of indenter  33 . Fewer moving parts allow for fewer failure points. The number and size of raised and lowered areas can be modified for different desired indenter travels. Indenter shaft  45  contains a rounded tip to allow for smooth operation on the raised and lowered sections of wheel  44 .  
         [0039]     An alternative embodiment of the torsion tester  11  of  FIG. 5  is shown in  FIG. 12  and utilizes a cam-operated twisting mechanism. Cam arm  32  drives rotation arm  46  to allow the torsion force to be smoothly transferred to the tubing assembly, including tubing  13  loaded with a test article  54 , such as a stent. This drive assembly allows for movement up to just less than 90° in each direction, that is clockwise and counterclockwise. Bearings  47  at each tubing holder joint allow for increased efficiency of force application.  
         [0040]      FIG. 10  illustrates a self-contained testing system  12  that combines the structure and functions of axial compression/elongation tester  11  of  FIG. 4  with the environmental test chamber  10  of  FIG. 3 . System  12  comprises a transparent cover  42  to create, along with circulation fan  17 , heating element  21  and thermocouple  20 , a self contained tester/testing environment. Cover  42  allows system  12  to operate independent of a larger heating environment. Seals  43  may be provided on all edges to ensure consistent temperatures during testing.  
         [0041]     A multi-axis tester, not shown, may be constructed to test all of the forces at one time. Such a multi-axis tester would preferably have independent parameter adjustment for each of the forces.  
         [0042]     The tubing, heat chamber and testers work together to provide a repeatable means to assess the durability of stent implants in a repeatable, accelerated time frame. Specific parameter settings are adjustable to the latest clinical information regarding appropriate parameter values. These parameter values (e.g., force, displacement, artery bend radius) can come from angiographic measurements, peer-reviewed, biomedical engineering and cardiovascular literature. The advantage of having a test system that can simulate the clinical parameters allows for rapid assessment of potential design changes of a device, as well as validating the durability of a selected design.