Abstract:
Prostheses are fatigue tested using an apparatus under simulated physiological loading conditions. A fluid housing defines an entrance chamber having fluid outflow ports and an exit chamber having opposing fluid inflow ports and a central flow conduit in communication with the entrance chamber and the exit chamber. A plurality of housing tubes into which prosthesis are deployed may extend between the fluid outflow and inflow ports. Alternatively, tubular prostheses may be connected directly between the inflow and outflow ports. A reciprocating linear drive pump having a flexible diaphragm is provided to cyclically pressurize fluid through a common closed loop within the fluid housing and drive the pressurized fluid through the prosthesis being tested. The test system is capable of rotation independent of the motor drive for accurate diameter measurements of all test samples at elevated frequencies.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of priority pursuant to 35 U.S.C. §119(e) of U.S. provisional application No. 61/289,135 filed 22 Dec. 2009 entitled “Fatigue evaluation of prostheses by radial excitation of tubular structures,” which is hereby incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure concerns fatigue testing of prosthetic devices, e.g., prosthetic stents, grafts, stent-grafts, and other prosthesis (collectively referred to hereinafter as “prostheses”), under simulated physiological loading conditions and high-cycle applications. 
       BACKGROUND 
       [0003]    The Food &amp; Drug Administration (FDA) and other worldwide regulatory agencies require medical device manufacturers to submit clinical and in vitro test data before commercial approval of prosthetic devices. As a part of this action, these devices are typically tested to 400,000,000 cycles simulating 10 years of life in the human body at an average heart rate of 80 beats per minute. Prosthetic testing apparatus and methods, such as those outlined by Vilendrer in U.S. Pat. No. 5,670,708 and Conti in U.S. Pat. No. 4,972,721, require significant capital investment and, in the case of the system outlined in U.S. Pat. No. 4,972,721, offer limited operating frequencies and measurement capabilities. Additionally, these test systems are typically built to order based on specific target prosthetic device sizes and configurations, limiting testing flexibility. Furthermore, current systems employ a flexible metallic bellows or conventional piston and cylinder as drive members to provide the pressure actuation. 
         [0004]    These traditional fluid drive technologies have several shortcomings. For example, flexible metallic bellows are not ideal because they require high forces to operate and resonate at specific frequencies, necessitating the use of larger driving systems and limiting the available test speeds. Also, piston and cylinder arrangements employ traditional seals which are subject to friction and thus have severely limited life in high cycle applications. Additionally, known single drive systems create standing waves along the length of the prosthetic devices being tested, which is not a natural pressure waveform found in the human body. Therefore, the test sample is not excited in a clinically relevant manner. 
         [0005]    The information included in this Background section of the specification, including any references cited herein and any description or discussion thereof, is included for technical reference purposes only and is not to be regarded subject matter by which the scope of the claimed invention is to be bound. 
       SUMMARY 
       [0006]    Implementations of fatigue testing systems and devices herein simulate physiologic loading conditions on prosthetic devices at elevated testing frequencies. Generally, fatigue testing is accomplished by first deploying the prosthesis in an appropriately sized flexible housing tube or other appropriate structure. The housing tube with the prosthesis being tested are then subjected to physiologically appropriate conditions, which may include, but are not limited to, pressure, radial strain, temperature, and flow. Testing and test conditions are controlled by a computer that permits both input of test conditions and monitors feedback of the test conditions during testing. System control may be either an open loop paradigm that requires user intervention in the event a condition falls outside specified condition parameters or a closed loop model in which the system monitors and actively controls testing outputs in order to ensure that the testing parameters remain within specified conditions. 
         [0007]    A working fluid, which may be water, saline, a saline/glycerin solution, a glycerin/water solution, or a blood analog or substitute, is employed within the testing system. The working fluid may be selected to simulate one or more attributes of human blood, such as density, viscosity, or temperature. For example, in certain instances, physiological saline which does not simulate the viscosity of blood, but simulates density, may be used. In other cases a saline/glycerin solution may be employed to simulate blood density and viscosity. 
         [0008]    Plural prosthesis housing tubes, or the prostheses themselves, are coupled in parallel to a main housing having plural fluid distribution channels in communication with each of the housing tubes or prostheses. The main housing consists generally of a single fluid reservoir in fluid flow communication with each of the prosthesis housing tubes or prosthesis itself. The single fluid reservoir includes an entrance section and an exit section in fluid communication via a central flow conduit. The entrance section includes a plurality of fluid outlet ports, a single fluid flow inlet port and single fluid port in communication with the central flow conduit. The exit section includes a plurality of fluid outlet ports, a single fluid flow outlet port and single fluid port in communication with the central flow conduit. An external fluid reservoir provides a fluid draw source for the circulation pump and maintains the working fluid at the specified temperature. 
         [0009]    Implementations of fatigue testing devices generally include a linear motor coupled with a fluid drive member. The fluid drive member impinges upon the entrance section of the fluid reservoir to provide a motive force to drive the working fluid through its cycles within the main housing and the housing tubes. In one implementation, the fluid drive member is coupled to an opening in the entrance chamber and is reciprocally moveable to pressurize and depressurize fluid within the entrance housing. The fluid drive member is a flexible diaphragm which is highly compliant with low resistance to axial deformation across its entire axial range of motion. 
         [0010]    These components operate together to act as a fluid pump and when combined with the fluid control system, provide the pressure, flow, and temperature environment necessary to cycle the prosthesis under physiologic conditions. The internal conditions, which include, among other things, temperature and pressure, are electrically communicated to monitoring and controlling software on a test system computer. The external tube housing diameter or prosthesis is directly monitored through an optical micrometer system, consisting of a LED or laser-based, high-accuracy, optical micrometer, paired with a precise liner positioning system. The main housing may rotate about the system central axis allowing individual tube measurements at all test locations. The dynamics of the fluid pump and, therefore, the system dynamics are controlled via test system control software. The pressure field resulting from the pump motion is easily adjusted and controlled. All system inputs and outputs may be continuously monitored and directed into a software-based control and alarm system, allowing the system to automatically adjust and halt if any signal deviates outside of the specified test conditions. 
         [0011]    This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. A more extensive presentation of features, details, utilities, and advantages of this technology is provided in the following written description of various embodiments, illustrated in the accompanying drawings, and defined in the appended claims to the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    Various features and functions of the disclosed technology may be better understood when considered in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the several views. 
           [0013]      FIG. 1  is a combination block diagram and isometric view illustrating a main testing apparatus and related control systems of an implementation of a fatigue-testing system for prostheses. 
           [0014]      FIG. 2  is a cross-section view of the fatigue-testing apparatus of  FIG. 1  showing the internal fluid chamber coupled with the linear drive system. 
           [0015]      FIG. 3  is an enlarged partial cross-section view of the fatigue-testing apparatus of  FIG. 1  detailing the fluid drive coupled with the rotational mechanism. 
           [0016]      FIG. 4  is an enlarged partial cross-section view of the fatigue-testing apparatus of  FIG. 1  showing the linear drive and support structure. 
           [0017]      FIG. 5  is a partial isometric view of the fatigue-testing apparatus of  FIG. 1  detailing the large fluid drive member. 
           [0018]      FIG. 6  is a partial perspective view of an alternate embodiment of a fluid-testing apparatus incorporating a small fluid drive member. 
           [0019]      FIG. 7  is an isometric view of the optical micrometer measurement system of the fatigue-testing apparatus of  FIG. 1 . 
           [0020]      FIG. 8  is an isometric view illustrating an alternate embodiment of a fatigue-testing apparatus of a fatigue-testing system for prostheses. 
           [0021]      FIG. 9  is a cross-section view of the fatigue-testing apparatus of  FIG. 1  showing a telescoping internal fluid chamber coupled with a rotary drive system. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]      FIGS. 1 and 2  depict a fatigue-testing system  60  having a fatigue-testing device  20  operably connected to a data acquisition (DAQ) device  3  and to an amplifier and control system  4 . These components are, in turn, operably connected to a microprocessor-based computer  2 . All systems are preferably connected to an uninterrupted power supply (UPS)  1 . The fatigue testing device  20  is composed of a pressurizable fluid housing  54  formed as a disk-shaped manifold or entrance chamber  11  and a disk-shaped manifold or exit chamber  12  connected by a cylindrical central flow conduit  29 . The entrance and exit chambers  11 ,  12  are supported, respectively, by an entrance support structure  15  and an exit support structure  16 . The support structures  15 ,  16  are affixed to a base plate  8 . 
         [0023]    A plurality of flexible tubes  36 , or other prosthesis-housing structures, or the prostheses themselves, extend between and are in fluid communication with the entrance chamber  11  and the exit chamber  12 . The tubes  36  are parallel to and arranged circumferentially around and spaced apart from the central flow conduit  29 . A plurality of connection adapters  32  corresponding to respective tubes  36  fit within a plurality of apertures  55  on opposing faces of the entrance chamber  11  and the exit chamber  12  for attachment of the tubes  36  in fluid communication with the entrance chamber  11  and exit chamber  12 . In an alternate implementation for the testing of tubular prosthesis devices that are formed of materials that remain substantially nonporous under the pressure induced by the fatigue-testing system  60 , the prosthesis devices may be directly attached to the connection adapters  32  to be placed in fluid communication with the entrance chamber  11  and the exit chamber  12 . 
         [0024]    A fluid flow pathway  38  is defined from the entrance chamber  11  to the exit chamber  12  passing through the central flow conduit  29  and also through the purality of tubes  36 . When the prostheses  30  being tested are positioned within the tubes, the fluid flow path  38  may further include passage through the prostheses  30 . In implementations in which tubular prostheses are attached directly to the adapters  32  between the entrance chamber  11  and the exit chamber  12  (rather than within prosthesis-housing structures), the fluid flow pathway is directly through the prostheses. 
         [0025]    Testing pressures are created through a fluid drive member  10 , which in the exemplary implementation shown is powered by a linear motor mounted inside a motor housing  9 . The linear motor is composed of a primary  17   a  (i.e., the stator) and a secondary  17   b  that translates linearly within the primary. A circulation pump  37  has an outlet port in fluid communication with the entrance chamber  11 . The circulation pump  37  provides controllable system flow for testing purposes and also helps ensure uniform temperature distribution. An emergency stop switch  14  is mounted on the base plate  8  and severs power to the system  60  in the case of an emergency. 
         [0026]    The fatigue-testing device  20  may be pressurized, for example, by introducing pressurized air from an external air source  6 , such as an air compressor or sealed pressurized volume. The system air pressure may be controlled via a pressure regulator  5 . Alternatively, the system may be pressurized through the circulation pump  37  by controlling the flow rate and restricting outlet flow from a fluid exit valve  33 . Before pressurization, a working fluid (not shown) is introduced into the entrance chamber  11  and/or the exit chamber  12 , completely filling the device  20 . 
         [0027]    A heating source and fluid level safety switch are contained in a heat and circulation chamber  7 . The heat and circulation chamber  7  also has inflow and outflow ports communicating with the exit chamber  12  and inlet port on the circulation pump  37 , respectively. The heat and circulation chamber  7  is pressurized via a pressure regulator  5  and is completely sealed. A monitoring port allows the temperature inside the heat and circulation chamber  7  to be directly monitored. 
         [0028]    The entrance and exit chambers  11 ,  12  along with the primary fluid system and flow path  38  are shown in  FIG. 2 . The plurality of prosthesis-containing housing tubes  36  are connected in fluid flow communication between the entrance and exit chambers  11 ,  12  as shown in  FIG. 1 . In exemplary embodiments, the inner diameters of the housing tubes  36  may preferably range from 1-50 mm. The plurality of housing tubes  36  are coupled in parallel between the entrance and exit chambers  11 ,  12  for simultaneous testing of prostheses  30 . 
         [0029]    The fluid flow pathway  38  within the main housing is illustrated by phantom lines in  FIG. 2 . A fluid drive member  10  is provided to pressurize and depressurize the system. The fluid drive member  10  is in direct fluid communication with the entrance chamber  11  and thereby with the exit chamber  12  and central flow conduit  29 . Sample adapters  32 , which allow the housing tubes  36  to be affixed to the fatigue-testing device  20  in a leak-free manner, are connected to the entrance chamber  11  and exit chamber  12 . The sample adapters  32  can be adjusted, allowing the system to be easily configured for various prosthesis sizes. The entrance and exit chambers  11 ,  12  may also be configured to accommodate various sample quantities and geometries. A plurality of manifold plugs  31  in each of the entrance and exit chambers  11 ,  12  serve as fluid filling and air purge locations, as well as locations for monitoring ports. 
         [0030]    It will be understood that during the primary or pressurization portion of a testing cycle, the fluid drive member  10  moves in a positive direction toward the entrance chamber  11 , decreasing the system volume and creating system pressurization. During a secondary or depressurization portion of the test cycle the fluid drive member  10  moves in a negative direction away from the entrance chamber  11 , increasing the system volume and depressurizing the system. These actions serve to pressurize and depressurize the housing tubes  36 , applying the appropriate radial strain and/or pulse pressure to the prostheses. The central flow conduit  29  creates an alternate path for energy from the pressurization cycle such that the prostheses may be excited from both ends, which mitigates the formation of standing waves within the test prostheses. In this manner, the test prostheses are excited in a more natural and clinically relevant manner. The drive member  10  returns to its starting position and the process is repeated, cycling the fluid pressure on the prostheses. A single test cycle may consist of completion of both the first and secondary portions of the test cycle such that the prostheses complete a physiologically relevant expansion and contraction. 
         [0031]    Monitoring transducers  52  can be inserted for continuous or periodic measurements through sample access valves  34  in the exit chamber  12 . Typically, transducers  52  are used for temperature and pressure monitoring. However, it should be understood that a variety of sensing elements can be inserted in a similar fashion. The working fluid temperature is controlled via the fluid heater and a temperature transducer contained in the heat and circulation chamber  7  shown in  FIG. 1 . Upper and lower temperature bounds are set in the test software. At startup, the system  60  will begin to heat until the upper bound is reached. As the input temperature falls below the lower bound, the heater  7  may again be activated, thus maintaining a mean temperature within acceptable bounds. This mean temperature is typically set to 37° C. to simulate physiologic conditions. Other monitoring transducers  52  may be used to provide feedback to the computer  2  or control system  4  to monitor the status of any number of system variables to provide active control over the system  60 , for example, to vary pump speed or control the stroke of the driver to provide consistent loading on the system  60 . 
         [0032]    Turning to  FIG. 3 , the fluid drive member  10  is mounted to an entrance rotational support  25 . Both, in turn, are affixed to the entrance support structure  15 . The entrance rotational support  25  acts as a rotational bearing surface and allows the entrance chamber  11 , which is affixed to the rotational member  28 , to rotate freely about the central axis without the need for the drive member  10 , linear motor  17 , or motor housing  9  to rotate. The exit chamber  12  structure is supported by and configured to rotate about the central axis on exit chamber support wheels  24  shown in  FIGS. 1 and 2 . The entrance chamber  11  is connected to the rotational member  28  which maintains internal pressure through the entrance seals  27  and is held in place by the entrance retaining clips  26 . 
         [0033]    Fluid enters the fatigue testing device  20  through an inflow port  40  defined in the entrance rotational support  25  and exits the fatigue testing device  20  through the exit flow valve  33  contained in the exit chamber  12  as shown in  FIG. 2 . Alignment of the entrance and exit chambers  11 ,  12  is maintained through the central flow conduit  29 . The central flow conduit  29  may be constructed of a single rigid section or composed of multiple telescoping sections (see  FIG. 8 ) which allow for adjustment of the length of the housing tube  36 . 
         [0034]    In one implementation, the fluid drive member  10  has a flexible diaphragm drive system as illustrated in  FIG. 3 . The diaphragm  44  is housed inside a diaphragm cylinder  45 . A rigid cap  42  clamps the diaphragm  44  to the moveable piston  43  mounted to a linear drive adapter  21  extending from the linear motor  17 , while peripheral edges of the diaphragm  44  are sealed between the entrance rotational support  25  and a flange  56  about one end of the diaphragm cylinder  45 . The diaphragm  44  is preferably a cap-like member constructed of a non-reactive and flexible thin rubber, polymeric or synthetic based material. The flexible diaphragm  44  is highly compliant with low resistance to axial deformation across its entire axial range of motion within the diaphragm cylinder  45  and entrance rotational support  25 . However, alternative configurations of the diaphragm  44  may be employed so long as the configuration is capable of low friction and low resistance to deformation under the influence of the piston  43 . 
         [0035]    Many advantages of a low friction flexible diaphragm  44  as opposed to a rigid metallic bellows or traditional piston and cylinder drive may be appreciated. The lateral surfaces of the diaphragm  44  evert as the piston  43  reciprocates within the diaphragm cylinder  45  and entrance rotational support  25 . This eversion exerts very little resistance to piston  43  movement. These components are affixed to the entrance support structure  15  and maintain the pressure seal along the circumference of the diaphragm  44 . 
         [0036]    The motor support structure along with the linear motor  17  and alignment mechanisms are shown in detail in  FIG. 4 . The linear motor  17 , which in some embodiments is electromagnetic, has a drive shaft  53  that is connected to the linear drive adapter  21 , which may be configured to connect with linear drive shafts  53  of varying diameter. The linear drive adapter  21  is clamped onto the linear drive shaft  53  by the drive shaft clamp  39 . Alignment is maintained through the motor alignment shaft  18  attached to a linear motor support structure  19  at one end and the housing  9  at the other. Rotation about the central axis may be prevented by the anti-rotation mechanism  23 , consisting of a linear guide affixed to the motor support structure  19 . Positional feedback may be provided by a linear encoder  22 , which in one embodiment may be a non-contact, optical type. It should be understood that the linear motor  17  is not restricted to this particular configuration and various drive technologies may be employed with similar effect. 
         [0037]    The fluid drive member  10  may be sized based on the volumetric requirements of the test by use of adaptor manifolds which are affixed to the main housing. Two possible drive member configurations are shown in  FIGS. 5 and 6 .  FIG. 5  shows a large fluid drive member  47 , typically used in conjunction with housing tubes  36  with a diameter of greater than 30 mm.  FIG. 6  depicts an alternate embodiment of a small fluid drive member  46  coupled with an adapter manifold  41  to the entrance rotational support  25 . This drive size is typically used in conjunction with housing tubes  36  with an internal diameter of 30 mm or less. It should be understood that the fluid drive member  10  is not restricted to these particular configurations and that any necessary volumetric displacement can be easily achieved. 
         [0038]    The optical micrometer system  13  is illustrated in  FIG. 7 . The optical micrometer  48 , which in one embodiment may be a high accuracy LED or laser type, is affixed to an optical micrometer support rail  49 . The optical micrometer support rail  49  is joined to a precision slide  51 . The precision slide  51  provides a structure for accurately and repeatedly positioning the optical micrometer  48 . The precision slide  51  is affixed to the optical micrometer base  50 . The optical micrometer base  50  is keyed to provide an accurate reference point when connected to an exit support structure reference datum  35  shown in  FIG. 2 . The optical micrometer  48  may thereby be used to inspect the prostheses  30  as they are placed under pressure in the fatigue-testing device  20 . The optical micrometer  48  may be used to measure expansion and contraction sizes of the prostheses  30  along their lengths. The fatigue-testing device  20  may be rotated on the entrance and exit support structures  15 ,  16  during a test run to place each of the prostheses  30  being tested within the scanning range of the optical micrometer  48 . 
         [0039]    An alternate embodiment fatigue-testing device  70  of a fatigue-testing system is shown in  FIGS. 8 and 9  along with an alternate embodiment of a drive system  71 . The fatigue testing device  70  is composed of a pressurizable fluid housing  61  formed as a disk-shaped manifold or entrance chamber  11  and a disk-shaped manifold or exit chamber  64  connected by a cylindrical, telescoping central flow conduit  62 . The entrance and exit chambers  11 ,  64  are supported, respectively, by an entrance support structure  15  and an exit support structure  16 . The support structures  15 ,  16  are affixed to a base plate  8 . 
         [0040]    A plurality of contoured tubes  73  (e.g., curved or bent, either regularly or irregularly), or other prosthesis-housing structures, or the prostheses themselves, extend between and are in fluid communication with the entrance chamber  11  and the exit chamber  64 . The tubes  73  are arranged circumferentially around and spaced apart from the central flow conduit  62 . A plurality of connection adapters  72  corresponding to respective contoured tubes  73  fit within a plurality of apertures  55  on opposing faces of the entrance chamber  11  and the exit chamber  64  for attachment of the tubes  73  in fluid communication with the entrance chamber  11  and exit chamber  64 . In this exemplary embodiment, the tubes  73  are U-shaped in order to meet FDA requirements for testing of certain types of prostheses (e.g., coronary stents). In order to accommodate the U-shaped tubes  73 , the connection adapters  72  may be formed as angled connectors with various bend angles. In an alternate implementation for the testing of tubular prosthesis devices that are formed of materials that remain substantially nonporous under the pressure induced by the fatigue-testing system, the prosthesis devices may be directly attached to the connection adapters  72  to be placed in fluid communication with the entrance chamber  11  and the exit chamber  64 . 
         [0041]    In the exemplary implementation of  FIGS. 8 and 9 , the central flow conduit  62  is telescopically formed of an entrance half  68  connected to the entrance chamber  11  and an exit half  69  connected to the exit chamber  64 . As shown, the exit half  69  is configured with an outer diameter slightly smaller than the inner diameter of the entrance half  68 , thereby allowing the exit half  69  to be received within the lumen of the entrance half  68 . It should be apparent that in an alternate embodiment, the entrance half  68  could be sized and configured to be received within the exit half  69 . The interface between the entrance half  68  and the exit half  69  forms a seal to prevent fluid leakage from the central flow conduit. The fluid seal may be provided by O-rings or other seal structures (not shown) disposed between the entrance half  68  and the exit half  69 . The telescoping central flow conduit  62  is thus able to move axially during system setup, allowing different testing lengths of prostheses to be easily configured. 
         [0042]    The alternate embodiment of the central flow conduit  62  shown in  FIG. 9  has a compliance flow control membrane  63  disposed therein. The flow control membrane  63  separates the central flow conduit  62  into two portions, which allows energy to pass through the central flow conduit  62 , but blocks the passage of fluid. This controls the circulatory flow of the system ensuring an even temperature distribution throughout the test system. It should be apparent that the flow control membrane  63  may be provided in either a telescoping or fixed-length central flow conduit design. As noted in  FIG. 9 , the flow control membrane  63  is preferably mounted within the inner portion of the telescoping central fluid conduit  62 . 
         [0043]    An alternate embodiment of the exit chamber  64  is also shown in  FIG. 9 . In this embodiment, the exit chamber  64  is provided with a primary manifold  74  that is in direct fluid communication with the central flow conduit  62  and a backchannel  75  that is separated from the primary manifold by a wall  76 . The backchannel  75  is in fluid communication with the apertures  55  at which the sample adapters  72  and sample access valves  34 . An additional set of manifold plugs  77  may be provided directly in line with the backchannel  75  adjacent to the manifold plugs  34  that provide access to the primary manifold  74  at each aperture  55 . The backchannel  75  provides an additional flow channel in the exit chamber  64  to provide greater mixing of the fluid between the sample adapters  72  and access valves  34  to provide for more uniform temperature distribution. Again, it should be apparent that the backchannel  75  can be provided on either a telescoping or fixed-length pressurizable fluid housing design. 
         [0044]    An alternate drive system  71  is also shown in  FIGS. 8 and 9 . In this exemplary embodiment, a shaft  66  of a rotary motor  65  (e.g., a servo or brushed motor) is coupled to a linkage system  67 , in this case a crank and slider mechanism, that is further coupled to the linear drive adapter  21 . In this manner, rotational motion from the rotary motor is translated to linear motion in order to drive the diaphragm inside the fluid drive member. Other types of motors with appropriate linkage systems may also be used to drive the fatigue-testing systems disclosed herein. 
         [0045]    Embodiments of the fatigue-testing system disclosed herein are capable of simulating physiologic conditions on a prosthesis at an accelerated rate. This accelerated rate may be achieved through a combination of one or more of a variety of features. For example, the use of a low-inertia, flexible diaphragm drive system reduces burden on the motor allowing for more frequent cycling. The uniform pressure field provided across the sample housing by connecting the entrance and exit manifolds through the central flow conduit helps maintain consistent conditions across multiple prostheses simultaneously tested. Further, by providing an automated test interface capable of running without direct management, proper testing conditions and safety mechanisms are ensured over the course of the testing cycle. 
         [0046]    The fatigue-testing system  60  is also flexible and capable of testing various prosthesis sizes and configurations. The design of the fluid housing with a central flow conduit allows for equal pressure assertion on prostheses from both ends while only needing a single driver on one side. Further, because the fatigue-testing device  20  is capable of rotation about a central axis by means of a stationary drive member and a rotary seal system, it allows for accurate external diameter measurements of the prostheses in the housing tubes at high frequency. An accurate reference feature for measurement of the prostheses by the optical measurement device  13  also aids in the efficiency of the system. 
         [0047]    While the present invention has been described with reference to the particular embodiments set forth above, it will be understood that variations, such as those in construction, configuration, dimension, material selection and assembly, may be employed without departing from the spirit and scope of the present invention. All directional references (e.g., proximal, distal, upper, lower, upward, downward, left, entrance, exit, right, lateral, longitudinal, front, back, top, bottom, above, below, vertical, horizontal, radial, axial, clockwise, and counterclockwise) are only used for identification purposes to aid the reader&#39;s understanding of the present invention, and do not create limitations, particularly as to the position, orientation, or use of the invention. Connection references (e.g., attached, coupled, connected, and joined) are to be construed broadly and may include intermediate members between a collection of elements and relative movement between elements unless otherwise indicated. As such, connection references do not necessarily infer that two elements are directly connected and in fixed relation to each other. The exemplary drawings are for purposes of illustration only and the dimensions, positions, order and relative sizes reflected in the drawings attached hereto may vary. 
         [0048]    The above specification, examples and data provide a complete description of the structure and use of exemplary embodiments of the invention. Although various embodiments of the invention have been described above with a certain degree of particularity, or with reference to one or more individual embodiments, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the spirit or scope of this invention. Other embodiments are therefore contemplated. It is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative only of particular embodiments and not limiting. Changes in detail or structure may be made without departing from the basic elements of the invention as defined in the following claims.