Patent Publication Number: US-7913573-B2

Title: Orthopedic simulator with a multi-axis slide table assembly

Description:
RELATED APPLICATIONS 
     The present application claims priority to Provisional Patent Application 60/760,595 filed Jan. 20, 2006, U.S. patent application Ser. No. 11/332,407, filed Jan. 13, 2006 and U.S. patent application Ser. No. 11/335,974 filed Jan. 20, 2006 the contents of which are incorporated herein, by reference, in their entirety. This application is a continuation-in-part of U.S. patent application Ser. No. 11/332,407, filed Jan. 13, 2006 now U.S. Pat. No. 7,617,744 and continuation-in-part of U.S. patent application Ser. No. 11/335,974 filed Jan. 20, 2006 now U.S. Pat. No. 7,654,150. 
    
    
     FIELD 
     There is an ever increasing need for testing of orthopedic devices. Such testing may be required for certification of the devices. For example, wear testing of spinal implants are subject to ISO and ASTM standards. In the example of a spinal wear implant, the test procedure defines the relative angular movement between articulating components, and specifies the pattern of the applied force, speed and duration of testing, sample configuration and test environment to be used for the wear testing of total intervertebral spinal disk prostheses. While the test method focuses on wear testing, additional mechanical tests such as fatigue testing and others can be required. 
     Spinal implants are only one type of orthopedic device. Others include, for example, hip-joint prostheses, knee-joints, etc. Such devices also need to be tested. For a spinal implant, for example, the testing may be a wear test in which the spinal implant is subjected to forces and loads that are repeated over thousands of cycles. The application of certain motions and forces to a test specimen may involve the use of an “x-y slide assembly” that operates within an orthopedic simulator as a translational assembly. Forces and motions in the “x” and “y” directions, e.g., anterior/posterior and lateral translation motions, may take place in such a slide assembly. 
     Previous assemblies have employed ball bearings in the slide design, which leads to fretting and skidding when translating. Such fretting and skidding can cause inconsistency between test stations in a simulator with multiple test stations. 
     SUMMARY 
     There is a need for an orthopedic simulator that reduces or eliminates fretting and skidding corrosion, and can operate in multiple modes of operation, such as a self-centering free-floating mode, a positive axis lock mode, and a simultaneous shear plane loading mode. 
     The above stated needs and others are met by embodiments of the present invention which provide an orthopedic simulator comprising a test station configured to hold a test specimen, and a multi-axis slide table on which the test station is mounted for multi-axis movement. The multi-axis slide table includes a bas, a lower translation plate and an upper translation plate. The lower translation plate is mounted to the base by a first linear slide and rail arrangement for movement along a first axis. The upper translation plate is mounted to the lower translation plate by a second linear slide and rail arrangement for movement along a second axis. 
     The earlier stated needs and others are met by embodiments of the present invention which provide a slide table assembly comprising a base, a first ball-bearing-less linear slide and rail arrangement coupled to the base, and a first translation plate coupled to the base via the first linear slide and rail arrangement for movement relative to the base along a first axis. A second ball-bearing-less linear slide and rail arrangement is coupled to the first translation plate. A second translation plate is coupled to the first translation plate via the second linear slide and rail arrangement for movement relative to the first translation plate along a second axis, different than the first axis. 
     The foregoing and other features, aspects and advantages of the disclosed embodiments will become more apparent from the following detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front, perspective view of an orthopedic simulator in accordance with certain embodiments of the invention, with an external housing removed for illustrative purposes, and with forces being schematically depicted. 
         FIG. 2   a  is a top view of the orthopedic simulator of  FIG. 1 ;  FIG. 2   b  is a front view;  FIG. 2   c  is a bottom view and  FIG. 2   d  is a side view. 
         FIG. 3  is a view similar to  FIG. 1 , illustrating the removability of a specimen containment module. 
         FIG. 4  depicts an exemplary embodiment of an assembled specimen containment module. 
         FIG. 5  is an exploded view of the specimen containment module of  FIG. 4 . 
         FIG. 6  is a side, partially cross-sectional view of the specimen containment module of  FIG. 4 . 
         FIG. 7  is a top view of a base of the specimen containment module of  FIG. 4 . 
         FIG. 8  is a schematic depiction of an embodiment of a circulation loop for circulating a temperature control fluid in a temperature control circuit. 
         FIG. 8   a  is a schematic depiction of a temperature control arrangement for circulating temperature control fluid in accordance with another embodiment of the present invention. 
         FIG. 9  depicts two test stations, with one test station having a specimen containment module releasably attached thereto. 
         FIG. 10  schematically depicts an exemplary arrangement for circulating bath fluid. 
         FIG. 11  depicts an embodiment of a specimen containment module in an installed position. 
         FIG. 12  is a perspective view of the orthopedic simulator of  FIG. 1 , with an indication of the flexion and extension motion. 
         FIG. 13  is a cross-sectional view of a portion of a flexion/extension motion linkage in accordance with embodiments of the invention. 
         FIG. 14  is a perspective view of the orthopedic simulator of  FIG. 1 , with an indication of the lateral bending motion around an axis of rotation. 
         FIG. 15  is a rear perspective view of the orthopedic simulator of  FIG. 1 . 
         FIG. 16  is a perspective view of the orthopedic simulator of  FIG. 1 , with an indication of anterior/posterior and lateral translation motions. 
         FIG. 17  depicts a portion of an x-y slide assembly in accordance with embodiments of the present invention. 
         FIG. 18  is a perspective view of the x-y slide assembly in accordance with embodiments of the present invention. 
         FIG. 19  is an exploded view of the x-y slide assembly of  FIG. 18 . 
         FIG. 20  is a perspective view of the orthopedic simulator of  FIG. 1 , with an indication of loading in a vertical direction. 
         FIG. 21  is a perspective view of an embodiment of an actuator in isolation. 
         FIG. 22  is a top view of the actuator of  FIG. 21   
         FIG. 23  is a side view of the actuator of  FIG. 21 . 
         FIG. 24  is a cross-sectional view of the actuator of  FIG. 21 . 
         FIG. 25  is a perspective view of the orthopedic simulator of  FIG. 1 , with an indication of the axial rotation linkage and a moment provided at a test specimen. 
         FIG. 26  is a rear perspective view of the orthopedic simulator of  FIG. 1 , illustrating an embodiment of a central manifold in accordance with embodiments of the present invention. 
         FIGS. 27-29  schematically depict different approaches to linkages. 
         FIG. 30  schematically depicts a nesting order of forces in accordance with embodiments of the present invention. 
         FIG. 31  shows the required forces for application to a test specimen intended for a lumbar region according to an exemplary set of curves. 
         FIG. 32  shows the same information as  FIG. 31 , but for cervical data. 
         FIG. 33  shows curves for non-sinusoidal input data in accordance with exemplary embodiments of the invention. 
         FIG. 34  depicts the orthopedic simulator within a housing. 
     
    
    
     DETAILED DESCRIPTION 
     The orthopedic simulator of the present invention may be employed, for example, as a spinal implant wear test machine. In such a configuration, the intent of ISO 18192 is satisfied. The orthopedic simulator is designed for accuracy as well as increased speed. 
     In the following description, it is assumed that the orthopedic simulator is a spinal implant wear test machine, but it should be apparent to those of ordinary skill in the art that this is exemplary only. The features, concepts and designs depicted in the following figures and description may be employed in other types of machines and orthopedic simulators. 
     The embodiments of the present invention address and solve problems related to the translation of a test specimen in an orthopedic simulator and more particularly to fretting and skidding corrosion present in previous translation assemblies, and the provision of multiple operational modes with a single slide table assembly. The embodiments of the invention solve these problems, at least in part, by providing an orthopedic simulator with a test station configured to hold a test specimen, and a multi-axis slide table on which the test station is mounted for multi-axis movement. The multi-axis slide table includes a base, a lower translation plate mounted to the base by a first linear slide and rail arrangement for movement along a first axis and an upper translation plate mounted to the lower translation plate by a second linear slide and rail arrangement for movement along a second axis. 
       FIG. 1  depicts an orthopedic simulator  10  for testing of test specimens of orthopedic devices. The orthopedic simulator  10  has a plurality of test stations  12 . In the illustrated embodiment, there are six test stations  12  in which specimens are subjected to the forces applied by the machine  10 , and a control station  14  that holds a specimen that is not subjected to all of the forces provided at the other test stations  12 . 
     The orthopedic simulator  10  is able to provide forces Fx, Fy, and Fz in the x, y and z directions as depicted in  FIG. 1 , shown with the x, y and z axes at one of the test stations  12 . Additionally, torques may be applied around the x, y and z axes, as depicted. The test specimen is not shown in  FIG. 1  so as not to obscure the present invention. In the spinal implant wear testing machine according to certain embodiments of the invention, a specimen containment module is provided that contains fluids in which the test specimen is immersed. Upper and lower adapters  18  (only seen clearly at one of the test stations  12  in which the specimen chamber is removed for illustrative purposes) hold the test specimens between them within the specimen containment module  16 . 
     A linkage  20  provides forces in the x direction with the linkage  22  providing forces in the y direction. Gimbals  24  are connected to the upper adapters  18  and may be moved around the y axis and around the x axis to provide moments around the x and y axes. 
     Vertical loads, represented by forces along the z axis, are provided by vertical load actuators  26 , as shown in  FIG. 1 . Although different types of actuators may be employed, a friction-free axial actuator is preferable to provide for a friction-free axial/torsion actuation system. The vertical load actuator  26  applies a vertical loading along the z axis through components  28  to the test specimen via the lower adapter  18 . In the illustrated embodiment, which will be described in more detail later, the components  28  include an x-y slide table and a load cell. 
     In is desirable to provide a transmission of drive torque with little deflection related error, having high torsional stiffness. At the same time, low axial stiffness is desirable so that there is little cross-talk onto the vertical loading and so the cross-talk is not seen at the load cell. An axial rotation linkage  30  is coupled to the vertical load actuator  26 . The motion of the axial rotation linkage  30  is around the vertical axis z, as depicted in  FIG. 1 . Although the axial rotation linkage  30  is depicted at the bottom of  FIG. 1 , it should be apparent to those of skill in the art that the structure depicted in  FIG. 1  is suspended vertically so that the axial rotation linkages  30  are free to rotate. This will become more apparent in later-described figures. 
       FIGS. 2   a - 2   d  depict alternate views of the orthopedic simulator  10 .  FIG. 2   a  is a top view which best shows the arrangement of the linkages  20  with the gimbals  24 . A crosshead  32  is provided, which may also best be seen in  FIG. 2   d .  FIG. 2   a  is a top view, while  FIG. 2   b  is a front view,  FIG. 2   c  is a bottom view and  FIG. 2   d  is a side view. 
       FIG. 3  depicts a perspective view of the orthopedic simulator of  FIG. 1 , with a specimen containment module  16  that is remote from the orthopedic simulator  10 . The specimen containment modules  16  are releasably attachable to the test station  12 . The releasable attachment feature of each of the specimen containment modules  16  enables bench top preparation work on the test specimen to be performed remotely from the environment of the orthopedic simulator  30 . This remote loading and preparation capability allows for careful removal and insertion of delicate test specimens. Further, the mounting of one-piece specimens is facilitated with this arrangement. An important consideration is the reduction in the contamination potential created by remotely mounting a specimen within the specimen containment module. The specimen containment module  16  also contains adapters  18  that are designed for flexibility, ease of manufacturing and low cost. 
     An exemplary embodiment of a specimen containment module  16  is shown in isolation in  FIG. 4 , and in exploded view in  FIG. 5 . The specimen containment module contains a base  34  and upper connector  37  that interface to a test station  12  and at which the specimen containment module  16  is releasably attached to the orthopedic simulator  10 . A chamber  36 , when inserted into the moat  38  in the base  34 , forms a fluid container with the base  34 . A test specimen  40  is depicted with a lower portion  40   a  and an upper portion  40   b . However, certain test specimens may also be one-piece specimens. 
     Releasable fasteners  42 , such as thumb screws, may be employed to releasably attach the specimen containment module  16  to the orthopedic simulator  10 . Fluid connections  44  are used to provide fluid as will be described in more detail in the following figures. 
       FIG. 6  is a side, partially cross-sectional view of the specimen containment module  16  of  FIGS. 4 and 5 . The test specimen  40  is shown with the upper and lower portions coupled together, as seen in  FIG. 6 . 
       FIG. 7  is a top view of the base  34 . A specimen mounting platform  46  is provided which includes two pins  48 , with one pin piloting and another pin interacting with a slot in the lower adapter  18   a  for anti-rotation purposes. Screw holes  50  are depicted and may be employed to provide a specimen hold down function. 
     The base  34  also includes a recess  52  that is able to interact with a pin  54  on the orthopedic simulator  10 . This provides a slidable installation of the specimen containment module  16 . A tubing loop  56  is provided within the base to provide a temperature control of the bath in which the test specimen  40  is immersed. As will be described in more detail, a temperature control fluid is circulated through the tubing loop  56  to precisely control the temperature of the bath. The temperature control fluid does not intermix with the bath fluid. A temperature probe  60  provides feedback on the temperature of the bath and can be used to control the temperature control fluid. The signal from the temperature probe  60  is provided as a feedback signal to a heather (not shown in  FIG. 7 ). 
     Recesses  58  provide for thumb screws or other releasable fasteners to secure the specimen containment module  16  to the orthopedic simulator  10 . Bath fluid circulation tubes  62  are used to circulate bath fluid within the fluid container formed by the base  34  and the chamber  36 , as will be described in more detail later with respect to  FIG. 9 . 
       FIG. 8  depicts a circulation loop for circulating the temperature control fluid in the temperature control circuit. The temperature control fluid is circulated in each of the specimen containment modules  16  through the tubing loops  56 , seen in  FIG. 7 . The tubing loops  56  are connected to a single circulation loop  64  that circulates a temperature control fluid, such as water, through the closed loop system. Although water is an exemplary temperature control fluid, other fluids may be employed as a temperature control medium in different embodiments. The tempered water is circulated through the heat exchangers in each of the baths of the specimen containment modules  16 . A heater  66  provides a precise control and circulation of the tempered water. The heater  66  receives temperature signals from the temperature probes  60  and employs this information to control the temperature of the temperature control fluid, and hence, the bath in each of the specimen containment modules  16 . 
     The daisy-chained approach depicted in  FIG. 8  produces a very stable temperature in each of the baths at the specimen containment modules  16 . In addition to stability, a consistency of temperature from station  12  to station  12  is achieved since the entire circulation loop  64  reaches a stabilized temperature. Also, a single heater may be employed, reducing costs, 
     In certain embodiments, such as depicted in  FIG. 8   a , each of the baths of the specimen containment modules  16  may be individually controlled with separate circulation loops  65  for each bath. Each circulation loop  65  has its own heater  66 , which receives temperature feedback signals from the respective temperature probe  60 . However, the embodiment depicted in  FIG. 8  is preferred. The arrangement of  FIG. 8   a , like that of  FIG. 8 , also has the advantage over electric heating elements or other types of heating, in preventing overtemperature related fluid degradation. 
       FIG. 9  depicts two test stations  12 , one of which has a specimen containment module  16  releasably attached thereto. A non-contact level sensor  68 , such as those known in the sensing art, are provided on posts  70  near the chamber  36 . The height of the non-contact level sensor  68  may be adjusted along the pillar  70  in the direction of arrow  72 . This allows the desired fluid height within the chamber  36  to be precisely adjusted. The non-contact level sensor  68  provides its signals to a fill controller  74 , schematically indicated as being connected to a non-contact level sensor  68 . The fill control  74 , based upon the signals received from the non-contact level sensors  68 , determines whether the fluid in the specimen containment module  16  needs to be replenished. The test fluid, such as bovine fluid, for example, may evaporate to some extent, thereby increasing the concentration of the fluid. Distilled water is furnished (through a fill tube, not shown) under the control of the fill control  74 . 
     An arrangement for the circulation of the bath fluid is depicted in  FIG. 10 . Unlike the temperature control fluid, individual loops are preferred in order to maintain each test specimen and bath in its own environment. In other words, cross-contamination of wear particles is avoided by providing the individual loops for each specimen module. In certain embodiments, peristaltic pumps  76  are employed for each of the individual loops. A stirring action is provided. 
       FIG. 11  shows a specimen containment module  16  (without the chamber  36  for illustrative purposes) in an installed position within the orthopedic simulator  10 . The specimen containment module is releasably attached at its base  34  to a load cell module  78 . The load cell module is designed to accommodate either a single or multi-axis force transducer. In the illustrated embodiment, a single axis transducer is depicted. 
       FIG. 12  depicts the orthopedic simulator  10  and exemplifies the flexion/extension motion. The linear actuator  20   a  of the linkage  20  extends back and forth in an axial manner, causing the connecting link  20   b  to translate in an axial direction. This causes the inner gimbals  24  at the test stations  12  to move and rotate around an axis of rotation depicted in  FIG. 12 . 
     Although not shown, the connecting link  20   b  and connections to the inner gimbals  24  employ high quality bearings, such as long life needle bearings used at key points. The design insures a long life and low lash, creating an accurate machine for a long term use. The low moving mass linkage depicted maximizes performance and is designed for ease of maintenance. 
       FIG. 13  depicts a cross-sectional portion of the flexion/extension motion linkage. The inner gimbal  24  is depicted as being connected to the upper specimen adapter  18   b . A stationary bearing housing  80  houses the needle bearings mentioned before. A radial needle bearing  84  is provided, as well as a needle roller thrust bearing  82 , which are provided in two places. A tubular shaft  86  permits rotation of the gimbals  24 . 
     A lateral bending motion around the axis of rotation is depicted in  FIG. 14 . A moving cross-head  32  (also seen in  FIGS. 2   a - 2   d ) is coupled via a connecting link  88  that is moved by linear actuator  90  in an up-and-down motion. This causes the inner gimbals  24  to be pivoted around the axis of rotation. 
     A rear view of the orthopedic simulator  10  is provided in  FIG. 15 . The moving cross-head  32  is shown as extending across the orthopedic simulator  10 . Also shown in this figure is a central manifold  92 , which will be discussed in more detail later. As with the flexion/extension linkages, it is preferred to use long life needle bearings that are of high quality at the key points in the lateral bending motion linkages. These designs ensure long life and low lash, creating an accurate machine for long term use. The low moving mass crosshead assembly maximizes performance. For example, the crosshead assembly  32  may be made of aluminum to provide a very light weight moving mass. In motion, the moving crosshead  32  pivots around the x-axis depicted in  FIG. 15 . 
       FIG. 16  shows the orthopedic simulator and depicts the anterior/posterior and lateral translation motions. A translation stage  96  is illustrated in this drawing. The translation stage includes an x-y slide assembly as will be see in the following figures. 
       FIG. 17  depicts a portion of the x-y slide assembly  100  that shows linear slides  102  with spaces  104  being provided for springs  107  that produce a biasing force if desired. The springs  107  can be placed between the translation plates  110 ,  112  themselves, and between the translation plate  110  and certain lock screw posts  105  of the base  114 , as will be better appreciated in  FIG. 18 . The configuration of the x-y slide assembly  100  with springs  107  places the x-y slide assembly  100  into a shear plane loading operational mode. 
     In other embodiments, the passive control provided by the springs is replaced with an active control by appropriately placed electric, pneumatic or servo-hydraulic actuators (not depicted). For example, such actuators may be provided in the spaces  104 . The actuators are controlled in force and/or displacement via an external control system, such as the controller  200 . One of the operating modes that is available with such embodiments is shear force control, and another operating mode is shear displacement control. 
     In the mechanism of the present invention, the Fx and Fy motions and forces take place in the x-y slide assembly  200 , when in the spring-loaded configuration described above. An adjustment system allows an operator to set the amount of force in each of the x and y axes. This is not a controlled degree of freedom, but rather, there is free translation if an external force overcomes the spring setting. For spinal implants that are simple ball-in-socket joints located coincident with the Mx, My and Mz centers of the machine, the spring is not engaged. However, some specimens would generate crosstalk loading into the Fx/Fy or Fz axes. This spring constraining force allows a user to simulate the soft tissue surrounding a specimen, or intentionally sideload an implant to simulate mis-implantation. In other embodiments, discussed above, the passive control provided by the springs  107  is replaced by active control through the use of electric, pneumatic or servo-hydraulic actuators. 
     A first lock screw post  103  on the base  114  receives an adjustable lock screw  101  that is adjusted to interact with the lower translation plate  110 . The lock screw  101  allows the x-y slide assembly  100  to be placed in an operational mode that provides for infinite positive axis locking of the slide assembly  100  within a dynamic range. The second lock screw posts  105  also permit locking screws to be received that will interact with the upper translation plate  112  to provide positive axis locking along a different axis from that provided by the lock screw  101 . 
       FIG. 18  is a perspective view of the x-y slide table  100  constructed in accordance with embodiments of the present invention.  FIG. 19  shows the x-y slide table  100  in an exploded view. The x-y slide assembly  100  forms a very compact package, with a very light weight assembly. There is a high torsion and shear capability of assembly with high axial dynamic load ratings for each x-y slide assembly  100 . Each slide assembly  100  also has high moment load ratings, due to its efficient design. There is an ultra-low coefficient of linear static and dynamic friction provided by the design. Double-row/side miniature roller bearings reduce or eliminate fretting corrosion. Grease may be provided to assist in the elimination of fretting corrosion and further reduce the coefficient of friction and the start up “stiction.” 
     The x-y slide assembly  100  of the present invention may incorporate a number of different modes of operation. These include free-floating to self-center a specimen; a positive axis lock within dynamic range; an ability to produce a large amount of static shear force, on each axis, for simultaneous shear plane loading of specimens; a shear force control and a shear displacement control. The x-axis translation plate has a built-in capability to align the upper specimen tooling and the load cell radially. 
     The x-y slide assembly  100  of the present invention overcomes particular concerns. For example, other such assemblies in orthopedic simulators used ball bearings in the slide design which lend themselves to fretting and skidding when translating. Other advantages of the present invention include the production of simultaneous transverse shear in a compact design, while producing friction-free stage floating, but yet is infinitely lockable within a dynamic range. The lowest inertia assembly for Mz rotation is produced, at all six test stations  12 . The design of the x-y slide assembly  100  can withstand a large amount of lbsF in compression. Further, the x-y slide assembly  100  is a translation assembly that can be easily removed from the Fz actuator  26 . It also provides a translation assembly that has over-turning moment capability to react moments caused by side loads that are off-centered loading. 
     The x-y slide assembly  100  is shown in  FIG. 18  in a free-floating configuration, as springs  107  are not provided in the spaces  104 , and adjustable lock screws are not provided in the first lock screw post  103  or the second lock screw posts  105 . The x-y slide assembly  100  includes the lower translation plate  110  and the upper translation plate  112 . In certain embodiments, the lower translation plate  110  translates along the x-axis while the upper translation plate  112  translates along the y axis. The base  114  supports the x-y slide assembly  100  and may be mounted on the load cell depicted earlier. Pins  116  are provided and pressed into base  114  and lower translation plate  110 . The pins  116  aid in assembly of the first mounted slide/rail at each axis and ensures squareness of the first rail to the first lock screw post  103 , and establish orthogonality between axis platforms, within the limits of the small screw clearances. Screws  118  are provided, as well as pin dowels  120 . Linear rail bearings  122  are provided for linear rails  124 . 
       FIG. 20  depicts the orthopedic simulator  10  and illustrates the loading in the z direction that is provided in the direction of arrows  128  by the vertical load actuator  26 . The integral actuator  26  is integral in nature and may be a precision, seal-less actuator design in certain preferred embodiments. The piston rod is floated on an oil film, and the near zero friction maximizes the load accuracy. A low mass rod may be employed to maximize the performance of axial rotation and vertical load channels. The individual test stations  12  have their own on-off valves. A perspective view of an actuator  26  in isolation is provided in  FIG. 21 . A top view of the actuator  26  is depicted in  FIG. 22  and a side view of the actuator  26  is depicted in  FIG. 23 . A cross-sectional view of the actuator  26  is depicted in  FIG. 24 , with an enlargement of a portion from  FIG. 24  shown in  FIG. 24   a.    
     In certain preferred embodiments, each actuator  26  has a handle  130  on the outside of the actuator  26  that operates a built-in hydraulic valve that allows a user to shut off any station individually. Hence, if a user desires to operate with fewer than six test specimens, or a specimen fails midway through the testing process and it is therefore desirable to remove that specimen from the remainder of the test cycles, the individual test station  12  may be turned off separately from the other test stations  12  without stopping the operation of the machine  10  and the testing of the other specimens. As best seen in  FIG. 24 , the actuator  26  includes a piston  132  that may be moved axially and rotated. The hydraulic actuator  26  includes a bottom end cap  134  and a top end cap  136 . The hydrostatic bearings  138  and  140  are provided. Thrust bearings  142  provide support for a test station  12  when the device is shut off. In such a case, a test station can be removed and the machine operated without the non-operation test station  12  influencing the other test stations  12 . 
     Pressure to extend the piston  132  along the z-axis is provided at port  144 , while pressure to retract the piston  132  is provided at port  146 . 
     The hydraulic pressure in return ports  144 ,  146  are connected to and fed from the central manifold  92  (see  FIG. 15 ) in preferred embodiments. The hydraulic actuator  26  is hydrostatic and is completely without seals, including high-pressure piston seals. The hydrostatic bearings “float” the piston rod and also provide some over-turning moment capabilities. The unique design produces an actuator without seal drag (as in a typical hydraulic actuator), resulting in a device that has extremely low linear and torsional friction. The only friction is the friction that is produced from viscous oil shear. With this design, an equal Fz force is provided across all seven actuators. 
     Thrust bearings are provided in the end of each end cap  134 ,  136 . The upper end cap  136  has thrust bearings lubricated by a blow-by actuator rod oil leakage. If one specimen should fail before others, an operator can turn off the station  12 . The actuator  26  retracts and the assembly will ride on the thrust bearings for a continued Mz motion. The Mz motion is common for all six Fz actuators  26  at the six test stations  12 . The seventh test station  14 , which operates as a load and soak station for control purposes, is not connected to the Mz drive apparatus. 
     The central manifold  92 , depicted, for example, in  FIG. 26 , provides an integral manifold for multiple connections and fluid tubing for the orthopedic simulator. The use of a central integral manifold greatly reduces plumbing, provides a performance improvement since there is a greater balancing of fluid and less plumbing is required, a size reduction, a cost reduction and also serves as a structural element. In other words, the central manifold  92  provides a strong cross-brace for the orthopedic simulator  10 . Examples of the plumbing include providing the fluid to the extension and retraction fluid connections of the vertical load actuators  26 . The central manifold  92  also provides for lubrication fluid circulation. 
       FIG. 25  shows the orthopedic simulator  10  and highlights the axial rotation linkage  30  originally shown in  FIG. 1 . The axial rotation linkage  30  provides a moment Mz at the test specimen. Referring now to  FIG. 26 , which shows a rear view of the orthopedic simulator  10 , a linear actuator  150 , via connecting link  152 , provides the driving force that causes the axial rotation linkages  30  to rotate around the z-axis. 
     It is desirable to provide a transmission of drive torque with little deflection related error, having high torsional stiffness. At the same time, low axial stiffness is desirable so that there is little cross-talk onto the vertical loading end and so that cross-talk is not seen at the load cell. The axial rotation linkage includes a rotational transfer link  154  that is coupled to the connecting link  152 . Movement of the connecting link  152  in a linear fashion causes the rotational transfer link  154  to freely rotate on bearings around the z-axis. A flexure assembly  156  that is torsionally stiff but axially compliant is coupled to the bottom of the piston  132  of the vertical load actuator  26 . The flexure assembly is torsionally stiff so as to rigidly transfer torque between the rotational transfer link  154  and the piston  132  of the actuator  26 . A friction free axial/torsion actuation is provided by the combination of the actual rotation linkage  30  and the friction-free vertical force actuator  26 . In operation, the vertical load actuator  26  applies a load to the test specimen  40  along the z-axis by moving the piston  132  along the z-axis. Driven by linear actuator  150  through the connecting link  152 , the rotational transfer link  154  and the flexure assembly  156  facilitate rigid torque transfer to the piston  132  to the test specimen (not shown) at the test station  12 . The piston  132  is allowed to translate along its axis freely due to the high axial compliance provided by the flexure assembly  56  of the axial rotation linkage  30 . 
       FIGS. 27-29  depict linkage approaches and highlight the differences between embodiments of the present invention and alternate linkage approaches which provide greater joint serialization error. In  FIG. 27 , a common sublinkage is provided for the flexion/extension (My) and axial rotation (Mz) to thereby create the fewest common number of joints between each specimen, between the displacement measuring device and each specimen, and between the drive actuator and each specimen. In this manner, variability is minimized. The approach provided in the present invention is depicted in  FIG. 27 . As can be seen, the solid cross-piece  160  provides force to all the linkages  162  at once, from the actuation mechanism  164 . By contrast,  FIG. 28  employs three separate connecting bars  166  which are connected by two links  168 . Hence, those test specimens at the left side of  FIG. 28  have a larger number of joints (8) than the number of joints (4) for the left-most specimen in  FIG. 27 . This increases the variability in the forces and motions applied to the test specimens from test station  12  to test station  12 . A similar variability is provided in  FIG. 29 , in which a large number of joints are provided for the various test stations, with each test station having a different number of joints. Hence, the arrangement of the present invention reduces variability in force and motion application from station  12  to station  12 . 
       FIG. 30  schematically depicts the nesting order of forces in accordance with embodiments of the present invention. This nesting order of forces is achieved by the arrangement of the linkages as depicted in the figures throughout this application. 
     The mechanism system generates relative motions and forces between the lower (inferior) and upper (superior) portions of orthopedic devices, such as multiple intervertebral disc implants, simultaneously to generate wear on the artificial bearing surfaces over similar motion and force induced degradation with time. The mechanism applies these motions and forces in such a way as to maximize the accuracy, test speed and durability of the linkage. The full six degree of freedom linkage system is nested as shown in  FIG. 30  to maximize performance and accuracy. Typical spinal implant tests in conventional systems require higher displacements in the flexion/extension direction (My), as compared to the lateral bending (Mx) and axial rotation (Mz) rotations. These motions are often performed at a common or similar frequency and wave shapes. Therefore, the flexion/extension motion represents the most demanding performance. The mechanism system of the present invention is nested, however, so as to place the sub-mechanism with the highest required performance closest to the specimen. This thereby minimizes the moving mass and any related inertial induced error. Hence, as seen in  FIG. 30 , the schematically induced specimen is indicated by reference numeral  170 . The closest sub-mechanism to the superior (upper) portion of the test specimen  170  is the flexion/extension (My). The lateral bending (Mx) is further from the superior portion of the specimen  170 , as indicated by  FIG. 30 . Finally, the drive for the Mx and My forces is furthest away from the specimen  170 . For the lower (or inferior) portion of the specimen  170 , the force in the y direction is free, fixed or biased and has a minimized moving mass and has the highest required performance. The forces in the x direction Fx is then nested further from the specimen  170  than the Fy force. The vertical force provided by the actuator  26 , Fz, is still further from the inferior portion of the test specimen  170 , with the moment around the z-axis, Mz, being provided in a nesting arrangement still further from the test specimen  170 . The drive for all these forces is provided as indicated. 
     The Euler sequence of rotational motion as applied by the mechanism of the present invention is flexion/extension→lateral bending→axial rotation. In the field of testing of spinal implants, this ordering of the mechanism promotes maximum performance and minimizes the additive joint error. The independency of linkages reduces or eliminates cross-talk and allows accurate control of the phases between the individual mechanisms. This is important to create the desired and controlled loading of the test specimen  170 . 
       FIG. 31  shows the required forces for applying to a test specimen of a spinal implant intended for the lumbar region according to the an exemplary set of curves. Similarly,  FIG. 32  shows the same information for cervical data. Duty cycle loading involves inserting high loads and displacement activity into a more typical repeating activity, such as lifting a heavy box periodically. This allows for the insertion of periodic overload states. Such overload states are known to potentially induce damage, but are relatively rare so that their rarity should be considered and the overload states placed in the context of other daily activity when included. In addition to duty cycle loading, embodiments of the present invention provide for re-creating any sinusoidal or non-sinusoidal curve, which allows for more accurate simulation (e.g., a “walking simulation”). The embodiments of the invention allow for inputting non-sinusoidal data with varying phase, amplitude and frequency content, such as real walking profiles. These curves, such as shown in  FIG. 33 , can be repeated for a large number of cycles, and hence are fatigue or wear generating. The representation of activity is not limited to walking, as one of ordinary skill in the art will readily appreciate, but may be used to simulate any number of replicated activities in a serial or repetitive fashion. Accordingly, a controller  200 , seen only in  FIG. 1 , is used to independently and individually control each of the motion devices. Hence, the flexion/extension, lateral bending, rotation, and loading of the test specimen  170  may be controlled to any desirable curve through the use of control software and the mechanisms provided in the orthopedic simulator  10 . This allows for the testing of an orthopedic device that simulates actual conditions that the orthopedic device will be subjected to rather than the constant forces depicted in  FIGS. 31 and 32  applied over 10 million cycles. For example, a test may account for the typical day for humans. Such a day may include sitting for hours at a time with intermittent periods of activity, including walking and sleeping periods. Strenuous physical activity, such as for athletes, may also be better modeled. The controller  200  thereby more accurately causes the orthopedic simulator  10  to simulate the forces that a spinal implant or other orthopedic device will actually be expected to see for a typical implant recipient. 
       FIG. 34  depicts the orthopedic simulator  10  within a housing  178 . The use of a housing  178  prevents contamination and reduces oil within the environment. Switches  180  allow a test station to be shut down very quickly in order to prevent invalidating of a test if an individual test station  12  should experience difficulty in operation. 
     The embodiments of the present invention described above provide an orthopedic simulator with a temperature control arrangement for maintaining a specimen bath at a precise temperature, without subjecting the bath fluid to potential over-temperature fluid degradation. Stability and consistency are provided in certain embodiments, as well as cost savings due to use of a single heater and controller. 
     Although the present invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation.