Abstract:
An adjustable-length orthopaedic strut apparatus having minimal x-ray absorption, the capability to produce small length adjustments with minimal axial backlash, and a body devoid of exposed threads, the apparatus having an outer telescoping strut element, an inner telescoping strut element, a threaded drive element rotationally mounted inside the outer telescoping strut element and engaging threads inside the inner telescoping strut element, and an input gear-train arranged to produce fine adjustment of the strut length by generating rotation of the threaded drive element that can be smaller than the input gear rotation. A preferred embodiment also includes compliant preload structures for reducing axial backlash between all moving elements, and one or more locking pins which can be selectively disengaged to adjust the compressive stiffness of the adjustable length strut.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Application 60/962,540, filed Jul. 30, 2007, the contents of which are expressly incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates generally to the field of adjustable length struts and more specifically to adjustable length strut apparatus for external orthopaedic fixator applications. 
       BACKGROUND 
       [0003]    External orthopaedic fixators are used to position two or more bone elements relative to one another, in either a fixed or an adjustable position, and often use one or more adjustable length struts to define the relative location of the two separate bone segments. 
         [0004]    One example of a highly adjustable external fixation device is the Taylor Spatial Frame sold by Smith &amp; Nephew (http://global.smith-nephew.com/us/TAYLOR_SPATIAL_FRAME — 7441.htm) which uses 6 adjustable-length struts mounted between two circular frames (in a configuration known to the robotics community as a “Stewart platform”) to provide full 6-degree-of-freedom (DOF) positioning capability. The struts in this example use metal threaded rods extending from a metal tubular structure, with a threaded ring rotatably captured on the end of the tubular structure defining the adjustable axial extension of the threaded rod. The design of the threaded ring provides a mechanical detent allowing a repeatable adjustment of one full revolution of the ring, with a full revolution typically corresponding to an axial extension of 1 mm. 
         [0005]    Another example of an adjustable frame, this time of the Ilizarov type, is also provided by Smith &amp; Nephew (http://global.smith-nephew.com/us/deformitycorrection/ILIZAROV_EXTERNAL_FXR_OVW — 13957.htm). In this design, the frame is not capable of full 6-DOF positioning, but instead uses 3 or 4 extendable struts which are rigidly mounted to at least one circular frame, and which provide control of the axial displacement and 2 tilt angles of a second frame relative to the first frame. This particular design also uses a metal threaded rod extending from a metal tubular structure, but in this case the rotating threaded ring is adapted with a spring loaded latch lever which engages with detents to enable repeatable adjustment in increments of ¼ revolution. 
         [0006]    Another example of a partially adjustable external fixator is the Sheffield Ring Fixator from Orthofix (http://www.orthofix.com/products/sheffield.asp). 
         [0007]    Yet another type of external strut, generally called a unilateral fixator, uses a single adjustable-length strut to define the position of two swiveling mounting brackets, each of which holds half-pins which engage bone segments. Examples of this type of strut include the Jet-X unilateral fixator from Smith &amp; Nephew (http://global.smith-nephew.com/us/deformitycorrection/JET_X_BAR_UNILATERAL_FIX — 7243.htm) and the XCaliber Articulated Ankle Fixator from Orthofix (http://www.orthofix.com/products/xcaliber_fixator2.asp). Both of these systems use molded plastic body elements constructed from radiolucent fiber-reinforced polymer materials to provide a slidably adjustable structure. Both of these systems also use a radio-opaque metal bolt or similar element to clamp the sliding elements together to define a fixed length. 
         [0008]    Existing adjustable length struts for orthopaedic applications suffer from several deficiencies. The struts used in the Taylor Spatial Frame provide a manually operable input ring for adjusting length, but this design provides little security against accidental rotation (e.g., as a result of an object brushing up against the strut), and provides no ability to prevent unwanted adjustment by, for example, curious or fidgety child patients. Furthermore, there is no way to provide a reliable adjustment of less than one full turn, or less than one full millimeter of length adjustment. Other disadvantages of this design include the presence of exposed threads which can gather dirt, and the relatively large weight and radio-opaque characteristics of the all-metal struts. 
         [0009]    The struts used in the Smith&amp;Nephew Ilizarov frame do provide for ¼ turn increments, but the structure used to provide this capability adds quite substantially to the weight of the strut. Furthermore, there is still no means for preventing undesired manual adjustment; the threaded rods are still exposed in the same manner as the struts in the Taylor Spatial Frame; and the all-metal design strongly absorbs X-rays used to image the bone locations. 
         [0010]    All of the above struts also suffer from axial backlash, which limits the precision and stiffness with which the two bone segments can be held in position. This unavoidable backlash is caused by manufacturing clearances between the threaded rod and the threaded adjuster elements, and in the rotational mounting of the adjuster to the outer tube structure. While these clearances are each generally small (well under 1 mm) the combination of several clearances can result in total backlash that is a significant fraction a millimeter, or more, and which can potentially have an adverse effect on bone healing. 
         [0011]    The slidable polymer struts used in the XCaliber and Jet-X products are mostly radiolucent, but they still require a metal locking element somewhere near the mid-point of the strut length, and this metal element can still obscure X-ray images in inconvenient locations. The manual clamping design of these struts does eliminate backlash but the length adjustment of these devices is not simple, and generally cannot be done safely by the patient. 
       SUMMARY 
       [0012]    An adjustable-length orthopaedic strut is provided that enables precise length adjustments which can be significantly smaller than the typical daily adjustment increment of 1 mm, but that is also resistant to accidental length adjustment. In accordance with one embodiment, there is disclosed a strut apparatus comprising: an outer telescoping strut element, an inner telescoping strut element, means for preventing significant rotation of the outer telescoping strut element relative to the inner telescoping strut element, a threaded drive element rotationally mounted inside the outer telescoping strut element, and means to enable rotation of the threaded drive element relative to the outer telescoping strut element; wherein the threaded drive element rotation causes axial motion of the inner telescoping strut element relative to the outer telescoping strut element. Also in accordance with an embodiment of the invention, the outer telescoping strut element has a first end joint and the inner telescoping strut element has a second end joint, wherein both end joints are adapted to allow at least partial rotation around at least two orthogonal axes. 
         [0013]    In accordance with a preferred embodiment, the drive element is coarsely threaded, typically having a thread pitch greater than 1 mm and thus producing more than 1 mm of axial motion when rotated one full revolution, and the means to enable rotation of the threaded drive element comprises an output gear rotatably fixed to the coarsely threaded drive element and an input gear adapted for external adjustment, wherein one rotation of the input gear produces less than one rotation of the output gear and the coarsely threaded drive element. 
         [0014]    One advantage of the preferred embodiment is that a light-weight, low-cost, and largely radiolucent orthopaedic strut can be conveniently manufactured wherein the telescoping inner and outer strut elements and the coarsely threaded drive element are molded out of optically opaque, translucent, transparent, or color-tinted plastic. Preferably, the telescoping strut elements and the means for adjusting the length of the adjustable length strut apparatus form a substantially radiolucent central portion extending in both directions from a mid-point equidistant between the end joints, and extending the majority of the distance from the mid-point to the first end joint or the second end joint. The substantially radiolucent central portion can be comprised of relatively thick sections of radio-transparent materials such as but not limited to plastic, as disclosed above, but can also be comprised of very thin sections of radio-opaque materials such as but not limited to metal. 
         [0015]    Also provided are means to improve the positioning precision of the adjustable length orthopaedic strut by internally preloading locations having mechanical clearance, so as to minimize significant sources of axial backlash. In one embodiment, the axial backlash between the threaded drive element and the internally threaded strut element is minimized by segmenting and expanding a portion of the internally threaded drive element so as to remove radial and axial thread clearance. The axial clearance at the rotating joint between the threaded drive element and the outer telescoping element is minimized by axially preloading the rotatable joint with a spring washer or other compliant element for example. And in a further embodiment, mechanical clearances between a spherical end-joint on each strut element and the body of the strut element are also minimized through the use of elastic preloading elements. Lastly, in yet another embodiment, the stiffness of the strut in response to compressive axial loading can be easily reduced (e.g., to allow more load-sharing by the healing bone prior to removal of the fixator) by removing or otherwise disengaging one or more locking pins that fix the position of one or more rigid elements which are stacked on top of elastic elements and which together act to limit the axial motion of at least one of the spherical end-joints. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The drawings constitute a part of this specification and include exemplary embodiments to the invention, which may be embodied in various forms. It is to be understood that in some instances various aspects of the invention may be shown exaggerated or enlarged to facilitate an understanding of the invention. 
           [0017]      FIG. 1A  is an elevation view of a Prior Art adjustable length strut. 
           [0018]      FIG. 1B  is a cross sectional view of a portion of the Prior Art adjustable length strut of  FIG. 1A . 
           [0019]      FIG. 2  is a perspective view of one embodiment of an adjustable length strut of the present invention. 
           [0020]      FIG. 3  is a partially cut-away view of  FIG. 2 . 
           [0021]      FIG. 4  is a magnified, partially transparent view of an adjustment mechanism of the strut of  FIG. 2 . 
           [0022]      FIG. 5  is a partially cut-away view of another embodiment of a strut of the present invention adapted for unidirectional action. 
           [0023]      FIG. 6  is a perspective view of an optically translucent version of a strut of the invention. 
           [0024]      FIG. 7  is an exploded view of an embodiment of the present invention illustrating preloading for elimination of backlash. 
           [0025]      FIG. 8  is a section view of an embodiment of the invention illustrating preloading for elimination of backlash. 
           [0026]      FIG. 9  is a section view of an embodiment of the invention illustrating preloading for elimination of backlash and adjustability of compressive stiffness. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0027]    Detailed descriptions of the preferred embodiment are provided herein. It is to be understood, however, that the present invention may be embodied in various forms. Therefore, specific details disclosed herein are not to be interpreted as limiting, but rather as a basis for the claims and as a representative basis for teaching one skilled in the art to employ the present invention in virtually any appropriately detailed system, structure or manner. 
         [0028]      FIG. 1A  shows an example of a prior art strut  5  primarily comprising an exposed threaded metal rod  10 , which telescopes out of a metal tube structure  20 . A threaded adjustment ring  30  is rotatably mounted to the end of the metal tube  20  and engages with the threads on the threaded rod  10 . A multi-axis pivot  15  is attached to the threaded rod  10 , and another multi-axis pivot  25  is built into the end of the metal tube  20 . The threaded rod  10  has an extension pin  12  which rides in slot  22  formed in the tube  20  in order to prevent rotation of the rod  10  relative to the tube  20 . 
         [0029]      FIG. 1B  shows a sectional view of the axial adjustment region of the same prior art strut  5 . This view illustrates that the internally threaded adjustment ring  30  engages with the externally threaded rod  10 , and is rotatably attached to the end of the tube  20  using a retaining ring  32 . Also shown is a mechanical detent mechanism comprising a small spring  34  and a ball  36  which fits into a small depression formed on the end face of the tube  20 . 
         [0030]    As can be seen from these figures, this prior art strut  5  has exposed threads on the outside of the structure which can collect dirt. While the adjustment ring  30  is adapted to be easily adjusted in increments of one full turn, the ring can also be easily rotated by accident. This design also has unavoidable clearances in the mating threads between the ring  30  and the rod  10 , and between the retaining ring  32  and both the tube  20  and the ring  30 . Additional clearances are present between individual moving elements in the multi-axis pivots  15  and  25 . All of these clearances together produce backlash (also known in the art as “slack” or “lost motion”) that limits the precision with which the axial length of the strut can be adjusted, because it creates a region of very low stiffness around the desired adjustment length. Changes to the length of the strut in response to external forces can occur with little or no resisting force until the all of the clearances are compressed and solid contact is established between all mating pairs of elements. 
         [0031]      FIG. 2  shows the exterior of one embodiment of a strut  75  comprising an outer telescoping strut element  100  with a multi-axis “rod-end” type of joint  110  at one end, and an inner telescoping strut element  200  with multi-axis end-joint  210  at an opposite end. The joints  110  and  210  contain threaded mounting studs  112  and  212  having a partially spherical portion, each with a socket head indentation  111  and  211  or similar feature to enable convenient mounting of the studs to a circular frame or other fixator element (not shown). The inner telescoping strut element  200  slides inside of the outer telescoping element  100 , while axial ridges  205  on the outside of the inner telescoping element  200  engage with mating features on the inside of the outer telescoping element  100  to prevent rotation of the inner telescoping strut element  200  relative to the outer telescoping strut  100 . A visual length indicating scale  201  can be etched, engraved, molded, printed or attached onto the inner telescoping element  200  in order to provide a visual indication of the total length of the adjustable strut assembly. Other methods of providing such a scale  201  are contemplated. Adjustment of the strut length is accomplished by rotation of adjusting element  120 , (shown in this embodiment as having the form of a Phillips-type screw head) which rotates within the outer telescoping tube  100 . As can be seen in  FIG. 2 , there are no exposed threads in this embodiment, only generally smooth external surfaces which will be unlikely to collect dirt or grime. Any small ridges or discontinuities that remain, such as the axial ridges  205  or ridges that may be present in this illustrative example, can easily be designed to minimize the potential for capture of dirt or debris. 
         [0032]    It will be appreciated that the geometry of the adjusting element  120  is not limited to a Phillips head screw, but could also take any form of hand-operated knob or crank, or any one of a variety of fittings for rotation by any tool, including but not limited to a socket head (for use with an Allen wrench), a hex head, a Torx drive socket, a cylindrical lock feature, or any other geometry, without departing from the scope of the invention. Those skilled in the art will also appreciate that the rod-end mounting features  110  and  210  need not be mounted orthogonally to the axis of the strut, and can be replaced with any type of single-axis or multi-axis joint, or even a rigid non-pivoting connection where desired, without departing from the scope of the invention. 
         [0033]      FIG. 3  shows a partial cut-away view of the strut shown in  FIG. 2  and illustrates a rotating threaded drive element  130  which is rotatably attached inside the outer telescoping tube  100  and which engages with threads  202  on the inside surface of the inner telescoping tube  200 . The drive element  130  can be a hollow tube with a smaller diameter aperture at the end, and in this case the rotatable connection can be made using a shoulder screw  132  or other similar means which allows the threaded drive element  130  to rotate inside the outer telescoping tube  100  without translating substantially in the axial direction. If the drive element  130  is solid, the rotatable connection can be made using a smaller diameter extension of the drive element and a mating nut or other fastener, or using any other means which are known to those skilled in the art. A first bevel gear  140  (gear teeth not shown) is attached to, and rotates with, the threaded drive element  130 . The first bevel gear  140  mates with a second bevel gear  150  (gear teeth not shown) which is mounted inside the outer telescoping strut  100 . The adjusting element  120  is attached to, or built into, the second bevel gear  150  such that rotation of the adjusting element  120  about an axis generally orthogonal to the axis of the adjustable strut causes rotation of the second bevel gear  150 , which in turn causes rotation of the first bevel gear  140  and the attached threaded drive element  130  about the longitudinal axis of the adjustable strut assembly  75 . 
         [0034]    In a preferred embodiment, the outer telescoping strut  100 , the inner telescoping strut  200 , and the threaded drive element  130  are all fabricated from a radiolucent material. An example of such fabrication is the molding of polymer material with the possible addition of reinforcing fibers also made of radiolucent material. In this manner, even if any of the bevel gears  140  and  150 , the shoulder screw  132 , or rod-end elements  112  or  212 , are made of metal or other radio-opaque material, the main central portion of the strut assembly, which is the area most likely to obscure the healing bone region of orthopaedic interest, would be radiolucent. This radiolucent region can be seen to extend from the strut mid-point (defined here as the point equidistant from the two rod-ends) in both directions for the majority of the distance from the mid-point to the rod-end joints. Thus, the assembled strut is radiolucent over at least the central half of the region between the rod-ends. 
         [0035]    In one embodiment, the threaded drive element  130  and inner telescoping element  200  with matching threads could be made of metal with relatively fine-pitch threads (on the order of 1 mm pitch). In another embodiment, it may be desirable to mold the parts out of a polymer material which may not have the strength or precision to enable the use of fine threads. In a molded embodiment, the threads may have a larger pitch (perhaps 2 mm or more) than the desired daily adjustment increment of approximately 1 mm. Therefore, it is desirable to provide a reliable means for making adjustments of significantly less than 1 full revolution of the threaded drive element  130 . In a preferred embodiment, this is achieved by choosing the “input” or second bevel gear  150  to have significantly fewer teeth than the first bevel gear  140  which is mounted to threaded drive element  130 . In this manner, the angular rotation of the threaded drive element  130  is substantially less than the angular rotation of the input gear  150 . 
         [0036]    This is illustrated more clearly in  FIG. 4 , which shows an enlarged, partially transparent view of a portion of the adjustable strut  75  in  FIG. 3 . In this non-limiting example, the threads on the threaded drive element  130 , and the mating threads on the inside of inner telescoping strut element  200  have a pitch of 2 mm. The input bevel gear  150  is sized to contain one-half as many teeth as the first bevel gear  140 . Thus, one revolution (arrow A) of the input bevel gear  150  (via the adjustment feature  120 ) produces only one half of a revolution (arrow B) of the bevel gear  140  and threaded drive element  130 , which in turn produces 1 mm of axial displacement (arrow C; corresponding to ½ of the thread pitch) of the inner telescoping tube  200 . Similarly, a one-quarter turn rotation of the adjuster element  120  (which would be conveniently indicated by the four-spoke pattern of a standard Phillips head screw) produces a one-quarter millimeter change in the overall strut length. Of course, other gear ratios and gear configurations can be substituted without departing from the scope of the invention. With a reasonable amount of gear reduction, a reasonably fine thread pitch on the threaded drive element  130 , and markings (not shown) to indicate small fractional-rotation inputs to the adjustment element  120 , length-adjustment increments of 1/10 mm or less could be achieved. 
         [0037]    Many orthopaedic procedures for gradual bone lengthening or repositioning implement a program of length changes having an average rate of approximately 1 mm per day. However, a single 1 mm increment performed once per day represents a fairly large and instantaneous repositioning of the healing bone. It is believed that a more gradual adjustment, such as four adjustments of ¼ mm per day for example, would be easier for the healing bone to tolerate. Thus, the ability to easily and consistently implement adjustments of ¼ mm or less, or to divide out a single day adjustment into multiple partial-day adjustments, is an important capability. The use of a gear reduction system enables small axial displacement increments to be made even if the thread pitch is relatively large, as might be necessary to provide acceptable strength or tolerances in a strut constructed with molded polymer parts. 
         [0038]      FIG. 5  shows an alternative embodiment of a strut  175 , which is adapted to act as a preloading strut that is designed to define a minimum length and to be able to impart force in the lengthening direction, but designed so as to not restrict the strut from extending to be longer than the adjusted minimum length. In this embodiment, the telescoping portion of the inner telescoping strut element  200  with internal threads  202  of  FIG. 3  has been replaced by inner telescoping element  300 , which is sub-divided into an unthreaded portion  310  and a short threaded portion  320 . Both portions  310 ,  320  have axial ridges or other features  305 , for example, which mate with corresponding grooves or other features on the inside of the outer telescoping strut element  100  to prevent rotation of both the threaded portion  320  and the unthreaded portion  310  relative to the outer telescoping strut element  100 . As will be apparent to those skilled in the art, rotation of the inner threaded drive element  130  will cause translation (without rotation) of the threaded portion  320 , and this threaded portion  320  will define a minimum overall strut length X. Rotation of the drive element  130  can thus generate forces in the “lengthening” direction, but will not exert any “retracting” force on the inner telescoping strut element  300 . It should be noted that a portion of the outer telescoping element  100  in  FIG. 5  has been shown detached and shifted away from the unthreaded and threaded portions  310  and  320  for purposes of clarity only, and does not represent the operational orientation of the elements, where the outer telescoping element  100  would be closely fitted around the inner telescoping elements  310  and  320 , with mating axial features engaged to prevent relative rotation. 
         [0039]    In some orthopaedic applications, it is important that the strut be capable of withstanding very large forces. In these cases, such as leg reconstruction of large adult patients, it may be preferable for the outer telescoping strut element  100 , the inner telescoping strut element  200 ,  300 , and the internal threaded drive element  130 , to be constructed of metal or another high-strength material. In some cases, it may be acceptable to have a radio-opaque strut, and solid or thick-walled metal elements can be used. In other cases where a reduced level of x-ray absorption is desired, the strut elements  100 ,  130 , and  200 ,  300  can be fabricated in the form of thin-walled tubes (i.e., less than 1 mm thick) in order to reduce weight and x-ray absorption. Accordingly, structures described herein can be made from metals and other radio-opaque materials, as well as from radiolucent materials, for example. 
         [0040]    In some other orthopaedic applications, the maximum load-carrying strength of a strut may be low enough that the strut elements can be made of reinforced polymer materials. In other orthopaedic applications, the maximum load carrying capacity of a strut may be low enough that the major strut elements can be made of a strong but unfilled polymer such as polycarbonate.  FIG. 6  shows an embodiment of the invention that is constructed of color-tinted polycarbonate  402 . Of course, the tinting can be removed if it desired to choose an optically clear polycarbonate (not shown). Such optically clear and/or color tinted struts provide a less intimidating and imposing visual appearance, and thus may be judged to be more attractive by patients who will be “wearing” an external fixation frame for months at a time. 
         [0041]    It is important that the strut assembly can precisely hold a defined total length without excessive clearances and associated mechanical backlash, which limits mechanical positioning accuracy and stiffness.  FIG. 7  illustrates an exploded view of a self-preloaded strut embodiment  575 , which reduces or eliminates axial backlash. In this embodiment, the hollow threaded drive element  130  in  FIG. 3  has been modified to a threaded drive element  530 , such that a portion of the threaded region has been cut along at least one radial line. This is illustrated in  FIG. 7  where threaded drive element  530  contains one or more axial slots  560  at one end. A compliant preload plug  570  inserted into the open end of the hollow and slotted drive element  530  acts to radially expand a portion of the threads near the end of the drive element  530 . When the threaded drive element  530  is then threaded inside the inner telescoping strut element  200 , the preload plug  570  acts to expand the threads of the drive element  530  until they are in intimate contact with the threads  202  (shown most clearly on  FIG. 3 ) on the inner telescoping strut  200 . If the threads have an angled profile, as is commonly used for example, the elimination of the radial clearance also results in the elimination of axial thread clearance, and the axial backlash in the threaded connection is thus eliminated or substantially eliminated. It will also be apparent to those skilled in the art that, without departing from the scope of the invention, the thread clearance could also be removed by radially compressing a portion of the outer threaded element, instead of radially expanding a portion of the inner threaded element. It will also be apparent to those skilled in the art that other methods can be employed for producing the desired preload, including but not limited to, the use of springs of any type, pressurized bladders, or the intentional interference fit between the parts, without departing from the scope of the invention. 
         [0042]    If pivoting joints such as rod-ends  110  and  210  are required, they can also be preloaded to remove axial backlash using compliant plugs, springs, material interference, or other common design techniques to minimize or eliminate clearance between the rod-end post elements  112  and  212  and the body of the inner and outer telescoping strut elements,  100  and  200  respectively. It will be apparent to those skilled in the art that other detailed forms of preloading or backlash elimination can be used to remove the clearance between the threaded drive element  130 ,  530  and both the inner telescoping strut element  200  and the outer telescoping strut element  100  without departing from the scope of the invention. 
         [0043]      FIG. 8  illustrates a section view of an assembled, fully preloaded embodiment of a strut assembly  575 . Axial preloading of the threaded connection between the inner telescoping strut element  200  and the threaded drive element  530  is created by the compliant plug  570  which acts to radially expand a region of threads on threaded drive element  530 , which may have a slot  560  to reduce the circumferential stiffness of the end of the threaded drive element  530 . Because there is a risk that the slotted portion  560  of the threaded drive element  530  could crack under large loads, it is preferred to have both a preloaded section and an un-preloaded section of threads on drive element  530  which engage with threads on inner telescoping strut element  200 , so that the un-preloaded thread section will still be available to maintain axial position (albeit with some backlash) if for some reason the preloaded section of threads were to fail. 
         [0044]      FIG. 8  also illustrates the use of a wavy washer  134 , or other compliant element, placed under the head of shoulder screw  132  which rotatably connects the threaded drive element  530  (together with the affixed bevel gear  140 ) to the end portion of the outer telescoping strut element  100 . The wavy washer  134  acts to preload the threaded drive element  530  (and affixed bevel gear  140 ) against the outer telescoping strut element  100 , and thus eliminates any free backlash resulting from clearances and tolerances in the length of the shoulder on shoulder screw  132 . If a compliant element such as a wavy washer is used with the shoulder screw  132  which rotatably mounts the threaded drive element  530  to the outer telescoping strut element  100 , then the axial “free play” or backlash of that joint is also eliminated. 
         [0045]    When preloading an axial device in this manner, the clearance is removed by forcing one element towards the other with some amount of preload force. If the elements are pushed in one direction, they will already be in intimate contact and the stiffness of the system is defined by the material properties and the stiffness of the material-to-material contact at the joint. If the elements are pushed in the opposite direction, any motion is initially prevented by the preload force. If, however, the applied force is larger than the preload force, the elements will move axially relative to one another. Thus, it can be seen that axial preloading is asymmetric. If the strut is to be used primarily to resist large compressive forces (i.e., forces acting to shorten the strut) then the wavy washer or other compliant element should be placed underneath the head of the shoulder screw  132  which is placed inside of the hollow threaded drive element  530 . This ensures that the threaded drive element  530  is held in intimate contact with the solid end portion of the outer telescoping element  100  and there will be no axial backlash even under large compressive loads. If, on the other hand, the strut is to be used primarily to resist tensile forces (those acting to lengthen the strut), then the wavy washer should be placed over the threaded shoulder screw  132  in the region between the bevel gear  140  and the end of the outer telescoping strut element  100 . 
         [0046]      FIG. 8  also illustrates the use of compliant elements  118  and  218  in the cavities of the rod-end joints, to preload any internal stud or other mating element (such as  112  and  212  shown on  FIG. 7 ) which would be fitted inside the rod-end cavities. 
         [0047]      FIG. 9  illustrates a further embodiment of a strut assembly  675 , wherein the lower spherical rod-end is held in place by a stack of rigid and elastic elements, comprising solid elements  610  and  612  interleaved with elastic elements  620  and  622 , and with the entire stack fitted inside inner telescoping strut element  200 . Also shown are two tapered pins  630  and  632 , which can be engaged to lock the rigid elements  610  and  612  into position relative to the telescoping element  200 . When pin  630  is engaged, the system has maximum stiffness because rigid element  610  blocks the (upward) motion of the lower spherical rod end. If pin  630  is not present while pin  632  is inserted, however, the strut has an intermediate stiffness in the strut-shortening direction because the elastic element  620  sandwiched between solid elements  610  and  612  can compress under load. Furthermore, if both pins  630  and  632  are removed or otherwise disengaged, the system will have further reduced stiffness because both compliant elements  620  and  622  can compress under load. Of course, the number and thickness of compliant elements and locking pins can be changed to provide additional stiffness options. It will be understood by those skilled in the art that disengagement of a locking pin may be achieved by complete removal of the pin, partial removal or other shift in the position of the pin, rotation of a flattened or other non-round pin, or by other means. 
         [0048]    While the invention has been described in connection with a preferred embodiment, it is not intended to limit the scope of the invention to the particular form set forth, but on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.