Abstract:
A device for removable attachment to a bone of a living patient, useful as a mounting support for a trackable part of a computerized surgical navigation system and a method for attaching an object removably to a bone of a living patient. A shaft having a self drilling, self-tapping screw tip can be inserted transversely of the bone and securely screwed into the cortex. A screw drives a sharp-edged sleeve along the shaft to engage the near cortex. A stepping mechanism allows incremental angular movement of a portion of an adjustment mechanism and indicates when the sleeve is securely engaged by resisting further tightening.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to releasably attaching an object to a bone in a living patient, and relates particularly to a device for fastening an object such as a component of a computerized surgical navigation system stably to a large bone such as a femur. 
         [0002]    In surgical procedures involving articulated skeletal bone joints, such as knee replacement procedures, it is often helpful to use computer-aided navigation, based on infrared or electromagnetic imaging systems, to locate bones or parts of bones correctly with respect to each other during the procedure. In order to establish and monitor the position of a large bone as a reference for such computer-aided navigation, a reference device must be securely and stably, but removably, mounted on the large bone. 
         [0003]    Attachment of objects to bones has been addressed by the prior art, as disclosed, for example, by Glossop U.S. Pat. No. 6,203,543 and Sasso U.S. Patent Application Publication No. US2005/0119566. 
         [0004]    In attaching some known devices to a bone as a mounting support for a navigation system reference target or beacon, a screw is inserted transversely through a large bone such as the femur, using a hole drilled entirely through the thickness of the bone. In order to be secure the screw must be long enough to extend beyond a shoulder or other depth-limiting stop far enough to extend into the cortex of the bone on the far side. At the same time, however, the screw should not be so long as to extend unnecessarily beyond the far side of the bone when the shoulder or other stop is seated on the near side of the bone. Accordingly, such a device must be available in various different lengths for use with bones of different sizes. While an estimate can be initially made of the size of a bone, time may be wasted if a mounting device of the wrong size is chosen initially and must be replaced. 
         [0005]    Adjustment of some previously available devices to securely support an object such as a part of a navigation system has required separate operations of first drilling and thereafter tapping screw holes in bones, taking more time than is desired during a surgical procedure. 
         [0006]    In other devices for anchoring an object to a bone a bone engaging part of the device must be rotated as it is moved through soft tissue surrounding the bone in order to bring that part of the device into engagement against the bone. This rotary motion during movement through the soft tissue causes undesired trauma to the soft tissue and may delay healing in that area. 
         [0007]    Adjusting a mounting fastening device to attach it securely to a bone has thus been a difficult and inexact procedure in the past. 
         [0008]    What is desired, then, is a device and a method for its use to attach an object securely to a large bone, and to accomplish such attachment without taking an undue amount of time, and with a predictable result, so as to achieve a secure attachment in a device which is readily removable when its presence is no longer required. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention answers some of the needs mentioned above by providing a device and a method for its use for securely but removably mounting an object on a bone of a living patient. 
         [0010]    As an aspect of one embodiment of the device disclosed herein a central shaft may have a screw threaded tip to extend through a bone and be fastened in the cortex of the opposite side of the bone. A bone-engaging member is mounted so as to slide longitudinally along the central shaft without being free to rotate about the shaft and includes a sharpened front end shaped to engage the near side of the bone securely to prevent the central shaft from moving with respect to the bone. 
         [0011]    As another aspect of the device, an adjustment mechanism may include a rotatable member mounted on a screw thread formed on the central shaft, so as to push the bone-engaging member along the central shaft until it engages the bone, and the bone-engaging member and the rotatable member may have mutually confronting faces providing for rotation of the driver about the central shaft in discrete angular steps, so as to indicate the force with which the bone-engaging member is urged toward the bone. 
         [0012]    As another aspect of the device the central shaft may include a shoulder to limit the depth to which the threaded tip of the central shaft can be engaged in a bone. 
         [0013]    As an aspect of another embodiment of the device the central shaft may include a combined drilling and tapping tip and a threaded distal end portion. 
         [0014]    A method of use of the device to removably mount an object on a bone in accordance with the present disclosure includes forming a hole of a first size extending into a bone for a first distance, providing threads within the hole, placing a shaft through the hole, and seating the shaft in the bone with a tip in contact with a far side cortex, and thereafter forcing a bone-engaging member longitudinally along the shaft to force a bone gripping front end of the bone-engaging member into gripping engagement in the bone defining and surrounding the first hole, while providing an indication of the force being applied to move the bone-engaging element. 
         [0015]    One aspect of the method includes advancing the bone-engaging element by moving it along the shaft by small discretely defined steps. 
         [0016]    The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL DRAWINGS 
         [0017]      FIG. 1  is a side elevational view of a device for mounting an object removably on a bone which is an exemplary embodiment of the present invention. 
           [0018]      FIG. 2  is a side elevational view of a deep socket wrench useful in operating the device shown in  FIG. 1 . 
           [0019]      FIG. 3  is a perspective view of a sharpened front end of a bone-engaging sleeve portion of the device shown in  FIGS. 1 and 2 . 
           [0020]      FIG. 4  is a fragmentary oblique view of adjacent portions of a bone-engaging member and an adjustment mechanism included in the device shown in  FIGS. 1 and 2 . 
           [0021]      FIG. 5  is a side elevational view of a portion of the device shown in  FIG. 1 , with portions thereof shown in sectional view taken on a diametrically aligned plane. 
           [0022]      FIG. 6  is a partially cutaway view, at an enlarged scale, of a portion of the coupling arrangement between the bone-engaging sleeve and a sleeve-moving member of an adjustment mechanism of the device shown in  FIGS. 1 ,  4 , and  5 . 
           [0023]      FIG. 7  is a view similar to  FIG. 6  showing the coupling with the sleeve-moving member in a different position with respect to the sleeve. 
           [0024]      FIG. 8  is a view of a portion of a person&#39;s upper leg, showing a first step in the procedure of installing the mounting device shown in FIGS.  1  and  3 - 7 . 
           [0025]      FIG. 9  is a view of the portion of a patient&#39;s leg shown in  FIG. 8 , showing a second step in a procedure of installation of the mounting device. 
           [0026]      FIG. 10  is a view similar to  FIG. 9 , showing performance of a subsequent step in the installation of the mounting device. 
           [0027]      FIG. 11  is a view similar to  FIG. 10  showing the mounting device installed and in use to support an object in a fixed position with respect to the patient&#39;s leg. 
           [0028]      FIG. 12  is a view similar to  FIG. 1  showing a device which is an alternative embodiment of the device shown in  FIG. 1 . 
           [0029]      FIG. 13  is a simplified view of a portion of a person&#39;s upper leg, showing a first step in the procedure of installing the mounting device shown in  FIG. 12 . 
           [0030]      FIG. 14  is a view of the portion of a patient&#39;s leg shown in  FIG. 13 , showing a second step of installation of the mounting device shown in  FIG. 12 . 
           [0031]      FIG. 15  is a further simplified view of the portion of a leg shown in  FIG. 14 , showing the mounting device after completion of installation. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0032]    Referring now to the drawings which form a part of the disclosure herein, an anchoring device  12 , shown in its entirety in  FIG. 1 , includes a central shaft  14 . At a distal end portion  16  of the shaft  14  a threaded tip  18  extends axially. The threaded tip  18  may be of a self-drilling, self-tapping configuration, but need not necessarily be so. The threaded tip  18  may include a central portion with a diameter  20  significantly smaller than a diameter  22  of the central shaft  14 , and the central shaft  14  thus may define a shoulder  24 . A helical thread defined on the threaded tip  18  may extend radially to a diameter  26  which also should be at least slightly less than the diameter  22  for best performance. 
         [0033]    A bone-engaging member in the form of a tubular sleeve  30  is located on the central shaft  14  with a sliding fit. A slot  32  extends radially and longitudinally of the sleeve  30  on at least one side thereof, and a pin  34  mounted in and extending radially from the shaft  14  is engaged in the slot  32 , so that the sleeve  30  is unable to rotate about the central shaft  14 , although it is able to move longitudinally along the shaft  14  as indicated by the arrow  36 . The sleeve  30  has a bone-gripping distal end portion  38 , which may be conically tapered to a sharp edge  40 , and may have sharpened ribs  42  extending longitudinally and protruding radially from the tapered distal end portion  38 . The sleeve  30  defines a central bore  44  which should be large enough to slide easily along the central shaft  14 , but small enough to provide only a minimal amount of free radial movement of the sleeve  30  with respect to the shaft  14 . A coupling  46  includes one or more legs  48  and interconnects the sleeve  30  with a suitable sleeve position adjusting member, such as a sleeve positioner  50 , by engaging circumferential grooves  52  and  54  defined respectively in the sleeve  30  and the positioner  50 . 
         [0034]    The sleeve positioner  50  may have a hexagonal portion including paired opposite flats  56  spaced a short distance away from the groove  54 , or may otherwise be configured to be engaged by a suitable wrench, to rotate the sleeve positioner  50  about the central shaft  14 . The sleeve positioner  50  includes internal threads engaged matingly with an external thread  58  defined on the central shaft  14 , so that rotating the positioner  50  with respect to the central shaft  14  moves the positioner  50  longitudinally along the central shaft  14 . 
         [0035]    One or more pins  60  or other protruding members may be provided on the central shaft  14  near its proximal end to receive and mate with or stabilize an object such as a portion of a navigation system (not shown), when such an object is mounted on the anchor device  12 . Similarly, a circumferential groove  62  is defined around the central shaft  14  near its proximal end, in order to receive a mating device included in an object to be mounted on the anchor device  12 . A square or otherwise suitably shaped proximal tip  64  may be provided on the central shaft  14  to mate with a suitable power-driven wrench to rotate the central shaft  14  during installation of the anchor device  12 . 
         [0036]    A T-wrench  70  shown in  FIG. 2  may include a handle  72 , and an open end  74  opposite the handle  72  is open to define a deep socket  76  designed to mate drivingly with the flats  56  or otherwise drivably configured portion of the positioner  50 , so as to rotate the positioner  50  with respect to the shaft  14  once the shaft  14  has been installed, as will be explained presently in greater detail. 
         [0037]    In one embodiment, the tapered distal end portion  38  of the sleeve  30  may be configured as shown at an enlarged scale in  FIG. 3 . While a longitudinally middle portion  78  of the sleeve is generally cylindrical, the distal or leading end portion  38  as shown in  FIG. 3  is tapered generally conically, but the several longitudinally extending ribs  42  each has a sharp edge  80  directed radially outwardly. For example, there may be four ribs  42  spaced evenly about the tapered distal end  38 , although as few as two or as many as six or more ribs  42  may be useful. A leading margin portion  82  of the tapered distal end portion  38  may be ground at a steeper angle to form the sharp edge  40 , and each of the ribs  42  may also have a tip of its distal end tapered to coincide with the sharp edge  40  and shaped to include a sharp leading edge  86 , so that as the sleeve  30  is moved into contact with a portion of a bone surrounding the central shaft  14  during installation of the anchor device  12 , the sharp leading edges  86  and the sharp edge  40  can engage the outer surface of the cortex of the bone and cut slightly into the cortex of the bone so as to engage the ribs  42  in the bone securely to prevent rotation of the sleeve  30 , and thus prevent rotation of the anchor device  12  with respect to the bone. 
         [0038]    Referring next to  FIG. 4 , the proximal face  90  of the sleeve  30  and the distal end face  92  of the positioner  50  have sets of opposed radially extending ramp surfaces or facets which engage each other closely at each of several positions of rotation, determined by equally spaced radially extending ridges  94  and  95 , and corresponding notches  96  and  97 . These permit the positioner  50  to rotate with respect to the central shaft  14  and the sleeve  30 , and also limit slippage of the faces  90  and  92  with respect to each other, to provide an indication to a surgeon of when sufficient torque has been applied to the positioner  50  to force the sleeve  30  to engage the tapered end portion  38  securely in a bone yet not extract the threaded tip  18 , as will be explained in greater detail presently. It will be understood that the number of ridges  94  and  95  and notches  96  and  97  will determine the incremental angle of rotation between angularly adjacent positions of engagement of the faces  90  and  92  with respect to each other, so that in the embodiment shown in  FIG. 4 , with  28  ridges  94  shown on the face  90  and  28  ridges  95  on the face  92 , positions of stable engagement of the faces  90  and  92  with each other will be separated by slightly less than 13 degrees of rotation of the positioner  50  with respect to the central shaft  14  and the sleeve  30 . 
         [0039]    In actual practice, the number of ridges  94  and notches  96  may be different from that shown in  FIG. 4 , and, for example,  40  ridges  94  and  40  notches  96  may be provided, as well as equal numbers of ridges  95  and notches  97 , with an angular separation of 9 degrees between adjacent fully mated positions of stability. For example, each ridge  94  or  95  may have height  98  of about 0.010 inch (0.25 mm) at the radially outer surface of the sleeve  30  and the driver  50 . While the ridges  94 ,  95  are shown herein as having equal slopes of 45 degrees at the outer circumferences of the mating faces  90  and  92 , with respect to a plane  104  extending diametrically through the sleeve  30  and a particular ridge  94 , it will be understood that other combinations of ridge height  98 , facet slopes, and angular separation of stable positions may also be chosen. For example, a shallower angle  100  may be chosen in combination with a steeper angle  102  in order to require more torque to be applied to the positioner  50  in one direction of rotation than in the other, in moving from one mated position to the next. 
         [0040]    As shown in  FIG. 5 , for example, the coupling  46  has includes four legs  48  interconnected with each other by a belt or ring. Each leg  48  has a catch  110  at each end, extending radially inward and engaged in the respective groove  52  or  54 . The catches  110  are separated axially apart from each other along each leg  48  by a distance  112  which is greater than the distance  114  between the surfaces defining the nearer sides, respectively, of the grooves  52  and  54  on the sleeve  30  and the driver  50 . The distance  112  is thus great enough to allow the ridges  94  of the faces  90  and  92  to pass by each other freely, with the faces  90  and  92  separated from contact with each other. 
         [0041]    The coupling  46 , however, prevents the positioner  50  from being separated completely from the sleeve  30 , as the catches  110  engage the shoulder surfaces defining the grooves  52  and  54 . Turning the positioner  50  in a direction in which the thread  58  moves the sleeve positioner  50  toward the proximal end of the shaft  14  thus brings the sleeve  30  along with the positioner  50 , so as to disengage the tapered distal end portion  38  of the sleeve  30  from a bone. 
         [0042]    As shown in  FIG. 7 , rotation of the positioner  50  in the direction indicated by the arrow  116  in  FIG. 7  causes the ridges  95  of the face  92  of the positioner  50  to ride up along the sloping faces of the ridges  94  of the face  90  of the sleeve  30 , which is prevented from rotating with respect to the shaft  14  by the engagement of the pin  34  in the slot  32  as shown in  FIGS. 1 and 5 . As the opposing ridges  95  pass by the ridges  94  with continued rotation of the positioner  50 , any upward pressure on the sleeve  30 , that is force urging the sleeve  30  toward the positioner  50 , will urge the ridges  94  of the face  90  of the sleeve  30  to slide along the following slope of the ridge  95  of the face  92  of the positioner  50  in an angular stepping manner until the ridges  94  and grooves  97  are again mated as shown in  FIG. 6 , but with the sleeve  30  in a position slightly further toward the distal end  16  of the shaft  14 . 
         [0043]    The anchor device  12  is utilized, for example, to support a portion of an infrared navigation system on a bone in connection with procedures such as surgical replacement of an injured knee joint, in which case the anchor device  12  is installed in the femur  125  of a patient. To install the anchor device  12 , as shown in  FIGS. 8-11 , for example, soft tissue  120  is dissected and retracted to expose a surface  122  of the anterior cortex  124  of a femur  125  as previously mentioned. A drill  126  may be used to bore a hole  128  having a diameter  130  about equal to or slightly greater than the diameter  22  of the central shaft  14 , so that the central shaft  14  can be inserted through the cortex  124  and into the marrow  132  a short distance. The drill  126  is then retracted and, as shown in  FIG. 9 , the distal end  16  of the central shaft  14  of the anchoring device  12  is inserted into the bone through the hole  128 , led by the self drilling, self tapping threaded tip  18 . Once the threaded tip  18  engages the cortex  134  on the opposite, posterior, side of the femur  125 , the central shaft  14  is rotated to drill a hole  129  into the opposite, posterior, cortex portion  134  of the femur  125 , simultaneously tapping the hole  129  and screwing the tip  18  into the posterior cortex  134  until the shoulder  24  at the distal end of the shaft is brought to bear securely against the interior of the posterior cortex  134 . 
         [0044]    When it is determined by the user, as by an increase in the torque required to turn the central shaft  14 , that the threaded tip  18  has drawn the shoulder  24  of the distal end  16  of the central shaft  14  firmly into contact against the interior of the cortex  134  of the opposite or posterior side of the femur  125 , rotation of the central shaft  14  is stopped, and a suitable wrench, for example the T wrench  70 , is utilized to rotate the positioner  50  about the central shaft  14 . The rotation of the positioner  50  about the central shaft  14  is accomplished initially without applying any significant amount of torque to the central shaft  14  itself, but it may be desirable to grasp the sleeve  30  to steady it and resist any tendency of the sleeve and the central shaft  14  to rotate as the positioner  50  is moved along the central shaft  14  by the threads  58 . It may also be desirable to urge the sleeve  30  downward along the central shaft  14  to keep the notched opposite faces  90  and  92  of the sleeve  30  and the positioner  50  separated from each other until the tapered distal end portion  38  of the sleeve  30  engages the surface  122  of the femur  125  surrounding the hole  128 . As the sleeve  30  moves downward along the central shaft  14  and through the soft tissue surrounding the bone  125 , the sleeve  30  is not required to rotate with respect to the soft tissue  120 , but only moves downward through the soft tissue without causing unnecessary trauma to the surrounding soft tissue  120 . 
         [0045]    Once the tapered distal end portion  38  engages the surface  122  of the anterior portion  124  of the cortex of the bone, the tips  84  of the ribs  42  begin to cut into the bone tissue surrounding the hole  128 , and by engaging the surrounding bone tissue, the ribs  42  prevent the sleeve  30  from rotating. Because the slot  32  of the sleeve  30  is engaged with the pin  34  on the shaft  14 , both the sleeve and the shaft  14  are prevented from rotating with respect to the bone as the wrench  70  continues to rotate the positioner  50  about the shaft  14 . 
         [0046]    As the distal end portion  38  of the sleeve  30  contacts and penetrates the surface of the bone, the distal end face  92  of the positioner  50  unavoidably is moved into contact against the proximal end face  90  of the sleeve  30 , bringing the ridges  95  on the distal end face  92  into contact against the ridges  94  on the proximal face  90  of the sleeve  30 . Greater resistance to movement of the sleeve along the shaft  14  naturally requires greater torque to be applied to the positioner  50  to rotate it in the direction of the arrow  116  in the next subsequent position of mating engagement between opposing notches  94  and grooves  96 . Thus, as the sharpened tapered distal end  38  is more deeply engaged and the bone-engaging sleeve  30  meets greater opposition from the bone it becomes more difficult to turn the positioner  50  about the central shaft  14 . Continued rotation of the positioner  50  drives the sleeve  30  toward the bone  125  forcing the tapered distal end portion  38  of the sleeve  30  into deeper and firmer engagement in the anterior cortex  124  of the bone around the hole  128 , but the torque required to turn the handle  72  of the T wrench  70  increases as rotation of the positioner  50  is continued ridge-by-ridge, until the tapered distal end  38  is securely engaged in the bone  125  and further rotation to move the ridges  95  through the next small angular increment, to the next position of mating engagement in the notches  96 , requires a noticeable increase in torque to be applied to the wrench  70 . At a point before a significant increase in torque would have to be applied to the wrench  70  to move the positioner  50  through the next incremental angle, so that further rotation might risk extracting the threaded tip  18  from the cortex on the far side of the bone, the anchor device  12  will be firmly engaged in the bone  125  and further rotation of the wrench would thereafter not be necessary. 
         [0047]    Once firm installation of the anchor device  12  is accomplished, the wrench  70  may be removed and thereafter the anchor is available for use as shown in  FIG. 11  to support an object  140  such as in infrared navigation system element, which may be supported by the positioner  50  or otherwise engaged on the proximal end of the central shaft  14 , as by being mated with the pins  60  or the groove  62 . 
         [0048]    As shown in  FIGS. 12-15 , an alternative anchoring device  150  is similar to the device  12  in many respects, but has a continuous external thread  152  extending proximally from the distal end  154  of its central shaft  156 . Instead of having a shoulder such as the shoulder  24  on the central shaft  14  of the device  12  described above, the distal end  154  has a short drilling tip  158  which may be helically fluted. A tap portion  160  which may also be fluted is located adjacent to the drilling tip  158  and is tapered, extending between the drilling tip  158  and the full diameter threaded portion  152  of the central shaft  156 . 
         [0049]    Use of the anchoring device  150  is somewhat simpler than use of the anchoring device  12 , as it is unnecessary to use a separate drill to form an initial hole in the near or anterior cortex  124  of the bone. Instead, as shown in  FIG. 13 , a simple opening is first cut through the surrounding soft tissue  162  to expose the surface of a bone such as a human femur  125  on which the device  150  is to be installed. 
         [0050]    Next with the sleeve positioner  50  and the sleeve  30  retracted toward the proximal end  159  of the central shaft  156 , the distal end  154  of the central shaft  156  is inserted through the opening in the soft tissue  162 , and the drill tip  158  of the anchor device  150  is brought into contact against the surface  122  of the anterior cortex  124 . Using a suitable driver  164  to engage the proximal end  64  of the shaft  156 , the shaft  156  is rotated so that the drill tip  158  bores a hole through the anterior cortex  124  and the tap  160  cuts threads  166  in the anterior cortex  124 , thus forming a threaded bore through the anterior cortex  124  with the central shaft  156  directed toward the opposite, posterior, side of the femur  125 . 
         [0051]    With continued rotation, the external thread  152  of the central shaft  156  becomes engaged in the threads  166 , and rotation of the central shaft is continued, moving the central shaft into the interior of the femur  125 , so that the drill tip  158  is eventually brought to bear against the interior surface of the posterior cortex  134 , as shown in  FIG. 14 . Rotation of the central shaft  156  is continued until the drill tip  158  has definitely encountered and may have begun to cut into the cortex  134 . In reaching this position, as shown in  FIG. 14 , the drill tip  158  and tap portion  160  have engaged the spongy or cancellous tissue  170  adjacent the posterior cortex  134 , as can be determined by an increase in the effort needed to rotate the central shaft  156 . 
         [0052]    Rotation of the central shaft  156  is then ceased, and a tool such as the T-wrench  70  is then used, as described before with respect to the mounting device  12 , to engage the flats  56  of the nut-like portion of the sleeve positioner  50  and rotate the positioner  50  relative to the central shaft  156 , thus driving the sleeve  30  along the central shaft  156  to engage the tapered distal end portion of the sleeve  30  in the cortex  124  of the anterior side of the femur  125 . The sleeve positioner  50  is rotated until the slip limiting effect of the mutual contact between the ridges  94 ,  95  and notches  95 ,  97  of the faces  92  and  90  of the positioner  50  and the sleeve  30  provides definitely increased resistance to further rotation of the positioner  50 , with the tapered distal end portion  38  of the sleeve  30  embedded in the cortex  124  far enough so that the tips  84  and sharp edges  80  of the ribs  42  engage the cortex  124  securely to prevent rotation of the sleeve  30  with respect to the femur  125 . As in the anchor device  12 , because of the engagement of the pin  34  extending through the central shaft  156  and engaged in the slot  32 , the sleeve  30  prevents the central shaft  156  from rotating, so that an object such as a portion of a computer aided navigation system is held securely in its position with respect to the femur  125  when the object is attached to the proximal end  159  of the central shaft  156  in the same manner described above with respect to the central shaft  14  of the anchoring device  12 . 
         [0053]    As with the device  12 , removal from engagement in the femur  125  or other bone is simply the reverse of the procedure for mounting the anchoring device  150 , as rotation of the positioning device  50  in the opposite direction along the thread  52  on the proximal end portion of the central shaft  156  first disengages the torque-resisting limited slip latching faces  90  and  92  from each other, while further movement of the positioner  50  along the central shaft  156  causes the coupling  46  to withdraw the distal end portion  38  of the sleeve  30  from engagement in the cortex  124  of the femur. Once the sleeve  30  has been withdrawn from engagement in the surface of the bone the central shaft  156  can be rotated in the opposite direction from that used to install it and thus can be removed from the threaded bore  168 . 
         [0054]    Because the central shaft  156  can be installed in a bone such as the femur  125  before the sleeve  30  is moved longitudinally along the shaft  156  through the soft tissue and toward the bone, the soft tissue is exposed to only a limited amount of rotation of the sleeve as the central shaft is being installed, and movement of the sleeve along the central shaft is accomplished thereafter without rotation, causing only a minimal amount of trauma to the soft tissue  162  surrounding the sleeve  30 . 
         [0055]    The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.