Abstract:
An instrument for seating taper junctions of modular implants engages two components and provides a sustained assembly force along the junction axis with great mechanical advantage. In one embodiment the instrument comprises a lever mechanism for generating the mechanical advantage. In another embodiment, the instrument provides for an indicator of the amount of force being applied to the junction. In another embodiment, the instrument engages the components for such that both assembly and disassembly can be accomplished.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a tool for assembling a multicomponent prosthesis. More particularly, the present invention relates to a tool for compressing a taper junction between two components of an orthopaedic joint replacement component. 
     It is known in the art of orthopaedic joint replacement to provide an implant having multiple components that are assembled at the time of surgery. For example, it is known to provide a stemmed implant in which the stem is provided separately from a body portion. Typically, the junction between the components includes corresponding male and female tapers. An example of such an implant for hip replacement surgery is taught in U.S. Pat. No. 3,067,740. An example of such an implant for knee replacement surgery is taught in U.S. Pat. No. 5,290,313. Note that in this patent, the tapers are self-locking tapers. A screw (not shown) is described as optionally useful to further secure the junction against loosening. Other exemplary prior art taper junctions include hip femoral head-to-stem junction, shoulder humeral head-to-stem junction, knee femoral component-to-stem junction, segmental long bone component-to-component junction, and many others. In these prior art taper junctions, the junction is held with threaded fasteners, self-locking tapers, or a combination of threaded fasteners and self-locking tapers. It is important that the mating tapers be well seated for a tight assembly. In the case of self-locking tapers, it is known to impact the tapers together by using a mallet until they lock. Where threaded fasteners are used, it is taught, as in U.S. Pat. No. 3,067,740, to seat the taper junction by tightening the fastener. U.S. Pat. No. 5,290,313 teaches first seating the junction to lock the tapers and then applying the threaded fasteners. 
     SUMMARY OF THE INVENTION 
     The present invention improves on the use of taper junctions by providing an instrument for seating these junctions more consistently and with more force than is possible with impact techniques or the use of threaded implant fasteners. 
     The junction assembly instrument engages first and second components and provides a sustained assembly force along the junction axis with great mechanical advantage. In one embodiment the instrument comprises a lever mechanism for generating the mechanical advantage. In another embodiment, the instrument provides for an indicator of the amount of force being applied to the junction. In another embodiment, the instrument engages the components such that both assembly and disassembly can be accomplished. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partially sectioned frontal view of an embodiment of the present invention engaged with a modular hip stem prosthesis. 
     FIG. 2 is a partially sectioned frontal view of the embodiment of FIG. 1 with the operating handle moved to the compressed position. 
     FIG. 3 is a detail view showing the operation of an indicator according to one embodiment of the invention. 
     FIG. 4 is a partially sectioned detail view of the embodiment of FIG. 1 prior to compression of the implant junction. 
     FIG. 5 is a partially sectioned detail view of the embodiment of FIG. 1 after compression of the implant junction. 
     FIG. 6 is a frontal view of the embodiment of FIG. 1 shown in use to assemble a modular implant in vivo. 
     FIG. 7 is a frontal view of the embodiment of FIG. 1 shown in use to assemble a modular implant on an operating room table. 
     FIG. 8 is a bottom view of an alternative embodiment of an engagement end of the present invention. 
     FIG. 9 is a top view of a mating implant component for use with the engagement end of FIG.  8 . 
     FIG. 10 is a side view of the engagement end of FIG. 8 received in the implant component of FIG.  9 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1-7 depict an illustrative junction assembly instrument for assembling first and second joint components. This particular illustrative example is shown being adapted for compressing a self-locking taper junction between a stem component and a proximal body component of a modular hip stem. 
     A modular hip stem implant  2  includes a stem component  4  and a proximal body component  6 . The stem component  4  has a bone contact portion  8  and a male taper junction portion  10 . A threaded stud  12  extends from the male taper  10 . The proximal body  6  includes a joint portion  14 , a female taper  16 , and a through hole  18  communicating with the female taper  16 . The through hole  18  is axially aligned with the female taper  16 . The through hole is preferably enlarged  19  proximally and includes a shoulder  17 . The hip stem implant  2  is assembled by axially aligning and seating the male taper  10  within the female taper  16 . Preferably the taper junction is self-locking such that upon being firmly seated the male and female tapers  10 ,  16  require great force to separate. The threaded stud  12  can be fitted with a nut (not shown) seated against shoulder  17  within enlarged opening  19  to further secure the junction. 
     A junction assembly tool  20  is advantageously used to seat modular components such as in the above described hip stem implant  2 . The tool  20  includes stationary handle  22  having a shaft  24  terminating in an engagement end  26  and a grip end  28 . The engagement end  26  is threaded for engaging the threaded stud  12  in axial force transmitting relationship. A pivot handle  30  includes a grip end  32 , a shaft  33 , and a working end  34 . The working end includes an L-shaped pivot block  36 . The pivot block  36  is connected to the stationary handle  22  via a connecting link  38  pinned at one end to the pivot block  36  to form a fulcrum  40  and pinned  42  at the other end to a mounting ring  44  affixed to the stationary handle  22 . A second engagement member  46  is mounted adjacent engagement end  26  of stationary handle  22  and is movable relative to engagement end  26 . In the exemplary embodiment, the second engagement member  46  is a sleeve coaxially mounted on engagement end  26  for longitudinal translation relative to engagement end  26 . A first end  48  of the second engagement member  46  is linked to the pivot block  36  and thus to the working end  34  of the pivot handle  30  by a connecting pin  50 . A second end  52  of the second engagement member  46  engages proximal body component  6  at shoulder  17  through enlarged portion  19  of through hole  18 . 
     An indicator  60  includes a pointer  62  having a first end  61  attached to the pivot handle  30  near the working end  34  and a second end  63  cantilevered away from the working end  34 . The pointed extends adjacent the pivot handle shaft  33 . Preferably, the pivot handle shaft  33  includes a longitudinal channel  64  in which the pointer  62  is positioned. The pivot handle shaft  33  includes a scale  66  adjacent the second end  63  of the pointer  62 . In the example, the scale  66  comprises a post  67  projecting from the shaft  33  and including an indicia mark  68 . 
     FIGS. 2-7 illustrate the use of the junction assembly tool  20  to assemble the modular hip stem implant  2  of FIG.  1 . Proximal body  6  is placed on stem  4  with the female taper  16  engaging the male taper  10  and threaded stud  12  extending through through hole  18 . Engagement end  26  of stationary handle  22  is threaded onto threaded stud  12 . If the handles  22 , 30  are held loosely, pivot handle  30  will swing away from stationary handle  22  as the second end  52  of second engagement member  46  presses against shoulder  17 . This separation of the handles  22 ,  30  is a result of the second engagement member  46  moving back along the engagement end  26 . As it moves back it pivots the pivot block  36  and thus the pivot handle  30  about the fulcrum  40 . By connecting the pivot block via the elongate connecting link  38 , the fulcrum  40  is permitted to move up and down slightly to prevent binding of the mechanism. Once the engagement end  26  securely engages the implant stem  4 , the handles are brought together to seat the stem  4  and proximal body  6  components. Forcing the handles together moves second engagement member  46  outwardly relative to engagement end  26 . The second end  52  presses against the proximal body  6  causing the proximal body  6  to move relative to the stem  4  into taper seating arrangement. 
     The coaxial arrangement of engagement member  46  and engagement end  26  is advantageous since it uniformly loads the tapers with a centrally aligned force through the threaded stud  12  and a uniform annular force against the shoulder  17 . 
     The axial arrangement of the handles in the illustrated embodiment is advantageous in that it allows for an elongate narrow tool. This configuration facilitates entry into narrow confines such as when the tool is used to seat implant components in-vivo as shown in FIG.  6 . However, the configuration is still easily used for back table assembly in the operating room as shown in FIG.  7 . In addition, the axial handle arrangement allows for large forces to be generated at the taper junction due to the relatively long distance from the grips  28 , 32  to the fulcrum  40  and the relatively short distance from the fulcrum  40  to the connecting pin  50 . The axial arrangement further contributes to high force capacity since a two-handed grip can be employed to make use of the entire upper body strength of the user if necessary. 
     Force applied to the pivot handle  30  tends to flex the pivot handle shaft  33 . Since the pointer  62  is cantilevered away from the working end  34 , it does not flex with the pivot handle shaft  33 . The amount of deflection of the pivot handle shaft  33  relative to the pointer  62  is a function of the amount of force applied to the handles and consequently is a function of the opposing forces applied to seat the tapers. By operating the handles to produce a predetermined relative deflection, a predetermined taper seating force be reproducibly applied. The scale  66  provides a convenient way to measure handle deflection. When the pointer  62  is aligned with the indicia mark  68  on the post  67  a predetermined force is applied to the taper junction. When the junction assembly tool is not in use, the pointer  62  is housed in the channel  64  which protects against damage to the pointer and its surroundings. 
     The exemplary embodiment has illustrated a tool for seating a junction between implant components. With only minor modification, the same tool can also be used for unseating the components. In the embodiment of FIGS. 1-7, the second end  52  of the second engagement member  46  presses against the proximal body  6  at shoulder  17  to seat the junction. If the handles are then moved apart, second end  52  retracts away from the shoulder. This is because although the threaded engagement between engagement end  26  and threaded stud  12  is capable of bi-directional force transmission, the pressing engagement of the second end  52  with the shoulder  17  is not bi-directional. If, on the other hand, second end  52  were bi-directionally engageable with proximal body  6 , then moving the handles apart would cause the joint components to move out of taper seated arrangement. FIG. 8 illustrates an alternate exemplary engagement end  70  for second engagement member  46  capable of bi-directional force transmission. FIG. 9 illustrates an alternative configuration  72  for the enlarged opening  19  of the proximal body  6 . The engagement end  70  has an oval tab  74  projecting radially from it. Enlarged opening  19  has a corresponding oval shaped side wall  76  for receiving the tab  74 . An undercut slot  78  is formed in the side wall  76 . The tab  74  can be axially inserted into the opening  19  and then rotated so that the tab engages the undercut slot for bi-directional force transmission as shown in FIG.  10 . 
     It will be understood by those skilled in the art that many other variations in design and construction may be made to the preferred embodiment without departing from the spirit and scope of the invention defined by the appended claims.