Abstract:
An instrument for recording and replicating the spatial orientation of a dialable component of an implant relative to a bone engaging component of the implant includes a base assembly configured to support the trial implant so that the datum is in a predetermined orientation, a fixture supported on said base assembly, said fixture including an alignment tool configured to engage the first dialable component of the trial implant when the trial implant is supported by said base assembly, and a locking assembly configured to record the angular orientation of said alignment tool relative to said fixture with said alignment tool engaged with the first dialable component, and to replicate the recorded angular orientation with the alignment tool and a second dialable component.

Description:
This application is a divisional application of application Ser. No. 10/879,261 filed Jun. 29, 2004 (now U.S. Pat. No. 8,273,093 issued Sep. 25, 2012), the disclosure of which is herein totally incorporated by reference in its entirety. 
    
    
     FIELD 
     The present disclosure relates to prosthetic devices particularly shoulder prostheses and, more particularly, to a method and apparatus for replication of angular position of a humeral head of a shoulder prosthesis. 
     BACKGROUND INFORMATION 
     The state of the prosthetic shoulder market has progressed such that a surgeon generally approaches shoulder replacement surgery according to one of two strategies. One strategy is to perform the shoulder replacement surgery in accordance with a manufacturer&#39;s shoulder prosthesis or shoulder prosthesis product line. Particularly, a surgeon is provided with instrumentation and technique guidelines for the particular shoulder prosthesis or prosthesis line. The guidelines and/or instrumentation direct or dictate the angle of humeral head resection for the implant (prosthesis). This angle is in relation to the humeral intramedullary (IM) canal and is designed to match an optimum set of angles already present in the design of the prosthesis. 
     Another approach is to perform the shoulder replacement surgery in accordance with the patient&#39;s anatomy. Particularly, the humeral head is resected according to angles perceived to be “anatomic” in the opinion of the surgeon, not according to angles already present in the prosthesis itself. With this approach, the prosthesis is designed so that its configuration is intraoperatively adjustable. This allows the prosthesis to be adjustable in situ so that it can match the bony preparation. 
     Even with respect to these two divergent manners of surgical strategy, a common problem in shoulder surgery is matching the humeral resection angle across the articular margin to the predetermined angle designed into the prosthesis. This angle may include the angle between a prosthetic collar and the diaphyseal section of the stem. In the case of a collarless stem, the angle may inscribe the difference between the longitudinal axis of the stem and the inferior surface of the prosthetic head. It is considered optimal for fixation and biomechanics if the resected angle and the angle of the prosthesis are identical, thereby allowing intimate contact between the superior surface of resected bone and the inferior surface of the implant. 
     Moreover, the angular version in which the prosthesis is implanted will have a significant impact on the biomechanics of the prosthetic joint. Currently, most shoulder prosthesis systems on the market dictate the varus/valgus angle of the bone cut. This strategy does not allow the surgeon to intraoperatively match the implant to the patient&#39;s biomechanics after the prosthesis has been trialed, much less implanted. There are two known products currently marketed that attempt to resolve at least one of the above-noted issues. First, the Tornier-Aequalis system provides a modular junction within the metaphyseal region of the stem which allows a small block between the stem and humeral head to be interchanged. This block is available in multiple angles, thus allowing the surgeon to select the block that best fits the bony anatomy as resected. This system, however, has two primary weaknesses. First, the use of modular blocks obviously forces the design to only allow angular adjustments in finite increments. Second, the need to adjust the angle through modular blocks forces the surgeon to remove the stem, change out a component, and reset the stem. 
     A second product currently marketed provides a humeral head that is infinitely adjustable in varus/valgus and anterior/posterior angles relative to the stem portion of the prosthesis. This is accomplished through a spherical shaped protrusion on the superior surface of the stem that fits into a spherical recess in the humeral head. These mating surfaces allow the head to be articulated about the stem, thus allowing adjustable positioning of the head. The head can be locked in a position relative to the stem. This solution provides the ability to adjust the neck-shaft angle as well as the version through flexibility in the anterior/posterior angle. The locking mechanism, however, is sub-optimal since it requires the turning of a locking screw that has its head facing lateral and inferior, for which there is no access once the stem has been cemented. This eliminates the ability to adjust head position on the fly, and forces a total revision if articular surfaces ever need to be revised. Lastly, the protrusion on the humeral stem even when the humeral head is not in place limits the surgeon&#39;s access to the glenoid in preparation for a glenoid replacement. 
     It should be appreciated that it is desired to have a prosthesis that is intraoperatively adjustable so that it can match the bony preparation. One such prosthesis that has attempted to provide this design feature provides a complicated and cumbersome jig to permit replication of the head angular position between the trial prosthesis and the final implant. One problem with this jig is that the head position is taken directly from the long axis of the humeral stem. Thus, the trial and implant stems are required to adjust and replicate this position. This means that the system has a large number of components to handle, and the position cannot be adjusted with the stem in vivo. Another problem is that the jig itself is a quite complicated table-top device. The jig includes an adjustment gage and a triplanar disc that must be manipulated to effect replication of the head orientation between trial and final implant. 
     With a shoulder prosthesis that allows a surgeon to adjust the angular position of the humeral head (i.e. adjust the articular surface of the humeral head relative to the humeral stem/component and/or bone) a method must be available for trialing the prosthesis. When the trial prosthesis is implanted, several adjustments can be made to set the angular position of the prosthetic head relative to the humeral stem. A means must be available for transferring the settings obtained during the trialing process to the final implant. 
     What is thus needed in view of the above is a better method for trialing a shoulder prosthesis, and more particularly for replicating the orientation of the trial implant components in the final implant. There is also a need for a replication system that is quick and easy to operate, as well as for a system that can be utilized with implants utilizing locking taper fixation. 
     SUMMARY 
     These and other needs are met by the present inventive recording and replication instrumentation that is capable of replicating the orientation of a dialable component of an implant relative to the bone engaging component of the implant. For instance, the present invention has particular value in the preparation of a final humeral implant having a stem configured for fixation within the humerus bone and a spherical head that is “dialably” mounted to the stem—i.e., is adjustably mounted so the three-dimensional angulation of the head can be adjusted as necessary for the particular patient anatomy. 
     The instrumentation contemplated by the present invention includes a base assembly for supporting the stem of a trial implant and a final implant, and a replication fixture that is supported on the base assembly, preferably so that it can be removed. The base assembly includes means for supporting the stem of an implant with a pre-determined datum in a pre-determined orientation. The same datum is used between the trial and final implants so that the configuration of the implant can be accurately repeated. The base assembly can include a clamp arrangement for clamping the implant stem in at least two degrees of freedom, and preferably in the medial-lateral, superior-inferior and anterior-posterior directions. The stem of the implant can include an array of notches or grooves that are engaged by clamp elements of the base assembly to fix the stem in those degrees of freedom. 
     The replication assembly is supported by several legs that are fixed to the base assembly or most preferably are removably fit within corresponding recesses in the base assembly. The replication assembly includes a guide member that supports an alignment tool. The alignment tool is configured to engage the dialable aspect of the implant so that the tool “records” the angular orientation of the dialable aspect relative to the datum. A locking mechanism locks the position of the guide member so that the alignment tool can be removed and replaced when a final implant is substituted in the base assembly. The alignment tool can then be used to position the dialable aspect of the final implant. In one embodiment, the alignment tool is an impaction tool to impact the dialable aspect within the implant stem to lock the components together at the replicated orientation. 
     In accordance with one embodiment of the invention, a method is provided for preparing an implant for implantation within a joint. The implant includes a stem for implantation within a bone and a dialable component mounted to the stem. In certain embodiments, the dialable component can include a head configured to engage an opposing portion of the joint, and a mounting assembly configured to fix the head to the stem in variable angular orientations to fit the joint anatomy. The inventive method comprises the steps of placing a stem of a trial implant within a bone of the joint, positioning a dialable component on the stem of the trial implant within the joint, using a jig to record an orientation of the dialable component relative to a pre-determined datum on the trial implant, and using the same jig to replicate the orientation for a dialable component of a final implant relative to the same datum on the final implant. 
     In certain aspects of the invention, the positioning step includes fixing the position of the trial dialable component relative to the datum of the trial implant, and the recording step includes supporting the trial implant within the jig with the datum in a predetermined orientation. The recording step can further include fixing the trial implant within the jig, using the jig to guide an alignment tool to engage the dialable component of the trial implant, and locking the angular orientation of the alignment tool relative to the jig when the alignment tool is in engagement with the dialable component. 
     The replicating step may include fixing a final implant within the same jig, using the jig to guide an alignment tool along the angular orientation obtained in the locking step, and manipulating the dialable component of the final implant and the alignment tool until the alignment tool engages the dialable component of the final implant. In certain embodiment, the replicating step includes fixing the dialable component to the stem of the final implant. This fixing step is preferably performed using the alignment tool. For instance, where the mounting assembly of the final implant forms a press-fit engagement between the stem and the dialable component, the step of fixing the dialable component includes impacting the alignment tool to impact the dialable component into press-fit engagement with the stem of the final implant. 
     In certain embodiments, the dialable component includes a mounting assembly for mounting a head to the stem of the implant and the mounting assembly includes a fixed component fixed in the stem and a dialable component capable of assuming various angular orientations relative to the fixed component. In accordance with these embodiments, the datum is a predetermined axis passing through the fixed component. For example, the fixed component is a tapered bore and the datum is the centerline axis through the bore. 
     The present invention further contemplates an instrument for recording the position of a dialable component of an implant relative to a predetermined datum on a stem of the implant configured to be engaged in a bone. This instrument comprises, in certain embodiments, a base assembly for supporting the stem with the datum in a predetermined orientation, a fixture removably mountable on the base assembly when supporting the stem, the fixture including an alignment tool for engaging the dialable component of the implant when the stem is supported by the base assembly, and a locking assembly for locking the angular orientation of the alignment tool relative to the fixture with the alignment tool aligned with the dialable component so that the alignment tool defines an angular orientation of the dialable component relative to the datum. 
     The base assembly includes an assembly configured to clamp the stem to the base assembly. Preferably, the assembly is a clamp assembly that is self-centering so that it is configured to center the stem relative to the base assembly in at least two degrees of freedom. In one feature of the invention, the clamp assembly includes a v-shaped clamping element configured to engage angled grooves defined in the stem. 
     In one feature of the inventive instrument, the locking assembly is configured to permit removal of the alignment tool after the angular orientation has been locked. The fixture may include a domed element defining an opening for extension of the alignment tool therethrough, and the locking assembly includes a guide member defining a bore for extension of the alignment tool therethrough, the guide member defining a spherical surface for nested engagement with the domed element. In some embodiments, the guide member includes a post projecting from the spherical surface, the post defining the bore and the opening is sized to receive the post therethrough to permit movement of the guide member relative to the domed element in at least two degrees of freedom. Preferably, the permitted movement relative to the domed element is a spherical angle. The post of the guide member includes external threads and the locking assembly further includes a nut configured to engage the external threads. A washer can be provided between the nut and the domed element with the dome surface disposed between the washer and the spherical surface of the guide member. 
     Preferably, the fixture includes at least one support leg and the base assembly defines at least one recess corresponding to the at least one support leg for removable engagement of the leg therein. With this feature, the fixture can be removed to permit placement in or removal of an implant in the base assembly. 
     It is one object of the invention to provide a system and method for readily and easily preparing a final implant that accurately duplicates the orientation or configuration of a trial implant that has been used to determine the proper implant configuration. Another object resides in features of the invention that ensure virtually identical replication of the position of a dialable component of an orthopaedic implant. 
     One benefit of the invention is that the inventive system and method permits use of standard trial and final implants. Another benefit is that the final implant can be prepared apart from the patient without sacrificing an optimal anatomical fit for the implant, which can be especially valuable where locking tapers are used. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is side elevational view of an exemplary shoulder prosthesis for use with the present invention. 
         FIG. 2  is an exploded partial cross-sectional view of a portion of the shoulder prosthesis shown in  FIG. 1 . 
         FIG. 2   a  is an exploded partial view of a modified head and dialable component for use with the prosthesis shown in  FIGS. 1-2 . 
         FIG. 3  is a top perspective view of a portion of the shoulder prosthesis shown in  FIG. 1 . 
         FIG. 4  is a top perspective view of a base assembly for a recording and replication instrument in accordance with one embodiment of the present invention. 
         FIG. 5  is a side perspective view of the base assembly shown in  FIG. 4 . 
         FIG. 6  is a side view of the base assembly shown in  FIG. 4 . 
         FIG. 7  is a top view of the base assembly shown in  FIG. 4 . 
         FIG. 8  is an end view of the base assembly shown in  FIG. 4 . 
         FIG. 9  is a side perspective view of a replication fixture for the recording and replication instrument in accordance with one embodiment of the present invention. 
         FIG. 10  is an exploded partial cross-sectional view of the replication fixture shown in  FIG. 9 . 
         FIG. 11  is a side elevational view of a base assembly as depicted in  FIG. 4  and a replication fixture as depicted in  FIG. 9  forming the recording and replication instrument of one embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention. 
     For purposes of illustration, the preferred embodiment of the invention is described in connection with a shoulder prosthesis, and particularly the humeral component of the prosthesis. However, the inventive concepts disclosed herein can be used at other joints or bone interfaces of the body. The common feature among these alternative uses of the invention is that they include components that can assume a range of angular orientations relative to each other—angular orientations that must be duplicated from a trial implant or prosthesis to a final implant. 
     Referring now to  FIG. 1  there is shown an exemplary embodiment of a shoulder prosthesis, generally designated  10 . The shoulder prosthesis  10  includes a bone-engaging portion or stem  12  terminating in a neck  14 , a spherical head  16 , and a mounting assembly  18  mounting the head to the neck. The stem is configured for implantation within a bone, such as the humerus. The head  16  is configured to engage a mating joint component in the glenoid of the shoulder joint. The mounting assembly is “dialable”, which means, in the context of the present invention, that the angular orientation of the head  16  relative to the stem  12  can be adjusted, preferably in multiple degrees of freedom. For instance, the head  16  can be adjusted in the medial-lateral direction ML and the superior-inferior direction SI or degree of freedom. This three-dimensional adjustability allows the implant to be configured to the particular patient anatomy. 
     The dialable mounting assembly provides a mounting interface between the stem neck  14  and the head  16 . Preferably, the mounting assembly can be removably engaged to both the stem and the head. In certain implants, the angular adjustment feature is situated between the mounting assembly and the head. In other implants, the angular adjustment is effected between the mounting assembly and the stem. For purposes of explanation of the present invention, this latter adjustability feature is illustrated. In particular, the mounting assembly contemplates a tapered bore  20  defined in the neck  14  of the stem  12 . This tapered bore defines a centerline which is used as a datum D, as explained in more detail below. The bore terminates in a concentric threaded bore  22 . 
     The tapered bore receives a spherical joint  24  that is configured to be pressed into the tapered bore to form a locking engagement. This locking engagement can be enhanced by a mounting screw  30  that passes through a spherical seat  26  in the joint element and engages the threaded bore  22  in the stem. Tightening the screw into the bore pushes the spherical joint element  24  further into the tapered bore  20 . 
     In the illustrated embodiment, the head  16  is engaged to the mounting assembly  18  by a taper-fit engagement. In particular, the mounting assembly defines a tapered bore  28  to receive a complementary tapered post  32  in a known manner. Alternatively, the head can incorporate the female locking component, as depicted in  FIG. 2   a . In this alternative configuration, the head  16 ′ includes a tapered bore  32 ′ that mates with the tapered exterior  28 ′ of the mounting assembly  18 ′. The other elements of the construction of the dialable elements can be the same as illustrated in  FIG. 2 . 
     It is understood that the implant  10  as thus far described can be a trial implant used to size and orient the components of the implant. As is known, the associated bone is prepared to receive the stem  12  of the implant. Thus, for the illustrated shoulder implant, the humerus bone is resected, broached and reamed to receive the stem  12  with the neck  14  positioned in alignment with the glenoid component of the shoulder joint. The head  16  is selected to mate with the glenoid component of the joint, and the dialable mounting assembly  18  is adjusted to properly position the head on the neck within the shoulder joint. The implant components are locked together so their relative angular positions are fixed. The trial implant is then removed and the orientation recording and replication instrument of the present invention is used to duplicate the trial configuration in a final implant. 
     In accordance with one aspect of the invention, the orientation recording and replication instrument  110  (see  FIG. 11 ) comprises a base assembly  40  and a replication fixture  70  that is removably supported on the base assembly. Referring to  FIGS. 4-8 , details of the base assembly  40  will be discussed. In the illustrated embodiment, the base assembly includes two base halves  42  and  44  that are connected through a spacer  46 . Mounting bolts  48  pass between the two halves and through the spacers to form a solid support base construction. At least the base half  42 , and preferably both halves  42 ,  44 , are generally U-shaped to define a slot  50 . This slot  50  is sized to receive the stem  12  of an implant  10  passing therethrough when the implant is mounted within the base assembly, as depicted in  FIG. 11 . 
     The base assembly  40  includes an assembly that is configured to engage and clamp the neck  14  of an implant (trial or final). In one aspect of the invention, this assembly includes a self-centering clamp assembly  51  that is configured to clamp the implant to maintain a pre-determined datum in a pre-determined and repeatable orientation. The datum is a fixed location or axis on the implant that is common between the trial implant and the final implant, and is used to establish a known starting point for replicating the position of the dialable components of the implant. In the preferred embodiment, the datum is the central axis passing through the tapered bore  20  to which the dialable mounting assembly  18  mates. This datum is identified as the axis line D, as seen in  FIGS. 2 and 11 . As shown in  FIG. 11 , the base assembly  40  and clamp assembly  51  are preferably configured so that the datum D is vertically oriented when the implant  10  is supported by the base assembly. Of course, other pre-determined orientations of the datum D can be used, provided that the orientation is repeatable between all trial and final implants. 
     In order to ensure repeatability of the orientation of the datum D, the self-centering clamp assembly  51  contemplates clamp elements integrated with features on the implant itself. In particular, in accordance with the preferred embodiment, the clamp assembly includes a rear clamp element  52  with a clamping edge  53  configured to engage a rear clamping groove  37  (see  FIGS. 2-3 ) defined in the neck  14 . Similarly, the clamp assembly includes a centering clamp element  55  that includes a generally V-shaped clamping edge  56 . This V-shaped edge is configured to engage angled grooves  35  defined in the neck  14  on the opposite side of the implant from the rear groove  37  (see,  FIGS. 2-3 ). The grooves  35 ,  37  and the clamp elements  52 ,  55  are arranged so that the datum D is established at a pre-determined orientation when the implant is engaged by the clamp assembly  51 . This pre-determined orientation of the datum is repeatable for every implant having similarly configured clamping grooves  35  and  37 . 
     The V-shaped edge  56  of the centering clamp element  55  ensures that the neck  14  is laterally centered within the base assembly—i.e., the neck is centered along the axis L shown in  FIG. 7 . In accordance with the preferred embodiment, the rear clamp element  52  is fixed to the base half  44  so that the rear clamping edge  53  has a fixed position in along the fore-aft axis F ( FIG. 7 ). Thus, the rear clamp element establishes the fore-aft position of the datum D when the rear clamping edge  53  is engaged within the rear groove  37  of the implant. In order to fix the implant within the clamp assembly, the centering clamp element  55  is slidably mounted within the base half  42  so that centering clamp element can move along the fore-aft axis F. 
     In the illustrated embodiment, the centering clamp element  55  is carried on a slidable carriage  58  that is disposed within a dovetail notch  60  defined in the base half  42 , as seen best in  FIG. 4 . The carriage includes angled legs  62  that slide within the notch  60 . A clamping nut  64  bears against a plate  63  extending from the legs  62  so that tightening the nut pushes the centering clamp element  55  toward the rear clamp element  52 . The clamping nut is threaded onto a clamping post  66  that is mounted to the base half  42  by a drive plate  67  ( FIG. 7-8 ). As the clamping nut  64  is rotated, it pushes against the drive plate  67  to push the carriage  58  along the dovetail notch  60  toward the implant neck  14  until the V-shaped clamping edge  56  engages the angled grooves  35  in the implant. As the clamping nut is tightened further, the V-shaped clamping edge manipulates the implant until the grooves  35  are centered relative to the clamping edge  56 . At this point, the implant  10  is firmly held on the base assembly  40  by the self-centering clamp assembly  51  and the datum is automatically fixed in its pre-determined orientation so that the angular position of the dialable elements of the implant can be recorded. 
     The recording and replication feature of the invention is accomplished using the replication fixture  70  illustrated in  FIGS. 9-10 . The fixture includes a mounting platform  72  that is supported on the base assembly  40  by legs  74 . In one feature, the legs fit within recesses  69  ( FIGS. 4 ,  6 - 7 ) formed in the upper faces  43 ,  45  of the base halves  42 ,  44 . The recesses  69  and legs  74  are accurately sized so that the mounting platform  72  assumes a pre-determined orientation relative to the base assembly  40 , the clamp assembly  51 , and more importantly the implant  10  supported by those two assemblies. Preferably, at least one of the legs  74  and associated recesses  69  are indexed so the legs are placed in the same orientation each time the fixture is removed and replaced on the base assembly. For example, one leg and recess can be differently sized form the other legs and recesses. In a specific embodiment, the indexed leg and recess have a smaller mating diameter than the other legs and recesses. As a further alternative, one leg and one recess can include a visual indicia or the leg and recess can include a key between the two components, such as a key and slot arrangement, to ensure that the fixture can be placed on the base assembly in only one orientation. 
     In the preferred embodiment, the replication fixture  70  is removably supported on the base assembly  40 , which can facilitate positioning and removal of a trial or final implant within the base assembly. However, it is also contemplated that the replication fixture is essentially fixed to the base assembly—i.e., the base assembly and replication fixture are one-piece or fastened together. It is also contemplated that the overall configuration of the base assembly and replication fixture will depend upon the form of the implant being supported and replicated. 
     The replication fixture  70  utilizes an alignment tool  100  to interdigitate with the dialable feature of an implant mounted within the base assembly  40 . The fixture further includes a locking assembly  75  that is operable to lock the angular orientation of the alignment tool to the fixture once it has been oriented relative to the implant  10 , as described in more detail herein. The locking assembly  75  includes a dome  76  integrated into the mounting platform  72 . The dome defines an opening  77  through which the alignment tool  100  extends. A spherical washer  80  rides on the dome  75  with its inner surface  81  in contact with the outer surface  76   b  of the dome. The washer also defines an opening  83  through which the alignment tool extends. 
     The alignment tool is carried by a guide member  85  having a spherical base  86  that nests with the dome  76 . In particular, the guide base  86  includes an upper surface that bears against the inner surface  76   a  of the dome. A cylindrical post  90  projects from the spherical base  86  of the guide member. The post includes external threads  91  and an internal bore  92 . The bore is sized to slidingly receive the alignment tool  100  therethrough. The opening  83  in the spherical washer  80  is sized to closely receive the post  90  therethrough. The opening  77  in the dome  76  is larger in size to permit angulation of the guide member  85  relative to the mounting platform  72 . 
     In a specific embodiment, the post  90  of the guide member has an outer diameter of 0.875 inches, while the opening  83  in the washer has a diameter of 0.895 inches. On the other hand, the opening  77  in the dome has a diameter of 1.75 inches, or twice the diameter of the guide post, which will permit nested movement of the guide member  85  relative to the dome through a spherical angle of about ten degrees. 
     As shown in  FIG. 10 , the three components are successively assembled so that the dome  76  is sandwiched between the spherical washer  80  and the spherical base  86  of the guide member  85 . This construction ensures controlled movement of the guide member and alignment tool relative to the fixed datum D of the implant  10  clamped within the base assembly  40 . In other words, the construction only permits spherical angulation, while preventing transverse shifting of the tool relative to the datum. 
     The locking assembly  75  further includes a locking nut  94  with internal threads  95  to engage the external threads  91  of the guide post  90 . The locking nut includes a face  96  that bears against a flat  82  of the spherical washer  80 . When the nut is tightened it draws the washer and spherical base  86  of the guide member together with the dome  76  therebetween. When tightened enough, the nut  94  essentially locks the position of the guide member relative to the mounting platform  72  and to the base assembly  40 , and ultimately to the datum D that was fixed relative to the base assembly when the implant was clamped in place by the self-centering clamp assembly  51 . 
     It can be seen from the foregoing description that the two components of the instrument  110 , namely the base assembly  40  and the replication fixture  70 , work together to: a) establish a datum D on the implant that is fixed regardless of the implant; b) provides a movable guide member for an alignment tool; and c) provides means for locking the guide member once its position relative to the datum has been established by the alignment tool. 
     These features can be understood with reference to  FIG. 11 . Once the trial implant has been configured in situ it is removed from the patient and placed within the base assembly  40 . In particular, the neck  14  of the implant is positioned within the self-centering clamp assembly in the orientation shown in  FIG. 11 . In this position, the stem  12  extends through the slot  50  in the base half  42  so the datum D can be vertically aligned in its fixed position. The head component can be removed, leaving the dialable mounting assembly  18  in the orientation achieved during trialing. 
     With the trial implant clamped in place, the replication fixture  70  is placed on the base assembly  40  with the legs  74  seated within the corresponding recesses  69  in the base halves  42 ,  44 . The fixture  70  is thus positioned above the implant and more specifically above the dialable mounting assembly  18  (or the assembly  18 ′ depicted in  FIG. 2   a ). The alignment tool  100  is inserted through the bore  92  in the guide member post  90 . The tool includes a guide shaft  102  with a diameter sized for close running fit within the bore  92 . The tool  100  is provided with an insertion tip  103  at one end and a handle  105  at an opposite end. With the tool extending through the guide member, the handle  105  is manipulated until the tip  103  can be inserted into the tapered bore  28  in the mounting assembly  18 . The tip  103  can be appropriately sized to form a tight, but removable, fit with the bore  28 . Once the tip is firmly engaged with the mounting assembly, the locking assembly  75  can be manipulated to lock the position of the guide member relative to the mounting platform  72  and the datum D. Specifically, the nut  94  can be threaded onto the post  90  to tighten the spherical washer  80 , dome  76  and spherical base  86  together. The alignment tool  100  is then removed. 
     The replication fixture can now be removed from the base assembly. The trial implant is removed from the clamping assembly and a final implant is positioned within the base assembly. It is understood that the final implant is substantially similar to the trial implant so that the datum D on the final implant will have the same orientation within the shoulder joint as the trial implant. Moreover, it is understood that the trial and final implants include the same configuration of the rear clamping groove  37  and the angled grooves  35  so that when any implant is fixed within the self-centering clamp assembly  51  the datum D consistently achieves the same orientation. Without this control, the trial orientation of the dialable components of the implant cannot be accurately replicated. 
     The final implant is lodged within the clamp assembly  51  in the same manner as the trial implant so that the datum D is in the vertical orientation shown in  FIG. 11 . The same replication fixture  70  is now placed onto the base assembly with the guide member  85  in the clamped position it was placed in when aligned with the trial implant. The same alignment tool  100  is introduced into the bore  92  of the guide member  85  and is manipulated until the insertion tip  103  engages the tapered bore  28  of the dialable mounting assembly. It is understood that the dialable component of the mounting assembly is loosely disposed within the tapered bore  20  in the neck  14  of the final implant. Thus, once the insertion tip contacts the tapered bore  28 , manipulation of the alignment tool will alter the angular orientation of the dialable component. When the tip is fully seated within the bore  28 , the position of the dialable component of the final implant exactly replicates the trialed position. The dialable component can then be fixed within the implant. 
     In accordance with the illustrated embodiment, the dialable component relies upon a taper fit between the dialable component and the tapered bore  20 . In particular, the position of the dialable component is established when the spherical joint  24  is pressed into the tapered bore  28 . In order to facilitate this press-fit engagement without sacrificing the replicated orientation, the present invention contemplates that the alignment tool  100  also serves as an impacting tool. Thus, the handle includes an impaction end  106  that can be struck by a mallet. The insertion tip  103  is hardened so that it can transmit the mallet impact into a force driving the spherical joint  24  into the tapered bore  20 . Once the dialable mounting assembly is firmly seated, the alignment/impaction tool  100  can be retracted and the entire replication fixture  70  removed from the base assembly. 
     The fixation of the dialable component of the mounting assembly can be completed in accordance with the assembly—i.e., the mounting screw  30  can pass through the spherical seat  26 , with the spherical head engaged within the sear, and into the threaded bore  22  of the neck  14 . In addition, the head  16  can be added to the implant while the implant is still held within the clamp assembly  51 . Once the final implant has been assembled it can be removed from the base assembly  40  and implanted within the patient&#39;s shoulder joint. It should be understood that the present invention can be utilized with other means for fixing or engaging the dialable components other than the taper-fit or press-fit engagement described above. Moreover, it is understood that the impaction aspect of the invention can be utilized on other forms of fixation, such as interdigitating elements or teeth, to complete the assembly. In addition, the engagement can be by threading or similar mechanism, in which case the impaction tool would be appropriately configured to accomplish the threaded engagement. 
     The instrument  110  provides s simple and easily managed device for replicating the orientation of dialable components of an orthopaedic implant. In accordance with the preferred embodiment, the implant itself incorporates landmark features that are fixed relative to a pre-determined datum and that are keyed on to clamp and support the implant with the datum in a known and repeatable orientation. In other words, in accordance with the illustrated embodiment, the neck  14  of the implant  10  (both trial and final) is provided with clamping grooves  35  and  37 . Moreover, the clamping assembly includes clamp elements  52  and  55  that are configured to lodge within these corresponding grooves. It is understood, however, that the implant can be provided with alternative landmark features and that the clamp elements can be appropriately configured to key on the alternative landmarks. Furthermore, the clamp elements can be configured to key on existing fixed and repeatable landmark features that are integrated into the existing implant design. It is important that the clamp assembly be able to achieve repeatable positioning of the same datum on any implant, whether trial or final. 
     The illustrated embodiment also contemplates one type of dialable component for a mounting assembly to mount a humeral head to the stem of a humeral implant. Of course, other implants include other types of dialable components of the implant with other types of clamping mechanisms to fix the position of the dialable component. For instance, in certain embodiments, the dialable component is integrated into the humeral head itself—e.g., the spherical joint  24  is integral with the underside of the head  16 . If this modified head is configured to accept a mounting screw, such as the screw  30 , then the head will include a bore, like the bore  28 , that can be engaged by the alignment tool  100  in the manner described above. 
     In other embodiments, the dialable component of the mounting assembly may not be configured to mate with the insertion tip  103  of the alignment tool  100  described above. It is contemplated that the working end of the alignment tool  100  be configured as necessary to engage the dialable component when fixed within a trial implant and when loosely situated within a final implant. For example, the dialable component may include a stem and the working end of the alignment tool can include a bore configured for a tight fit with the stem. 
     The alignment tool  100  can also include an alignment groove  108  defined in its working end. The alignment groove can mate with a corresponding key in the dialable mounting assembly. The key and groove can be used to locate a rotational angle or to perform a rotating driving function, such as to fix a threaded engagement. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same should be considered as illustrative and not restrictive in character. It is understood that only the preferred embodiments have been presented and that all changes, modifications and further applications that come within the spirit of the invention are desired to be protected.