Abstract:
A joint prosthesis includes a bone engaging portion having a first recess portion, and an internal wall defining a second recess portion, an articulating component, an insert component having (i) a first insert portion configured to snugly fit within said second recess portion in contact with the internal wall, and (ii) a projecting portion that fixedly projects from said first insert portion and is configured to snugly fit with the first recess portion so as to rotationally fix the first insert portion within the second recess portion, said insert component also having a first coupling portion, and a mating component configured to mate with said articulating component and having a second coupling portion configured to mate with the first coupling portion to form a fixed male/female couple at variable version and inclination angular orientations with respect to the insert component.

Description:
This application is a continuation of prior application Ser. No. 11/025,185, filed Dec. 29, 2004, (now U.S. Pat. No. 8,444,698 issued May 21, 2013), the entire contents of which are herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to joint prosthesis, and particularly to prosthesis having an articulating head component. More specifically, the invention relates to a system for achieving infinitely variable positions for the head component relative to a bone engaging portion of the prosthesis. 
     Repair and replacement of human joints, such as the knee, shoulder, elbow and hip, has become a more and more frequent medical treatment. Longer life spans mean that the joints endure more wear and tear. More sports activities mean greater likelihood of serious joint injuries. Treatment of injuries, wear and disease in human joints has progressed from the use of orthotics to mask the problem, to fusion of the joint, to the use of prostheses to replace the damaged joint component(s). 
     As the success rate for total or partial joint replacements has increased, so too has the need for modularity and universality in the joint prosthesis. Patient variety means that no single size or configuration of joint prosthesis will suffice. The physical dimensions of a patient&#39;s joint components vary, as well as the bio-mechanic relationship between these components. For instance, in a shoulder prosthesis, the relationship between the articulating humeral and glenoid components can be significantly different between patients. These relationships are especially important where only one component of the joint is being replaced and must integrate with the existing natural opposing joint component. 
     In joint replacement procedures, the proximal end of a bone, such as the humerus, is resected to form a stable platform to receive a joint implant. In some cases, it is discovered after the implant has been fixed within the bone that the resection was inappropriate for the patient&#39;s joint. Correction of this problem requires, at a minimum, removal of the implant and implantation of a new implant to fit the resected surface. The availability of a differently sized or configured implant component is very beneficial, and even more important where further resection of the bone is necessary. 
     For instance, in many shoulder surgeries, only the humeral component is replaced, leaving the glenoid component intact. In this case, it is imperative that the articulating surface of the humeral component match the articulating surface of the glenoid component as perfectly as possible, both statically and dynamically. With a typical humeral prosthesis, version and inclination are adjusted by the geometry of the head of the prosthesis. In other words, certain pre-determined head geometries are available that can be selected for a mating glenoid component. Absent an infinite variety of pre-determined head geometries, the resulting humeral prosthesis can often only achieve a best-fit relationship to the glenoid component of the shoulder joint. 
     In a typical surgical procedure, a trial component will be used to determine the optimum final component to be fixed to the bone. In most cases, the surgeon is able to make a good selection that fits the joint very well. However, in some cases, the accuracy of the fit cannot be determined until the surgery is completed and the patient has had an opportunity to exercise the repaired joint. Where significantly problems arise, a revision surgery may be necessary to replace an improperly sized or configured joint component. One typical revision surgery requires removal of the entire prosthesis from the bone and replacement with a different prosthesis. 
     There is a significant need for a joint prosthesis that is both modular and universal. Such a prosthesis would be easily manipulated during the surgery and capable of achieving nearly infinite version and inclination angles. Moreover, an optimum prosthesis would be readily available for modification in a revision surgery without having to remove the entire prosthesis. 
     SUMMARY OF THE INVENTION 
     These and other needs of the prior art are met by the present invention in which a joint prosthesis includes a removable component to which the articulating component of the prosthesis is connected. The removable component permits adjustment of the angular orientation of the articulating component so that the joint prosthesis is truly universal. 
     In one aspect of the invention, the joint prosthesis includes a bone engaging portion having a first recess portion, and an internal wall defining a second recess portion, an articulating component, an insert component having (i) a first insert portion configured to snugly fit within said second recess portion in contact with the internal wall, and (ii) a projecting portion that fixedly projects from said first insert portion and is configured to snugly fit with the first recess portion so as to rotationally fix the first insert portion within the second recess portion, said insert component also having a first coupling portion, and a mating component configured to mate with said articulating component and having a second coupling portion configured to mate with the first coupling portion to form a fixed male/female couple at variable version and inclination angular orientations with respect to the insert component. 
     In a specific embodiment of the invention, a joint prosthesis includes a bone engaging portion configured for engagement within a bone of a patient, said bone engaging portion having a substantially cylindrical cavity and a platform surface defining a platform plane, an articulating component configured for articulating engagement with an opposing aspect of a joint, an insert component having a cylindrical portion configured to mate with said cavity by insertion of the cylindrical portion into the cavity through the platform plane, wherein said insert component is configured to mate with said bone engaging portion in a rotationally keyed configuration which rotationally fixes the insert component in the cavity at a predetermined rotational angle, and a mating component configured to mate with said articulating component and having a spherical portion configured to mate in a press-fit engagement with a tapered bore of said insert component. 
     In accordance with a method of the present invention, a joint prosthesis is constructed by placing an insert component into a complementary configured cavity defined in the proximal portion of a bone engaging implant, such as a stem. A fixation element, such as a screw, is used to fix the insert within the stem. A mating component is engaged with the insert component, such as by a press-fit engagement between a tapered bore in the insert and a compressible ball portion on the mating component. An articulating component, such as a femoral head, is then mated with the mating component, such as through a press-fit engagement. 
     In a further feature of the present invention, a revision procedure includes the step of accessing the fixation element through openings defined in at least the mating component. The fixation element is released from engagement with the stem so that the insert component is no longer fastened thereto. The insert component is then removed, preferably with the mating component and head components fastened undisturbed. 
     In yet another aspect, the removed insert component with the undisturbed mating component and head component can be transported to a replication instrument. The angular position of at least the mating component may be ascertained relative to a fixed datum using the instrument. That angular position can be conveyed to a new insert and mating component using the instrument. Once the three-dimensional angles have been properly replicated in the new prosthesis components, the mating component can be fixed within the insert component, preferably by impaction. The head component may also be engaged to the mating component, also preferably by impaction. The completed assembly is then conveyed to the stem that has not been removed from the patient&#39;s bone. The insert component is placed within the insert cavity in the stem and the fixation element is used to rigidly connect the insert component to the stem with the mating component and head component in their proper anatomic relation to the patient&#39;s bone. These steps can be implemented in a true revision surgery to replace an existing prosthesis, or can be carried out during an original joint replacement procedure. 
     It is one object of the invention to provide a joint prosthesis that is both modular and universal. This object is achieved by features that permit infinitely variable positioning of a mating joint component relative to a bone engaging portion of the prosthesis. 
     Another object is to provide a prosthesis that is readily available for modification, whether during initial implantation or during a subsequent revision procedure. One benefit of the invention is that this modification can occur without removing or disturbing the bone engaging component, or stem, of the implant. 
     These and other objects and benefits of the invention will be appreciated upon consideration of the following written description together with the accompanying figures. 
    
    
     
       DESCRIPTION OF THE FIGURES 
         FIG. 1  is a side view of a prior art humeral prosthesis. 
         FIG. 2  is an enlarged cross-sectional view of a portion of a joint prosthesis with a mounting element configured for articulating engagement with the stem of the prosthesis to permit angular positioning of a head component in multiple degrees of freedom. 
         FIG. 3  is a side exploded view of a modular prosthesis in accordance with one embodiment of the present invention that is adapted to facilitate modification or revision of the implant. 
         FIG. 4  is a front cross-section view of the modular prosthesis shown in  FIG. 3  in an assembled configuration. 
         FIG. 5  is a front perspective of a stem component of the modular prosthesis shown in  FIGS. 3-4 . 
         FIG. 6  is an enlarged cross-section view of a portion of the stem depicted in  FIG. 5 . 
         FIG. 7  is a top perspective view of an insert component of the modular prosthesis illustrated in  FIGS. 3-4 . 
         FIG. 8  is a side cross-section view of the insert component shown in  FIG. 7 . 
         FIG. 9  is a side cross-section view of a mating component of the modular prosthesis shown in  FIGS. 3-4 . 
         FIG. 10  is a side view of a fixation component of the modular prosthesis shown in  FIGS. 3-4 . 
         FIG. 11  is a side view of a replication instrument for use in replicating the orientation of the mating component of the prosthesis shown in  FIGS. 3-4 . 
         FIG. 12  is a perspective view of a dummy stem for use in the replication instrument shown in  FIG. 11 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and described in the following written specification. It is understood that no limitation to the scope of the invention is thereby intended. It is further understood that the present invention includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the invention as would normally occur to one skilled in the art to which this invention pertains. 
     The present invention contemplates a joint prosthesis with an articulating component that must be positioned at a particular angular orientation to replicate and accommodate anatomic features of the patient&#39;s joint. In the following description, the prosthesis is identified as a humeral prosthesis for a shoulder implant. It is understood, however, that the principles of this invention can be applied to other prosthesis that include an adjustable component. The present invention is particularly suited for prostheses that are amenable to replacement or adjustment in a revision surgery. 
     By way of background, a typical joint prosthesis of the prior art is illustrated in  FIG. 1 . The prosthesis  10  is the humeral component of a shoulder prosthesis that can be implanted in the humerus bone for articulating engagement with the natural glenoid or with a glenoid prosthesis. The prosthesis  10  includes a stem  12  configured to be implanted within the humerus bone in a conventional manner. The stem  12  forms a platform surface  15  that faces the glenoid component of the joint when the prosthesis is in its operative position. The platform surface  15  defines a tapered bore for use in mounting the articulating head component  14 . The head component includes a tapered post  18  that can be press-fit or friction-fit within the tapered bore  16  to firmly mount the head component to the stem  12 . 
     The prosthesis  10  can be a modular prosthesis, meaning that a number of stem and head geometries can be provided from which a selection can be made that most closely approximates the natural joint components of the patient. Thus, the angle of the platform surface  15  can be different among stems  12 . While all head components  14  will include a generally spherical bearing surface  19 , the orientation of this surface relative to the platform surface  15  can be changed. Specifically, the location of the post  18  relative to the bearing surface  19  can be offset from the center of the surface (i.e., an eccentric head). In some cases, the angle of the post can be different between head components  14 . 
     An improved modular prosthesis introduces an articulating mounting element  30  between the stem  12  and a head component  20 , as shown in  FIG. 2 . This mounting element  30  is shown and described in co-pending application Ser. No. 10/748,448 (the &#39;448 application), entitled JOINT PROSTHESIS WITH INFINITELY POSITIONABLE HEAD, filed on Dec. 30, 2003, and owned by the assignee of the present invention. While the &#39;448 application provides a more detailed disclosure of the mounting element, which disclosure is incorporated herein by reference, following is a general description to facilitate an understanding of the present invention. 
     This mounting element  30  of the &#39;448 application includes a proximal portion  33  that mates with the head component  20 . In a specific embodiment, the proximal portion  33  defines a tapered surface that is press-fit or friction-fit within a complementary bore  21  defined in the head component. 
     The mounting element  30  further includes an articulating portion  34  that is preferably in the form of a spherical ball joint. The articulating portion is sized to achieve a press-fit engagement within a tapered bore  16  of the stem  12  when the portion  34  is pushed sufficiently far into the bore. The spherical shape of the articulating portion  34  allows the mounting element  30  to rotate about three dimensional axes x, y, z. Thus, the mounting element can rotate about its own axis (the x axis), pivot about a version axis (they axis) or pivot about an inclination axis (the z axis). 
     In addition to the press-fit engagement, a second fixation capability is disclosed in the &#39;448 application that augments the engagement between the articulating portion  34  and the tapered bore  16 . In particular, a machine screw  40  may be provided that includes a threaded portion  46  configured to mate with a threaded bore  18  in the stem  12 . The bore  18  is concentrically disposed at the base of the tapered bore  16 . The screw  40  is introduced into the threaded bore  18  through the articulating mounting element  30 . 
     As shown in  FIG. 2 , the mounting element  30  defines a central passageway  36  that extends through the length of element and that is open at its proximal and distal ends. The passageway defines an internal bearing surface  38  at the distal end of the element, or more specifically at the base of the articulating portion  34 . The screw includes a head  42  that includes an underside surface  44  that is complementary with the internal bearing surface. These two surfaces form a spherical bearing interface that allows the mounting element  30  to experience its full range of angular motion without interference from the screw  40 , even when the screw is loosely threaded into the threaded bore  18 . The articulating portion  34  defines a relief  39  at the distal end of the passageway  36  to facilitate this full range of movement of the mounting element. 
     The passageway  36  in the mounting element allows introduction of the screw  40  through the mounting element and into the threaded bore  18 . The screw can be loosely threaded into the bore to permit movement of the mounting element. Once the proper position for the mounting element  30  has been achieved, the screw can be tightened using a tool engaged within the tool recess  43  on the head  42  of the screw. As the screw is tightened, it drives the articulating portion  34  deeper into the angled bore  16 , thereby fixing the mounting element against further articulation. The screw thus combines with the friction or press-fit feature to lock the construct. 
     The mounting element  30  disclosed in the &#39;448 application represents a significant improvement over the prior art prosthesis  10  in that it greatly simplifies the process of aligning the mounting element, and ultimately the humeral head, at the proper anatomic angle for the patient&#39;s shoulder joint. Moreover, the mounting element  30  allows infinite positioning of the humeral head, in lieu of the limited selection of pre-defined angles available with the prosthesis of the prior art. 
     Even though the mounting element  30  presents a significant advance over the prior prostheses, problems still arise when a revision surgery is indicated. During some primary implant procedures, the surgeon may discover that a different humeral head is needed after the final implant stem has been fixed within the humerus. In some cases, the accuracy of the fit of the prosthetic components cannot be determined until the surgery is completed and the patient has had an opportunity to exercise the repaired joint. Where significant problems arise, a revision surgery may be necessary long after the primary surgery to replace an improperly sized or configured joint component. In most cases, the modular components of the prosthesis cannot be removed without also removing the component, or stem, fixed within the bone. Removal and replacement of an implanted stem is often problematic and runs the risk of creating a revision construct of poor integrity. 
     The present invention addresses the problem of revision surgeries on prosthetic implants by providing an insert component that allows the bone implanted component to remain within the bone. In accordance with one embodiment of the invention, a prosthesis  50  is provided as illustrated in  FIGS. 3-4  that includes a stem  52 , an insert component  54 , a fixation element  56  and a mating component  58 . The stem  52  is configured to be implanted within a bone of a patient and may be identical in most respects to prior stems used for similar joint replacement procedures. More particularly, the portion of the stem  52  that is implanted within the prepared intramedullary canal of the humerus may be identical to the prior art stem  12  shown in  FIG. 1 . As with the prior art stems, the stem  52  includes a platform surface  60  that is aligned toward the mating aspect of the joint, or the glenoid aspect in the case of a shoulder prosthesis. 
     However, the platform surface  60  of the stem  50  in the present invention takes on different characteristics from the prior art. In particular, the platform surface is configured to receive an insert component  54  and a fixation element  56  operable to rigidly fix the insert component to the stem. The insert component  54  is adapted for engagement with the mating component  58  under conditions that allow adjustment of the angular orientation of that component. The mating component  58  is configured to receive an articulating component, such as the humeral head  20  shown in  FIG. 2 . 
     Referring to  FIGS. 5-6 , details of the platform surface  60  of the stem  50  are illustrated. The platform surface defines an insert cavity  62  with a base recess  64  embedded within the stem and a plate recess  66  opening into the platform surface. As shown in  FIG. 5 , the base recess  64  is preferably cylindrical, for ease of manufacturing and to facilitate placement of the insert component  54  within the insert cavity  62 . However, other cross-sectional configurations for the base recess may be acceptable. 
     The plate recess  66  is generally rectangular with an edge  67  that opens at the superior end  61  of the platform surface  60 . The plate recess preferably includes a rounded inboard end to facilitate manufacture of the recess  66 . For instance, the base recess  64  can be formed by drilling to a certain depth into the platform surface  60  of the stem  52 . The plate recess  66  can be initially formed by drilling concentrically with the base recess, but at a larger diameter and to a shallower depth. The platform surface can then be milled to carve out the open edge  67  of the plate recess. 
     The insert component  54  is configured to fit snugly within the insert cavity  62 , as can be seen from  FIGS. 7-8 . In particular, the insert component includes a base portion  70  that is configured to be snugly received within the base recess  64 . Thus, the cross section of the base portion preferably emulates the cross section of the base recess—i.e., the base portion  70  is cylindrical in the illustrated embodiment. The insert component further includes a plate portion  72  that is also configured to be snugly received within the plate recess  66 . As with the base portion, the plate portion  72  follows the configuration of the plate recess  66  so that the base portion is generally rectangular with a rounded inner edge. In the preferred embodiment, the plate portion  54  includes a tab  80  that extends from the cylindrical base portion  70  so that the free end  81  of the tab is accessible at the open edge  67  of the plate recess. Preferably, the free end  81  is substantially coincident with the open edge. 
     The plate portion  72  defines a lower surface  78  that rests within the plate recess  66 . The insert component is preferably sized so that the base portion  70  is slightly offset from the bottom wall  77  of the base recess  64  when the lower surface  78  of the plate portion  72  is situated within the plate recess. The free end  81  of the plate portion  72  includes a lower rounded edge  79  to provide a small access for a removal tool between the insert component and the insert cavity, as discussed in more detail herein. 
     In one feature of the invention, the insert component  54  defines a tapered bore  74 . The tapered bore mates in press-fit engagement with engagement surface  85  of a ball portion  84  of the mating component  58  ( FIG. 9 ). This press-fit engagement accomplishes final fixation of the mating component  58  with the stem  52 . This interface may be similar to the press-fit engagement described in the &#39;448 application incorporated by reference. The mating component preferably includes a tapered cylinder  82  that is configured for press-fit engagement within the complementary bore  21  of the humeral head  20  ( FIG. 2 ). The mating component includes a central bore  87  that may be configured for a press-fit engagement with a male feature on the humeral head, in lieu of or in addition to the press-fit against the outer surface of the tapered cylinder  82 . 
     In order to secure the mating component to the stem  52 , a fixation element  56  is provided that fixes the insert component  54  to the stem. In the preferred embodiment, the insert cavity  62  of the stem defines a threaded bore  68  in the base recess  64 . The fixation element  56  constitutes a screw, as shown in  FIG. 10 , with a threaded stem  92  adapted to engage the threaded bore  68 . The head  94  of the screw preferably includes a hex recess  96  for receiving a hex driving tool of known design. The insert component  54  includes a fastener bore  76  through the bottom wall  77  of the component to receive the fastener therethrough. Thus, the insert component is fixed to the stem  52  using the fixation element or screw  56 , as shown in  FIG. 4 . The ball portion  84  of the mating component  58  preferably defines a flared opening  89  to prevent contact between the mating component and the head of the screw when the mating component  58  is impacted within the tapered bore  74 . 
     The fixation element  56  represents one beneficial feature of the prosthesis  50  of the present invention. Specifically, the fixation element allows removal of the insert component  54  from the prosthesis stem  52  at any time, including when the mating component  56  is in solid engagement with the insert component. This feature facilitates revision of the articulating component, or humeral head, at any time by simply unscrewing the fixation element  56  from the threaded bore  68  in the stem. When the fixation element  56  is removed, the insert component  54  can be readily extracted from the insert cavity  62  in the stem. Preferably, a tool can be pressed between the rounded edge  79  of the tab portion  80  of the insert component and the platform surface  60  of the stem to help dislodge the insert without contacting the mating component  58 . Once removed, the insert component and mating component can serve as a trial component that is replicated in a final prosthesis. 
     Whether as a final implant or a trial implant, when the mating portion  58  is installed in the tapered bore  74 , the ball portion  84  may be initially loosely situated within the bore  74  so that the angular orientation of the mating component  58  can be adjusted. This adjustment may occur with the articulating head component  20  mounted on the mating component. Once the proper angles have been determined, the mating component can be fixed within the tapered bore by impaction in a known manner, and the humeral head can be added in a similar fashion. It can be appreciated that since the mating component is engaging a removable insert component  54  the impaction steps may occur apart from the implant stem  52 . Thus, the impaction of the mating component into the insert component, and the impaction of the articulating head onto the mating component can occur on a fixture. Rigid fixation of the final implant may be accomplished through means other than impaction, but this fixation may still occur apart from the stem implanted within the patient&#39;s bone. 
     Preferably, the adjustment of the angular position of the mating component for use in a final prosthesis can occur using a replication instrument, such as the instrument disclosed in co-pending application Ser. No. 10/879,261 (the &#39;261 application), entitled INSTRUMENTATION FOR RECORDING AND REPLICATING ORTHOPAEDIC IMPLANT ORIENTATION, owned by the assignee of the present invention, the disclosure of which is incorporated herein by reference. 
     While details of the instrument are found in the &#39;261 application, following is a general description of the instrument  100  as depicted in  FIG. 11 . In particular, the instrument includes a base assembly  102  that carries a stationary clamp element  104  and a movable clamp element  106 . An adjustment mechanism  108  may be manually operated to move the movable clamp element toward the stationary element  104 . The neck of the prosthesis stem  52  is provided with positioning grooves  53   a  and  53   b . The superior groove  53   a  accepts the fixed clamp element  104 , while a pair of inferior grooves  53   b  are configured to mate with the movable clamp element  106 . When the neck of the stem is engaged by the clamp elements  104 ,  106 , a fixed datum D is established that is perpendicular to the platform surface  60 . The spatial angular orientation of the mating component  58  is gauged relative to this datum. The base assembly  102  thus establishes a fixed spatial position for this datum that can be used to replicate the angles of the mating component. 
     To achieve this replication, the instrument  100  further includes a replication fixture  110  that is mounted on the base assembly  102 . The fixture includes a platform  112  with legs  114  that are supported on the base assembly. The platform  110  includes an annular dome  116  which supports a spherical washer  118  on one surface and a cannulated guide member  120  on the opposite surface. The guide member includes a hollow stem portion  121  that passes through the dome  116  and washer  118 . The stem portion  121  is threaded to receive a locking nut  122  to fix the angular orientation of the guide member  120  relative to the datum D. 
     As explained in more detail in the &#39;261 application, the guide member  120  cannula allows passage of an alignment tool  125 , and more particularly the guide shaft  127  of the tool. The distal end of the guide shaft is sized to fit snugly within the bore  87  of the tapered cylinder  58 . When the guide shaft  127  is situated within the cylinder of the mating component used as part of the trial assembly, the guide member  120  and spherical washer  118  assume a corresponding spatial angle relative to the dome  116 . At this point in the method, the locking nut is tightened, thereby fixing the three-dimensional angular position of the guide member  120 . The replication fixture  110  is then removed and stem  52  is released from the base assembly. A final humeral prosthesis configured as the prosthesis  50  shown in  FIGS. 3-4  may then clamped within the base assembly with a final mating element  58  loosely engaged within the tapered bore  74  of a final insert component  54 . The alignment tool is reinserted into the guide member and the guide shaft is engaged with the mating component to replicate the angular orientation of the trial component. The alignment tool  125  is configured with an impaction end  129  that can be struck with a mallet to impact the mating element into the insert component to form the replicated final construct. Once the humeral head is impacted onto the mating component, the insert component can be positioned within the insert cavity  62  of a stem  52  implanted within the prepared intramedullary canal of the patient&#39;s bone. The insert component is then fixed in place using the fixation element  56 . 
     As shown in  FIG. 11 , the replication instrument  100  engages a prosthesis  50  that can be configured as a final or a trial prosthesis. However, for the purposes of providing a baseline for replicating the angular orientation of the articulating components of the joint, an entire bone implant is not necessary. Thus, in an alternative method for replicating the necessary angles, a dummy prosthesis  150  is provided as shown in  FIG. 12 . The dummy prosthesis  150  meticulously emulates the proximal portion of the trial or final prosthesis  50  to provide the proper alignment of the datum line D ( FIG. 11 ). Thus, the dummy prosthesis includes a truncated stem  152  that includes positioning grooves  153   a ,  153   b  that are identical to the grooves  53   a ,  53   b  described above. These dummy grooves are engaged by the clamp elements  104 ,  106 , in the manner described above. The proximal end of the dummy prosthesis  150  defines an insert cavity  162  with a base recess  164  and plate recess  166 , all configured to receive the insert component  54 . 
     The dummy prosthesis  150  functions the same as a final or trial prosthesis when mounted within the replication instrument  100 . However, the dummy stem  152  does not require the features found on an implantable stem, since the dummy prosthesis  150  is not configured for implantation within the patient&#39;s bone. Preferably, the stem  152  is about ⅓ the length of the final prosthesis stem so that the dummy prosthesis is easy to manipulate and fix within the replication instrument. 
     As explained above, the illustrated embodiment provides a prosthesis for the humeral aspect of the shoulder joint. Thus, the prosthesis  50  and its components are appropriately dimensioned for implantation within the humerus bone of the patient. In a specific embodiment, the base portion  70  of the insert component  54  has a diameter of 0.5 inches and a height of 0.183 inches to fit within a comparably dimensioned base recess  64 . The plate portion  72  has a width of 0.525 inches, a thickness of 0.1 inches, and an overall length of 0.752 inches to fit within a plate recess  66  of the same dimensions. The threaded stem  92  of the fixation screw  56  has a length of 0.197 inches to pass through the bottom wall  77  of the insert portion and into a bore  76  threaded to a depth of 0.175 inches. Preferably, the plate portion  72  of the insert is sized to sit substantially flush with the platform surface  60  of the prosthesis  50   
     Furthermore, the components of the prosthesis are formed of acceptable medical grade materials appropriate for the particular function being served by the components. For instance, the stem is formed of a material appropriate for implantation within a prepared intramedullary canal. The insert component and mating component are formed of a biocompatible material appropriate for the mating engagement between these components. 
     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. 
     For instance, the mating interface between the insert component and the mating component can be reversed. Specifically, the tapered bore may be incorporated into the mating component, while the ball portion projects from the insert component. The same modification can be made to the mating interface between the humeral head and the mating portion. 
     In the preferred embodiment, the fixation element is a machine screw; however, other forms of fixation or fastening are contemplated. For instance, rather than a screw that requires multiple turns for complete fixation, the element can incorporate a rotating locking cam or bayonet mount arrangement. As a further alternative, the fixation element can incorporate a press-in feature in which the element is pressed into the bore and locks in place, such as a spring clip construction. The fixation element must be capable of achieving a rigid attachment of the insert component to the stem. Moreover, it is preferred that the fixation element be capable of removal without disturbing or damaging the implanted stem. 
     In accordance with the preferred embodiment of the invention, the insert component is removable to facilitate revision or replacement of the angularly adjustable components. In one specific application, the insert component is implemented solely as a trial implant wherein the insert component is removably fixed to the stem to permit positioning of the mating component and femoral head in a proper anatomic orientation. With the mating component locked in its acceptable position, the insert component can be removed and placed within a replication instrument. The orientation of the mating component may then be replicated in a final prosthesis that does not include the insert component. 
     The present invention provides advantages even if the insert component is permanently fixed to the stem. Where the final implant includes the insert component, the insert component may be permanently fixed to the implanted stem with an appropriate fixation element. This variation still takes advantage of the ability to establish a final angular orientation of the mating component outside the surgical site.