Abstract:
A trial implant system includes a stem component and an articulating component forming part of a joint of a patient. A trial mounting assembly mounts the articulating component to the stem component at variable relative angular positions. The trial mounting assembly includes an expandable ball portion and an expansion element configured to expand the ball portion upon rotation of the expansion element. In an expanded state, the ball portion contacts a cavity within the stem at three points of contact to fix the center of rotation of the ball portion at a pre-determined location within the stem. The stem cavity defines an inwardly projecting circumferential lip that bears against ball portion as it expands to ensure the three-point contact.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to prosthetic devices and, more specifically, to a system and method for replication of angular position of an articulating head of a prosthesis. The invention has particular application to the humeral component of a shoulder prosthesis. 
     2. Background Information 
     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. 
     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. 
     Presently, two strategies are available to a surgeon for shoulder replacement surgery. One strategy is to perform the shoulder replacement surgery in accordance with the design of a particular manufacturer&#39;s shoulder prosthesis or shoulder prosthesis product line. In this case, 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 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 the 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. Many 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. 
     An improvement to this latter product places an adjustable mounting element between the stem and the humeral head. The mounting element is configured for articulating engagement with the stem to permit angular positioning of the head component in multiple degrees of freedom. Details of this prosthesis are found 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, the disclosure of which is incorporated herein by reference. 
     As disclosed in the &#39;448 application, the humeral head is fixed to the mounting element by a press-fit engagement. The mounting element is fastened to the humeral stem by two mechanisms. In the first mechanism, the mounting element achieves a friction fit with a tapered bore in the neck of the humeral stem. The second fixation mechanism includes a screw that is threaded into a threaded bore portion of the tapered bore in the stem. The screw bears against the mounting element to lock the element in position within the tapered bore. The joint prosthesis in this &#39;448 application is both modular and universal in that it permits infinitely variable positioning of a mating joint component relative to a bone engaging portion of the prosthesis. Moreover, this improved prosthesis is readily available for modification, whether during initial implantation or during a subsequent revision procedure. 
     With shoulder prostheses that allow a surgeon to adjust the angular position of the humeral head, such as those described above, a method must be available for trialing the prosthesis. When the trial prosthesis is implanted, several adjustments may be made to set the angular position of the prosthetic head relative to the humeral stem. In a typical trialing system, the trial prosthesis includes a broach configured to be tightly received within a previously prepared intramedullary (IM) canal of the humerus. In current systems, an articulating element is oriented relative to the neck of the broach and locked in place by a press-fit taper. Locking the articulating trial element thus requires impaction of the element within the broach. This method produces galling of the broach which can significantly limit the useful life of the broach. Moreover, the impaction step frequently causes the trial broach to sink further into the IM canal. This displacement of the trial broach results in an indeterminate offset of the center of rotation of the trial element. In addition, discrepancies between the amount of impaction of the trial element vis-à-vis the final implant element results in an unknown offset of this center of rotation, which ultimately leads to a poor anatomic fit and improper alignment of the humeral head prosthesis. 
     There is a need for a trialing system that avoids these problems of the current trialing approaches. There is a further need for a trialing system that can ensure accurate duplication of the angles of the trial implant without using impaction to fix the trial components. 
     SUMMARY OF THE INVENTION 
     These and other needs are met by the trial system and method of the present invention. In one embodiment, this trial system includes a trial implant for the joint of a patient, comprising a stem component configured for placement within a bone of a patient, an articulating component configured for articulating contact with a mating aspect of the joint, and a mounting assembly for mounting the articulating component to the stem component at variable angular orientations relative thereto. The mounting assembly includes a cavity defined in the stem portion, a mounting element having a portion configured to support the articulating component and an expandable portion configured to expand within the cavity, and an expansion element rotatable within the mounting element and cooperating with the expandable portion upon rotation to expand the expandable portion within the cavity. 
     In certain embodiment, the expandable portion of the mounting element includes an expanding ball and a bore within the ball. In one feature of these embodiments, mating portions of the expansion element and the bore define a rotational engagement so that rotation of the expansion element drives the expansion element into the bore. In addition, the expansion element and the bore include cooperating portions that cooperate to expand the ball when the expansion element is driven into the bore. In a more specific embodiment, the bore is a tapered threaded bore and the expansion element is a screw configured to be threaded into the threaded bore. Alternatively, the screw can be tapered to expand the bore and the ball. In another specific embodiment, the mating portions define a threaded engagement and the cooperating portions include a tapered portion of the bore and a peg on the expansion element sized to expand the tapered portion when the expansion element is driven into the bore. 
     In a further feature of the invention, the expanding ball includes a first ball portion adjacent the portion of the mounting element configured to support the articulating component defining a first spherical diameter. The ball portion further includes a second ball portion attached to the first ball portion and defining a second spherical diameter smaller than the first spherical diameter. 
     According to one novel aspect, the cavity in the stem includes an annular rim extending into the cavity. The annular rim defines an inner diameter that is less than the outer diameter of the expandable portion of the mounting element in both its un-expanded and expanded states. The expandable portion has a contracted state that defines an outer diameter less than the inner diameter of the rim so that the expandable portion can be inserted past the rim into the cavity. Once past the rim, the expandable portion may assume its un-expanded configuration in which the portion is trapped within the cavity but still movable to various angular orientations. 
     The invention further contemplates an improvement for a trial implant for the joint of a patient, the trial implant including a stem component configured for placement within a bone of a patient, an articulating component configured for articulating contact with a mating aspect of the joint and a mounting assembly for mounting the articulating component to the stem component at variable angular orientations. In accordance with one aspect of the invention, the improvement comprises a cavity defined in the stem portion, the cavity including a side wall, a base and an annular rim extending into the cavity opposite the base, and an expandable portion on the mounting assembly having an expanded configuration within the cavity in which the expandable portion is in contact with each of the side wall, base and annular rim of the cavity. 
     Preferably, the annular rim defines an inner diameter and the expandable portion defines an expanded diameter in the expanded configuration that is greater than the inner diameter. Moreover, the expandable portion has an unexpanded configuration defining a diameter greater than the inner diameter to retain the expandable portion within the cavity. In yet another aspect of this embodiment, the expandable portion has a compressed configuration defining a diameter that is less than the inner diameter to permit passage of the expandable portion past the annular rim into the cavity. 
     In accordance with certain aspects of this improvement, the expandable portion includes an expanding ball and an expansion element extending into the expanding ball and configured to expand the ball to the expanded configuration. The expanding ball may define a tapered threaded bore and the expansion element is a screw configured to be threaded into the threaded bore. in a preferred embodiment, the expanding ball includes a first ball portion arranged to contact the annular rim and the side wall when the expandable portion is expanded within the cavity and defining a first spherical diameter. The expanding ball further includes a second ball portion connected to the first ball portion and arranged to contact the base of the cavity when the expandable portion is expanded within the cavity and defining a second spherical diameter less than the first spherical diameter. 
     The invention also contemplates a method for establishing an angular orientation of an articulating component of an implant for the joint of a patient relative to a stem component configured for placement within a bone of a patient, comprising the steps of: 
     providing a trial mounting assembly for mounting the articulating component to the stem component, the trial mounting assembly having an expandable portion configured to expand within a cavity defined in the stem component; 
     disposing the expandable portion within the cavity in the stem component; 
     introducing an expansion element into the expandable portion; 
     rotating the expansion element relative to the expandable portion to drive the expansion element into the expandable portion causing the expandable portion to expand within the cavity. 
     Another method of the invention is directed to establishing an angular orientation of an articulating component of an implant for the joint of a patient relative to a stem component configured for placement within a bone of a patient, and comprises the steps of: 
     providing a trial mounting assembly for mounting the articulating component to the stem component, the trial mounting assembly having an expandable portion configured to expand within a cavity defined in the stem component; 
     disposing the expandable portion within the cavity in the stem component; 
     expanding the expandable portion within the cavity so that the expandable portion contacts the cavity at three points of contact. 
     It is one object of the invention to provide a trial system for use in obtaining the angular orientation of an articulating component for a joint prosthesis. Another object is to provide such as system that does not require impaction to fix the trial components in their proper angular orientation. Other objects and specific benefits of the invention can be discerned from the following written description and the accompanying figures. 
    
    
     
       DESCRIPTION OF THE FIGURES 
         FIG. 1  is a side view of a humeral implant for a shoulder prosthesis. 
         FIG. 2  is an enlarged partial cross-section view of a trial assembly in accordance with one embodiment of the present invention. 
         FIG. 3  is a cross-sectional view of a ball cylinder of the trial assembly shown in  FIG. 2 . 
         FIG. 4  is a bottom view of the ball cylinder shown I  FIG. 3 . 
         FIG. 5  is an enlarged partial cross-section view of the interface between the ball cylinder of  FIG. 2  and the neck of a trial broach in accordance with one aspect of the present invention. 
         FIG. 6  is an enlarged cross-section view of an alternative embodiment of the mounting cavity for receiving the trial ball cylinder. 
         FIG. 7  is an enlarged cross-section view of a ball cylinder according to another embodiment of the invention for use with the humeral implant shown in  FIG. 1 . 
         FIG. 8  is a side view of a replication instrument for use in replicating the angular orientation of the ball cylinder in the trial assembly 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. 
     As shown in  FIG. 1 , the present invention has particular application to a humeral prosthesis  10  for use in a shoulder joint replacement. The prosthesis includes a stem  12  configured to be disposed within the prepared IM canal of the humerus bone. Where the prosthesis  10  is a final implant, the stem is configured for permanent implantation within the IM canal, often accompanied by the introduction of bone cement into the canal. Where the prosthesis  10  is a trial implant, the prosthesis may be a broach that combines its function as a bone working tool to prepare the bore in the IM canal with its function as a trial stem. 
     The prosthesis  10  includes a neck  13  that is angled relative to the stem but arranged to sit flush with the resected head of the humerus bone when the prosthesis is implanted. An articulating head component  16 , or humeral head for a shoulder prosthesis, is supported on the stem  12  by a mounting assembly  18 . Where the prosthesis is a final prosthesis, the mounting assembly is configured for a final fixation, such as by impaction into a cavity  20  in the neck  13  of the stem. Where the prosthesis  10  is a trial broach, the mounting assembly  18  can be constructed according to the present invention disclosed herein. 
     In order to achieve a joint prosthesis that emulates the natural joint as closely as possible, the articulating component  16  is infinitely positionable. Where the prosthesis  10  is a humeral prosthesis, the humeral head component  16  must be variably angularly positionable in the medial-lateral M-L rotation and superior inferior S-I rotation degrees of freedom. In a typical shoulder joint, the humeral head will be positioned at an angle of 135° to the axis of the humerus bone. However, normal variations in patient anatomy can yield humeral head angles of 120°-150°. Thus, a universal humeral prosthesis  10  will be capable of 15° variations in all angular directions from the mean datum line D passing perpendicular to the platform surface  14  of the neck  13 . 
     The neck  13  of the stem  12  includes a superior positioning groove  22  and a pair of inferior positioning grooves  24 . As explained in more detail herein, these grooves establish the position of the trial and final implants in a replication instrument, serving as a means to provide a repeatable orientation for the datum line D relative to which the humeral head angular orientation is established. 
     In accordance with the present invention, a trial assembly  30  is provided that does not require impaction of the trial components, as depicted in  FIG. 2 . The assembly  30  includes a trial ball cylinder  32  having a cylinder portion  34  and an expanding ball portion  36 . The cylinder portion  34  defines an outer surface  38  that is received within a mating recess  17  formed in the humeral head component  16 . Preferably, the cylinder portion and mating recess form a tight fit, but not a press-fit engagement so that no impaction of the trial assembly  30  is required. The cylinder portion  34  can be provided with an alignment notch  65  to receive a tab  66  formed on the interior of the mating recess  17  in the humeral head. The notch and tab interlock to ensure that the trial ball cylinder  32  rotates and pivots with the humeral head as the head is manipulated within the patient&#39;s shoulder joint to find the optimum fit with the glenoid aspect of the joint. 
     The trial ball cylinder defines an inner cavity  40  which provides access to a tapered bore  45 . The tapered bore is coincident with expansion slots  42  formed in the expanding ball portion  36 . In accordance with the preferred embodiment, four such expansion slots  42  are provided, as best seen in  FIG. 4 . The slots are configured in a conventional manner to allow the radial or diametrical expansion of the ball portion  32  as an expansion element is driven into the tapered bore  45 . In the preferred embodiment, the expansion element is an expansion screw  48  with a uniform diameter threaded stem  49  that is threaded into internal threads  46  of the tapered bore. The screw  48  includes a head portion  50  that is preferably larger than the threaded stem  49  so the head portion contacts the angled portion  41  ( FIG. 3 ) of the cavity to limit the passage of the screw into the bore. The head portion may be provided with an internal hex recess to receive a known hex driving tool for screwing the expansion element  48  into the tapered threaded bore  45 . 
     It should be readily understood that as the expansion screw  48  is threaded into the tapered bore  45 , the threaded stem  49  causes the walls of the bore to expand outward, thereby opening each of the expansion slots  42  (as shown in  FIG. 5 ). In accordance with the preferred embodiment of the invention, the expansion element  48  is a screw driven into a tapered bore. Alternatively, the bore  45  in the expanding ball portion  36  can have a uniform diameter and the expansion screw can incorporate a tapered threaded stem. Regardless of which component is tapered or uniform, a screw-threaded bore interface is a preferred mechanism to enlarge the expanding ball portion  36  since it relies upon the application of torque rather than on an impact force to drive the expansion element  48  into the bore  45 . Other expansion elements are contemplated that rely upon the application of a torque or rotational force to expand the slots  42  in the ball portion  36 . 
     In accordance with one feature of the trial assembly  30 , the expanding ball portion  36  is configured so that the center of rotation C ( FIGS. 2 and 5 ) will automatically substantially coincide with the center of rotation of the final implant. Ideally, this center of rotation C is fixed relative to the humerus bone when either the trial broach or the final implant is engaged with the IM canal. Maintaining this reference point constant ensures that the replicated angle of the humeral head in the final implant is anatomically accurate and appropriate for the patient&#39;s shoulder joint anatomy. As is apparent, the expanding ball portion  36  of the trial ball cylinder  32  is sized to freely slide in and out of the mounting cavity  20  formed in the neck  13  of the trial broach. Thus, unless the expanding ball portion is properly positioned and constrained, the center of rotation C can shift up and down and radially side to side within the cavity. 
     In one feature of the present invention, the expanding ball portion is positively positioned and constrained at three points of contact within the cavity  20 . In accordance with the preferred embodiment, the expanding ball portion includes a larger ball portion  54  and a smaller ball portion  56  projecting from a truncated base  55  ( FIGS. 1 and 5 ) of the larger ball portion. The two ball portions share their origin with the center of rotation C. The smaller ball portion  56  is provided to reduce the overall height of the expanding ball portion  36 , which translates into a minimized depth for the mounting cavity  20 . As can be seen in  FIG. 1 , the size of the neck  13  limits the depth for the mounting cavity  20 . On the other hand, strength, stability and accuracy considerations suggest an optimum diameter for the expanding ball portion, and particularly for the larger ball portion  54 . If the entire expanding ball portion  36  were formed at the same diameter as the larger ball portion, the cavity  20  would have to be deeper to accommodate the ball portion, which necessarily would exceed the structural limit for the cavity. Thus, truncating the larger diameter ball portion  54  and integrating the smaller diameter ball portion  56  sufficiently reduces the height of the expanding ball portion so that the cavity can fall within the preferred depth limits discussed above. 
     The mounting cavity  20  includes a radial lip or ring  62  at the opening of the cavity adjacent the platform surface  14 . As the ball portion  36  expands, the larger ball portion  54  contacts this lip at point P 1 . It is understood that this point P 1  represents a circumferential line of contact between the spherical ball portion  54  and the cylindrical cavity  20 . As the larger ball portion  54  expands further into the lip  62 , the reaction pushes the ball portion  36  deeper into the cavity  20  until the smaller ball portion  56  bottoms on the cavity base  58  at point P 2 . Again, it is understood that the point of contact P 2  represents a circumferential line of contact between the spherical ball portion  56  and the flat base  58 , broken at 90° intervals by the gaps formed by the expanded slots  42 . Further expansion of the larger ball portion  54  stops when the portion contacts the cavity side wall  60  at the circumferential point of contact P 3 . These three points of contact P 1 , P 2 , and P 3  fix the location of the center of rotation C and provide a solid engagement of the ball portion  36  within the cavity  20 . 
     In a specific embodiment of the invention, the larger ball portion  54  is formed at a spherical diameter of 0.364 inches, while the smaller ball portion has a spherical diameter of 0.314 inches. The expansion slots  42  have a width of 0.025 inches. The expansion slots also accommodate compression of the ball portion  36  so that the inner diameter of the lip  62  can be smaller than the spherical diameter of the larger ball portion. In other words, in the specific embodiment, the cylindrical wall  60  of the cavity  20  is formed at a diameter of about 0.375, which is greater than the largest diameter of the expanding ball portion  36 . The lip  62  projects inward from the cavity side wall  60  at an inner diameter of about 0.355 inches, which is less than the diameter of the larger ball portion  54 . When the slots  42  are fully compressed, the diameter of the larger ball portion decreases by about ½ the slot width, or by about 0.012 inches. This reduced outer diameter of 0.352 inches is less than the inner diameter of the lip so that compressed ball portion can slide past the lip  62  and into the cavity  20 . Once inside, the ball portion  36  is restored to its normal size so that the cylinder portion  34  is loosely retained on the trial broach  12 . 
     In the preferred embodiment, the lip  62  is integrally formed within the cavity  20 , such as by machining an undercut in the trial broach  12 . Alternatively, the circumferential lip can be created by a snap-ring  68  mounted within a groove  69  adjacent the platform surface  14 , as shown in  FIG. 6 . The snap-ring projects inward into the interior of the cavity to define an inner diameter comparable to the inner diameter of the lip  62  shown in  FIG. 5 . The snap ring preferably bottoms within the groove so that there is no variation in the inner diameter defined by the ring, which is important to ensure proper positioning of the center of rotation C. 
     In the embodiment of  FIGS. 2-4 , the expanding ball portion  36  relies upon a tapered threaded bore and an expansion screw to expand the ball portion and lock it within the cavity. In an alternative embodiment, a peg and cam approach is used to expand the ball portion. As shown in  FIG. 7 , a trial assembly  70  includes a trial ball cylinder  72  having a cylindrical portion  74  and an expanding ball portion  76 . The cylindrical portion  74  has an outer surface  78  that is adapted to mate with the bore  17  in the humeral head component  16 . The cylindrical portion also defines an inner cavity  80  and the ball portion  76  includes expansion slots  82  similar to the prior embodiment. 
     In this embodiment, the cavity  80  opens into a bore  85  passing through the expanding ball portion  76 . The upper portion of the bore bears internal threads  86 , while the lower portion of the bore defines a circumferential cam surface  87 . As can be seen in  FIG. 7 , the cam surface is inwardly curved relative to the bore  85 . The expansion element is a screw  88  having an upper threaded portion  89  configured to mate with the internal threads  86 . A hex recess  90  accepts a hex driving tool to thread the expansion element into the bore. 
     The lower portion of the screw  88  defines a peg  92  that bears against the cam surface  87  as the screw is driven into the bore. In the preferred embodiment, the peg is tapered, as shown in  FIG. 7 . As the peg traverses the cam surface it expands the surface, and ultimately expands the slots  82  in the same manner as described above to lock the ball portion  76  within the cavity  20 . 
     It can be appreciated that the present invention contemplates a trial assembly, such as the assemblies  30  and  70 , which allow a full range of articulation or rotation of the trial components, and more specifically the trial head component  16 . The expanding ball portions  36 ,  76  positively establish the center of rotation C, which coincides with the origins for the larger and smaller spherical ball portions. As with other trial implants, the trial broach  10  is positioned within the prepared IM canal with the platform surface  14  aligned with the resected surface of the humerus bone. The trial ball cylinder  32  is then positioned within the mounting cavity  20 . The three-dimensional angle of the trial cylinder relative to the broach stem can be adjusted with the trial humeral head  16  mounted on the cylinder portion  34 . Once the trial head is properly oriented relative to the glenoid aspect of the shoulder joint, the expansion element  48 ,  88  is tightened using an appropriate tool. The mating bore  17  of the trial head  16  preferably passes through the head so that the expansion element can be accessed by the driving tool with the trial head in position on the cylinder portion. The interdigitating notch  65  and tab  66  help hold the trial head in position as the head is manipulated and the expansion element tightened. 
     In accordance with one aspect of the method of the present invention, the expansion element is tightened within the expanding ball portion without the application of an impaction force. In the preferred embodiment, the expansion element is tightened by applying torque or a rotational force to the element. Most preferably, a threaded interface is provided between the expansion element and the expanding ball portion so that the amount of tightening torque can be controlled. In an alternative approach, the rotational interface can be in the form of a bayonet engagement in which the expansion element is rotated through a fixed angle and mating cam surfaces propel the expansion end of the element into the bore of the expanding ball portion. With this alternative, the amount of rotation of the expansion element is calibrated so that the ball portion expands enough to ensure a solid fixation within the mounting cavity. 
     Once the angular orientation of the trial ball cylinder  32 ,  72  has been established, the entire trial broach is removed from the humerus bone. The trial assembly  30 ,  70  may then be mounted within a replication instrument to ascertain the angular position of the prosthetic head relative to the fixed datum D and established center of rotation C. Any replication instrument may be used to ascertain this orientation and translate that position to a final prosthesis. One exemplary replication instrument  100  shown in  FIG. 8  is particularly suited for use with the trial assemblies  30  and  70  described above. The details of this instrument are 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. 
     As described in the &#39;261 application, the instrument  100  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 . As explained above, the neck  13  of the trial broach  10  (as well as the final humeral implant) is provided with positioning grooves  22  and  24 . The superior groove  22  accepts the fixed clamp element  104 , while the pair of inferior grooves  24  are configured to mate with the movable clamp element  106 . When the neck of the trial broach is engaged by the clamp elements  104 ,  106 , the position of the datum D is fixed at a known orientation. 
     The replication 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 cavity  40 ,  80  of the cylinder portion  34 ,  74 . When the guide shaft  127  is situated within the cylinder portion 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 the trial broach  10  released from the base assembly. The final humeral prosthesis is then clamped within the base assembly with a final head mounting assembly loosely engaged to the final implant stem. The alignment tool is reinserted into the guide member and the guide shaft is engaged with the mounting assembly to replicate the angular orientation of the trial ball cylinder  32 ,  72 . The alignment tool  125  is configured with an impaction end  129  that can be struck with a mallet to impact the final implant mounting assembly within the final implant stem. Once the humeral head is impacted onto the mounting assembly, the prosthesis is ready to be implanted in the humerus bone. 
     It can be appreciated that with the present invention, all of the impaction steps occur outside the patient&#39;s body and with the use of separate fixtures. Thus, the invention allows for highly accurate replication of the appropriate anatomic angle for the humeral head relative to the humerus and glenoid aspects of the shoulder joint. These same principles can be used for other joint prostheses where the angle of an articulating component is critical. 
     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.