PATENT ABSTRACT
Adjustable prostheses and related methods provide a wide range of adjustment along or about multiple axes. The prostheses and related methods make possible a straightforward, yet robust way of securing, e.g., a humeral head prosthesis in a desired position and maintaining the prosthesis in the desired position during use.

PATENT DESCRIPTION
RELATED APPLICATIONS  
       [0001]    This application is a divisional of co-pending U.S. application Ser. No. 10/041,722, filed on 8 Jan. 2002, which claims the benefit of provisional Application Serial No. 60/271,895, filed on 27 Feb. 2001, entitled “Adjustable Head Prosthesis for the Shoulder.” 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention generally relates to an adjustable mounting assembly and alignment system for a bone prosthesis and related methods.  
         BACKGROUND OF THE INVENTION  
         [0003]    A shoulder joint consists of a ball-and-socket type coupling of the humerus to the scapula. The humerus forms the ball, and the socket is formed at the glenoid cavity of the scapula. Injury or disease to the joint often results in destruction or deterioration of the head of the humerus, leading to pain and a corresponding loss of mobility and function. In such cases, it is often necessary to provide a replacement joint surface, i.e., a prosthesis, for the head of the humerus that mates with the glenoid cavity.  
           [0004]    The proper alignment of the prosthesis is generally useful to effective performance of the replacement procedure. Typically, the position of the mount is adjusted until the desired position is achieved. The mount is fixed in the desired position and the prosthesis is then secured onto the mount.  
           [0005]    However, conventional mounts provide only a limited range of adjustment, typically allowing only two degrees of freedom, i.e., linearly along an X-axis and Y-axis. The devices that do have more degrees of freedom require multiple trials and a fixture to be used away from the surgical site for proper alignment of the prosthesis to the humerus.  
           [0006]    Further, even upon locking the device in a desired position, conventional mounts may not hold the desired position. This is especially true when force is exerted, e.g., hammering the prosthesis to secure its placement on a mount.  
           [0007]    There remains a need for mounting systems and methods that permit a wide range of adjustment of a humeral head prosthesis while enabling the mount, and attached prosthesis, to remain securely fixed in a desired position.  
         SUMMARY OF THE INVENTION  
         [0008]    The invention provides various adjustable prostheses and related methods that provide a wide range of adjustment along or about multiple axes. The invention makes possible a straightforward, yet robust way of securing, e.g., a humeral head prosthesis in a desired position and maintaining the prosthesis in the desired position during use.  
           [0009]    Other features and advantages of the inventions are set forth in the following specification and attached drawings. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is an exploded view of the components of an adjustable locking mount system that embodies features of the invention, in which the mounting hub is centric.  
         [0011]    [0011]FIG. 2 is an assembled perspective view of the system shown in FIG. 1.  
         [0012]    [0012]FIG. 3 a  is a side sectional view of the assembled components of the system shown in FIG. 2.  
         [0013]    [0013]FIG. 3 b  is a view similar to FIG. 3 a  and illustrating the spherical radii of the stacked washers.  
         [0014]    [0014]FIGS. 4 a - 4   e  illustrate rotational movement of the cooperating components of the assembled system shown in FIG. 2.  
         [0015]    [0015]FIG. 5 a  is a side sectional view of the assembled components of the system shown in FIG. 3 and illustrating the system components in a level position.  
         [0016]    [0016]FIG. 5 b  is a sectional view as shown in FIG. 5 a , illustrating the position of the system components and the movement of the mounting hub and lock washer when the mounting hub is rotated about the x or y axis.  
         [0017]    [0017]FIG. 5 c  is a sectional view as shown in FIG. 5 b , illustrating the procedure of locking the system in a desired position.  
         [0018]    [0018]FIG. 6 is an exploded view of the components of an alternative embodiment of an adjustable locking mount system that embodies features of the invention, in which the mounting hub is eccentric.  
         [0019]    [0019]FIG. 7 is an assembled perspective view of the system shown in FIG. 6.  
         [0020]    [0020]FIG. 8 is side sectional view of the assembled components of the system shown in FIG. 7.  
         [0021]    [0021]FIGS. 9 a - 9   e  illustrate rotational movement of the cooperating components of the assembled system shown in FIG. 7.  
         [0022]    [0022]FIG. 10 is an exploded view of an adjustable locking mount system embodying features of the invention incorporated in a shoulder replacement assembly.  
         [0023]    [0023]FIG. 11 is a perspective view of the assembled components of the system shown in FIG. 10.  
         [0024]    [0024]FIG. 12 a  is an enlarged perspective view of the top portion of the trial ring shown in FIG. 10.  
         [0025]    [0025]FIG. 12 b  is an enlarged perspective view of the bottom portion of the trial ring shown in FIG. 10.  
         [0026]    [0026]FIG. 13 a  is an enlarged perspective view of the top portion of the artificial head shown in FIG. 10.  
         [0027]    [0027]FIG. 13 b  is an enlarged perspective view of the bottom portion of the artificial head shown in FIG. 10, and further illustrating the interior surface of the artificial head.  
         [0028]    [0028]FIG. 14 a  is an exploded view of the components of an alternate embodiment of a shoulder replacement system embodying features of the invention and viewed from the head to the stem.  
         [0029]    [0029]FIG. 14 b  is a view similar to FIG. 14 a  and viewed from the stem to the head.  
         [0030]    [0030]FIG. 15 is a view similar to FIGS. 14 a  and  14   b  and illustrating a partially assembled view of the system components.  
         [0031]    [0031]FIG. 16 is a perspective view of a humerus bone, with a line representing a cut in the ball portion of the humerus made during shoulder replacement surgery.  
         [0032]    [0032]FIG. 17 illustrates a humerus as shown in FIG. 16, illustrating the head cut and removed from the humerus and a bore reamed into the bone.  
         [0033]    [0033]FIG. 18 is a perspective view illustrating a humerus as shown in FIG. 17, and further illustrating the insertion into the bore of a stem carrying an adjustable mount of the present invention.  
         [0034]    [0034]FIGS. 19 a  and  19   b  are perspective views illustrating a humerus as shown in FIG. 18, and further illustrating a trial ring engaging the mount and being rotated simultaneously with the mount.  
         [0035]    [0035]FIG. 19 c  illustrates the trial being and the mount rotated independently of each other.  
         [0036]    [0036]FIG. 20 illustrates a humerus as shown in FIGS. 19 a  and  19   b , illustrating the trial ring being simultaneously tilted with the mount.  
         [0037]    [0037]FIG. 21 illustrates a humerus as shown in FIG. 20, and further illustrates the procedure of locking the mount in a desired position.  
         [0038]    [0038]FIG. 22 shows a humerus as in FIG. 21, with the trial ring removed and illustrating the placement of an artificial head onto the mount.  
         [0039]    [0039]FIG. 23 illustrates a humerus as shown in FIG. 22, with the artificial head placed on the mount and further illustrating the use of a hammer to secure the artificial head on the mount.  
         [0040]    [0040]FIG. 24 a  is an exploded view of the components of an alternative embodiment of a shoulder replacement system embodying features of the invention and viewed from the head to the stem.  
         [0041]    [0041]FIG. 24 b  is a view similar to FIG. 24 a  and viewed from the stem to the head.  
         [0042]    [0042]FIG. 25 is a view similar to FIGS. 24 a  and  24   b  and illustrating the use and placement of the pivot pin component of the system to secure the bottom insert component onto the stem component.  
         [0043]    [0043]FIG. 26 is a view similar to FIG. 25 and illustrating the placement of the eccentric mount component onto the bottom insert component.  
         [0044]    [0044]FIGS. 27 a - 27   e  are partially assembled views of the system shown in FIGS. 24 a  and  24   b  and illustrating rotational movement of the partially assembled system.  
         [0045]    [0045]FIG. 28 is a partially assembled view of the system shown in FIGS. 24 a  and  24   b  and illustrating the placement of the top insert on the bottom insert.  
         [0046]    [0046]FIG. 29 is a perspective view of the components of the system shown in  24   a  and  24   b  assembled.  
         [0047]    [0047]FIG. 30 a  is an exploded view of the components of an alternative embodiment of a shoulder replacement system embodying features of the invention and viewed from the head to the stem.  
         [0048]    [0048]FIG. 30 b  is view similar to FIG. 30 a  and viewed from the stem to the head.  
         [0049]    [0049]FIG. 31 is a view similar to FIGS. 30 a  and  30   b  illustrating the use of the pivot pin component to secure the mounting ring and the bottom disk to the stem.  
         [0050]    [0050]FIG. 32 is a view similar to FIG. 31 and illustrating the placement of the top disc on the bottom disk.  
         [0051]    [0051]FIGS. 33 a - 33   e  are views similar to FIG. 32 and illustrating the placement of the head component onto the mounting ring component and further illustrating the rotational movement of the assembled system.  
         [0052]    [0052]FIG. 34 is a view similar to FIGS. 33 a - 33   e  and illustrating the locking of the assembled system in a desired position.  
         [0053]    [0053]FIG. 35 a  is an exploded view of an alternative embodiment of a shoulder replacement system embodying features of the invention viewed from the head to the stem.  
         [0054]    [0054]FIG. 35 b  is a view similar to FIG. 35 a  and viewed from the stem to the head.  
         [0055]    [0055]FIG. 36 is an exploded view of the bottom and top plate components of the system shown in FIGS. 35 a  and  35   b  and illustrating the major and minor axes of the top and bottom plates.  
         [0056]    [0056]FIG. 37 is a partially assembled view of the system shown in FIGS. 35 a  and  35   b  and illustrating the use of the pivot pin to secure the placement of the bottom plate onto to stem.  
         [0057]    [0057]FIG. 38 is a view similar to FIG. 37 and illustrating the placement of the top plate on the bottom plate.  
         [0058]    [0058]FIGS. 39 a - 39   e  are views similar to FIG. 38 and illustrating rotational movement of the partially assembled system.  
         [0059]    [0059]FIG. 40 is an assembled view of the system shown in FIGS. 35 a  and  35   b.   
     
    
       [0060]    The invention may be embodied in several forms without departing from its spirit or essential characteristics. The scope of the invention is defined in the appended claims, rather than in the specific description preceding them. All embodiments that fall within the meaning and range of equivalency of the claims are therefore intended to be embraced by the claims.  
       DETAILED DESCRIPTION  
       [0061]    Although the disclosure hereof is detailed and exact to enable those skilled in the art to practice the invention, the physical embodiments herein disclosed merely exemplify the invention that may be embodied in other specific structure. While the preferred embodiment has been described, the details may be changed without departing from the invention, which is defined by the claims.  
         [0062]    I. The Adjustable Locking Mount System  
         [0063]    A. System 1:  
         [0064]    Interior Hub Centrally Located with Respect to Mounting Surface  
         [0065]    [0065]FIG. 1 shows the individual components of an adjustable locking mounting system  10 A. FIGS. 2 and 3 a  illustrate the system  10 A when assembled. As will be described in detail later, the system  10 A permits adjustment in three directions or three degrees of freedom (rotational around axes x, y, and z, where the z-axis is represented by the axis of the pivot pin  12 ) (see FIGS. 4 a - 4   e ).  
         [0066]    The system  10 A comprises the pivot pin  12 , at least one slip washer  14 , at least one lock washer  16 , a mounting hub  18 , and a locking screw  20 . Each of these components of the system  10 A will now be described in detail.  
         [0067]    1. System Components  
         [0068]    As seen in FIG. 1, the pivot pin  12  is a rigid, generally cylindrical or rod-like member. The pivot pin  12  is convex, e.g., domed, at one end to couple with the mounting hub  18  (see, e.g., FIG. 3 a ). In a representative embodiment, the arc of curvature is 0.400″ diameter (0.200″ radius).  
         [0069]    In particular, the convex arrangement permits adjustment of the mounting hub  18  by swinging or tilting across the axis of the pivot pin  12  (i.e., rotation about the x-axis and y-axis) as well as by rotating or twisting about the axis of the pivot pin  12  (i.e., rotation about the z-axis) (see FIGS. 4 a - 4   e ).  
         [0070]    As best seen in FIGS. 1 and 3, the pivot pin  12  has a threaded central bore  26  that serves to receive the locking screw  20 . Thus, the pivot pin  12  serves to receive both the mounting hub  18  and the locking screw  20  (see FIG. 3 a ).  
         [0071]    The pivot pin  12  can be made of suitable metal, plastic, or ceramic materials and formed by conventional molding or machining techniques.  
         [0072]    As shown in FIG. 1, the mounting hub  18  is a rigid member comprising a mounting surface  24 , an interior hub  22 , and an exterior pivot surface  28 . The center of the mounting hub  18  serves to receive the locking screw  20 .  
         [0073]    The mounting surface  24  is configured to mate with an object or device being mounted on the hub and therefore can take on a variety of shapes. Thus, the mounting hub  18  serves as a base for mounting of another object or device. For example, the mounting surface  24  can be circular or geometric. In the illustrated embodiment, the mounting surface  24  is generally circular.  
         [0074]    Additionally, the mounting surface  24  can be stepped to further aid in positioning and securing the object or device on the mounting surface  24  (not shown). In this arrangement, the object or device being mounted would have a complementary stepped surface. The stepped surface provides greater control of any adjustment by permitting adjustment to be in uniform increments and reducing the risk of inadvertent movement. The mounting surface  24  could alternatively be a threaded surface to facilitate engagement with a mating part.  
         [0075]    As best illustrated in FIG. 1, the interior hub  22  is open. The bottom surface of the interior hub  22  is configured to conform to the shape of the convex end of the pivot pin  12  and sized to receive the slip washer(s)  14  and lock washer(s)  16 . That is, the interior hub  22  permits a slip washer  14  and lock washer  16 , or multiple slip washers  14  and lock washers  16 , to be alternately stacked upon one another (see FIG. 3 a ).  
         [0076]    As shown in FIGS. 1-3 a , the exterior pivot surface  28  of the mounting hub  18  is configured to nest on and to conform to the convex end of the pivot pin  12 , thus permitting a wider range of motion, as previously described.  
         [0077]    As best seen in FIG. 3 a , the exterior pivot surface  28  is located centrally with respect to the interior hub  22 . Further, the interior hub  22  is centrally located with respect to the mounting surface  24 , such that the geometric center of the mounting hub  18  coincides with the center of rotation of the mounting hub  18  about the pivot pin  12 .  
         [0078]    The mounting hub  18  serves to engage and pivot about the pivot pin  12 , thus permitting adjustment of the position of the mounting hub  18  with respect to the pivot pin  12 , as will be described later. Upon obtaining the desired position, the position of the mounting hub  18  can be locked by use of the locking screw  20 , as will also be described in greater detail later.  
         [0079]    The mounting hub  18  can be made of any suitable metal or plastic and formed by conventional machining or molding techniques.  
         [0080]    As shown in FIG. 1, the system  10 A also provides at least one slip washer  14 . The slip washer  14  is preferably a rigid annular ring or doughnut-like member. As FIGS. 1 and 3 a  best show, the slip washer  14  is configured to conform to the bottom surface of the interior hub  22 .  
         [0081]    The center of the slip washer  14  serves to receive the locking screw  20 . The center of the slip washer  14  is of a diameter only slightly larger than the outside diameter of the locking screw  20 . The slip washer  14  also serves to provide a frictional surface, which upon tightening of the locking screw  20 , serves to further secure the mounting hub  18  in a desired position.  
         [0082]    The slip washer  14  permits the lock washer  16  to slide across the surface of the slip washer  14  (see FIGS. 5 a  and  5   b ). The slip washer  14  is similar in function yet physically different in top and bottom spherical radii from the lock washer  16 .  
         [0083]    As seen in FIG. 3 b , additional washers  14  and  16  in the assembly would also have different spherical radii, represented by R1-R5 in FIG. 3 b , as they are stacked further from the center of rotation or pivot point on the pivot pin  12 . In a representative embodiment, R1 is 0.200, R2 is 0.250, R3 is 0.300, R4 is 0.350, and R5 is 0.400.  
         [0084]    The radii of the washers  14  and  16  can be varied to accommodate the thickness of the individual washers  14  and  16 . Regardless of the thickness or radii of the washers  14  and  16 , the washers  14  and  16  are configured to rotate about the same pivot point.  
         [0085]    Desirably, as illustrated in FIGS. 1 and 3 a , a second slip washer  14 , similar in function but differing in spherical radii from the first slip washer  14  is placed over the lock washer  16 . As illustrated in FIGS. 5 a  and  5   b , the lock washer  16  is able to slide between the slip washers  14 .  
         [0086]    In this arrangement, the second slip washer  14  provides an additional frictional surface, which upon tightening of the locking screw  20 , serves to further secure the desired position.  
         [0087]    The slip washer(s)  14  can be made of any suitable metal or plastic and formed by conventional machining or molding techniques.  
         [0088]    As also seen in FIG. 1, the system  10 A further provides a lock washer  16 . The lock washer  16  is a rigid, annular ring or doughnut-like member similar to the slip washer  14 .  
         [0089]    As FIGS. 1 and 3 a  best illustrate, the lock washer  16  is configured to conform to the surface of the slip washer  14 . This arrangement permits the lock washer  16  to be stacked on top of the slip washer  14 .  
         [0090]    As in the case of the slip washer  14 , the center of the lock washer  16  serves to receive the locking screw  20 . The center of the lock washer  16  is also sized larger than the center of the slip washer  14 . That is, the center of the lock washer  16  not only serves to receive the locking screw  20 , but also permits the lock washer  16  to pivot about the pivot pin  12 .  
         [0091]    The lock washer  16  also provides two additional frictional surfaces when sandwiched between two slip washers  14 , which upon tightening of the locking screw  20 , serve to further secure the desired position.  
         [0092]    As also seen in FIGS. 1 and 3 a , the lock washer  16  is of a larger diameter than the slip washer  14 . This arrangement allows the lock washer  16  to fit over the slip washer  14 . In a representative embodiment, the lock washer  16  is sized to approximate or be slightly less than the diameter of the interior hub  22 , thereby providing a secure fit of the lock washer  16  within the interior hub  22  and allowing only minimal translation in the x and y axes, yet not restricting z-axis translation of the lock washer  16  within the interior hub  22  and with respect to the axis of the pivot pin  12 , as will later be described in detail.  
         [0093]    This arrangement secures/couples the lock washer  16  to the interior hub  22  and permits the lock washer  16  to slide with the mounting hub  18  over the slip washer  14  (see, e.g., FIGS. 5 a  and  5   b ). Thus, the lock washer  16  serves to provide an additional rotational and rocking surface for the mounting hub  18 .  
         [0094]    Like the slip washer  14 , the lock washer  16  can be made of any suitable plastic or metal and formed by conventional molding or machining techniques.  
         [0095]    Desirably, as previously noted, a second slip washer  14  similar in function but differing in spherical radii from the first slip washer  14  can be provided. In this arrangement, as seen in FIGS. 1 and 3 a , the lock washer  16  also serves to receive the second slip washer  14 . It will be apparent that any number of slip washers  14  and lock washers  16  can be similarly alternately stacked upon each other and thereby accommodate variations in the depth of the interior hub  22 .  
         [0096]    As also shown in FIG. 1, the system  10 A provides a locking screw  20 . The locking screw  20  is a screw that is adapted for passage through the mounting hub  18 , the slip washer(s)  14 , the lock washer(s)  16 , and the pivot pin  12  when the system is assembled (see FIG. 3 a ). In inside the diameter of the slip washer  14  is sized to approximate or be slightly larger than the diameter of the locking screw  20 . This arrangement secures/couples the slip washer  14  to the locking screw  20  and the pivot pin  12 .  
         [0097]    As illustrated in FIG. 3 a , the locking screw  20  is desirably threaded to fit the threaded bore  26  of the pivot pin  12 . As FIG. 5 c  illustrates, rotation (represented by arrow in FIG. 5 c ) of the screw  20 , e.g., by an Allen wrench  30 , advances the screw into the pivot pin  12  to fix the mounting hub  18  in a desired position.  
         [0098]    The locking screw  20  can be made of any suitable plastic or metal and formed by conventional molding or machining techniques.  
         [0099]    The locking screw  20 , when not fully tightened, serves to hold the assembly while the desired position is determined. Tightening of the locking screw  20  compresses the washers  14  and  16 , hub  18 , and pin  12  together, thereby creating multiple frictional forces between the mating surfaces. These frictional forces and the compression of the screw  20  are what limit movement in the locked position.  
         [0100]    It will be apparent that the components just described can be used in any combination. For example, plastic slip washers  14  may be alternated with metal lock washers  16 .  
         [0101]    2. Adjustment of the Orientation of the Mounting Hub  
         [0102]    The system  10 A as previously described enables the mounting hub  18  to be oriented in a variety of directions with respect to the pivot pin  12 . The types of movement, and thus the types of adjustments permitted, will now be discussed.  
         [0103]    The system  10 A permits movement of the mounting hub  18  in at least three rotational directions.  
         [0104]    First, as represented by arrows in FIGS. 4 a - 4   b , the mounting hub  18  can be rocked or rotated, i.e., tilted, about the x-axis (i.e., side to side rotation). This motion is permitted by the convex surfaces of the pivot pin  12 , mounting hub  18 , slip washer(s)  14 , and lock washer(s)  16 .  
         [0105]    Second, as represented arrows in FIGS. 4 c - 4   d , the mounting hub  18  can be rocked or rotated, i.e., tilted, about the y-axis (i.e., front to back rotation). This motion is permitted by the convex surfaces of the pivot pin  12 , mounting hub  18 , slip washer(s)  14 , and lock washer(s)  16 .  
         [0106]    Third, as represented by arrows in FIG. 4 e , the mounting hub  18  can be rotated 360° in either a clockwise or counterclockwise direction about the z-axis (i.e., axis of the pivot pin  12 ).  
         [0107]    It is to be understood that the rotational and rocking movements permit adjustment in virtually an infinite number of rotational directions.  
         [0108]    B. System 2:  
         [0109]    Interior Hub Eccentrally Located with Respect to Mounting Surface  
         [0110]    1. System Components  
         [0111]    [0111]FIG. 6 shows the individual components of an alternative system  10 B providing an adjustable locking mount system. FIGS. 7 and 8 illustrate the system  10 B when assembled.  
         [0112]    Like system  10 A, the system  10 B comprises a pivot pin  12 , at least one slip washer  14 , at least one lock washer  16 , a mounting hub  18 , and a locking screw  20 .  
         [0113]    Also like system  10 A, the mounting hub  18  has an exterior pivot surface  28  that is located centrally with respect to the interior hub  22 . In this embodiment, as FIGS. 6-8 best show, the interior hub  22  is eccentric with respect to the mounting surface  24 , such that the geometric center of the mounting hub  18  does not coincide with the center of rotation of the mounting hub  18  about the pivot pin  12 . The eccentric configuration permits a broader range of adjustment.  
         [0114]    2. Adjustment of the Orientation of the Mounting Hub  
         [0115]    The system  10 B as previously described enables the mounting hub  18  to be oriented in a variety of directions with respect to the pivot pin  12 . The types of movement, and thus the types of adjustments permitted, will now be discussed.  
         [0116]    The system  10 B permits movement of the mounting hub  18  in at least five directions.  
         [0117]    First, as represented by arrows in FIGS. 9 a - 9   b , the mounting hub  18  can be rocked or rotated about the x-axis, as previously described for system  10 A.  
         [0118]    Second, as represented by arrows in FIGS. 9 c - 9   d , the mounting hub  18  can be rocked or rotated about the y-axis, as also previously described for system  10 A.  
         [0119]    Third, as represented by arrows in FIG. 9 e , the mounting hub  18  can be rotated up to 360° in either direction about the z-axis, as previously described for system  10 A.  
         [0120]    As best illustrated in FIGS. 7 and 8, when the mounting hub  18  includes an interior hub  22  that is eccentric relative to the mounting surface  24 , the distance from the pivot pin  12  to the mounting surface  24  increases to a maximum value, depicted as point A 1  and then decreases to a minimum value, depicted as point A 2 .  
         [0121]    Reorientation or translation of the linear position of point A 1  and point A 2  with respect to the pivot pin  12  is possible when the mounting hub  18  is rotated about the z-axis.  
         [0122]    Reorientation of points A 1  and A 2  with respect to the x-axis provides a fourth degree of freedom. Similarly, reorientation of points A 1  and A 2  with respect to the y-axis provides a fifth degree of freedom.  
         [0123]    It is to be understood that the rotational and rocking movements just described permit adjustment in virtually an infinite number of directions.  
         [0124]    After the desired position is obtained, the locking screw  20  is tightened to secure the mounting hub  18  in the desired position, as previously described for System  10 A (see FIG. 5 c ).  
         [0125]    II. Use of the System in Shoulder Replacement  
         [0126]    [0126]FIGS. 10-23 detail the use of either of the previously-described systems  10 A or  10 B in shoulder replacement surgery. Desirably, system  10 B would be employed, thereby providing the greatest range of adjustment. In the embodiment illustrated in FIGS. 10-23, the mount of system  10 B is employed.  
         [0127]    The long bone of the upper or proximal arm, as shown in FIG. 16, is known as the humerus  38 . The proximal end of the humerus  38  comprises a ball-shaped head  40  that normally nests within the glenoid cavity of the shoulder bone, or scapula.  
         [0128]    Through disease or injury, the head  40  of the humerus  38  can become damaged such that the shape of the head  40  is altered or the head  40  does not fit properly within the glenoid cavity. Such damage typically results in the shoulder joint becoming painful and a corresponding reduction in mobility of the joint.  
         [0129]    Conventional techniques provide for replacement of the head  40  of the humerus  38  with a prosthesis, or artifical head  42 . As seen in FIG. 10, the system  10 B, comprising a pivot pin  12 , a mounting hub  18  (with eccentrally located interior hub  22 ), slip washers  14 , a lock washer  16 , and a locking screw  20 , can be employed within a shoulder replacement assembly  44  suitable for implantation into a humerus  38 . The system  10 B would permit a physician to mount, position, and secure an artificial head  42 .  
         [0130]    As shown in FIG. 10, the replacement assembly comprises a stem  46  including tendon attachment holes  50 , an assembled system  10 B implanted within the stem  46 , a trial ring  48 , and an artificial head  42 . FIG. 11 illustrates the replacement assembly  44  in assembled form.  
         [0131]    The stem  46  is a conventional stem  46  suitable for implantation within a humerus  38 . The stem  46  desirably includes tendon attachment holes  50  that serve to secure attachment of tendons (not shown) to the stem  46 .  
         [0132]    The stem  46  serves to hold the system  10 B. That is, the pivot pin  12  is implanted within the stem  46  such that the convex portion protrudes at a pre-selected angle from the stem  46  (e.g., 35°).  
         [0133]    The pivot pin  12  can be implanted within the stem  46  by various techniques. In one embodiment, the pin  12  is integrally molded with the stem  46 . Alternatively, the pin  12  can be a separate member configured to mate with an existing stem  46 . In a representative embodiment, the pin  12  includes a Morse taper, as seen in FIG. 10, configured to mate with a complementary tapered surface within the stem  46 . In yet another embodiment, the pin  12  is configured to mate with the stem  46  by threaded engagement (not shown).  
         [0134]    As also shown in FIG. 10, a trial ring  48  is desirably provided. The trial ring  48  is a rigid, generally ring-like member having an inner surface  52  and an outer surface  54 . The inner surface  52  is desirably eccentric relative to the outer surface  54 . The trial ring  48  can be made of plastic or any other suitable material.  
         [0135]    The trial ring  48  is adapted to mate with the mounting hub  18 , i.e., the trial ring&#39;s  48  inner surface  52  geometry approximates the geometry of the mounting surface  24 . In the embodiment illustrated in FIG. 10, the mounting surface  24  is circular and conically tapered and the trial ring  48  has an inner surface  52  that is complementary circular and tapered.  
         [0136]    Optionally, the inner surface  52  of the trial ring can be of a geometric or stepped formation adapted to mate with a complementary surface on the mounting surface  24 , as previously described (not shown).  
         [0137]    As shown in FIG. 12 a , the outer surface  54  of the trial ring  48  desirably has reference markers  56 , e.g., A, B, C, and D, spaced circumferentially around the outer surface  54 .  
         [0138]    Optionally, as also seen in FIG. 12 a , the outer surface  54  is tapered or radiused outward toward the bottom of the trial ring  48  for better visualization of the markers  56 .  
         [0139]    In the embodiment illustrated in FIGS. 12 a  and  12   b , the outer surface  54  of the trial ring  48  contains knurls  58 . The knurls  58  provide for easier grasping of the trial ring  48 . Optionally, the outer surface  54  does not contain knurls  58  or the outer surface  54  is otherwise adapted for grasping (not shown). The outside diameter  57  of the trial ring  48  corresponds or is equivalent to the outside diameter of the humeral head  42 .  
         [0140]    The trial ring  48  is adapted to engage the mounting hub  18  and pivot simultaneously with the mounting hub  18 . In this arrangement, the reference markers  56  can be utilized for evaluation and recording of the desired position, as will be described in greater detail later.  
         [0141]    As seen in FIG. 10, an artificial head  42  is also provided. The artificial head  42  is a rigid, dome-like member having interior  60  and exterior surfaces  62 . The artificial head  42  can be made of stainless steel or other suitable materials.  
         [0142]    As best illustrated in FIGS. 11 and 13 a , the exterior surface  62  is domed to mimic the ball-like head  40  of the humerus  38 .  
         [0143]    As seen in FIG. 13 b , the interior surface  60  is recessed and adapted to mate with the mounting surface  24 . In the embodiment illustrated in FIG. 13 b , the inner surface  60  is circular. Optionally, the interior surface  60  can be stepped to mate with a complementary mounting surface  24 , as previously described (not shown).  
         [0144]    As FIG. 13 b  also shows, the interior surface  60  desirably has reference markers  56 ′ that are complementary to, i.e., mirror, the reference markers  56  on the trial ring  48 . This assures that, when complementary markers  56  and  56 ′ on the trial ring  48  and the artificial head  42  are similarly orientated with respect to the mounting hub  18 , the position of the artificial head  42  will be the same as the position of the trial ring  48 , as will be explained in greater detail later.  
         [0145]    Desirably, as in the embodiment illustrated in FIG. 13 b , the recessed inner surface  60  of the artificial head  42  is eccentrally located with respect to the outer surface  62 .  
         [0146]    When used in combination with the eccentrally located interior hub  22  of system  10 B, this arrangement provides a “double-eccentric” system. The double-eccentric configuration provides a maximum range of adjustment from O axes offset to up to the maximum axes offset.  
         [0147]    In an alternate embodiment, shown in FIGS. 14 a - 14   b  and  15 , the inner surface  60  of the artificial head  42  is centrally located with respect to the outer surface  62 . In this arrangement, an intermediate collar  63  having an interior surface  59  and an exterior surface  61  can be provided.  
         [0148]    The interior surface  59  of the collar  63  is eccentrally located with respect to the exterior surface  61  and configured to mate with the mounting surface  24 . The exterior surface  61  is desirably configured to mate with the interior surface  60  of the artificial head  42 . This arrangement also results in a double-eccentric configuration.  
         [0149]    In use, as seen in FIG. 16, the physician makes a cut  65  through the head  40  of the humerus  38  by conventional techniques. Next, as shown in FIG. 17, an interior bore  64  is reamed in the humerus  38  by conventional techniques to prepare the bone for receiving the stem  46 .  
         [0150]    The stem  46 , incorporating the system  10 B, is then inserted within the bore  64 , as shown in FIG. 18. Tendons can then be attached to the stem  46  using the tendon attachment holes  50  (not shown).  
         [0151]    The trial ring  48  is then placed on the mounting hub  18 . The eccentric interior hub  22  of the mounting hub  18 , together with the eccentric inner surface of the trial ring  48  form a double-eccentric system, as shown in FIGS. 19 a - 19   c . As represented by arrows in FIGS. 19 a  and  19   b , the trial ring  48  is then rotated simultaneously with the mounting hub  18  until the desired position relative to the cut surface of the humerus  38  is achieved (e.g., center of trial ring  48  is centered with cut surface of humerus  38 ).  
         [0152]    As FIG. 19 c  shows, the trial ring  48  is also adapted to rotate independently of the mounting hub  18 .  
         [0153]    Then, as shown in FIG. 20, the trial ring  48  is tilted (represented by arrows and phantom lines in FIG. 20) with the mounting hub  18  until the desired position relative to the cut is achieved (e.g., parallel to cut).  
         [0154]    As seen in FIG. 21, the mounting hub  18  is then secured in the desired position by tightening (represented by arrow in FIG. 21) the locking screw  20 , e.g., with an Allen wrench  30 .  
         [0155]    As also seen in FIG. 21, the physician can then make a mark  66  on the humerus  38  corresponding to the position of a given reference marker  56  on the trial ring  48  when the mounting hub  18  is properly aligned.  
         [0156]    For example, FIG. 21 illustrates a mark  66  made on the humerus  38  corresponding to the position of reference marker “B” when the trial ring  48  is properly aligned.  
         [0157]    Next, as illustrated in FIG. 22, the artificial head  42  is then orientated so that the desired reference marker on the interior surface  60  of the artificial head  42  is aligned with the mark  66  previously made on the humerus  38 .  
         [0158]    For example, FIG. 22 illustrates the reference marker “B” on the interior surface  60  of the artificial head  42  being aligned with the mark  66  previously made on the humerus  38 .  
         [0159]    The artificial head  42  is then placed (represented by phantom lines in FIG. 22) on the mounting hub  18  in this desired orientation.  
         [0160]    Finally, as shown in FIG. 23, the physician seats and secures the aligned artificial head  42  in place by hitting the artificial head  42  with a hammer  68  to lock the tapers together before placing the artificial head  42  into position within the glenoid cavity.  
         [0161]    III. Alternate Mounting Systems  
         [0162]    A. Embodiment #1:  
         [0163]    Eccentric Mechanism  
         [0164]    [0164]FIGS. 24 a - 29  detail an alternate embodiment of a shoulder prosthesis mounting system  10 C embodying features of the invention. With reference to FIGS. 24 a  and  24   b , the system  10 C comprises a stem  46 , a pivot pin  12 , a bottom eccentric insert  108 , an eccentric mount  110 , a top eccentric insert  112 , at least one fastener  114 , at least one guidepin  116 , and an artificial head  42 .  
         [0165]    The stem  46  is a conventional stem suitable for implantation into a humerus and serves to receive the pivot pin  12 . The pivot pin  12  comprises a ball component  118  and a post component  120 . The post  120  extends from the ball  118  and is sized to pass through the mount  110  and an eccentric opening  122  on the bottom insert  108  to mate with the stem  46 , e.g., by threaded engagement (see e.g., FIG. 24 a ) or Morse taper (not shown).  
         [0166]    In an alternate embodiment, the post  120  and the ball  118  are not integral. The post  120  is integral with the stem  46  and extends from the stem  46 . The ball  118  is configured to mate with the post  120 , e.g., by threaded engagement, and thus is selectively removable from the post  120 .  
         [0167]    In either embodiment, the stem  46  is configured to carry the post  120  such that the ball  118  protrudes at a pre-selected angle from the stem  46 , e.g., 35°. Desirably, a portion of the post  120  remains exterior to the stem  46 , enabling the mount  110  to pivot freely on the ball  118  (see FIG. 29).  
         [0168]    The eccentric opening  122  is of a larger diameter than the post  120  and sized to permit rotation of the mount  110  about the x, y, and z axes, as will be described in greater detail later.  
         [0169]    As seen in FIG. 25, the ball  118  is a spherical member sized to rest on the eccentric opening  122  of the bottom insert  108 . This arrangement allows the ball  118  to serve as a pivot surface permitting adjustment of the eccentric mount  110 .  
         [0170]    The eccentric mount  110  is a ring-like member having an outer surface  124  and an inner surface  126 , as seen in FIGS. 24 a  and  24   b . As best illustrated in FIG. 24 b , the inner surface  126  of the mount  110  is eccentric with respect to the outer surface  124 . This arrangement allows the head  42  to be positioned eccentrally with respect to the mount  110 . As FIGS. 25 and 26 show, the bottom insert  108  has an outer surface  128  adapted to mate with the inner surface  126  of the mount, e.g., by recessed slip fit that is free to rotate.  
         [0171]    With reference again to FIG. 26, at least one guidepin  116  extends from the bottom insert  108 . In the illustrated embodiment, three guidepins  116  are employed. The guidepins  116  are adapted to pass through complementary guidepin holes  130  on the top insert  112  when the top and bottom inserts  112  and  108  are properly aligned. Thus, the guidepins  116  serve to help align and secure the top and bottom inserts  112  and  108 .  
         [0172]    As best seen in FIG. 24 b , the top eccentric insert  112  has a top surface  132  and a bottom surface  134 . The bottom surface  134  has an eccentric recessed area  136  configured to mate with the ball  118 . The top insert  112  is further adapted to rest on the bottom insert  108 .  
         [0173]    As best shown in FIG. 26, the bottom and top inserts  112  and  108  each further comprise at least one fastener opening  138  adapted for passage of a fastener  114 , e.g., a screw. The fastener  114 , when tightened, serves to secure the mount  110  in a desired position by compressing the top and bottom inserts  112  and  108  together around the ball  118  and the mount  110 . The “stacking” arrangement of the top and bottom inserts  112  and  108  serves to maximize the surface area compressed, thereby aiding in securing the mount  110  in a desired position.  
         [0174]    The eccentric mount  110  along with the eccentric opening  122  of the bottom insert  108  and the eccentric recessed area  136  of the top insert  112  provide a double-eccentric system.  
         [0175]    The artificial head  42  serves as a prosthesis for the head of a humerus, as previously described (see, e.g., FIG. 23). As FIG. 24 b  shows, the recessed interior surface  60  of the head  42  is desirably concentric with respect to the outer surface  62  and is threaded to mate with the outer surface  124  of the mount. Placement of the head  42  onto the mount  110  secures the head to the mount  110  (see FIG. 28).  
         [0176]    The system  10 C provides at least five degrees of freedom, thereby allowing a wide range of adjustment in multiple dimensions.  
         [0177]    First, as illustrated by arrows in FIGS. 27 a - 27   b , the mount  110  can be rocked or rotated, i.e., tilted, about the x-axis (i.e., side to side rotation).  
         [0178]    Second, as illustrated by arrows in FIGS. 27 c - 27   d , the mount  110  can be rocked or rotated, i.e., tilted, about the y-axis (i.e., front to back rotation).  
         [0179]    Third, as illustrated by arrows in FIG. 27 e , the mount  110  can be rotated up to 360° in either direction about the z-axis.  
         [0180]    Fourth and fifth, the double eccentric arrangement permits translation of the linear position of points A 1  and A 2  with respect to the pivot pin  12  when the inserts  108  and  112  and mount  110  are rotated, as previously described for system  10 B (see FIGS. 7 and 8). This action permits translation along the x and y axes.  
         [0181]    The double-eccentric configuration serves to maximize the range of translational adjustment possible under the fourth and fifth types of movement.  
         [0182]    In use, as shown in FIG. 25, the pivot pin  12  is passed through the bottom insert  108  and the mount  110 . The pivot pin  12  is then coupled to the stem  46 , e.g., by screwing the post  120  into the stem  46 . As FIG. 26 shows, the top insert  112  is then aligned with the bottom insert  108  by aligning the fastener openings  138  on the top and bottom inserts  112  and  108 , the guidepins  116  with the guidepin holes  130 , and the recessed area  136  with the ball  118 .  
         [0183]    The position of the mount  110  is then adjusted by rotating or rocking the mount about the x, y, and z axes (see FIGS. 27 a - 27   e ). The fastener  114  is then tightened to secure the mount  110  in a desired position (not shown). Finally, the head  42  is mounted onto the mount  110  (see FIGS. 28 and 29).  
         [0184]    B. Embodiment #2:  
         [0185]    Disk Slide Mechanism  
         [0186]    [0186]FIGS. 30 a - 34  detail another embodiment of a shoulder prosthesis mounting system  10 D embodying features of the invention. With reference to FIGS. 30 a  and  30   b , the system  10 D comprises a stem  46 , a pivot pin  12 , a mounting ring  140 , a bottom disk  142 , a top disk  144 , an artificial head  42 , and a locking tool  146 .  
         [0187]    The stem is a conventional stem  46  and serves to receive a pivot pin  12 , as previously described for system  10 C The pivot pin  12  is similar in configuration to the pivot pin of System  10 C. The post  120  is adapted to pass through the bottom disk  142  and the mounting ring  140  to mate with the stem  46 , e.g., by threaded engagement.  
         [0188]    As FIG. 31 shows, the ball  118  is sized to rest within the bottom disk  142 . This arrangement allows the ball  118  to serve as a pivot surface, thereby permitting adjustment of the mounting ring  140 .  
         [0189]    As best seen in FIG. 30 a , the mounting ring  140  is comprised of an outer ring  148  having a circular marginal surface and an integrally-formed upstanding inner annular ring  150 . The center of the inner ring defines a chamber  152  and includes an opening  154  permitting passage of the post  120 .  
         [0190]    With reference again to FIG. 31, the chamber  152  is configured to receive the bottom disk  142  and the ball  118 . The outer surface  156  of the inner ring  150  is desirably configured, e.g., threaded, to mate with the interior surface  60  of the head  42 .  
         [0191]    In the illustrated embodiment, the inner ring  150  is concentric with respect to the outer ring  148 . However, the invention also contemplates embodiments in which the inner ring  150  is eccentric with respect to the outer ring  148 .  
         [0192]    As best seen in FIG. 34, the center opening  154  of the mounting ring  140  is of a larger diameter than the diameter of the post  120  and sized to permit translation of the mounting ring  140  about the x and y axes and rotation about the z-axis, as will be described in greater detail later.  
         [0193]    As seen in FIG. 30 a , the mounting ring  140  desirably has a locking aperature  158 . The aperature  158  is a bore that transverses the circumferential margin of the mounting ring  140  and serves to receive the locking tool  146 . The locking tool  146  is configured for insertion into the locking aperature  158  and allows rotation of the mounting ring  140  to tighten the head  42  onto the mounting ring  140  (see also FIG. 34).  
         [0194]    The bottom disk  142  is a ring-like member having an open center permitting passage of the post  120  and is configured to rest within the chamber  152  and receive the ball  118  (see FIGS. 30 a - 31 ). It is further configured to receive the top disk  144 , as illustrated in FIG. 32.  
         [0195]    Referring again to FIGS. 30 a  and  30   b , the top disk  144  has a top surface  160  and a bottom surface  162 . The top surface  160  is desirably flat or otherwise configured to permit compression of the top and bottom disks  144  and  142  upon mounting of the head  42  onto the mounting ring  140 . The bottom surface  162  has a recessed area  164  configured to mate with the ball  118 . The top disk  144  is further configured to rest on the bottom disk  142  (see also FIG. 32).  
         [0196]    This stacking arrangement permits compression of the top and bottom disks  144  and  142  as the head  42  is mounted onto the mounting ring  140  and serves to maximize the surface area compressed, thereby securing the mounting ring  140  in a desired position.  
         [0197]    The artificial head  42  serves as a prosthesis for the head of a humerus, as previously described. As seen in FIG. 30 b , the recessed interior surface  60  of the head  42  is desirably concentric with respect to the outer surface  62  of the head  42 . The invention also contemplates, however, embodiments in which the interior surface  60  is eccentric. The interior surface  60  of the head  42  is also desirably threaded or otherwise configured to mate with the inner ring  150  of the mounting ring  140 .  
         [0198]    Similar to system  10 C, the system  10 D provides at least five degrees of freedom.  
         [0199]    First, as illustrated by arrows in FIGS. 33 a - 33   b , the mounting ring  140  can be rocked or rotated, i.e., tilted, about the x-axis (i.e., side to side rotation).  
         [0200]    Second, as illustrated by arrows in FIGS. 33 c - 33   d , the mounting ring  140  can be rocked or rotated, i.e., tilted, about the y-axis (i.e., front to back rotation).  
         [0201]    Third, as illustrated by arrows in FIG. 33 e , the mounting ring  140  can be rotated up to 360° in either direction about the z-axis.  
         [0202]    The difference between the outside diameter of the top and bottom disks  144  and  142  and the inside diameter of recessed chamber  152  forms a gap, as seen in FIG. 32. This arrangement permits linear translation along the x-axis, providing a fourth degree of freedom, and the y-axis, providing a fifth degree of freedom.  
         [0203]    In use, with reference to FIGS. 30 a - 32 , the post  120  is passed through the bottom disk  142  and the mounting ring  140 . The post  120  is then coupled to the stem  46 , e.g., by screwing. The top disk  144  is then aligned with the bottom disk  142  by aligning the recessed area  164  with the ball. Next, the head  42  is mounted onto the mounting ring  140 .  
         [0204]    The position of the head  42  is then adjusted by rotating and rocking the head  42  about the x, y, and z axes (see FIGS. 33 a - 33   e ). As FIG. 34 illustrates, the locking tool  146  is then inserted into the locking aperture  158 . As represented by arrows in FIG. 34, the mounting ring  140  is then rotated by use of the locking tool  146  to tighten the head  42  onto the mounting ring  140 . This action places all the components in compression and fixes the head  42  in place.  
         [0205]    C. Embodiment #3:  
         [0206]    Slotted Mechanism  
         [0207]    [0207]FIGS. 35 a - 40  detail another embodiment of a shoulder prosthesis mounting system  10 E embodying features of the invention. With reference to FIGS. 35 a  and  35   b , the system comprises a stem  46 , a pivot pin  12 ), a bottom plate  166 , a top plate  168 , at least one fastener  170 , and at least one fastening element  172  for securing the fastener  170 .  
         [0208]    The stem  46  and pivot pin  12  are configured as previously described for systems  10 C and  10 D. The post  120  is adapted to pass through the bottom plate  166  to mate with the stem  46 , e.g., by threaded engagement. The ball  118  is sized to rest on the bottom plate  166 . This arrangement allows the ball  118  to serve as a pivot surface that permits adjustment of the bottom plate  166 .  
         [0209]    As shown in FIG. 36, the bottom plate  166  is a circular member having a major axis A 1  and a minor axis A 2 . An elongated eccentric slot  174  is provided along the major axis A 1 . The bottom plate  166  also provides a pair of elongated fixation slots  176  radially spaced from the center and parallel to the major axis A 1 . The fixation slots  176  allow the position of the top plate  168  to be laterally adjusted with respect to the bottom plate  166 . The fixation slots  176  also serve to receive fasteners  170 , e.g., bolts, to secure the position of the top plate  168 .  
         [0210]    As shown in FIG. 37, the eccentric slot  174  receives the ball  118  and allows lateral, i.e., side to side, adjustment (represented by arrows and phantom lines in FIG. 37) of the position of the ball  118  within the eccentric slot  174 .  
         [0211]    The bottom plate  166  includes a circumferential outer surface  178  configured to mate with the head  42 , e.g., by threaded engagement (see e.g., FIG. 35 b ). The bottom plate  166  serves to receive the top plate  168  in a stacked configuration.  
         [0212]    Referring again to FIG. 36, the top plate  168  is a generally elliptical member having a major axis A 3  and a minor axis A 4 . The major axis A 3  parallels the minor axis A 2  of the bottom plate  166  and the minor axis A 4  parallels the major axis A 1  of the bottom plate  166  when the top plate  168  is aligned with bottom plate  166 . The top plate  168  further provides fastener receiving openings  180  sized and configured to receive the fasteners  170 .  
         [0213]    The top plate  168  further provides a top surface  182  and a bottom surface  184 . The top surface  182  is configured to receive a fastening element  172  for the fastener  170 , e.g., a nut. The bottom surface  184  includes a recessed area  186  configured to mate with the ball  118 . The recessed area  186  desirably includes an opening  188  adapted for viewing the ball  118 , thereby aiding in aligning the top plate  168  with respect to the bottom plate  166 . The top plate  168  is further configured to rest on the ball  118 , leaving a gap between the top plate  168  and bottom plate  166 .  
         [0214]    The fasteners  170 , when tightened, serve to secure the plates  166  and  168  to the ball  118  in a desired position by compressing the top and bottom plates  166  and  168  together. The stacked arrangement of the plates  166  and  168  serves to maximize the surface area compressed, thereby aiding in securing the plates  166  and  168  in the desired position relative to the ball  118 .  
         [0215]    The artificial head  42  serves as a prosthesis for the head of a humerus, as previously described. The recessed interior surface  60  of the head  42  is desirably concentric with respect to the exterior surface  62  of the head  42 , as shown in FIG. 35 b . It should be understood, however, that the invention also contemplates embodiments in which the interior surface  60  is eccentric.  
         [0216]    Similar to systems  10 C and  10 D, the system  10 E provides at least five degrees of freedom.  
         [0217]    First, as illustrated by arrows in FIGS. 39 a - 39   b , the bottom plate  166  can be rocked or rotated, i.e., tilted, about the x-axis (i.e., side to side rotation).  
         [0218]    Second, as illustrated by arrows in FIGS. 39 c - 39   d , the bottom plate  166  can be rocked or rotated, i.e., tilted, about the y-axis (i.e., front to back rotation).  
         [0219]    Third, as illustrated by arrows in FIG. 39 e , the bottom plate  166  can be rotated up to 360° in either direction about the z-axis.  
         [0220]    The slots  176  in the base  166  permit translation of the linear position of the major axis A 1  and minor axis A 2  with respect to the pivot pin  12  when the bottom plate  166  is slid along the x axis, providing a fourth degree of freedom, or the y axis, providing a fifth degree of freedom.  
         [0221]    In assembling the system  10 E, the post  120  is passed through the eccentric slot  174  of the bottom plate  166 , thereby resting the ball  118  within the slot  174 , as seen in FIG. 37. The bottom plate  166  is then slid (illustrated by arrows in FIG. 37) along the slot  174  until the desired lateral position is obtained. The fasteners  170  are then passed through the fixation slots  176  of the bottom plate  166 .  
         [0222]    Next, the top plate  168  is aligned with the bottom plate  166  by aligning the recessed area  186  with the ball  118  and the fastener receiving holes  180  with the fasteners  170 . The fasteners  170  are then passed through the fixation slots  176  of the bottom plate  166  and the fastener receiving openings  180  on the top plate  168 . The top plate  168  is thereby positioned to rest on the ball  118  and over bottom plate  166 , as FIG. 38 illustrates. The position of the plates  166  and  168  is then adjusted by rotating or rocking the bottom plate  166  about the x, y, and z axes (see FIGS. 39 a - 39   e ).  
         [0223]    The components of the system  10 E can be provided in a fully assembled form in which the user only need tighten the fasteners  170  after adjusting the position of the plates  166  and  168  to secure the plates  166  and  168  in the desired position.  
         [0224]    Fastening elements  172 , e.g., nuts, can be used if desired to tighten and secure the fasteners  170 . This action compresses the plates  166  and  168  around the ball  118  to secure the plates  166  and  168  in the desired orientation and location relative to the ball  118 .  
         [0225]    Finally, as seen in FIG. 40, the head  42  is mounted onto the bottom plate  166 .  
         [0226]    The foregoing is considered as illustrative only of the principles of the invention. Furthermore, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described. While the preferred embodiment has been described, the details may be changed without departing from the invention, which is defined by the claims.