Abstract:
Joint prostheses and associated methods that have a medialized center of rotation, inhibit subluxation of the implant while facilitating full range of motion and normal articular function, are able to be implanted using standard bone preparation techniques, and/or provide increased implant lifetime.

Description:
BACKGROUND 
       [0001]    A standard shoulder joint prosthesis includes an artificial ball-and-socket joint with the ball portion replacing the humeral head and the socket portion implanted in the glenoid cavity of the scapula. Generally, this type of arrangement is appropriate where the rotator cuff is relatively intact and functional for stabilizing the implant. The reverse arrangement—the ball portion secured to the scapula and the socket portion secured to the humeral head—is termed a “reverse shoulder prosthesis” and is often used where the rotator cuff of the patient is relatively less functional. In both the standard and reverse configurations, however, long term loosening of the muscles supporting the prosthesis is a concern. For example, a common failure mode of a reverse shoulder prosthesis is continued degradation of the deltoid muscle, which eventually allows the prosthesis to sublux, or separate, thereby interfering with proper functioning of the joint. 
       SUMMARY 
       [0002]    Some embodiments relate to joint prostheses and associated methods that have a medialized center of rotation, inhibit subluxation of the implant while facilitating full range of motion and normal articular function, are able to be implanted using standard bone preparation techniques, and/or provide increased implant lifetime. 
         [0003]    Some embodiments relate to a joint prosthesis adapted to be secured to a first bone and a second bone for facilitating relative articulation between the first and second bones. The joint prosthesis includes a first articulation component defining a first articulation surface that is substantially convex and a second articulation component defining a second articulation surface that is substantially concave and a third articulation surface that is substantially convex. The first articulation surface of the first articulation component is engaged with the second articulation surface of the second articulation component such that the first articulation component is substantially limited in angulation relative to the second articulation component within a first plane. The prosthesis also includes a third articulation component defining a fourth articulation surface that is substantially concave, the third articulation surface of the second articulation component being engaged with the fourth articulation surface of the third articulation component such that the third articulation component is substantially limited in angulation relative to the first articulation component within a second plane that is angularly offset from the first plane. 
         [0004]    Other embodiments relate to a virtual ball-and-socket prosthesis for replacing a joint between a first bone and a second bone. The prosthesis includes means for limiting angular articulation of a first articulation component in sliding contact with a second articulation component to changes in pitch and means for limiting angular articulation of a third articulation component in sliding contact with the second articulation component to changes in yaw. The prosthesis also includes first bone anchor means for securing the first articulation component to a first bone and second bone anchor means for securing the third articulation component to a second bone, as well as means for allowing changes in roll between the first bone anchor means and the second bone anchor means. 
         [0005]    Some embodiments relate to a virtual ball-and-socket prosthesis for replacing a natural joint between two bones. The prosthesis includes a first bone anchor component, a second bone anchor component, and a plurality of articulation components that articulatably join the first and second bone anchor components, the plurality of articulation components defining a pitch bearing interface, a yaw bearing interface separate from the pitch bearing interface, and a roll bearing interface separate from both the pitch and yaw bearing interfaces. The plurality of articulation components are secured relative to one another such that articulation between the first and second bone anchors in pitch is borne by the pitch bearing surface, articulation between the first and second bone anchors in yaw is borne by the yaw bearing interface, and medial rotational articulation between the first and second bone anchors is borne by the rotational bearing interface. 
         [0006]    Still other embodiments relate to a method of assembling an artificial joint between bones. The method includes securing a first articulation component having a first articulation surface that is convex to a second articulation component having a second articulation surface that is concave such that the first articulation surface of the first articulation component is engaged with the second articulation surface of the second articulation component and the first articulation component is limited in angulation relative to the second articulation component to a first plane. The method also includes securing a third articulation component defining a fourth articulation surface that is concave to the second articulation component such that a third articulation surface of the second articulation component that is convex is engaged with the fourth articulation surface of the third articulation component and the third articulation component is limited in lateral angulation relative to the first articulation component to a second plane that is angularly offset from the first plane. 
         [0007]    While multiple embodiments are disclosed, still other embodiments of the present invention will become apparent to those skilled in the art from the following detailed description, which shows and describes illustrative embodiments of the invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not restrictive. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a perspective view of a first joint prosthesis, according to some embodiments. 
           [0009]      FIG. 2  is a perspective view of the joint prosthesis of  FIG. 1  in an unassembled state, according to some embodiments. 
           [0010]      FIGS. 3 and 4  show a first articulation component of the joint prosthesis of  FIG. 1 , according to some embodiments. 
           [0011]      FIGS. 5 and 6  show a second articulation component of the joint prosthesis of  FIG. 1 , according to some embodiments. 
           [0012]      FIGS. 7 and 8  show a third articulation component of the joint prosthesis of  FIG. 1 , according to some embodiments. 
           [0013]      FIG. 9  is a sectional view of the joint prosthesis of  FIG. 1 , according to some embodiments. 
           [0014]      FIG. 10  is a perspective view of a second joint prosthesis, according to some embodiments. 
           [0015]      FIG. 11  is a cutaway view of the joint prosthesis of  FIG. 10 , according to some embodiments. 
           [0016]      FIG. 12  is a perspective view of a first articulation component of the joint prosthesis of  FIG. 10 , according to some embodiments. 
           [0017]      FIG. 13  is a perspective view of a second articulation component of the joint prosthesis of  FIG. 10 . 
           [0018]      FIG. 14  is a perspective view of a third articulation component of the joint prosthesis of  FIG. 10 . 
       
    
    
       [0019]    While the invention is amenable to various modifications, permutations, and alternative forms, specific embodiments have been shown by way of example in the drawings and are described in detail below. The intention, however, is not to limit the invention to the particular embodiments described. On the contrary, the invention is intended to cover all modifications, equivalents, and alternatives falling within the scope of the invention as defined by the appended claims. 
       DETAILED DESCRIPTION 
       [0020]    As described in greater detail, some embodiments relate to an artificial, virtual ball-and-socket joint that includes a linked articulation assembly adapted to reduce and/or prevent subluxation of the artificial joint due, for example, to relaxation of the muscles supporting the artificial joint. Additionally, in some implementations, the linked articulation assembly of the artificial joint includes a plurality of distinct articulation interfaces for bearing movement of the artificial joint in distinct coordinate directions, such as a first interface for supporting articulation along the anteroposterior direction, a second interface for supporting articulation along the inferosuperior direction, and one or more interfaces for supporting rotational articulation. The separate interfaces provide means for reducing wear, as the bearing surfaces need only support movement along one discrete direction, which can be contrasted to the bearing surfaces of a typical ball-and-socket joint. While various features associated with some embodiments have been described above, it should be understood that various additional or alternate features are contemplated. 
         [0021]    The terms pitch, roll, and yaw are also used, where roll generally refers to angulation, or rotation, in a first plane through which a longitudinal axis of a body orthogonally passes (e.g., rotation about a longitudinal axis passing through the glenoid), pitch refers to angulation, or rotation, in a second plane orthogonal to the first plane, and yaw refers to angulation, or rotation, in a third plane orthogonal to the first and second planes. In some embodiments, pitch is angulation in the anteroposterior direction, yaw is angulation in the inferosuperior direction, and roll is medial rotational articulation. 
         [0022]      FIG. 1  shows a joint prosthesis  10  (also described as an artificial joint or a prosthesis) in an assembled state and  FIG. 2  shows the joint prosthesis  10  in an unassembled state, according to some embodiments. As shown in  FIGS. 1 and 2 , the joint prosthesis  10  is adapted as a reverse shoulder prosthesis for replacing the gleno-humeral joint of a patient. Though the joint prosthesis  10  is adapted as a reverse shoulder prosthesis according to some embodiments, in other embodiments the joint prosthesis  10  is adapted as a traditional shoulder prosthesis or as a prosthesis for other bodily joints, such as the hip, for example. 
         [0023]    As shown, the prosthesis  10  includes a first bone anchor  12  (also described as a base plate or an articulation component), a first articulation component  14  (also described as a glenosphere), a second articulation component  16  (also described as a liner or a disk), a third articulation component  18  (also described as a rotational plate), a locking ring  20  (also described as a locking member), a peg  22  (also described as a fastener, a guide, or a locking bolt), and a second bone anchor  24  (also described as a stem). The prosthesis  10  is generally adapted as a virtual ball-and-socket joint, being able to articulate through a wide range of motion similar to that of a traditional ball-and-socket joint by supporting freedom of movement between the first and second bone anchors  12 ,  24  in at least three coordinate directions, such as X-, Y-, and Z-axis angular articulation. For example, the prosthesis  10  optionally facilitates angular articulation relative to the X-axis (also described as pitch), or parallel to the Y-Z plane, relative to the Y-axis (also described as yaw), or parallel to the X-Z plane, and relative to the Z-axis (also described as roll), or parallel to the X-Y plane. In some embodiments, angular articulation relative to the X-axis corresponds to front-back motion or anteroposterior articulation, angular articulation relative to the Y-axis corresponds to up-down motion or inferosuperior articulation, and angular articulation relative to the Z-axis corresponds to medial rotational articulation. 
         [0024]    As shown in  FIG. 2 , the first bone anchor  12  includes a body  30  and a post  32 . The body  30  and the post  32  are optionally adapted to assist with securing the first bone anchor  12  directly to a scapula (not shown). For example, the first bone anchor  12  is optionally adapted to be secured to boney structures forming a glenoid cavity of the scapula. As shown, the body  30  includes one or more apertures  34  for receiving a fastener or fasteners (e.g., bone screws) for securing the body  30  to the scapula or other structure. The body  30  also defines an upper, shoulder portion  36  that is formed as a substantially flat plate and a lower, insert portion  38  that is reduced in diameter relative to the shoulder portion  36  and is formed as a hollow cylinder. In some embodiments, the bone anchor  12  is formed of titanium, for example, although other materials are contemplated. 
         [0025]    In some embodiments, the post  32  is adapted to be secured directly to the scapula (e.g., including male threads or an appropriate geometry for assisting in attaching the first bone anchor  12  to the boney structures of the scapula). In other embodiments, the body  30  and/or the post  32  are adapted to interface with a secondary anchoring device (not shown) for securing the first bone anchor  12  to the scapula or other suitable structure, such as the secondary anchoring devices described in U.S. application Ser. No. 12/765,347, “Joint Prosthesis Attachment System, Device, and Method,” filed Apr. 22, 2010, the entire contents of which are incorporated herein by reference. 
         [0026]      FIGS. 3 and 4  show the first articulation component  14  from a top view and a bottom view, respectively. As shown, the first articulation component  14  is a substantially hollow bowl, or is substantially cup-shaped, the first articulation component  14  having an inner surface  40  and an outer surface  42 , the inner surface  40  being described as a first articulation surface and the outer surface  42  being described as a second articulation surface of the prosthesis  10 . The outer surface  42  is substantially smooth overall and adapted for repeated articulation. As shown in the cross-sectional view of  FIG. 9 , the inner surface  40  has an upper portion  44  that is substantially cylindrical and a lower portion  46  that is substantially concave, where the upper portion  44  is adapted to form a complementary fit with the insert portion  38  of the first bone anchor  12 . 
         [0027]    In some embodiments, the upper portion  44  and the insert portion  38  are secured together using an interference or frictional fit, detents, fasteners, adhesives, combinations thereof, or other fastening means. As shown in  FIG. 4 , the outer surface  42  forms a track  48  (also described as a projection, a tenon, or a rail), that extends through an arcuate path diametrically (e.g., along a centerline or diameter of the first articulation component  14 ) across the first articulation component  14  in the X-axis direction, although the track  48  is also optionally comprised of one or more projections that extend along one or more parallel chords of the component  16 . In some embodiments, the track  48  has a substantially rectangular cross-section, although a variety of shapes, such as dovetail cross-sections, are contemplated. 
         [0028]    The first articulation component  14  is optionally formed of cobalt-chrome alloy and/or other suitable materials having low friction and/or wear characteristics for the outer surface  42 , such as PTFE. Though some specific examples have been provided, a variety of materials are contemplated. 
         [0029]    As shown in  FIGS. 3 and 4 , the first articulation component  14  also includes a slot  50 , or opening, that is centered on the first articulation component  14 , for example at an apex of the first articulation component  14 . In some embodiments, the slot  50  is formed through the track  48 , from the inner surface  40  to the outer surface  42 , and has an arc length that is substantially shorter than that of the track  48 . The slot  50  extends from a first end  52  to a second end  54 , the first and second ends  52 ,  54  defining limits, or a range of movement, of the prosthesis  10  in a first direction, such as angular articulation relative to the Y-axis, or parallel to the X-Y plane, also described as a change in pitch. 
         [0030]      FIGS. 5 and 6  show the second articulation component  16  from a top view and a bottom view, respectively. In some embodiments, the second articulation component  16  is a substantially hollow bowl, or is substantially cup-shaped, the second articulation component  16  having an inner surface  60  (also described as a third articulation surface) and an outer surface  62  (also described as a fourth articulation surface). The inner surface  60  is substantially concave and forms a first recess  64  and the second outer surface  62  is substantially convex and defines a second recess  66 , each of the recesses  64 ,  66  also being described as guides, mortises, or channels. The second articulation component  16  also has an aperture  68  through the second articulation component  16  from the inner surface  60  to the outer surface  62 . In some embodiments, the inner and outer surfaces  60 ,  62  are generally smooth, being adapted for repeated articulation. 
         [0031]    As shown in  FIGS. 2 and 5 , the first recess  64  extends through an arcuate path diametrically (e.g., along a centerline or diameter of the second articulation component  16 ) across the inner surface  60  in the X-axis direction. In other embodiments, the second articulation component  16  includes one or more parallel recesses extending along one or more parallel chords of the component  16  in the X-axis direction. The first recess  64  has a substantially rectangular cross-section that is complementary to that of the track  48  of the first articulation component  14 , although a variety of cross-sections, such as complementary, interlocking dovetail cross-sections, are also contemplated. 
         [0032]    As shown in  FIG. 6 , the second recess  66  extends through an arcuate path diametrically (e.g., along a centerline or diameter of the second articulation component  16 ) across the outer surface  62  of the second articulation component  16  in the Y-axis direction. As shown, the second recess  66  extends in a substantially orthogonal direction to the first recess  64  of the second articulation component  16 . In other embodiments, the second articulation component includes one or more parallel recesses extending along one or more parallel chords of the component  16  in the Y-axis direction. As shown, the second recess  66  has a substantially rectangular cross-section that is complementary to a track feature of the third articulation component  18 , although a variety of cross-sections, such as complementary, interlocking dovetail cross-sections, are also contemplated. 
         [0033]    As shown in  FIGS. 5 and 6 , the aperture  68  is formed through the second articulation component  16 , from the inner surface  60  to the outer surface  62 . In some embodiments, the aperture  68  is optionally positioned at an apex or center of the second articulation component  16  and has a substantially non-circular cross-section, such as a generally square cross-section, for example. 
         [0034]    The second articulation component  16  is optionally formed of ultra-high-molecular-weight-polyethylene (UHMWPE) or other suitable materials having low friction and/or wear characteristics for the inner and outer surfaces  60 ,  62 , such as PTFE. Though some specific examples have been provided, a variety of materials are contemplated. 
         [0035]      FIGS. 7 and 8  show the third articulation component  18  from a top view and a bottom view, respectively. As shown, the third articulation component  18  includes a central portion  80  that is shaped as a substantially hollow bowl, or is substantially cup-shaped and a perimeter portion  82  that is shaped as a substantially flat rim extending from the central portion  80 . The third articulation component  18  also has an inner surface  84  and an outer surface  86 . In some embodiments, the inner and outer surfaces  84 ,  86  are substantially smooth overall and adapted for repeated articulation. As subsequently described, the inner surface  84  is adapted to engage and articulate with the outer surface  62  of the second articulation component  16  to define a second articulation interface. 
         [0036]    In some embodiments, at the central portion  80 , the inner surface  84  is substantially concave and defines a fifth articulation surface  84 A. At the perimeter portion  82 , the inner surface  84  is substantially flat, or planar, and defines a sixth articulation surface  84 B. And, at the central portion  80 , the outer surface  86  is substantially convex and defines a seventh articulation surface  86 A and, at the perimeter portion  82 , the outer surface  86  is substantially flat, or planar and defines a eighth articulation surface  86 B. 
         [0037]    As shown in  FIG. 7 , the inner surface  84  of the third articulation component  18  forms a track  90  (also described as a projection, a strip, a tenon, or a rail), that extends through an arcuate path diametrically (e.g., along a centerline or diameter) across the third articulation component  18  in the Y-axis direction, although the track  90  is also optionally comprised of one or more projections that extend along one or more parallel chords of the component  18 . In some embodiments, the track  90  has a substantially rectangular cross-section, although a variety of shapes, such as dovetail cross-sections, are contemplated. 
         [0038]    As shown in  FIGS. 7 and 8 , the third articulation component  18  also includes a slot  92 , or opening, that is centered on the third articulation component  18 , for example at an apex of the third articulation component  18 . In some embodiments, the slot  92  is formed through the track  90 , from the inner surface  84  to the outer surface  86 , and has an arc length that is substantially shorter than that of the track  90 . The slot  92  extends from a first end  94  to a second end  96 , the first and second ends  94 ,  96  defining a limit, or range of movement, of the prosthesis  10  in a second direction that is angularly offset from the first direction in which the slot  50  of the first articulation component  14  extends. For example, in some embodiments, the second direction is orthogonal to the first direction and corresponds to angular articulation relative to the X-axis, or in the Y-Z plane, also described as a change in yaw. 
         [0039]    The third articulation component  18  is optionally formed of cobalt-chrome alloy or other suitable materials having low friction and/or wear characteristics for the inner and outer surfaces  84 ,  86 , such as PTFE. Though some specific examples have been provided, a variety of materials are contemplated. 
         [0040]      FIG. 9  is a cross-section of the prosthesis  10  shown in an assembled state, according to some embodiments. As shown in  FIGS. 2 and 9 , the locking ring  20  is ring-shaped, having a substantially circular profile with an open interior, and includes an upper, cap portion  100  (also described as a lid or a retainer portion) and a lower collar portion  102  (also described as a lip or a shoulder portion). The cap portion  100  has an inner surface  104  and is adapted to retain the perimeter portion  82  of the third articulation component  18  in a seated position with the second bone anchor  24 . In some embodiments, an inner surface  104  the cap portion  100  defines a ninth articulation surface of the prosthesis  10 , the inner surface  104  being adapted to slide against the perimeter portion  82  of the third articulation component  18 . 
         [0041]    The collar portion  102  is adapted to fit with the second bone anchor  24  for securing the locking ring  20  to the second bone anchor  24 . In some embodiments, the collar portion  102  includes female threads for securing the collar portion  102  to the second bone anchor  24 . In other embodiments, the collar portion  102  additionally or alternative is adapted to be secured to the second bone anchor  24  using an adhesive or other fixation means. In some embodiments, the locking ring  20  is formed of titanium, although a variety of materials are contemplated. 
         [0042]    As shown in  FIGS. 2 and 9 , the peg  22  includes a female connector  110  and a male connector  112 . In some embodiments, the male and female connectors  110 ,  112  are formed of cobalt-chrome with a UHMWPE liner for suitable wear and/or low friction characteristics, although other materials are contemplated. The female connector  110  includes body  114  and a cap  116 . The body  114  is substantially elongate, hollow, and has a non-circular outer cross-section according to some embodiments (e.g., substantially square-shaped). The body  114  also optionally defines an internal lumen  118  ( FIG. 9 ) that is cylindrical in shape. The body  114  is generally adapted to be received through the three articulation components  14 ,  16 ,  18 —through the slot  50 , the aperture  68  ( FIGS. 5 and 6 ), and the slot  92 . The cap  116  is substantially wider than the slot  50  and has a lower surface  120  ( FIG. 9 ), also described as a tenth articulation surface, for sliding against the inner surface  40  of the first articulation component  14 . 
         [0043]    As shown in  FIG. 2 , the male connector  112  includes a post  126  and a cap  128 . The post  126  is substantially complementary in shape to the cross-section of the internal lumen  118  of the female connector  112  (e.g., substantially cylindrical). In some embodiments, the post  126  is adapted to be received in the body  114  of the female connector  110  in a complementary fit to secure the male and female connectors  110 ,  112  together. If desired, a fastener (not show), such as a screw, is driven through the cap  116  of the female connector  110  into the post  126  to secure the peg  22  together. In other embodiments, the male and female connectors  110 ,  112  are additionally or alternatively secured together with adhesive or using other fastening means. As shown in  FIG. 9 , the cap  128  is larger than the slot  92  of the third articulation component  18  such that the cap  128  is adapted to ride on the seventh articulation surface  86 A of the third articulation component  18 . 
         [0044]    As shown in  FIGS. 2 and 9 , the second bone anchor  24  includes a head portion  140  and a stem portion  142 . In some embodiments, the second bone anchor  24  is formed of titanium, although a variety of materials are contemplated. The head portion  140  and/or the stem portion  142  is optionally adapted to assist with securing the second bone anchor  24  directly to a humerus. For example, the stem portion  142  of the second bone anchor  14  is optionally substantially elongate and adapted to be secured within the proximal medullary canal of the humerus, though the second bone anchor  24  is optionally adapted to be secured to other boney structures, such as the femur in cases where the prosthesis  10  is adapted for hip replacement, for example. 
         [0045]    The head portion  140  is substantially conical in shape and forms an outer flange  148 , a support surface  150  (also described as an eleventh articulation surface), and a recessed pocket  152 . In some embodiments, the head portion  140  is adapted to serve as a fourth articulation component and is rotatable with respect to the third articulation component  18  as subsequently described. 
         [0046]    The outer flange  148  is substantially vertically oriented relative to the support surface  150 . As shown, the outer flange  148  includes a top wall  148 A adapted to support the cap portion  100  of the locking ring  20  and an outer wall  148 B adapted to be secured to the collar portion  102  of the locking ring  20 . The outer flange  148  also defines an inner wall  148 C which helps retain the perimeter portion  82  of the third articulation component  18  in the head portion  140  and against which an edge of the perimeter portion  82  optionally slides. The support surface  150  is adapted to slidingly support and engage the eighth articulation surface  86 B on the perimeter portion  82  of the third articulation component  18 . The recessed pocket  152  is adapted to receive portions of the first, second, and third articulation components  12 ,  14 ,  16 , as well as the peg  22 , such that the components are free to angularly articulate as desired. 
         [0047]    Assembly of the prosthesis  10  from the unassembled state of  FIG. 2  to the assembled state includes mating the track  48  of the first articulation component  14  with the first recess  64  of the second articulation component  16  such that the first articulation component  14  is able to articulate relative to the Y-axis while being substantially constrained from angular articulation relative to the X- or Z-axes. Upon mating the track  48  and recess  64 , the inner surface  60  of the second articulation component  16  slides against the outer surface  42  of the first articulation component  14  to define a first articulation interface between the surfaces  42 ,  60 , where the second articulation component  16  provides a bearing surface or acts as a bushing for repeated articulation with the first articulation component  14 . Thus, the track  48  and the first recess  64  optionally provide means for limiting angular articulation of the first articulation component  14 , which is in sliding contact with the second articulation component  16 , to changes in pitch. 
         [0048]    The track  90  of the third articulation component  18  is mated with the second recess  66  of the second articulation component  16  such that the third articulation component  18  is able to articulate with the second articulation component  14  relative to the X-axis while being substantially constrained from articulating in rotational or other directions relative to the Y- or Z-axes. In some embodiments, upon mating the track  90  and second recess  66 , the outer surface  62  of the second articulation component  16  engages and slides against the inner surface  84  of the third articulation component  18  to define a second articulation interface between the surfaces  62 ,  84  where the second articulation component  16  provides a bearing surface or acts as a bushing for repeated articulation with the third articulation component  18 . Thus, the track  90  and second recess  66  optionally provide means for limiting angular articulation of the third articulation component  18 , which is in sliding contact with the second articulation component  16 , to changes in yaw. 
         [0049]    The first, second, and third components are secured together with the peg  22  by inserting the female connector  110  through the three articulation components  14 ,  16 ,  18 —through the slot  50  ( FIG. 3 ), the aperture  68  ( FIG. 5 ), and the slot  92 . The male connector  112  is inserted into the female connector  110  and secured in place. In some embodiments, the non-circular cross-sections of the body  114  of the female connector  110 , the slots  50 ,  92 , and the aperture  68  help ensure that the three components  14 ,  16 ,  18  do not articulate relative to the Z-axis, or change in roll, with respect to one another while still leaving the components  14 ,  16 ,  18  free to angularly articulate relative to Y- and X-axes, respectively. 
         [0050]    In some embodiments, the three articulation components  14 ,  16 ,  18  are secured between the first and second bone anchors  12 ,  24  such that the first and second bone anchors  12 ,  24  are able to rotate, or angulate relative to the Z-axis as well as angulate relative to the X- and Y-axes as described, where the second bone anchor  24  is optionally described as fourth articulation component and the first bone anchor  12  is optionally described as a fifth articulation component. For example, in some embodiments, implantation of the prosthesis  10  includes securing the first bone anchor  12  to a first bone (not shown) such as a scapula. The first bone anchor  12  is optionally secured directly to the first bone (e.g., using bone screws) or using a secondary anchoring device, such as those previously described. The first bone anchor  12  is secured to the first articulation component  14  by positioning the insert portion  38  of the first bone anchor  12  into the upper portion  44  of the first articulation component such that the first bone anchor  12  is fixed to, and moves with, the first articulation component  14  as a single piece. In some embodiments, the insert portion  38  and the upper portion  44  are secured together with the help of adhesives and/or mechanical fasteners (not shown). 
         [0051]    In some embodiments, the second bone anchor  24  is secured to a second bone (not shown), such as a humerus, using known techniques. For example, in some embodiments, the stem portion  142  of the second bone anchor  24  is secured in a proximal medullary cavity of a humerus. 
         [0052]    As shown in  FIG. 9 , the head portion  140  of the second bone anchor  24  is secured to the third articulation component  18  by receiving the perimeter portion  82  of the third articulation component against the support surface  150  of the second bone anchor  24 . The locking ring  20  is secured over the perimeter portion  82  and onto the head portion  140  of the second bone anchor  24  such that the third articulation component  18  is free to rotate with respect to the second bone anchor  24 , the perimeter portion  82  and the support surface  150  engaging to form a third articulation interface and the perimeter portion  82  and the locking ring  20  engaging to form a fourth articulation interface of the prosthesis  10 . Thus, the third and fourth articulation interfaces optionally provide means for allowing changes in roll between the first bone anchor  12  and the second bone anchor  24 . 
         [0053]    Upon securing the articulation components  14 ,  16 ,  18  to the bone anchors  12 ,  24 , the prosthesis  10  is linked, forming a fixed assembly that limits subluxation between the first and second bone anchors  12 ,  24  and is able to freely articulate. For example, in some embodiments, the prosthesis is adapted such that substantially no subluxation is allowed between the first and second bone anchors  12 ,  24 . 
         [0054]    Articulation of the prosthesis  10  includes articulating the first and second articulation components  14 ,  16  relative to one another such that the first articulation component  14  angulates and shifts laterally relative to the second articulation component  16  along a first arcuate path extending in the X-Z plane. In particular, the first component  14  is guided in the X-Z plane as the track  48  rides within the first recess  64  of the second articulation component  16  such that the first articulation component  14  only articulates in the X-Z plane relative to the second articulation component  16 , or only changes in pitch, and is substantially constrained from articulating in other directions relative to the second articulation component  16 . The peg  22  rides in the slot  50  in the first articulation component with the first and second ends  52 ,  54  of the slot  50  serving as stops, or limits to the range of travel of the prosthesis in the X-Z plane. Substantially all of the X-Z plane articulation of the prosthesis  10  occurs at the first articulation interface between the first and second articulation components  14 ,  16 , including the track  48  and the first recess  64 , such that the inner surface  60  of the second articulation component  16  is only exposed to wear in one direction, the X-axis direction, rather than all directions as would otherwise be the case in a traditional ball-and-socket joint, helping increase wear life of the prosthesis  10 . 
         [0055]    In some embodiments, the third articulation component  18  is articulated relative to the second articulation component  16  such that the third articulation component  18  angulates and shifts laterally relative to the second articulation component  16  along a second arcuate path extending parallel to the Y-Z plane. In particular, the third articulation component  18  is guided in the Y-Z plane as the track  90  rides within the second recess  66  of the second articulation component  16  such that the third articulation component  18  only articulates in the Y-Z plane relative to the second articulation component  16 , or only changes in yaw, and is substantially constrained from articulating in other directions relative to the second articulation component  16 . 
         [0056]    In some embodiments, the peg  22  rides in the slot  92  in the third articulation component  18  with the first and second ends  94 ,  96  of the slot  92  serving as stops, or limits in the range of travel of the prosthesis in the Y-Z plane. Thus, according to some embodiments, substantially all of the Y-Z plane articulation of the prosthesis  10  occurs at the first articulation interface between the third and second articulation components  18 ,  16 , including the track  90  and the second recess  66 , such that the outer surface  62  of the second articulation component  16  is only exposed to wear in one direction, the Y-axis direction, rather than all directions as would otherwise be the case in a traditional ball-and-socket joint, also helping increase wear life of the prosthesis  10 . 
         [0057]    In some embodiments, the third articulation component  18  is articulated relative to the head portion  140  of the second bone anchor  24 , also described as a fourth articulation component, such that the third articulation component rotates, or angulates, in the X-Y plane relative to the Z-axis. In particular, the perimeter portion  82  of the third articulation component  18  is maintained between, and engages, the locking ring  20  and the support surface  150  at third and fourth articulation interfaces such that the third articulation component  18  only articulates in the X-Y plane, or changes in roll, relative to the head portion  140  and is substantially constrained from articulating in other directions relative to the head portion  140 . In some embodiments, limits (not shown) such as slots or guides are provided to limit the range of travel of the prosthesis in the X-Y plane, or to limit roll of the prosthesis  10 . Thus, according to some embodiments, substantially all of the X-Y plane articulation of the prosthesis  10  occurs at the third and fourth articulation interfaces between the third articulation component  18 , the head portion  140 , and the locking ring  20  such that the perimeter portion  82  of the third articulation component  18  is only exposed to wear in the rotational direction, rather than all directions as would otherwise be the case in a traditional ball-and-socket joint, also helping increase wear life of the prosthesis  10 . 
         [0058]    The three degrees of freedom (X-Z plane, Y-Z plane, and X-Y plane) help the prosthesis  10  act as a virtual ball-and-socket joint, with comparable mobility and a substantially medialized center of rotation, while maintaining the articulation components in a linked, substantially non-subluxating configuration. For example, the prosthesis  10  is optionally adapted to facilitate articulation between the humerus and the scapula through a natural range of motion, including flexion, extension, adduction, abduction, and rotation. 
         [0059]    While certain components have been referred to as forming a track and others a recess for receiving the track according to various embodiments, it should be understood that in other embodiments the track(s) and recess(es) are optionally reversed on the components. 
         [0060]      FIG. 10  is a perspective view and  FIG. 11  is a cut away view of another prosthesis  210 , according to some embodiments. As shown, the prosthesis  210  includes, a first articulation component  214  (also described as a glenosphere), a second articulation component  216  (also described as a liner or a disk), a third articulation component  218  (also described as a rotational plate), and a second bone anchor  224  (also described as a stem). Though not shown, the prosthesis  210  also includes a base plate (e.g., formed of titanium) a locking ring (e.g., formed of titanium) and a first bone anchor (e.g., formed of titanium) substantially similar to those of the prosthesis  10 , according to some embodiments. Moreover, the prosthesis  210  optionally includes any of the features described in association with the prosthesis  10  and vice versa, as desired. 
         [0061]    The prosthesis  210  is generally adapted as a virtual ball-and-socket joint, being able to articulate through a wide range of motion similar to that of a traditional ball-and-socket joint by supporting freedom of movement between in at least three coordinate directions, such as X-, Y-, and Z-axis angular articulation. For example, the prosthesis  210  optionally facilitates angular articulation relative to the X-axis (also described as pitch), or parallel to the Y-Z plane, relative to the Y-axis (also described as yaw), or parallel to the X-Z plane, and relative to the Z-axis (also described as roll), or parallel to the X-Y plane. In some embodiments, angular articulation relative to the X-axis corresponds to front-back motion or anteroposterior articulation, angular articulation relative to the Y-axis corresponds to up-down motion or inferosuperior IS articulation, and angular articulation relative to the Z-axis corresponds to medial rotational articulation. 
         [0062]      FIG. 12  shows the first articulation component  214  from a perspective view. As shown, the first articulation component  214  is a substantially hollow bowl, or is substantially cup-shaped, the first articulation component  214  having an inner surface  240  and an outer surface  242 , the inner surface  240  being described as a first articulation surface and the outer surface  242  being described as a second articulation surface of the prosthesis  210 . The outer surface  242  is substantially smooth overall and adapted for repeated articulation. As shown, the inner surface  240  has an upper portion  244  that is substantially cylindrical and a lower portion  246  that is substantially concave, where the upper portion  244  is adapted to form a complementary fit with an insert portion of a bone anchor, such as the first bone anchor  12 . 
         [0063]    In some embodiments, the upper portion  244  is secured to an insert portion using an interference or frictional fit, detents, fasteners, adhesives, combinations thereof, or other fastening means. As shown, the outer surface  242  forms a track  248  (also described as a projection, a tenon, or a rail), that extends through an arcuate path diametrically (e.g., along a centerline or diameter of the first articulation component  14 ) across the first articulation component  214  in the X-axis direction, although the track  248  is also optionally comprised of one or more projections that extend along one or more parallel chords of the component  216 . As shown, the track  248  has a dovetail shaped cross-section adapted to interlock with a complementary cross-section, although a variety of interlocking shapes (e.g., interlocking D-shapes, star-shapes, or others), are contemplated. 
         [0064]    The first articulation component  214  is optionally formed of cobalt-chrome alloy or other suitable materials having low friction and/or wear characteristics for the outer surface  242 , such as PTFE. Though some specific examples have been provided, a variety of materials are contemplated. 
         [0065]      FIG. 13  shows the second articulation component  216  from a perspective view, according to some embodiments. As shown, the second articulation component  216  is a substantially hollow bowl, or is substantially cup-shaped, the second articulation component  216  having an inner surface  260  (also described as a third articulation surface) and an outer surface  262  (also described as a fourth articulation surface). The inner surface  260  is substantially concave and forms a first recess  264  and the second outer surface  262  is substantially convex and defines a second recess  266 , each of the recesses also being described as guides or mortises. In some embodiments, the inner and outer surfaces  260 ,  262  are generally smooth, being adapted for repeated articulation. 
         [0066]    As shown, the first recess  264  extends through an arcuate path diametrically (e.g., along a centerline or diameter of the second articulation component  216 ) across the inner surface  260  in the X-axis direction. In other embodiments, the second articulation component  216  includes one or more parallel recesses extending along one or more parallel chords of the component  216  in the X-axis direction. As shown, the first recess  264  has a substantially dovetail shaped cross-section that is complementary to that of the track  248  of the first articulation component  214 , although a variety of interlocking cross-sections are contemplated. 
         [0067]    In some embodiments, the second recess  266  extends through an arcuate path diametrically (e.g., along a centerline or diameter of the second articulation component  216 ) across the outer surface  262  of the second articulation component  216  in the Y-axis direction. The second recess  266  extends in a substantially orthogonal direction to the first recess  264  of the second articulation component  216 . In other embodiments, the second articulation component  216  includes one or more parallel recesses extending along one or more parallel chords of the component  216  in the Y-axis direction. As shown, the second recess  266  has a substantially dovetail shaped cross-section that is complementary to a track feature of the third articulation component  218 , although a variety of shapes are also contemplated. 
         [0068]    The second articulation component  16  is optionally formed of UHMWPE or other suitable materials having low friction and/or wear characteristics for the inner and outer surfaces  260 ,  262 , such as PTFE. Though some specific examples have been provided, a variety of materials are contemplated. 
         [0069]      FIG. 14  shows the third articulation component  218  from a top view and a bottom view, respectively. As shown, the third articulation component  218  includes a central portion  280  that is shaped as a substantially hollow bowl, or is substantially cup-shaped and a perimeter portion  282  that is shaped as a substantially flat rim extending from the central portion  280 . The third articulation component  218  also has an inner surface  284  and an outer surface  286  ( FIG. 11 ). The inner and outer surfaces  284 ,  286  are substantially smooth overall and adapted for repeated articulation. As subsequently described, the inner surface  284  is adapted to engage and articulate with the outer surface  262  of the second articulation component  216  to define a second articulation interface. 
         [0070]    As shown in  FIG. 14 , at the central portion  280 , the inner surface  284  is substantially concave and defines a fifth articulation surface  284 A. At the perimeter portion  282 , the inner surface  284  is substantially flat, or planar, and defines a sixth articulation surface  284 B. As shown in  FIG. 11 , at the central portion  280 , the outer surface  286  defines a seventh articulation surface  286 A and at the perimeter portion  282 , the outer surface  286  is substantially flat, or planar, and defines a eighth articulation surface  286 B. 
         [0071]    As shown in  FIG. 14 , the inner surface  284  of the third articulation component  218  forms a track  290  (also described as a projection, a strip, a tenon, or a rail), that extends through an arcuate path diametrically (e.g., along a centerline or diameter of the third articulation component  218 ) across the third articulation component  218  in the Y-axis direction, although the track  290  is also optionally comprised of one or more projections that extend along one or more parallel chords of the component  218 . In some embodiments, the track  290  has a substantially dovetail shaped cross-section, although a variety of shapes are contemplated. 
         [0072]    The third articulation component  218  is optionally formed of cobalt-chrome alloy or other suitable materials having low friction and/or wear characteristics for the inner and outer surfaces  284 ,  286 , such as PTFE. Though some specific examples have been provided, a variety of materials are contemplated. 
         [0073]    As shown in  FIG. 11 , the second bone anchor  224  includes a head portion  340  and a stem portion  342 . The head portion  340  and/or the stem portion  342  is optionally adapted to assist with securing the second bone anchor  224  directly to a humerus. For example, the stem portion  342  of the second bone anchor  224  is optionally substantially elongate and adapted to be secured within the proximal medullary canal of the humerus, though the second bone anchor  224  is optionally adapted to be secured to other boney structures, such as the femur in cases where the prosthesis  210  is adapted for hip replacement, for example. In some embodiments, the second bone anchor  224  is formed of titanium, although a variety of materials are contemplated. 
         [0074]    The head portion  340  is substantially conical in shape and forms an outer flange  348 , a support surface  350  (also described as an eleventh articulation surface), and a recessed pocket  352 . In some embodiments, the head portion  340  is adapted to serve as a fourth articulation component and is rotatable with respect to the third articulation component as subsequently described. 
         [0075]    The outer flange  348  is substantially vertically oriented relative to the support surface  350 . As shown, the outer flange  348  includes a top wall  348 A adapted to support a cap portion of a locking ring, such as the locking ring  20 , and an outer wall  348 B adapted to be secured to a collar portion of a locking ring, such as the locking ring  20 . The outer flange  348  also defines an inner wall  348 C which helps retain the perimeter portion  282  of the third articulation component  218  in the head portion  340  and against which an edge of the perimeter portion  282  optionally slides. The support surface  350  is adapted to slidingly support and engage the eighth articulation surface  286 B on the perimeter portion  282  of the third articulation component  218 . The recessed pocket  352  is adapted to receive portions of the first, second, and third articulation components  212 ,  214 ,  216 , such that the components are free to angularly articulate as desired. 
         [0076]    Assembly of the prosthesis  210  from an unassembled state to the assembled state shown in  FIG. 11  includes mating the track  248  of the first articulation component  214  with the first recess  264  of the second articulation component  216  such that the first articulation component  214  is able to articulate relative to the Y-axis while being substantially constrained from angular articulation relative to the X- or Z-axes. In some embodiments, upon mating the track  248  and recess  264 , the inner surface  260  of the second articulation component  216  slides against the outer surface  242  of the first articulation component  214  to define a first articulation interface between the surfaces  242 ,  260 , where the second articulation component  216  provides a bearing surface or acts as a bushing for repeated articulation with the first articulation component  214 . The interlocking shapes of the track  248  and the recess  264  links the first and second components  214 ,  216  such that subluxation, or separation between the first and second articulation components  214 ,  216  is substantially prevented, or otherwise limited. Thus, the track  248  and the first recess  264  optionally provide means for limiting angular articulation of the first articulation component  214 , which is in sliding contact with the second articulation component  216 , to changes in pitch. 
         [0077]    The track  290  of the third articulation component  18  is mated with the second recess  266  of the second articulation component  216  such that the second and third articulation components  216 ,  218  are able to articulate relative to the X-axis while being substantially constrained from articulating in rotational or other directions relative to the Y- or Z-axes. The interlocking shapes of the track  290  and the recess  266  links the second and third components  216 ,  218  such that subluxation, or separation between the second and third articulation components  216 ,  218  is substantially prevented, or limited. In some embodiments, upon mating the track  290  and second recess  266 , the outer surface  262  of the second articulation component  216  engages and slides against the inner surface  284  of the third articulation component  218  to define a second articulation interface between the surfaces  262 ,  284  where the second articulation component  216  provides a bearing surface or acts as a bushing for repeated articulation with the third articulation component  218 . Thus, the track  290  and second recess  266  optionally provide means for limiting angular articulation of the third articulation component  218 , which is in sliding contact with the second articulation component  216 , to changes in yaw. 
         [0078]    In some embodiments, the three articulation components  214 ,  216 ,  218  are secured between a first bone anchor, such as the bone anchor  12 , and the second bone anchor  224  such that the bone anchors are able to change in roll, or angulate relative to the Z-axis, or in the X-Y plane, as well as change in relative pitch and yaw. For example, in some embodiments, implantation of the prosthesis  210  includes securing a first bone anchor (e.g., the first bone anchor  12 ) to a first bone (not shown) such as a scapula. The first bone anchor is optionally secured directly to the first bone (e.g., using bone screws) or using a secondary anchoring device, such as those previously described. The first bone anchor is secured to the first articulation component  214  by positioning the insert portion of the first bone anchor into the upper portion  244  of the first articulation component  214 . In some embodiments, the insert portion and the upper portion  244  are secured together with the help of adhesives and/or mechanical fasteners (not shown). 
         [0079]    In some embodiments, the second bone anchor  224  is secured to a second bone (not shown), such as a humerus, using known techniques. For example, in some embodiments, the stem portion  342  of the second bone anchor  224  is secured in a proximal medullary cavity of a humerus. In other embodiments, the bone anchors are secured between another set of bones, such as between a femur and a pelvis to serve as an artificial hip. 
         [0080]    As shown in  FIG. 11 , the head portion  340  of the second bone anchor  224  is secured to the third articulation component  218  by receiving the perimeter portion  282  of the third articulation component  218  against the support surface  350  of the second bone anchor  224 . The locking ring (not shown) is secured over the perimeter portion  282  and onto the head portion  340  of the second bone anchor  224  such that the third articulation component  218  is free to rotate with respect to the second bone anchor  224 , the perimeter portion  282  and the support surface  350  engaging to form a third articulation interface and the perimeter portion  282  and the locking ring engaging to form a fourth articulation interface of the prosthesis  310 . Thus, the third and fourth articulation interfaces optionally provide means for allowing changes in roll between the first bone anchor and the second bone anchor  224 . 
         [0081]    Upon securing the articulation components  214 ,  216 ,  218  to the bone anchors, the entire prosthesis  210  is linked, forming a fixed assembly that limits subluxation between the bone anchors and is able to freely articulate. For example, in some embodiments, the prosthesis  210  is adapted such that substantially no subluxation is allowed between the bone anchors, and thus between the bones to which they are secured (e.g., the humerus and scapula). 
         [0082]    Articulation of the prosthesis  210  includes articulating the first and second articulation components  214 ,  216  relative to one another such that the first articulation component  214  angulates and shifts laterally relative to the second articulation component  216  along a first arcuate path extending in the X-Z plane. In particular, the first component  214  is guided in the X-Z plane as the track  248  rides within the first recess  264  of the second articulation component  216 . The track  248  only permits the first articulation component  214  to articulate in the X-Z plane relative to the second articulation component  216 , or only to change in pitch, and substantially constrains articulation between the first and second articulation components  214 ,  216  in other directions. 
         [0083]    In some embodiments, substantially all of the X-Z plane articulation of the prosthesis  210  occurs at the first articulation interface between the first and second articulation components  214 ,  216 , including the track  248  and the first recess  264 , such that the inner surface  260  of the second articulation component  216  is only exposed to wear in one direction, the X-axis direction, rather than all directions as would otherwise be the case in a traditional ball-and-socket joint, helping increase wear life of the prosthesis  210 . 
         [0084]    In some embodiments, the third articulation component  218  is articulated relative to the second articulation component  216  such that the third articulation component  218  angulates and shifts laterally relative to the second articulation component  216  along a second arcuate path extending parallel to the Y-Z plane. In particular, the third articulation component  218  is guided in the Y-Z plane as the track  290  rides within the second recess  266  of the second articulation component  216  such that the third articulation component  218  only articulates in the Y-Z plane relative to the second articulation component  16 , or only changes in yaw, and is substantially constrained from articulating in other directions relative to the second articulation component  216 . 
         [0085]    According to some embodiments, substantially all of the Y-Z plane articulation of the prosthesis  210  occurs at the first articulation interface between the third and second articulation components  218 ,  216 , including the track  290  and second recess  266 , such that the outer surface  262  of the second articulation component  216  is only exposed to wear in one direction, the Y-axis direction, rather than all directions as would otherwise be the case in a traditional ball-and-socket joint, also helping increase wear life of the prosthesis  210 . 
         [0086]    In some embodiments, the third articulation component  218  is articulated relative to the head portion  340  of the second bone anchor  224 , also described as a fourth articulation component, such that the third articulation component rotates, or angulates, in the X-Y plane relative to the Z-axis. In particular, the perimeter portion  282  of the third articulation component  218  is maintained between, and engages, the locking ring (not shown) and the support surface  350  at third and fourth articulation interfaces such that the third articulation component  218  only articulates in the X-Y plane, or changes in roll, relative to the head portion  340  and is substantially constrained from articulating in other directions relative to the head portion  340 . 
         [0087]    According to some embodiments, substantially all of the X-Y plane articulation of the prosthesis  210  occurs at the third and fourth articulation interfaces between the third articulation component  218 , the head portion  340 , and the locking ring, such that the perimeter portion  282  of the third articulation component  218  is only exposed to wear in the rotational direction, rather than all directions as would otherwise be the case in a traditional ball-and-socket joint, also helping increase wear life of the prosthesis  210 . 
         [0088]    The three degrees of freedom (X-Z plane, Y-Z plane, and X-Y plane) help the prosthesis  210  act as a virtual ball-and-socket joint, with comparable mobility and a substantially medialized center of rotation, while maintaining the articulation components in a linked, substantially non-subluxating configuration. For example, the prosthesis  210  is optionally adapted to facilitate articulation between the humerus and the scapula through a natural range of motion, including flexion, extension, adduction, abduction, and rotation. 
         [0089]    While certain components have been referred to as forming a track and others a recess for receiving the track according to various embodiments, it should be understood that in other embodiments the track(s) and recess(es) are optionally reversed on the components. For example, the track  48  is optionally formed on the second articulation component  16  with the corresponding recess  64  on the first articulation component  14 , and so forth. 
         [0090]    Various embodiments and features thereof have been described with reference to relational terms. Unless context specifically dictates otherwise, the terms “first,” “second,” “third,” etc. used with reference to various features are not intended to require a particular order, but are used in a general sense to designate the different features for description purposes. Similarly, the terms “upper,” “lower,” “front,” “back,” “vertical,” “horizontal,” etc. are not intended to be limiting in nature, but are instead used to provide relative orientation between features being described. 
         [0091]    Various modifications, permutations, and additions can be made to the exemplary embodiments and aspects of the embodiments discussed without departing from the scope of the present invention. For example, while the embodiments described above refer to particular features, the scope of this invention also includes embodiments having different combinations of features and embodiments that do not include all of the features. Accordingly, the scope of the present invention is intended to embrace all such alternatives, modifications, permutations, and variations as fall within the scope of the claims, together with all equivalents thereof.