Patent Publication Number: US-9901453-B2

Title: Elbow prosthesis

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 13/465,690, filed on May 7, 2012, which is a continuation-in-part of U.S. patent application Ser. No. 12/562,616, filed on Sep. 18, 2009, which is a continuation-in-part of U.S. patent application Ser. No. 12/391,904, filed on Feb. 24, 2009, which is a continuation-in-part of U.S. patent application Ser. No. 11/384,943, filed on Mar. 17, 2006 (now U.S. Pat. No. 8,585,768), which is a continuation-in-part of U.S. patent application Ser. No. 10/333,140, filed on Jan. 15, 2003 (now U.S. Pat. No. 7,247,170), which is a National Stage of International Application No. PCT/US01/22338 (published as WO 02/05728), filed Jul. 17, 2001, which claims priority to U.S. Provisional Application No. 60/219,103, filed Jul. 18, 2000. U.S. patent application Ser. No. 13/465,690 also claims priority to U.S. Provisional Application No. 61/505,360 filed, on Jul. 7, 2011. Each of these applications is incorporated herein by reference in its entirety. 
     U.S. patent application Ser. No. 11/780,365 filed on Sep. 19, 2007 which is now U.S. Pat. No. 7,625,406 and U.S. patent application Ser. No. 11/780,370 filed on Sep. 19, 2007 which is now U.S. Pat. No. 7,604,666 disclose related subject matter. These applications are also incorporated by reference. 
    
    
     FIELD 
     The present teachings relate generally to prosthetic devices used in arthroplasty and more particularly to a modular elbow prosthesis. 
     BACKGROUND 
     The present teachings relate generally to prosthetic devices used in arthroplasty and more particularly to a modular elbow prosthesis. 
     Linked or constrained elbow prostheses are known which comprise simple hinge arrangements, one component of which is attached to the end of the humerus and the other component of which is attached to the end of the ulna. The humeral component includes a shaft, which is cemented into a prepared cavity in the end of the humerus, and the ulnar component includes a shaft, that is cemented to the end of the ulna. The components of the prosthesis are connected together by means of a hinge pin so that the prosthesis allows a single degree of freedom of movement of the ulna relative to the humerus. 
     One example of a linked elbow prostheses is disclosed in U.S. Pat. No. 6,027,534 to Wack et al. In several respects, the linked embodiment of the &#39;534 patent is typical of the designs for linked elbow prostheses in that it includes a humeral stem that terminates at a yoke at its distal end, a bearing component, a retaining pin and an ulna stem. The bearing component includes an oversized hole that is aligned with the longitudinal axis of the bearing and adapted to accept the retaining pin in a slip-fit condition. The distal end of the bearing component is coupled to the ulna stem. Despite the relatively widespread use of designs of this type, several drawbacks have been noted. 
     One significant drawback concerns the assembly of the elbow prosthesis after the surgeon has cemented the humeral and ulna stems to their respective bones. In using such conventionally configured linked elbow prosthesis devices, it is frequently necessary for the surgeon to drill a fairly large hole through the humerus so that the retaining pin may be inserted to the yoke of the humeral stem and the humeral bearing component. As a high degree of accuracy is typically required to ensure proper alignment between the hole in the humerus and the hole in the yoke of the humeral stem, a significant cost can be associated with this step in the installation of an elbow prosthesis due to the cost of the tooling used and the amount of time required to complete this step. The other method for attaching the prosthetic device includes inserting the device in its linked condition or placing the remaining piece into the yoke prior to fully seating the humeral component into the bone. This later method is typically somewhat difficult, given the limited amount of joint space that is available and the time constraints associated with the use of a PMMA bone cement. 
     Unlinked, or unconstrained, elbow prostheses are known which are similar to linked elbow prostheses but do not have a specific component which mechanically couples the humeral and ulnar stems together. Rather, the prosthetic device is held together by the patient&#39;s natural soft tissues. One example of an unlinked elbow prostheses is also disclosed in U.S. Pat. No. 6,027,534 to Wack et al. In several respects, the unlinked embodiment of the &#39;534 patent is similar to the linked embodiment discussed above in that it includes a humeral stem that terminates at a yoke at its distal end, a humeral bearing component, a retaining pin, an ulnar bearing component and a ulnar stem. The outer surface of the humeral bearing is contoured to match the contour of the ulnar bearing component. Despite the relatively widespread use of designs of this type, several drawbacks have been noted. 
     For instance, a retaining pin that is transverse to the longitudinal axis of the patient is employed, thereby making its removal difficult if a bearing needs to be replaced. 
     SUMMARY 
     An elbow prosthesis can comprise a stem structure and an articulating component. The stem structure can be operable to be positioned in a bone of a joint. The stem structure can include a stem portion and a C-shaped body portion. The stem portion can be operable to be positioned in the bone. The C-shaped body portion can have a first articulating surface that is bound by a medial wall and a lateral wall that are separated by a first distance. The articulating component can have a second articulating surface positioned between a medial side surface and a lateral side surface that are separated by a second distance. The first distance can be greater than the second distance. The second articulating surface of the articulating component can be configured to slidably communicate in a medial/lateral direction along the first articulating surface of the C-shaped body portion. The articulating component can be limited from further medial movement by engagement of the medial side surface with the medial wall and limited from further lateral movement by engagement of the lateral side surface with the lateral wall. 
     According to one aspect of the present disclosure, an elbow prosthesis is provided. The elbow prosthesis includes a stem structure and an articulating component. The stem structure is operable to be positioned in a bone of a joint and includes a stem portion and a C-shaped body portion. The stem portion is operable to be positioned in the bone. The C-shaped body portion includes a first articulating surface bound by a medial wall and a lateral wall. The medial and lateral walls are separated by a first distance. The articulating component includes a second articulating surface positioned between a medial side surface and a lateral side surface. The medial and lateral side surfaces are separated by a second distance that is less than the first distance. The second articulating surface is configured to slidably communicate in a medial/lateral direction along the first articulating surface of the C-shaped body portion. The articulating component is limited from further medial movement by engagement of the medial side surface with the medial wall and limited from further lateral movement by engagement of the lateral side surface with the lateral wall. 
     According to another aspect of the present disclosure, an elbow prosthesis is provided. The elbow prosthesis includes a stem structure, an articulating component, and an unlinked humeral component. The stem structure includes a stem portion and a C-shaped body portion. The stem portion is operable to be positioned in a bone. The C-shaped body portion includes a first articulating surface. The articulating component includes a humeral opposing articulating surface and a second articulating surface opposite the humeral opposing articulating surface. The second articulating surface engages the first articulating surface. The humeral opposing articulating surface includes first and second pairs of diagonally opposed articulating surfaces. The unlinked humeral component includes a humeral articulating surface configured to engage the first pair of diagonally opposed articulating surfaces in a first position, and configured to engage the second pair of diagonally opposed articulating surfaces in a second position. 
     According to yet another aspect of the present disclosure, an elbow prosthesis is provided. The elbow prosthesis includes a stem structure, an articulating component, and an unlinked humeral component. The stem structure includes a stem portion and a C-shaped body portion. The stem portion is operable to be positioned in a bone. The C-shaped body portion includes a first articulating surface bound by a medial wall and a lateral wall. The articulating component includes a humeral opposing articulating surface and a second articulating surface opposite the humeral opposing articulating surface. The second articulating surface is configured to slidably communicate along the first articulating surface between the medial and lateral walls. The unlinked humeral component is configured to engage the humeral opposing articulating surface, and is operable to form a first angle relative to the second articulating surface in a first position, and operable to form a second angle relative to the second articulating surface in a second position. 
     Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       DRAWINGS 
       Additional advantages and features of the present teachings will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a perspective view of a modular unlinked ulnar stem assembly constructed in accordance to one example of the present teachings; 
         FIG. 2  is an assembled view of the modular unlinked ulnar stem assembly of  FIG. 1 ; 
         FIG. 3  is an exploded perspective view of another modular unlinked ulnar stem assembly constructed in accordance to the present teachings; 
         FIG. 4  is an assembled view of the modular unlinked ulnar stem assembly of  FIG. 3 ; 
         FIG. 5  is an exploded perspective view of another modular unlinked ulnar stem assembly constructed in accordance to the present teachings; 
         FIG. 6  is an assembled view of the modular unlinked ulnar stem assembly of  FIG. 5 ; 
         FIG. 7  is an exploded medial side view of a stemless modular unlinked ulnar prosthesis according to the present teachings; 
         FIG. 8  is a medial side view of the stemless modular unlinked ulnar prosthesis shown in  FIG. 7  and implanted into an ulna; 
         FIG. 9  is an exploded medial side view a modular ulnar stem assembly that incorporates a universal stem and is shown with a linked bearing component; 
         FIG. 10  is a medial side assembled view of the modular unlinked ulnar stem assembly shown with an unlinked bearing component in an assembled position; 
         FIG. 11  is a medial side assembled view of the modular ulnar stem assembly of  FIG. 9  shown with the linked bearing component secured to the universal stem; 
         FIG. 12  is an exploded medial side view of a modular ulnar prosthesis that incorporates a mobile bearing; 
         FIG. 13  is a medial side view of the modular ulnar prosthesis shown with the mobile bearing in an engaged position with the ulnar stem component; 
         FIG. 14  is a perspective view of the mobile bearing of  FIG. 12 ; 
         FIG. 15  is an anterior view of the mobile bearing of  FIG. 14 ; 
         FIG. 16  is a cross-sectional view of the modular ulnar stem assembly taken along line  16  of  FIG. 13  and shown with the mobile bearing in an intermediate position between a medial and lateral wall of the ulnar stem; 
         FIG. 17  is a cross-sectional view of the modular ulnar prosthesis of  FIG. 13  and shown with the mobile bearing slidably translated into engagement with the medial wall of the ulnar prosthesis; 
         FIG. 18  is a cross-sectional view of the modular ulnar prosthesis of  FIG. 13  and shown with the mobile bearing slidably translated into engagement with the lateral wall of the ulnar prosthesis; 
         FIG. 19  is a cross-sectional view of the modular ulnar prosthesis of  FIG. 13  and shown with an exemplary humeral component and radial component; 
         FIG. 20  is a cross-sectional view of the assembly of  FIG. 19  and shown with the arm rotated into a varus position; and 
         FIG. 21  is a cross-sectional view of the assembly of  FIG. 19  and shown with the arm rotated into a valgus position. 
     
    
    
     DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS 
     The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features. 
     With reference to  FIGS. 1, 2 and 3  of the drawings, an unlinked prosthetic joint device constructed in accordance with the teachings of a first aspect is generally indicated by reference number  2500 . The following description is directed to various linked and unlinked prosthetic elbow joints. Additional features suitable for implementation with the following examples may be found in commonly owned and copending U.S. Ser. No. 12/780,424, filed May 14, 2010, which is expressly incorporated herein by reference. Although the particular prostheses illustrated and discussed relate to prosthetics for use in reconstructing an elbow, it will be understood that the teachings have applicability to other types of linked and unlinked prosthetic devices. As such, the scope of the present teachings will not be limited to applications involving elbow prosthesis but will extend to other prosthetic applications. 
     With reference now to  FIGS. 1 and 2 , a modular unlinked ulnar stem assembly  2500  constructed in accordance to one example of the present teachings will be described. In general, the modular unlinked ulnar stem assembly  2500  can include a stem structure  2502 , an articulating component  2504  and a fastener  2506 . As will become appreciated from the following discussion, the articulating component  2504  can be modular, such that a series or set of articulating components can be provided that are each selectively attachable to the stem structure  2502  according to a specific patient&#39;s needs. Such examples can include articulating components having different shapes, surfaces, sizes, constraints, etc. The modular unlinked ulnar stem assembly  2500  can be favorable in some circumstances as it has the potential to remove less native bone than other elbow prostheses, such as a semi-constrained total elbow, for example. Furthermore, the modular unlinked ulnar stem assembly  2500  can be favorable in circumstances where one side of the elbow joint includes satisfactory bone and/or cartilage and it is only desirable to replace the other side of the elbow joint. 
     With specific reference now to  FIG. 1 , exemplary features of the stem structure  2502  will be described. The stem structure  2502  can generally include a stem portion  2510  and a C-shaped body portion  2512 . In this regard, the body portion  2512  can generally include an ulnar ring  2514  that is in the form of a partial or semi-circular cylinder. The body portion  2512  can generally extend between a first end  2516  and a second end  2518 . The body portion  2512  can further comprise a first retaining mechanism  2520  formed thereon. The first retaining mechanism  2520  can include an extension portion  2522  and a first anti-rotation portion  2524 . The extension portion  2522  can generally be in the form of a rail or keel that extends between the first end  2516  and the second end  2518  of the body portion  2512 . The first anti-rotation portion  2524  can generally include a series of first undulations  2526  having a series of peaks and valleys. In the example shown, the first undulations  2526  can be generally in the form of a wave-like pattern formed around the ulnar ring  2514 . The specific geometry and wave-like pattern is merely exemplary and can consist of other formations that are configured to cooperatively engage complementary structure on the articulating component  2504  as will be described in detail herein. 
     The first end  2516  of the body portion  2512  can include a threaded bore  2530  formed therein. The second end  2518  of the body portion  2512  can include a catch  2532 . As will be become appreciated from the following discussion, the first retaining mechanism  2520  that includes the first anti-rotation portion  2524  and the extension portion  2522  can cooperatively mate with complementary features formed on the articulating component  2504  and therefore inhibit rotation of the articulating component  2504  around the ulnar ring  2514  as well as inhibit medial/lateral translation of the articulating component  2504  on the ulnar ring  2514 . The catch  2532  can also assist in confining the articulating component  2504  to a fixed position relative to the ulnar ring  2514 . The configuration of the first retaining mechanism  2520  can be particularly advantageous during assembly of the articulating component  2504  onto the stem structure  2502  to maintain the articulating component  2504  in a static position relative to the ulnar ring  2514  prior to securing the articulating component  2504  further to the stem structure  2502  with the fastener  2506 . 
     The articulating component  2504  will now be described in greater detail. In general, the articulating component  2504  can include a body  2534  that generally takes the shape of a partial or semi-circular cylinder complementary to the shape of the ulnar ring  2514 . The body  2534  can extend between a first end  2536  and a second end  2538 . The first end  2536  of the body  2534  can define a passage  2540  therein for receiving the fastener  2506  in the assembled position. The second end  2538  of the body  2534  can include a relief  2542 . The articulating component  2504  can further comprise a second retaining mechanism  2544  that has a groove  2546  configured to receive the extension portion  2522  on the ulnar ring  2514  and a second anti-rotation portion  2548  for mating with the first anti-rotation portion  2524  on the ulnar ring  2514 . 
     The second anti-rotation portion  2548  can include a series of second undulations  2550  having a series of peaks and valleys that may be in the form of a wave-like pattern or other geometry that can suitably mate or nest with the first anti-rotation portion  2524  on the ulnar ring  2514 . In one example, the articulating component  2504  can be formed of UHMWPE or PEEK. In other examples, however, the articulating component  2504  may be a combination of a first polyethylene portion on the articulating side and a molded metallic substrate that forms the second anti-rotation portion  2548 . The molded configuration can be similar to the combination polyethylene and metallic component described in pending U.S. Ser. No. 12/780,424, filed May 14, 2010 identified above. 
     With specific reference to  FIG. 2 , assembly of the articulating component  2504  to the stem structure  2502  according to one example of the present teachings will be described. At the outset, a surgeon can advance the second retaining mechanism  2544  of the articulating component  2504  onto the first retaining mechanism  2520  of the stem structure  2502 . In this regard, the second anti-rotation portion  2548  can cooperatively mate with the first anti-rotation portion  2524  as the extension portion  2522  of the first retaining mechanism  2520  nests within the groove  2546  of the second retaining mechanism  2544 . At this time, the catch  2534  on the ulnar ring  2514  can locate partially into the relief  2542  of the articulating component  2504 . Those skilled in the art will readily appreciate that the cooperatively mating structures formed on the stem structure  2502  and the articulating component  2504  may be reversed. For example, the extension portion  2522  may alternatively be provided on the articulating component  2504  while the groove  2546  may alternatively be located on the body portion  2512  of the stem structure  2502 . Likewise, the catch  2532  and the relief  2542  may be provided on opposite components. 
     Nonetheless, once the first and second retaining mechanisms  2520  and  2544  have been positioned against each other ( FIG. 2 ), translation of the articulating component  2504  in the medial/lateral direction can be inhibited by the interaction of the extension portion  2522  and the groove  2546 . Likewise, rotation of the articulating component  2504  around the ulnar ring  2514  can be inhibited by the interaction of the first and second anti-rotation portions  2524  and  2548 . The catch  2532  can assist in further inhibiting rotational movement of the articulating component  2504 . Moreover, as the catch  2532  is angled toward the second end  2538  of the articulating component  2504 , the articulating component  2504  can be inhibited from lift-off from the stem structure  2502 . Next, the fastener  2506  can be advanced through the passage  2540  in the articulating component  2504  and threadably advanced into the threaded bore  2530  of the stem portion  2510  to couple the articulating component  2504  to the stem portion  2510 . 
     Turning now to  FIGS. 3 and 4 , another modular unlinked ulnar stem assembly  2600  according to the present teachings will be described. In general, the modular unlinked ulnar stem assembly  2600  can include a stem structure  2602 , an articulating component  2604  and a fastener  2606 . The stem structure  2602  can generally comprise a stem portion  2610  and a generally C-shaped body portion  2612 . The stem structure  2602  can generally include an ulnar ring  2614  that is in the form of a partial or semi-circular cylinder. The ulnar ring  2614  can generally extend between a first end  2616  and a second end  2618 . The body portion  2612  can further comprise a first retaining mechanism  2620 . The first retaining mechanism  2620  can include a groove  2622  and a first anti-rotation portion  2624 . The first anti-rotation portion  2624  can be in the form of first undulations  2626 . A threaded bore  2630  can be formed on the stem structure  2602  at the first end  2616  of the body portion  2612 . 
     The articulating component  2604  will now be further described according to one example. The articulating component  2604  can be modular, such that a series of articulating components can be provided that are selectively attachable to the stem structure  2602  according to a specific patient&#39;s needs. In general, the articulating component  2604  can include a body  2634  that generally takes the shape of a partial or semi-circular cylinder complementary to the shape of the ulnar ring  2614 . The body  2634  can generally extend between a first end  2636  and a second end  2638 . The body  2634  can define a passage  2640  configured to receive the fastener  2606  at the first end  2636 . The articulating component  2604  can further comprise a second retaining mechanism  2644  that can comprise an extension portion  2646  in the form of a rail or keel and a second anti-rotation portion  2648 . The second anti-rotation portion  2648  can be in the form of a series of second undulations  2650  that are provided with a geometry suitable to cooperatively mate or nest with the first undulations  2626  of the first anti-rotation portion  2624 . Again, it will be appreciated that the configuration and location of some or all of the various structures of the first and second retaining mechanisms  2620  and  2644  can be swapped between components. In one example, the articulating component  2604  can be formed of UHMWPE or PEEK. In other examples, however, the articulating component  2604  may be a combination of a first polyethylene portion on the articulating side and a molded metallic substrate that forms the second anti-rotation portion  2648 . The molded configuration can be similar to the combination polyethylene and metallic component described in pending U.S. Ser. No. 12/780,424, filed May 14, 2010 identified above. 
     Assembly of the articulating component  2604  to the stem structure  2602  will now be described according to one example. Initially, a surgeon can locate the second retaining mechanism  2644  onto the first retaining mechanism  2620 . In this regard, the extension portion  2646  can be nestingly received by the groove  2622  on the body portion  2612  of the stem structure  2602 . Concurrently, the second undulations  2650  on the second anti-rotation portion  2648  can be matingly engaged with the first undulations  2626  of the first anti-rotation portion  2624 . Again, the interaction of the extension portion  2646  and the groove  2622  can inhibit medial/lateral translation of the articulating component  2604  relative to the stem structure  2602 . Similarly, the interaction of the first and second anti-rotation portions  2624  and  2648  can inhibit rotation of the articulating component  2604  around the body  2612  of the stem structure  2602 . Next, the fastener  2606  can be advanced through the passage  2640  and threadably mated into the threaded bore  2630  of the stem structure  2602  to couple the articulating component  2604  to the stem structure  2602 . 
     Turning now to  FIGS. 5 and 6 , another modular unlinked ulnar stem assembly  2700  according to the present teachings will now be described. In general, the modular unlinked ulnar stem assembly  2700  can include a stem structure  2702 , an articulating component  2704  and a fastener  2706 . Again, as with the other examples discussed herein, the articulating component  2704  can be modular, such that a series of articulating components having various geometries can be provided that are selectively attachable to the stem structure  2702  according to a specific patient&#39;s needs. The stem structure  2702  can include a stem portion  2710  and a body portion  2712 . The body portion  2712  can be in the form of an ulnar ring  2714  that extends between a first end  2716  and a second end  2718 . The body portion  2712  can comprise a first retaining mechanism  2720  that can include a groove  2722  and a first anti-rotation portion  2724 . The first anti-rotation portion  2724  can be in the form of a series of depressions  2726 . In one example, the depressions  2726  can be perpendicular to the groove  2722 . The body portion  2712  can include a threaded bore  2730  formed at the first end  2516 . 
     The articulating component  2704  can include a body  2734  that generally takes the shape of a partial or semi-circular cylinder complementary to the shape of the ulnar ring  2714 . The body  2734  can extend between a first end  2736  and a second end  2738 . The articulating component  2704  can further define a passage  2740  formed therethrough at the first end  2736  for receiving the fastener  2706 . The articulating component  2704  can further comprise a second retaining mechanism  2744  that can include an extension portion  2746  and a second anti-rotation portion  2748 . The second anti-rotation portion  2748  can be in the form of a series of fingers  2750  that are shaped to be cooperatively received into the depressions  2726  of the first retaining mechanism  2720 . In one example, the extension portion  2746  can be perpendicular to the fingers  2750 . Again, it is appreciated that some or all of the features of the first and second retaining mechanisms  2720  and  2744  can be swapped between the stem structure  2702  and the articulating component  2704 . Moreover, while the number of depressions  2726  shown around the ulnar ring  2714  is four, fewer or additional depressions  2726  may be used for cooperatively mating with a like amount of fingers  2750 . Additionally, some or all features of the second retaining mechanism  2744  on the articulating component  2704  can be formed of a metallic material while the remainder of the articulating component  2704  can be formed of UHMWPE. 
     Assembly of the articulating component  2704  with the stem portion  2710  will now be described. Initially, a surgeon can locate the second retaining mechanism  2744  of the articulating component  2704  onto the first retaining mechanism  2720  of the stem structure  2702 . In this regard, the extension portion  2746  and the fingers  2750  can cooperatively nest into the groove  2722  and depressions  2726 , respectively. As with the other examples described above, the interaction of the extension portion  2746  and the groove  2722  can inhibit medial/lateral translation of the articulating component  2704  relative to the stem structure  2702 . Similarly, interaction of the fingers  2750  and the depressions  2726  can inhibit rotation of the articulating component  2704  around the ulnar ring  2714 . Next, the fastener  2706  can be located through the passage  2740  and threadably advanced into the threaded bore  2730  of the stem portion  2710 . 
     With reference now to  FIGS. 7 and 8 , a modular unlinked ulnar prosthesis assembly  2800  constructed in accordance to one example of the present teachings will be described. In general, the modular unlinked ulnar prosthesis assembly  2800  can include a tray structure  2802 , an articulating component  2804 , a fastener  2806  and a securing member  2808 . As will become appreciated from the following discussion, the articulating component  2804  can be modular, such that a series of articulating components can be provided that are each selectively attachable to the tray structure  2802  according to a specific patient&#39;s needs. The modular unlinked ulnar prosthesis assembly  2800  can be favorable in some circumstances as it has the potential to remove less native bone than other elbow prostheses, such as a semi-constrained total elbow, for example. Moreover, the modular unlinked ulnar prosthesis assembly  2800  has the potential to remove less native bone than other unlinked ulnar prosthesis assemblies that require the use of a stem, such as disclosed herein. In this regard, the modular unlinked ulnar prosthesis assembly  2800  can be referred to as a “stemless” ulnar prosthesis as the only structure that ultimately is required to penetrate into the native bone is a boss portion  2810  and related securing member  2808  as will be discussed in greater detail. As such, a conventional stem that may be configured to pass into an IM canal of the ulna is not required for the modular unlinked ulnar prosthesis assembly  2800 . As with some of the modular unlinked ulnar prostheses discussed above, the modular unlinked ulnar prosthesis assembly  2800  can be favorable in circumstances where one side of the elbow joint includes satisfactory bone and/or cartilage and it is only desirable to replace the other side of the elbow joint. 
     With specific reference now to  FIG. 7 , exemplary features of the tray structure  2802  will be described. The tray structure  2802  can generally include a boss portion  2810  and a C-shaped body portion  2812 . In this regard, the body portion  2812  can generally include an ulnar ring  2814  that is in the form of a partial or semi-circular cylinder. The body portion  2812  can generally extend between a first end  2816  and a second end  2818 . The body portion  2812  can further comprise a first retaining mechanism  2820  formed thereon. The first retaining mechanism  2820  can include a receiving portion or arcuate groove  2822  and a first anti-rotation portion  2824 . The groove  2822  can generally be in the form of a centrally positioned channel that extends between the first end  2816  and the second end  2818  of the body portion  2812 . The first anti-rotation portion  2824  can generally include a series of first undulations  2826 . In the example shown, the first undulations  2826  can be generally in the form of a wave-like pattern formed around the ulnar ring  2814  and project perpendicular to the groove  2822 . The specific geometry and wave-like pattern is merely exemplary and can consist of other formations that are configured to cooperatively engage complementary structure on the articulating component  2804  as will be described in detail herein. 
     The first end  2816  of the body portion  2812  can include a threaded bore  2830  therein. As will become appreciated from the following discussion, the first retaining mechanism  2820  that includes the first anti-rotation portion  2824  and the groove  2822  can cooperatively mate with complementary features formed on the articulating component  2804  and therefore inhibit rotation of the articulating component  2804  around the ulnar ring  2814  as well as inhibit medial/lateral translation of the articulating component  2804  on the ulnar ring  2814 . The configuration of the first retaining mechanism  2820  can be particularly advantageous during assembly of the articulating component  2804  onto the tray structure  2802  to maintain the articulating component  2804  in a static position relative to the ulnar ring  2814  prior to securing the articulating component  2804  further to the tray structure  2802  with the fastener  2806 . 
     The articulating component  2804  will now be described in greater detail. In general, the articulating component  2804  can include a body  2834  that generally takes the shape of a partial or semi-circular cylinder complementary to the shape of the ulnar ring  2814 . The body  2834  can extend between a first end  2836  and a second end  2838 . The first end  2836  of the body  2834  can define a passage  2840  therein for receiving the fastener  2806  in the assembled position ( FIG. 8 ). The articulating component  2804  can further comprise a second retaining mechanism  2844  that has an arcuate keel or extension portion  2846  configured to be nestingly received into the groove  2822  on the ulnar ring  2814 . Similarly, the second retaining mechanism  2844  can also include a second anti-rotation portion  2848  for mating with the first anti-rotation portion  2824  on the ulnar ring  2814 . The second anti-rotation portion  2848  can include a series of second undulations  2850  that may be in the form of a wave-like pattern or other geometry that can suitably mate with the first anti-rotation portion  2824  on the ulnar ring  2814 . The keel  2846  can extend generally perpendicular to the second undulations  2850 . In one example, the articulating component  2804  can be formed of UHMWPE or PEEK. In other examples, however, the articulating component  2804  may be a combination of a first polyethylene portion on the articulating side and a molded metallic substrate that forms the second anti-rotation portion  2848 . The molded configuration can be similar to the combination polyethylene and metallic component described in pending U.S. Ser. No. 12/780,424, filed May 14, 2010 described above. 
     The securing member  2808  will now be described in greater detail. In general, the securing member  2808  can include a body  2860  that extends between a first terminal end  2862  and a second terminal end  2864 . The body  2860  can have a shank  2866  and a head  2868 . At least portions of the shank  2866 , initiating at the first terminal end  2862  can have threads  2869  formed thereon. The securing member  2808  can extend a length L 1  between the respective first and second terminal ends  2862  and  2864 . 
     Additional features of the boss portion  2810  of the tray structure  2802  will be described. The boss portion  2810  can have a bore  2870  that has a countersink  2872  and a shaft receiving portion  2874 . The boss portion  2810  can have a length L 2  that extends from the body portion  2812  of the tray structure  2802  to a terminal end of the boss portion  2810 . The boss portion  2810  can be roughened, porous coated and/or plasma sprayed to facilitate boney ingrowth. 
     Locating the tray structure  2802  relative to an ulna  2880  according to one example will now be described. In some examples, the ulna  2880  may be minimally reamed to create a counterbore  2882  for receipt of the boss portion  2810 . Nevertheless, once the ulna  2880  has been suitably prepared for receipt of the tray structure  2802 , the tray structure  2802  can be located onto the ulna  2880  to resurface the ulna  2880 . Next, the surgeon can select the securing member  2808  and pass the shank  2866  into the bore  2870  formed in the boss portion  2810  of the tray structure  2802 . It will be appreciated that a plurality of distinct length securing members may be provided such that a surgeon can select an appropriate length according to the application. 
     The surgeon can then threadably advance the securing member  2808  into the ulna  2880  until the head  2868  can locate into the countersink  2872  while the shank  2866  is received through the shaft receiving portion  2874  of the boss portion  2810 . In this regard, because the length L 1  of the securing member  2808  is greater than the length L 2  of the boss portion  2810 , at least a portion of the shank  2866  having the threads  2869  can extend proud through the boss portion  2810  and into the host ulna  2880 . Preferably, the head  2868  can be advanced to a distance where the second terminal end  2864  of the securing member  2808  is at least flush with or recessed into the countersink  2872  of the boss portion  2810  so as not to interfere with the articulating component  2804 . 
     With continued reference to  FIGS. 7 and 8 , assembly of the articulating component  2804  to the tray structure  2802  according to one example of the present teachings will be described. At the outset, a surgeon can advance the second retaining mechanism  2844  of the articulating component  2804  onto the first retaining mechanism  2820  of the tray structure  2802 . In this regard, the second anti-rotation portion  2848  can cooperatively mate with the first anti-rotation portion  2824  as the extension portion  2846  of the second retaining mechanism  2844  nests within the groove  2822  of the first retaining mechanism  2820 . Again, those skilled in the art will readily appreciate that some or all of the cooperatively mating structure formed on the tray structure  2802  and the articulating component  2804  may be reversed. Once the first and second retaining mechanisms  2820  and  2844  have been positioned against each other, translation of the articulating component  2804  and the medial/lateral direction can be inhibited by the interaction of the extension portion  2846  and the groove  2822 . Likewise, rotation of the articulating component  2804  around the ulnar ring  2814  can be inhibited by the interaction of the first and second anti-rotation portions  2824  and  2848 . 
     With reference now to  FIGS. 9-11 , a modular ulnar stem assembly  3000  constructed in accordance to one example of the present teachings will be described. In general, the modular ulnar stem assembly  3000  can include a universal stem structure  3002 , an unlinked bearing component  3004  ( FIG. 10 ), a linked bearing component  3006  ( FIG. 9 ) and a fastener  3008 . As will be described herein, the universal stem  3002  can be configured to selectively and alternatively couple with either of the unlinked bearing component  3004  or the linked bearing component  3006 . In this regard, in some instances where a patient may have the universal stem  3002  implanted with the unlinked bearing component  3004  and, over time, it may become desirable to convert the prosthesis to a configuration having a linked bearing component, a surgeon can access the elbow and merely replace the unlinked bearing component  3004  with a linked bearing component  3006 . In doing so, the surgeon need not substantially disturb the universal stem  3002  as it is configured to selectively and alternatively couple with either of the unlinked bearing component  3004  and the linked bearing component  3006 . As with the other modular ulnar assemblies described herein, the unlinked bearing component  3004  and the linked bearing component  3006  can both be modular, such that a series of unlinked and linked bearing components can be provided that are each selectively attachable to the universal stem  3002  according to specific patient&#39;s needs. 
     With specific reference now to  FIGS. 9 and 10 , exemplary features of the universal stem  3002  will be described. The universal stem  3002  can generally include a stem portion  3010  and a C-shaped body portion  3012 . In this regard, the body portion  3012  can generally include an ulnar ring  3014  that is in the form of a partial or semi-circular cylinder. The body portion  3012  can generally extend between a first end  3016  and a second end  3018 . The body portion  3012  can further comprise a first retaining mechanism  3020  thereon. The first retaining mechanism  3020  can include an extension portion  3022  and a first anti-rotation portion  3024 . The extension portion  3022  can generally be in the form of an arcuate rail or keel that centrally extends between the first end  3016  and the second end  3018  of the body portion  3012 . The first anti-rotation portion  3024  can generally include a series of first undulations  3026 . In the example shown, the first undulations  3026  can be generally in the form of a wave-like pattern formed around the ulnar ring  3014 . The specific geometry and wave-like pattern is merely exemplary and can consist of other formations that are configured to cooperatively engage complementary structure on either of the unlinked bearing component  3004  and the linked bearing component  3006  as will be described in detail herein. 
     The first end  3016  of the body portion  3012  can include a threaded bore  3030  formed therein. The second end  3018  of the body portion  3012  can include a catch  3032 . As will become appreciated from the following discussion, the first retaining mechanism  3020  that includes the first anti-rotation portion  3024  and the extension portion  3022  can cooperatively mate with complementary features formed on either of the unlinked bearing component  3004  or the linked bearing component  3006 . In this regard, the cooperating geometries can therefore inhibit rotation of the unlinked bearing component  3004  (or the linked bearing component  3006 ) around the ulnar ring  3014 , as well as inhibit medial/lateral translation of the unlinked bearing component  3004  (or the linked bearing component  3006 ) on the ulnar ring  3014 . Both ends of the linked bearing component  3006  can therefore be positively secured to the universal stem  3002 . The catch  3032  can also assist in confining the unlinked bearing component  3004  or the linked bearing component  3006  to a fixed position relative to the ulnar ring  3014 . The configuration of the first retaining mechanism  3020  can be particularly advantageous during assembly of either of the unlinked bearing component  3004  or the linked bearing component  3006  onto the universal stem  3002 . For example, the first retaining mechanism  3020  can maintain the respective unlinked or linked bearing components  3004  and  3006  in a static position relative to the ulnar ring  3014  prior to securing the respective unlinked or linked bearing components  3004  and  3006  further to the universal stem  3002  with the fastener  3008 . 
     The unlinked bearing component  3004  will now be described in greater detail. In general, the unlinked bearing component  3004  can include a body  3034  that generally takes the shape of a partial or semi-circular cylinder complementary to the shape of the ulnar ring  3014 . The body  3034  can extend between a first end  3036  and a second end  3038 . The first end  3036  of the body  3034  can define a passage  3040  therein for receiving the fastener  3008  in the assembled position ( FIG. 10 ). The second end  3038  of the body  3034  can include a notch or relief  3042 . The unlinked bearing component  3004  can further comprise a second retaining mechanism  3044  that has a groove  3046  configured to receive the extension portion  3022  on the ulnar ring  3014  and a second anti-rotation portion  3048  for cooperatively mating with the first anti-rotation portion  3024  on the ulnar ring  3014 . 
     The second anti-rotation portion  3048  can include a series of second undulations  3050  that may be in the form of a wave-like pattern or other geometry that can suitably mate with the first anti-rotation portion  3024  on the ulnar ring  3014 . The second undulations  3050  can be perpendicular to the groove  3046 . In one example, the unlinked bearing component  3004  can be formed of UHMWPE or PEEK. In other examples, however, the unlinked bearing component  3004  may be a combination of a first polyethylene portion on the articulating side and a molded metallic substrate that forms the second anti-rotation portion  3048 . The molded configuration can be similar to the combination of polyethylene and metallic component described in pending U.S. Ser. No. 12/780,424, filed May 14, 2010 described above. 
     With specific reference now to  FIG. 10 , assembly of the unlinked bearing component  3004  to the universal stem  3002  according to one example of the present teachings will be described. At the outset, a surgeon can advance the second retaining mechanism  3044  of the unlinked bearing component  3004  onto the first retaining mechanism  3020  of the universal stem  3002 . In this regard, the second anti-rotation portion  3048  can cooperatively mate with the first anti-rotation portion  3024  as the extension portion  3022  of the first retaining mechanism  3020  nests within the groove  3046  of the second retaining mechanism  3044 . At this time, the catch  3032  on the ulnar ring  3014  can locate partially into the relief  3042  of the unlinked bearing component  3004 . Those skilled in the art will readily appreciate that the cooperatively mating structures formed on the universal stem  3002  and the unlinked bearing component  3004  may be reversed. For example, the extension portion  3022  may alternately be provided on the unlinked bearing component  3004  (and the linked bearing component  3006 ) while the groove  3046  may alternatively be located on the body portion  3012  of the universal stem  3002 . Likewise, the catch  3032  and the relief  3042  may be provided on opposite components. 
     Nonetheless, once the first and second retaining mechanisms  3020  and  3044  have been positioned against each other, translation of the unlinked bearing component  3004  in the medial/lateral direction can be inhibited by the interaction of the extension portion  3022  and the groove  3046 . Likewise, rotation of the unlinked bearing component  3004  around the ulnar ring  3014  can be inhibited by the interaction of the first and second anti-rotation portions  3024  and  3048 . The catch  3032  can assist in further inhibiting rotational movement of the unlinked bearing component  3004 . Next, the fastener  3008  can be advanced through the passage  3040  in the unlinked bearing component  3004  and threadably advanced into the threaded bore  3030  of the stem portion  3010  to further secure the unlinked bearing component  3004  to the stem portion  3010 . 
     With specific reference now to  FIG. 9 , the linked bearing component  3006  will be described in greater detail. In general, the linked bearing component  3006  can include a substrate or body  3054  that generally takes the shape of a closed ring that has a mating portion  3055  that is complementary to the shape of the ulnar ring  3014 . The body  3054  can generally include a bearing portion  3056  that is molded to the body  3054 . The bearing portion  3056  can have a closed bearing surface  3058 . The body  3054  can further include a notch or relief  3064  and retaining mechanism  3066  that has a groove  3068  configured to receive the extension portion  3022  of the universal stem  3002  and a second anti-rotation portion  3070  configured to mate with the first anti-rotation portion  3024  on the ulnar ring  3014 . 
     The second retaining mechanism  3066  can include a series of second undulations  3074  that may be in the form of a wave-like pattern or other geometry that can suitably mate with the first anti-rotation portion  3024  on the ulnar ring  3014 . In one example, the bearing portion  3056  can be formed of UHMWPE or PEEK that is molded over the body  3054 . The remainder of the body  3054 , such as including the mating portion  3055 , can be formed of a metallic substrate that can encompass the bearing portion  3056 . The body  3054  can include a passage  3080  therein for receiving the fastener  3008  in the assembled position (see  FIG. 11 ). 
     With reference now to  FIGS. 9 and 11 , assembly of the linked bearing component  3006  to the universal stem  3002  according to one example of the present teachings will be described. Again, it will be appreciated that it may be desirable to replace the unlinked bearing component  3004  with a linked bearing component  3006  to provide increased constraint. At the outset, a surgeon may remove the unlinked bearing component  3004  from the universal stem  3002 . In this regard, the fastener  3008  can be threadably retracted from the threaded bore  3030  and the universal stem  3002  and the unlinked bearing component  3004  subsequently removed from the universal stem  3002 . It will be appreciated that in other examples, a surgeon can intra-operatively decide whether to select the unlinked bearing component  3004  or the linked bearing component  3006  based on a given patient. 
     Once the unlinked bearing component  3004  has been suitably removed, the linked bearing component  3006  can be coupled to the universal stem  3002 . Again, as with the other embodiments described herein, a linked bearing component  3006  can be selected from a plurality of linked bearing components according to the needs of a particular patient. The surgeon can advance the second retaining mechanism  3066  of the linked bearing component  3006  onto the first retaining mechanism  3020  of the universal stem  3002 . In this regard, the second anti-rotation portion  3070  can cooperatively mate with the first anti-rotation portion  3024  as the extension portion  3022  of the first retaining mechanism  3020  nests within the groove  3068  of the second retaining mechanism  3066 . At this time, the catch  3032  on the ulnar ring  3014  can locate partially into the notch  3064  of the linked bearing component  3006 . Those skilled in the art will readily appreciate that the cooperatively mating structure is formed on the universal stem  3002  and the linked bearing component  3006  may be reversed. Moreover, it will be appreciated that the complementary mating surfaces of the universal stem and the unlinked bearing component  3004  and linked bearing component  3006  can be formed with various geometries as long as the complementary mating surfaces of both the unlinked bearing component  3004  and the linked bearing component  3006  can selectively and alternatively locate onto the first retaining mechanism  3020  of the universal stem  3002 . 
     Once the first and second retaining mechanisms  3020  and  3066  have been positioned against each other, translation of the linked bearing component  3006  in the medial/lateral direction can be inhibited by the interaction of the extension portion  3022  and the groove  3068 . Likewise, rotation of the linked bearing component  3006  around the ulnar ring  3014  can be inhibited by the interaction of the first and second anti-rotation portions  3024  and  3070 , respectively. The catch  3032  can assist in further inhibiting rotational movement of the linked bearing component  3006 . Next, the fastener  3008  can be advanced through the passage  3080  in the linked bearing component  3006  and threadably advanced into the threaded bore  3030  of the universal stem  3002  to further secure the linked bearing component  3006  to the universal stem  3002 . 
     With reference now to  FIGS. 12-18 , a modular unlinked ulnar stem assembly  3100  constructed in accordance to one example of the present teachings will be described. In general, the modular unlinked ulnar stem assembly  3100  can include a stem structure  3102  and a mobile ulnar bearing  3104 . As will become appreciated from the following discussion, the mobile ulnar bearing  3104  can be configured to slidably communicate along a surface of the stem structure  3102  in a medial/lateral direction. In addition, the mobile ulnar bearing  3104  can be configured to include dedicated surface areas that can be adapted to articulate against a surface of a distal unlinked humeral component  3106  ( FIG. 19 ). The mobile ulnar bearing  3104  therefore can be configured to provide two articulations. A first articulation provided during medial/lateral translation of the mobile ulnar bearing  3104  along an opposing surface of the stem structure  3102  and a second articulation with the distal unlinked humeral component  3106 . Again, as with the other embodiments disclosed herein, the mobile ulnar bearing  3104  can be modular, such that a series of mobile ulnar bearings  3104  can be provided that are each selectively configured for slidable movement along the stem structure  3102  according to a specific patient&#39;s needs. The modular unlinked ulnar stem assembly  3100  can be favorable in some circumstances as it has the potential to remove less native bone than other elbow prostheses, such as a semi-constrained total elbow, for example. 
     With specific reference now  FIGS. 12, 13 and 16  exemplary features of the stem structure  3102  will be described. The stem structure  3102  can generally include a stem portion  3110  and a C-shaped body portion  3112 . In this regard, the body portion  3112  can generally include an ulnar ring  3114  that is in the form of a partial or semi-circular cylinder. The body portion  3112  can generally extend between a first end  3116  and a second end  3118 . The body portion  3112  can further comprise a first wall  3120  arranged at the first end  3116  and a second wall  3122  arranged at the second end  3118 . The body portion  3112  can further comprise a medial wall  3124  and a lateral wall  3126  ( FIG. 16 ). The first wall  3120 , second wall  3122 , medial wall  3124  and lateral wall  3126  can cooperate to provide a boundary that confines the mobile ulnar bearing  3104  as will be described more fully herein. A width W 1  can be defined between the medial wall  3124  and the lateral wall  3126 . The body portion  3112  can further provide an ulnar ring articulation surface  3130  that the mobile ulnar bearing  3104  slidably communicates therealong. As with the other examples provided herein, the stem structure  3102  can be formed of a biocompatible material, such as cobalt or titanium. The ulnar ring articulation surface  3130  of the stem structure  3102  can be highly polished to facilitate smooth translation of the mobile ulnar bearing  3104  therealong. 
     With general reference now to  FIGS. 12-21  and specific reference to  FIGS. 14 and 15 , the mobile ulnar bearing  3104  will be described in greater detail. In general, the mobile ulnar bearing  3104  can include a body  3150  that generally takes the shape of a partial or semi-circular cylinder complementary to the shape of the ulnar ring  3114 . The body  3150  can extend between a first end  3152  and a second end  3154 . The body  3150  can further include a medial side surface  3156  and a lateral side surface  3158 . The body  3150  can include an ulnar opposing articulating surface  3160  that slidably communicates along the ulnar ring articulating surface  3130  ( FIG. 12 ) and a humeral opposing articulating surface  3162  that articulates with a humeral articulating surface  3166  of the distal unlinked humeral component  3106  ( FIG. 19 ). The ulnar opposing articulating surface  3160  can be linear or spherical along a cross-section taken through a medial/lateral plane of the mobile ulnar bearing  3104 . Likewise, the ulnar ring articulation surface  3130  can have a similar cross-sectional profile that matches the ulnar opposing articulating surface  3160 . 
     The humeral opposing articulating surface  3162  can be collectively formed by a first articulating surface  3170 , a second articulating surface  3172 , a third articulating surface  3174  and a fourth articulating surface  3176 . In the example shown, the first and second articulating surfaces  3170  and  3172  are arranged generally diagonally relative to each other while the third and fourth articulating surfaces  3174  and  3176 , respectively are also arranged generally diagonally relative to each other. In this regard, the first, second, third and fourth articulating surfaces  3170 ,  3172 ,  3174  and  3176  can provide four quadrants that can be configured to selectively articulate along the humeral articulating surface  3166  according to a given varus or valgus movement. The configuration of the first, second, third and fourth articulating surfaces  3170 ,  3172 ,  3174  and  3176  can be particularly advantageous as they can selectively provide a relatively larger surface area contact with the humeral articulating surface  3166  of the distal unlinked humeral component  3106  as compared to a general line-to-line contact that may be achieved from other conventional mobile ulnar bearing configurations as will be described in detail herein. 
     With reference now to  FIGS. 16-18 , movement of the mobile ulnar bearing  3104  relative to the ulnar ring  3114  will be described. As identified above, the mobile ulnar bearing  3104  can be configured to slidably translate along the ulnar ring articulating surface  3130  in a medial/lateral direction. In this regard, the mobile ulnar bearing  3104  can define a width W 2  between the medial side surface  3156  and the lateral side surface  3158 . The width W 2  can be less than the width W 1  provided between opposing medial and lateral walls  3124  and  3126  of the ulnar ring  3114 , such that the mobile ulnar bearing  3104  can slidably communicate along the ulnar ring articulating surface  3130  between the medial wall  3124  and the lateral wall  3126  of the ulnar ring  3114 . As shown in  FIG. 17 , the mobile ulnar bearing  3104  can translate to a medial-most position where the medial side surface  3156  of the mobile ulnar bearing  3104  is in contact with the medial wall  3124  of the ulnar ring  3114 . As can be appreciated, further translation of the mobile ulnar bearing  3104  in the medial direction is precluded by contact with the medial wall  3124 . Similarly, the mobile ulnar bearing  3104  is shown in a lateral-most position in  FIG. 18  where the lateral side surface  3158  is moved into contact with the lateral wall  3126  of the ulnar ring  3114 . Again, further translation of the mobile ulnar bearing  3104  in the lateral direction is precluded by contact with the lateral wall  3126 . 
     With reference now to  FIGS. 14 and 19-21 , articulation of the mobile ulnar bearing  3104  with the distal unlinked humeral component  3106  will be further described. By way of example, the modular unlinked ulnar stem assembly  3100  is shown with the distal unlinked humeral component  3106  in a neutral position in  FIG. 19 . An exemplary radial component  3180  can be implanted if desired. In  FIG. 20 , the distal unlinked humeral component  3106  is shown during varus arm movement. In the exemplary configuration, the varus arm position can have an angle  3184  of about 3.5 degrees. In  FIG. 21 , the distal unlinked humeral component  3106  is shown in a valgus arm position. More specifically, the distal unlinked humeral component  3106  is shown rotated in angle  3186  of about 3.5 degrees. When the arm is in the varus position shown in  FIG. 20 , the humeral articulating surface  3166  of the distal unlinked humeral component  3106  can engage the first and second articulation surfaces  3170  and  3172  of the mobile ulnar bearing  3104 . The surface profile of the first and second articulation surfaces  3170  and  3172  can provide a diagonally opposed surface contact with the humeral articulating surface  3166 . Explained further, the cross-hatched surface areas  3170  and  3172  can both be contacting the humeral articulating surface  3166  thereby providing a supportive interface between the mobile ulnar bearing  3104  and the distal unlinked humeral component  3106  in the varus position. Such a configuration can provide a more stable surface-to-surface contact versus a line-to-line contact. Similarly, when the distal unlinked humeral component  3106  is moved to the corresponding valgus arm position shown in  FIG. 21 , the third and fourth articulation surfaces  3174  and  3176  of the mobile ulnar bearing  3104  can communicate with the humeral articulation surface  3166  of the distal unlinked humeral component  3106 . Again, the surface profile of the third and fourth articulation surfaces  3174  and  3176  provided by the mobile ulnar bearing  3104  can provide a surface contact with the humeral articulation surface  3166  that can improve stability. Explained further, the cross-hatched surface areas  3174  and  3176  can both be contacting the humeral articulating surface  3166  thereby providing a supportive interface between the mobile ulnar bearing  3104  and the distal unlinked humeral component  3106  in the valgus position. 
     While the description in the specification and illustrated in the drawings are directed to various embodiments, it will be understood that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the teachings and the appended claims. In addition, many modifications may be made to adapt a particular situation or material to the teachings without departing from the scope thereof. Therefore, it is intended that the teachings and claims are not to be limited to any particular embodiment illustrated in the drawings and described in the specification, but that the teachings and claims can include any embodiments falling within the foregoing description and the appended claims.