Abstract:
An insert ( 11 ) of a joint ball receptacle of a prosthetic shoulder joint according to the invention has a first side (I) with a depression ( 15 ) for receiving a joint ball, and a second side (II) which is provided and formed for being received in a shell ( 13 ) of the joint ball receptacle. According to the invention, a snap action mechanism ( 19 ) which extends around a central axis ( 17 ) of the insert ( 11 ) is arranged on the second side (II), which snap-action mechanism ( 19 ) has, as seen from the second side (II), and together with a collar ( 21 ), an undercut ( 23 ) which extends around the central axis ( 17 ), in such a way that the collar ( 21 ) can engage with elastic deformation behind a correspondingly formed mating element ( 25 ) of the shell ( 13 ), wherein the snap-action mechanism ( 19 ) is discontinuous in the peripheral direction of the insert ( 11 ) at at least one point ( 31 ); according to the invention, the insert ( 11 ) has a cylindrical guide region ( 27 ) at the outer periphery in the region provided for being received in the shell ( 13 ).

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a U.S. National Phase Patent Application based on International Application Serial No. PCT/EP2006/066410 filed Sep. 15, 2006, the disclosure of which is hereby explicitly incorporated by reference herein. 
     
    
     SUMMARY 
       [0002]    The invention relates to an insert of a joint ball mount of, for example, a shoulder joint prosthesis. In exemplary forms thereof, the invention furthermore relates to a shell of a joint ball mount of a shoulder joint prosthesis for use with such an insert, to a joint ball mount of a shoulder joint prosthesis consisting of such an insert and such a shell and also to a complete shoulder joint prosthesis. 
         [0003]    With shoulder joints, the selection of the suitable implant and the manner of the implantation decisively depends on the condition in which the involved bones are, with the condition of the muscles, in particular of the rotator cuff, also playing a role. With the shoulder joint, the condition of the shoulder blade is particularly important, and above all in this connection the joint socket, the glenoid, which cooperates with the head of the upper arm, the humerus in the healthy joint. The necessity of a part replacement or complete replacement of the shoulder joint can be present for various reasons. Typical causes are, for example, advanced wear of the joint surfaces or fractures, e.g. due to an accident. Depending on the type and degree of damage, a so-called inverse prosthesis configuration can also be indicated in which the artificial joint ball and the artificial joint mount are swapped over with respect to their positions in a natural joint. 
         [0004]    Before possible embodiments of the joint ball mount proposed here are presented with reference to  FIGS. 2 to 15 , an introductory overview will first be given with reference to  FIGS. 1   a ; ,  1   b  and  1   c,    
         [0005]    The bony structure of the shoulder joint consists of the head of the upper arm bone E and of the joint socket B of the shoulder blade A. In addition, two bone projections of the shoulder blade A, namely the acromion C and the coracoid D, are important for the function of the shoulder joint. These bone projections, together with a ligament, not shown, connecting them, form the so-called “ceiling” of the shoulder which has an arch-like shape and prevents an upward migration of the head of the upper arm from the joint socket.  FIG. 1  a shows a so-called anatomical configuration in which the prosthesis reproduces the shoulder joint in its natural structure, i.e. the humerus E is provided with an artificial joint head  116  and the shoulder blade A is provided with an artificial joint ball mount or joint socket  114 . The anchorage of the joint socket  114  at the shoulder blade A, more precisely at the correspondingly prepared glenoid B, takes place via screws  114   a  in the example shown. The anchorage of the artificial joint head  116  at the humerus E takes place by means of a shaft  112  which can have different types of design. 
         [0006]      FIG. 1   b  shows a so-called inverse configuration in which the artificial joint head and the artificial joint socket  114 ′ have been swapped over with respect to their positions in the natural shoulder joint. The joint head is here formed by an artificial joint part which includes a base platform  111  and a ball component  117  firmly connected to the platform  111 . The anchorage of the platform  111  at the shoulder blade A, more precisely at the correspondingly prepared glenoid B, takes place in the example shown via a spigot  119  only indicated schematically of the platform  111  and by means of screws  123  for which corresponding screw mounts are provided in the platform  111 . Whereas the platform  111  therefore supports the artificial ball component  117  here, the artificial joint socket  114 ′, which forms the ball joint mount, is anchored to the humerus E by means of a shaft  112 . 
         [0007]    It is known to make joint ball mounts with a shell to be anchored in the bone and an insert to be fastened therein. It is known to make the shell from metal and the insert, which has a recess, for example, for the reception of a joint ball, from plastic. It is known, for example, in accordance with  FIG. 1   c  for the anchorage of an insert in a shell to provide the inner contour of the shell  13  with an undercut and to provide the insert  11  with a peripherally formed and radially outwardly extending collar such that an undercut element is formed which extends in the peripheral direction, extends radially outwardly and is hook-shaped in cross-section. The element can be brought into engagement with the undercut of the shell  13  by elastic deformation, and indeed such that the insert  11  is anchored in the shell  13 , as  FIG. 1   c  shows. 
         [0008]    With a joint ball mount of the kind set forth here, the collar of the snap-action mechanism is interrupted at at least one point of its periphery. This has the effect that the collar can be deformed more pronouncedly on the introduction of the insert into a shell, which is then expressed in a larger restoring movement of the undercut after the complete introduction of the insert. Due to this larger restoring movement, the undercut can engage behind the corresponding counter-element in the shell to a comparatively large degree or with a relatively large “depth”. A good and large-area shape matching can thus ultimately be achieved between the insert and the shell or between the undercut of the insert and the counter-element of the shell. The insert is furthermore provided with a cylindrical guide region. The insert can be combined with a shell in accordance with one of the claims directed to a shell. In this manner, a largely full surface contact of the outer side of the cylindrical guide region of the insert at the inner side of the cylindrical inner guide region of the shell is obtained when the insert is coupled to the shell. The cylindrical guide region takes over a guidance function and/or centering function of the insert in the shell. The function of the coupling of insert and shell achievable with the snap-action mechanism is in particular separate from the function of the guidance and/or centering of the insert. An insert for the joint ball mount of a shoulder prosthesis has become known for GB 2,405,346 which has a snap-action mechanism at its axial end and furthermore has a cylindrical guide region. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0009]      FIG. 1  is an elevational view of a shoulder joint including an anatomical prosthesis; 
           [0010]      FIG. 1   b  is an elevational view of the shoulder joint including an inverse shoulder prosthesis; 
           [0011]      FIG. 1   c  is a sectional view of an insert in a shell of a prosthetic ball mount; 
           [0012]      FIG. 2  is a perspective view of a prosthetic shell useable in a prosthetic ball mount; 
           [0013]      FIG. 3  is an elevational view of the prosthetic shell of  FIG. 2 ; 
           [0014]      FIG. 4  is a sectional view of the prosthetic shell of  FIG. 2 ; 
           [0015]      FIG. 5  is a partial detailed view of the prosthetic shell of  FIG. 2 ; 
           [0016]      FIG. 6  is a perspective view of an insert adapted for cooperation with the prosthetic shell illustrated in  FIGS. 2-5 ; 
           [0017]      FIG. 7  is an elevational view of the insert of  FIG. 6 ; 
           [0018]      FIG. 8  is a sectional view of the insert of  FIG. 6  taken along line A-A of  FIG. 7 ; 
           [0019]      FIG. 9  is a sectional view of the insert of  FIG. 6  taken along line B-B of  FIG. 7 ; 
           [0020]      FIG. 10  is a partial detailed view of the cross-sectional view of  FIG. 8 ; 
           [0021]      FIG. 11  is a partial cross-sectional view of the insert of  FIG. 6  taken along line E-E of  FIG. 7 ; 
           [0022]      FIG. 12  is a perspective view of an alternative embodiment prosthetic shell; 
           [0023]      FIG. 13  is a perspective view of an alternative embodiment insert adapted for cooperation with the prosthetic shell of  FIG. 12 ; and 
           [0024]      FIGS. 14 and 15  are sectional views illustrating securement of the insert illustrated in  FIG. 13  in the prosthetic shell illustrated in  FIG. 12 . 
           [0025]    Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate preferred embodiments of the invention, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
       
    
    
     DETAILED DESCRIPTION  
       [0026]    An embodiment is shown in the  FIGS. 2 to 11 . A further embodiment is shown in the  FIGS. 12 to 15 . 
         [0027]      FIGS. 2 to 5 ,  12  show a shell,  FIGS. 6 to 11 ,  13  an insert, and  FIGS. 14 ,  15  a joint ball mount. 
         [0028]    The shell  13  in accordance with  FIGS. 2 to 5  has a conically converging spigot  39  with which the shell  13  can be anchored to a humerus shaft, not shown, or to the shoulder blade, optionally with a platform arranged therebetween. The outer cross-sectional shape of the conical spigot  39  is elliptic. The shell  13  has at its side provided for the mounting of the insert  11  described in the following with reference to  FIGS. 6 to 11  a cylindrical section  41  in the form of a peripheral wall whose center axis  43  in accordance with  FIG. 4  is inclined with respect to a center axis of the conical spigot  39 . The cylindrical section  41  is interrupted at two mutually diametrically opposed points. The cut-outs  35  arising in this manner serve for the reception of corresponding projections  37  of the insert  11  in accordance with  FIG. 6 . A security against rotation of the insert  11  coupled to the shell  13  is hereby achieved. The inner side of the cylindrical peripheral wall  41  serves as a cylindrical inner guide region  33  for a corresponding cylindrical guide region  27  of the insert  11  in accordance with  FIG. 6 . At the “base” of the mount, a further cylindrical recess (without reference numeral) can be recognized which, as can be recognized in  FIGS. 14 and 15 , is provided for the reception of a second guide of the insert. 
         [0029]    In  FIG. 3 , which shows a view of the shell  13  from “below” in accordance with  FIG. 2 , that is from its anchorage side, in particular the elliptical outer cross-sectional shape of the conical spigot  39  can be recognized. 
         [0030]    It can in particular be seen from  FIG. 4 , which shows a section along the line “A-A” of  FIG. 3 , and from  FIG. 5 , which shows the detail “B” of  FIG. 4 , that the mount side of the shell  13  for the insert  11  is substantially formed by two cylindrical regions with different inner diameters. The region with a larger inner diameter is bounded by the already mentioned cylindrical inner guide region  33  and is separated from the region with a smaller inner diameter by a radially inwardly projecting peripheral projection which provides an undercut and serves as a counter-element  25  for a snap-action mechanism  19  of the insert  11  described in more detail in the following, see  FIG. 6 . 
         [0031]    The insert  11  shown in  FIGS. 6 to 11  is provided with a peripheral cylinder section  45  which has a cylindrical guide region  27  at its outer side which extends parallel to the center axis  17  of the insert  11 , said center axis coinciding with the center axis of the cylinder section  45 . The cylinder section  45  of the insert  11  is dimensioned in accordance with the mount region with a larger inner diameter of the shell  13  so that, in the assembled state, the cylindrical guide region  27  of the insert  11  and the cylindrical inner guide region  33  of the shell  13  are mutually oppositely disposed and cooperate, as described above. The dimensions are selected such that the insert  11  is inserted into the shell  13  with a small tolerance. The diameter tolerances are in particular dimensioned here such that the cylindrical guide region of the insert can be introduced into the shell without excessive force exertion and, on the other hand, a good centration and guidance is ensured. In a possible embodiment, the diameter of the cylindrical inner guide region  33  of the shell  13  can have an excess dimension with respect to the diameter of the cylindrical guide region  27  of the insert  11  of a maximum of 1/10 mm, or a maximum of 1%, for example less than 0.4%, of the diameter of the cylindrical guide region  27  of the insert  11 . As is in particular shown in  FIG. 10 , the one axial end of the cylindrical guide region  27  adjoins, via a first ring-shaped surface  47  extending perpendicular to the axis of the cylindrical guide region  27 , a section  53  of the insert  11  which is formed in the region of the first side I, see  FIGS. 8 and 9 , and at which this insert  11  has its maximum outer diameter, with the first ring-shaped surface  47  extending radially outwardly from the cylindrical guide region  27  such that the outer diameter of the cylindrical guide region  27  corresponds to the inner diameter of the first ring-shaped surface  47 . The section  53  of the insert  11  is arranged outside the shell  13  in the condition coupled to the shell  13  such that the outer diameter of the cylindrical guide region  27  forms the largest diameter of the insert  11  disposed inside the shell  13  in the condition coupled with the shell  13 . With its other end, the cylindrical guide region  27  adjoins the snap-action mechanism  19  of the insert  11  described in more detail below via a second ring-shaped surface  49  extending perpendicular to the axis of the cylindrical guide region  27 , with the second ring-shaped surface  49  extending radially inwardly away from the cylindrical guide region  27  such that the outer diameter of the cylindrical guide region  27  corresponds to the outer diameter of the second ring-shaped surface  49 . One of the two ring-shaped surfaces  47  or  49  acts in this connection as an axial abutment for a correspondingly formed counter-surface of the shell  13 . A plurality of snap-action elements  29  of the insert  11  which are each made as a hook-like peripheral segment and are arranged radially inside the cylindrical guide region  27  cooperate with the counter-element  25  of the shell  13 . The snap-action elements  29  are arranged uniformly distributed in the peripheral direction, with in each case an interruption  31  being present between two adjacent snap-action elements  29 . In a possible embodiment, the interruptions  31  can each have a width of at least 1 mm. The interruptions  31  extend in each case in the axial direction from the end face of the cylinder section  45  extending perpendicular to the center axis  17  up to the free axial end of the snap-action elements  29 . The totality of the snap-action elements  29  form a snap-action mechanism  19  in the form of a peripheral, interrupted collar  21  with an undercut  23 , see  FIG. 10 , with the maximum outer diameter of the snap-action mechanism  19  being smaller than the outer diameter of the cylindrical guide region  27  which is arranged closer to the first side I than the snap-action mechanism  19 . The projections  37  of the insert  11  which project radially with respect to the cylinder section  45  and are mutually diametrically oppositely disposed cooperate with the cut-outs  35  of the shell  13  in order—as already mentioned above—to hold the insert  11  with rotational security in the shell  13  in the coupled state. The snap-action mechanism  19  of the insert  11  is formed on a second side II of the insert  11  facing the shell  13  in the assembled state, whereas—as in particular  FIGS. 8 and 9  show—a recess  15  is formed on the oppositely disposed first side I of the insert  11 , said recess forming the actual mount for the joint ball of the prosthesis not shown here. Furthermore, a second guide region (without reference numeral) is arranged at the axial end of the second side II and can cooperate with a corresponding mount of the shell. 
         [0032]    The following statements are made with respect to  FIGS. 8 and 9 . The recess  15  is made in the shape of a conical section and has a spherical surface region  51  which extends with respect to a center point M of the spherical surface region over an angular region Ω of, for example, 110°, i.e. over a semi-angular region Ω/2 of, for example, 55°. The angle Ω in particular does not exceed an angle of 120° with shoulder joint support shells. In other embodiments, Ω is limited to maximum values of 110° or even 100° or 90°, in particular not to restrict the movability range of the shoulder joint and because a seating and the centration of the joint ball anyway takes place to a high degree by the ligament and muscular apparatus in shoulder joints; this in comparison, for example, with hip joints where a comparable rim width angle of the support shell comes in the order of magnitude of 180°. The recess  15  furthermore has a pole  55  which is disposed on the point of intersection of the center axis  17  with the spherical surface region  51 , with the axial position of the axial end of the cylindrical second side guide region  27  facing the second side II being slightly offset, for example by 0.5 mm, with respect to the axial position of the pole  55  in the direction of the first side I. 
         [0033]    It is generally also possible for the axial position of the axial end of the cylindrical guide region  27  facing the second side II to be slightly offset in the direction of the second side II. The axial end of the cylindrical guide region  27  facing the second side II can therefore also be arranged “lower” than the pole  55  of the recess  15 —considered from the first side I.  FIG. 9  furthermore shows a gradient angle ω which is formed between a perpendicular  57  to the center axis  17  and a tangent  59  at the spherical surface region  51 . The tangent  59  is applied at an axial position of the spherical surface region  51  which corresponds to the axial position of the end of the cylindrical guide region  27  facing the first side I. 
         [0034]    The gradient angle ω amounts in the embodiment shown, for example, to less than 45° and can amount to the angle Ω/2. Other gradient angles, which are present at axial positions at the spherical surface region  51 , which correspond to the axial positions of other regions of the cylindrical guide regions  27 , are consequently disposed in the range between 0° and the gradient angle ω which corresponds to the axial position of the end of the cylindrical guide region  27  facing the first side I. It can furthermore be seen from  FIG. 10  that the axial extent x of the cylinder section  45 , and thus of the cylindrical guide region  27  of the insert  11 , amounts to a multiple of the depth a of the undercut  23  of the collar  21  formed by the snap-action elements  29  measured in the radial direction with respect to the center axis  17 . In a possible embodiment, the depth a of the undercut amounts to at least 1 mm, for example more than 1.3 mm. With respect to the outer diameter of the snap-action mechanism  19 , the depth a of the undercut  23  can amount in a possible embodiment to at least 3%, at least 3.5% in an embodiment, of this outer diameter. The axial extent x of the cylindrical guide region  27  parallel to the center axis  17  of the insert  11  can amount to at least 3 mm, in particular to at least 3.5 mm. With respect to the outer diameter of the cylindrical guide region  27 , its axial extent x parallel to the center axis  17  can amount to at least 10% of this outer diameter. As regards the axial extent of the cylindrical inner guide region  33  of the shell  13 , this is at least substantially just as large in an embodiment as the axial extent x of the cylindrical guide region  27  of the insert  11  to be received. 
         [0035]    In a further embodiment in accordance with the  FIGS. 12 and 13 , a security against rotation is provided whose components, cut-out and projection, are particularly swapped over with respect to their arrangement at the shell and insert with respect to the security against rotation explained in connection with the embodiment in accordance with  FIGS. 2 to 11 . The shell  213  shown in  FIG. 12  includes a cylinder section  241  which has a respective radially inwardly projecting projection  237  at two mutually diametrically opposed points. The projections  237  are received in the assembled state by corresponding cut-outs  235  of the insert  211  which are provided at two mutually diametrically opposed points in a cylinder section  245  of the insert  211 . Otherwise, the embodiment in accordance with  FIGS. 12 and 13  corresponds to the embodiment in accordance with  FIGS. 2 to 11 , with aspects already explained in the above again being taken up or repeated in the following with respect to  FIGS. 14 and 15 . 
         [0036]    In  FIGS. 14 and 15 , a longitudinal section is shown through a joint ball mount made up of the shell  213  in accordance with  FIG. 12  and the insert  211  in accordance with  FIG. 13  along the center axis of the insert  211  or of the center axis of the cylinder section  241  of the shell  213 . The cylinder section  245  of the insert  211  has a cylindrical guide region  227  at its outer side which cooperates with a cylindrical inner guide region  233  of the cylinder section  241  of the shell  233  such that a full-area mutual contact of the cylinder sections  241  and  245  is realized. The insert  211  furthermore has a plurality of undercut, elastically deformable snap-action elements  229  which engage behind a peripheral projection  225  of the shell  213  so that a snap-in connection is formed between the insert  211  and the shell  213 . 
         [0037]    The insert  11  shown in the Figures is made of plastic in one embodiment, in particular of polyethylene. The minimal material thickness of the supporting region, adjacent to the recess  15  provided for the reception of the joint ball, amounts, for example to not less than 3 mm and lies in specific embodiments at 3.4 mm to 4.0 mm, more specifically at 3.5 mm, at most in a tolerance range of ±0.5 mm. The material thickness is measured in this context in the radial direction of the ball-shaped or sphere-shaped recess  15  for the reception of the joint ball, that is in the direction of the surface normal force introduction. 
         [0038]    The joint ball mount  11 ,  13  described here can be fastened conventionally—in a configuration also called “anatomical”—to the scapula. The described components are also very well suited for the formation of a shoulder joint prosthesis in which the joint ball mount  11 ,  13  is provided—in a configuration also called “inverse”—for fastening to the humerus. In this context, the joint ball mount  11 ,  13  is fastened, for example, to a shaft known per se such as is used for fastening in a long bone and which can be anchored easily in the humerus—cemented or not cemented depending on the embodiment.