Abstract:
A shoulder prosthesis assembly includes a metaglenoid element ( 7 ) having a plate ( 13 ) with a central tubular bushing ( 14 ) and members for anchoring it in the glenoid cavity. The assembly also includes a glenoid sphere ( 9 ) that fits over the metaglenoid element which has an axial screw that is ( 35 ) screwed into the bushing. The glenoid sphere has a member to facilitate installation thereof on the metaglenoid element. The member may include a pin ( 46 ) adapted to slide in the tubular bushing ( 14 ) and in an axial channel of the screw, so that the glenoid sphere ( 9 ) can be screwed into place without any risk of damaging the thread of the screw or the tapping of the bushing.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of co-pending application Ser. No. 11/183,826 filed on Jul. 19, 2005, which is a continuation of Ser. No. 10/168,802, filed on Jun. 24, 2002, which is a National Stage of PCT/FR00/03539 filed on Dec. 14, 2000, which claimed priority to French Application No. 9916420 filed Dec. 23, 1999. The entire contents of each of the above-identified applications are hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a shoulder prosthesis assembly of the type comprising a metaglenoid element formed by a plate provided with a central tubular bushing together with members for anchoring in the glenoid cavity, and a glenoid sphere adapted to be fitted over the metaglenoid element. The glenoid sphere is provided with an axial screw that is free to turn relative to the glenoid sphere. This screw is adapted to be screwed into the bushing of the metaglenoid element. The glenoid sphere has a convex joint surface for co-operating with a concave cup of an associated humeral prosthesis. 
         [0003]    The invention also provides a prosthesis forming part of the prosthesis assembly. 
         [0004]    A prosthesis of this type is known, for example, from French patent No. 89/13366 (2 652 498) filed in the name of Medinov. 
         [0005]    It is known that the rotator cuff is constituted by the set of shoulder muscles which comprise the deltoid, the supraspinatus, the infraspinatus, the infrascapular, and the smaller round. When massive rupture occurs of the cuff, only the deltoid remains, and that is insufficient for enabling the shoulder joint to operate properly, which requires a shoulder prosthesis to be implanted. 
         [0006]    Prostheses are known in which the metaglenoid element is fastened to the glenoid cavity of the scapula by a plurality of screws and receives a glenoid sphere having a convex joint surface, which sphere is screwed to the periphery of the metaglenoid element. 
         [0007]    Unfortunately, it is found that after being in use for a certain length of time, the glenoid sphere tends to become unscrewed. This causes the glenoid sphere to tilt under the metaglenoid element and destroys the threads . . . . This damage caused to the prosthesis can be sufficient to require a surgeon to remove the entire prosthesis in order to replace it. 
         [0008]    To solve this problem, proposals were initially made to cause the facing contacting surfaces of the glenoid sphere and the metaglenoid element to be rough. That technique has turned out to be insufficient. 
         [0009]    Proposals were then made to associate the screw of the glenoid sphere with an arrangement of complementary male and female conical bearing surfaces made respectively on the metaglenoid element and the glenoid sphere. The two conical bearing surfaces can then be moved apart or towards each other as a function of the direction in which the screw is driven in order to lock or unlock the system comprising the glenoid sphere and the metaglenoid element. A shoulder prosthesis arranged in that way is described in French patent No. 95/09395 (2 737 107) in the name of Medinov. That prosthesis gives satisfaction. 
         [0010]    However, the heads of the screws can pass right through the metaglenoid element at the end of screw tightening if the surgeon cannot feel this end appropriately, since no special retaining means are provided. Under such circumstances, the metaglenoid element is no longer fixed to the glenoid cavity. 
         [0011]    Another problem lies in giving the surgeon access to the metaglenoid element, which raises a difficulty with putting the glenoid sphere properly into place on the metaglenoid element. The patient is in a semi-seated position during surgery, such that the surgeon has to approach the metaglenoid element from a cantilevered-out position, with the metaglenoid element being, as it were, at the bottom of a well defined by the tissues and the muscles of the shoulder. Access to the metaglenoid element is narrow and relatively difficult, with poor visibility that makes it impossible to tell whether the glenoid sphere screw is exactly on the axis of the metaglenoid bushing. 
         [0012]    It results that the surgeon can position the glenoid sphere incorrectly, so that the axial screw of the glenoid sphere is not exactly on the axis of the corresponding hole in the metaglenoid element. Under such circumstances, screw tightening is started when the thread on the glenoid sphere screw and the tapping in the central hole of the metaglenoid element are not properly engaged. Once the surgeon becomes aware of this faulty insertion, the threads have already become sufficiently damaged to require the entire prosthesis to be replaced. Not only must two prostheses be used, thus considerably increasing equipment cost, but the time required for the operation is doubled. 
       SUMMARY OF THE INVENTION 
       [0013]    The object of the invention is to provide a glenoid prosthesis that is arranged in such a manner as to eliminate those various drawbacks. 
         [0014]    According to the invention, the shoulder prosthesis includes means for guiding the glenoid sphere and adapted to facilitate proper placement of the screw on the metaglenoid element on the axis of the bushing. 
         [0015]    In an embodiment of the invention, said guide means comprise a pin adapted to be capable of being slid in the tubular bushing, and the screw of the glenoid sphere presents a longitudinal axial channel opening out to both ends of the screw and allowing the pin to be inserted into said channel, such that the pin serves to guide the glenoid sphere when the screw slides along the pin until it is inserted into the tubular bushing. 
         [0016]    Preliminary insertion of the pin in the central bushing of the metaglenoid element makes it possible to provide complete guidance of the glenoid sphere onto the axis of the central bushing of the metaglenoid element. The glenoid sphere screw can then be screwed properly into the metaglenoid bushing without any risk of damaging the threads, thereby almost completely eliminating any risk of needing to have recourse to a second prosthesis. 
         [0017]    According to another characteristic of the invention, the members for anchoring the metaglenoid element in the glenoid cavity are screws, and means are provided to ensure that each screw comes into abutment against the metaglenoid element at the end of screw tightening. 
         [0018]    In an embodiment of the invention, the abutment means comprise a collar formed around the head of at least one screw and projecting radially relative to a thread of the screw head, and a corresponding annular bearing shoulder arranged around the edge of a tapped hole for insertion of the screw in the metaglenoid element. 
         [0019]    According to another possible characteristic of the invention, the abutment means comprise at least one head of a screw presenting a convex outside surface adapted to bear against a complementary concave seat formed in the wall of a hole for inserting said screw in the metaglenoid element. 
         [0020]    These arrangements thus avoid any risk of the screw passing through the metaglenoid element at the end of screw tightening, thus guaranteeing security in the fixing of the metaglenoid element to the glenoid cavity. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    Other features and advantages of the invention appear from the following description given with reference to the accompanying drawings which show an embodiment by way of non-limiting example. 
           [0022]      FIG. 1  is an elevation view in a front plane and on a small scale of a total shoulder prosthesis including a glenoid implant in accordance with the invention. 
           [0023]      FIG. 2  is a view on a larger scale, showing the glenoid prosthesis of  FIG. 1  in axial section on  2 / 2  of  FIG. 4 . 
           [0024]      FIG. 3  is a view of the glenoid prosthesis in section on  3 / 3  of  FIG. 4 . 
           [0025]      FIG. 4  is a plan view of the metaglenoid element of the shoulder prosthesis of  FIGS. 1 to 3 . 
           [0026]      FIG. 5  is a view of the shoulder prosthesis in axial section on  3 / 3  of  FIG. 4 , showing the beginning of the operation of inserting the axial screw of the glenoid sphere into the central bushing of the metaglenoid element. 
           [0027]      FIG. 6  is a view analogous to  FIG. 5  showing the operation of engaging the glenoid sphere on the metaglenoid element by means of a guide pin. 
           [0028]      FIG. 7  is a fragmentary axial section view on a larger scale than  FIG. 5  showing the free end of the axial screw of the glenoid sphere. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0029]    The total shoulder prosthesis shown in  FIG. 1  comprises a glenoid implant  1  and a humeral implant  2 , respectively secured to the glenoid cavity  3  of a scapula  4  and in the medullar canal  5  of a humerus  6 . 
         [0030]    The glenoid implant  1  comprises both a metaglenoid element  7  formed by a plate  13  ( FIGS. 2 to 6 ) provided with a plurality of screws  8  for anchoring it in the glenoid cavity  3 , two of which screws can be seen in  FIG. 1 , and also a glenoid sphere  9  adapted to be fitted over the metaglenoid element  7 . 
         [0031]    The glenoid sphere  9  presents a joint surface  11  which is convex in this embodiment, for co-operating with a complementary surface of a cup  12  of the associated humeral prosthesis  2 . In this embodiment, the cup  12  has a concave surface complementary to the convex surface  11 . 
         [0032]    The plate  13  is preferably conical, being circularly symmetrical about an axis X-X and being fitted with a central tubular adapter or bushing  14  that is preferably integrally formed with the plate  13 . The central bushing  14  projects from the plate  13  and has circular ribs  15  to anchor in the bone of the metaglenoid element  7 . The bushing  14  presents an axial bore  16  that has tapping  17  over a fraction of its length. The tapping  17  is extended by a bore  49  constituting a smooth zone. An opening  16   a  communicates with the bore  16  and a countersink  16   b  communicates with the opening  16   a  and the bore  16 , as seen in  FIGS. 5 and 6 . 
         [0033]    The plate  13  is pierced by four holes for passing the glenoid anchor screws of the metaglenoid element  7 . Two of these holes  18  are diametrically opposite and present tapping  19 . Their axes diverge symmetrically about the axis X-X. The other two holes  21  are diametrically opposite and are offset from the holes  18  by angular intervals that are preferably equal to 90°. The non-tapped holes  21  are defined by walls  22  constituting concave bearing seats for complementary convex surfaces  23  defining the heads  24  of the bone-anchor screws  25  for anchoring the metaglenoid element  7  in the glenoid cavity  3  ( FIG. 3 ). 
         [0034]    The seats  22  are oriented in such a manner that once the screws  25  have been put into place thereon they are disposed so as to diverge relative to the central bushing  14  and symmetrically thereabout. Sockets  26 , e.g. hexagonal sockets, are formed in the heads  24  to enable the screws  25  to be tightened by means of a corresponding tool (not shown). 
         [0035]    The holes  18  can receive diverging screws  27  each constituted by a head  28  and a threaded shank  29 . The head  28  has a thread  31  suitable for screwing into the tapping  19 , and an end collar  32  formed around the head  28 . The collar  32  projects radially relative to the tapping  31  and can be brought to bear against a corresponding angular shoulder  33  arranged around the edge of the hole  18  in which the screw  27  is inserted. 
         [0036]    The collar  32  constitutes abutment means for the screw  27  in the glenoid cavity  3  at the end of screw tightening, preventing the screw  27  from being moved in further as soon as the collar  32  comes into abutment on the associated shoulder  33 . Similarly, the concave seats  22  act relative to the associated screws  25  as abutments for stopping them in the metaglenoid element  7  once they have been screwed into the glenoid cavity  3 , as soon as the screws  25  have been engaged far enough for the surfaces  23  of the heads  24  to come into contact with the seats  22 . 
         [0037]    This arrangement prevents the screws  25  and  27  from passing through the metaglenoid element  7  at the end of screw tightening. 
         [0038]    The glenoid sphere  9  is made of a substantially hemispherical part whose spherical surface  11  is complementary to the corresponding joint surface of the cup  12 . The glenoid sphere  9  has arranged therein a conical recess  34  that is circularly symmetrical about the axis X-X, and that is dimensioned in such a manner as to enable the plate  13  to be inserted therein. The glenoid sphere, which is circularly symmetrical about the axis X-X, is then fitted over the metaglenoid element  7 . 
         [0039]    The glenoid sphere  9  is provided with an axial screw  35  that is free to rotate relative to the glenoid sphere  9 . The shank  36  of the screw  35  presents a thread  10  over a fraction of its length and it can be screwed into the tapping  17  of the bushing  14 . The screw  35  has a head  37  suitable for being inserted in a housing  38  that is circularly symmetrical about the axis X-X and that opens out into the polar cap of the surface  11  via an axial orifice  39 . This orifice allows the end of a tool for tightening the screw  35  to be inserted, which tool can be received in a socket  41  of complementary shape. 
         [0040]    After the head  37  has been inserted in the housing  38 , the screw  35  is held in place by a threaded washer  42  placed in a tapped housing  43  that is coaxial with the housing  38  about the axis X-X. 
         [0041]    A longitudinal axial channel  45  is arranged in the screw  35  opening out at both ends of the screw, i.e. at one end in the socket  41  and in the orifice  39 . The prosthesis assembly also comprises means for guiding the glenoid sphere  9 , and adapted to make it easier to install on the metaglenoid element  7 . In the embodiment shown, these guide means comprise a pin  46  that preferably presents some flexibility, being adapted to be slid initially into the axial bore  16  of the bushing  14  and secondly through the longitudinal channel  45  of the screw  35  ( FIG. 6 ). The pin  46  is preferably made of metal, and its diameter is advantageously slightly smaller than the diameter of the channel  45  in the screws  35 . 
         [0042]    The screw  35  has a smooth free end  48  whose tip is chamfered so as to facilitate insertion into the inlet of the tubular bushing  14 . The guide means for the screw  35  further comprise the bore constituting a smooth cylindrical zone  49  arranged in the inlet of the bushing  14  and followed by the tapping  17 . The chamfered tip  51  preferably has an annular surface  52  whose longitudinal section is rectilinear (i.e. a conical surface) and which is followed to the free end of the tip  51  by a rounded annular surface  53 , preferably constituted by a spherical fillet. At its end remote therefrom, the conical section  52  joins a smooth zone  50  followed by the thread  10 . 
         [0043]    The pin  46  is used by the surgeon as follows. 
         [0044]    The metaglenoid element  7  is initially anchored in the glenoid cavity  3  by impacting the bushing  14  and by tightening the screws  25 ,  27 . Thereafter, the surgeon inserts one end of the pin  46  in the bore  16  of the bushing  14 . Using a screwdriver that has a cannula, the surgeon then presents the glenoid sphere  9  to the inlet of the well formed in the tissues and the muscles of the shoulder, with the bottom of the well being constituted by the metaglenoid element  7 , the end of the pin  46  being threaded into the axial channel  45 . The surgeon then causes the glenoid sphere  9  to slide along the pin  46  until the free end  48  of the screw  35  engages in the bore constituting a smooth zone  49  ( FIG. 6 ). The chamfered and rounded tip  51  makes it easier to insert the screw  35  into the bushing  14  without damaging the tapping  17  because of the bore constituting a smooth zone  49 , and without damaging the thread  10  because of the smooth zone  50 . As soon as the screw  35  is properly on the axis X-X of the bushing  14 , the surgeon tightens the screw  35  until the glenoid sphere  9  becomes impacted on the metaglenoid element  7  ( FIGS. 3 and 4 ) and then withdraws the screwdriver and the pin. 
         [0045]    The arrangement of the rounded fillet  53  of the conical zone  52 , of the smooth zone  50 , and of the bore constituting a smooth zone  49  of the bushing  14  can be arranged to provide proper guidance for the screw  35  in the bushing  14 , either in combination with a guide pin  46  ( FIGS. 5 and 6 ) or without one. Under such circumstances, the screw  35  can have the axial channel  45  or it can be solid. Nevertheless, the combination of the bore  16 , the pin  46 , the end  48  with its smooth zone  50 , and the bore constituting a smooth zone  49  of the bushing  14  provides the most satisfactory guidance for the glenoid sphere  9  up to its proper position on the axis X-X of the metaglenoid element  7 . 
         [0046]    In other words, the following variants are possible:
       the screw  35  can have a cannula, i.e. can present the axial channel  45  and can be used together with the guide pin  46  without any offset between the thread of the screw  35  and the tapping of the bushing  14 : under such circumstances, the thread  10  extends almost to the free end of the screw  35 , and the bore constituting a smooth zone  49  is replaced by additional tapping  17 ;   the screw  35  does not have a cannula, and therefore does not present an axial channel  45 , but the thread  10  and the tapping  17  are offset as shown in the drawings, i.e. a smooth zone  50  is provided on the screw  35  and a bore constituting a smooth inlet zone  49  is arranged in the bushing  14 ; and   the screw  35  has a cannula and in addition the thread  10  and the tapping  17  are offset, i.e. the smooth zone  50  and the tip  51  are arranged on the end  48  of the screw, while a bore constituting a smooth zone  49  is arranged in the inlet of the bore  16  in the bushing  14 .       
 
         [0050]    The end  48  of the screw  35  can have a different profile, for example it could have a simple conical chamfer such as  52 , or it could have no more than a rounded end fillet of appropriate size. 
         [0051]    The indications for the above-described glenoid prosthesis  1  are essentially as follows: traumatisms, arthrosis between the humerus and the scapula, rheumatoid polyarthritis. In general, the shoulder prosthesis of the invention is indicated for pathologies of the rotator cuff.