Abstract:
A shoulder prosthesis comprising a socket and a condyle that cooperates with the socket. The condyle also cooperates with a base plate facing the scapula similar to a clamping connection. The condyle is plugged onto the base plate by means of a receptacle that is embodied on the condyle. The base plate can be anchored to the glenoid fossa of the scapula with the aid of two peg-type protrusions.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a shoulder prosthesis having a condyle and an articulation socket which co-operates therewith. In particular, the present invention relates to a so-called inverse shoulder prosthesis. 
     Shoulder prostheses of the generic type are generally constructed in such a manner that an articulation socket which co-operates with a condyle is secured to an upper arm bone, the so-called humerus, which co-operates with a condyle which is secured in an anatomical glenoid cavity (Cavitas Glenoidalis) of a shoulder blade (scapula). In contrast to the anatomical shoulder joint, in which the ball-like humerus head of the upper arm bone forms a ball and socket joint with the glenoid cavity of the shoulder blade, the arrangement between the condyle and the articulation socket is therefore transposed in this shoulder prosthesis and accordingly constitutes an inverse shoulder joint. 
     In these inverse shoulder prostheses, there is the problem that, when the upper arm is moved towards the upper body (adduction), the articulation socket which is fixed to the humerus slides along the condyle with, if the upper arm is adducted to a very high degree, a portion of the articulation socket protruding beyond an edge of the condyle in such a manner that it abuts against a lower edge of the Cavitas Glenoidalis of the shoulder blade. This may first of all cause occurrences of wear and damage to the bone material. However, a far more significant problem is that there is also the risk that the edge of the articulation socket will at a later stage collide with screws which are used to anchor the condyle in the shoulder blade. This may ultimately lead to these screws becoming damaged or even broken so that secure anchoring of the condyle in the shoulder blade is no longer ensured. 
     In order to avoid this effect of so-called inferior notching, attempts have already been made to arrange the condyle in an offset manner on the Cavitas Glenoidalis in the direction towards a lower corner of the shoulder blade, the so-called Angulus Inferior. In this regard, EP 1 607 070 A1, for example, discloses a shoulder prosthesis in which the condyle is fitted to a base plate which is in turn anchored to the shoulder blade by means of a pin-like protrusion. The protrusion in this instance is arranged on the base plate centrally relative to the center axis of the base plate at the shoulder blade side, the condyle having, for the base plate, a receptacle which is orientated eccentrically relative to the center axis thereof. Although it is possible to enlarge the movement clearance in which a collision of the articulation socket with the bone material is prevented owing to the resultant offset arrangement of the condyle, the problem of inferior notching has not been completely overcome thereby. In this solution too, if additional screws are used to anchor the base plate to the shoulder blade in addition to the pin-like protrusion, there is the danger that they will become damaged over time. 
     Another problem with the shoulder prosthesis described in EP 1 607 070 A1 mentioned above is that a reliable arrangement of the condyle with the desired orientation can be achieved only with difficulty on the shoulder blade. The conventional procedure during the operation involves a doctor first forming in the shoulder blade a hole which corresponds to the pin-like protrusion and into which the base plate is then introduced and anchored with the pin. It is not possible in this instance for the doctor to determine a preferred rotational orientation of the base plate, for which reason there is a danger that the base plate will be arranged on the shoulder blade with poor orientation. Ultimately, this leads to the shoulder prosthesis not being able to be inserted with the desired level of precision. 
     GENERAL DESCRIPTION OF THE INVENTION 
     Accordingly, the problem addressed by the present invention is to avoid the disadvantages of the previously known shoulder prosthesis. In particular, a new type of shoulder prosthesis is intended to be provided which has a particularly good and reliable implantation technique and force transmission at the anchoring interfaces with the anatomical shoulder blade in order to enable increased service-life of the material and more precise and improved reliability in terms of anchoring. 
     The problem is solved by a shoulder prosthesis having an articulation socket and a condyle which co-operates therewith and which co-operates with a scapula-side base plate in the manner of a clamping connection, the condyle being fitted to the base plate by means of a receptacle which is formed thereon and which can be anchored to the Cavitas Glenoidalis of the scapula by means of two pin-like protrusions which are arranged on the base plate. 
     In principle, the present invention is based on the notion of anchoring the base plate, which is provided to secure the condyle to the shoulder blade, to the Cavitas Glenoidalis of the scapula using two pin-like protrusions. In contrast to the solution known from EP 1 607 070 A1, in which the base plate is secured using only one protrusion, the solution according to the invention affords clear advantages with regard to the procedure for securing the shoulder prosthesis and with respect to the anchoring which can be achieved in this case. As will be explained in greater detail below, the use of two protrusions according to the invention enables damage caused to additional anchoring elements by the inferior notching described above to be prevented. Furthermore, the use of the two projections also allows anchoring elements which are additionally used to be arranged and orientated in such a manner that they can engage in an optimum manner in the bone material of the shoulder blade. This therefore again allows further improved anchoring of the shoulder prosthesis to the shoulder blade. Finally, by means of the two protrusions, together with the pilot holes which are defined beforehand and which are formed for the two protrusions, a correct rotational orientation of the base plate on the shoulder blade is ensured, for which reason it is far simpler for a doctor to secure the shoulder prosthesis correctly in the desired manner when fitting the shoulder prosthesis. 
     According to the invention, a shoulder prosthesis is therefore proposed with an articulation socket and a condyle which co-operates therewith, the condyle co-operating with a scapula-side base plate in the manner of a clamping connection, and the condyle further being fitted to the base plate by means of a receptacle which is formed therein and which can be anchored to the Cavitas Glenoidalis of the scapula by means of two pin-like protrusions which are arranged on the base plate. 
     Preferred embodiments of the invention relate in particular to the configuration of the base plate. 
     For instance, firstly in accordance with a particularly preferred configuration of the present invention, there is provision for the base plate to be able to be secured on the Cavitas Glenoidalis an offset towards an Angulus Inferior of the scapula. To this end, the base plate has in particular a dome-like base face which is provided for abutment against the Cavitas Glenoidalis of the Scapula, the center of the dome-like shape being arranged eccentrically relative to the center axis of the base plate. The center of this dome-like shape is preferably arranged on a straight connecting line between the two protrusions and in particular displaced in the opposite direction to the Angulus Inferior of the scapula. 
     The receptacle of the condyle for the base plate is in contrast preferably formed centrally relative to the center axis thereof. Together with the above-mentioned feature of the eccentric configuration of the base face of the base plate, this enables the desired offset arrangement of the condyle on the shoulder blade to be achieved in a simple manner. The particular nature of this consists in the fact that the contact face of the Cavitas Glenoidalis can be constructed in a simple manner so as to be shaped in the manner of a spherical portion in order to fit together with the dome-like base face. This also contributes to simplified assembly of the condyle on the shoulder blade, since the required processing of the Cavitas Glenoidalis can be carried out by the surgeon in a simple manner. 
     As already mentioned above, the use of the two protrusions in particular advantageously allows additional anchoring elements to be used to secure the base plate to the shoulder blade. According to another preferred embodiment, there is accordingly provision for the base plate to be able to be additionally anchored to the Cavitas Glenoidalis by means of anchoring elements which can be received in sleeves provided on the base plate. At least one of the two sleeves is arranged to the side of the straight connecting line between the two protrusions. A sleeve for receiving an anchoring element is preferably arranged at both sides of the straight connecting line between the two protrusions. A particular advantage of this configuration is that, owing to the use of the two protrusions, the two anchoring elements, which are preferably screws, can be offset further towards the center of the base plate. This ensures that the anchoring elements can penetrate reliably and securely into the bone material of the shoulder blade. In this manner, the securing of the shoulder prosthesis is therefore again further optimized. 
     The sleeves may have, at a side of the base plate facing away from the scapula, recesses for heads which are provided on the anchoring elements. At least one of the sleeves may be constructed in such a manner that the corresponding anchoring element is spread apart in the manner of a finger of an open hand after the final assembly of the shoulder prosthesis, whereby the anchoring of the prosthesis in the shoulder blade is again improved. This sleeve for angularly stable support of an anchoring element preferably has a screw thread for forming a screw connection with the associated anchoring element. The additional sleeves are in contrast preferably constructed in such a manner that the associated anchoring elements are orientated substantially parallel with the protrusions, but with a slight clearance being provided in the alignment of these anchoring elements in order to facilitate assembly. 
     A receptacle side face on the condyle and an outer face of the base plate that co-operates therewith are preferably constructed so as to be circular in order to form the clamping connection. In order to allow the condyle to be arranged on the base plate in a manner which is secure in terms of rotation, the condyle preferably has, at a side facing away from the Angulus Inferior of the scapula, a groove which co-operates with a tongue or protuberance which is formed on the base plate. There may in particular also be provision for an additional sleeve for receiving an additional anchoring element to be provided in the region of this tongue or protuberance on the base plate. The receptacle on the condyle is preferably sized in such a manner that the scapula-side base face of the base plate and a scapula-side edge face of the condyle adjoin each other in a substantially flush manner. 
     Seen as a whole, owing to these configurations a shoulder prosthesis is obtained which can be arranged and anchored on the shoulder blade in a simple but particularly reliable manner. Furthermore, damage to the elements required for anchoring on the shoulder blade can be eliminated to the greatest possible extent. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is to be explained in greater detail below with reference to the appended drawings, in which: 
         FIG. 1  is a first view of a shoulder prosthesis according to the invention having an articulation socket and a condyle and a base plate which is connected to the condyle with the corresponding anchoring elements; 
         FIG. 2  is another view of the shoulder prosthesis according to the invention; 
         FIGS. 3 and 4  are two exploded views of the shoulder prosthesis according to the invention; 
         FIG. 5  is a view of the base plate with the two protrusions and the anchoring elements; 
         FIG. 6  is a view of the base plate without the anchoring elements; 
         FIG. 7  is a plan view of the base face of the base plate; 
         FIGS. 8 and 9  are sections through the illustration of the base plate according to  FIG. 7 ; 
         FIG. 10  is a view of the condyle connected to the base plate; 
         FIG. 11  is a plan view of the combination comprising the base plate and condyle; 
         FIGS. 12 and 13  are two sections through the illustration according to  FIG. 11 ; 
         FIG. 14  is a view of an anatomical scapula (shoulder blade); 
         FIG. 15  is a sectioned view of the condyle which is secured to the scapula; 
         FIG. 16  is another sectioned view of the shoulder prosthesis according to the invention mounted on the shoulder blade; and 
         FIGS. 17 to 19  are views of an alternative configuration of a shoulder prosthesis according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 to 4  illustrate different views of the shoulder prosthesis according to the invention, generally designated  2 .  FIGS. 1 and 2  show the shoulder prosthesis  2  in the joined-together state, whilst  FIGS. 3 and 4  are exploded views. 
     Significant integral components of the shoulder prosthesis  2  are an articulation socket  4  and a condyle  6  which co-operate in the manner of a ball and socket joint. The condyle  6  is fitted to a base plate  8 . The articulation socket  4  is in turn connected, by means of a coupling piece  10 , to a rod  12  which is recessed in the upper arm (humerus) by means of a humerus head which is not illustrated in  FIGS. 1 to 4 . The articulation socket  4  is arranged so as to be able to be pivoted in and counter to the directions indicated with the directional arrows  14  and  16  on the surface of the condyle  6 , the so-called glenosphere.  FIG. 1  shows the articulation socket  4  in a position in which the upper arm is adducted, but not adducted to such an extent that there would be any risk of damage to the bone material. 
     In order to anchor the shoulder prosthesis  2  according to the invention to a shoulder blade not illustrated in  FIGS. 1 to 4 , the so-called scapula, the base plate  8  has various elements which are introduced into the corresponding bone material of the shoulder blade. 
     These are firstly two pin-like protrusions  18  which are integrally formed with the base plate  8  and which are arranged substantially at right-angles to the base face  22  of the base plate  8 . These pins  18  are constructed in the manner of a peg and have ribs  24  on their periphery. When assembling the shoulder prosthesis  2  on the shoulder blade, these pins  18  are recessed into openings which are formed during an operation and clamped by means of press-fitting. 
     Three screws  26 ,  28  and  30  constitute additional anchoring elements which are received in sleeves  36 ,  38  and  40  formed on the base plate  8 . After the base plate  8  has been anchored using the two protrusions  18 , the anchoring screws are screwed into the bone material of the shoulder blade in order to provide additional anchoring. They are guided on the base plate  8  in such a manner that the screw  26  is splayed slightly towards the upper side and the two additional screws  28  and  30  are orientated substantially parallel to the protrusions  18 . These two screws  28  and  30  are primarily used to screw the base plate  8  securely to the shoulder blade initially. Subsequently, the screw  26 , which is guided in an angularly stable manner, is introduced and the base plate  8  is secured to the bone, whereby particularly reliable anchoring of the base plate  8  and consequently the entire prosthesis  2  to the shoulder blade is brought about. 
     The condyle  6  comprises, for assembly with the base plate  8 , a receptacle  42  (see  FIG. 3 ) whose size for forming a clamping connection  44  between the condyle  6  and the base plate  8 . The receptacle  42  is formed on the condyle centrally relative to the center axis  46  of the condyle  6 . In order to form the clamping connection  44  between the condyle  6  and the base plate  8 , an outer face  50  of the base plate  8  accordingly co-operates with a receptacle side face  52 . The outer face  50  and the receptacle side face  52  are constructed so as to be circular in such a manner that a condyle wall  54  delimited by the edge face  48  has a consistent wall thickness in the region of the edge face  48  of the glenosphere. This is the consequence of the central receptacle for the base plate  8  and results in particularly uniform force transmission being ensured between the base plate  8  and the condyle  6 . In this context, it should be noted that the connection between the base plate  8  and condyle  6  could also be produced in another manner by means of elements which co-operate in a positive-locking and/or non-positive-locking manner. Another example of this will be explained below. In order to produce the two elements, it is possible to use all the materials which are conventional in prosthetics, it also being completely possible to use a combination of elements of different materials. 
     As will be explained in greater detail below, the base face  22  of the base plate  8  has a slight dome-like shape, the center of this dome-like shape being offset relative to the center axis  45  of the base plate  8  and consequently also relative to the center axis  46  of the condyle. This affords particular advantages with respect to the assembly of the shoulder prosthesis  2  on the shoulder blade, which will also be explained in greater detail below. 
     It should further be noted that the receptacle  42  of the condyle  6  is sized in such a manner that the base plate  8  almost completely disappears in the receptacle  42  when the condyle  6  is fitted to the base plate  8 . In particular, the edge face  48  of the condyle  6  adjoins the base face  22  of the base plate  8  in a practically flush manner. 
     In order to prevent as far as possible the bone material of the shoulder blade from becoming damaged when the arm is adducted to a high degree, the base plate  8  is intended to be fixed in an offset manner on the Cavitas Glenoidalis (not illustrated in  FIGS. 1 to 4 ) counter to the direction of the arrow  14 . This means that the base plate  8  having a lower portion  56  cannot be supported on the Cavitas Glenoidalis with the base face  22 . To this end, it is desirable to increase the anchoring stability of the base plate  8 , which is achieved by the sleeve  36  for receiving one of the anchoring screws  26  being formed in an upper portion  58  of the base plate  8 . In order to form this sleeve  36  on the base plate on the upper portion  58  of the base plate in a manner which is secure in terms of detachment, the base plate has a protuberance or a tongue  60  which first provides the necessary space for the sleeve  36 . Furthermore, this tongue  60  co-operates with a groove  62  which is provided on the condyle, in order to form a tongue-and-groove connection  64 , which allows the condyle  6  to be fitted to the base plate  8  in a correct manner. Further, by means of this tongue-and-groove connection  64 , twisting of the condyle  6  on the base plate  8  owing to forces acting radially on the condyle  6  is also prevented. 
     Another element that serves to fit the condyle  6  to the base plate  8  in the correct position is a centering pin  87  which protrudes from the inner side of the condyle  6  and engages in an opening  82  of the base plate  8 . This opening  82  on the base plate  8  extends into the lower protrusion  18  so that sufficient length is provided to receive the centering pin  87 . These are optional auxiliary means which facilitate the fitting of the condyle  6  to the base plate  8  which is secured to the bone. 
     Optionally, it would also be possible to dispense with these auxiliary means and it would also be conceivable to use alternative auxiliary means. One possible variant is illustrated by way of example in  FIGS. 17 to 19 . 
       FIG. 5  is a plan view of the base plate  8  in a state in which the anchoring screws  26 ,  28 ,  30  have also been introduced into the corresponding sleeves  36 ,  38 ,  40 . The corresponding openings for introducing the screws  26 ,  28 , can be seen in the exploded illustration of  FIG. 4 , in which the rear side  23  of the base plate  8  is shown. In addition to the three openings for the anchoring screws  26 ,  28 ,  30 , the opening  82  for receiving the centering pin  87  is also shown. 
     With reference to  FIGS. 6 to 9 , the structure of the base plate  8  is to be described in greater detail below. It is particularly significant for the base face  22  of the base plate  8  to have the dome-like shape mentioned above. Furthermore, it should be emphasized in this context that the center  47  of the dome-like shape does not coincide with the geometric center or the center axis  45  of the substantially circular base plate  8 . Instead, the center  47  of the dome-like shape is displaced by a distance L slightly in the direction towards the sleeve  36  for the upper anchoring screw  26 . In this direction therefore the base face  22  of the base plate  8  has a degree of eccentricity. The center  47  of the dome-like shape is located on the straight connecting line  49  between the two protrusions  18  or on the axis of symmetry  51  which extends through the protrusions  18  and the tongue  60 , as can be seen, on the one hand, from the illustration of  FIG. 7  and, on the other hand, from the sectioned illustration of  FIG. 9 . 
     It can also be seen from the illustration of  FIG. 7  that the sleeves  38  and  40  for the two additional anchoring screws  28  and  30  are arranged symmetrically relative to the axis of symmetry  51 . In particular, the sleeves  38 ,  40  are located at both sides of the axis of symmetry  51  in the region between the two protrusions  18 , respectively. Since neither of the two protrusions  18  is formed directly in the center of the base plate  8 , it is accordingly possible to move the receiving sleeves  38 ,  40  closer to the axis of symmetry  51  which—as will be explained in greater detail below—affords particular advantages when anchoring the base plate  8  to the bone material of the shoulder blade. 
     It should further be emphasized that the sleeve  36  which, as mentioned above, is provided for angularly stable arrangement of the screw  26 , has a screw thread  66 . Using this thread  66 , the screw  26 , which is provided to secure the base plate  8  to the bone, can co-operate with the sleeve  36  by means of a screw connection. To this end, the sleeve  36  is further provided with a recess  68  for receiving the corresponding screw head. 
     The two additional sleeves  38 ,  40  are, however, constructed differently from the sleeve  36  since they are provided for retaining the screws  28  and  30 , which perform a different function. These screws  28  and  30  are primarily used first to screw the base plate  8  securely to the shoulder blade before the splayed connection is brought about using the screw  26 . The sleeves  38  and  40  are therefore orientated substantially parallel with the protrusions  18 , have no threads and are further formed in the manner of a spherical portion at the upper region thereof. If screws with a spherical head are used as anchoring elements, they can be pivoted within a small angular range. This slight clearance allows the screws to be introduced into the shoulder blade in a more simple manner, thereby facilitating the assembly of the prosthesis. 
     This configuration of the sleeves  38  and  40  can in particular also be seen in  FIGS. 10 to 13 , which illustrate the co-operation of the base plate  8  with the condyle  6  and which are to be explained below. 
     The condyle accordingly comprises the receptacle  42  which—as can be seen in particular from the illustration of FIG.  13 —is formed on the condyle centrally relative to the center axis  46  thereof. With the exception of the tongue-and-groove connection  64 , the peripheral wall of the condyle  6  accordingly has a uniform thickness, whereby particularly secure retention of the condyle  6  on the base plate  8  is enabled. The clamping connection  44  between the receptacle side face  52  of the condyle  6  and the outer face  50  of the base plate  8  contributes to this when the condyle  6  is fitted to the base plate  8 . 
     In particular from the illustration in  FIG. 12  it is possible again to see the eccentric configuration of the base face  22  in the base plate  8 . It can clearly be seen that the center  47  of the protruding base face  22  is displaced slightly relative to the center axis  45  of the base plate  8  in the longitudinal direction towards the tongue-and-groove connection  64 . However, a lateral deviation relative to the axis of symmetry is not present according to the illustration of  FIG. 13 , that is to say, the center of the dome-like shape coincides in this direction with the center axes  45  and  46  of the base plate  8  and the condyle  6 . 
     From the illustration of  FIG. 12 , it is further possible to see the bore  82  of the base plate  8 , which bore has an open end  84  at the condyle-side and extends at the scapula-side into the corresponding protrusion  18 . In a continuation of this bore  82 , a corresponding recess  86  is also formed in the condyle  6  and serves to receive the centering pin  87  illustrated in  FIGS. 1 to 4 , for centering the condyle  6  when it is fitted to the base plate  8 . Together with the tongue-and-groove connection  64 , an arrangement of the condyle  6  on the base plate  8  is thereby enabled which is secure in terms of rotation and the fitting of the condyle  6  to the pre-assembled base plate  8  is facilitated. 
     Another specific feature which can be seen in particular from  FIGS. 10 to 12  is that the outer face of the condyle  6  is pulled slightly in a downward direction in a region opposite the tongue-and-groove connection  64 . This continuation of the outer face in the form of a lip  70 , when the upper arm is adducted, results in an additional or extended guide being provided for the articulation socket  4 . Another advantage of this configuration consists in that the contact face between the surface of the condyle  6  and the articulation socket  4  is increased when the arm is abducted so that ultimately a more uniform transmission of force can occur. In particular, the occurrence of punctual or linear contacts, which would lead to increased wear of the material, is reduced. In this context, it should also be mentioned that the condyle  6  forms a closed surface over its entire outer side. In particular, there are no openings or the like, which could potentially lead to punctual or linear contacts with the articulation socket  4 , which affords clear advantages with respect to any potential wear of the material. 
       FIG. 14  is a plan view of an anatomical scapula  88 , the shoulder blade, whose basic shape is that of a triangle. The lower corner of the triangle is referred to as the Angulus Inferior  90 , which a right-hand edge “Margo Lateralis  92 ” and a left-hand edge “Margo Medialis  94 ” adjoin. The Margo Lateralis  92  and the Margo Medialis  94  delimit the upper edge “Margo Superior  96 ” at a right-hand corner “Angulus Lateralis  98 ” and at a left-hand corner “Angulus Superior  100 ”. At the Angulus Lateralis  98 , an anatomical articulation socket “Cavitas Glenoidalis  102 ” is positioned and has a coracoid process “Processus Coracoideus  104 ” which is curved forwards and outwards. 
       FIGS. 15 and 16  respectively illustrate the condyle  6  secured to the shoulder blade using the base plate  8 , and the shoulder prosthesis  2  completely secured to the shoulder joint. The articulation socket  4  which is secured to the rod  12  by means of the coupling piece  10  is fixed to the upper arm bone (humerus) which is not illustrated. The base plate  8  is in turn anchored to the Cavitas Glenoidalis  102  of the scapula  88  with the protrusions  18  and the anchoring screws  26 ,  28  and  30 . The condyle  6  is fitted to the base plate  8  by means of the receptacle  42 . The receptacle  42  is formed on the condyle centrally around the center axis  46  of the condyle  6  and laterally delimited by a receptacle side face  52  which extends in a circular manner around the center axis  46 . 
     The base plate  8  is in turn anchored to the Cavitas Glenoidalis  102  by means of the protrusions  18  and the anchoring screws  26 ,  28 ,  30 . In this instance, it is particularly significant—as can be seen from the illustration of  FIG. 15  in particular—for the base plate  8  to be arranged on the Cavitas Glenoidalis  102  in an offset state in the direction towards the Angulus Inferior. This prevents to the greatest possible extent abutment of the edge  106  of the articulation socket when the arm is abducted towards the upper body. Even when the upper arm is adducted to an extremely high degree, as illustrated in  FIG. 16 , it is not possible for the articulation socket edge  106  to abut the anchoring elements. 
     This desired offset arrangement of the base plate  8  is particularly promoted by the fact that the center  47  of the dome shape of the base face  22  of the base plate  8  is formed eccentrically relative to the center axis  45  of the base plate  8 . In particular, this center  47  is displaced in the opposite direction to the Angulus Inferior  90  of the scapula  88 . This particular configuration results in the surface of the Cavitas Glenoidalis  102  being able to have a spherical-portion-shaped recess which is particularly preferred for assembly of the shoulder prosthesis  2  since, owing to the offset arrangement, the center of the support region of the base face  22  now coincides with the center  47  of the dome shape. The processing of the Cavitas Glenoidalis  102  which is required prior to assembly of the shoulder prosthesis  2  is therefore relatively simple for a surgeon to carry out, uniform abutment of the base face  22  on the Cavitas Glenoidalis  102  over the entire contact face region nonetheless being achievable. It is significant for the shape of the base plate  8  to be adapted in an optimum manner to the specific bone conditions, whereby simple but also particularly reliable securing of the prosthesis  2  to the shoulder blade is enabled. This is a significant advantage compared with solutions according to the prior art in which the base face of the base plate was, for example, planar and the bone accordingly had to be highly processed and manipulated in order to allow the prosthesis to be secured. 
     The use of the two protrusions  18  on the base plate  8  according to the invention also further allows the use of an anchoring screw which is directed downwards to be dispensed with. Even if the arm—as illustrated in FIG.  16 —is adducted to an extremely high degree and accordingly, over a period of time, wear of the bone material  88  of the shoulder blade occurs at the corresponding contact location, it is not possible at any time for the articulation socket edge  106  to collide with the protrusions  18  and/or one of the anchoring screws  26 ,  28 ,  30 . The risk of one of the screws  26 ,  28 ,  30  becoming damaged over time and accordingly secure anchoring of the condyle  6  on the shoulder blade no longer being ensured is prevented in this manner. 
     The use of the two protrusions  18  also further allows the slightly more central arrangement of the two central anchoring screws  28 ,  30 . Since the bone material of the shoulder blade is relatively thin at the Cavitas Glenoidalis  102 , it is thus nonetheless ensured that the screws  28 ,  30  are securely introduced into the bone material and can penetrate therein over a sufficient length. Therefore, the anchoring of the condyle  6  is thereby again further improved. 
     Finally, the two protrusions  18  also allow the base plate  18  to be arranged on the previously processed Cavitas Glenoidalis  102  in the correct position in a manner which is secure in terms of rotation. Prior to the final assembly of the shoulder prosthesis  2 , the corresponding holes for the protrusions  18  are generally marked using a template or positioning gauge, predrilled and then formed in the bone material. When a single protrusion  18  is used according to the previous solutions, it was not possible to exclude, during the subsequent assembly of the base plate  8 , twisting thereof relative to the anterior/posterior and inferior/superior rotational orientation defined beforehand, which ultimately led to incorrect positioning or incorrect orientation of the screws and consequently the whole condyle  6 . This resulted in the anchoring screws often protruding into regions in which there was no bone material. This problem does not occur when two protrusions  18  are used according to the invention since the two openings for the protrusions define a rotational orientation for the base plate  8  so that, with a relatively low level of complexity, it is possible to fix the shoulder prosthesis to the shoulder blade in a precise and secure manner. 
     The use of two protrusions on the base plate therefore also in principle affords advantages even when the base face does not have the eccentricity described above. Although this specific configuration affords additional advantages, the invention generally relates to inverse shoulder prostheses—for example, also to conventional configurations in which the base face of the base plate is constructed in a planar manner or in which the base face of the base plate is constructed in a dome-like manner, but without any eccentricity of the center point of the dome relative to the center axis of the base plate. 
     An alternative configuration for the base plate  8  and the condyle  6  is illustrated, for example, in  FIGS. 17 to 19 , elements of these two components which are identical to the components of the base plate  8  and condyle  6  described above being given the same reference numerals. Only the differences in this variant with respect to the first embodiment are to be referred to below. 
     The significant difference of this second variant involves the configuration of the contact region between the base plate  8  and the condyle  6 . At the side of the base plate  8  facing the condyle  6 , there is provided an indentation or circular recess  118  in which a corresponding cylinder-like protrusion  116  engages at the lower side of the condyle  6 . The absorption of radial forces in the joined-together state is again improved by this engagement. 
     A further difference consists in that a central hole  120  extends through the condyle  6  and merges into a corresponding bore  122  in the base plate  8 . The two bores  120 ,  122  can be used to receive a guiding or centering pin (not illustrated) which facilitates the arrangement of the condyle  6  on the base plate  8 . 
     With regard to the additional configurations, the second variant corresponds to the first embodiment. Again, therefore, the base face  22  of the base plate  8  is constructed with two protrusions  18  and is provided with an eccentric dome-like shape. However, as already mentioned, the dome-shape could also be formed centrally or the base face  22  of the base plate  8  could even be planar. 
     In sum, the solution according to the invention therefore allows the condyle to be mounted on the shoulder blade in a manner which is not only simple to implement but which is also particularly reliable. Furthermore, the individual components of the shoulder prosthesis according to the invention are configured in such a manner that it is possible to prevent damage to the elements which are intended for anchoring on the shoulder blade, brought about in particular by inferior notching which can never be completely avoided.