Abstract:
An acetabular cup includes an articular insert ( 1 ) having a substantially semi-spherical articular cavity ( 1   c ), and a spherical head ( 2 ) engaged in the articular cavity ( 1   c ). A retaining device ( 10   a ) opposes the axial release ( 4 ) of the spherical head ( 2 ) from the articular cavity ( 1   c ) of the articular insert ( 1 ). For example, the retaining device can take the form of a back-tapered segment ( 10   a ) of the articular insert ( 1 ). The spherical head ( 2 ) is factory mounted in the articular insert ( 1 ), by temporarily heating the articular insert ( 1 ) and performing sterilization after assembly. In this way, the spherical head ( 2 ) retention capacity in the articular insert ( 1 ) is increased substantially in order to reduce the risk of luxation.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to a prosthetic acetabulum for replacing the natural acetabulum of the hip. 
     A total hip prosthesis comprises two parts constituting a ball-and-socket joint, namely a first part intended to be implanted in the pelvis of a patient, and a second part intended to be implanted in the femur. 
     The first part of the prosthesis generally has a stem which is intended to engage in the medullary canal of the femur, and of which the proximal end is connected by a neck to a spherical head intended to engage in the female part of the joint. 
     The second part of the prosthesis, which has to be implanted in the pelvis, and which will be designated generally by the word acetabulum, normally comprises a hemispherical insertion cup, which is placed in a prepared cotyloid cavity of the pelvic bone, and in which is placed an articular insert, which is designed to receive the spherical head. The insertion cup is commonly made of metal. The articular insert is made of a material with a low coefficient of friction, such as polyethylene or a ceramic. 
     In the known devices, a distinction can be made between single-mobility acetabulums, dual-mobility acetabulums, and acetabulums with a movable cup. 
     In the single-mobility acetabulums, the polyethylene or ceramic insert is fixed in an insertion cup and has a coaxial and substantially hemispherical articular cavity permitting the engagement and pivoting of the spherical head of the first part of the prosthesis. 
     The rotation movements of the joint then take place between the spherical head of the first part of the prosthesis and the articular cavity of the articular insert. 
     In a dual-mobility acetabulum, the articular insert is itself mounted rotatably in the insertion cup, thereby providing a first sliding surface between the insertion cup and the articular insert, and a second sliding surface between the articular insert and the spherical head. 
     In the acetabulums with a movable cup, the articular insert made of ceramic has a spherical outer surface so as to be rotatably mounted directly in the cotyloid cavity of the pelvis of a patient. Alternatively, an articular insert made of polyethylene is engaged fixedly in a metal cup which has a spherical outer surface and is itself mounted rotatably in the cotyloid cavity of the pelvis of a patient. 
     Therefore, the main problem during use of a hip prosthesis is the risk of luxation. Luxation means the release of the spherical femoral head from the articular cavity. 
     Various means have already been proposed for reducing the risks of luxation. 
     For example, dual-mobility acetabulums have the effect of slightly reducing the risk of luxation. However, this reduction is insufficient. 
     An additional means for reducing the risks of luxation has been proposed in the form of an increase in the depth of the articular cavity of the articular insert, such that the head is engaged across a spherical cap slightly larger than the hemisphere. In practice, it is then necessary to engage the head with force in the articular cavity, and this requires a specific tool of the press type. 
     Hitherto, the spherical heads of the male part of the joint have formed an interchangeable element which is attached to the end of the neck and which allows the practitioner, when implanting the prosthesis, to easily adapt the diameter of the head and the length of the neck by simply selecting a suitable head from among a series of heads with different diameters and with seats of different depths in which the neck end is engaged. The choice of head by the practitioner is necessarily made according to the specific anatomical features presented by the patient receiving the prosthesis. 
     This means that it is therefore the practitioner who then has to engage the head with force in the articular cavity of the articular acetabulum, by means of the tool of the press type. This operation of engagement with force poses considerable risks of damaging the outer surface of the head and the inner surface of the articular insert, such that the articular surfaces are then degraded, thus reducing the lifetime of the prosthesis. 
     In addition, since this is an operation carried out by the practitioner in the operating theater, using a relatively basic tool of the press type, the admissible force for engaging the head in the cavity of the articular acetabulum is necessarily limited, and this at the same time limits the retention capacity of the head in the articular insert. There is therefore still a need to increase this retention capacity. 
     The teaching of document DE 41 02 510 A1 discloses increasing the force by which the head is retained in the articular insert by providing a retaining means in the form of a continuation of the acetabulum, which increases the angle of its cavity beyond 180°, on a portion of its periphery. However, the document does not describe a solution to the resulting difficulty of introducing the head. 
     Alternatively, it has been proposed to increase the force of retention of the head in the articular insert by fitting a retaining ring in a groove formed at the entrance of the cavity of the articular insert. The ring is made of polyethylene. It can be slit or can have snap-fit means, as are described in the document WO 88/07845 A1. By virtue of its transverse slit or its snap-fit means, and since polyethylene is a relatively elastic material, the ring can be easily deformed by reducing its diameter in order to introduce it into the entrance of the cavity, and it is then relaxed such that it recovers its initial diameter and engages with its periphery in the corresponding groove of the articular insert. The ring then opposes the withdrawal of the head from the articular insert. However, there is still a risk of the retaining ring itself being ejected in the event of a force tending to withdraw the head from the articular insert, this being due to the flexibility of the ring, which is a result of the presence of the transverse slit. Consequently, the retention capacity is still insufficient. 
     SUMMARY OF THE INVENTION 
     The invention results from the observation that all the means available to the practitioner in the operating theater for fitting the head in the articular insert do not permit a sufficient retention capacity, and it appears that sterilization has a negative influence on this retention capacity. 
     The problem addressed by the present invention is that of substantially reducing the risk of luxation of a hip prosthesis by providing the acetabulum with a very much increased retention capacity, so as to eliminate any risk of luxation, by avoiding the negative influence that sterilization has on this retention capacity. 
     In order to achieve these aims, and others, the invention thus proposes a prosthetic hip acetabulum, comprising:
         an articular insert, with a receiving face in which is opened a cavity with a substantially hemispherical, concave articular surface,   a spherical head, which can be engaged in the articular cavity of the articular insert,   a retaining means for opposing the axial release of the spherical head from the articular cavity of the articular insert when it is engaged in the articular cavity,       

     in which:
         the assembly composed of articular insert and spherical head is packed in a sterile state in a protective envelope,   and, in the protective envelope, the spherical head is engaged in the articular cavity of the articular insert and is retained by the retaining means.       

     This arrangement makes it possible to assemble the acetabulum at the time of manufacture in the factory and to perform the sterilizing step after the assembly step involving fitting the spherical head in the articular insert. It has been found, according to the present invention, that the assembly step, involving forced engagement of one element in another, reduces the mechanical properties of at least one of the two elements if it is made of polyethylene or another equivalent plastic material and if it has been subjected beforehand to a sterilizing step. Thus, performing the sterilizing step after assembly avoids damaging the mechanical properties of the element and increases the retention capacity of the head in the articular insert. 
     The increase in the retention capacity has been able to be demonstrated in tests, of which the results are set out later in the description. 
     In practice, by virtue of these arrangements, the force retaining the spherical head and exerted by the retaining means can be greater than a defined force threshold, this force threshold being considerably greater than the retention force obtained by the currently known devices, for given materials and given geometries of the elements forming the prosthesis. 
     According to a first embodiment, in the articular cavity and the retaining means, the spherical head is engaged across a spherical cap larger than the hemisphere. 
     The spherical cap can advantageously extend by an angle greater than 190°. 
     In this way, the retaining means can be formed by the articular insert itself, which is made of polyethylene or another equivalent plastic material and has a retaining segment undercut in the articular surface. 
     In this case, the articular insert can also have an outer surface in the shape of a spherical cap in order to be mounted rotatably in an insertion cup and in order thereby to constitute a dual-mobility acetabulum. 
     According to another embodiment, the retaining means comprises a continuous annular ring, having a retaining surface in the shape of a spherical crown and able to engage functionally with play against the spherical head, and having peripheral fixing projections able to interlock functionally with corresponding recesses of the articular insert or with corresponding recesses of a peripheral cup surrounding the articular insert, in order to prevent withdrawal of the continuous annular ring. 
     The absence of a transverse slit in the continuous annular ring provides the ring with great stiffness, which avoids the risk of withdrawal of the ring itself in the event of the spherical head being withdrawn from the articular insert. 
     In this case, the continuous annular ring can advantageously be made of polyethylene or of another equivalent plastic material. 
     On account of the continuous nature of the annular ring constituting the retaining means in the second embodiment, and on account of the angle of more than 190° of the spherical cap across which the head is engaged in the articular cavity and the retaining means, a high retention capacity of the spherical head in the articular insert is achieved. 
     According to the invention, this retention capacity is increased still further by making available a method of assembly for producing a prosthetic hip acetabulum according to the invention, with a spherical head engaged in a cavity of an articular insert and retained by a retaining means, the method having a step of sterilizing the components of the acetabulum and an assembly step involving fitting the spherical head in the articular insert and the retaining means, in which method the sterilizing step is performed after the assembly step. 
     To further reduce or to avoid deterioration of the mechanical properties of the elements to be assembled upon engagement of the head in the articular insert and the retaining means, it is possible, during the assembly step, for the retaining means to be subjected advantageously to a differential thermal stress which temporarily modifies its dimensions in order to facilitate assembly, and then, after assembly, the retaining means is allowed to return to room temperature so as to oppose the withdrawal of the spherical head from the articular insert. 
     In this method, according to a first embodiment:
         the retaining means is the articular insert itself, which is made of polyethylene or another equivalent plastic material,   the differential thermal stress involves heating the articular insert in order to increase its internal diameter and thereby facilitate the axial introduction of the spherical head.       

     In this method, according to another embodiment:
         the retaining means is a continuous annular ring made of polyethylene or another equivalent plastic material,   the differential thermal stress involves cooling the continuous annular ring in order to reduce its internal diameter and thereby facilitate the engagement of the peripheral projections of the ring in the corresponding recesses of the articular insert or of the peripheral cup.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, features and advantages of the present invention will become clear from the following description of particular embodiments, with reference being made to the attached figures in which: 
         FIGS. 1 to 3  illustrate, in a perspective view, a front view and a lateral cross section, respectively, a prosthetic hip acetabulum according to a first embodiment of the present invention; 
         FIGS. 4 to 6  illustrate, in a perspective view, a front view and a lateral cross section, a spherical head such as the one from the embodiment in  FIGS. 1 to 3 ; 
         FIGS. 7 to 9  illustrate, in a perspective view, a front view and a lateral cross section, respectively, the articular insert of the prosthetic acetabulum from  FIGS. 1 to 3 ; 
         FIGS. 10 to 12  illustrate, in a perspective view, a front view and a lateral cross section, respectively, the assembled unit composed of articular insert and spherical head, packed in a sterile state in a protective envelope for its distribution to the place of use; 
         FIGS. 13 to 15  illustrate, in a perspective view, a front view and a lateral cross section, respectively, a prosthetic hip acetabulum according to a second embodiment of the present invention; 
         FIGS. 16 to 18  illustrate, in a perspective view, a front view and a lateral cross section, respectively, a prosthetic acetabulum of the movable cup type according to a third embodiment of the present invention; 
         FIGS. 19 to 21  illustrate, in a perspective view, a front view and a lateral cross section, respectively, a prosthetic hip acetabulum of the movable cup type according to a fourth embodiment of the present invention; and 
         FIG. 22  illustrates, in a perspective view, a continuous annular ring. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference is made to  FIGS. 1 to 12 , which illustrate the structure of a dual-mobility prosthetic acetabulum according to a first embodiment of the present invention. 
     After complete assembly, the prosthetic acetabulum is as illustrated in  FIGS. 1 to 3 . It comprises an articular insert  1 , a spherical head  2 , and an insertion cup  3 . 
     The insertion cup  3  is made of metal. It has a convex outer anchoring face  3   a  which is substantially hemispherical and is designed to be anchored in a cotyloid cavity in the pelvis of a patient. As will be seen more clearly in  FIGS. 1 and 2 , the outer anchoring face advantageously has projections  3   b  to facilitate its anchoring in the cotyloid cavity of the pelvis. The insertion cup  3  has an inner receiving face  3   c  of spherical shape which is mirror-polished to constitute an articulation surface receiving the articular insert  1 . The articular insert  1  is made of polyethylene or another equivalent plastic material. It has a spherical outer face  1   a  engaging in the inner receiving face  3   c  of the insertion cup  3  in order to permit the pivoting of the articular insert  1  in the insertion cup  3 . The articular insert  1  has a receiving face  1   b , in which is opened an articular cavity  1   c  with a substantially hemispherical concave articular surface  1   d.    
     The spherical head  2  is engaged in the articular cavity  1   c  of the articular insert  1 , in which it is retained by a retaining means that opposes its axial release under the effect of an extraction force illustrated by the arrow  4 . 
     In this embodiment, the retaining means is formed by the articular insert  1  itself, which for this purpose has an articular surface  1   d  extending by an angle A greater than 190°. On account of the value of the angle A being greater than 190°, the articular surface  1   d  has an undercut segment  10   a , which narrows toward the receiving face  1   b . This has the result that the diameter D 1  of the opening of the articular insert  1  is smaller than the diameter D 2  of the spherical head  2 , which can therefore be released from the articular cavity  1   c  only under the effect of an extraction force  4  above a high value. 
     The spherical head  2  has a spherical outer articular surface  2   a , which is thus engaged in the articular cavity  1   c  across a spherical cap  2   b  larger than the hemisphere and which extends by the angle A. 
     The spherical head  2  can be made of ceramic or of metal. Metal will be preferred for reasons of cost. The metal can be stainless steel or titanium, for example. The same applies to the insertion cup  3 . 
     The spherical head  2  has a frustoconical recess  2   c  for receiving, with force, a neck end of the first part of a hip prosthesis. 
       FIGS. 4 to 6  show more clearly the structure of the spherical head  2 , isolated from the other elements of the acetabulum. The same elements are designated by the same reference numbers as in  FIGS. 1 to 3 . 
       FIGS. 7 to 9  show more clearly the structure of the articular insert  1 , isolated from the other elements of the acetabulum. The same elements are designated by the same reference numbers as in  FIGS. 1 to 3 . 
       FIG. 9  shows more clearly the articular surface  1   d , and the portion thereof corresponding to the spherical cap  2   b , limited by a cone whose summit is at the center O of the spherical head  2  and of the articular surface  1   d  and which is indicated by the lines OX and OY. This spherical cap  2   b  is larger than the hemisphere, and it extends by the angle A or the angle at which the spherical head  2  is engaged in the articular insert in  FIG. 3 . At right angles to the intersection with the cone OX-OY, the articular surface  1   d  is continued by a cylindrical segment  1   e  of diameter D 1 , then by a beveled edge joining the receiving face  1   b.    
     According to the invention, the articular insert  1  and the spherical head  2  are factory-assembled and are then sterilized and packed in a protective envelope  5 , as illustrated in  FIGS. 10 to 12 . The insertion cup  3  is packed separately. 
     To use the acetabulum, the practitioner selects an insertion cup  3  suitable for the morphology of the cotyloid cavity formed in the patient who is to be treated, chooses an assembly of articular insert  1  and spherical head  2  whose recess  2   c  is suitable for adjusting the length of the first part of a prosthesis as a function of the anatomical features of the patient who is to be treated, places the insertion cup  3  in the cotyloid cavity formed in the pelvis of the patient, and assembles the other elements of the hip prosthesis without first having to fit the spherical head  2  with force into the articular insert  1 . 
     A second embodiment of a prosthetic hip acetabulum according to the invention, as illustrated in  FIGS. 13 to 15 , is now described. 
     This embodiment again has an articular insert  1  receiving and retaining a spherical head  2 . The articular insert  1  is engaged with rotation in an intermediate insert  6 , which is itself engaged fixedly in an insertion cup  3 . 
     In this embodiment, the spherical head  2 , the articular insert  1  and the intermediate insert  6  can be made of ceramic. The insertion cup  3  is made of metal, for example stainless steel or titanium. 
     The spherical head  2  has the same shape as the head  2  in the embodiment in  FIGS. 1 to 12 . 
     The articular insert  1  is similar to the one in the preceding embodiment in  FIGS. 1 to 12  in that it again has the articular surface  1   d , the articular cavity  1   c , in which the spherical head  2  is engaged, the receiving face  1   b , and the spherical outer surface  1   a.    
     The difference in this second embodiment lies essentially in the structure of the retaining means. 
     In this case, the retaining means comprises a continuous annular ring  10   b , having a retaining surface  10   c  in the shape of a spherical crown able to engage functionally with play  10   d  against the spherical head  2  and having peripheral fixing projections  10   e  able to interlock functionally with corresponding recesses if of the articular insert  1 . 
     In practice, the continuous annular ring  10   b  is engaged in a seat of corresponding shape formed at the entrance of the articular cavity  1   c  of the articular insert  1 . The peripheral projections  10   e  comprise, for example, a peripheral rib of the continuous annular ring  10   b , which rib engages in a corresponding annular groove of the articular insert  1 . When the continuous annular ring  10   b  is placed in the articular insert  1 , its retaining surface  10   c  constitutes an undercut segment which retains the spherical head  2  inside the articular cavity  1   c  of the articular insert  1 . 
     As in the embodiment in  FIGS. 1 to 12 , the articular insert  1  and the spherical head  2  are delivered in an assembled and sterile state in a protective envelope, with the continuous annular ring  10   b  in place, such that, when the prosthesis is being implanted, the practitioner does not have to fit the head  2  with force into the articular insert  1 . 
     A third embodiment of a prosthetic hip acetabulum according to the invention is now described, as illustrated in  FIGS. 16 to 18 . This once again has a spherical head  2 , identical to the head in the preceding embodiment, an articular insert  1  and a cup  3 . A first difference lies in the fact that the cup  3  is a movable cup whose outer face  3   a  is spherical and smooth in order to constitute a sliding surface that can be engaged and pivoted in the cotyloid cavity of a patient. 
     In this embodiment, the articular surface  1   d  of the articular insert  1  is hemispherical. The articular insert  1  is made of polyethylene or another equivalent plastic material and is engaged fixedly in the cup  3 . The retaining means is once again a continuous annular ring  10   b , similar to the one in the embodiment in  FIGS. 13 to 15 , with a retaining surface  10   c  in the shape of a spherical crown that is able to engage functionally with a play  10   d  against the spherical head  2 , and having peripheral fixing projections  10   e.    
     In this case, the peripheral projections  10   e  lock themselves in recesses  3   f  formed in the cavity of the cup  3 . Thus, the continuous annular ring  10   b  is retained by the cup  3 . Its retaining surface  10   c  in the shape of a spherical crown constitutes an undercut segment of the articular surface  1   d , which opposes the withdrawal of the spherical head  2  from the articular insert  1 . 
       FIGS. 19 to 21  illustrate a fourth embodiment of a prosthetic hip acetabulum according to the invention. This once again has a spherical head  2  identical to those of the preceding embodiments. The articular insert  1  is made of ceramic and has a substantially hemispherical articular surface  1   d . The outer surface  1   a  of the articular insert  1  made of ceramic is spherical, so as to constitute an articular surface that is able to pivot in a cotyloid cavity in the pelvis of the patient. 
     In this embodiment, the retaining means is once again formed by a continuous annular ring  10   b  made of polyethylene or another equivalent plastic material, with a retaining surface  10   c  in the shape of a spherical crown forming an undercut segment of the articular surface  1   d , and with peripheral fixing projections which lock functionally with corresponding recesses if of the articular insert  1 . An acetabulum with a movable cup made of ceramic is thus obtained. 
       FIG. 22  illustrates, in a perspective view, the continuous nature of the annular ring  10   b , the latter being like the rings used in the embodiments in  FIGS. 13 to 21 . 
     In all the embodiments that have been described, the retaining means is a component made of polyethylene or of equivalent plastic material, which component has to be deformed when fitting the spherical head  2  into the articular insert  1 . It is possible, for example, to use polyethylene, polyether ether ketone (PEEK), etc. 
     In the embodiment in  FIGS. 1 to 3 , it is necessary to deform the mouth of the articular insert  1  in order to expand it and thereby facilitate the insertion of the spherical head  2 . 
     In the other embodiments, it is necessary to contract the continuous annular ring  10   b  in order to facilitate its engagement in the articular insert  1  or in the peripheral cup  3 . 
     For this purpose, the retaining means is subjected to a differential thermal stress which temporarily modifies its dimensions and thus facilitates its assembly. After assembly, the retaining means is allowed to return to room temperature, such that it recovers its original shape and then opposes the withdrawal of the spherical head  2  from the articular insert  1 . 
     In the embodiment in  FIGS. 1 to 3 , in which the articular insert  1  itself constitutes the retaining means  10   a  and is made of polyethylene, the differential thermal stress consists in heating the articular insert  1  in order to temporarily increase its internal diameter and thereby facilitate the axial introduction of the spherical head  2 . 
     In the case of a retaining means in the form of a continuous annular ring  10   b , the differential thermal stress consists in cooling the continuous annular ring  10   b  in order to temporarily reduce its external diameter and thereby facilitate the engagement of the peripheral projections  10   e  of the ring in the corresponding recesses  1   f  or  3   f , respectively, of the articular insert  1  or the peripheral cup  3 . 
     According to the invention, the assembly step as defined above is performed before the step of sterilizing the unit composed of spherical head  2  and articular insert  1 . 
     The sterilizing step can comprise a step of sterilization by bombardment with gamma rays. 
     It has been found, according to the invention, that assembly prior to sterilization allows the retention capacity of the spherical head in the articular insert to be substantially increased, which reduces the risks of luxation. 
     This increasing of the retention capacity has been demonstrated by carrying out comparative traction tests on a head engaged in an articular insert, under the following conditions:
         head made of stainless steel measuring 28 mm;   articular insert made of polyethylene, with an undercut retaining segment measuring 4 mm in length, and with an external diameter of 48, 54 or 62 mm;   axial traction force exerted between the head and the articular insert.       

     The table below shows the limit values of the force bringing about the release of the spherical head from the articular insert for the three external diameters of the articular insert and in each of two cases: head introduced into the articular insert after sterilization, or head introduced into the articular insert before sterilization: 
     
       
         
               
               
               
             
           
               
                   
               
               
                   
                 Force producing 
                 Force producing 
               
               
                   
                 withdrawal of a head 
                 withdrawal of a head 
               
               
                 Insert 
                 impacted after 
                 impacted before 
               
               
                 diameter 
                 sterilization (in daN) 
                 sterilization (in daN) 
               
               
                   
               
             
             
               
                 48 
                 42.71 
                 50.10 
               
               
                 54 
                 54.01 
                 65.32 
               
               
                 62 
                 65.21 
                 73.03 
               
               
                   
               
             
          
         
       
     
     The present invention is not limited to the embodiments that have been explicitly described and instead it includes the various modifications and generalizations contained within the scope of the attached claims.