Abstract:
A component, e.g. a prosthetic ball and socket joint like an acetabular cup of a prosthetic joint, having a male or female location feature in a surface thereof for mutual interfit with a fixing member of a fixing assembly for securing the fixing member to the surface of the component and optionally for securing the component to a second component by means of a fixing screw.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional application of U.S. application Ser. No. 10/559,950 filed Nov. 8, 2006 entitled “Fixing Assembly,” which claims priority to PCT Application No. PCT/GB04/02539 filed Jun. 11, 2004 entitled “Fixing Assembly,” which claims priority to GB Application No. 0313444.2 filed Jun. 11, 2003, the contents of all of which are incorporated herein by this reference. 
     
    
     SUMMARY 
       [0002]    Embodiments relate to a fixing assembly for securing a fixing member to a surface of a component, and particularly, but not exclusively, to a fixing assembly for securing a fixing member to a prosthetic cup fitted during the resurfacing or replacement of the bearing surfaces of a patient&#39;s hip joint. 
         [0003]    Conventionally, during the resurfacing or replacement of the bearing surfaces in a hip joint, a standard prosthetic cup component is driven into a prepared location in the acetabulum, and the stability of the cup within the acetabulum assessed by the Surgeon. Occasionally, the stability of the cup within the acetabulum is judged to be inadequate, indicating that there would be insufficient bone fully to support the cup during the healing period. In these circumstances, the Surgeon may decide that the standard cup is to be removed, and a dysplasia cup having a number of integral fixing members fitted in its place. The integral support lugs are drilled and tapped to allow dysplasia support screws to be fitted. These support screws are then anchored into prepared points in the acetabulum. It can be seen that a disadvantage of such a method is that the Surgeon has to fit and then remove the standard cup before the dysplasia cup is fitted. It would be advantageous to provide a standard acetabular cup, to which support lugs may be fitted when required, thus allowing for the transformation of a standard acetabular cup into a dysplasia cup, thereby obviating the need to remove a standard cup. 
         [0004]    A further disadvantage associated with dysplasia cups having integral fixing members, is that the location and angular disposition of the support screws is dictated by the location and orientation of the integral fixing members, and thus cannot be adjusted by the Surgeon. It would be desirable for the Surgeon to be able to adjust the location and/or the angle of the support screws in order to achieve the best fixation of the screws, and thus of the cup, in the acetabulum. 
         [0005]    It is an object of the present invention to provide an improved fixing assembly. 
         [0006]    According to a first aspect of the present invention, there is provided a fixing assembly for securing a fixing member to an external surface of a component, said fixing assembly comprising said surface of the component, a fixing member and a fixing screw, said surface and said fixing member being formed with a female location feature and a male location feature respectively, or vice versa, such that in use, said male and female location features fit together, and said fixing screw passes through an aperture in said fixing member to secure said fixing member to said surface, with said fixing screw being external to said surface. 
         [0007]    According to a second aspect of the present invention, there is provided a fixing assembly for securing a fixing member to a surface of a component, said fixing assembly comprising said surface of the component, a fixing member and a fixing screw, said surface and said fixing member being formed with a female location feature and a male location feature respectively, or vice versa, such that in use, said male and female location features fit together, and said fixing screw passes through an aperture in said fixing member to secure said fixing member to said surface, with said fixing screw being available for securing said component to a second component. 
         [0008]    Preferably, said male and female location features each have at least one corresponding undercut, and desirably, said male and female location features each have two corresponding such undercuts, so that in use, an interference fit dovetail joint is formed between said male and female locating features. Advantageously, the location feature of the component is at least partly located on a rim of said surface. 
         [0009]    Preferably, said male location feature has a slit, such that the male location feature is divided into two arms. Advantageously, said slit allows deformation of the male location feature upon the interference fitting of the male and female location features, and desirably said male location feature is of resiliently deformable material. 
         [0010]    Preferably, the male location feature having a slit is located on the fixing member, and desirably, said fixing member is formed such that the slit extends radially from the aperture, through said male location feature. Conveniently, said slit is formed in a plane substantially parallel to the axis of the aperture. Alternatively, the slit may be formed in a plane substantially perpendicular to the axis of the aperture. 
         [0011]    Preferably, in use, when said fixing screw is screwed into said fixing member aperture, the arms of the male location feature are locked in position. Advantageously, when the fixing screw is partly or fully screwed out of said fixing member aperture, the orientation of the location feature of the fixing means relative to the location feature of the surface, and thus the orientation of the fixing means itself in relation to the surface, may be adjusted. 
         [0012]    Preferably, said fixing screw has a head, and said screw has a thickened portion near the head, such that when the screw is screwed into the aperture of said male fixing member having at least one male location feature with a slit, the arms of the location feature are pushed apart. 
         [0013]    Conveniently, said surface has a threaded portion adjacent the at least one location feature, such that, in use, as said fixing screw is screwed into said aperture of the fixing member, said fixing screw engages with said threaded portion. 
         [0014]    Preferably, said surface has a circumferential groove forming said female location feature to lock the fixing member to the component. 
         [0015]    According to a third aspect of the present invention, there is provided a kit of parts for a fixing assembly for securing a fixing member to an external surface of a component, the kit comprising an external surface of a component, a fixing member and a fixing screw, said surface and said fixing member being formed with a female location feature and a male location feature respectively, or vice versa, such that when the kit of parts is assembled, said male and female location features fit together, and said fixing screw passes through an aperture in said fixing member to secure said fixing member to said surface, with said fixing screw being external to said surface. 
         [0016]    According to a fourth aspect of the present invention, there is provided a kit of parts for a fixing assembly for securing a fixing member to a surface of a component, the kit comprising a surface of a component, a fixing member and a fixing screw, said surface and said fixing member being formed with a female location feature and a male location feature respectively, or vice versa, such that when the kit of parts is assembled, said male and female location features fit together, and said fixing screw passes through an aperture in said fixing member to secure said fixing member to said surface, with said fixing screw being available for securing said component to a second component. 
         [0017]    Preferably, said surface of a component is the external surface of a cup of a prosthetic ball and socket joint, and advantageously, said cup is an acetabular cup of a prosthetic hip joint. More preferably, said acetabular cup is a dysplasia cup. Desirably, said second component is bone. 
         [0018]    According to a fifth aspect of the present invention, there is provided a component having a male or female location feature in a surface thereof, for mutual interfit with a fixing member of a fixing assembly of the first or second aspects of the present invention. Preferably, said component of said fifth aspect is an acetabular cup. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
           [0020]      FIG. 1  is an exploded perspective view of a fixing assembly according to a first embodiment of the present invention 
           [0021]      FIG. 2  is a perspective view of the assembled fixing assembly of  FIG. 1 . 
           [0022]      FIG. 3  is an enlarged fragmentary schematic vertical sectional view of the assembled fixing assembly of  FIG. 2 . 
           [0023]      FIG. 4  is an enlarged perspective view of a female location feature as shown in  FIGS. 1 to 3 . 
           [0024]      FIG. 5  is an enlarged fragmentary schematic transverse view of the fixing assembly in use, showing detail of the female location feature of the cup, male location feature of the fixing member, and fixing screw of  FIGS. 1 to 4 . 
           [0025]      FIGS. 6 to 10  are views corresponding to  FIGS. 1 to 5  respectively, showing a second embodiment of the fixing assembly according to the present invention. 
           [0026]      FIGS. 11 to 14  show a third embodiment of the fixing assembly according to the present invention, more specifically,  FIGS. 11 to 13  are views corresponding to  FIGS. 1 to 3  respectively, and  FIG. 14  is an enlarged fragmentary schematic longitudinal view of the fixing assembly in use, showing detail of the female location feature of the cup, male location feature of the fixing member, and fixing screw of  FIGS. 11 to 13 . 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0027]    Referring to the drawings, an example embodiment of a fixing assembly  10  according to the present invention is shown in  FIGS. 1 to 5 . The fixing assembly comprises the external surface of a component, which is an acetabular cup  11  in the preferred embodiment, a fixing member in the form of a lug  12 , fixing screw  13  and male and female location features  14  and  15  located on the lug and cup respectively. In use, the fixing assembly secures the lug  12  to the external surface of the cup  11  by means of the male and female location features  14  and  15 . Fixing screw  13  passes through an aperture  16  in the lug. Aperture  16  has internal threads  18 , which engage with external threads  19  of the fixing screw  13 . 
         [0028]    The lug  12  has a slit  21 , which extends radially from the aperture  16 , through said male location feature  14  to divide the male location feature  14  into two arms  22 ,  23 , the relevance of which will be discussed below. The slit is formed in a plane substantially parallel to and including the axis of the aperture. 
         [0029]    The location features  14 ,  15  according to a preferred embodiment of the present invention each have at least one undercut. As can be seen best from  FIG. 4 , the male location feature  14  of lug  12  has two undercut side portions  27 ,  28 . The female location feature  15  has two corresponding undercut side portions  25 ,  26  such that, in use, when the male and female location features are fitted together, an interference fit dovetail joint  29  is formed. 
         [0030]    In order to assemble the fixing assembly, the male location feature  14  of the lug  12  is inserted into the female location feature  15  of the cup  11 . The male location feature  14 , and at least part of the remainder of the lug through which the slit extends is made from a suitable material to allow the arms  22 ,  23  to be slightly pushed or squeezed together during location of the male location feature  14  into the female location feature  15 . The slit allows deformation of the male location feature upon the interference fitting of the male and female location features. 
         [0031]    The female location feature is a recess in an outer surface of a rim  31  around the exterior of the cup at its open end. The recess breaks through the horizontal surface  31   a  of the rim and runs out into the external surface of the cup below the rim. The recess is conical (convex) in front view with the undercut side portions  25 ,  26  each extending from the surface  31   a  to where the rim runs out. To match the conical (convex) recess, the side of each undercut is correspondingly conical (convex). The male location feature  14  of lug  12  is introduced into the recess forming the female location feature  15  with the lug orientated such that the two corresponding undercut side portions  27 ,  28  are perpendicular to the axis of the cup. The lug  12  is then attached to the external cup surface  11  by rotating the lug through approximately 90 degrees, so that side portions  27 ,  28  of the lug  12  engage with undercut side portions  25 ,  26  of the female location feature  15 . The lug may then be articulated to the desired angle by rotation of the lug in its location through a limited angle, the slit  21  allowing for any article manufacturing variations between the interference fit of the male location feature  14  with female location feature  15 . In the example of the present invention in the context of an acetabular cup, the fixing screw  13  location point in the bone of a patient (not shown) would be machined conventionally. 
         [0032]    The fixing screw  13  is introduced into the lug aperture  16 . Fixing screw  13  has a head portion  32 , and a thickened portion  33  towards the head, so that when the screw is screwed into the lug aperture  16 , the external threads  19  of thickened portion  33  engage with the internal threads  18  of aperture  16 , and arms  22 ,  23  are pushed apart and locked in place by the screw  13 . The male location feature of the lug is thus held fixed in position inside the female location feature  15 . Unscrewing fixing screw  13  so that the thickened portion  33  is no longer engaged with the aperture  16  allows arms  22 ,  23  to move together, so that the articulation of the lug  12  may be adjusted relative to the female location feature  15  and thus the surface of the cup  11 , or detached from the cup. 
         [0033]    Referring to  FIGS. 6 to 10 , a second embodiment of the fixing assembly  110  according to the present invention comprises an external surface of a component in the form of a cup  111 , a fixing member in the form of a lug  112 , and a fixing screw  113  and male and female location features  114  and  115  located on the lug and cup respectively. As with the previously described embodiment, in use, the fixing assembly secures the lug  112  to the external surface of the cup  111  by means of the male and female location features  114  and  115 . Fixing screw  113  passes through an aperture  116  in the lug. Aperture  116  has internal threads  118 , which engage with the external threads  119  of the fixing screw  113 . 
         [0034]    The female location feature  115  is of very similar form to that of the first embodiment, having two undercut side portions  125 ,  126 , and the male location feature  114  has two corresponding undercut side portions  127 ,  128 , such that, in use, when the male and female location features are fitted together, a dovetail joint  129  is formed, as seen best from  FIG. 10 . However, wherein side portions  27  and  28  of the lug of the first embodiment are conical in profile, side portions  127 ,  128  of the lug of the second embodiment are prismatic (flat) in profile, and it will be appreciated that undercut portions  125 ,  126  of the female location feature  115  are correspondingly profiled. It will be appreciated from the illustrations, that the lug herein illustrated and described according to this particular embodiment of the present invention does not feature a slit equivalent to slit  31  of the first embodiment. However, it will be appreciated that such a slit may be incorporated into this embodiment in order to accommodate manufacturing variation. 
         [0035]    In use, the male location feature  114  of the lug  112  is slid into the correspondingly formed female location feature  115  of the cup  111 . In order to secure the male location feature to the cup, an externally threaded portion  130 , with concentric raised threads, is disposed adjacent the female location feature  115  of the cup  111 . In the embodiment shown, the threaded portion is disposed on the external surface of the cup  111  directly below the recess forming the female location feature  115 . Fixing screw  113  is introduced into lug aperture  116 . Although a thickened portion of the fixing screw is not illustrated or described for this particular embodiment of the present invention, it is envisaged that such a thickening may be present. The external threads  119  of the fixing screw  113  engage with the internal threads  118  of the aperture  116 , and as the screw  113  is screwed into the aperture, the threads  119  of the fixing screw also engage with those of the complimentarily threaded portion  130  of the external surface of the cup  111 . When the screw  113  is screwed into the aperture  116  and engaged with threaded portion  130 , the lug  112  is locked in position, attached to the external surface of the cup  111 . The threads of portion  130  extend angularly to a sufficient degree to allow engagement of the screw therewith even if the lug is at a slight angle. If the Surgeon wishes to alter the angle of the lug  112  with respect to the cup, an alternative lug (not shown), with its apertured part at an angle to the male location feature, may be employed and fitted as described above. 
         [0036]    Referring to  FIGS. 11 to 14 , a third embodiment of a fixing assembly  210  according to the present invention is shown. The fixing assembly comprises the external surface of a component, which is an acetabular cup  211  in the preferred embodiment, a fixing member  212  in the form of a lug, a fixing screw  213 , and male and female location features  214  and  215  located on the lug and cup respectively. As with the previously described embodiments, in use, the fixing assembly secures the lug  212  to the external surface of the cup  211  by means of the male and female location features  214  and  215 . Fixing screw  213  passes through an aperture  216  in the lug. Aperture  216  has internal threads  218 , which engage with the external threads  219  of the fixing screw  213  which is conventional in form. 
         [0037]    The lug  12  has a slit  221 , which extends radially from the aperture  216 , through said male location feature  214  to divide the male location feature  214  into upper and lower arms  222 ,  223 , the relevance of which will be discussed below. The slit is formed in a plane substantially perpendicular to the axis of the aperture. The external surface of the cup  211  has a circumferential groove  215  defined by upper and lower circumferential ridges  231 ,  232  respectively, the groove forming the female location feature. This groove  215  has a circumferential undercut portion  225  in the under surface of the upper circumferential ridge  231 , for mutual interfit with a correspondingly undercut portion  227  of the upper arm  222  of the male location member  214  of lug  212 . 
         [0038]    In use, in order to assemble the fixing assembly, the male location feature  214  of the lug  212  is inserted into the groove  215  of the cup  211 . The male location feature  214  and the remainder of the lug through which the slit  221  extends, is advantageously made from a resiliently deformable material, and the entire lug may be made from such a resiliently deformable material. Forming the male location feature from a resiliently deformable material enables the arms  222 ,  223  to be pushed or squeezed together in order to facilitate location of the male location feature  214  into the groove  215 . As can be seen best from  FIGS. 13 and 14 , the undercut portion  225  of the groove is located on the underside of the horizontal surface  231  of the rim of the cup. With the arms  222 ,  223  squeezed together, the undercut portion  227  of the upper arm  222  of the male location feature  214  of the lug  212  is introduced into the corresponding undercut  225  of the groove  215 . The profile of the male location feature  214  is similar in section to that of the groove  215 , as can be seen from  FIGS. 13 and 14 . The lug  212  is then clipped into place by rotating the lug relative to the central axis of the cup, and the pressure on the arms  222 ,  223  released to allow the arms to separate as far as the profile of the groove will allow. A circumferential outer lip  233  with an upper, outwardly directed arcuate surface  234  is located on the upper surface of the lower circumferential ridge  232 . The lower arm  223  of the male location feature of the lug has a downwardly directed ramp  235  with an outwardly facing ramp surface  236 . On insertion of the male location feature into the grove  215 , the surface  234  engages the surface  236  to move the lower arm upwards. The ramp thus moves past the lip  233 , which is then received in a recess  237  of the lower arm disposed behind the ramp. It will be understood that, as an alternative to the specific form of the lug  212  as described and illustrated herein for this particular embodiment, it is envisaged that a folded spring clip may be employed. 
         [0039]    The fixing screw  213  is then introduced into the lug aperture  216 . When the screw is screwed into the aperture  216 , the external threads  219  engage with the internal threads  218  of the aperture to lock the position of the arms  222 ,  223  apart, so as to hold the male location feature  214  of the lug  212  in position inside the groove  215 , as shown in  FIG. 14 . 
         [0040]    It will be understood that although the example embodiments depict the female location feature  15  on the cup and male location feature  14  on the lug, it is equally possible that the female location feature could be on the lug and the male location feature on the cup, or a combination thereof. In addition, although only one location feature is shown on the cup, it is envisaged that more than one may be present, so that a plurality of lugs may be secured to one cup. 
         [0041]    For each of the embodiments described herein, in use, the fixing screw remains external to the surface of the cup, and the fixing screw remains available for use to secure the cup to a second component. In the example of an acetabular cup, this second component would be a patient&#39;s bone. Thus it can be seen that the present invention is a particularly convenient arrangement, wherein the fixing screw that secures the fixing means to the external surface of the component is the same fixing screw that may secure the cup to a second component. However, in one alternative the surface could be an internal surface rather than an external one, but with the screw available for securement to a further component. 
         [0042]    It is envisaged that a fixing device according to the present invention may be available as a kit form. 
         [0043]    Finally, it will be appreciated that a fixing assembly according to the present invention need not relate specifically to an assembly for fixing a lug to an acetabular cup. 
         [0044]    It is considered that the component in which the male and/or female location feature(s) is or are provided, such as an acetabular cup, is inventive in its own right.