Patent Abstract:
A buttress plate for retaining an acetabular cup implanted in an acetabulum is provided. The buttress plate includes a base having a first end and a second end. The first end extends along a plane and includes at least one passage for receiving a fastener to secure the base to the acetabulum. The second end includes a lip extending transverse to the plane for securing the acetabular cup within the acetabulum.

Full Description:
RELATED APPLICATION 
     This application claims priority from U.S. Provisional Patent Application Ser. No. 61/036,487, filed on Mar. 14, 2008, the subject matter of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention is directed to bone fixation and, in particular, is directed to a method and apparatus for buttressing a prosthetic acetabular component for a deficient acetabulum. 
     BACKGROUND OF THE INVENTION 
     Every year, thousands of individuals undergo total hip replacement (THR) as a result of degeneration or otherwise excessive damage to the acetabulum. This damage causes irregular and/or painful articulation between the femoral head and the acetabulum during normal movement of the leg and hip. It therefore becomes necessary to replace the damaged articulation site with a prosthetic implant that can adequately manage the normal forces encountered at the site while maintaining a sufficient range of motion between the femur and pelvic girdle. 
     A THR procedure typically involves resecting a portion or all of the acetabulum so that a prosthetic component mimicking the acetabulum can be installed in its place. The cup is contoured and constructed such that its functionality closely resembles that of the native acetabulum. The femoral head may also be partially or fully resected to accommodate a prosthetic stem and head that will articulate with the prosthetic cup. 
     The efficacy of current acetabular cups lies in the ability to retain a secure connection between the cup and the pelvic girdle. By maintaining a strong connection, the ability of the surrounding bone to heal is improved, as is the reliability of the replaced articulation site. However, due to constant and variable articulation between the femoral head and the acetabular cup, the cup may loosen over time. This movement may contribute to a weakened articulation site or may make the femoral head susceptible to slippage out of the cup, resulting in severe pain and discomfort. Another contributing factor to the performance decline of an acetabular cup can be the complex anatomical geometry of the pelvic girdle. This geometry may make it difficult to ensure a lasting, rigid connection between the cup and the bone. Current cups use bone cement, friction, and/or fasteners/screws to directly secure the cup to the pelvic girdle. For the above reasons, these fastening constructions can fail to ensure proper fixation of the cup within the acetabulum. 
     Accordingly, there is a need for an apparatus and method for ensuring that prostethic acetabular cups are securely fitted to the pelvic girdle to ensure proper healing and reliable articulation with the femoral head. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a buttress plate for retaining an acetabular cup implanted in an acetabulum is provided. The buttress plate includes a base having a first side portion and a second side portion. The first side portion extends along a plane and includes at least one passage for receiving a fastener to secure the base to the acetabulum. The second side portion includes a lip extending transverse to the plane for securing the acetabular cup within the acetabulum. 
     In accordance with another embodiment of the present invention, a method is provided for retaining an acetabular cup implanted in an acetabulum. The acetabular cup is placed within the acetabulum, wherein the cup is pre-contoured to the acetabulum. A buttress plate is fastened to a lateral surface of the acetabulum, the plate including a base having a first side portion and a second side portion, the first side portion extending along a plane and including at least one passage for receiving a fastener to secure the base to the acetabulum, the second side portion including a lip extending transverse to the plane for securing the acetabular cup within the acetabulum. 
     In accordance with another embodiment of the present invention, a buttress plate for retaining an acetabular cup implanted in an acetabulum is provided. A buttress plate includes an arcuate base having a first side portion and a second side portion. The first side portion extends along a plane and includes at least one tab extending outward from the first side portion. Each of the at least one tabs includes at least one passage for receiving a fastener to secure the base to the acetabulum. The second side portion includes an arcuate lip extending transverse to the plane for securing the acetabular cup within the acetabulum. The base further includes means for engaging bone and means for promoting bone growth 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other features and advantages of the present invention will become apparent to those skilled in the art to which the present invention relates upon reading the following description with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic illustration of an acetabular buttress plate in accordance with an embodiment of the present invention; 
         FIG. 2  is a top view of the plate of  FIG. 1 ; 
         FIG. 3  is a sectional view of the plate of  FIG. 2  taken along line  3 - 3 ; 
         FIG. 4  is an exploded assembly view of the plate securing a support cup to the pelvis; 
         FIG. 5A  is a sectional view of the plate retaining the cup within the acetabulum; 
         FIG. 5B  is a sectional view of the plate retaining the cup within the acetabulum in accordance with another embodiment of the present invention; and 
         FIG. 6  is an enlarged view of the plate in accordance with another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention is directed to bone fixation and, in particular, is directed to a method and apparatus for buttressing a prosthetic cup for an acetabulum within the pelvis of a patient. An embodiment of a buttress plate  10  in accordance with the present invention is illustrated in  FIG. 1 . In operation, the buttress plate  10  is adapted to maintain a typical acetabular cup  20  within the acetabulum  32  of a pelvic girdle  30  to allow the cup to articulate with a prosthetic femoral head (not shown). 
     The buttress plate  10  has an arcuate construction extending generally about an axis  16  and may, for example, be pre-contoured to the shape of the pelvis  34  of the patient. In particular, the buttress plate  10  may be pre-contoured to the posterior column of the acetabulum. The plate  10  includes a base  40  having a top surface  42  longitudinally spaced from a bottom surface  44 . A first side portion  46  (or end) is laterally spaced from a second side portion  48  (or end) ( FIGS. 2-3 ). The first side portion  46  and second side portion  48  extend generally parallel to one another or may have an oblique relation to one another. As shown in  FIG. 2 , the first side portion  46  and the second side portion  48  have a concentric relation to one another, with the first side portion being disposed radially outward of, and being laterally spaced from, the second side portion relative to the axis  16 . Stated differently, the first and second side portions  46  and  48  are depicted in  FIG. 2  as being curved and having coincident centers. The second side portion  48  in this concentric relationship is interposed laterally between the first side portion  46  and the axis  16 . 
     The first side portion  46  of the base  40  includes at least one tab  50  which projects radially outward from the first side portion. Although three tabs  50  are illustrated in  FIG. 2 , it will be understood that the plate  10  may comprise more or fewer tabs, including a single tab. The tabs  50  are radially spaced from one another along the first side portion  46  in a uniform or non-uniform manner. Each tab  50  includes one or more passages  52  which extend longitudinally from the top surface  42  of the base  40  to the bottom surface  44  of the base. The passages  52  are sized to accommodate a fastener  80  to allow the plate  10  to be fixed to bone and in particular, to the pelvis  34 . 
     The first side portion  46  of the base  40  and the tabs  50  extend generally along a plane  60  extending substantially parallel to the top surface  42  and the bottom surface  44  of the base. The second side portion  48  of the base  40  includes a lip  62  which extends at an angle relative to the plane  60 . In particular, the lip  62  extends longitudinally downward and away from the top surface  42  of the base  40 . Although  FIG. 3  illustrates that the transition between the lip  62  and the remainder of the base  40  is arcuate, it will be understood that this transition may likewise be linear. 
     The bottom surface  44  of the base  40  may include means for engaging bone  90 . The means  90  may include ribs, projections, or barbs, or may otherwise be textured or adapted to increase the ability of the bottom surface  44  of the base  40  to engage bone. Furthermore, the plate  10  may be mechanically, chemically, and/or biologically treated to promote bone growth once the plate has been installed within the patient. 
     The plate  10  is constructed of any biocompatible material capable of providing a secure fixation site at the pelvic girdle  30 . The plate  10  can be made of metal, polymers, or combinations thereof. The plate  10  should be strong enough to maintain engagement with the pelvis  30  while the femoral head is articulating within the acetabular cup  20 . 
     As shown in  FIG. 4 , the plate  10  is used as a buttress to ensure that the acetabular cup  20  remains in place within the pelvic girdle  30 . Acetabular cups  20  are well known in the art. Typically, the cup  20  is pre-contoured to the shape of the native acetabulum  32  such that the cup can closely mimic the functionality of the acetabulum. The cup  20  has a generally cup-shaped construction and may include a plurality of holes  66  for facilitating the fastening of the cup to the acetabulum  32  via fasteners, screws, bone cement, etc. 
     In operation, the cup  20  is placed within the acetabulum  32  such that a rim  70  of the cup is recessed from a lateral surface  36  of the pelvis  30  which helps define the acetabulum  32 . As shown in  FIG. 5A , the plate  10  is then positioned on the pelvis  34  such that the first side portion  46  of the base  40  overlies the lateral surface  36  and the lip  62  on the second side portion  48  of the base extends over the rim  70  of the acetabular cup  20  and into the acetabulum  32 . In this configuration, the lip  62  overlies and engages a portion of the acetabular cup  20  below the rim  70 , thereby placing a portion of the cup between the lip and the native acetabulum  32 . Stated differently, the top surface  42  may be substantially located in a first plane, and at least a portion of the acetabular cup  20  is then interposed laterally between the lip  62  and the acetabulum  32  as these structures are laterally concentrically arranged (with respect to the axis  16 ) within a cross-section taken in a second plane parallel to, and longitudinally spaced from, the first plane. 
     A fastener or screw  80  is inserted into each passage  52  on the tabs  50  of the base  40 . The fastener  80  includes a biocompatible screw or other structure suitable for penetration into bone. Each fastener  80  is screwed into the lateral surface  36  of the pelvis  34  until the head of the fastener applies a compressive force to the top surface  42  of the base  40  of the plate  10  to thereby pin the plate against the lateral surface ( FIG. 5A ). When the plate  10  becomes fixed to the pelvis  34 , the lip  62  becomes fixed in an engaging fashion with the portion of the acetabular cup  20  below the rim  70 , thereby preventing the cup from moving within the acetabulum  32 . This configuration can ensure that the articulation between the acetabular cup  20  and the femoral head remains stable and more closely resembles natural articulation. Due to this stable connection and/or the use of bone growth-promoting materials, this construction may contribute to proper healing at the implant site. 
     Alternatively, the lip  62  on the base  40  of the plate  10  may be configured such that when the plate is placed across the pelvis  34 , the lip overlies and engages a top surface  72  of the rim  70  of the acetabular cup  20  ( FIG. 5B ). In this construction, each fastener  80  is screwed into the lateral surface  36  of the pelvis  34  until the head of the fastener  80  applies a compressive force to the top surface  42  of the base  40  of the plate  10  to thereby pin the plate against the lateral surface. When the plate  10  becomes fixed to the pelvis  34 , the lip  62  applies a compressive force to the top surface  72  of the rim  70  of the cup  20 , thereby pinning the cup against the acetabulum  32  and preventing the cup from moving within the acetabulum. 
     In accordance with another aspect of the present invention, the lip  62  of the base  40  of the plate  10  and the rim  70  of the cup  20  may be configured with additional structure to mate with one another. The mating connection between the plate  10  and the cup  20  provides a more rigid engagement between the cup  20  and the acetabulum  32 . In particular, the rim  70  of the cup  20  may include one or more projections  90  which engage one or more projection  92  on the lip  62  of the plate  10  ( FIG. 6 ). The projection  92  on the lip  62  may be configured to snap over the projection  90  on the rim  70  as the base  40  is secured onto the lateral surface  36  of the pelvis  34 . Those in the art will understand, however, that the plate  10  and the cup  20  can include other or additional mating features that increase the likelihood that the cup remains rigidly secured in the acetabulum  32 . 
     From the above description of the invention, those skilled in the art will perceive improvements, changes and modifications. Such improvements, changes and modifications within the skill of the art are intended to be covered by the appended claims.

Technology Classification (CPC): 0