Abstract:
A prosthetic joint system includes a body having at least one aperture into which an elongate sleeve component is engageable. A screw for securing the body to bone is insertable into the bone via a bore in the sleeve. The bore has a length that is sufficient to accommodate movement of the screw as the prosthesis subsides over time. More particularly, the sleeve allows the screw head to move longitudinally such that the screw is prevented from damaging an inner member of the prosthesis system that provides an articulation surface for a corresponding prosthetic joint component. In one embodiment, a prosthesis system includes an acetabular cup for a prosthetic hip joint.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     Not Applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not Applicable. 
     FIELD OF THE INVENTION 
     The present invention relates generally to prosthetic components, and more particularly, to prosthetic joints. 
     BACKGROUND OF THE INVENTION 
     Joint arthroplasty is a well known surgical procedure by which a diseased and/or damaged natural joint is replaced by a prosthetic joint. Joint arthroplasty is commonly performed for hips, knees, elbows, and other joints. The health and condition of the joint to be replaced dictate the type of prosthesis necessary to replace the natural joint. For example, in a total hip arthroplasty an acetabular cup may be implanted in the pelvis to replace the natural acetabulum. 
     To implant an acetabular cup, an acetabular cavity is reamed in the acetabulum. The reamed cavity generally conforms to an outer surface of the acetabular cup. The acetabular cup is then inserted into the formed cavity and is then further secured by mechanical means, such as one or more fixation screws. The acetabular cup is positioned in the pelvis at a relatively fixed orientation with respect to patient anatomy and should remain stable. 
     FIGS. 1-4 show one type of prior art acetabular cup  10  implanted in a patient&#39;s acetabulum  12 . The cup  10  includes a hemispherical outer member  14  for interfacing to bone and a mating inner member  16  for providing an articulation surface for the ball  18  of a femoral component  20 . The inner member  16  is formed from polyethylene, for example, to provide a durable, low friction interface for allowing the femoral component to move freely. Apertures  22  are formed in the outer member  14  of the cup to provide passageways for fixation screws  24  that secure the acetabular cup  10  to the bone. 
     While the apertures allow the fixation screws to penetrate bone, the screws must be inserted at locations corresponding to the apertures. In addition, the fixations screws must be introduced into the bone within a limited angular range to allow proper seating of the screw head in the aperture. 
     Further, while the implanted cup may initially be secured in position, movement of the implanted acetabular cup over time can contribute to erosion of the surrounding bone. One effect of such bone erosion is the loosening of the acetabular cup, allowing it to shift in position. More particularly, the implanted cup tends to subside into the surrounding bone so as to adversely affect conditions in the prosthetic joint. Typically, an implanted acetabular cup will subside several millimeters within a few years after implantation, which can result in one or more fixation screw impinging on the cup liner. Screw/liner contact can cause fretting of the liner and possibly catastrophic failure of the liner. 
     For example, as shown in the prior art prosthetic hip joint of FIG. 4, as the cup  10  subsides into the surrounding bone a head  26  of the fixation screw  24  begins to impinge upon the polyethylene inner cup member  16 . The action of the screw  24  against the polyethylene inner member  16  as the joint is subjected to loads may cause wear debris to develop, which can ultimately result in osteolysis within the joint. In extreme cases, the screw  24  can fracture the inner member  16  causing catastrophic failure of the joint. 
     It would, therefore, be desirable to provide an implantable prosthesis system having features to optimize the positioning of fixation screws into bone, to minimize the effects of implant subsidence into the surrounding bone, and to increase safety margins. 
     SUMMARY OF THE INVENTION 
     The present invention provides a prosthetic joint system that enhances the long term fixation properties of the implant by providing a structure to allow fixation screws to be inserted at a range of angles and to accommodate subsidence of the prosthesis into bone. Although, the invention is primarily shown and described as an acetabular cup implantation system, it is understood that the invention has other applications as well, such as for use with prosthetic knee systems. 
     In one embodiment, an implantable prosthesis system includes an acetabular cup having a convex outer component for interfacing with bone and an inner component that is matable with the outer component to provide an articulation surface for a corresponding femoral component. The acetabular cup includes at least one aperture to provide a passageway for a fixation element, such as a screw, to secure the cup to bone. A plurality of sleeve components are provided to mate with the apertures formed in the outer component. Each sleeve is of a substantially elongate shape having an outer surface and a longitudinal bore extending therethrough from a proximal opening to a distal opening in the sleeve. The longitudinal bore is adapted to receive a fixation element, e.g., a bone screw, such that a head of the screw seats within the sleeve adjacent the distal opening of the sleeve. Each sleeve is adapted to mate with one of the apertures such that the distal opening of the sleeve is spaced away from an outer surface of the acetabular cup. 
     The sleeve component can be positioned at a range of angles with respect a central axis of an aperture of the acetabular cup with which it is mated, thus allowing the fixation screws to be inserted into a desired region of bone. In addition, the sleeve compensates for subsidence of the implanted acetabular cup by providing a region to accommodate longitudinal movement of the fixation screw to prevent the screw head from contacting, and thereby possible damaging, the inner cup component. 
     To implant the acetabular cup system, an acetabular cavity is formed in the patient&#39;s acetabulum and the prosthetic cup component is inserted into the formed cavity. The surgeon then drills one or more holes in the bone via the apertures in the cup outer surface at predetermined angles to optimize fixation of the cup to bone by way of the screws. The formed holes are then used to align a larger diameter drill for enlarging a portion of the holes to receive the sleeve components. The sleeve components are engaged with respective fixation screws, which are then rotated into the holes formed in the bone to secure the acetabular cup to the acetabulum. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a partly cross-sectional view of a prior art prosthetic hip joint; 
     FIG. 2 is a diagrammatic view of a prior art acetabular cup that forms a part of the prior art prosthetic hip joint of FIG. 1; 
     FIG. 3 is a cross-sectional view of a part of the prior art acetabular cup of FIG. 2 further showing a screw in a first position; 
     FIG. 4 is a cross-sectional view of a part of the prior art acetabular cup of FIG. 2 further showing a screw in a second position; 
     FIG. 5 is a perspective view of an acetabular cup system in accordance with the present invention; 
     FIG. 6 is a cross-sectional view of a part of the acetabular cup system of FIG. 5 along line  6 — 6  with a screw, which forms a part of the system, shown in a first position; 
     FIG. 7 is a cross-sectional view of a part of the acetabular cup system of FIG. 5 along line  6 — 6  with a screw, which forms a part of the system, shown in a second position; 
     FIG. 8 is a cross-sectional view of a sleeve component that forms a part of the acetabular cup system of FIG. 5; and 
     FIG. 9 is a cross-sectional view of the sleeve component of FIG. 8 shown mated with an acetabular cup and oriented at a range of angles. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 5 shows an acetabular cup system  100  in accordance with the present invention. The system includes a convex outer member  102  and an inner bearing member  104 . A series of apertures  106  are formed in and extend through the outer member  102  to provide a passageway for fixation screws  108  into the surrounding bone. One or more sleeve components  110  are provided, each of which is matable within a respective aperture. The sleeve member  110 , when mated with an aperture of the outer member  102 , is effective to accommodate subsidence of the implant into bone over time by preventing damage to the inner member  104  as a result of impingement of the screw  108  against the inner member due to migration of the implanted cup. In addition, the sleeve component  110  can be positioned at various angles in relation to the implanted acetabular cup to optimize placement of screws within bone. 
     The outer member  102  of the acetabular cup is generally hemispherical so as to facilitate its implantation into a complementary cavity reamed in a patient&#39;s acetabulum. In general, an outer surface  112  of the cup interfaces with bone to secure the cup within the formed acetabular cavity. The contour of the outer member  102  and/or formed acetabular cavity can be adapted for an interference fit of the cup if desired. It is understood that the outer surface  112  of the cup can include various surface features to enhance bone ingrowth. 
     The apertures  106  are formed at various locations in the outer member  102  of the cup to provide a surgeon with a range of options for inserting the fixation screws  108  through the acetabular cup into the surrounding bone. The screws  108  enhance fixation of the implanted cup  100  to improve the likelihood of achieving long term fixation of the prosthetic component. 
     As shown in FIGS. 6,  7 , and  9 , the aperture wall  114  engages the sleeve component  110  and prevents its passage through the cup outer member  102 . In general, the respective geometries of the sleeve component  110  and the aperture wall  114  should, upon engagement, cooperate to position a distal end  113  of the sleeve  110  a predetermined distance from the outer surface  112  of the acetabular cup. A proximal or mating end  126  of the sleeve includes a structure adapted for seating in the aperture wall  114 . The sleeve/aperture structure should also allow the sleeve to be positioned at a selected angle with respect to the cup outer surface  112 . Exemplary structures for the sleeve mating end  126  include arcuate, spherical, and tapered. 
     In an exemplary embodiment, the aperture wall  114  is tapered so as mate with a complementary mating surface  116  at a proximal or mating end  126  of the sleeve component. The taper is such that the inner-most end  118  of the aperture is larger than the outer-most end  120 . It is understood that the taper angle can vary about the longitudinal axis of the sleeve to facilitate seating of the mating end  126  within the aperture wall at a predetermined position. 
     The structure of the aperture wall  114  and the complementary sleeve mating surface  116  allows the sleeve  110  to be positioned at a range of angles in relation to the outer member  102  of the acetabular cup, as shown in FIG.  9 . By providing a range of angles, the sleeve component  110  can be positioned so as to provide access to desired regions of bone in the acetabular cavity, such as those having the deepest and/or best quality bone. In an exemplary embodiment, an angle A formed by a longitudinal axis  127  of the sleeve and a normal to the convex outer component  102 , i.e., a central axis of the aperture, ranges from zero degrees (axis  127  coincident with the normal) to about ten degrees. It is understood that the sleeve  110  can be rotated in any direction from the normal for allowing the surgeon to insert the screw into a bone at a selected angle for optimal fixation of the cup. 
     Referring again to FIG. 8, the elongate sleeve  110  has a longitudinal bore  122  formed therein through which the screw  108  passes. The bore extends from a proximal opening  123  to a distal opening  125 . In an intermediate region  124  of the sleeve, the bore  122  has a diameter that allows passage of a threaded region  128  and head  130  of the fixation screw  108 . In an exemplary embodiment, the bore  122  is flared, thus increasing in diameter, at the proximal opening  123  so as to facilitate insertion of the screw into the sleeve. The distal opening  125  of the bore has a tapered seating surface  132  that complements the geometry  134  of the screw head  130  such that the screw is properly seated within the sleeve  110  and is prevented from exiting the sleeve  110 . 
     To implant the acetabular cup system  110 , a cavity is reamed in the patient&#39;s acetabulum using conventional techniques and instruments. In one embodiment, the formed acetabular cavity is adapted for interference fit engagement with the acetabular cup  10 . After the cup is inserted into the formed cavity, the surgeon drills holes in the bone via the apertures  106  in the cup outer member. The holes are formed at an angle to allow insertion of the fixation screws into selected regions of bone, such as those having the deepest and/or best quality bone. The holes are sized to be slightly smaller than the screws to facilitate insertion of the screws into the bone. A larger diameter drill is then used to enlarge an upper region of the formed holes so as to form enlarged and unenlarged regions of each hole. The enlarged region  129  of the hole is shown in phantom in FIG.  6 . The enlarged region of the hole conforms to an outer diameter of the sleeve intermediate portion  124 , which protrudes from the cup outer surface. After implantation, there is a gap between an end  131  of the enlarged region and the distal end  113  of the sleeve. 
     The sleeve components  110  are then engaged with respective fixation screws  108 . The surgeon then inserts the sleeve/screw assembly into an aperture  106  in the cup and rotates the screw within the formed hole until the mating surface  116  of the sleeve is securely seated within the aperture  106 . The intermediate portions  124  of the sleeve are disposed within the enlarged regions of the holes. Each sleeve/screw assembly is secured in place to completely affix the outer member  102  of the acetabular cup to the bone. 
     The polyethylene inner member  104  is then mated to the cup outer member  102  using conventional techniques and components, such as a bearing insert. The implanted acetabular cup can then receive a corresponding femoral component. 
     As the acetabular cup subsides into bone (FIG.  7 ), the gap between the distal end  113  of the sleeve and the end  131  of the enlarged region of the hole decreases. As the gap shrinks, the fixation screw  108  recedes from the bone and the head  130  of the screw moves longitudinally in the sleeve bore. However, the length of the bore is of sufficient length to accommodate movement of the screw without the screw head  130  contacting the bearing member  104 . 
     It is understood that the overall dimensions of the acetabular cup system components can vary. In an exemplary embodiment, the sleeve component  110  has a length in the range of about eleven millimeters to about sixteen millimeters and protrudes about five millimeters to about ten millimeters from the outer surface  112  of the implanted cup. Initially, the screw is inserted into bone to provide a distance from the head  130  of the screw  108  to the surface of the inner member  104  that can range from about seven millimeters to about twelve millimeters. 
     One skilled in the art will appreciate further features and advantages of the invention based on the above-described embodiments. Accordingly, the invention is not to be limited by what has been particularly shown and described, except as indicated by the appended claims. All publications and references cited herein are expressly incorporated herein by reference in their entirety.