Patent Publication Number: US-2005143835-A1

Title: Junction for a modular implant

Description:
BACKGROUND  
      Medical implants to replace or augment various parts of the mammalian body have been successfully used to reduce pain and improve function. For example, orthopaedic implants for replacing portions of bones and joints damaged by disease and/or trauma often eliminate pain and/or increase mobility. Orthopaedic implants for hips, knees, shoulders, ankles, elbows, wrists, the digits of the hands and feet, vertebral bodies, spinal discs, and other bones and joints have been developed. Many medical implants are made more versatile by providing them as separate modular components that can be combined to form an implant suited to a particular patient&#39;s condition. Where such modular components are supplied, a means for attaching them to one another is provided.  
     SUMMARY  
      The present invention provides a junction for modular implant components.  
      In one aspect of the invention, a modular joint implant includes a male/female junction between first and second joint components. The first component includes a bore having a longitudinal junction axis and a bore opening. The second component includes a projection engageable with the bore in male/female seating arrangement. A first portion of the bore opening is offset axially relative to a second portion of the bore opening.  
      In another aspect of the invention, the first portion is offset in a direction of increasing stiffness of the wall surrounding the bore.  
      In another aspect of the invention, the first portion is offset in a direction of increasing wall thickness.  
      In another aspect of the invention, the first portion is offset on a side of the implant that is generally in tension when the implant is loaded.  
      In another aspect of the invention, a modular joint implant includes a male/female junction having a side that is predominately in compression in use and a side that is predominately in tension in use. The implant includes a first component including a bore having a bore opening and an interior surface forming a female side of the male/female junction. The first component further has an exterior surface. The interior and exterior surfaces define a wall between them having a wall thickness. The wall thickness on the tensile side of the implant being greater than the wall thickness on the compressive side of the implant.  
      In another aspect of the invention, a modular joint implant includes a male/female junction having a side that is predominately in compression in use and a side that is predominately in tension in use. The implant includes a first component including a bore having a bore opening and an interior surface forming a female side of the male/female junction. The bore has a side on the tensile side of the implant that is shifted axially relative to a side of the bore on the compressive side of the implant.  
      These and other aspects of the invention will be described in reference to the appended drawings.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Various embodiments of the present invention will be discussed with reference to the appended drawings. These drawings depict only illustrative embodiments of the invention and are not to be considered limiting of its scope.  
       FIG. 1  is an exploded perspective view of a modular implant junction according to the present invention;  
       FIG. 2  is a side section view of the modular implant junction of  FIG. 1 ; and  
       FIG. 3  is a side section view of an optional configuration of the lateral side of the modular implant junction of the present invention.  
    
    
     DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS  
      Embodiments of a junction for a modular implant are applicable to a variety of implants for use throughout the body. A femoral hip stem has been used to illustrate the invention. However, the invention may also be applied to various other implants including orthopaedic implants for hips, knees, shoulders, ankles, elbows, wrists, the digits of the hands and feet, vertebral bodies, spinal discs, and other suitable implants.  
       FIGS. 1-2  depict a modular femoral hip implant  10  for replacing the proximal head and neck of a femur of a hip joint that has been damaged due to injury or disease. In use, the proximal head and neck are surgically removed and the femoral hip implant  10  is inserted into the proximal femur. The femoral hip implant  10  supports a femoral head  12  that may be a modular and separate component as shown. Optionally, the femoral head  12  may be integral to the femoral hip implant  10 . An acetabular component  14  may be implanted in the acetabulum of the pelvis to articulate with the femoral head  12 . Optionally, the femoral head  12  may articulate with the natural acetabulum. The femoral hip implant  10  has a medial aspect  16  and a lateral aspect  18 . When the patient loads the joint, such as by standing, walking, or other activities, forces are transmitted to the femoral hip implant  10  through the head  12 . These forces tend to create a bending moment that places the medial aspect  16  of the femoral hip implant in compression and the lateral aspect  18  in tension.  
      The femoral hip implant  10  may include modular components such as a proximal body  20  and a stem  22 . The proximal body  20  has a top end  24  and a bottom end  26 . A neck  28  extends upwardly and medially from the top end  24  to support the femoral head  12  for articulation with the acetabular component  14 . The proximal body  20  and stem  22  include a male/female junction for holding them together. In the illustrative embodiment, the female side of the junction is depicted in the proximal body  20  and the male side of the junction is depicted on the stem  22 . It is contemplated that the male/female portions may be reversed and still fall within the scope of the invention. The proximal body  20  includes a bore  30  ( FIG. 2 ) having a bore opening  32  and an interior surface  34  forming the female side of the male/female junction. The bore  30  has a longitudinal junction axis  36 . The proximal body  20  has an exterior surface  38  spaced from the interior surface  34  of the bore  30 . The exterior  38  and interior  34  surfaces define a wall  40  between them. The wall  40  has a wall thickness  42  that may be constant or that may increase from the bore opening  32  toward the top end  24  as shown in  FIG. 2 . An increasing wall thickness may be accomplished by tapering the bore such that it narrows from the bore opening  32  toward the top end  24  as in the illustrative embodiment. A tapered bore can be made self-locking as is known in the art.  
      The stem  22  includes a bottom end  44  and a top end  46 . The bottom end  44  is configured for insertion into the intramedullary canal of the patient&#39;s femur. The top end  46  includes a projection  48  having an exterior surface  50  forming the male side of the male/female junction. The projection  48  is engageable with the bore  30  in male/female seating arrangement along the junction axis  36 . A threaded stud  33  extends from the projection  48  and is received by a counter bore  35  formed in the proximal body  20 . A nut  37  threads onto the stud  33  to secure the male/female junction.  
      When the femoral hip implant  10  is loaded, the medial aspects of the proximal body  20  and stem  22  are placed in compression and the lateral aspects of the proximal body  20  and stem  22  are placed in tension. Due to differences in the bending stiffness of the proximal body  20  and stem  22  in the region of the male/female junction, the bore wall  40  may move relative to the exterior surface  50  of the projection  48 . Cyclic loading can lead to fretting between the interior surface  34  of the bore  30  and exterior surface  50  of the projection  48 . This may be more problematic on the tension side since tensile forces may initiate and propagate fatigue cracks.  
      The magnitude of the fretting motion is related to the relative stiffness of the male and female parts of the junction. The relative motion at the opening  32  of the bore  30  is an accumulation of the relative motion along the entire length of the junction. This accumulated relative motion may be decreased by decreasing the length along which the relative motion accumulates on the tensile side of the junction. However, it is ineffective to simply make the junction shorter, because this will create higher stresses on the smaller diameter male cross section where such a shortened junction would end. However, by shortening only the tensile side, the compressive side is still supported and the tensile stress at the end of the tensile side increases only slightly while the relative fretting motion decreases significantly. In the illustrative embodiment, a lateral portion  52  of the bore opening  32  is offset axially upwardly relative to a medial portion  54 . This offset, or relieved, portion  52  can be created by stepping up the lateral portion  52 , sloping up the lateral side such that the bore opening  32  is transverse to the junction axis  36 , or by forming the bore opening in some other suitable shape.  
      Fretting in the male/female junction can also be reduced by better matching the stiffness of the male and female sides of the male/female junction. The present investigators have found that one way to better match the stiffness of the male and female sides is to increase the stiffness of the bore wall  40  adjacent the bore opening  32  on the tensile side of the implant  10 . Increasing the stiffness can be accomplished by increasing the outer diameter of the proximal body  20  adjacent the bore opening  32  to move material radially away from the junction axis  36  such that the bending moment of inertia is increased. Increasing this stiffness can also be accomplished by increasing the wall thickness of the proximal body  20  at the junction of the proximal body  20  and projection  48 . In the illustrative embodiment, the bore  30  is tapered so that it narrows proximally and the wall thickness  42  increases proximally. By offsetting a lateral portion  52  of the bore opening  32  axially upwardly relative to a medial portion  54  the wall thickness on the lateral side  18  of the bore opening  32  is increased. This offset, or relieved, portion  52  can be created by sloping the lateral side such that the bore opening  32  is transverse to the junction axis  36  as shown, by stepping up the lateral portion  52 , or by forming the bore opening in some other suitable shape. In the illustrative embodiment, the projection  48  forming the male side of the junction narrows upwardly. Thus, moving the lateral portion  52  upwardly also moves the lateral interface to an area of decreased stiffness of the projection  48  to further match the stiffness of the male and female portions laterally. Finally, by moving the lateral side upwardly while having the medial, anterior, and posterior sides of the junction extend further downwardly, the fatigue properties of the lateral side are improved while the overall interface of the male and female parts is kept relatively large to facilitate secure engagement between them.  
      Another way increase the stiffness of the bore wall  40  is to enlarge the bore  30  on just the tensile side of the junction as shown in  FIG. 3  such that the bore wall is offset away from the projection  48 . This has the same effect as moving the tensile side axially upwardly as described above. Other ways of better matching the stiffness of the male and female sides of the junction may also be used and are considered within the scope of this invention.  
      It will be understood by those skilled in the art that the foregoing has described illustrative embodiments of the present invention and that variations may be made to these embodiments without departing from the spirit and scope of the invention defined by the appended claims.