Abstract:
The present invention provides an improved prosthetic stem for implantation in a bone. Specifically, the present invention provides a prosthetic femoral stem having a strengthening rib protruding from the substrate thereof and being flush with a porous coating formed thereon. The strengthening rib is advantageously positioned so as to increase the cross-sectional moment of inertia of the prosthetic femoral stem and thereby decrease the stress on the anterio-lateral surface of the femoral stem. A protrusion in accordance with the present invention may also be utilized with a prosthetic femoral stem absent a porous coating. In such an embodiment, the height of the protrusion is such that the protrusion will be surrounded by the cement mantle formed when the prosthetic femoral component is cemented in a femoral canal.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a prosthetic stem for implantation in a bone. More particularly, the present invention relates to a prosthetic hip stem (i.e., femoral stem) having a strengthening rib to increase the fatigue strength of the prosthetic stem.  
           [0003]    2. Description of the Related Art  
           [0004]    Orthopedic implants utilized to replace all, or a portion of, a patient&#39;s joint (e.g., the hip) are commonly utilized to restore the use of, or increase the use of a joint which has deteriorated due to, e.g., aging, illness or injury. In the case of hip replacement, femoral components are utilized to replace a portion of the patient&#39;s femur including, e.g., the femoral head and neck. A femoral stem is positioned within a canal of the femur and is secured thereto. The femoral stem includes a femoral neck adapted to receive a prosthetic femoral head to complete the femoral prosthesis. Prosthetic femoral stems are generally either cemented in the femoral canal or are interference fit therein.  
           [0005]    Femoral stems may advantageously include a porous external surface to accommodate bone ingrowth or cement interdigitation. Various porous substances are utilized to coat the substantially nonporous outer surface (i.e., substrate) of a femoral stem including, e.g., wire mesh, or beaded or dimpled surfaces. For the purposes of this document, “substantially nonporous” signifies a material having less porosity relative to the porous coating of a prosthetic stem.  
           [0006]    Femoral stems are susceptible to fatigue failure after repeated loading over time. Stems having a porous coating are generally not as strong as a similarly sized stem absent a porous coating since, e.g., the substantially nonporous core of a coated stem (which is stronger than the porous coating) is smaller than the substantially nonporous core of a similarly sized stem absent porous coating. Furthermore, the porous coating of a femoral stem creates sharp corners between the porous coating and the substrate of the femoral stem. These sharp corners cause stress risers which can weaken the stem.  
           [0007]    What is needed in the art is a femoral stem having a structure which increases the fatigue strength of a femoral stem having a porous coating without increasing the external dimensions thereof.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention provides an improved prosthetic stem for implantation in a bone. Specifically, the present invention provides a prosthetic femoral stem having a strengthening rib protruding from the substrate thereof and being flush with any porous substance formed thereon. The strengthening rib is advantageously positioned so as to increase the cross-sectional moment of inertia of the prosthetic femoral stem and thereby increase the fatigue strength of the prosthetic femoral stem. The strengthening rib of the current invention increases the fatigue strength of the femoral stem not only by increasing the effective core area at a critical area of the stem (i.e., a high stress area where fatigue failure is likely to occur), but also by decreasing the stress risers associated with a porous coating at the aforementioned critical area. The strengthening rib is formed from a substantially nonporous material, and, in one exemplary embodiment is formed from a material substantially identical to the substrate material.  
           [0009]    The invention, in one form thereof, comprises a prosthetic stem for implantation in a bone. The prosthetic stem of this form of the current invention includes a neck connected to a shaft, with the neck and shaft forming an obtuse angle. A porous substance protrudes outwardly from a substrate of the prosthetic stem and a substantially nonporous protrusion also protrudes outwardly from the substrate. The substantially nonporous protrusion is substantially flush with the porous substance so that the nonporous protrusion does not increase the external dimensions of the prosthetic stem.  
           [0010]    The invention, in another form thereof, comprises a prosthetic hip stem for implantation in a femur utilizing bone cement to form a mantle about the portion of the hip stem inserted into the canal in the femur. The hip stem of this form of the current invention includes a neck connected to a shaft, with the neck extending from a medial side of the hip stem to form an obtuse angle with the shaft. A transition section is positioned intermediate the neck and the shaft and has a transverse cross-sectional area larger than the transverse cross-sectional area of the shaft. A protrusion is positioned on the external surface of the hip stem and has a height whereby the protrusion is covered by the mantle of bone cement when the prosthetic hip stem is implanted in a femur.  
           [0011]    The present invention advantageously increases the fatigue strength of a prosthetic femoral stem without increasing the external dimensions thereof. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    The above-mentioned and other features and objects of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:  
         [0013]    [0013]FIG. 1 is a perspective view of a femoral stem in accordance with the present invention;  
         [0014]    [0014]FIG. 2 is a side view thereof;  
         [0015]    [0015]FIG. 3 is a side view of a second embodiment of a femoral stem in accordance with the present invention;  
         [0016]    [0016]FIG. 4 is a medial view of a third embodiment of a femoral stem in accordance with the present invention  
         [0017]    [0017]FIG. 5 is a lateral view of the femoral stem illustrated in FIG. 4;  
         [0018]    [0018]FIG. 6 is a cross-sectional view thereof;  
         [0019]    [0019]FIG. 7 is a cross-sectional view of a fourth embodiment of a femoral stem in accordance with the present invention;  
         [0020]    [0020]FIG. 8 is a cross-sectional view of the prosthetic femoral stem illustrated in FIG. 2;  
         [0021]    [0021]FIG. 9 is a cross-sectional view of the prosthetic femoral stem illustrated in FIG. 3; and  
         [0022]    [0022]FIG. 10 is a side view of a fifth embodiment of a prosthetic femoral stem in accordance with the present invention. 
     
    
       [0023]    Corresponding reference characters indicate corresponding parts throughout the several views. Although the drawings represent embodiments of the present invention, the drawings are not necessarily to scale and certain features may be exaggerated to better illustrate and explain the invention. The exemplifications set out herein illustrate exemplary embodiments of the invention only, and such exemplifications are not to be construed as limiting the scope of the invention in any manner.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0024]    Referring now to the drawings and particularly to FIG. 1, there is illustrated femoral stem  20  having porous substance  28  affixed to substrate  26  and protrusion  40  extending from substrate  26 . As illustrated in FIGS. 2 and 8, protrusion  40  is flush with porous substance  28 . The larger effective transverse cross-sectional area of substrate  26  created by protrusion  40  increases the transverse cross-sectional moment of inertia of femoral stem  20  and thereby effects a decrease in stress on the anterio-lateral surface of femoral stem  20 . For the purpose of this document, “transverse cross-sectional area” denotes the area of a cross-section taken along a plane substantially perpendicular to the longitudinal axis of femoral shaft  26 . Femoral stem  20  generally includes distal end  50 , femoral shaft  36 , transition section  30  (having medial curve  32 ), flange  24 , shoulder  23 , and neck  22  terminating in proximal end  52 . The geometry of femoral stem  20  is illustrated by way of example only. It is contemplated that the teachings of the present invention will have applicability to femoral stems of varying geometry.  
         [0025]    In the exemplary embodiment of FIGS. 1, 2, and  8 , protrusion  40  of femoral stem  20  has a generally oval shape to maximize the coverage of porous substance  28  and minimize the stress concentrations at the edges of protrusion  40 . The oval shape of protrusion  40  eliminates sharp comers about the periphery thereof and thereby decreases the stress concentrations at the edges of protrusion  40 . The greater transverse cross-sectional area at the midpoint of protrusion  40  is positioned at a critical area of femoral stem  20  (i.e., a high stress area where fatigue failure is likely to occur). The critical area of femoral stem  20  will vary depending upon whether femoral stem is implanted in a right or a left femur. The relatively wide midpoint of protrusion  40  allows femoral stem  20  to accommodate both right and left femur implantations as well as slight rotations of the femoral stem during implantation. Advantageously, protrusion  40  is positioned on the medial side of femoral stem  20  so that protrusion  40  is loaded in compression. In this way, the detrimental effects of stress risers in protrusion  40  are effectively negated.  
         [0026]    Femoral stem  20  is, e.g., formed of a cobalt-chromium alloy. However, femoral stem  20  may be constructed of other bio-compatible metals or alloys, such as titanium. Similarly, porous coating  28  comprises, e.g., a beaded coating formed from a cobalt-chromium alloy, although various other porous coatings may be utilized in conjunction with the teachings of the present invention. The generally oval shape of protrusion  40  allows for maximum application of the porous outer surface while providing sufficient material to adequately increase the transverse cross-sectional moment of inertia of femoral stem  20 . As stated above, the midpoint of protrusion  40  comprises the portion of protrusion  40  having the greatest transverse cross-sectional area. The midpoint of protrusion  40  is positioned at medial curve tangency  34 . Medial curve tangency  34  comprises the portion of medial curve  32  tangent to cylindrical femoral shaft  36 . In other words, medial curve tangency  34  is located at the point where medial curve  32  ends and cylindrical femoral shaft  36  begins. In one exemplary femoral stem, medial curve tangency  34  comprises a critical area of the femoral stem (i.e., a high stress area where fatigue failure is likely to occur).  
         [0027]    Less protrusion material is required toward the proximal end of the stem because the transverse cross-sectional area of the stem in transition section  30  is larger than the transverse cross-sectional area of femoral shaft  36 , while less material is needed toward the distal end of the stem since this end of femoral stem  20  will be solidly fixed in the femur. With this in mind, the transverse cross-sectional area of protrusion  40  of the exemplary embodiment illustrated in FIGS. 1, 2, and  8  generally decreases from the midpoint thereof to the proximal and distal ends thereof to allow for maximum application of the porous coating. In one exemplary embodiment, protrusion  40  extends from it&#39;s midpoint approximately 2.5 centimeters (1 inch) into transition section  30  and approximately 2.5 centimeters (1 inch) into femoral shaft  36 .  
         [0028]    [0028]FIGS. 3 and 9 illustrate femoral stem  20   a  in accordance with a second embodiment of the present invention. The several embodiments of the present invention include similar components to the embodiment illustrated in FIGS. 1, 2, and  8 . These similar components are denoted with a reference numeral having a letter appended thereto. For the sake of brevity, these similar components will not all be discussed in conjunction with the various alternative embodiments disclosed herein. Femoral stem  20   a  includes lateral protrusion  42  of similar shape to medial protrusion  40  illustrated in FIG. 1. Lateral protrusion  42  is positioned on the lateral side of femoral stem  20   a  with the midpoint thereof generally lying opposite medial curve tangency  34  of femoral stem  20 . Various placements of the protrusions of the current invention may be utilized to increase the cross-sectional moment of inertia of the femoral stem and therefore increase the strength of a femoral stem. Furthermore, plural protrusions may be utilized to further increase the strength of a femoral component in accordance with the present invention. For example, medial protrusion  40  (FIG. 1) may be used in conjunction with lateral protrusion  42  (FIGS. 3) to form a femoral stem in accordance with the present invention.  
         [0029]    FIGS.  4 - 6  illustrate a third embodiment of the present invention having medial rib  44  and lateral rib  46 . As illustrated in FIG. 6, medial rib  44  and lateral rib  46  extend from substrate  26   b  and are flush with porous coating  28   b.  Ribs  44 ,  46  run substantially the length of femoral stem  20   b  and include end points which gradually taper to transition into substrate  26   b.  The smooth transitions provided by the tapering of the end points of ribs  44 ,  46  function to decrease stress risers therein. As illustrated in FIG. 4, medial rib  44  generally runs from distal end  50   b  to the distal side of flange  24   b.  Similarly, lateral rib  46  runs from distal end  50   b  to shoulder  23   b.  Although illustrated as running substantially the length of femoral stem  20   b,  ribs  44 ,  46  may be partial ribs running along only a portion of the length of femoral stem  20   b.  It is further contemplated that ribs  44 ,  46  could be of varying width, with the greatest transverse cross-sectional area (associated with the greatest width) being positioned at a critical area.  
         [0030]    [0030]FIG. 7 illustrates a cross-sectional view of femoral stem  20   c  having a single rib  45 . Rib  45  may be either a medial or lateral rib as discussed above with respect to femoral stem  20   b.  FIG. 7 is provided to illustrate a single rib configuration in accordance with the present invention, as opposed to the dual rib configuration of FIGS.  4 - 6 .  
         [0031]    [0031]FIG. 10 illustrates femoral stem  20   d  having helical rib  48 . Helical rib  48  includes a proximal end adjacent the distal side of flange  50   d  and a distal end adjacent distal end  50   d  of femoral stem  20   d.  The proximal end of helical rib  48  is rotated 90° from the distal end of helical rib  48 . Helical rib  48  will have particular applicability to longer femoral stems which experience substantial bending in the anterior-posterior plane of the distal portions thereof. The helical arrangement of rib  48  allows for rib placement in both the medial-lateral plane and the anterior-posterior plane of femoral stem  20   d,  with the anterior-posterior placement of rib  48  advantageously occurring in the distal portion of the femoral stem.  
         [0032]    While described above with respect to a femoral stem having a porous coating, the ribs of the current invention may be utilized with a femoral stem absent such a porous outer coating. In such applications, the protrusions of the present invention will extend outwardly from the external surface of the femoral stem. In such situations, the height of the ribs will be sized so that the protrusion will be covered by the cement mantle formed by the bone cement utilized to implant the femoral stem.  
         [0033]    While this invention has been described as having exemplary designs, the present invention may be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains.