Patent Publication Number: US-6911048-B2

Title: Modular hip prosthesis

Description:
RELATED APPLICATION 
   This application is a Continuation application of U.S. Ser. No. 09/524,341, filed Mar. 13, 2000 now U.S. Pat. No. 6,319,286. 

   BACKGROUND AND SUMMARY OF THE INVENTION 
   The present invention relates generally to the field of total hip arthroplasty, and, more particularly, to a three segment modular hip stem that allows full size interchangeability between component parts, yet provides superior resistance to component disengagement during use. 
   Modularity in total hip arthroplasty design is an evolving concept that is receiving increased citation in the clinical literature. The advantages of these systems include off the shelf flexibility for customizing proximal and distal canal filling, as well as accommodating difficult situations of proximal deformity and bone loss. These designs, however, raise concerns that include structural compromise at the metal-metal interconnections due to stresses and intercomponent disengagement. 
   To address these concerns, the present invention, in summary, provides a modular hip prosthesis comprising the following components: (a) a proximal segment having an axial bore therethrough, the proximal segment including a neck lockingly engageable with a femoral head component, and further including a male tapered portion extending distally of said neck; (b) a distal segment having a proximal end and a distal tip, the distal segment further being formed with a male tapered portion adjacent the proximal end thereof; and (c) a metaphyseal segment having a proximal end and a distal end, the metaphyseal segment including a bone engaging outer surface portion, and further including an axial bore therethrough, the axial bore including first and second female tapered portions, the first female tapered portion located adjacent the proximal end of the metaphyseal segment and dimensionally configured to lockingly engage the male tapered portion of the proximal segment, the second female tapered section located adjacent the distal end of the metaphyseal segment and dimensionally configured to lockingly engage the male tapered portion of the distal segment. 
   The male and female tapered portions of the corresponding proximal, metaphyseal and distal segments each comprises a conical section blending into a generally parabolic-shaped section. The blended conical taper/parabolic taper geometry of each tapered portion ensures sufficient taper contact area, and decreases the interfacial contact stresses and internal body stresses under bending loading of the male/female taper junction. The conical tapered sections each have taper angles ranging from about 1° to about 2.5° to provide enhanced torsional resistance at the taper junctions. The proximal segment is lockingly engageable with the proximal end of the metaphyseal segment to align the axial bores formed through the proximal and metaphyseal segments. The proximal end of the distal segment is lockingly engageable with the distal end of the metaphyseal segment to align the axial bores formed through the distal and metaphyseal segments. 
   Optionally, the proximal segment is formed with a throughbore, and the distal segment is formed with a threaded bore adjacent the proximal end thereof. These bores are alignable with the axial bore of the metaphyseal segment. A screw, dimensionally configured to pass through the aligned bores, is threadably engaged with the threaded bore formed in the distal segment to further enhance locking engagement of the prosthesis components if desired. 
   The present invention provides the following advantages: (a) superior resistance to component disassociation by increasing taper contact area and reducing contact stresses due to bending and torsional loads at the taper junctions; (b) intraoperative flexibility through its modularity; (c) full interchangeability of any segment with any other segment; (d) adjustability of each segment for anteversion and retroversion independent of the position of other segments, thus allowing a universal design for left and right hip applications; (e) independent selection of leg length and offset of the prosthesis; (f) primary and revision application with the same system; (g) allows the surgeon to tailor the device to the anatomy of the patient even in the face of a revision surgery that might leave a bone deficit; and (h) the use of all styles and sizes of femoral head components. 
   The accompanying drawings, which are incorporated in and constitute part of the specification, illustrate the detailed description and preferred embodiments of the invention, and together with the detailed description, serve to explain the principles of the invention. It is to be understood, however, that both the drawings and the description are explanatory only and are not restrictive of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an exploded, perspective view of one embodiment of the modular hip prosthesis of the invention. 
       FIG. 2  is a cross-sectional, side elevation view of one embodiment of the proximal component of the invention. 
       FIG. 3  is a cross-sectional, side elevation view of one embodiment of the metaphyseal component of the invention. 
       FIG. 4  is a transverse cross-sectional view of one embodiment of the metaphyseal component of the invention taken along lines A—A of FIG.  3 . 
       FIG. 5  is a cross-sectional, side elevation view of one embodiment of the distal component of the invention. 
       FIG. 6  is a transverse cross-sectional view of one embodiment of the distal component of the invention taken along lines B—B of FIG.  5 . 
       FIG. 7  is a cross-sectional, side elevation view of the engaged proximal, metaphyseal, and distal components of one embodiment of the modular hip prosthesis of the invention. 
       FIG. 8  is a cross-sectional, side elevation view of the proximal, metaphyseal, and distal components of  FIG. 7  showing illustrative taper and blend dimensions. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to  FIGS. 1-8 , wherein like reference numerals are used to identify like components throughout the various views, a first embodiment of the modular hip prosthesis of the invention is shown generally at  10 . As shown in  FIG. 1 , hip prosthesis  10  generally includes: (a) a proximal segment  12 ; (b) a metaphyseal segment  14 ; and (c) a distal segment  16 . A threaded screw  18  may optionally be used to enhance locking engagement of segments  12 ,  14 , and  16  as described below. As here embodied, proximal segment  12 , metaphyseal segment  14 , and distal segment  16  are each constructed as separate parts. As a result, the segments may each be sized independently of one another. Such independent sizing capability gives the prosthesis modularity—that is, it provides the surgeon with a wide selection of prosthesis configurations to accommodate virtually every anatomical condition encountered during surgery. Advantageously, the modular prosthesis  10  of the invention may be implanted using well known bone cement implantation techniques, or, in the alternative, may be implanted in an uncemented mode, using bone engaging surface applications well known to persons skilled in the art. 
   Referring now to  FIG. 2 , proximal segment  12  includes a neck  20  formed with: (a) an angularly offset arm  21  terminating in a male tapered column  22 ; (b) an extension member  24  extending distally of neck  20  formed with a male tapered portion  25 , and terminating in a cylindrical nipple  26 ; and (c) a segmented bore  27  formed through neck  20 , extension member  24 , and nipple  26 . Preferably, proximal segment  12  is constructed from a biocompatible, high strength titanium alloy. However, proximal segment  12  may be constructed from other biocompatible materials such as cobalt chromium alloy, stainless steel, and composite materials. The outer surface finish of proximal segment  12  is preferably polished, with a surface roughness average of 32 microinches or less as determined by profilometry. The outer surface finish may also be smooth matte or machined using surface preparation techniques well known in the art. 
   Tapered column  22  of proximal segment  12  is dimensionally configured for locking engagement with the complimentary female tapered portion of a femoral head component (not shown). One skilled in the art will readily recognize that proximal segment  12  may be constructed to accommodate all styles and materials of femoral head components. An undercut  23  is formed in arm  21  and column  22  on each side of proximal segment  12  to increase the range of motion between neck  20  and the acetabular component (not shown) of a total hip joint replacement system, and to facilitate engagement of a femoral head removal tool (not shown) when it is necessary to disassemble the femoral head from proximal segment  12  during repair or revision of hip prosthesis  10 . 
   As preferably embodied, tapered portion  25  of extension member  24  comprises a male conical tapered section  25   a  blending into a generally parabolic-shaped male tapered section  25   b  having a blend radius R 2  of about 0.25 inch (see FIGS.  2  and  8 ). The parabolic geometry of tapered section  25   b  decreases the interfacial contact stresses and internal body stresses under bending loading between tapered portion  25  and complementary female tapered portion  33  of metaphyseal segment  14  (described below). As preferably embodied, the conical taper section  25   a  has a taper angle ranging from about 1° to about 2.5° to provide enhanced torsional resistance at the proximal/metaphyseal taper junction. In the illustrative embodiment of the invention shown in  FIG. 8 , conical tapered section  25   a  has a length of about 0.43 inch, and parabolic tapered section  25   b  has a length of about 0.09 inch. For these illustrative taper lengths, the ratio of parabolic taper length to conical taper length is about 21%. As preferably embodied, the parabolic taper/conical taper length ratio should range from about 5% to about 30%. This range ensures sufficient taper contact area, and minimizes the presence of sharp corners on the parabolic tapered section  25   b  which can lead to high point contact stresses at the proximal/metaphyseal taper junction when the prosthesis is subject to bending stresses. As preferably embodied, the conical tapered section  25   a  has a blend radius R 1  of about 0.09 inch (see FIG.  8 ). The complementary conical tapered section  33   a  of female tapered segment  33  has a blend radius R 3  of about 0.05 inch. These differing radii create a reduced stress condition at the proximal/metaphyseal taper junction in the vicinity of gap G (see  FIG. 7 ) that is created when the proximal and metaphyseal segments are joined. Advantageously, the same geometries and radii for tapered portions  25  and  33  can be used for all sizes of proximal segment  12  and metaphyseal segment  14 , thereby enhancing size interchangeability, and thus modularity, between the proximal and metaphyseal segments. 
   As preferably embodied, nipple  26  has a length of about 0.18 inch to increase the moment arm of extension member  24  (see FIGS.  2  and  8 ), and thereby, assist in unloading the proximal/metaphyseal taper junction upon inducement of bending stresses in the prosthesis. As with the taper geometries and blend radii described above, the same length for nipple  26  can be used for all sizes of proximal segment  12 . Nipple  26  is dimensionally configured smaller than the diameter of sections  32   a ,  32   b  and  32   c  of throughbore  32  in metaphyseal segment  14  (described below) so that, when extension member  24  of proximal segment  12  is slidingly received in throughbore  32  upon assembly of the prosthesis components (see FIG.  7  and discussion below), nipple  26  will not initially engage the sidewall of bore  32 . Upon application of sufficient load to the femoral head of the prosthesis (not shown), nipple  26  will contact the sidewall of intermediate bore segment  32   b  of bore  32 , and thereby, transfer a portion of the induced bending stress away from the proximal/metaphyseal taper junction. 
   Referring again to  FIG. 2 , segmented bore  27  of proximal segment  12  includes a first straight section  27   a , a tapered intermediate section  27   b , and a second straight section  27   c . As preferably embodied, section  27   b  tapers inwardly toward bore section  27   c  at an angle of about 60°. Bore sections  27   a ,  27   b  and  27   c  are dimensionally configured to allow screw  18  to pass through proximal segment  12 . Bore section  27   a  also acts as a countersink for the head of screw  18 , and should be dimensioned large enough to comfortably accommodate a mechanical driver such as a screw driver or drill bit to threadably engage screw  18  with threaded bore  42  formed in distal segment  16  (discussed more fully below) when screw  18  is used as part of the prosthesis  10  assembly. 
   Referring now to  FIG. 3 , metaphyseal segment  14  has a proximal end  14 , a distal end  14   b , and is configured with a trapezoidal truncated pyramidal section  30 , integrated with a conical section  31 . As shown in  FIG. 4 , this profile presents itself in transverse cross-section as a generally trapezoidal section  36  offset from a generally circular section  35 . Alternatively, the pyramidal section  30  may be constructed so that the metaphyseal segment  14  has a generally rectangular transverse cross section offset from a generally circular transverse cross section. Metaphyseal segment  14  is preferably constructed from a biocompatible, high strength titanium alloy, but may also be constructed from other biocompatible materials such as cobalt chrome alloy, stainless steel, and composite materials. Metaphyseal segment  14  also includes a bore  32  comprising proximal bore section  32   a , intermediate bore section  32   b , and distal bore section  32   c . Referring to  FIGS. 3 and 8 , bore segment  32   a  is formed with a female tapered portion  33  comprising a conical tapered section  33   a  blending into a generally parabolic-shaped tapered section  33   b . Female tapered sections  33   a  and  33   b  are complementary to male tapered sections  25   a  and  25   b , respectively, of cylindrical section  24 . As here embodied, conical tapered section  33   a  has a taper angle ranging from about 1° to about 2.5°, a length of about 0.50 inch, and a blend radius R 3  (referred to above) of about 0.05 inch. Parabolic tapered section  33   b  has a length of about 0.09 inch, and a blend radius R 4  of about 0.25 inch (see FIG.  8 ). For the foregoing illustrative taper lengths, the ratio of parabolic taper length to conical taper length is about 18%. Tapered sections  33   a  and  33   b  are dimensionally configured to lockingly engage tapered sections  25   a  and  25   b , respectively, upon insertion of cylindrical section  24  into bore  32 . As with tapered sections  25   a  and  25   b  of cylindrical section  24 , the parabolic taper/conical taper length ratio for tapered sections  33   a  and  33   b  should range from about 5% to about 30% to ensure reduced contact stresses and internal stresses in the region of the proximal/metaphyseal taper junction. Also, as discussed above with respect to proximal segment  12 , the same taper geometries and blend radii for tapered sections  33   a  and  33   b  can be used for all sizes of metaphyseal segment  14  to enhance interchangeability of the proximal and metaphyseal components, and thereby, modularity of the prosthesis  10 . 
   Referring again to  FIGS. 3 and 8 , bore segment  32   c  of metaphyseal bore  32  is formed with tapered portion  34  comprising a conical tapered section  34   a  and a generally parabolic-shaped tapered section  34   b . Tapered sections  34   a  and  34   b  are dimensionally configured to lockingly engage the corresponding male tapered sections  43   a  and  43   b  of distal segment  16 , respectively, upon insertion of proximal end  16   a  of distal segment  16  into bore  32  of metaphyseal segment  14  (as more fully discussed below). As here embodied, the conical tapered section  34   a  has a length of about 0.51 inch, a taper angle ranging from about 1° to about 2.5°, and a blend radius R 5  of about 0.50 inch. Parabolic tapered section  34   b  has a length of about 0.09 inch and a blend radius R 6  of about 0.25 inch (see FIG.  8 ). For the foregoing illustrative taper lengths, the ratio of parabolic taper length to conical taper length is about 18%. As with the other tapered portions of the prosthesis  10  discussed above, the parabolic taper/conical taper length ratio should range from about 5% to about 30% to ensure sufficient taper contact area and minimize high point contact stresses at the proximal/metaphyseal taper junction. Also, as with the other tapered portions described above, the same taper geometries and blend radii for tapered sections  34   a  and  34   b  can be used for all sizes of metaphyseal segment  14  to enhance interchangeability of components, and thereby, modularity of the prosthesis  10 . 
   The geometry of metaphyseal segment  14  increases torsional stability of the component during use in the body, and provides better fill of the proximal intramedulary canal. The outer surface finish of metaphyseal segment  14  may be polished, with a surface roughness average of about 32 microinches or less as determined by profilometry. The outer surface finish may also be smooth matte or machined using surface preparation techniques well known in the art. As preferably embodied, the outer surface of metaphyseal segment  14  contains a bone engaging surface coating, such as, for example, grit blasted surface, plasma spray coating, sintered metal bead coating, hydroxylapatite coating, or other bioactive coatings such as bio-glass ceramics, demineralized bone and carrier, and growth factor and carrier. The application of such coatings to metallic implant surfaces is well known in the art. Optionally, metaphyseal segment  14  may be constructed with a distal ring  37 . Distal ring  37  is a region of raised material equal in thickness to the minimum thickness of the bone engaging coating applied to the outer surface of the metaphyseal segment. Distal ring  37  increases the wall thickness of conical section  31  of metaphyseal segment  14 . This in turn will increase the fatigue strength of conical section  31  by increasing the local wall thickness and shielding it from notches that may result from the porous coating process. As preferably embodied, distal ring  37  should be used in smaller sizes of metaphyseal segment  14 , wherein the sidewall of conical section  31  in the vicinity of distal end  14   b  may be relatively thin. The local stress levels on conical section  31  that may necessitate use of distal ring  37  for a particular size of metaphyseal segment  14  can be readily determined by persons skilled in the art. 
   Referring now to  FIG. 5 , distal segment  16  is formed with a proximal end  16   a , a distal tip  16   b , and includes a plurality of sharpened longitudinal flutes  40  formed along an incremental length of the outer surface thereof. The sharp edges of flutes  40  dig into the cortical bone wall of the intramedulary canal to increase the torsional stability of distal segment  16  during use of the prosthesis in the body. Distal segment  16  is also optionally formed with a coronal slot  41  beginning at distal tip  16   b , and proceeding proximally for an incremental length thereof. Coronal slot  41  increases the flexibility of distal segment  16 . This increased flexibility inhibits the concentration of stresses at distal tip  16   b  when the prosthesis is loaded, and allows the prosthesis to better accommodate the curvature of the intramedullary canal. Those skilled in the art will recognize that the length of longitudinal flutes  40  can readily be adjusted as desired, in light of the overall prosthesis design scheme, to facilitate resistance to torsional loadings on the prosthesis. In the illustrative embodiment of distal segment  16  shown in the Figures, the length of longitudinal flutes  40  is about 80% of the overall length of distal segment  16 . Advantageously, the same ratio of flute length to distal segment length can be used for all sizes of distal segment  16 . Those skilled in the art will also recognize that the length of coronal slot  41  can be readily adjusted to provide the desired degree of flexibility in distal segment  16  without unduly compromising the fatigue strength of the distal segment. 
   As preferably embodied, distal tip  16   b  has a generally parabolic axial cross-section which also serves to reduce contact stresses between distal segment  16  and the bone in the vicinity of the distal tip. As shown in  FIG. 6 , distal segment  16  has a generally round transverse cross-section, but may be constructed with other cross-sectional geometries such as, for example, hexagonal or oval. Optionally, distal segment  16  may be formed with longitudinal channels instead of sharp longitudinal flutes to facilitate both increased stem flexibility and engagement of cortical bone in the intramedulary canal. Although distal segment  16  shown in the Figures has a straight profile, it may also be curved to better match the natural curvature of the patient&#39;s intramedulary canal. Distal segment  16  is preferably constructed from a biocompatible, high strength titanium alloy, but may also be constructed from other biocompatible materials such as cobalt chrome alloy, stainless steel, and composite materials. Further, distal segment  16  is preferably provided with a polished outer surface finish having a surface roughness average of 32 microinches or less as determined by profilometry. The distal segment may also be provided with a smooth matte or machined outer surface finish using surface preparation techniques well known in the art. To facilitate fixation of distal segment  16  to the cortical bone wall of the intramedulary canal, if desired, distal segment  16  may also be constructed without longitudinal flutes, and instead provided with a porous bone engaging surface coating, such as, for example, grit blasted surface, plasma spray coating, sintered metal bead coating, hydroxylapatite coating, or other bioactive coating such as bio-glass ceramics, demineralized bone and carrier, and growth factor and carrier. 
   Referring now to  FIGS. 5 and 8 , distal segment  16  is also formed with a threaded bore  42  adjacent proximal end  16   a  thereof. Bore  42  is dimensionally configured to threadably engage screw  18  upon insertion through the aligned bores of proximal segment  12 , metaphyseal segment  14 , and distal segment  16  (see discussion below). Distal segment  16  is also formed with a male tapered portion  43  adjacent proximal end  16   a . Tapered segment  43  comprises a conical tapered section  43   a  and a generally parabolic-shaped tapered section  43   b . Male tapered sections  43   a  and  43   b  are dimensionally configured to lockingly engage the corresponding female tapered sections  34   a  and  34   b  of metaphyseal segment  14 , respectively, upon insertion of proximal end  16   a  of distal segment  16  into bore  32  of metaphyseal segment  14 . As here embodied, conical tapered section  43   a  has a length of about 0.48 inch, a taper angle ranging from about 1° to about 2.5°, and a blend radius R 7  of about 0.09 inch. Parabolic tapered section  43   b  has a length of about 0.09 inch, and a blend radius R 8  of about 0.25 inch (see FIG.  8 ). For the foregoing illustrative taper lengths, the ratio of parabolic taper length to conical taper length is about 19%. The parabolic/conical taper length ratio should range from about 5% to about 30% to ensure sufficient taper contact area and minimize high point contact stresses at the metaphyseal/distal taper junction. Also, as with the other tapered portions described above, the same taper geometries and blend radii for tapered sections  43   a  and  43   b  can be used for all sizes of distal segment  16  to enhance interchangeability of the distal and metaphyseal components, and thereby, modularity of the prosthesis  10 . 
   Referring now to  FIGS. 7 and 8 , cross-sectional views of proximal segment  12 , metaphyseal segment  14 , and distal segment  16  are shown to more clearly illustrate the internal relationship between these components upon assembly. As shown in the Figures, extension member  24  of proximal segment  12  is received in close-fitting, sliding relationship in bore section  32   a  of metaphyseal segment  14 , with tapered sections  25   a  and  25   b  of extension  24  lockingly engaging tapered sections  33   a  and  33   b  of bore segment  32   a , respectively. Similarly, proximal end  16   a  of distal segment  16  is received in close-fitting, sliding relationship in bore segment  32   c  of metaphyseal segment  14 , with tapered sections  43   a  and  43   b  of distal segment  16  lockingly engaging tapered sections  34   a  and  34   b  of bore segment  32   c , respectively. Before a taper lock relationship is established between proximal segment  12  and metaphyseal segment  14 , the angular orientation of arm  21  and column  22  of proximal segment  12  is established to place column  22  in the desired position to receive a conventional femoral head component (not shown). Upon locking engagement of the complimentary tapered portions of the proximal, metaphyseal and distal segments, bores  27 ,  32 , and  42  will be in axial alignment. Thereupon, screw  18  is inserted through the aligned bores into threaded engagement with the complimentary threaded section of bore  42 . Screw  18  has a countersunk head  19  receivable in countersink  28  formed in section  27   a  of metaphyseal bore  27 . Screw  18  is securely tightened to further enhance locking engagement of the proximal, metaphyseal and distal segments if desired. 
   The present invention may be embodied in other forms than disclosed in the detailed description of the invention without departing from the spirit or essential characteristics of the invention. Accordingly, the described embodiments of the invention are to be considered in all respects as illustrative and not restrictive. The scope of the present invention is therefore indicated by the claims set forth below, and not by the foregoing description of the invention. All modifications which come within the meaning and range of equivalency of the claimed subject matter are to be embraced within the scope of the claims.