Variable modulus prosthetic hip stem

• US 6,228,123 B1
• Filed: 08/19/1998
• Issued: 05/08/2001
• Est. Priority Date: 08/19/1998
• Status: Expired due to Fees

• Current Assignee: DePuy Orthopaedics Incorporated (Johnson & Johnson)
• Original Assignee: DePuy Orthopaedics Incorporated (Johnson & Johnson)
• Inventors: Dezzani, Michael Massino
• Primary Examiner(s): Nguyen, Dinh X.
• Assistant Examiner(s): STEWART, ALVIN J
• Application Number:
  US09/136,349
• Time in Patent Office: 993 Days
• Field of Search: 623/23, 623/22, 623/18, 623/16, 623/11, 623/12, 623/15, 623/26, 623/32
• US Class Current: 623/23.32
• CPC Class Codes: A61F 2/30742 Bellows or hose-like seals; Sealing membranes A61B17/8808 takes precedence; sealing membranes for retaining bone substitute A61F2/2846
• CPC Class Codes: A61F 2/30771 applied in original prostheses, e.g. holes or grooves
• CPC Class Codes: A61F 2/3662 Femoral shafts
• CPC Class Codes: A61F 2/367 Proximal or metaphyseal parts of shafts trochanteric devices connected to said proximal parts A61F2/30739
• CPC Class Codes: A61F 2/3672 Intermediate parts of shafts
• CPC Class Codes: A61F 2/3676 Distal or diaphyseal parts of shafts
• CPC Class Codes: A61F 2/389 Tibial components A61F2/3868 takes precedence
• CPC Class Codes: A61F 2002/30014 differing in elasticity, stiffness or compressibility
• CPC Class Codes: A61F 2002/30125 elliptical or oval
• CPC Class Codes: A61F 2002/30327 differing in diameter
• CPC Class Codes: A61F 2002/30332 Conically- or frustoconically-shaped protrusion and recess
• CPC Class Codes: A61F 2002/304 with additional means for limiting said sliding
• CPC Class Codes: A61F 2002/30451 soldered or brazed or welded
• CPC Class Codes: A61F 2002/30602 Shaft made of a bundle of anchoring rods or bristles, e.g. fasciculate shaft
• CPC Class Codes: A61F 2002/30616 Sets comprising a plurality of prosthetic parts of different sizes or orientations
• CPC Class Codes: A61F 2002/30682 Means for preventing migration of particles released by the joint, e.g. wear debris or cement particles
• CPC Class Codes: A61F 2002/30683 Means for collecting wear particles in a hollow cavity inside the prosthesis
• CPC Class Codes: A61F 2002/30836 knurled
• CPC Class Codes: A61F 2002/30917 for avoiding direct contact between bone and implant, e.g. extensible membrane for preventing contamination or for easy removal
• CPC Class Codes: A61F 2002/365 Connections of heads to necks
• CPC Class Codes: A61F 2002/3674 : Connections of proximal parts to distal parts
• CPC Class Codes: A61F 2220/0025 : Connections or couplings between prosthetic parts, e.g. between modular parts; Connecting elements
• CPC Class Codes: A61F 2220/0033 : made by longitudinally pushing a protrusion into a complementary-shaped recess, e.g. held by friction fit
• CPC Class Codes: A61F 2220/0058 : soldered or brazed or welded
• CPC Class Codes: A61F 2230/0008 : elliptical or oval
• CPC Class Codes: A61F 2250/0018 : differing in elasticity, stiffness or compressibility
• CPC Class Codes: A61F 2250/0029 : differing in bending or flexure capacity
• CPC Class Codes: A61F 2250/0039 : differing in diameter
• CPC Class Codes: A61F 2310/00017 : Iron- or Fe-based alloys, e.g. stainless steel
• CPC Class Codes: A61F 2310/00023 : Titanium or titanium-based alloys, e.g. Ti-Ni alloys
• CPC Class Codes: A61F 2310/00029 : Cobalt-based alloys, e.g. Co-Cr alloys or Vitallium

Abstract

A prosthetic stem insertable in an intramedullary cavity to support an articulation component includes a proximal neck portion and a distal root portion. The proximal neck is solid metal and extends for a length effective to reach into the cavity and couple to surrounding bone for load bearing engagement therewith, while the distal root portion includes a stranded cable which fills the bone cavity but flexes to avoid significant transfer of bending stresses. The cable is tightly bunched at its junction with the neck, providing a transitional degree of stiffness to its distal part which is significantly more flexible and bends to accommodate natural displacement of the surrounding bone. The prosthesis may be formed of a compatible metal such as titanium, cobalt chromium, stainless steel or the like. The prosthesis has a section modulus characterized by three distinct regions. The proximal end region has the largest cross section and presents a stiff modulus of solid material, while the distal region is composed of strands and presents a flexible bending stiffness. An intermediate region of relatively short length where the cable attaches to the upper portion has a bending modulus that changes quickly from fairly stiff to flexible. The location of this portion may be varied by changing the relative lengths of the proximal solid and distal cable portions thus determining or limiting to a small local region the area of bone which may experience any stress shielding. The stiffness of the cable may also be varied by use of different gauge wires or the addition of welds or circumferential bands. In various embodiments, the cable may be entirely enclosed within a surrounding shell, may have its distal end crimped within a surrounding ring or cup, or may have its filaments welded or otherwise fused or bonded together at one or more regions. In one embodiment the cable is formed of asymmetric fibers which run parallel to each other to provide a bending modulus that is stiffer in one plane than in the transverse direction.

Claims

1. A prosthesis of the type including at least a stem having a distal root portion for insertion into an intramedullary cavity and a proximal portion for supporting a head or junction component

wherein said proximal portion is solid and extends into the intramedullary cavity for a length effective to couple to surrounding bone for stress-transmissive engagement therewith, and

said distal root portion extends from said proximal solid portion and includes a cable comprising a plurality of strands extending in a generally densely packed space-filling contiguity centrally in said cavity, the strands being selected to achieve a bending modulus of the distal root less than a bendibg modulus of said proximal portion and the natural femur such so to provide a stimulating level of strain transmission to distal bone while limiting load bearing function and thereby avoid stress shielding.

2. A prosthesis according to claim 1, wherein at least said proximal portion is formed of a metal selected from among titanium, cobalt chromium, stainless steel and compatible metal.

3. A prosthesis according to claim 1, wherein the proximal solid portion has a first bending stiffness A and the distal root portion has a bending stiffness B wherein B<

<

A.

4. A prosthesis according to claim 3, further having an intermediate portion between said proximal neck and said distal root portion, said cable extending in said intermediate portion, and having a bending stiffness C such that B<

C<

A.

5. A prosthesis according to claim 3, wherein said cable is formed of strands having anisotropic bending stiffness.

6. A prosthesis according to claim 3, wherein said cable is bundled into cross-section having anisotropic bending stiffness.

7. A prosthesis according to claim 3, wherein said cable is formed of strands having isotropic bending stiffness.

8. A prosthesis according to claim 3, wherein said cable is bundled into cross-section having isotropic bending stiffness.

9. A prosthesis according to claim 3, wherein said cable is crimped into said solid proximal portion.

10. A prosthesis according to claim 3, wherein said cable is welded to said solid proximal portion.

11. A prosthesis according to claim 3, wherein said cable is bound together at plural positions along its length.

12. A prosthesis according to claim 3, wherein said cable is covered by an enclosure along its length.

13. A method of forming a prosthesis for a bone ending, such method comprising the steps of

providing a solid member having a distal end shaped to extend into a prepared bone opening and anchor in load-transmissive engagement with surrounding bone, said member having an articulation surface or means for receiving an articulation surface at a proximal end, and

providing a cable extending from the distal end of said solid member, said cable being fixedly attached to the solid member and comprising a plurality of strands extending in a generally densely packed space-filling contiguity centrally in the bone opening

wherein the step of providing a cable includes selecting the strands to achieve a bending modulus of the distal root less than a bending modulus of said proximal portion and the natural femur such as to provide a stimulating level of strain transmission to distal bone while limiting load bearing function and thereby avoid stress shielding.