Dual modulus hip stem and method of making the same

Abstract

An orthopaedic prosthesis for use in a hip replacement surgery. The orthopaedic prosthesis includes a metallic foam shell and a metallic core. The metallic core includes a neck configured to receive a femoral head component and a stem extending through the metallic foam shell.

Claims

1. A method of manufacturing an orthopaedic prosthesis for a patient, comprising: producing a shed from a metallic foam material having a first elastic modulus, producing a stem core from a metallic material having a second elastic modulus greater than the first elastic modulus, the stem core including a neck configured to receive a femoral head component, a collar, and a stem body that extends from the collar to a distal tip, the stem body comprising a proximal segment that extends distally from the collar and a distal segment that extends distally from a distal end of the proximal segment to the distal tip, and securing the shell to the stem body such that (i) the shell completely encases the proximal segment of the stem body, (ii) the shell covers a lateral surface of the distal segment of the stem body from the distal end of the proximal segment through an area adjacent the distal tip, and (iii) a medial surface of the distal segment of the stem body from the distal end of the proximal segment through the area adjacent the distal Up is devoid of the shell.

2. The method of claim 1, wherein securing the shell to the stem body includes sintering the shell to the stem body.

3. The method of claim 1, wherein producing the shell from the metallic foam material includes: compressing the metallic foam material around the stem core, and machining the metallic foam material to form the shell.

4. The method of claim 1, wherein producing the shell from the metallic foam material includes: compressing the metallic foam material to form an outer geometry of the shell, and machining a channel in the metallic foam material sized to receive the stem core.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The detailed description particularly refers to the following figures, in which:

(2) FIG. 1 is a perspective view of an orthopaedic implant;

(3) FIG. 2 is a perspective view of a core of the orthopaedic implant of FIG. 1;

(4) FIG. 3 is a cross-sectional view of the orthopaedic implant taken along the line 3-3 in FIG. 1;

(5) FIG. 4 is a cross-sectional view of the orthopaedic implant taken along the line 4-4 in FIG. 3;

(6) FIG. 5 is a cross-sectional view of the orthopaedic implant taken along the line 5-5 in FIG. 3;

(7) FIG. 6 is a cross-sectional view of the orthopaedic implant taken along the line 6-6 in FIG. 3;

(8) FIG. 7 is a cross-sectional view of the orthopaedic implant taken along the line 7-7 in FIG. 3; and

(9) FIG. 8 is a simplified block diagram of a process for manufacturing the orthopaedic implant of FIGS. 1-7.

DETAILED DESCRIPTION OF THE DRAWINGS

(10) While the concepts of the present disclosure are susceptible to various modifications and alternative forms, specific exemplary embodiments thereof have been shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit the concepts of the present disclosure to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

(11) Terms representing anatomical references, such as anterior, posterior, medial, lateral, superior, inferior, etcetera, may be used throughout the specification in reference to the orthopaedic implants or prostheses and surgical instruments described herein as well as in reference to the patient's natural anatomy. Such terms have well-understood meanings in both the study of anatomy and the field of orthopaedics. Use of such anatomical reference terms in the written description and claims is intended to be consistent with their well-understood meanings unless noted otherwise.

(12) Referring to FIG. 1, an orthopaedic prosthesis is illustratively embodied as a femoral orthopaedic implant 10 of a hip prosthesis. The femoral orthopaedic implant 10 (hereinafter implant 10) includes a head component 12 and an elongated stem component 14 that is configured to be inserted into an intramedullary canal of a patient's surgically-prepared femur (not shown). The head component 12 includes a spherical outer surface 16 configured to engage a patient's natural acetabulum (not shown) or a prosthetic acetabular cup implanted into the patient's pelvic bone. The head component 12 also includes a distal surface 18 having an opening 20 defined therein, and an inner wall (not shown) extends inwardly from the opening 20 to define an aperture 22 in the head component 12.

(13) The stem component 14 of the implant 10 includes a core 24 having a neck 26 configured to be coupled to the head component 12. In the illustrative embodiment, the neck 26 includes a plurality of external threads 28 that are configured to engage with a plurality of internal threads (not shown) lining the aperture 22 of the head component 12. It should be appreciated that in other embodiments the neck and the head component may be configured to be press fit, taper fit, or secured together by other fastening means.

(14) As shown in FIG. 2, the core 24 of the stem component 14 also includes a collar 30 and a core body 32 extending distally from the collar 30. As shown in FIG. 1, the neck 26 extends medially and proximally from the collar 30. In the illustrative embodiment, the neck 26, the collar 30, and the core body 32 are formed as a monolithic structure (e.g., a single molded or cast part). It should be appreciated that in other embodiments the components of the core 24 (e.g., the neck 26, the collar 30, and the core body 32) may be formed as separate components secured to one another by a mechanical fastener (e.g., screw, bolt, taper fit, etc.), adhesive, or other suitable fastener.

(15) The stem core 24 is formed from an implant grade metallic material having a high tensile strength and a high elastic modulus (i.e., a high material stiffness). As used herein, the term “high tensile strength” refers to a tensile strength that is greater than 650 MPa. Additionally, as used herein, the term “high elastic modulus” refers to an elastic modulus or modulus of elasticity that is greater than or equal to 100 GPa. In the illustrative embodiment, the core 24 is formed from cobalt-chromium alloy (“CoCr”) having a minimum ultimate tensile strength of 650 MPa and an elastic modulus of approximately 195 GPa. It should be appreciated that in other embodiments the core 24 may be formed from any material having a high tensile strength and a high elastic modulus, including, for example, a titanium alloy such as Ti-6A1-4V, which has a minimum ultimate tensile strength of 750 MPa and an elastic modulus of approximately 105 GPa.

(16) As described above, the core 24 of the stem component 14 includes a core body 32, which lies generally in the coronal plane of a patient's body when the implant 10 is secured to the patient's femur. As shown in FIG. 2, the core body 32 of the core 24 extends from a proximal end 34 attached to the collar 30 to a distal end 36. The core body 32 includes a medial surface 38 and a lateral surface 40 positioned opposite the medial surface 38. When the core 24 of the stem component 14 is viewed in the coronal plane, the core body 32 has a thickness 42 at the proximal end 34, which is defined between the surfaces 38, 40 of the core body 32. The core body 32 has another thickness 44 defined between the surfaces 38, 40 at the distal end 36. In the illustrative embodiment, the thickness 44 is less than the thickness 42, and the core body 32 tapers to decrease in thickness between the proximal end 34 and the distal end 36.

(17) In the illustrative embodiment, the medial surface 38 of the core body 32 is convex. As described in greater detail below, the medial surface 38 is defined by a radius 46 (see FIGS. 4-7) that decreases in magnitude as the medial surface 38 extends from the proximal end 34 of the core body 32 to the distal end 36. The lateral surface 40 is also convex in the illustrative embodiment. The lateral surface 40, like the medial surface 38, is defined by a radius 48 (see FIGS. 4-7) that decreases in magnitude as the lateral surface 40 extends from the proximal end 34 of the core body 32 to the distal end 36. While the radii 46, 48 decrease in magnitude, the magnitude of the radius 48 of the lateral surface 40 is greater than the magnitude of the radius 46 of the medial surface 38.

(18) Returning to FIG. 1, the stem component 14 of the implant 10 also includes a shell 50 that is secured to the core 24. The shell 50 has a sheath 52 that has a proximal end 54 attached to the collar 30 and a distal end 56 positioned between the collar 30 and the distal end 36 of the core 24. The shell 50 also includes a cover layer 58 that extends distally from the sheath 52 to the distal end 36 of the core body 32. In the illustrative embodiment, the sheath 52 and the cover layer 58 are formed as a monolithic structure. It should be appreciated that in other embodiments the components of the shell 50 (e.g., sheath 52 and the cover layer 58) may be formed as separate components. The separate components may be secured to one another by a mechanical fastener (e.g., screw, bolt, taper fit, etc.), adhesive, or other suitable fastener or secured separately to the core 24.

(19) The shell 50 of the stem component 14 is formed from a metallic foam matrix having a low elastic modulus. As used herein, a “low elastic modulus” refers to an elastic modulus or modulus of elasticity similar to that of a patient's natural femur (i.e., between 10 GPa and 20 GPa). In the illustrative embodiment, the shell 50 is formed from a foam matrix of titanium having an elastic modulus of approximately 10 GPa and an ultimate tensile strength of the foam matrix of titanium is approximately 35 MPa. In that way, the shell 50 has an elastic modulus that is closer to that of a patient's femur. It should be appreciated that in other embodiments the shell 50 may be formed any metallic foam matrix having a low elastic modulus, such as, for example, a CoCr foam matrix having an elastic modulus of approximately 19 GPa, a CoCr alloy foam matrix, a titanium foam alloy matrix, or other foam matrix.

(20) As described above, the core 24 of the stem component 14 in the illustrative embodiment is formed from CoCr having an elastic modulus of approximately 195 GPa while the shell 50 is formed from a foam matrix of titanium having an elastic modulus of approximately 10 GPa. Thus, in the illustrative embodiment, the elastic modulus of the shell 50 is approximately 1/20 of the elastic modulus of the core 24. In still other embodiments, the core may be formed from CoCr having an elastic modulus of approximately 195 GPa and the shell may be formed from a CoCr foam matrix having an elastic modulus of approximately 19 GPa. In such embodiments, the elastic modulus of the shell is approximately 1/10 of the elastic modulus of the core. In other embodiments, the core may be formed from Ti-6 Al-4V having an elastic modulus of approximately 105 GPa and the shell may be formed from a titanium foam matrix having an elastic modulus of approximately 10 GPa. In such embodiments, the elastic modulus of the shell is approximately 1/10 of the elastic modulus of the core.

(21) As shown in FIG. 1, the sheath 52 of the shell 50 has an outer surface 60, and the cover layer 58 has an outer surface 62. The outer surfaces 60, 62 define a portion of the external geometry of the implant 10. As such, the outer surfaces 60, 62 engage the portion of the patient's femur defining the intramedullary canal when the implant 10 is inserted into the proximal end of the patient's surgically-prepared femur. In the illustrative embodiment, the outer surface 60 of the sheath 52 is porous to enable bone ingrowth fixation, and the outer surface 62 of the cover layer 58 is non-porous. It should be appreciated that in other embodiments the cover layer 58 may also be porous.

(22) As shown in FIG. 3, the core body 32 of the stem core 24 extends through the sheath 52 of the shell 50. The core body 32 includes a core segment 64 that is positioned in the sheath 52, and a core segment 66 that is positioned distal of the sheath 52. The sheath 52 is attached to and encases a medial surface 68 and a lateral surface 70 of the core segment 64. The surfaces 68, 70 form proximal sections of the medial surface 38 and lateral surface 40, respectively, of the core body 32.

(23) The core body 32 (i.e., the core segments 64, 66) and the shell 50 (i.e., the sheath 52 and cover layer 58) cooperate to define a longitudinal axis 72 of the stem component 14, which extends between the proximal end 34 and the distal end 36. The core body 32 has a longitudinal axis 74 that is defined between the ends 34, 36, and the axis 74 is offset from the axis 72. In the illustrative embodiment, the axis 74 is offset in the medial direction from the axis 72 such that the core body 32 is biased toward the medial side 80 of the stem component 14 and away from the lateral side 82 of the stem component 14. Additionally, the thickness of the shell 50 on the lateral side 82 of the stem component 14 is greater than the thickness of the shell 50 on the medial side 80 of the stem component 14.

(24) For example, as shown in FIG. 4, the sheath 52 has a lateral thickness 90 and a medial thickness 92 when viewed in a transverse plane extending through the stem component 14 between the proximal end 54 and the distal end 56 of the sheath 52. The lateral thickness 90 of the sheath 52 is defined between a lateral-most point 94 of the lateral surface 70 of the core segment 64 and a lateral-most point 96 of the outer surface 60 of the sheath 52. The medial thickness 92 of the sheath 52 is defined between a medial-most point 98 of the medial surface 68 of the core segment 64 and a lateral-most point 100 of the outer surface 60 of the sheath 52. Each of the points 94, 96, 98, 100 lies in the coronal plane, as indicated by an imaginary line 102.

(25) As shown in FIG. 4, the lateral thickness 90 is greater than the medial thickness 92. In other words, the thickness 90 of the shell 50 on the lateral side 82 of the stem component 14 is greater than the thickness 92 of the shell 50 on the medial side 80 of the stem component 14. In the illustrative embodiment, the lateral thickness 90 is greater than 5 millimeters, and the medial thickness 92 is between 2 and 4.5 millimeters.

(26) As shown in FIGS. 3-5, the thickness of the sheath 52 of the shell 50 on the medial side 80 of the stem component 14 generally decreases from the proximal end 54 and the distal end 56 of the sheath 52. As shown in FIG. 5, the sheath 52 has a lateral thickness 110 and a medial thickness 112 when viewed in a transverse plane extending through the stem component 14 at the distal end 56 of the sheath 52. The lateral thickness 110 of the sheath 52 is defined between a lateral-most point 114 of the lateral surface 70 of the core segment 64 and a lateral-most point 116 of the outer surface 60 of the sheath 52. The medial thickness 112 of the sheath 52 is defined between a medial-most point 118 of the medial surface 68 of the core segment 64 and a lateral-most point 120 of the outer surface 60 of the sheath 52. Each of the points 114, 116, 118, 120 lies in the coronal plane, as indicated by the imaginary line 102.

(27) As shown in FIG. 5, the lateral thickness 110 of the sheath 52 is again greater than the medial thickness 112 of the sheath 52. In the illustrative embodiment, the lateral thickness 10 is greater than five millimeters. Additionally, the medial thickness 112 at the distal end 56 of the sheath 52 is less than the medial thickness 92, which is shown in FIG. 4 between the proximal end 54 and the distal end 56 of the sheath 52. As described above, the medial thickness 92 of sheath 52 in the illustrative embodiment is between 2 and 4.5 millimeters, and, in the illustrative embodiment, the medial thickness 112 of the sheath 52 is between 1 and 1.5 millimeters.

(28) Returning to FIG. 3, the outer surface 60 of the sheath 52 has a curved or rounded distal surface section 130 at the distal end 56 of the sheath 52. The distal surface section 130 has an edge 132 that extends around the core body 32 of the stem core 24 and the cover layer 58 of the shell 50. As described above, the core body 32 also includes a core segment 66 that is positioned distal of the end 56 of the sheath 52. The core segment 66 extends from the edge 132 of the sheath 52 to the distal end 36 of the core body 32.

(29) The core segment 66 has a medial surface 134 that forms a distal section of the medial surface 38 of the core body 32. The core segment 66 also has a lateral surface 136 that forms a distal section of the lateral surface 40 of the core body 32. As shown in FIG. 3, the cover layer 58 of the shell 50 is attached to only the lateral surface 136 of the core segment 66. The medial surface 134 of the core segment 66, like the shell 50, forms a portion of the external geometry of the implant 10 such that the medial surface 134 may engage the patient's bone when the implant 10 is inserted into the intramedullary canal.

(30) As shown in FIG. 3, the cover layer 58 has a body 140 that extends from a proximal end 142 attached to the sheath 52 to a distal end 144 secured to the distal end 36 of the core body 32. In the illustrative embodiment, the thickness of the body 140 decreases between the ends 142, 144. For example, as shown in FIG. 6, the body 140 has a lateral thickness 150 when viewed in a transverse plane that extends through the stem component 14 between the ends 142, 144 of the cover layer 58. The lateral thickness 150 of the body 140 is defined between a lateral-most point 152 of the lateral surface 136 of the core segment 66 and a lateral-most point 154 of the outer surface 62 of the cover layer 58. As shown in FIG. 6, the lateral thickness 150 is greater than 4.5 millimeters.

(31) As shown in FIG. 7, the body 140 has a lateral thickness 158 when viewed in a transverse plane extending through the stem component 14 at the distal end 144 of the cover layer 58. The lateral thickness 158 of the body 140 is defined between a lateral-most point 160 of the lateral surface 136 of the core segment 66 and a lateral-most point 162 of the outer surface 62 of the cover layer 58. In the illustrative embodiment, the lateral thickness 158 of the body 140 is between 4 millimeters and 3 millimeters. In other words, the lateral thickness 158 of the body 140 at the distal end 144 of the cover layer 58 is less than the lateral thickness 150 of the body 140 between the ends 142, 144.

(32) In use, the implant 10 is inserted into a proximal end of a patient's surgically-prepared femur. The elongated stem component 14 is received in the intramedullary canal and the sheath 52 and the cover layer 58 of the shell 50 engage the portion of the patient's femur surrounding the canal. The core 24 is sized and shaped to meet the minimum strength requirements of the implant 10, while the shell 50 is configured to possess the external geometry necessary to fit into the intramedullary canal. The combination of the high tensile strength/high elastic modulus core 24 with the low modulus shell 50 results in a reduced stiffness for the implant 10 such that stress shielding of the patient's bone is reduced.

(33) Referring now to FIG. 8, a method 200 of manufacturing the elongated stem component 14 of the implant 10 is illustrated. In the illustrative method, the performance requirements of the stem component 14 are determined and the stem core 24 of the stem component 14 is procured. The stem core 24 and the shell 50 may then be assembled to form the stem component 14.

(34) In block 210, the performance requirements of the stem component 14 are determined. The performance requirements for the stem component 14 may vary between different patients such that a customized stem component 14 may be required. The performance requirements include the external geometry and the minimum endurance and performance requirements, such as, for example, the minimum tensile strength and the minimum stiffness of the stem component 14.

(35) In block 212, the core 24 of the stem component 14 is selected. The core 24 may be an off-the-shelf, generic core that may be used with multiple implant sizes. The core 24 may be a customized, patient-specific component designed to satisfy the performance requirements of a particular patient. Whether the core 24 is generic or custom, the selected core 24 of the component 14 is sized and shaped to provide the minimum strength of the implant 10. The minimum strength of the core 24 is determined in accordance with International Organization for Standardization Standard No. 7206-4:2010 “IMPLANTS FOR SURGERY—PARTIAL AND TOTAL HIP JOINT PROSTHESES—PART 4: DETERMINATION OF ENDURANCE PROPERTIES AND PERFORMANCE OF STEMMED FEMORAL COMPONENTS” and Standard No. 7206-6:1992 “IMPLANTS FOR SURGERY—PARTIAL AND TOTAL HIP JOINT PROSTHESES—PART 6: DETERMINATION OF ENDURANCE PROPERTIES OF HEAD AND NECK REGION OF STEMMED FEMORAL COMPONENTS.”

(36) In block 214, the stem component 14 is assembled. To do so, a metallic foam matrix, which will form the shell 50 of the stem component 14, is procured. The metallic foam matrix may be compressed around the core 24 such that the core 24 is received in a channel within the foam matrix. The foam matrix may then be machined to the required external geometry of the shell 50. Alternatively, the metallic foam matrix may be compressed separately into the shape of shell 50. After the matrix is compressed, a channel sized to receive the core 24 may be machined in the shell 50 before the shell 50 is assembled with the core 24.

(37) A sintering operation may be used to secure the shell 50 to the core 24. It should also be appreciated that the shell 50 and the core 24 may be secured via a brazing operation, a press-fit, or other securing means.

(38) While the disclosure has been illustrated and described in detail in the drawings and foregoing description, such an illustration and description is to be considered as exemplary and not restrictive in character, it being understood that only illustrative embodiments have been shown and described and that all changes and modifications that come within the spirit of the disclosure are desired to be protected.

(39) There are a plurality of advantages of the present disclosure arising from the various features of the method, apparatus, and system described herein. It will be noted that alternative embodiments of the method, apparatus, and system of the present disclosure may not include all of the features described yet still benefit from at least some of the advantages of such features. Those of ordinary skill in the art may readily devise their own implementations of the method, apparatus, and system that incorporate one or more of the features of the present invention and fall within the spirit and scope of the present disclosure as defined by the appended claims.