MEDICAL IMPLANTS WITH 100% SUBSURFACE BORON CARBIDE DIFFUSION LAYER

Abstract

An orthopedic medical implant, implant part or surgical instrument includes a metallic body having a metal or a metal alloy. The metallic body includes a sub-surface that is a thermal diffused boron carbide layer, and the metallic body is void of an additive layer onto a surface of the metallic body.

Claims

1. An orthopedic medical implant, implant part or surgical instrument comprising: a metallic body comprising a metal or a metal alloy, wherein the metal or metal alloy is selected from the group consisting of cobalt, cobalt-chromium alloys, titanium, titanium-alloys, and mixtures thereof, the metallic body includes a sub-surface that is a thermal diffused boron carbide layer, the thermal diffused boron carbide layer comprises borides having a formula MeB, MeB.sup.2, or Me.sub.2B, where Me represents a metal present in the metallic body of the orthopedic medical implant, implant part or surgical instrument, the metallic body is void of an additive layer onto a surface of the metallic body, and a dimension of a pre-boronized implant, implant part, or surgical instrument is the same as a dimension of a post-boronized implant, implant part, or surgical instrument.

2. An orthopedic medical implant, implant part or surgical instrument comprising: a metallic body comprising a metal or a metal alloy, the metallic body including a sub-surface that is a thermal diffused boron carbide layer, and the metallic body is void of an additive layer onto a surface of the metallic body.

3. The orthopedic medical implant, implant part or surgical instrument as recited in claim 2 wherein the metal or metal alloy is selected from the group consisting of cobalt, cobalt-chromium alloys, titanium, titanium-alloys, and mixtures thereof.

4. The orthopedic medical implant, implant part or surgical instrument as recited in claim 2 wherein the thermal diffused boron carbide layer comprises borides having a formula MeB, MeB.sup.2, or Me.sub.2B, where Me represents a metal present in the metallic body of the orthopedic medical implant, implant part or surgical instrument.

5. The orthopedic medical implant, implant part or surgical instrument as recited in claim 2 wherein a dimension of a pre-boronized implant or implant part is the same as a dimension of a post-boronized implant or implant part.

6. The orthopedic medical implant, implant part or surgical instrument as recited in claim 2 wherein a surface hardness of the surface of the metallic body is at least 1500 HV.

7. The orthopedic medical implant, implant part or surgical instrument as recited in claim 2 wherein a boronization thickness of the metallic body is at least 100 microns.

8. The orthopedic medical implant, implant part or surgical instrument as recited in claim 2 wherein the surface of the metallic body has a coefficient of friction of 0.01 at 15,000 psi.

9. The orthopedic medical implant, implant part or surgical instrument as recited in claim 2 wherein a surface chemistry of the surface of the metallic body is 40% to 60% boron.

10. The orthopedic medical implant, implant part or surgical instrument as recited in claim 2 wherein the implant or implant part is selected from the group consisting of a femoral component of an uni-compartmental knee arthroplasty or a total knee arthroplasty, a tibial component of a uni-compartment knee arthroplasty or a total knee arthroplasty, a femoral head of hip arthroplasty, a Morse taper of a hip arthroplasty, an acetabular cup or liner of a hip arthroplasty, a humeral head of a shoulder arthroplasty, a humeral or ulnar component of an elbow arthroplasty, a metacarpal or radial stem of a wrist arthroplasty, a vertebral endplate components of a disc arthroplasty, and a tibial or talar component of an ankle arthroplasty.

11. The orthopedic medical implant, implant part or surgical instrument as recited in claim 2 wherein the metallic body is a cobalt-based alloy.

12. The orthopedic medical implant, implant part or surgical instrument as recited in claim 2 wherein the medical implant, implant part or surgical instrument has a hexagonal close packed crystal structure and is age hardened.

13. The orthopedic medical implant, implant part or surgical instrument as recited in claim 2 wherein the metallic body is a titanium-based alloy.

14. The orthopedic medical implant, implant part or surgical instrument as recited in claim 2 wherein orthopedic medical implant, implant part or surgical instrument is in an alpha+beta phase.

15. An orthopedic medical implant, implant part or surgical instrument comprising: a metallic body comprising a metal or metal alloy, the metallic body including a sub-surface including a boronized layer of the metal or metal alloy, wherein the orthopedic medical implant, implant part or surgical instrument is in an unannealed condition.

16. A method of forming an orthopedic medical implant, implant part or surgical instrument, the method comprising creating a thermal diffused boron carbide layer in a sub-surface of a metallic body of the orthopedic medical implant, implant part or surgical instrument, wherein the orthopedic medical implant, implant part or surgical instrument comprises a metal or a metal alloy, and the metallic body is void of an additive layer onto a surface of the metallic body.

17. The method as recited in claim 16 further comprising heat-treating the orthopedic medical implant, implant part or surgical instrument in a controlled atmosphere furnace.

18. The method as recited in claim 17 wherein heating-treating occurs between 1000° F. and 1700° F.

19. The method as recited in claim 17 wherein heating-treating occurs between 1000° F. and 1750° F., and the orthopedic medical implant, implant part or surgical instrument has a hexagonal close packed crystal structure and is age hardened.

20. The method as recited in claim 17 wherein heating-treating occurs at below 1100° F., and the orthopedic medical implant, implant part or surgical instrument is in an alpha+beta phase.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] These and other objects and features of the present invention will be more fully disclosed or rendered obvious by the following detailed description of the preferred embodiments of the invention, which is to be considered together with the accompanying drawings wherein like numbers refer to like parts and wherein:

[0026] FIG. 1 illustrates a table showing the different cobalt alloys typically used in medical applications;

[0027] FIG. 2 illustrates a table showing how the addition of molybdenum, chromium, iron, nickel, and carbon affect the crystal structure of a cobalt;

[0028] FIG. 3 illustrates a table showing the weight percent of the hexagonal close packed phase resulting from different thermos-mechanical reduction of area for a cobalt-based alloy with 28% chromium and 6% molybdenum;

[0029] FIG. 4 illustrates a table showing the weight percent of the hexagonal close packed phase at varying reductions of cross-sectional area;

[0030] FIG. 5 illustrates a graph showing hardness for percent reduction of cross-sectional area for a cobalt-based alloy bar;

[0031] FIG. 6 illustrates a graph showing the aging response of a cobalt-based alloy; the graph shows the hardness versus aging time;

[0032] FIG. 7 illustrates a schematic view showing the components of a traditional hip arthroplasty;

[0033] FIG. 8 illustrates a schematic view showing the boronization process;

[0034] FIGS. 9 and 10 illustrate schematics showing the diffused boronized layer;

[0035] FIG. 11 illustrates a schematic showing the surface hardness of the boronized layer;

[0036] FIG. 12 illustrates a schematic showing a typical femoral head manufactured from a cobalt-based alloy;

[0037] FIG. 13 illustrates a schematic showing a modular component of a femoral arthroplasty;

[0038] FIG. 14 illustrates the energy dispersive spectroscopy (EDS) spectrum analysis of the B.sub.4C layer through a scanning electronic microscope in cobalt alloy L605 per ASTM F90-14 Standard Specification for Wrought Cobalt-20-Chromium-15-Tungsten-10-Nickel Alloy for Surgical Implant Applications (UNS R30605);

[0039] FIGS. 15 and 16 illustrate scanning electronic microscope images of the B.sub.4C coated sample processed at 1600° F. and 1750° F.;

[0040] FIG. 17 illustrates the thickness of the Co.sub.2B layer and the hardness of the layer and the hardness of the core material; and

[0041] FIG. 18 illustrates an orthopedic cutting instrument having the Co.sub.2B layer.

DETAILED DESCRIPTION

[0042] Boronizing is a thermochemical diffusion process in which hard boride layers are generated by the diffusion of boron into the material surface. A medical implant, implant part or surgical instrument includes a fully diffused boronized subsurface of metal or metal alloy. The boronized subsurface provides an extremely hard surface that is highly lubricious and has a very low coefficient of friction. Additionally, a method of producing a medical implant, implant part or surgical instrument includes a diffused boronized subsurface of the metal or metal alloy.

[0043] A B.sub.4C diffusion process provides a fully diffused surface treatment that is extremely hard and does not alter the implant or implant part's dimensions, while penetrating deeper into the implant or implant part's substrate. Because the layer is diffused 100% into the implant's subsurface, there is no additive coating that 1) can delaminate or wear off, 2) can change the implant's size/shape, or 3) will need to be ground and/or polished during manufacturing.

[0044] Among other things, the diffusion surface treatment of the implant or implant part provides a slurry containing boron carbide and other elements. A method includes heat treating the implant or implant part in an inert atmosphere and initiating the migration of boron atoms into the substrate. The migration process results in a surface chemistry high in boron.

[0045] The implant surfaces can be finish machined, ground, and polished when still metallic (and thus still relatively soft), and then treated to increase the surface hardness with the boron carbide diffusion process. The resulting parts do not need to be ground or polished.

[0046] A B.sub.4C diffusion process transforms a surface of metal parts into an extremely hard and slick intermetallic non-brittle boride layer. The resulting diffused boride layer is not a coating. This eliminates the potential for bond failure that can result in delamination. Additionally, because the base material is transformed to an intermetallic boride from the original surface down to a desired penetration depth, zero dimensional change occurs through the process.

[0047] FIG. 1 illustrates a table defining several of the cobalt-based alloys used for medical applications. The cobalt-based alloys which are often used to produce orthopedic implants are well known and widely used in industry, primarily for wear applications. Many of the strength properties of the cobalt alloys arise from (1) the crystallographic texture of cobalt (Co), (2) the solid-solution-strengthening effects of Cr (chromium), W (tungsten) and Mo (molybdenum), (3) the formation of metal carbides, and (4) the corrosion resistance imparted by chromium. The cobalt alloy material has a high corrosion resistance, mainly due to the high chromium content that forms a thin passive chromium oxide layer with good adhesion, protecting the underlying matrix material.

[0048] These alloys contain generally around 15% chromium to ensure a good corrosion resistance, between 4-17% tungsten or molybdenum for solid solution strengthening, and between 0.1-3% carbon to form hard carbides. The high temperature crystal structure of pure cobalt in its stable phase is face-centered cubic (fcc). Below 800° F., the crystal structure of the stable phase is hexagonal close packed (hcp). However, as shown in FIG. 2, the addition of carbon, iron and/or nickel to cobalt lowers the allotropic transformation point of the cobalt to a point below room temperature. The addition of these elements in effective amounts causes the face-centered cubic structure to exist at room temperature instead of the hexagonal close packed structure.

[0049] Additions of both chromium and tungsten tend to increase the transformation temperature in which the hexagonal close packed structure reverts back to face-centered cubic structure (at temperatures between 1769° F. and 2260° F., depending on the cobalt alloy).

[0050] The face-centered cubic to hexagonal close packed crystallographic transformation occurs in cobalt alloys thru thermo-mechanical warm or cold working of the metal. Thermo-mechanical work is defined at processing/working (i.e. rolling, casting, forging, drawing, swaging) the metal at a temperature below its hexagonal close packed to face-centered cubic phase transformation temperature. However, the face-centered cubic crystal structure does not appear on the usual pressure-temperature phase diagrams of other hexagonal close packed alloys, such as titanium, zirconium, and hafnium (Hf), making this phenomenon unique to cobalt. For example, a cobalt-based alloy with 28% chromium and 6% molybdenum and having a 0.335″ diameter rod was cold drawn up to 30% in a single reduction operation. In this example, the microstructure underwent a 38% face-centered cubic to hexagonal close packed phase transformation. The percent of thermo-mechanical work reduction appears to increase the percent of face-centered cubic to hexagonal close packed phase transformation relatively linearly, such as shown in FIGS. 3 and 4.

[0051] Because the cold or warm-drawing process (or forging) on a solid bar can only perform about a 30% reduction before needing an anneal operation, the reduced bar has 38% hexagonal close packed structure in its microstructure and the texturing effect is significant. The hexagonal close packed crystals are tightly locked together in the microstructure mixture (38% hexagonal close packed and 62% face-centered cubic), and the basal planes are radially oriented, increasing bi-axial strength and wear resistance (as shown in FIGS. 5 and 6).

[0052] The stresses built up from the thermo-mechanical (cold or warm) work of the cobalt alloys can only be relieved by annealing, which for most cobalt alloys is above 2,000° F. The anneal will reverse the hexagonal close packed to face-centered cubic phase transformation at temperatures between 1769° F. and 2260° F., depending on the cobalt alloy. At these temperatures, the desirable hexagonal close packed will revert to face-centered cubic and will stress relieve or anneal the material which is not desirable from a hardness and wear perspective.

[0053] Therefore, it is desirable to apply the B.sub.4C thermal diffusion process to cobalt alloys at a temperature that is below the annealing temperature so not to soften the material and reverse the phase hexagonal close packed to face-centered cubic transformation, yet hot enough to allow for the boride layer to diffuse deep subsurface. Temperatures between 1000° F. and 1750° F. are appropriate for the B.sub.4C diffusion process, will not reverse the favorable hexagonal close packed phase transformation, and will desirably age harden the implant.

[0054] The present invention discloses a method for producing an implant or implant part that has a high surface hardness, is highly lubricious, has a low coefficient of friction, and resists delamination. In FIG. 7, the boron diffusion process 50 is shown. The process includes, cleaning the implant or implant part, creating a boron carbide slurry, coating the implant or implant part with the boron carbide slurry, allowing the slurry to dry, heat-treating the implant or implant part in a controlled atmosphere furnace, and removing residual material with water.

[0055] The heat-treatment may be achieved at a temperature of 1000° F. to 1700° F., with increasing temperatures resulting in increased depth of boron diffusion. The heat treatment may be performed under a protective-gas atmosphere such as argon, nitrogen, hydrogen, or a mixture of argon, nitrogen and hydrogen.

[0056] The B.sub.4C slurry should be made using boron carbide nanopowder. The average particle size (APS) should be preferably between 45 nm to 55 nm and may be as small as 20 nm to 40 nm. This allows for 100% subsurface diffusion. The B.sub.4C slurry may an activator, such as NaBF.sub.4, KBF4, (NH.sub.4).sub.3BF.sub.4, NH.sub.4Cl, Na.sub.2CO.sub.3, BaF.sub.2, and Na.sub.2B.sub.3O.sub.7. The slurry may also include a diluent, such as Al.sub.2O.sub.3.

[0057] It should be noted that if heat treatment at this temperature range results in the precipitation of carbides, the implant or implant part can be further processed to ensure that the ductility and toughness of the implant are not significantly negatively impacted. One method of doing this is by heating the implant or implant part to above 1200° C. to dissolve the carbides and then rapidly cooling the implant to about 800° C. to limit further carbide formation.

[0058] In one example, the boron carbide diffusion process occurs at below 1100° F., and the resulting medical implant or implant part is in the alpha+beta phase.

[0059] The boron diffusion process finds particular utility in increasing the surface performance of hip arthrodesis systems. FIG. 8 shows an exemplary hip arthrodesis system 5. The hip arthrodesis system consists of a stem 10, a femoral head 15, an acetabular cup 20, and an acetabular linear 25. The stem 10 is joined to the femoral head 15 by a taper 30. The femoral head 15 articulates inside the acetabular liner 25. Wear of the acetabular liner 25 can lead to failure of the hip arthrodesis system. It is highly desirable to have the femoral head 15 and the acetabular cup 20 have a high surface hardness, high lubricity, and a low coefficient of friction. It is further desirable to have a femoral head 15 that does not have an additive layer that can delaminate.

[0060] Now looking at FIGS. 9 and 10, the resulting implant or implant part is shown with an outer surface of diffused boron 60. The diffused boron layer 65 is shown adjacent to the substrate material 70. The diffused boron layer 65 may have a total depth of about 0.012″ (300 μm). It is important to note that the diffused boron coating does not change the dimensions of the implant or implant part.

[0061] The boron coating is very hard. Looking now at FIG. 11, the surface hardness 80 may be between 1500 HV and 2000 HV.

EXAMPLE 1

[0062] FIG. 12 shows a femoral head 90 that has been boronized using the B.sub.4C diffusion process previously described. The femoral head 90 has a surface hardness of 1800 HK, a surface chemistry of 40%-60% boron, and a coefficient of friction of 0.01 at 15,000 psi. The depth of boron diffusion is about 250 microns.

EXAMPLE 2

[0063] FIG. 13 shows the Morse taper region of a hip arthroplasty 100 that has been boronized using the process previously described. The more taper has a surface hardness of 1600 HK, a surface chemistry of 40%-60% boron, and a coefficient of friction of 0.01 at 15,000 psi. The depth of boron diffusion is about 150 microns.

[0064] FIG. 14 shows the EDS (energy dispersive spectroscopy) spectrum analysis of the B.sub.4C layer (shown through a scanning electronic microscope) in cobalt alloy L605 per ASTM F90—14 Standard Specification for Wrought Cobalt-20-Chromium-15-Tungsten-10-Nickel Alloy for Surgical Implant Applications (UNS R30605). An advantage to applying the B.sub.4C diffusion level to alloy L605 is that the boronizing temperature up to 1700° F. is not hot enough to anneal the material, so the metal alloy can remain hard in its cold, worked, and aged condition with a hexagonal close packed crystallographic texture while its surface is the diffusion layer can be Co.sub.3B or Co.sub.2B.

EXAMPLES 3 & 4

[0065] One B.sub.4C diffusion sample was processed at 1600° F., and the other sample was processed at 1750° F. The 1600° F. sample has an average surface roughness of 6.5 μin, and the 1750° F. sample has an average surface roughness of 14.4 μin. The difference in surface roughness can be seen in FIGS. 15 and 16. FIG. 15 shows a scanning electronic microscope image of the B.sub.4C diffusion process sample processed at 1600° F. The image shows a fairly smooth surface finish. FIG. 16 shows a scanning electronic microscope image of the B.sub.4C thermal diffused sample processed at 1750° F. FIG. 16 shows a slightly rougher surface finish.

[0066] Looking now at FIG. 17, the Co.sub.2B layer thickness was measured after B.sub.4C diffusion into medical grade Co—Cr alloy. The average layer thickness was found to be 0.00083″. Knoop hardness measurements were also taken of the Co.sub.2B layer and the core material. Knoop 25 g hardness values for the Co.sub.2B surface were found to be on average 1929 (>80 HRC). The core material was found to have a Knoop 25 g hardness of 477 (46 HRC).

[0067] FIG. 18 shows an orthopedic cutting instrument 64. In one example, the orthopedic cutting instrument 64 is a resector or shaver that cuts cartilage and bone during surgery. The orthopedic cutting instrument 64 includes an outer shell 66 and a blade 68 that rotates in the outer shell 66 to remove cartilage and bone. In one example, the blade 68 rotates at 8000 RPMs.

[0068] During operation, the inner blade 68 can hit the outer shell 66 as the blade 68 rotates. In one example, the outer shell 66 is made of a cobalt-chromium alloy, and an inner surface of the outer shell 66 has a B.sub.4C layer as described above. In this example, the blade 68 is made of a softer material than the inner surface of the outer shell 66. If the blade 68 and the outer shell 66 engage each other during operation, the softer blade 68 can wear, preventing breakage and any metal from entering the surgical site. The B.sub.4C layer does not increase any dimension of the orthopedic cutting instrument 64, allowing for a tight fit between the rotating blade 68 and the outer shell 66. In another example, the blade 68 is made of a cobalt-chromium alloy with a B.sub.4C layer, and the outer shell 66 is made of a softer material. In another example, both the outer shell 66 and the blade 68 are made of a cobalt-chromium alloy with a B.sub.4C layer, but they have different hardnesses.

[0069] In one example, an orthopedic medical implant, implant part or surgical instrument includes a metallic body including a metal or a metal alloy. The metal or metal alloy is selected from the group consisting of cobalt, cobalt-chromium alloys, titanium, titanium-alloys, and mixtures thereof. The metallic body includes a sub-surface that is a thermal diffused boron carbide layer. The thermal diffused boron carbide layer comprises borides having a formula MeB, MeB.sup.2, or Me.sub.2B, where Me represents a metal present in the metallic body of the orthopedic medical implant, implant part or surgical instrument. The metallic body is void of an additive layer onto a surface of the metallic body, and a dimension of a pre-boronized implant or implant part is the same as a dimension of a post-boronized implant or implant part.

[0070] In another example, an orthopedic medical implant, implant part or surgical instrument includes a metallic body including a metal or a metal alloy. The metallic body includes a sub-surface that is a thermal diffused boron carbide layer, and the metallic body is void of an additive layer onto a surface of the metallic body.

[0071] In another embodiment according to any of the previous embodiments, the metal or metal alloy is selected from the group consisting of cobalt, cobalt-chromium alloys, titanium, titanium-alloys, and mixtures thereof.

[0072] In another embodiment according to any of the previous embodiments, the thermal diffused boron carbide layer includes borides having a formula MeB, MeB.sup.2, or Me.sub.2B, where Me represents a metal present in the metallic body of the orthopedic medical implant, implant part or surgical instrument.

[0073] In another embodiment according to any of the previous embodiments, a dimension of a pre-boronized implant or implant part is the same as a dimension of a post-boronized implant or implant part.

[0074] In another embodiment according to any of the previous embodiments, a surface hardness of the surface of the metallic body is at least 1500 HV.

[0075] In another embodiment according to any of the previous embodiments, a boronization thickness of the metallic body is at least 100 microns.

[0076] In another embodiment according to any of the previous embodiments, the surface of the metallic body has a coefficient of friction of 0.01 at 15,000 psi.

[0077] In another embodiment according to any of the previous embodiments, a surface chemistry of the surface of the metallic body is 40% to 60% boron.

[0078] In another embodiment according to any of the previous embodiments, the implant or implant part is selected from the group consisting of a femoral component of an uni-compartmental knee arthroplasty or a total knee arthroplasty, a tibial component of a uni-compartment knee arthroplasty or a total knee arthroplasty, a femoral head of hip arthroplasty, a Morse taper of a hip arthroplasty, an acetabular cup or liner of a hip arthroplasty, a humeral head of a shoulder arthroplasty, a humeral or ulnar component of an elbow arthroplasty, a metacarpal or radial stem of a wrist arthroplasty, a vertebral endplate components of a disc arthroplasty, and a tibial or talar component of an ankle arthroplasty.

[0079] In another embodiment according to any of the previous embodiments, the metallic body is a cobalt-based alloy.

[0080] In another embodiment according to any of the previous embodiments, the medical implant, implant part or surgical instrument has a hexagonal close packed crystal structure and is age hardened.

[0081] In another embodiment according to any of the previous embodiments, the metallic body is a titanium-based alloy.

[0082] In another embodiment according to any of the previous embodiments, orthopedic medical implant, implant part or surgical instrument is in an alpha+beta phase.

[0083] In another example, an orthopedic medical implant, implant part or surgical instrument includes a metallic body including a metal or metal alloy. The metallic body includes a sub-surface including a boronized layer of the metal or metal alloy. The orthopedic medical implant, implant part or surgical instrument is in an unannealed condition.

[0084] In another example, a method of forming an orthopedic medical implant, implant part or surgical instrument, the method includes creating a thermal diffused boron carbide layer in a sub-surface of a metallic body of the orthopedic medical implant, implant part or surgical instrument. The orthopedic medical implant, implant part or surgical instrument includes a metal or a metal alloy, and the metallic body is void of an additive layer onto a surface of the metallic body.

[0085] In another embodiment according to any of the previous embodiments, the method includes heat-treating the orthopedic medical implant, implant part or surgical instrument in a controlled atmosphere furnace.

[0086] In another embodiment according to any of the previous embodiments, the heating-treating occurs between 1000° F. and 1700° F.

[0087] In another embodiment according to any of the previous embodiments, the heating-treating occurs between 1000° F. and 1750° F., and the orthopedic medical implant, implant part or surgical instrument has a hexagonal close packed crystal structure and is age hardened.

[0088] In another embodiment according to any of the previous embodiments, the heating-treating occurs at below 1100° F., and the orthopedic medical implant, implant part or surgical instrument is in an alpha+beta phase.

[0089] It should be understood that many additional changes in the details, materials, steps and arrangements of parts, which have been herein described and illustrated in order to explain the nature of the present invention, may be made by those skilled in the art while still remaining within the principles and scope of the invention.