Titanium porous surface bonded to a cobalt-based alloy substrate in an orthopaedic implant device

A femoral component of a knee prosthesis, including a cobalt-based alloy substrate and a titanium fiber metal pad bonded thereto by means of an interlayer of a cobalt-based alloy including nickel. More specifically, a method of bonding a titanium porous surface to a cobalt-based alloy in an orthopaedic implant device, by first applying an interlayer of a cobalt-based alloy including nickel to the substrate and then bonding a porous structure to the interlayer. In one embodiment, an interlayer of L-605 is first applied to a substrate of Co-Cr-Mo by diffusion bonding at approximately 2200.degree. F. and then a fiber metal pad of CP-titanium is diffusion bonded to the interlayer at approximately 1650.degree. F. A layer of CP-titanium may optionally be placed intermediate the fiber metal pad and interlayer before the second diffusion step. In an alternative embodiment, MP-35N alloy may be substituted for the L-605 alloy.

BACKGROUND OF THE INVENTION 
The present invention relates generally to orthopaedic implant devices of 
the type having a porous surface into which bone tissue can grow or 
surgical cement can enter and, more particularly, to a method of bonding a 
porous surface structure of titanium or a titanium alloy onto a substrate 
of a cobalt-based alloy, whereby enhanced bonding strength and corrosion 
resistance is achieved. 
Orthopaedic implant devices commonly include a porous surface to enhance 
attachment of the device to adjacent bone tissue. Consequently, various 
methods have developed for manufacturing an implant device having a porous 
surface. One such method involves securing a porous fiber metal structure, 
i.e., a wire mesh pad, to a portion of the surface of the device where 
bone ingrowth or attachment is desired. While the fiber metal pad 
generally provides a suitable porous surface, efforts to develop improved 
methods of attaching the pad to the implant surface continue. For 
instance, it is a desired to further improve the bonding strength and 
corrosion resistance of the attachment interface. 
In several orthopaedic implant applications, it is desired to combine 
dissimilar metals in order to take advantage of the particular strength, 
biocompatibility, and corrosion resistance properties of the respective 
metals. For instance, as disclosed in UK Patent GB 2142544 B to Medcraft, 
it is known to combine a cobalt-based alloy with titanium or a titanium 
alloy, wherein one constitutes the substrate of an implant device and the 
other constitutes a wire mesh that is diffusion bonded to the substrate to 
provide a porous surface. However, diffusion bonding of commercially pure 
titanium porous structure (such as a porous pad) onto cobalt-based alloys 
requires elevated temperatures which reduces corrosion resistance of the 
composite structure. Furthermore, diffusion bonding of titanium porous 
structure directly onto a cobalt-based alloy does not produce necessary 
bond strength in the structure. 
In another application involving a cobalt-based alloy and titanium, the 
femoral component of a knee prosthesis is fabricated by casting a 
cobalt-based alloy substrate and then securing a titanium fiber metal pad 
to the surface of the substrate by first plasma spraying a titanium 
coating on the surface. This bonding method is disclosed in U.S. Pat. No. 
4,969,907 to Koch et al. 
Both of the aforementioned bonding arrangements result in the formation of 
an alloy at the interface between components that tends to exhibit some 
corrosion at the interfacing layers. Nevertheless, it desired to provide 
an improved method of bonding a titanium porous surface to the surface of 
a cobalt-based alloy substrate, wherein bonding strength and corrosion 
resistance are enhanced. 
SUMMARY OF THE INVENTION 
The present invention provides an orthopaedic implant device having a 
porous surface, wherein a titanium porous structure is bonded to the 
surface of a cobalt-based alloy substrate by an interlayer comprising a 
cobalt-based alloy including nickel. Such interlayer may also include 
additional elements, such as tungsten. Generally, the nickel, or nickel 
and tungsten, contained in the bonding interlayer tends to form an alloy 
with the titanium porous structure at their bonding interface that 
enhances the corrosion resistance properties of the resulting assembly. 
More specifically, the bonding process of the present invention involves 
first applying the bonding interlayer to the substrate, and then bonding 
the porous structure to the interlayer. In this manner, a dual cycle 
process is established whereby a lower process temperature can be used for 
the titanium bonding step, thereby minimizing unwanted corrosion problems. 
An advantage of the orthopaedic implant device of the present invention is 
that a titanium porous surface is provided on a cobalt-based alloy 
substrate with enhanced bonding strength and corrosion resistance 
properties. 
An advantage of the bonding method of the present invention is that a 
titanium porous structure can be bonded to a cobalt-based alloy substrate. 
Another advantage of the bonding method of the present invention is that a 
dual cycle bonding process is employed whereby different bonding 
temperatures can be used, thereby protecting the metallurgical properties 
of the component alloys and determining bonding strengths at the 
respective interfaces between alloys. 
The invention, in one form thereof, provides an orthopaedic implant device 
having enhanced corrosion resistance properties. The device includes a 
substrate of a cobalt-based alloy, and a porous structure of titanium or a 
titanium alloy. According to the invention, the porous structure is bonded 
to the substrate by interlayer disposed intermediate the substrate and the 
porous structure. The interlayer comprises a cobalt-based alloy including 
nickel. In one aspect of this form of the invention, a layer of 
commercially pure titanium may be disposed intermediate the porous 
structure and the interlayer. 
The invention further provides, in one form thereof, a method of 
manufacturing an orthopaedic implant device having a porous surface. A 
substrate of a cobalt-based alloy in the form of an orthopedic implant 
device is first provided. An interlayer of a cobalt-based alloy including 
nickel is then applied onto the surface of the substrate. Following the 
application of the interlayer onto the surface of the substrate, a porous 
structure of titanium or a titanium alloy is bonded to the interlayer. In 
one aspect of the invention according to this form thereof, the porous 
structure is bonded to the interlayer by diffusion bonding. A titanium 
layer may be provided between the porous structure and the interlayer 
prior to the assembly being diffusion bonded together.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1, there is shown an orthopedic implant device 10, 
i.e., a femoral component of a knee prosthesis, including a substrate 12 
of a cobalt-based alloy to which a titanium porous surface structure 14 is 
bonded in accordance with the present invention. In the disclosed 
embodiment, device 10 is the femoral component of a knee prosthesis and 
generally comprises a forward shield portion 16, a lower articulating 
bearing portion 18, and a pair of rearward, upwardly extending condyles 
20, all of which define an inwardly facing surface 22 that contacts and 
attaches to the distal end of the femur (not shown). In accordance with a 
preferred embodiment of the invention, porous surface structure 14 is a 
titanium fiber metal pad 24 partially disposed within a recess 26 in 
surface 22. 
Pad 24 is secured to a bottom surface 28 of recess 26 by means of a bonding 
interlayer 30, as shown in FIG. 2, wherein interlayer 30 comprises a 
cobalt-based alloy including nickel. As will be more particularly 
described hereinafter, interlayer 30 is initially applied to surface 28, 
and is then bonded with titanium fiber metal pad 24 along an interface 32. 
At interface 32, the nickel in cobalt-based alloy interlayer 30 forms a 
new alloy with the titanium of pad 24 at bonding temperatures, thereby 
producing improved corrosion resistance properties as compared to direct 
bonding of the titanium pad with the cobalt-based alloy substrate. 
FIG. 3 illustrates an alternative embodiment of the present invention, 
wherein a layer 134 of commercially pure titanium (CP-titanium) is 
disposed between interlayer 130 and fiber metal pad 124. According to this 
embodiment, not only is there a corrosion resistive interface 132 
established between titanium layer 134 and interlayer 130, but the bonding 
strength between pad 124 and interlayer 130 is also enhanced. 
A method of bonding titanium porous surface structure 14 to cobalt-based 
alloy substrate 12 will now be described with reference FIG. 4, wherein an 
exemplary embodiment of the method of the present invention is 
diagrammatically illustrated. Generally, block 40 represents the first 
step of providing an orthopaedic implant device, or component part 
thereof, that is fabricated from a cobalt-based alloy, i.e., a 
cobalt-based alloy substrate. In the preferred embodiment, a cast alloy 
containing cobalt, chromium, and molybdenum is used, having the ASTM 
designation F75. One such commercial alloy of this designation is 
ZIMALOY.RTM. cobalt-chromium-molybdenum alloy, manufactured by Zimmer, 
Inc. of Warsaw, Ind., the assignee of the present invention. The cast 
substrate provided in the first step of the method is preferably cleaned 
by shot-blasting with stainless steel shot, and by mass tumbling. 
Block 44 of FIG. 4 represents the next step of the process, during which a 
layer of a cobalt-based alloy containing nickel is applied to the surface 
of the substrate. According to the preferred embodiment, the applied layer 
is L-605 alloy, comprising cobalt, chromium, tungsten, and nickel, and 
having the ASTM designation F90. When using the L-605, the nickel and 
tungsten in this interlayer 30 forms a new alloy with the titanium of pad 
24 at bonding temperatures. More specifically, an L-605 foil having a 
thickness of 0.005 inch is diffusion bonded to the surface of the 
substrate. It has been found that a desirable bonding strength is achieved 
by diffusion bonding at approximately 2200.degree. F. for 8 hours, in a 
vacuum furnace having a 400 .mu.m partial pressure of argon. A clamping 
pressure between the foil and substrate is applied by means of a suitable 
clamping fixture, e.g., a multi-piece bolted clamping fixture. 
It will be appreciated that the temperature, cycle time, partial pressure 
of argon, and clamping pressure in step 44 can be adjusted to vary the 
resulting bonding strength between the substrate and interlayer. However, 
optimum strength properties were observed using a dual diffusion bonding 
cycle (2 hours and 6 hours) at 2200.degree. F. in a 400 .mu.m partial 
pressure of argon, wherein clamping pressure is applied by a bolted 
fixture with the bolts tightened to 8-10 inch-pounds of torque. The two 
hour cycle may be applied, the materials cooled down, and then the six 
hour cycle applied. The resulting bonding strength, depending on the 
surface finish and cleanliness, exhibits a range of 10 ksi to 150 ksi. 
The next step of the process, represented by block 46 in FIG. 4, involves 
diffusion bonding a porous structure to the L-605 layer that has been 
applied to the substrate. The porous structure is in the form of titanium 
fiber metal pad 24, as provided for and represented in block 42. The term 
"titanium fiber metal pad" as used herein is intended to encompass both 
commercially pure titanium (CP-titanium) and other alloys based on 
titanium. In the preferred embodiment, fiber metal pad 24 is fabricated 
from CP-titanium, and has an approximate thickness of 0.055 inch. The 
fiber metal material from which pad 24 is cut to fit within recess 26, is 
commercially available as fine wires, manufactured by Astro Metallurgical 
of Wooster, Ohio. An example of such a suitable porous fiber metal 
structure is disclosed in U.S. Pat. No. 3,906,550 to Rostoker et al., 
although is not limited thereto. 
According to step 46 of FIG. 4, the CP-titanium fiber metal pad is then 
diffusion bonded to the pre-applied L-605 layer at a bonding temperature 
of approximately 1650.degree. F. The same cycle time (8 hours), partial 
pressure of argon (400 .mu.m), and clamping pressure (8-10 inch-pounds of 
torque for bolted fixture) that were used for bonding the L-605 to the 
substrate are again used for bonding the pad to the L-605 layer. Likewise, 
it is recommended that the same dual diffusion bonding cycle (2 hours and 
6 hours) be used to achieve optimum bonding strength. 
The bonding strength between the titanium fiber metal pad and the L-605 
layer can be further increased by introducing an interlayer of CP-titanium 
therebetween, as represented by the optional process step of block 48 in 
FIG. 4 and illustrated by the alternative structure of FIG. 3. 
Specifically, a CP-titanium foil having a thickness of 0.003 inch is 
placed between the pad and the L-605 layer prior to clamping and diffusion 
bonding of the pad. The titanium foil and the titanium pad may be bonded 
to the pre-applied L-605 layer at the same time. This additional layer 
increases the bonding surface area between the fiber metal pad and the 
L-605 layer, thereby enhancing bonding strength. Alternatively, a sheet or 
coating of CP-titanium or titanium alloy may be used. 
As represented by block 50 of FIG. 4, the process of the present invention 
results in an orthopaedic implant device having a cobalt-based alloy 
substrate and a titanium porous surface that are bonded together in such a 
way as to exhibit enhanced bonding strength and corrosion resistance 
properties. 
Interlayer 30, as disclosed herein, is an L-605 alloy layer and is applied 
to the cobalt-based alloy substrate by diffusion bonding. It is 
contemplated that MP-35N alloy, comprising cobalt, nickel, chromium, and 
molybdenum and having ASTM designation F562, may be substituted for the 
L-605 alloy. Instead of diffusion bonding, it is contemplated that L-605 
or MP-35N in the form of a foil or a sheet may also be applied to the 
substrate by sintering, welding, or cladding. Alternatively, a coating of 
these alloys may be applied by one or more of a variety of methods, 
including thermal spray coating, vacuum deposition coating, sputtering, 
and plating. 
Thermal spray coating methods for depositing a thin layer of L-605 or 
MP-35N alloy to the substrate include combustion flame spray, detonation 
gun, plasma arc, wire arc, high velocity oxygen fuel (HVOF), and low 
pressure (hypervelocity) plasma arc spray. The latter two methods have 
shown particularly good results. 
High velocity oxygen fuel (HVOF) is a combustion process which provides 
improved oxidation control and superior coating bond strength when 
compared to standard combustion flame processes. The coating alloy powder 
is introduced into the high speed and temperature combustion gas stream 
which carries it to the substrate face, where the alloy impacts and bonds. 
Oxide formation is controlled by using a "rich" fuel mixture, This makes 
extra fuel available to combine with the free oxygen surrounding the 
substrate being coated, thereby reducing the amount of oxygen available to 
form oxides. In addition, the coating particles are bonded to the 
substrate with a great deal more energy than is available with standard 
flame spray. Particle velocities approach 2500 feet per second as opposed 
to 400 feet per second in a typical flame spray. This greatly improves 
coating bond strength, and when combined with the oxide reduction, greatly 
reduces coating porosity. Several HVOF processes are commercially 
available, one in particular being JETCOAT.RTM. by Deloro Stellite, of 
Goshen, Ind. 
Low pressure plasma arc spray (LPPS), unlike HVOF, is not a combustion 
process, but rather uses an electric arc to heat a plasma carrier gas to 
high temperatures. As in the HVOF process, coating alloy powder is 
introduced into the high temperature and speed carrier stream. The coating 
takes place in an inert atmosphere at pressures greatly reduced below 
atmospheric and approaching a vacuum at 15-60 Torr. The inert atmosphere 
eliminates oxide formation and allows highly reactive metals, such as 
titanium, to be deposited without oxide contamination of the coating. The 
greatly reduced pressure allows the coating particles to reach speeds in 
excess of 3000 feet per second. The near vacuum inert atmosphere used in 
this process results in an extremely dense coating with no oxide 
formation. 
Thus, an alternative embodiment of this device is to use the previously 
described HVOF process to deposit an L-605 or MP-35N corrosion inhibiting 
interlayer. Alternatively, the low pressure plasma spray (LPPS) process 
may be used to deposit both an L-605 or MP-35N interlayer and a 
commercially pure titanium coating on top of the interlayer to enhance 
bonding between the interlayer and the titanium fiber metal. 
In accordance with the ASTM designation F90 referred to above, the chemical 
requirement for the L-605 alloy are as follows: 
______________________________________ 
Composition, % 
Element Minimum Maximum 
______________________________________ 
Carbon 0.05 0.15 
Manganese 1.00 2.00 
Silicon -- 0.40 
Phosphorus -- 0.040 
Sulfur -- 0.030 
Chromium 19.00 21.00 
Nickel 9.00 11.00 
Tungsten 14.00 16.00 
Iron -- 3.00 
Cobalt* Balance Balance 
______________________________________ 
*Approximately equal to the difference between 100% and the sum percentag 
of the other specified elements. 
In accordance with the ASTM designation F562 referred to above, the 
chemical requirement for the MP-35N alloy are as follows: 
______________________________________ 
Composition, % 
Element Minimum Maximum 
______________________________________ 
Carbon -- 0.025 
Manganese -- 0.15 
Silicon -- 0.15 
Phosphorus -- 0.015 
Sulfur -- 0.010 
Chromium 19.0 21.0 
Nickel 33.0 37.0 
Molybdenum 9.0 10.5 
Iron -- 1.0 
Titanium -- 1.0 
Cobalt* Balance Balance 
______________________________________ 
*Approximately equal to the difference between 100% and the sum percentag 
of the other specified elements. 
These chemical requirements for these two alloys are identified in the 
indicated ASTM standard. 
While the present invention has been illustrated with specific reference to 
a femoral component of a knee prosthesis, the present invention is equally 
applicable to other prosthesis, devices, e.g., hip, ankle, elbow, 
shoulder, wrist, finger, and toe joints, wherein a titanium porous 
structure is bonded to a cobalt-based alloy substrate by means of an 
interlayer comprising a cobalt-based alloy including nickel. 
It will be appreciated that the foregoing description of a preferred 
embodiment of the invention is presented by way of illustration only and 
not by way of any limitation, and that various alternatives and 
modifications may be made to the illustrated embodiment without departing 
from the spirit and scope of the invention.