Method for ceramic peening of orthopaedic titanium alloy implants

A method of preparing fretting wear resistant titaniumbase alloy orthopaedic implants includes the steps of shot peening the surface of the implant with spherical ceramic beads of a selected size and density, and with selected intensity such that the fretting wear resistance of the implant is increased and wherein the implant surface is uniformly peened to create a compressively-stressed region within the surface of approximately 100-300 microns depth.

SPECIFICATION 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to surgical prosthetic devices and more 
particularly to orthopaedic surgical implants. Even more particularly, the 
present invention relates to a method of ceramic shot peening of 
titanium-base alloy orthopaedic surgical implants that improves such 
implants' resistance to fretting wear. 
2. General Background 
"Fretting wear" can be defined as the surface damage phenomenon that occurs 
on two adjacent contacting surfaces which experience oscillatory relative 
motion of small amplitudes (fretting). Fretting may also lead to fretting 
corrosion and fretting fatigue. Fretting corrosion relates to chemical 
reactions which take place at the fretting interface as a result of 
fretting. Fretting fatigue is related to fretting-initiated fatigue cracks 
which may result in component fatigue failure, as well as surface and 
subsurface stress concentrations. 
Fretting wear is the loss of material (wear debris) that is associated with 
fretting conditions. When microscopic particles of metal are fretted from 
the metallic implant surface, the "fresh" metallic surface left behind by 
the newly removed particles releases metal ions there from. This ion 
release continues until the "fresh" surface becomes fully oxidized. 
The July, 1989 issue of "Metal Finishing" by Robert B. Heaton described 
shot peening as a cold working process in which the surface of a metal 
part is bombarded with spherical particles (or shot) formed from cast 
iron, cast steel, stainless steel, ceramic or glass. The Heaton article 
states that peening is used to create a uniform compressive stress layer 
on or near the surface of metals that prolongs surface life under cyclical 
loading conditions by resisting fatigue failures. The Heaton article also 
discusses peening for application to leaf and coil springs, gear fillets, 
drive shafts, torsional bars, axles, oil well drilling equipment, turbine 
blade root, foil sections, and other metal components exposed to cyclical 
stresses. 
A patent that describes a treatment process for a stainless steel 
orthopaedic implant device is U.S. Pat. No. 5,057,018 issued to H. Shetty, 
et al and entitled "Method of Surface Finishing Orthopedic Implant 
Devices". In the Shetty patent, a rough ground or machined stainless steel 
part is shot blasted with glass beads having a nominal size of between 
one-tenth and one-half that of the stainless steel shot. Electropolishing 
and passivation follow the shot blasting steps. A part thus treated 
includes a heavily cold-worked outer layer that is said to enhance the 
fatigue properties of the stainless steel orthopaedic implant device. The 
present invention is different from U.S. Pat. No. 5,057,018 in the 
following ways: i) it claims improvements for titanium alloy devices, not 
stainless steel devices; ii) the shot peening method described utilizes 
ceramic media, not stainless steel and glass media; and iii) improved 
surface properties which are claimed include only fretting wear resistance 
instead of fatigue and corrosion resistance. 
SUMMARY OF THE INVENTION 
The present invention provides a titanium-base alloy implant surface with 
improved resistance to fretting wear. A controlled ceramic shot peening 
process is used to induce a controlled surface roughness and uniform 
compressive stress on and into the surface of the titanium orthopaedic 
implant. These factors enhance the overall structural integrity of the 
implant and reduce the potential for metallic debris to be liberated from 
the implant as a result of metal-on-metal contact. 
The present invention provides a method of preparing a controlled surface 
roughness of titanium orthopaedic implants by peening the surface of the 
implant with spherically shaped ceramic shot of a size, density, and 
selected peening intensity so that the fretting wear resistance of the 
implant is increased. 
The present invention provides an improved method preparing fretting wear 
resistant titanium orthopaedic implants wherein the surface of the implant 
is shot peened with ceramic beads of a selected shot size, density, and 
selected peening intensity so that the fretting wear resistance of the 
implant is increased, and wherein the implant surface is peened such that 
a compressively-stressed region of approximately 100-300 microns in depth 
is produced. 
In the method of the present invention, residual compressive stresses of at 
least 100 kpsi are produced within the surface of the titanium implant. 
With the present invention, fretting wear debris generation is minimized. 
In the preferred method of the present invention, the shot size is between 
120 and 1200 microns diameter, the shot shape is spherical, the density 
(specific gravity) of the material from which the shot is composed is 
between 2.2 and 10.0 g/cm.sup.3, the shot material composition includes 
any number of oxide ceramics such as ZrO.sub.2, Al.sub.2 O.sub.3, 
SiO.sub.2, MgO, etc., the number fraction of non-spherical (including 
broken) shot is less than 5.0%, and the Almen intensity of the ceramic 
shot peening procedure is between 2.5 A and 18 A. In the preferred method 
of the present invention, the shot is filtered to remove any non-spherical 
or particulate shot material prior to peening operations, surface coverage 
is 100-200%, and the peening procedure is fully automated to ensure 
uniform coverage over the entire peened surface.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention provides a method for forming the surface of titanium 
alloy orthopaedic implants such as hip implants, knee implants, spinal 
implants, and the like. This material is used for such applications due to 
its high fatigue strength, low elastic modulus, and excellent 
biocompatibility and corrosion resistance. 
The ceramic shot used to peen the titanium-alloy surface is preferably 
120-1200 microns in diameter, the shot density (specific gravity) is 
preferably 2.2-10.0 g/cm.sup.3, the shot. composition is preferably any of 
a number of oxide ceramics or mixtures thereof. The number fraction of 
non-spherical shot is preferably less than 5.0%. The Almen intensity is 
preferably 2.5 A-18 A. Surface coverage is preferably 100-200%. The 
peening procedure is preferably automated to ensure uniform surface 
coverage. 
Referring now to FIG. 1, there can be seen a shot peening nozzle 12 and 
specimen fixture assembly 11 of the type used in conjunction with a shot 
peening apparatus (not shown) in carrying out the process and producing an 
orthopaedic implant workpiece 10 in accordance with the present invention. 
Nozzle 12 is used to carry and expel ceramic media 13 with high pressure 
air toward a target area of workpiece 10. The nozzle is connected to a 
hose 14, which is connected to a dry blast system with a cyclone media 
separator and dust blower (not shown) or similar features which are used 
to recirculate only undamaged spherical ceramic shot. 
Assembly 11 includes a fixture 15 for supporting and manipulating workpiece 
10 which in turn is connected to arms 16 which are compatible with an 
automated, programmed motion system (not shown) used to control the 
location, angle, and coverage of peening of workpiece 10. 
The metal substrate of implant workpiece 10 in FIG. 1 is preferably wrought 
and annealed titanium-6 aluminum-4 vanadium (Ti-6A1-4V) alloy (R.sub.c 
30-35), but the method of the present invention is applicable to any 
titanium-base alloy in any metallurgical condition. The surface finish of 
workpiece 10 is preferably polished or machined with a surface roughness 
(Ra)of 128 microinches or less. Following the ceramic peening surface 
treatment, the titanium-alloy surface is preferably not given any 
additional surface treatments which may physically alter the surface. The 
surface is, however, preferably cleaned and passivated in accordance with 
conventional methods. These steps are conducted to remove loosely attached 
debris, remove most surface contamination, and reinforce the passive 
surface oxide layer. 
In FIG. 2, there can be seen ceramic shot peened orthopaedic titanium alloy 
surfaces at 1.times. magnification. The alloy (Ti-6A1-4V) has been peened 
with spherical ceramic shot. 
In FIG. 3, a schematic of a machined titanium implant surface 17 is shown 
having microscopic surface defects 18. 
In FIG. 4, the same surface 17 is shown after some time "t" where fretting 
wear has occurred, producing larger cracks and defects 19, 20 and escaping 
debris 21, 22. 
In FIG. 4, a schematic of a machined and ceramic shot peened titanium 
implant surface 23 is shown having microscopic surface defects 24 and a 
compressively stressed sub-surface region 25. 
In FIG. 5, the same surface 23 is shown after some time "t" where fretting 
wear has occurred producing less cracks and defects 26 and less escaping 
debris 27 compared to the non-peened surface in FIG. 3. 
Because many varying and different embodiments may be made within the scope 
of the inventive concept herein taught, and because many modifications may 
be made in the embodiments herein detailed in accordance with the 
descriptive requirement of the law, it is to be understood that the 
details herein are to be interpreted as illustrative and not in a limiting 
sense.