Note: Descriptions are shown in the official language in which they were submitted.
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ZM0143
H. Ravi Shetty
METHOD OF SURFACE FINISHING ORTHOPEDIC IMPLANT DEVICES
BACKGROUND OF THE INVENTION
The present invention relates generally to stainless steel
orthopedic implant devices and, more particularly, to a surface
treatment process applicable to such devices, wherein both the
fatigue and, corrosion properties of an orthopedic implant device
are enhanced.
More specifically, the present invention relates to
orthopedic implant devices commonly used by orthopedic surgeons
to repair and replace fractured and deteriorating bones and
joints. For example, U.S. Patent No. 4,612,920, issued to Lower,
discloses a compression hip screw of the general type to which
the surface treatment process of the present invention is
applicable. Other orthopedr~c devices that the present invention
would be applicable to include, but are not limited to, hip
prostheses, bone plates, intramedullary nails, and other fracture
fixation devices fabricated from stainless steel.
Representative of the function of many orthopedic implant
devices, the aforementioned compression hip screw rigidly
connects a femoral head to the remaining portion of the femur
despite a fracture in the area of the femur neck. During a
normal walking cycle, substantial loads are imparted to and
transferred by the compression hip screw. Consequently, it is
desirable to reduce the possibility of a component failure that
might require the patient to undergo further surgery.
A variety of stainless steel alloys are used for fracture
fixation devices and are chosen on the basis of their high
strength, ductility, fracture toughness, biocompatibility, and
corrosion resistance. It is generally known that the fatigue and
corrosion properties of orthopedic implant devices fabricated
CA 02044758 2002-12-13
from these alloys can be affected by different surface treatment
processes. For instance, one such process includes the steps of
mechanical grinding, sisal buffing or color buffing,
electropolishing, and passivation.
Recently, orthopedic implant devices have been surface
treated by shot blasting with alumina, i.e., a form of aluminum
oxide having a hard crystalline structure. Alumina is typically
used as an abrasive and includes a sharp, irregular surface.
Shot blasted alumina tends to become imbedded in and/or leave a
1o residue on the surface of some stainless steels; therefore,
subsequent steps of glass bead blasting and electropolishing are
requited.
while the surface finishing processes employed in the
manufacture of stainless Steel orthopedic implant devices have
been generally successful ir~~providing devices having clean
surfaces, it is desired to develop a surface finishing process
that maintains presently attained levels of surface cleanliness
while further enhancing the fatigue properties of the device.
SUMMARY OF,~HE INVENTION
Generally, the present invention provides a process far
surface finishing a metal orthopedic ~.mplant device,
wherein the fatigue strength of the device is greatly enhanced
without compromising corrosion resistance .properties of the
metal. The invention also encompasses orthopedic
implant devices fabricated in accordance with the claimed
process.
More specifically, the process of the present invention
enhances the fatigue strength of a metal (e.g.lstainless steel)
by causing a heavily cold-worked outer layer to be formed on the
metal implant device as the result of shot blasting
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CA 02044758 2002-12-13
with steel shot. A further step of the inventive process
includes electropolishing the shot blasted surface.
In one aspect of the invention, the initial use of larger
stainless steel shot and the subsequent use of smaller glass
beads for shot blasting results in more thorough coverage and
cold-working of the outer surface of a stainless steel orthopedic
implant device. In another aspect of the invention, the step of~
electropolishing the cold-worked outer layer of the implant
device is intended to restore corrosion resistance properties to
the surface without diminishing gains in fatigue strength
produced by the shot blasting steps of the process.
An advantage of the method of the present invention is that
orthopedic implant devices having increased fatigue strength are
possible as the result of surface treatment of the device.
Another advantage of the method of the present invention is
that it is possible to significantly enhance the fatigue strength
of an orthopedic implant devices without compromising corrosion
resistance properties thereof.
A further advantage of the method of the present invention
is that aluminum oxide contamination of the surface of stainless
steel orthopedic implant devices is virtually eliminated.
Yet another advantage of the method of the present invention
is that a stainless steel orthopedic implant device having a
heavily cold-worked outer layer and a relatively soft center core
may be easily fabricated, thereby providing a device exhibiting
unique structural properties for orthopedic applications.
The invention, in one form thereof, provides a method of
manufacturing an orthopedic implant device having enhanced
fatigue properties. The method typically includes several steps,
including an initial step of providing a metal substrate in the
form of an orthopedic implant device, or component thereof. The
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CA 02044758 2002-12-13
metal substrate has a generally smooth outer surface which,
pursuant to another step of the invention, is shot blasted with
metal shot until a cold-worked outer layer is formed on the metal
substrate. The cold-worked layer has a textured outer surface
which, according to a further step of the invention, is
electropolished. In one aspect of the invention according to
this form, the textured outer surface is shot blasted with glass
beads prior to electropolishing.
The invention further provides, in one preferred form, a
method of increasing the fatigue strength of an orthopedic
implant device that is fabricated from stainless steel. The
outer surface of the device is first shot blasted with stainless
steel shot. Next, the outer surface is shot blasted with glass
bead shot having a nominal size less than the nominal size of the
stainless steel shot. After shot blasting the outer surface with
glass bead shot, electropolishing is performed on the outer
surface. In one aspect of the invention according to this form
thereof, the nominal size of the glass beads is preferably one-
tenth to one-half that of the stainless steel shot.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a perspective view of the spray nozzle and fixture
associated with a bead blasting apparatus of the type used in
accordance with the method of the present invention, wherein a
fixation plate is shown being processed;
Fig. 2 is a diagrammatic representation of the process steps
involved in an exemplary embodiment of the method of the present
invention;
Fig. 3 is a fragmentary sectional view of a hip joint
including a femur having a fracture at the neck thereof, wherein
a compression hip screw, manufactured in accordance with the
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msahod of the present invention, is shown providing fixation of
the femoral head to the femurs and
Fig. 4 is an enlarged transverse sectional view of the plate
of Fig. 3, taken along the line 4-4 in Fig. 3, particularly
showing in exaggerated scale a textured outer surface, a heavily
cold-worked outer layer, a less heavily cold-worked sublayer, and
an unaffected inner core are in accordance with the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to Fig. 1, there is shown a shot blasting
nozzle and fixture assembly 10 of the type used in conjunction
with a shot blasting apparatus (not shown) in carrying out the
process and producing an orthopedic implant device 12 in
accordance with the preset invention. Assembly 10 includes a
. ;.
nozzle 14 used to carry and~'expel blasting media, i.e., stainless
steel shot and glass beads, with high pressure air toward a
target area of device 12. Nozzle 14 is connected to a hose~l6,
which is operably connected to a dry blast system, such as a
Model 4228-F system manufactured by Cyclo-Blast Dry Honer Co. of
Belmont, California.
Assembly 10 also includes a fixture 18 for supporting a
workpiece, i.e., orthopedic implant device 12, during the shot
blasting steps of the present invention, which steps will be more
particularly described hereinafter. Fixture 18 includes a
support block 20 attached to a base member 22 by means of a neck
portion 24. As illustrated in Fig. 1, the top surface of support
block 20 includes a V-shaped groove 26 in which device 12 is
disposed during shot blasting aperations. Nozzle 14 is
selectively spaced above support block 20 by means of a
horizontal plate member 28 having an opening 30 through which
nozzle 14 is operably received and retained therein.
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Plate member 28 is slidably connected to a vertical spacing
bar 32, for selective positioning above support block 20, by
means of a clamping block 34 and a pair of screws 36. Spacing
bar 32 is fixedly mounted to support block 20 by a pair of screws
38. Accordingly, it can be seen from Fig. 1 that a nozzle
opening 40 of nozzle 14, from which blasting media is expelled by
high pressure air, is directed toward and selectively positioned
above the workpiece resting on support block 20.
Referring now to Fig. 2, the steps for surface treating a
stainless steel orthopedic implant device in accord with an
exemplary embodiment of the present invention are
diagrammatically illustrated. Generally, block 50 represents the
first step of providing a stainless steel orthopedic implant
device or a component par~,thereof, i.e., a metal substrate.
Block 52 represents the next step of shot blasting the outer
surface of the part with stainless steel shot. Block 54
represents the next step of shot blasting with glass beads. The
part then undergoes electropolishing and passivation, as
represented by blocks 56 and 58, respectively. The steps will
now be described in detail.
The metal substrate provided in the step of block 50 is
preferably hot forged or cold-worked 22Cr-l3Ni-5Mn (22-13-5)
stainless steel alloy (Rb 60 to Rc 50), or hot forged or cold-
worked 316L stainless steel alloy (Rb 60 to Rc 50). Both are
used in fracture fixation devices due to their high strength,
ductility, fracture toughness, biocompatibility, and corrosion
resistance. Inasmuch as the substrate has already been
fabricated into an orthopedic implant component part prior to
this step, the surface has been appropriately machined, surface
ground, and/or mass tumbled.
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The step of block 52 involves shot blasting the workpiece
with stainless steel shot of a uniform size. In one embodiment,
the shot used is cold-worked 304 stainless steel shot, the
approximate uniform nominal size of which is selected to be
l ~
r5 either 0.0070, 0.0110, 0.0170, or 0.0280 inche~i.e., U.S.
standard sieve sizes No. 80, No. 50, No. 40, and No. 25,
respectively. More particularly, the shot originally consist of
cylindrically-shaped particles which become spherically-shaped
upon repeated use as a blasting medium. Stainless steel shot of
the type disclosed herein is commercially available from Pellets,
Inc. of Tonawanda, New York.
Although larger size shot is typically used on the cold-
worked materials, the fatigue endurance limit of the treated
stainless steel part has not been found to increase significantly
with increased shot size. ':Essentially, shot blasting with steel
shot increases the fatigue strength of the part by cold-working
the outer surface layer, thereby introducing residual compressive
stress on the surface.
As represented by the step of block 54, the workpiece may be
shot blasted with glass beads, i.e., silicon oxide, after it has
been shot blasted with stainless steel shot. The uniform nominal
size of the glass beads used in the disclosed embodiment is
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approximately 0.0029 inch~(U.S. standard sieve size No. 200)
and, thus, is in the range of one-tenth to one-half the size of
the steel shot previously described. Consequently, this step
further improves the fatigue properties of the part by working
the surface areas not covered by the larger steel shot.
Additionally, the glass bead blasting helps clean the surface of
any steel shot residue that may have been transferred to the
target surface. Glass beads of the type disclosed herein are
commercially available from Potters, Inc. of Cleveland, Ohio.
The steps of electropolishing and passivation, represented
b;y blocks 56 and 58, respectively, are performed in accordance
with conventional methods. Specifically, the electropolishing
step is performed using 85% phosphoric acid to one part (by
volume) of Electro-Glo 300 concentrate solution or equivalent,
and the passivation step is performed using a nitric acid
solution so as to provide a protective oxide film on the finished
part. The electropolishing step is an important step in
restoring the corrosion resistance properties of the surface and,
therefore, should be controlled in such a way that it produces
the desired results, i.e., cleaning and smoothening the surface
and reducing stress concentration effects, without dissolving
away the cold-worked surface layer produced during previous
steps.
The corrosion properties of stainless steel orthopedic
implant parts treated in accordance herewith are similar to that
of the base alloys in a highly polished and passivated condition.
Although shot blasting with steel shot deforms the surface and
produces sharp corners that would ordinarily decrease corrosion
resistance properties, glass bead blasting and electropolishing
mechanically and electrochemically smoothen the surface and
restore corrosion resistance properties thereto.
Referring now to Figs. 3 and 4, a femur 60 is shown with a
femoral head 62 and a fracture 64 located at a neck 66. Femoral
head 62 forms a hip joint with an acetabular member 68. An
opening 70 is formed in femur 60 to extend from a lateral side of
the femur to wn internal position within femoral head 62. In the
disclosed embodiment of the present invention, orthopedic implant
device 12 comprises a compression hip screw that has been treated
according to the process of Fig. 2. Compression hip screw 12 is
of the type disclosed in U.S. Patent No. 4,612,920, assigned to
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CA 02044758 2003-03-28
the same assignee as the present invention.
Generally, compression hip screw 12 includes a lag screw 72
threadedly secured to femoral head 62, a plate 74 adapted for
5 attachment to the femur by means of bone screws 76, and a
compression screw (not shown) for coupling lag screw 72 and plate
74. As shown in Fig. 3, an upper portion of plate 74 defines an
integral barrel 78 disposed in opening 70, while a lower portion
of the plate is attached to the lateral side of the femur.
10 Referring now to Fig. 4, a cross-sectional view of plate 74
of compression hip screw 12 illustrates the structure of a
stainless steel orthopedic implant device in accordance with the
present invention. Generally, shot blasting a stainless steel
part with stainless steel shot, as described herein, results in a
15 heavily cold-worked outer layer and a less heavily cold-worked
sublayer (or diffused layer) having a thickness at least twice
that of the outer layer. Specifically, Fig. 4 shows in
exaggerated fashion a textured outer surface 80, a heavily cold-
worked outer layer 82, a less heavily cold-worked sublayer~84,
20 and an inner core or substrate 86 substantially unaffected by the
surface treatment process described herein. In one embodiment,
layer 82 is approximately .0035 to .0040 inches thick.
In a preferred embodiment of carrying out the process of the
present invention, air pressure at 90-100 psi is used with the
25 shot blasting apparatus to propel the steel shot, while air
pressure at 40 psi is used for the glass bead blasting. All
blasting is executed perpendicularly to the specimen surface,
i.e., an angle of incidence of 90°, at a distance of 1.5 and 2.0
inches from the tip of the nozzle. Parts machined from
3o stainless steel are stress relieved prior to surface
finishing.
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The time duration for shot blasting with stainless steel
shot to achieve thorough coverage of the target area varies
according to several variables, including the type of shot and
target materials, the size, shape, hardness, and density of the
shot, and the velocity, flow rate, and angle of impact. However,
for one exemplary shot blasting set-up, the fatigue strength
increased rapidly over an elapsed time of approximately two
minutes until a maximum fatigue strength was achieved at an
optimal 100% surface coverage. Thereafter, the fatigue strength
declined gradually. Accordingly, it is recommended that the
duration be set at between 100-120% of the elapsed time
experimentally determined to achieve 100% surface coverage, in
order to minimize variances in fatigue strength for slightly
different timed periods.,,
While the preferred embodiment of the process of the present
invention has been described as including the step of shot
blasting with glass beads, it will be understood that this
particular step is optional. In other words, orthopedic implant
devices in accordance with the present invention may be processed
without glass bead blastingt however, glass bead blasting has
been found to smoothen the surface of the stainless steel parts
and further enhance corrosion and fatigue properties.
It will be appreciated that the fatigue strength of the
surface finish produced by the process of the present invention
is much more consistent than finishes of the prior art methods,
due to the uniform shape and higher mass of steel shot and its
ability to induce uniform cold-work on the surface of the alloy.
It is noted that greater improvement of fatigue strength is
typically experienced in the case of hot forged or annealed
stainless steel. Lesser improvement will be experienced in cold-
worked stainless steel, depending upon the alloy type and the
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degree of cold-working put into the alloy prior to being finished
in accordance with the present invention.
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.
fi.
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