Note: Descriptions are shown in the official language in which they were submitted.
il3SOO~;
BONE CONNECTIV~ PROST~ESES ADAPTED TO MAXIMIZE
STRENGTH AND DURABILITY OF PROSTHESES-BONE
CEMENT INTERFACE; AND METHODS OF FORMING SAME
Background of the Invention
Field of the Invention
The present invention relates to prostheses
adapted to be fixedly attached to bone by means of a
bone cement. Specifically, the present invention is
directed towards prostheses adapted to maximize the
strength and durability of the prostheses/bone cement
adherence.
Description of the Prior Art
A In the field of orthopedic surgery, ZIMALOY,
a chromium-cobalt-molybdenum alloy, stainless ~teel~
titanium alloys~ and polymerized materials such as
ultra high molecular weight polyethylene (hereinafter
UHMWPE) have been used successfully to replace the ends
of long bones and joints, including the hip joint.
However, there exists a severe limitation with respect
to such orthopedic surgery, namely, coupling of the
prostheses to bone. Due to such factors as mechanical
stress, fatigue, corrosion, etc~, the prostheses/bone
cement joints have been prone to failure.
Present methods of utilizing such bone pros-
theses involve the use of a prosthesis having a stem
portion which is inserted into the interior of a bone.
~ t~Jd? ,f,~, .k
500f~
A bone cement comprising a mixture of polymethylmeth-
acrylate (hereinafter PMMA~ polymer and methyl meth-
acrylate monomer and optionally including a styrene co
polymer of PMMA is likewise inserted into the bone cavity
and is utili ~ed to couple the stem of the implant to the
the bone itself. Experience has demonstrated, however,
that serious drawbacks exist with respect to the coupling
between the prosthesis stem and the bone cement. At-
tempted solutions to this problem have been directed
primari ly toward strengthening the prosthesi s/bone cement
interface by means of gross mechanical interlock involv-
ing, for example, dove tails, small stems, etc. Such
devices result in stress concentrations that can exceed
the strength of the bone cement as well as cause non-
physiological force distribution in the bone.
Adherence at the interface between the implant
and PMMA is greatly restricted by current industrial and
surgical practices~ For instance, the PMMA cement is
typically applied in a highly viscous doughy state with
the result that the degree of contact between the implant
and the cement is inadequate. Moreover, the existence of
weak boundary layers such as contaminants and weak metal
oxides on the surface of the implant have also caused
problems. Weak boundary layers may be due to the compo-
sition of the implant or to the process of forming the
same. Thus, in the case of a metal implant, the surface
of the implant normally includes weak metal oxides as
weak boundary layers. In the case of a polymeric im-
plant, the surace of the implant normally includes a
-- 2 --
li;~500~;
weak boundary layer comprising monomer, partially poly-
merized or low molecular weight polymer and contaminants
comprising mold release agents, etc. Finally, the im-
plant may come in contact with air, blood, water, etc.
prior to being inserted into the bone thereby becoming
contaminated. The existence of weak boundary layers,
e.g., surface contaminants, is detrimental to the for-
mation of good implant-bone cement adherence. Thus, the
strength of such joints has been dependent upon gross
mechanical interlock. Such difficulties in the formation
of a satisfactory prosthesis/ bone cement connection have
also caused the result that mere resurfacing of a deter-
iorated joint, e.g~, a deteriorated hip joint due to
arthritis, was not readily accomplished~ Thus, in the
case of a deteriorated articular surface, e.g., surface
of the head or ball in a ball and socket joint, the en-
tire head of the bone is generally removed and a pros-
thetic head connected to the bone by means of a stem
inserted into the interior of the bone, although in some
instances, resurfacing implants have been used with bone
cement.
Summary of the Invention
It has now been discovered that prosthesis
fixation problems may be overcome by treating at least
that portion of the prosthesis which is adapted to be
connected ~o bone in order to provide a PMMA film fixedly
adhered to said portions of the prosthesis. Prior to the
application of the PMMA film, the surface to be coated
0~;
is treated to eliminate any weak boundary layer existing
thereon. Thereafter, a PMMA film is applied by dipping,
painting spraying, etc., and finally, after the film
has dried, it is annealed to remove any stresses in the
film.
The resultant prosthesis has a film of P.~MA
firmly adhered to the surface thereof. This PMMA film
adhesively interacts molecularly with PMMA bone cementO
Accordingly, the adherence of a prosthPsis adhesively
connected to bone by means of a PMMA cement can be dras-
tically increased.
In accordance with one aspect of this invention
there is provided a prosthesis comprising a prosthetic
element and a polymethylmethacrylate film fixedly adhered
to at least a portion of the surface of said prosthetic
element; said prosthesis comprising a surface adapted
to be fixedly attached to bone by means of bone cement;
said prosthetic element bearing said polymethylmethacrylate
film at least upon said attachment surface; said polymethyl-
methacrylate film being adhered to said prosthetic elementby a process which comprises: treating said prosthetic
element surface to elimina~e any weak boundary layer;
applying polymethylmethacrylate to said treated sur~ace;
and thereafter annealing said polymethylmethacrylate
film.
In accordance with another aspect of this
invention there is provided a process for fixedly adhering
a polymethylmethacrylate fi:Lm to a prosthetic element
to provide an improved prosthesis adapted ~o be joined
to bone by means of bone cement which comprises:
treating said prosthetic element surface to
eliminate any weak boundary layer;
4 -
50(~
applying polymethylmethacrylate to said treated
surface;
and thereafter annealing said polymethylmetha-
crylate film.
In accordance with another aspect of this
invention there is provided a process comprising joining
a prosthesis to bone by applying bone cement to a
polymethylmethacrylate film fixedly adhered to a
prosthetic element, said prosthesis having been prepared
by the steps of:
treating said prosthetic element to eliminate
any weak boundary layer;
applying polymethylmethacrylate to said treated
surface to form a film thereon;
and thereafter annealing said polymethylmethacry-
late film.
_ief Description of the Drawings
In the drawings which form a part of the original
disclosure of the present invention:
Fig. 1 is an elevational side view in longitudi-
nal section of a PMMA coated hip prosthesis prepared in
accordance with the present invention.
Fig. 2 illustrates an enlarged fragmentary view
of a P~ ~ coated bone implantl as shown in Fig. 1, which
has been fixedly adhered to the interior of a bone by m~ans
of a PMMA bone cement.
Fig. 3 is a side elevational view of a human femur
having a deteriorated head surface.
Fig. 4 is a perspective view of a prosthesis
having a PMMA coating on the bone connective surface thereofO
The prosthesis may be utilized for resurfacing a deteriorated
head surface of a ball and socket joint
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113S006
thus obviating the need for the removal of the head por-
tion of the joint.
Fig. 5 illustrates an enlarged fragmentary
cross-section of a deteriorated femur head bearing the
resurfacing prosthesis of Fig. 4.
Detailed Description of the Invention ~
According to the present invention, prostheses
exhibiting marked fixation improvements have been dis-
covered. Such a prosthesis comprises a prosthetic element
having a PMMA film fixedly adhered to at least a portion
of the surface of the prosthetic element. The prosthesis
includes a surface adapted to be fixedly attached to bone
or a bone attachment surface. At least the bone attach-
ment portion of the surface, in accordance with the
present invention, is coated with a PMMA film prior to
attachment to bone. The PMMA coating or film is adhered
to the prosthetic element by a process which comprises
treating the element surface to eliminate any weak bound-
ary layer which may be present; applying PMMA to the
treated surface; and annealing the PMMA film.
Fig. 1 is an elevational side view in longitud-
inal section of a stem insertion hip joint prosthesis
having a bone attachment surface bearing a PMMA film.
Also shown is a resurfacing prostheses for the socket
portion of a hip joint, with the bone attachmènt surface
having a PMMA film fixedly adhered thereto in accordance
with the present invention. Thus, in Fig. 1 there is
shown a stem insertion prosthesis 1, comprising a rigi~
li350()6
prosthetic element 2, which may be composed of a metal
alloy or a polymer such as UHMWPE, bearing a thin, high
strength PMMA film 3. Also shown is a resurfacing pros-
thesis 4 for the socket portion of a ball and socket
joint, comprising a rigid prosthetic element 5 and a
PMMA film 6.
The rigid prosthetic element which is coated
in accordance with the present invention may be chosen
from any suitable material including metal alloys and
plastic. Thus, the element may be composed of a titanium
alloy, stainless steel, a cobalt-chromium or cobalt-
chromium-molybdenum alloy, MP-35 (protozol~ or a poly-
meric material such as ultrahigh molecular weight poly-
ethylene (UHMWPE).
In order to provide a high strength PMMA film
in accordance with the present invention, the prosthetic
element must first be prepared. Preparation involves re-
moval of any contaminants which may act as weak boundary
layers so that the coating may be joined directly to
the prosthetic element with no intervening materialO In
the case of a metal prosthetic element, the weak boundary
layer may comprise contaminants such as dirts and oils
and additionally typically includes weak metal oxides.
In the case of a polymeric prosthetic element, thP weak
boundary layer typically comprises contaminants such as
unreacted monomer, anti-oxidation agents and mold release
agents and additionally low molecular weight polymer.
In the case of a metal prosthetic element, re-
moval of the weak boundary layer may involve a degreasing --
~135006
step. Removal of the weak metal oxides is accomplishedby an acid treating step which may be followed by a de-
smutting and passivation step. However, any treatment
which functions effectively to remove contaminants and
weak metal oxides may be utilized.
The degreasing treatment may be carried out
through the utilization of an aqueous alkaline solution,
such as, for example~ an aqueous solution of sodium hy-
droxide. Thus, the prosthetic element to be degreased
may be immersed in a 1 N solution of sodium hydroxide
which has been heated to its boiling point, for 30 min-
utes to remove contaminants and grease. Another degreas-
ing treatment which may be utilized with less contam-
inated elements comprises exposing the prosthetic ele-
ment to trichloroethylene vapor. In order to determine
whether or not degreasing is complete, the ~water break
test" may be utilized according to which the degreased
prosthetic element is rinsed in distilled water. When
the element is removed from the water, if the water beads
up and runs off in less than 30 seconds, the surface is
not clean enough. There should be no break in the film
of water nor any tendency of the film to crawl or pucker.
Subsequent to the degreasing treatment, the
metallic prosthetic element should preferably be treated
with an acid etching treatment in order to remove weakly
bound metal oxides. Such treatment may comprise immers-
ing the element in a sulfuric acid/water admixture at an
elevated temperature of, for example, 60C. for a period
of approximately 1/2 hour. Other treatments which may
1135~)06
be utilized include immersing the prosthetic element in
a sulfuric acid/sodium dichromate aqueous solution or
treatment with other acid solutions.
It is preferred that the acid etching treatment
be discontinued prior to the accomplishment of any gross
surface changes. Thus, it is preferred that the surface
which is designed to be attached to bone~ be smooth.
This results in a more continuous stress concentration
about the prosthetic element/bone cement interface. How-
ever, where it is desired to use an implant having a
rough surface thus promoting a greater degree of mechan-
ical interlock, the coating of the present invention
may be utilized and a stronger joint will result.
In the ca~e of an alloy prosthetic element
which has ~een acid etched such as with the sulfuric
acid solution discussed above, completion of the etching
reaction will be evidenced by a reaction which turns
the surface of the element black. This is due to the
presence of carbon which is a component of metal alloys.
Such presence of carbon indicates that the surface has
been sufficiently etched. If no carbon appears, etching
is not complete. In order to avoid any gross surface
changes, the element should be removed from the etching
solution within ten seconds of the appearance of carbon.
The etched element may be checked by means of a Hobsyn
Tally Surface Profile or an SEM to insure that no gross
surface changes have occurred.
Thereafter, any carbon remaining on the surface
of the element may be removed by means of a desmutting
~i3SOO~
and passivation treatment. Such desmutting and passiva-
tion treatment may be carried out by means of a hydro-
fluoric acid/nitric acid aqueous admixture heated to
an elevated temperature of approximately 60~C. Other
strong oxidation reagents may be utilized if desired.
When the etched element is immersed in such a solution,
there should be a reaction within seconds evidenced by a
burst of bubbles as carbon is removed. This is followed
by another sudden burst of bubbles evidencing a secondary
reaction. At this point, the element should be removed
from the desmutting and passivation solution. This
treatment functions not only to remove carbon but addi-
tionally promotes the formation of a well adhered, uni-
form, high strength oxide surface~ and is a preferred
treatment stepr
The initial removal of weak boundary layers
may be carried out not only by chemical means, i.e.,
degreasing and acid etching, but mechanical means may
be utilized if desired. Thus, the prosthetic element
may be treated by blasting with alumina grit to provide
a virgin metal surface. Other mechanical treatments such
as grinding, honing, machining, etc., may also be uti-
lized.
Following mechanical treatment of the pros-
thetic element, the treated surface should immediately
be immersed in a passivation solution comprising, e.g.,
nitric and hydrofluoric acid, as above. It is preferred
that the passivation treatment be carried out within
a short time from the mechanical treatment. The lapse
1~135~()6
of time between mechanical treatment and passivation
should preferably be less than one minute.
Thereafter, the treated element should be
rinsed in water until the water has a neutral pH. The
~reated element should thereafter be dried by any suit-
able means such as by heating in an oven or by blowing
the surface dry with a warm air stream.
Once the element has dried, it is allowed to
cool to room temperature prior to application of the
PMMA film thereto. Care must be taken that the clean
surface not be contaminated during drying or cooling.
Coupling agents such as siloxane derivatives may be
applied before the coating.
Thereafter, the PMMA film is applied to the
prosthetic element. The film may be applied by means
of painting, spraying, dipping, powder coating, electro-
static coating, or in any other suitable manner in the
form of a lacquer, powder or emulsion. The method and
form utilized will depend on a number of various factors
including the desired coating thickness, strength, im-
plant geometry and surface roughness.
The film consists essentially of PMMA. How-
ever, other materials may be included in the film such
as cross-linking agents, free radical catalysts, acti-
vators, plasticizers, chain transfer agents, inhibitors,
plasticizing co-polymers 6 as well as adhesion promoters
in the form of co-polymers, such as of acrylic acid and
other freely orienting polar molecules.
-- 10 --
11~50()6
One preferred method of applying the film to
the prosthetic element comprises the application of a
PMMA lacquer to the element. Application may take the
form of dipping, spraying, etc. A PMMA lacquer is pre-
pared by dissolving PMMA high molecular weight beads in
a solvent such as dichloromethane. A small amount of
barium sulfate may be added to the lacquer in order to
keep the coated ~urface from crazing as well as making
the coating radio opaque. The concentration of polymer
in the solution should be in the range of 0.01 g. per
ml. to about 0.8 9. per ml., preferably from about 0.2
g. per mlg to about 0.4 g. per ml., most preferably from
about 0.25 g. per ml. to about 0.35 gO per ml.
The element is immersed in the lacquer for a
period of time sufficient to form a suitable coating on
the surface of the element. Such period of time may
range from about 5 seconds to about 60 minutes, prefer-
ably from about 15 minutes to about 60 minutes, most
preferably from about 25 to 35 minutes.
Another method of applying the film to the
proæthetic element comprises the application of PMMA
dissolved in methylmethacrylate (MMA), and additionally
containing a conventional curing catalyst. Still another
method of applying the PMMA coating is to coat the pros-
thetic element with MMA and catalyst. In cases where
MMA is used, a high temperature curing step follows the
coating to polymerize the MMA to PMMA. This may consti-
tute the annealing step discussed hereinafter.
1135006
Upon completion of the application of PMMA to
the element, the PMMA film should be annealed by exposing
the coated element to a temperature above that of the
glass transition temperature of PMMA, i.e., 70-90~C,
preferably 80Co The curing or annealing treatment is
necessary to insure complete pvlymerization and removal
of any volatile components from the film. High pressures~
i.e., greater than 100 psi may be applied to inhibit
bubble formation. Moreover, by heating the film to a
temperature above the glass transition temperature of
PMMA, any mechanical stresses in the film developed
during the drying thereof will be eliminated.
The rate at which the coated element is cooled
following the annealing treatment is preferably carefully
controlled to insure that it does not exceed about 1.5C.
per minute until the coated element reaches a temperature
of about 80Co This insures that only minimal stresses
are formecl in the film during cooling. If desired, the
film may be crosslinked by chemical and/or radiation
techniques.
The thickness of the film thus produced is not
of critical importance; however, the preferred minimum
thickness of the film should be about 0.0001 inch, more
preferably about 0.001 inch, most preferably about 0.002
inch.
Upon completion of the annealing or ~uring of
the PMMA film, the coated prosthetic element is ready
for use as a prosthe~is. If the prosthesis is a bone
implant prosthesis, the interior of the bone is removed
so()~;
and cleaned and a PMMA bone cement is applied to the
interior of the bone . Thereafter, the implant portion
of the prostheses, coated in accordance with the present
invention is inserted into the interior of the bone.
If desired, the coating may be softened with a solvent
such as MMA monomer prior to insertion into the bone.
This causes the PMMA film to swell and so~ten, thus
allowing for greater mechanical and chemical interaction
between the coating and bone cement.
In Fig. 2, an enlarged fragmentary view of a
coated prostheses which has been fixedly adhered to bone
by means of a PMMA bone cement is illustrated. Pros-
thetic element 2 is connected to bone cement 7 via the
PMMA film 3. Bone 8 is shown to be adhered to the bone
cement. The interface 9 between the PMMA film and the
element is shown to be free of defects and any weak
boundary layer due to the precoating treatment of the
element. The interface between the PMMA coating and bone
cement 10, represents both a chemical and mechanical
adherence. Flaws 11 in the bone cement 7 may be dis-
placed away from the interface 9 due to the interaction
of the film and cement.
Where a prosthetic element comprising a poly-
mer, especially UHM~E is to be utilized, a somewhat dif-
ferent weak boundary layer removal treatment is utilized.
Such treatment comprises either an oxidation treatment or
a treatment referred to by the acronym "casing" (cross-
linking by activated species of inert gases)O The oxida-
tion treatment may be performed by corona discharge,
~i3500~
flame treatment or an acid treatment, such as chromic
acid, etc. The oxidation treatment accomplishes the re-
moval of contaminants and low molecular weight polymers,
i.e., polyethylene.
It is preferred that the polymeric element
be degreased prior to the oxidation or casing treatment.
Such degreasing treatment is usually readily accomplished
by immersing the polymeric element in trichloroethylene
liquid for several seconds.
As previously mentioned, in lieu of surface
oxidation treatment, the polymeric element may be treated
by "casingn. This process consists of allowing electron-
ically excited species of rare gasses to impinge upon
the surface of the polymer~ As these metastable and
ionic gasses come in contact with polyethylene, for ex-
ample, they cause abstraction of hydrogen atoms and for-
mation of polymer radicals at and near the surface of
the polymer. The radicals formed by this process inter-
act to form crosslinks and unsaturated groups without
appreciable scission of the polymer chain. The mechan-
ical strength of the surface region is increased remark-
ably by the formation of a gel matrix. Thus, a weak
boundary layer is transformed into a strong boundary
layer. Wettability of the surface is relatively un-
affected. Contact of the activated gas with the polymer
surface for a time as short as one second will remarkably
improve adhesive joint strength. Longer exposure times
may be necessary when utilizing a more inert polymeric
element such as, for instance, polytetrafluoroethylene.
- 14 -
11350()6
It is believed that substitution of casing for the oxida-
tion treatment results in a more well adhered PMMA film.
Upon completion of the oxidation and/or casing
treatment of the polymer surface, a P~MA coating is
applied to the polymeric element in the same manner as
previously discussed with respect to the metal element.
The PMMA film should thereafter be annealed or cured
as with the coated alloy element. Annealing at a tem-
perature of about 100C. is sufficient with a UHMWPE
element.
When a PMMA coating is applied to a prosthetic
element in accordance with the present invention, the
resulting prosthesis may be joined to bone cement and
will exhibit markedly superior adherence to the same when
compared to the adherence of bone cement to an uncoated
prosthesis or when compared to the adherence of a coated
prosthesis wherein the element pre-coating treatment
and/or where the annealing treatment is not utilized.
The effect of the improved adherence between
the prosthesis and bone cement results not only in im-
proved adhesion of bone implant prosthesis to the in-
terior of a bone but moreover may eliminate in some cases
the need for using an implant stem. Thus, stems have
been used in prosthetics to implant a steel object
securely into the bone. However~ typically, the reason
for the implant is a surface deterioration of the joint,
for example, due to arthritis, but a mere resurfacing
was not easily accomplished because of the fixation
problems.
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il350(~6
Recent advances in the application of cement
to bone involves the pressurization of cement deep into
the pores of bones creating intimate interlock. The
only problem remaining has been the attachment of a metal
surface to the cement wi~hout a stem. Such problem may
be solved utilizing the P~MA coated prostheses of the
present invention. Thus, i n Fig~ 3 there is shown a
human femur 21 in elevational side view, with the head
thereof having a deteriorated surface 22, due to, for
example, arthritis. In the past, it would have been
preferred to remove the entire head of the femur and
to substitute a prosthetlc head connected to the femur
by means of a stem insertionO However, by utilizing
the present invention, an exterior surface may be fixedly
attached to the deteriorated surface. The new prosthetic
surface shown in Fig. 4 comprises a prosthetic element
23 which may be an alloy, and an inner surface 24 com-
prising PMMA. It can be strongly adhered to the head
of the femur when prepared according to the process of
the present: invention, as shown in Fig. 5.
In Fig. 5 there is illustrated an enlargement
of a fragmentary cross-section of a femur head after
resurfacing. Bone cement 25 is shown extending deep
into the surface of the bone 26. The bone cement is
connected to the PMMA film 24; there being such forces
as molecular bonding at the PMMA/PMMA-bone cement inter-
face 27. The prosthetic element 23 i 5 thus connected
by means of the PMMA film ar.d the bone cement to the
resurfaced bone. Resurfacing prostheses of the present
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~i;3S006
invention may be used not only for deteriorated ball
and sock~t joints, but may be used in general on any
deteriorated articular surface including, for example,
a deteriorated knee.
The following examples serve to illustrate
the formation of the prosthetic devices of the present
invention and the improvement in results obtained by
the use thereof.
Example 1
Preparation of a PMMA Lac~uer - Thirty grams
of high molecular weight PMMA beads (manufactured by
Fisher Chemical) are dissolved in 100 ml of dichloro-
methane with stirring. When all of the beads have dis-
solved, one gram of barium sulfate is added. The barium
sulfate does not dissolve in the solution but rather,
a portion becomes suspended in the solution rendering
the solution opaque. The solution is allowed to stand
for one-half hour in order to allow the barium sulfate
to settle out. At the end of one-half hour the solution
containing suspended barium sulfate is decanted from
the barium sulfate sediment.
Pre aration of a Cobalt-Chromium Stem Insert
P
Prosthetic Element - The surface of a dry grit blasted
-
cobalt-chromium alloy is to be coated. The element is
first degreased by immersing it in 1 N solution of sodium
hydroxide which has been heated to its boiling point.
The alloy element is allowed to remain in the alkaline
- 17 -
~35l)0~
bath for 30 minutes. Thereafter, the element is removed
from the bath and rinsed with warm water until the water
tests neutral.
The thus treated element is thereafter acid
etched by means of a 50% by volume aqueous sulfuric acid
solution. The element is immersed in the solution at
a temperature of 60C. After about one-half hour a reac-
tion which turns the surface black occurs. The element
is removed from the acid solution and is rinsed in dis
tilled water.
Following acid etching, the element is treated
in a desmutting solution comprising an admixture of 15%
by volume of a 52~ hydrofluoric acid solution; 45% by
volume nitric acid and 40% by volume water. The treated
element is immersed in this solution at a temperature
of from about 50 to about 60C. After ten seconds, there
is a burst of bubbles indicating the removal of carbon.
This is followed by another sudden burst of bubbles indi-
cating a secondary reaction. At this point the element
is removed from the desmutting solution. Immediately
upon removal the element is rinsed with distilled water
until the water is neutral. The element is dried in
an oven at a temperature of 110C. for approximately
5 to 10 minutes. The element is removed from the oven
and allowed to cool to room temperaturer
Applicatisn of the Coating
The treated alloy element is immersed in the
PMMA lacquer prepared earlier. After 30 minutes the
~ 18 -
~3S~ 6
element is removed from the lacquer and placed into an
oven at 60~C. The element is left in the oven for two
hours.
Annealing Procedure
The coated element is placed in an oven at
160C. for 18 hours. The element is removed from the
oven and allowed to cool at a rate of 1 1/2C. per minute
until the temperature is 70C. Thereafter, the element
is cooled in open air. It is important that the coated
element not be cooled too quickly due to the differences
in rate of thermal contraction between the coating and
the substrate.
The PMMA film produced thereby has a thickness
of between 0.001 and 0.0015 inch.
Example 2
Preparation of a Polyethylene Element - A
UHMWPE prosthetic element is to be coated. The element
is passed over a butane-oxygen flame until all parts of
the surface have been exposed to the flame. Thereafter~
the treated element is exposed to activated species of
an argon gas which have been generated by means of a
high voltage discharge electrode in a vacuum chamber.
The treated polyethylene element is thereafter allowed
to cool to room temperature and is then coated with the
PMMA lacquer as described in Example 1. The coated poly
ethylene element is dried in an oven at a temperature
of 60C~ for two hours. Thereafter, the coating is an-
nealed by heating the coated polyethylene element in
.. .. .
~i350~6
an oven at 100C. for two hours. The substrate is slowly
allowed to cool as described in Example 1.
Example 3
A stem insert sample is prepared and coated as
described in Example 1. This sample and an uncoated stem
insert sample are bonded to bone cement under a~bient
conditions. The interface shear strength as measured
by a cylindrical lap shear strength test for each element
measured in mega Pascals and pounds/sq. in. is determined
to be as follows:
Uncoated Pretreated, Coated
and Annealed
MPa (Standard Deviation) psi MPa
2.1 ~.5) 305 15.2 (.7) 22~0
The cylindrical lap shear strength test (re-
ferred to above and in the following examples) is carried
out as follows: A cylindrical sample to be tested is
inserted into an annular mold containing PMMA/MMA bone
cement and the cement is allowed to harden. The mold
is removed to provide a cylindrical sample having an
annular ring of bone cement bonded to a portion of the
cylindrical surface. The sample is placed in a gripping
device wherein it is gripped by means of the annular
ring of bone cement. Force is applied axially to an
end of the cylinder and the amount of force necessary
to break the bond between the cylindrical sample and
the annular ring of bone cement is recorded.
- 20-
1~350~i
Example 4
An alloy stem insert sample is prepared by de-
greasing it, coating it with PMMA and annealing it, all
as described in Example 1. For purposes of comparison,
another prosthetic element is prepared in the same way,
except that instead of annealing treatment, the PMMA
coating is cured at ambient temperature and is thereafter
bonded to bone cement under ambient conditionsO The in-
terface shear strengths as measured by a cylindrical lap
shear strength test for the two elements are determined
to be as follows:
Ambient CuredAnnealed
MPa psi MPa psi
3-9 (~9) 4796.2 (.7) 900
Example 5
PMMA coa~ed stem insert samples are prepared
as in Example 1. One is joined to bone cement under am-
bient conditions while another is exposed to intermedul-
lary contents, wiped with a saline solution and there-
after joined to bone cement. The interface shear
strengths of each are measured. There is no statisti-
cally significant difference in values.
The above tests are performed on uncoated stem
inserts. The interface shear strength of the uncoated,
uncontaminated sample is significantly less than the
coated sample and moreover, the interface shear strength
of the contaminated sample shows a 50~ reduction in value
with respect to the uncoated, uncontaminated sample.
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1~51306
Thus~ it can be seen that prostheses prepared
in accordance with the present invention are unaffected
by contamination while untreated prior art prostheses
are greatly influenced by contamination.
Example 6
A UHMh~E sample is flame treated, coated with
PMMA, dried and annealed as described in Example 2.
This sample and an unoated UHMWPE sample are bonded to
bone cement under ambient conditions. The block shear
strength for each sample is determined to be as follows:
Uncoated Coated
MPa MPa psi
Less than .1 5~1 (.5~ 740
The "block shear strengths" referred to above
are measured as follows. A cubical or block sample having
a flat surface to be tested which is treated or untreated
as desired, is provided. A cube or block of bone cement
is formed on the surface to be tested, by means of, e.g.,
a mold. After hardening of the bone cement the sample
is gripped by the cube of bone cement. Force is applied
to the sample and the amount of force required to break
the cement bond between the sample and the cube of bone
cement is recorded.
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