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Description  |
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FIELD OF THE INVENTION
The present invention relates to the orthopedic field and the provision of
prostheses, such as hip and knee implants, as well as methods of
manufacture of such devices and material used therein.
BACKGROUND OF THE INVENTION
The use of synthetic polymers, e.g., ultra high molecular weight
polyethylene, with metallic alloys has revolutionized the field of
prosthetic implants, e.g., their use in total joint replacements for the
hip or knee. Wear of the synthetic polymer against the metal of the
articulation, however, can result in severe adverse effects which
predominantly manifest after several years. Various studies have concluded
that such wear can lead to the liberation of ultrafine particles of
polyethylene into the periprosthetic tissues. It has been suggested that
the abrasion stretches the chain folded crystallites to form anisotropic
fibrillar structures at the articulating surface. The stretched-out
fibrils can then rupture, leading to production of submicron sized
particles. In response to the progressive ingress of these polyethylene
particles between the prosthesis and bone, macrophage-induced resorption
of the periprosthetic bone is initiated. The macrophage, often being
unable to digest these polyethylene particles, synthesize and release
large numbers of cytokines and growth factors which can ultimately result
in bone resorption by osteoclasts and monocytes. This osteolysis can
contribute to mechanical loosening of the prosthesis components, thereby
sometimes requiring revision surgery with its concomitant problems.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an implantable prosthesis
device formed at least in part of melt-irradiated ultra high molecular
weight polyethylene (UHMWPE) which results in reduced production of fine
particles from the prosthesis during wear of the prosthesis.
It is another object of the invention to reduce osteolysis and inflammatory
reactions resulting from prosthesis implants.
It is yet another object of the invention to provide a prosthesis which can
remain implanted within a person for prolonged periods of time.
It is yet another object of the invention to provide improved UHMWPE which
can be used in the prostheses of the preceding objects.
Still another object of the invention is to provide improved UHMWPE which
has a highly entangled and low crystalline polymeric structure.
A still further object of the invention is to provide improved UHMWPE which
is relatively transparent and wear resistant.
According to the invention, a medical prosthesis for use within the body,
e.g., for hip or knee joint replacement, is provided. It is formed at
least in part of an UHMWPE which has a polymeric structure with less than
about 50% crystallinity, less than about 290 .ANG. lamellar thickness and
less than about 940 MPa tensile elastic modulus, so as to reduce
production of fine particles from the prosthesis during wear of the
prosthesis. In one embodiment the UHMWPE has a polymeric structure with
about 40% crystallinity, about 100 .ANG. lamellar thickness and about 200
MPa tensile elastic modulus. Preferably, the UHMWPE has substantially no
trapped free radicals, has reduced hardness, is substantially not
oxidized, has substantially no chain scission, has extensive cross-linking
so that a substantial portion of the polymeric structure does not dissolve
in Decalin.TM. (decahydronaphthalene) at 150.degree. C. over a period of
24 hours, and/or has a high density of entanglement so as to cause the
formation of imperfect crystals and reduce crystallinity. The UHMWPE
initially has a molecular weight greater than about 500,000, preferably
greater than about 1,000,000 and more preferably greater than about
2,000,000. In certain embodiments, part of the prosthesis is in the form
of a cup shaped article having a load bearing surface made of this UHMWPE.
This load bearing surface can be in contact with a second part of the
prosthesis having a mating load bearing surface of a metallic or ceramic
material.
Another aspect of the invention is UHMWPE having a unique polymeric
structure characterized by less than about 50% crystallinity, less than
about 290 .ANG. lamellar thickness and less than about 940 MPa tensile
elastic modulus. The UHMWPE can also have high transmissivity of light.
Yet other aspects of the invention are fabricated articles, e.g., with a
load bearing surface, and transparent and wear resistant coatings, made
from such UHMWPE. One embodiment is where the fabricated article is in the
form of a bar stock which is capable of being shaped into articles by
conventional methods, e.g., machining.
Yet another aspect of the invention includes a method for making highly
entangled and crosslinked UHMWPE. Conventional UHMWPE, in the form of,
e.g., a powder, a bar stock, a shaped bar stock, a coating or a fabricated
article, e.g., a cup shaped article for use in a prosthesis, is provided.
Preferably, the UHMWPE starting material has an average molecular weight
of greater than about 2 million. The UHMWPE is surrounded with an inert
material that is substantially free of oxygen. The UHMWPE is heated at or
above its melting temperature, preferably, at about 145.degree. C. to
about 230.degree. C., more preferably at about 175.degree. C. to about
200.degree. C., for a time sufficient to allow the polymer chains to
achieve an entangled state, e.g., for a period of about 5 minutes to about
3 hours. The heated UHMWPE is then crosslinked so as to trap the polymer
chains in the entangled state using irradiation, e.g., with gamma
irradiation or electron irradiation, preferably allowing a dose greater
than about 1 MRad, more preferably a dose greater than about 20 MRad. The
irradiated UHMWPE is then cooled to about 25.degree. C., preferably at a
rate equal to or greater than about 0.5.degree. C./min., more preferably
at a rate equal to or greater than about 120.degree. C./min. In certain
embodiments, the cooled UHMWPE is then machined or compression molded.
Yet another aspect of the invention includes the product made in accordance
with this method.
The invention also features a method of making a prosthesis from UHMWPE so
as to reduce production of fine particles from the prosthesis during wear
of the prosthesis. UHMWPE having a polymeric structure with less than
about 50% crystallinity, less than about 290 .ANG. lamellar thickness and
less than about 940 MPa tensile elastic modulus is provided. A prosthesis
is formed from this UHMWPE, the UHMWPE forming a load bearing surface of
the prosthesis.
Yet another aspect of the invention includes a method of treating a body in
need of a prosthesis. A shaped prosthesis formed of ultra high molecular
weight polyethylene having a polymeric structure with less than about 50%
crystallinity, less than about 290 .ANG. lamellar thickness and less than
about 940 MPa tensile elastic modulus, is provided. This prosthesis is
applied to the body in need of the prosthesis.
The above and other objects, features and advantages of the present
invention will be better understood from the following specification when
read in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view through the center of a medical hip joint
prosthesis in accordance with a preferred embodiment of this invention;
FIG. 2 is a side view of an acetabular cup liner as shown in FIG. 1;
FIG. 3 is a cross-sectional view through line 3--3 of FIG. 2;
FIG. 4 is a graph showing the crystallinity and melting point of
melt-irradiated UHMWPE at different irradiation doses;
FIG. 5 is an environmental scanning electron micrograph of an etched
surface of conventional UHMWPE showing its crystalline structure;
FIG. 6 is an environmental scanning electron micrograph of an etched
surface of melt-irradiated UHMWPE showing its crystalline structure at
approximately the same magnification as in FIG. 5; and
FIG. 7 is a graph showing the crystallinity and melting point at different
depths of a melt-irradiated UHMWPE cup.
DETAILED DESCRIPTION
This invention provides a medical prosthesis for use within the body which
is formed of ultra high molecular weight polyethylene (UHMWPE) which has a
polymeric structure with less than about 50% crystallinity, less than
about 290 .ANG. lamellar thickness and less than about 940 MPa tensile
elastic modulus, so as to reduce production of fine particles from the
prosthesis during wear of the prosthesis.
A medical prosthesis in the form of a hip joint prosthesis is generally
illustrated at 10 in FIG. 1. The prosthesis shown has a conventional ball
head 14 connected by a neck portion to a stem 15 which is mounted by
conventional cement 17 to the femur 16. The ball head can be of
conventional design and formed of stainless steel or other alloys as known
in the art. The radius of the ball head closely conforms to the inner cup
radius of an acetabular cup liner 12 which can be mounted in cement 13
directly to the pelvis 11. Alternatively, a metallic acetabular cup can be
cemented to the pelvis and the cup liner 12 can form a coating or liner
connected to the cup by cement or other means as known in the art.
The specific form of the prosthesis can vary greatly as known in the art.
Many hip joint constructions are known and other prostheses such as knee
joints are known. All such prior art prostheses can be benefited by making
at least one load bearing surface of such prosthesis of a high molecular
weight polyethylene material in accordance with this invention. Such load
bearing surfaces can be in the form of layers, linings or actual whole
devices as shown in FIG. 1. In all cases, it is preferred that the load
bearing surface act in conjunction with a metallic or ceramic mating
member of the prosthesis so that a sliding surface is formed therebetween.
Such sliding surfaces are subject to breakdown of the polyethylene as
known in the prior art. Such breakdown can be greatly diminished by use of
the materials of the present invention.
FIG. 2 shows the cup liner in the form of a half hollow ball-shaped device
better seen in the cross-section of FIG. 3. As previously described, the
outer surface 20 of the cup liner need not be circular or hemispherical
but can be square or of any configuration to be adhered directly to the
pelvis or to the pelvis through a metallic cup as known in the art. The
radius of the cup liner shown at 21 in FIG. 3 of the preferred embodiment
ranges from about 20 mm to about 35 mm. The thickness of the cup liner
from its generally hemispherical hollow portion to the outer surface 20 is
preferably about 8 mm. The outer radius is preferably in the order of
about 20 mm to about 35 mm.
In some cases, the ball joint can be made of the UHMWPE of this invention
and the cup formed of metal, although it is preferred to make the cup or
cup liner of UHMWPE to mate with the metallic ball. The particular method
of attachment of the components of the prosthesis to the bones of the body
can vary greatly as known in the art.
The medical prosthesis includes, e.g., orthopedic joint and bone
replacement parts, e.g., hip, knee, elbow or ankle replacements. The
prosthesis can be in the form of a cup shaped article which has a load
bearing surface. Medical prostheses are also meant to include any wearing
surface of a prosthesis, e.g., a coating on a surface of a prosthesis in
which the prosthesis is made from a material other than the UHMWPE of this
invention.
The prostheses of this invention are useful for contact with metal
containing parts formed of, e.g., stainless steel, titanium alloy or
nickel cobalt alloy, or with ceramic containing parts. For example, a hip
joint is constructed in which a cup shaped article having an inner radius
of 25 mm, is contacted with a metal ball having an outer radius of 25 mm,
so as to closely mate with the cup shaped article. The load bearing
surface of the cup shaped article of this example is made from the UHMWPE
of this invention, preferably having a thickness of at least about 1/4
inch and more preferably having a thickness of about 1/3 inch or more.
The prostheses can have any standard known form, shape, or configuration,
or be a custom design, but have at least one load bearing surface of
UHMWPE of this invention.
The prostheses of this invention are non-toxic to humans. They are not
subject to deterioration by normal body constituents, e.g., blood or
interstitial fluids. They are capable of being sterilized by standard
means, including, e.g., heat or irradiation.
By UHMWPE is meant linear non-branched chains of ethylene that have
molecular weights in excess of about 500,000, preferably above about
1,000,000, and more preferably above about 2,000,000. Often the molecular
weights can reach about 8,000,000. By initial average molecular weight is
meant the average molecular weight of the UHMWPE starting material, prior
to any irradiation.
The UHMWPE of this invention has a polymeric structure with less than about
50% crystallinity, preferably less than about 40% crystallinity, more
preferably less than about 35% crystallinity, and most preferably less
than about 30% crystallinity. By crystallinity is meant the fraction of
the polymer that is crystalline. The crystallinity is calculated by
knowing the weight of the sample (w, in g), the heat absorbed by the
sample in melting (E, in cal) and the heat of melting of polyethylene
crystals (.DELTA.H=69.2 cal/g), and using the following equation:
##EQU1##
The UHMWPE of this invention has a polymeric structure with less than about
290 .ANG. lamellar thickness, preferably less than about 200 .ANG.
lamellar thickness, and most preferably less than about 100 .ANG. lamellar
thickness. By lamellar thickness (1) is meant the calculated thickness of
assumed lamellar structures in the polymer using the following expression:
##EQU2##
where, .sigma..sub.e is the end free surface energy of polyethylene
(2.22.times.10.sup.-6 cal/cm.sup.2), .DELTA.H is the heat of melting of
polyethylene crystals (69.2 cal/g), .rho. is the density of the
crystalline regions (1.005 g/cm.sup.3), T.sub.m.sup.0 is the melting point
of a perfect polyethylene crystal (418.15 K) and T.sub.m is the
experimentally determined melting point of the sample.
The UHMWPE of this invention has less than about 940 MPa tensile elastic
modulus, preferably less than about 600 MPa tensile elastic modulus, more
preferably less than about 400 MPa tensile elastic modulus, and most
preferably less than about 200 MPa tensile elastic modulus. By tensile
elastic modulus is meant the ratio of the nominal stress to corresponding
strain for strains less than 0.5% as determined using the standard test
ASTM 638 M III.
In one embodiment, the UHMWPE of this invention has a polymeric structure
with about 40% crystallinity, about 100 .ANG. lamellar thickness and about
200 MPa tensile elastic modulus.
It is preferred that the UHMWPE have no trapped free radicals, especially
unsaturated trans-vinylene free radicals. It is preferred that the UHMWPE
have a hardness less than about 65 on the Shore D scale, more preferably a
hardness less than about 55 on the Shore D scale, most preferably a
hardness less than about 50 on the Shore D scale. By hardness is meant the
instantaneous indentation hardness measured on the Shore D scale using a
durometer described in ASTM D2240. It is preferred that the UHMWPE be
substantially not oxidized and/or have substantially no chain scission. By
substantially not oxidized is meant that the ratio of the area under the
carbonyl peak at 1740 cm.sup.-1 in the FTIR spectra to the area under the
peak at 1460 cm.sup.-1 in the FTIR spectra of the melt-irradiated sample
be of the same order of magnitude as the ratio for the sample before melt
irradiation. By substantially no chain scission is meant that the fraction
of the polymer sample dissolving in normal xylene at 130.degree. C. and
remaining in solution after cooling to room temperature be less than or
equal to the fraction of the conventional UHMWPE dissolving in normal
xylene at 130.degree. C. and remaining in solution after cooling to room
temperature within the margins of experimental error. In some embodiments,
the polymeric structure has extensive cross-linking such that a
substantial portion of the polymeric structure does not dissolve in
Decalin.TM. (decahydronaphthalene). By substantial portion is meant at
least 50% of the polymer sample's dry weight. By not dissolve in
Decalin.TM. is meant does not dissolve in Decalin.TM. at 150.degree. C.
over a period of 24 hours. Preferably, the UHMWPE has a high density of
entanglement so as to cause the formation of imperfect crystals and reduce
crystallinity. By the density of entanglement is meant the number of
points of entanglement of polymer chains in a unit volume; a higher
density of entanglement being indicated by the polymer sample's inability
to crystallize to the same extent as conventional UHMWPE, thus leading to
a lesser degree of crystallinity.
The polymeric structure of the UHMWPE used in the prostheses of this
invention results in the reduction of production of fine particles from
the prosthesis during wear of the prosthesis. As a result of the limited
number of fine polyethylene particles being shed into the body, the
prosthesis exhibits longer implant life. Preferably, the prosthesis can
remain implanted in the body for at least 10 years, more preferably for at
least 20 years and most preferably for the entire lifetime of the patient.
The invention also includes other fabricated articles made from UHMWPE
having a polymeric structure with less than about 50% crystallinity, less
than about 290 .ANG. lamellar thickness and less than about 940 MPa
tensile elastic modulus. Such articles include shaped articles and
unshaped articles, including, e.g., machined or molded objects, e.g.,
cups, gears, nuts, sled runners, bolts, fasteners and the like, and bar
stock, films, cylindrical bars, sheeting, panels, and fibers. Shaped
articles can be made, e.g., by machining. The fabricated articles are
particularly suitable for load bearing applications, e.g., as a load
bearing surface, and as metal replacement articles. Thin films or sheets
of UHMWPE, which have been melt-irradiated can also be attached, e.g.,
with glue, onto supporting surfaces, and thus used as a transparent, wear
resistant load bearing surface.
The invention also includes UHMWPE which has a unique polymeric structure
characterized by less than about 50% crystallinity, less than about 290
.ANG. lamellar thickness and less than about 940 MPa tensile elastic
modulus. Depending upon the particular processing used to make the UHMWPE,
certain impurities may be present in the UHMWPE of this invention,
including, e.g., calcium stearate, mold release agents, extenders,
anti-oxidants and/or other conventional additives to polyethylene
polymers. In certain embodiments, the UHMWPE has high transmissivity of
light, preferably a transmission greater than about 10% of light at 517 nm
through a 1 mm thick sample, more preferably a transmission greater than
about 30% of light at 517 nm through a 1 mm thick sample, and most
preferably a transmission greater than about 40% of light at 517 nm
through a 1 mm thick sample. Such UHMWPE is particularly useful for thin
films or sheets which can be attached onto supporting surfaces of various
articles, the film or sheet being transparent and wear resistant.
This invention also provides a method for making UHMWPE which is highly
entangled and crosslinked. Preferably, this UHMWPE has a polymeric
structure with less than about 50% crystallinity, less than about 290
.ANG. lamellar thickness and less than about 940 MPa tensile elastic
modulus. Conventional UHMWPE, e.g., a powder, a bar stock, a shaped bar
stock, a coating, or a fabricated article is provided. By conventional
UHMWPE is meant commercially available high density (linear) polyethylene
of molecular weights greater than about 500,000. Preferably, the UHMWPE
starting material has an average molecular weight of greater than about 2
million. By initial average molecular weight is meant the average
molecular weight of the UHMWPE starting material, prior to any
irradiation. This UHMWPE is surrounded with an inert material that is
substantially free of oxygen, e.g., nitrogen, argon or helium. The UHMWPE
is heated above its melting temperature for a time sufficient to allow the
UHMWPE chains to take up an entangled state. Preferably, the temperature
is about 145.degree. C. to about 230.degree. C., and more preferably, is
about 175.degree. to about 200.degree. C. The period of time of heating
should be sufficient to permit the UHMWPE polymer chains to take up a
random coil conformation. Preferably, the heating is maintained so to keep
the polymer at the preferred temperature for about 5 minutes to about 3
hours, and more preferably for about 30 minutes to about 2 hours. The
UHMWPE is then irradiated so as to trap the polymer chains in the
entangled state, e.g., with gamma irradiation or electron irradiation. In
general, gamma irradiation gives a high penetration depth but takes a
longer time, resulting in increased cost of operation and the possibility
of some oxidation. In general, electron irradiation gives more limited
penetration depths but takes a shorter time, and hence the cost and the
possibility of oxidation is reduced. The irradiation dose can be varied to
control the d | | |
