WEAR-RESISTANT ARTIFICIAL JOINT
The present invention relates to orthopedic prostheses utilized in joint replacements, and more particularly to prosthetic joint replacements wherein there is moving contact
between mating (or bearing) prosthetic surfaces.
BACKGROUND OF THE INVENTION
The bones of a human (or animal) skeleton provide rigid, structural reinforcement for the body. Despite this rigidity, movement of the body (walking, chewing, etc.) is still
permitted by the fact that bones are attached to each other at movable joints, where two or
more separate bones move relative to each other.
One common joint geometry is the hinge joint, such as the hinge joint of the human
elbow or knee. A pure hinge joint allows relative rotation, such that both bones maintain a constant, mutual planar orientation. Another common joint geometry is the ball-and-socket
joint, such as the human shoulder joint. A ball-and-socket joint allows rotation in more than
one plane. Then there are more complex joint geometries, which can facilitate complex
combinations of relative translational and rotational movement between adjacent bones.
Examples of these more complex geometries can be found in the human jaw and the human hip.
In all of these types of joints, bones come into direct, physical contact with each other.
At the joint, the bones move relative to each other, exhibiting rotational contact, rolling
contact, translational contact, or some combination of these. Of course, this moving contact
causes wear on the joints themselves.
However, the portions of bones that come into mutual contact at joints are generally
coated with cartilaginous material (or cartilage) The natural cartilage has a combination of
material properties, such as hardness, coefficients of friction, elasticity and compressive strength, so that the cartilage wear due to moving contact at the joint is relatively slow, and
the cartilage at the joint typically outlasts the life of the human or animal to whom the joint
belongs In this case, the wear at the joint is not considered very problematic
Nevertheless, some people live long enough that their joint cartilage deteriorates
and/or wears out More particularly, the cartilage can wear or deteriorate down to the bone material portion of the joint When this happens, catastrophic or semi-catastrophic failure of
the bone can occur due to the lack of capacity that bone matter has with respect to moving
contact and wear This wear problem is especially likely to occur if the joint (1) had less
cartilage than normal to begin with, (2) is subject to a disease that affects cartilage, (3) is
subject to an accident or other trauma affecting the cartilage, (4) is exercised more often than
average, and/or (5) is routinely placed under greater-than-average stress The problem of
cartilage wear is especially prevalent at the knee joint and hip joint, in part because the weight
of the human body bears on this joint as it moves during walking, running and even standing, thereby exacerbating wear
In an effort to restore functions of a deteriorated or damaged joint, surgeons have
devised numerous methods of partial and total joint replacements In these joint replacements,
portions of the bone at the existing joint are either removed or supplemented with prosthetic
components designed to restore some or all of the original functionality of the joint Today,
hip and knee joint replacements are common Increasingly, shoulder joints and other joints in
the body are being replaced as well
Existing joint prostheses generally have a protruding, or male, member (sometimes
called a "ball") and a recessed, or female, member (sometimes called a "socket") Initially,
both the male and female members were made of metal However, experience indicated that this metal-on-metal construction resulted in a high degree of wear in the earlier designs at
locations where there was moving contact between the male member and the female member
(The locations where mating joint surfaces contact are herein called "bearing surfaces ") This wear on bearing surfaces caused early artificial joint failures, and usually mandated additional
surgery to go back into the body and replace the worn artificial joint with a fresh, new artificial
joint
In order to reduce wear on the joint components, the female component was modified
to include a non-metallic (usually plastic) insert, which covered the bearing surface of the female component This has resulted in artificial joints that have metal-on-plastic contact,
which has been the prevalent artificial joint design for some time now One example of an
artificial joint that utilizes dissimilar materials at the bearing surfaces is presented in U S
patent 5,879,407 to Waggener In Waggener an artificial joint includes a ball and a socket
The surface of the ball is ceramic and the surface of the socket is made of a noble metal or
alloy, such as platinum An example of a more common plastic-on-metal artificial joint is
shown in U S patent 4,055,862 to Farling
In metal-on-plastic joints, considerable research has been directed toward improving
the characteristics of the plastic insert so that its moving contact with the metal surface of the
mating artificial joint component would undergo minimal wear in normal use
Notwithstanding some advances in the area of metal finishes, as well as in the area of polishes
for the plastic insert, the metal-on-plastic design still has significant wear and tends to fail over
time due primarily to wear. More particularly, there are two distinct concerns about wear.
One concern is that the bearing surface will wear out completely, such as when a plastic insert
bearing surface wears out and exposes an underlying stem. Another concern is that particles
that wear off of the joint may remain in the vicinity of the joint and cause problems such as
increased wear and loosening of the insert or prosthesis.
SUMMARY OF THE INVENTION
The inventor of the present invention has recognized a problem inherent in the above-
described artificial joint research. Specifically, the research appears to be fixated on metal-on-
plastic artificial joints. The possibility of plastic-on-plastic artificial joints appears to have been neglected or rejected as unfeasible.
According to the present invention, and ignoring the conventional wisdom that
artificial joints should be metal-on-metal or plastic-on-metal, it has been determined that
plastic-on-plastic artificial joints are feasible, and are sometimes superior to conventional
plastic-on-metal designs. This is especially true for plastic-on-plastic joints where the plastic material(s) are chosen to have superior wear characteristics. Generally speaking, less wear
means fewer artificial joint failures and associated artificial joint replacement surgeries.
More subtly, wear resistant artificial joints made according to the present invention
may facilitate the manufacture of artificial joints that precisely emulate the complex bearing
surface geometry of a healthy joint. This is good because it allows the body to move in a
natural fashion and will limit range of movement less. Also, if more precise emulation of
healthy joint geometry is achieved, the reduced wear of the present invention can help the joint
maintain this geometry over time. For example, if a normal joint bearing surface has a narrow
ridge, the plastic-on-plastic artificial joint of the present invention may be made to more easily
form the narrow ridge in the first place, and to also allow the narrow ridge to hold out against wear over time
According to a preferred aspect of the present invention, ultra high molecular weight
polyethylene ("UHMWPE") is used on both mating, bearing surfaces of an artificial joint This
UHMWPE-on-UHMWPE joint experiences very little wear and represents a tremendous
advance over conventional metal-on-plastic joints The decrease in artificial joint wear on
artificial joints made according to the present invention may mean fewer replacement surgeries and diminished surgery-related health risks
At least some embodiments of the present invention have one or more of the following
advantages, objects and/or benefits
(1) to provide an artificial joint that exhibits less wear in normal use,
(2) to provide an artificial joint that less frequently experiences catastrophic or
semi-catastrophic failure in normal use,
(3) to provide an artificial joint that generates less debris at its bearing surface,
(4) to provide an artificial joint that requires less frequent (surgical) replacement,
(5) to provide an artificial joint that is easier to move,
(6) to provide a less expensive artificial joint,
(7) to provide an artificial joint that can be more easily manufactured and sculpted
to better match the shape of the cartilaginous surface of a healthy joint, and
(8) to provide an artificial joint that can better maintain the (sometimes intricate)
shape of the cartilaginous surface of a healthy joint, notwithstanding wear caused by normal
use of the artificial joint
According to the present invention, an artificial joint includes a first joint member and a second joint member. The first joint member includes a first bearing surface made of artificial,
non-metallic material. The second joint member includes a second bearing surface made of
artificial, non-metallic material. The first bearing surface and the second bearing surfaces are
shaped to mate to guide relative movement of the artificial joint.
According to another aspect of the present invention, an artificial human knee joint
includes a first joint member and a second joint member. The first joint member includes a
first bearing surface made of artificial material. The second joint member includes a second
bearing surface made of artificial material. The first bearing surface and the second bearing
surfaces are shaped to mate to guide relative movement of the artificial joint. The first and
second bearing surfaces have a coefficient of friction of less than 0.1 when the joint is placed under normal use in a human body.
According to another aspect of the present invention, an artificial joint includes a first
joint member, which includes a first bearing surface and a first anchor member. The first
anchor member has a generally tubular shape, and includes an inner anchor surface and an
outer anchor surface. The inner anchor surface is disposed around an interior surface of the
first anchor member. The outer anchor surface is disposed around an exterior surface of the first anchor member.
Other advantages, objects and benefits will become apparent through a review of the rest of this document.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will become more fully understood from the detailed description
given below, together with the accompanying drawings, which are given by way of illustration
only, and thus are not to be construed as limiting the scope of the present invention In the
drawings
Fig 1 is a perspective view of a first embodiment of an artificial joint according to the present invention,
Fig 2 is a cross-sectional view of the first embodiment of an artificial joint according
to the present invention,
Fig 3 is a cross-sectional view of a second embodiment of an artificial joint according to the present invention,
Fig 4 is a perspective view of a third embodiment of an artificial joint according to the present invention,
Fig 5 is a cross-sectional view of the femoral component of the third embodiment artificial joint, and
Fig 6 is another cross-sectional view of the femoral component of the third embodiment artificial joint
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Before starting a description of the Figures, some terms will now be defined
DEFINITIONS
the present invention: at least some embodiments of the present invention; references
to various feature(s) of the "present invention" throughout this document do not mean that all
claimed embodiments or methods include the referenced feature(s). ultra high molecular weight polyethylene ("UHMWPE"): any polyethylene resin with a
molecular weight greater than 1,000,000.
"at least double melt": material that has undergone double melting, triple melting or
any greater number of melts.
"at least triple melt": material that has undergone triple melting or any greater number
of melts. bone: can include calcic matter, as well as cartilaginous matter, marrow and any other
type of matter included in a bone structure.
coefficient of friction: refers to the specific coeffecient of friction value as normally measured in the context of artificial joints.
open-ended interior space of an artificial joint component: refers to interior space that
is open ended at the time the artificial joint is inserted in a bone.
Artificial: means material not made by a body at the location where the material is
present in the body; for example, plastic is an artificial material because it is not manufactured by a body at all; as a further example, if material is taken out of a person's left knee and
inserted into their right knee, that material would be artificial, for purposes of this document,
because the material, while manufactured by the body, is not located at the location where the
body manufactured it.
To the extent that the definitions provided above are consistent with ordinary, plain
and accustomed meanings (as generally evidenced, inter alia, by dictionaries and/or technical
lexicons), the above definitions shall be considered supplemental in nature To the extent that
the definitions provided above are inconsistent with ordinary, plain and accustomed meanings
(as generally evidenced, inter alia, by dictionaries and/or technical lexicons), the above
definitions shall control If the definitions provided above are broader than the ordinary, plain
and accustomed meanings in some aspect, then the above definitions will control at least in
relation to their broader aspects
To the extent that a patentee may act as its own lexicographer under applicable law, it
is hereby further directed that all words appearing in the claims section, except for the above-
defined words, shall take on their ordinary, plain and accustomed meanings (as generally
evidenced, inter alia, by dictionaries and/or technical lexicons), and shall not be considered to be specially defined in this specification Notwithstanding this limitation on the inference of
"special definitions," the specification may be used to evidence the appropriate ordinary, plain
and accustomed meanings (as generally evidenced, inter alia, by dictionaries and/or technical lexicons), in the situation where a word or term used in the claims has more than one pre-
established meaning and the specification is helpful in choosing between the alternatives
Referring to Figs 1 (perspective view) and 2 (cross-sectional view), an artificial joint
100 is shown Artificial joint 100 is a ball and socket joint that allows rotation in more than
one plane Artificial joint 100 includes a ball component 102 and a socket component 104
Ball component 102 includes base plate 106 and ball insert 108 Base plate 106 includes base
portion 106a and stem portion 106b When artificial joint 100 is inserted in a body, at least a
portion of stem 106b is driven into the subconjural portion of one of the bones of the joint in
the conventional manner
Base plate 106 is preferably made of metal and is preferably made of titanium or
stainless steel Alternatively, base plate 106 could be made of other metals and alloys to achieve the desired characteristics and functionality For example, the same metals which are
used to make the stem portion of conventional plastic-on-metal joints can be similarly used to make base plate 106 Ball insert 108 is preferably made of UHMWPE and is adhered to base
plate 106 in the manner conventionally used to adhere polyethylene and metal parts in conventional plastic-on-metal artificial joints The preferred construction of ball insert 108
will be more fully described below
Socket component 104 comprises a base plate 112 and a socket insert 110 Base plate 112 includes base portion 112a and stem portion 112b When artificial joint 100 is
inserted in a body, at least a portion of stem 112b is driven into the subconjural portion of one
of the bones of the joint in the conventional manner Base plate 108, like base plate 106, is
preferably made of titanium or stainless steel
When both of stem 106b and stem 112b are properly driven into adjacent bones, ball
insert 108 and socket insert 110 contact to become bearing surfaces, whose shapes guide permitted motion of the artificial joint Socket insert 110 is preferably made of UHMWPE and
is adhered to base plate 112 in any manner conventionally used to adhere polyethylene and
metal parts in conventional plastic-on-metal artificial joints, such as by gluing, bonding, screwing, or press-fitting
An important difference between at least some embodiments of the present invention
and conventional artificial joints is that neither of the bearing surfaces 108, 110 are made of
metal Rather, the bearing surfaces are both non-metallic This is because metal bearing
surfaces wear (as in a metal-on-metal joint) or cause wear (as in a plastic-on-metal joint) This
use of a metal bearing surface is generally inconsistent with the extremely low characteristics
of friction, which are believed to be achievable with the present invention
More preferably, the bearing surfaces are both plastic, and even more preferably, both
bearing surfaces are polyethylene Polyethylene' s combination of strength, toughness,
smoothness, compressibility, elasticity, stiffness, pore density and other material properties is well-known, and polyethylene is conventionally preferred in plastic-on-metal artificial joints However, when polyethylene constitutes both bearing surfaces, as in the present invention, the
relevant coefficient of friction and associated wear is believed to decrease dramatically It is
believed that certain other plastics could also lead to similar advantageous results
The preferred polyethylene bearing insert surfaces of the present invention is UHMWPE It is believed that the use of UHMWPE can lead to a relevant coefficient of
friction of 0 1, 0 05 or even as low as 0 01 With coefficients of friction this low, there is
drastically reduced wear of the bearing surface Reduced wear of the bearing surface means
that the bearing surface will maintain its shape and integrity, and that there will be less debris
and associated particle wear It is believed that the low friction artificial joints of the present
invention may substantially outlast a human or animal lifetime so that surgeries to replace worn or particle-damaged artificial joints will no longer be necessary
Another preferred type of polyethylene according to the present invention is
polyethylene that has been melted down two or more times Multiple melt polyethylene
generally exhibits increased purity, cross-linking, elasticity, and strength, and a decreased
tendency to delaminate Accordingly, multiple melt polyethylene has been shown to be
capable of high repetition translational contact with an equivalent polyethylene product to
produce a virtually wearless system Double melt polyethylene is therefore preferred
according to the present invention Triple (or more) melt polyethylene is even more preferred
One brand of this multiple-melt polyethylene is known as Dursul (It is noted that the word
Dursul may be subject to trademark rights )
All artificial joint components are preferably sterilized in any conventional manner The inserts are preferably sterilized by gamma radiation
Referring now to Fig 3, an alternative embodiment of an artificial joint 300 is shown
Artificial joint 300 includes ball component 302 and socket component 304 Ball component 302 includes base plate 306 and ball insert 308 Socket component 304 includes base plate
312 and socket insert 310 Both ball insert 308 and socket insert 310 are made of triple melt
UHMWPE As shown in Fig 3, ball insert 308 completely surrounds base plate 306, in the
vicinity of reference numeral 309, for improved wear characteristics at both ball component
302 and socket component 304
Referring now to Fig 4, artificial knee joint 400 is shown Knee joint 400 includes
femoral component 402 and a tibial component 404 Femoral component 402 includes metal
base plate 406 and triple melt UHMWPE insert 408 Similarly, tibial component 404 includes metal base plate 412 and triple melt UHMWPE insert 410 Metal base plate 406 includes
anchor (or stem) members 405 for insertion into the subconjural bone of the femur Metal
base plate 412 includes anchor members 414 for insertion into the subconjural bone of the
tibia The mating, bearing surfaces of inserts 408 and 410 are preferably shaped to match the
natural geometry of a healthy knee as closely as possible
In order to form this geometry, the inserts may be formed by any conventional
UHMWPE forming technique, such as by molding and/or machining When choosing the
forming technique, the ability of the forming technique to accurately achieve the desired shape
should be considered The effect that the forming technique may have on surface wear should
also be considered For example, it is believed by some that molded polyethylene bearing surfaces have better wear characteristics than machined bearing surfaces, at least in the context
of conventional plastic-on-metal joints
It is further noted that the complex geometry shown in Fig 4, which attempts to
accurately emulate the shape of cartilage in a healthy knee, includes ridges and depressions It
is believed that the plastic-on-plastic joints of the present invention may be especially advantageous in the context of knee joint replacements because the complex geometry of the
artificial knee joint will hold its shape better under decreased friction and wear For example,
the anatomy of a natural knee can be duplicated all the way to the periphery of the cortical
rim, such that the artificial joint feels and performs more naturally Also, artificial knee joints
are subjected to a high degree of stress and associated wear because they carry most of the
body's weight and are subjected to both intermittent and repetitive motions Therefore, it is
especially advantageous to decrease wear in joints like the knees and hips, which tend to wear faster than lower stress joints
Anchor members 405 will now be discussed in more detail with reference to Figs 5 and 6 Fig 5 shows femoral component 402 just after it has been driven into a femur bone
500 As is conventional, material may be removed from the end of the femur bone prior to
insertion of femoral component 402 It is the anchor members 405 that secure femoral component 402 to femur 500
While conventional anchor members are cylindrically solid in construction, anchor
members 405 are tubular in construction so that they include an open-ended interior space
450 As shown in Fig 5 , bone matter begins to flow into this interior space when it is first
driven into femur 500
As shown in Fig 6, as time passes, the bone grows to fill this interior space 450, and bonds to the interior wall of interior space 450 By filling interior space 450 and bonding to it,
the bond between femur 500 and femoral component 402 is greatly strengthened as the bone
heals This is advantageous because the more secure attachment allows the artificial joint to
perform better, and can ward off repair work that is conventionally necessitated when a
prosthetic component becomes loose Preferably, the inside of the interior space is
constructed and treated to promote bonding with the regrowing bone
While anchor members 405 are cylindrically tubular in shape, other tube profiles are possible Also, non-tubular constructions with other types of interior spaces are also possible,
such as open ended or hollowed out spheroid anchors (although these may be more difficult to
drive into the bone) Furthermore, ridges or other surface discontinuities can be provided on
the interior anchor member surface to further promote tight bonding between bone and
prosthesis
Although the artificial joints of the present invention have been explained in terms of a
generic ball and socket joint geometry of Figs 1 to 3 and a knee joint of Fig 4, those skilled in
the art will appreciate that the range of joints for which the present invention can be utilized is
significant For example, the present invention could be utilized in not only a replacement
knee, but could equally be utilized to replace the joints in ankles, feet, hips, elbows, wrists,
hands, shoulders or mouth, as well as other joints in a human or animal Also, while joints
according to the present invention are preferably applied to joints that emulate natural joint
geometry, the non-metallic bearing surfaces and tubular stems of the present invention could
also be applied in the context of joints that do not precisely emulate a natural joint. For
example, U.S. patent 3,945,053 (incorporated by reference) discloses an artificial, rolling
contact knee joint which has bearing surfaces that are not particularly similar to the natural
bearing surfaces of a knee.
Another preferred embodiment of the present invention includes use of all poly male
and female components with roughened and similar anchoring parts that are bonded to bone
by bone cement (e.g., polymethyl-methacrylate based bone cement).
Many other variations on the above-described artificial joints are possible. Such
variations are not to be regarded as a departure from the spirit and scope of the invention, but rather as modifications intended to be encompassed within the scope of the following claims,
to the fullest extent allowed by applicable law.