Note: Descriptions are shown in the official language in which they were submitted.
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Background of the Invention
Field of the Invention
The present invention relates, in general, to prosthetic
devices such as artificial joints, particularly artifi-
cial knee joints, and, more particularly, to suchdevices in which the plastic portion of the device can
be replaced.
Prior Art
Artificial implantable prostheses to replace human joints
have been available for some time. These prostheses
generally consist of a metallic portion which interfaces
with a plastic member. For example, in a knee joint, the
natural knee joint includes the end of a femur which has
two knob-like processes, the medial condyle and the
lateral condyle, which are smooth and rest on the medial
condyle and lateral condyle of the tibia. The condyles
of the tibia are very smooth and slightly cupped to pro-
vide a surface on which the femur condyles can articulate.
In the event that a knee joint is replaced, the femur
and/or the tibial portions of the joint are replaced.
The femoxal component is usually a metal implant which is
anchored in the femur. The tibial component comprises an
ultra-high molecular weight polye-thylene member which may
be anchored in a metal tray. The polyethylene member
provides the smooth-cupped surfaces on which the femoral
component rests during norma~ motion. One of the draw-
backs in such joint replacement procedures is the~ Eact
tha-t the plastic components may slowly deform because of
cold flow or may wear. The deformation or wear may
require the eventual replacemen-t of at least the plas-tic
portion of the tibial knee prosthesis.
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The knee prosthesis must also be properly sized and
fitted to the patient by the orthopaedic surgeon when
the implant procedure is undertaken. Because of the
great variety of different configurations of the human
knee, there must be an extensive variety of different
sized knee prostheses available to -the surgeon during
the surgical procedure so that the proper prosthesis
may be selec-ted for implantation. One of the vari-
abilities in the prosthesis is the height of the tibial
component of the joint. Prior patents have suggested
that the tibial portion of the joint be composed of a
metal tray and a replaceable polyethylene insert, so
that the metal tray could be implanted into the tibia,
and the surgeon would have available plastic components
of different heights to properly size the prosthesis
during the surgical procedure. In order to provide this
flexibilityl it is necessary to have some system of
securing the plastic component of the tibia prosthesis
into the metal tray. U.S. Patent 4,257,129 discloses
such a system with a metal tray and a removable poly-
ethylene insert in the tray. The polyethylene insert
is secured to the tray using a dovetail joint in the
tray with a matching dovetail joint in the polyethylene
insert. In addition, there are two pins to secure the
insert into the tray. A first pin extends through the
polyethylene insert and into the tray in the vertical
direction, and a second pin extends in the horizontal
direction to secure the first pin in position. To re-
move the plastic insert, the pins are removed and the
polyethylene insert can be removed by sliding the poly-
ethylene insert outof the dove-tail grooves of the metal
tray.
U.S. Patent 4,207,627 discloses a knee prosthesis with a
metal tray and two plastic inserts. The plastic inserts
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hav~ a lip on one side which fits in a r~cess in the metal
tray to r~tain th~ ins~rt in th~ tray.
Summary of -th~ Inv~ntion
The present inv~ntion provides an improved prosthesis in
which the plastic insert can b~ readily fitted into the m~tal
tray. The prosthesis provides an improved locking system to
more positivcly secure the plastic insert into the tray wi-~h-
out using pins or similar locking components. In thc pr~s~nt
device, th~ plastic insert is snap fitted into thc metal tray
on two opposed sidcs of the ins~rt and, although sccur~ly
affix~d in the tray~ th~ ins~rt may readily be removed if
necessary.
According to a further broad aspect of th~ present invention
th~r~ is provided an implantabl~ joint prosthesis comprising
a metal tray and rigid plastic inscrt removably fitt~d into
the tray. The tray has an upper surface and a lower surfacc.
The lower surfac~ has at least one stem for insertion in-to a
human bonc. The upp~r surfac~ of -the m~tal tray has a
g~nerally flat continuous central region and an upwardly
~xtending wall enclosing the major portion of thc central
region. Rec~ss~s ar~ provided adjacent the base of the wall
on opposit~ sides of the tray. Th~ rig:id pl ast..i.c i.ns~.rl: has
a g~nerally flat conZinuous lower slurclc~ and a first- and
s~cond !.ab at t-hc lowcr c~dge of the insert at a location
which corresponds to the r~cesses in the tray. A narrow slot
is provid~d in the lowcr surface of th~ ins~rt and spaccd
immediatcly b~hind at l~ast the first tab to allow the first
tab to deflect when -the ins~rt is forccd into the tray.
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Descriptlon of the Drawings.
., . . . . . ~
E'ig. 1 shows an isometric view of an assembled prosthesis
made according to the present invention.
Fig. 2 shows an exploded view of the two components of the
prosthesis of the present invention.
Fig. 3 shows the bottom plan view of the plastic portion of
the present prosthesis.
Fig. 3A shows a fragmentary view of the bottom plan view of
the plastic portion of the prosthesis showing an alternative
construction.
Fig. 4 shows a top plan view of the metal tray component of
the present invention.
Figs. 5 and 6 show a technique to insert the plastic compo-
nent of th~ present invention into the metal tray.
Fig. 7 shows placement of an extraction tool to remove the
plastic insert from the metal tray.
Figs. 8 and 9 show a detailed fragmentary view of the
locking elements in the plastic portion of the present
- prosthesis.
Figs. 10 and 11 show details of the locking elements in
the metal portion of the tray.
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Detailed Descri tion of the Present lnvention
P
Although the present invention will be described in refer-
ence to a tibial component of an artificial knee, the
invention can be used for other implantable prostheses
where a removable plastic insert is used in combination
with a tray which is implanted in the human bone. An
example of such other prostheses would include a shoulder
prosthesis.
o
Fig. 1 shows the completelv assem~led prosthesis of the
present invention. It includes a metal tray component 15
into which is inserted a plastic element 16, which is
hereinafter referred to as an insert. The metal tray can
~5 be any of the metals commonly used in the manufacture of
implantable prostheses. The most common metals used for
this purpose are titanium, chrome-cobalt alloys and
stainless steel. Although the preferred material of
construction of the tray is metal, it should be understood
that reinforced plastics which are biologically compatible
with the human body could also be employed to make the
tray portion of the prosthesis. The plastic insert is
usually made of medical grade ultrahigh molecular weight
polyethylene. The polyethylene has the re~uired
properties of rigidity and smoothness that allows the
metal component of the prosthesis to ride on the plastic
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and be ca?able of moving readilv to provide the required
degree of articulation for a knee prosthesis. The up?er
surface of the tibial component has cup shaped bearing
surfaces 35 which the condylar portions of the femoral
component contact in the implanted prosthesis. There is a
raised post 38 between the bearing surfaces which con-
strains lateral movement of the knee. The upper surface
topography of the insert can be varied depending on the
degree of constraint desired in the prosthesis. The
height of the post 38 above the bearing surfaces 35 and
the configuration of the post can be varied to control the
degree of constraint of the implanted prosthesis. The
upper surface topography could be that shown in U.S.
Patent 4,213,209 or ~.~. Patent 4,2g8,99~.
Fig. 2 shows an exploded view of the two components. The
ste~s 17 are shown on the lower surface of the tray por-
tion 15 of the prosthesis. In the implantation of the
prosthesis, the upper surface of the tibial portion of the
~0 knee is usually cut to form a flat surface, and holes are
drilled into that flat surface to accept the stem portions
of the prosthesis. Although there are three stems shown
in the present drawings, there may be a single stem or two
stems, depending on the particular design of the prosthe-
sis. There may be a porous metal or plastic coating 34 0,7one or more of the stems and on the bone contact surface
of the tray. The pores in the porous coating are of
sufficient size to allow bone to grow into the pores to
more securely fix the prosthesis in place. ~1hen the
present invention is used for a prosthesis other than a
tibial prosthesis a stem would be designed to fit into the
particular bone into which it would be implanted. The
center region 18 on the upper surface of the tibial trav
is relatively flat. There is a wall l9 around the
periphery of the central region to hold the plastic insert
15 in place. There is a cutout portion 20 shown in the
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drawing which would be em?loyed in a tibial prosthesis
used in a surgical procedure in which the cruciate
ligaments of the patient would not be removed. This
cutout area provides clearance for the cruciate ligaments
after the implant is in place. There are recesses or
undercuts in the wall 19 on at least two opposed sides of
the prosthesis. In the drawings, these recesses are shown
as 21, 22 and 23 in the anterior or front area of the
metal portion of the prosthesis. Rather than three
separate recesses, a single continuous recess may be
employed. There are recesses 24 and 25 in Fig. 4 in the
back or posterior portion of the metal portion of the
prosthesis.
The lower surface of the plastic insert is best shown in
Fig. 3. As shown in Fig. 3, the plastic insert has tabs
27, 28 and 29 on the front or anterior section of the
insert. If there is a single recess in the anterior
portion of the tray, there may be a single table or more
than one tab on the anterior portion of the plastic
insert. These tabs will fit into the recess or recesses
on the tray portion of the insert. As shown in Fig. 3,
there are slots 32 cut in the insert immediately behind
each of the tabs on the anterior ~ortion of the insert.
~s shown in Fig. 3, the slot extends beyond the ends of
the tab throuqh the lower surface of the insert. As shown
in Fig. 3A, the slot 32A may only extend in the plastic
insert 16A for a length which is the length of the tab
29A. The slot is shown in cross section in Fig. 9. It is
necessary for these slots to be immediately in back or
rearward of of the tabs. A single tab, positioned in the
center of anterior portion of the insert, is sufficient to
lock the insert into the tray. The tabs on the posterior
section of the insert may be identical to the tabs on the
anterior section of the insert or they may be a tab of the
construction shown in Fig. 8 of the drawing. The tab
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shown in Fig. 8 is capable of sliding into the undercut or
recess in the posterior wall of the tray. In order for
the tabs to slide into the notches there m~st be clearance
between the top of the wall of the tray and that portion
of the insert which lies on the top of the wall. The
clearance is provided by an arcuate cutout 26 in the top
of the wall and a corresponding arcuate surface 33 made in
the portion of the insert that will overlie the top of the
wall of the tray. Figs. 8 and 9 show the details of the
posterior (Fig. ~) and anterior (Fig. 9) tabs which match
into corresponding recesses in the metal tray, which are
shown in Fig. 10 and Fig. 11. The purpose of the slot 32
is to allow the tab to deflect when the plastic insert is
pushed into the tray in the manner shown in Figs. 5 and 5.
The slots 32 extend from the bottom flat surface of the
insert into the body of the insert. The slot should
extend into the insert to a depth which is greater than
the dimension from the bottom of the insert to the hori-
zontal portion of the tab. The corners of the sliding tab
shown in Fig. 8 may be rounded rather than cut on angles.
The curved surface 33 above the tab matches the arcuate
cutout 26 in the tray. The curved surfaces allow the tabs
to slide into the undercut or recesses without
interference between the top of the wall 19 and the upper
edge of the insert.
The snap tab is shown in detail in Fig. ~. The forward
end of the tab may be cut on a radius to provide a camming
action against the top edge of the wall 19 to force the
tab to deflect rearward when a force is applied downward
to set the insert into the tray. In an alternate con-
struction, Fig. 5-7, the forward edge of the tab is angled
at an angle of between about 45 and 60. This also
provides a camming action to force the tab rearward when a
force is applied downward to seat the insert into the
tray. The width of the slot should be at least equal to
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the width of the table to allow the ta~ to be deflected
into the 510t and clear the wall of the tray.
~hen it is desired to insert the plastic component, the
posterior tabs are inserted into the posterior undercut or
recesses of the tray by sliding the insert from front to
bac~ of the tray until the tabs are in the recesses as
shown in Fig. 5. The anterior portion of the tray is then
positioned immediately over the anterior recesses and
forced downward. ~Jhen the plastic insert is pushed down-
ward, the tabs are deflected ~s they are capable of moving
backward because of the slot behind the tabs in the bottom
of the plastic insert. After the horizontal portion of
the tabs have cleared the top of the recess, the tab will
revert back to the position shown in Fig. ~, and the
plastic insert will be firmly locked into the tray.
The bottom surface of the insert is flat and has a lower
portion which is dimensioned to fit within the interior
wall of the tray. The height of the lower portion is
substantially the same as the height of the wall of the
tray. The upper portion of the insert is dimensioned so
that its outer periphery is the same as the outer
periphery of the wall of the tray. The ~unction between
the lower and the upper portions of the insert is a
hori20ntal ledge that rests on the top of the wall 19 when
the insert is in place in the tray. The dimensions of the
lower portion of the plastic insert are slightly smaller,
1 to 25 thousands of an inch, than the dimensions of the
space within the wall of the tray to compensate for the
differences in thermal expansion between the plastic of
the insert and the metal of the tray. The upper surface
of the plastic insert need not be coextensive with the ton
of the metal tray. The upper surface of the insert can
extend beyond the dimensions of ~he tray if desired.
However, the lower surface of the plastic ins~rt must be
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dimensioned to fit within the wall of the metal tray and
be locked into the tray.
As shown in Figs. 5 and 6 there is a cutout portion or
opening 30 located ln the center of the anterior surface
of the insert where the insert meets the top of the wall
of the tray. The opening 30 is of a height and width to
receive the forward end of the removal tool as shown in
Fig. 7. The forward end of the tool has a bifurcated
tip 36 with a portion of the tip integral with each of the
handles of the toolO To remove the insert from the tray,
the tip is inserted into the cutout portion and the
handles 37 forced towards one another moving the tip
portions 36 in opposite directions and forcing the insert
out of the tray.
The tip of the extraction tool may be of sufficient length
so that force may be exerted against the tab to push the
tab back into the slot to allow the horizontal portion of
the tab to clear the notch.
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