Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to prosthetic joints,
and more particularly to prosthetic joints used for total
human knee joint replacement and which permit the knee joint
to perform in an almost natural anatomical manner.
Medically, hinged knee joints are implanted when
the tissues of the knee joint are grossly deteriorated from
disease or other cause, particularly the loss of function
of the four major ligaments of the knee: 2 cruciate ligaments
and 2 collateral ligaments.
In the normal anatomical function of the healthy
knee, the geometry of motion is complex. The knee is essen-
tially a large knuckle joint, in fact, it is the largest joint
in the body. The upper surface of the tibia provides two
bearing surfaces which are essentially flat and essentially
at right angles to the axis of the shaft of the tibia. The
co-acting lower end of the femur consists of two large rounded
ends called condyles. The condyles roll and slide on the
two supporting surfaces of the tibia, called the tibial plateau.
There is no congruency or symmetry in either joint between
the two corresponding surfaces on the tibia and the two corres-
ponding condyles. Normally, bone joint surface pressure load-
ings are disbursed favourably through cartilage covered articu-
lar surfaces, and the menisci and synovium contribute to
adequate normal function. The two rolling and sliding actions,
plus the result of forces and constraints put on the knee
joint by muscles, ligaments, joint capsule, etc. result in
a relative axial rotation between the two cooperating bones,
the femur and the tibia in addition to the basic motion of
flexion. If one sits with the leg fully extended in front
of him, as he flexes the knee downward through 90 degrees,
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the tibia rotates inwardly, approximately 10 degrees. This
axial rotation of the tibia is relatively small and not at
all uniform per unit of flexion occurring mostly in the initial
phases of flexlon from the leg being fully extended. In yeneral,
hinge-type prosthetic joints do not incorporate this relative
axial rotation and therefore do not closely simulate the natural
action of the knee joint.
To reproduce a natural leg movement to the maximum
possible extent, a prosthetic knee joint must provide two
degrees of freedom, namely, bending or rotation of the tibia
about an axis transverse to the shaft of the femur, which
motion is called flexion, and rotation of the shaft of the
tibia about its axis relative to the axis of the shaft of
the femur. Furthermore, the prosthetic knee must accommodate
the large stresses placed on the bearing surfaces of the pros-
thesis. These stresses can cause inadequate bearing surfaces
to wear out. Obviously, wear of the bearing surfaces is un-
desirable, because movement of the prosthetic knee would be
impaired and the debris resultiny from wear would be hamful
to the body.
To date, knee prostheses either do not provide rota-
tion in more than one plane, or have bearing surfaces which
tend to wear away. The development of a prosthetic joint
involves a conundrum in that the greater the degree of success
of the operative procedure, the more likely the chance of
failure of the prosthesis. The preceding stems from the fact
that the patient candidate for the prosthetic joint has im-
paired joint function and therefore limited mobility. If
the implantation of an artificial joint reduces pain and im-
proves mobility, the patient becomes more active. The greater
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the degree of improvement of joint function, the greater theprobability of increased activity by the patient. Increased
activity will be accompanied by increased load and motion
on the joint, which combine to tend to cause mechanical failure
of the artificial joint. This condition is particularly pro-
nounced in the knee because of the complex geometry of motion
and the large load forces involved.
Considering only those prostheses permitting rotation
in two planes, prostheses can be divided into at least three
general classes.
A first class includes those knee prostheses in
the form of inserts or modules, or resurfacing devices which
replace the bicondylar joint of a human limb and are con-
structed to simulate the shape of that bicondylar joint. These
prostheses attempt to imitate the natural motion of a joint
by structuring the surfaces of the joint to imitate nature.
However, these joints have high local loading stresses at
selective points on the bearing surfaces and, therefore, tend
to wear away at these selective points, thus resulting in
degradation and ultimate failure of the prosthetic joint.
As a result, such prostheses may have to be replaced after
only a short period of use, depending of course on the degree
of activity of the patient.
A second class of prostheses includes a ball and
sGcket joint with means to limit motion to rotation in two
planes. In such a joint, the means to prevent rotation in
the third plane is commonly constructed so that unit bearing
loads are high, and early bearing failure generally result
when patient activity increases.
A third class of prostheses comprises a fixation
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stem implantable in a tibia and having an upstanding arm extend-
ing from the top of the fixation stem. The top of the upstanding
arm may be in the form of a ball. The ball fits into a socket
in the space between the condyloid members of the femoral
prosthesis. In this construction, the condyloid prosthetic
members of the prosthesis of the femur rotate and slide on
the prosthetic surfaces of the tibia. This relative motion
between the non-congruent prosthetic surfaces under the high
bearing loads imposed by the function of the joint may lead
to premature mechanical failure of the prosthetic bearing
surfaces.
The preferred embodiment of the present invention
overcomes the above-discussed disadvantages of the known art
by providing a prosthetic joint in which the bones can undergo
rotation in two planes, yet will not be subject to excessive
unit bearing stresses. Bearing surfaces are arranged to be
in contact over large areas of engagement so that the large
forces exerted by reason of the function and mechanics of
the knee are spread out over the entire area of the engagement.
Mated bearing surfaces are of large area and are of congruent
geometry. Therefore, local stresses (force per unit area)
are reduced from those values found in the prior art wherein
the bearing surfaces are not congruent. With the arrangement
of the present invention, the prosthetic device will continue
to function with a minimum of wear and simulate normal action
of a human limb for long periods of time under conditions
of normal patient activity.
The preferred embGdiment substantially eliminates
the transmission of shock torque loads through the prosthesis
by virtue of allowing relative axial rotation between the
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bones. Such torque loads are transmitted through tendons,
ligaments and other soft tissues covering the joint. In ad-
dition, this embodiment has no fixed coupling between the
two bones and is designed to extend or dist~act should flexion
beyond approximately 120 cause the bones to move apart or
move axially with respect to each other. Accordingly, the
present invention overcomes the two principal factors contri-
buting to loosening of hinge type prostheses.
Further, the preferred embodiment provides the very
practical advantage of allowing the surgeon improved access
to the posterior joint space during the surgical procedure.
One alternate form of the prosthesis includes a
tibia stem component which is implantable directly in the
tibia and which stem upper surface supports a bearing shoe.
The bearing shoe is axially rotatable relative to the tibia
and is confined by guide elements at the edge of the upper
surface of the tibia stem component. This embodiment can
be designed to allow the cruciate ligaments to remain intact.
In this embodiment, the femoral and tibial components are
not hingedly coupled.
A second alternate is similar to the above, and
permits removal of a minimum of bone from the tibia.
Accordingly, it is an object of the present invention
to provide a durable prosthetic joint which enables the limb
to undergo approximate natural movement.
It is another object of the present inventior to
enable a pair of human bones joined by a prosthetic joint
to undergo relative rotation in at least two planes and thereby
closely simulate the natural action of the joint.
It is yet another object of the present invention
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to provide a prosthetic device having an improved bearing
configuration.
The present invention prov:ides a prosthetic device
for forming a joint between a pair of human or animal bones
comprising first fixation means adapted for attachment to
one of the bones; first bearing means mounted on the first
fixation means; second fixation means adapted for attachment
to the other bone, and comprising a bearing surface means
for supporting weight and being subjected to force; an inter-
mediate load bearing member having two surfaces, one surface
comprising a thrust bearing surface means for matingly engaging
the bearing surface means, and adapted to distribute weight
and to transmit forces while rotating freely relative to the
bearing surface means independent o the position of the first
bearing means, the other surface of the intermediate load
bearing member comprising means for supporting the first bearing
means and adapted to support weight and be subjected to force
from the first bearing means while experiencing relative motion
with the first bearing means.
Preferably, the first fixation means is adapted
to be attached to the femur bone and a coupling means posi-
tioned at one end of the first fixation means. The coupling
means is pivotally attached to a flanged stem element forming
the intermediate load bearing member, which has a rod depending
thereon. A flanged sleeve element forming the second fixation
means is adapted to be implanted in a tibia bone and receives
the depending rod and flange of the stem element in a manner
which permits relative rotation between the stem and the sleeve
and distraction or separation of the stem from the sleeve.
Stop elements mounted on the flange of the sleeve are shaped
~lV205il
to correspond to the shape of the flange element of the stem
element. The stem flange and the stop elements cooperate
to limit the amount of relative rotation (axial to the shaft
of the tibia~ between the femur and the tibia.
Therefore, the device of the present invention enables
a human limb to approximate normal movements comprising rotation
in two planes while distributing load forces over substantial
bearing areas. By permitting rotation in two planes while
reducing local unit stresses placed on the bearing means,
the device embodying the present invention minimizes wear
and maintains improved prosthetic motion of a limb over long
periods of time.
For a better understanding of the present inventi~on,
together with other details and features thereof, reference
is made to the following description of a preferred embodiment
thereof taken in conjunction with the accompanying drawings,
in which:
Figure 1 is a diagram showing the kinematics of
a prosthetic joint embodying the teachings of the present
invention;
Figure 2 is an exploded perspective of a prosthetic
joint embodying the teachings of the present invention;
Figures 3 and 4 show two views of the prosthetic
joint joining a pair of bones;
Figure 5 shows the underneath side of the tibia
sleeve;
Figure 6 shows a bearing shoe adapted for use with
the prosthetic joint of the present invention;
Figures 7 and 8 show an alternative form of the
present invention;
11C~2~Sl
Figures 9 and lO show another alternative form of
a prosthesis according to -the teachings of the present inven-
tion; and
Figure ll shows two locking means for setting the
prosthesis to maintain a desired orientation of the tibia
with respect to the femur.
Figure l shows the kinematics of the prosthesis
embodying the present invention. As illustrated schematically
in Figure l, in the extended position of the leg or lower
limb, the femur "F" and the tibia "T" are generally aligned
in an essentially continuous line along the z-axis of the
co-ordinate system shown in Figure l. As used herein,
"transverse rotation" means flexion and refers tc rotation
of the tibia about the x-axis transverse thereto and occurs
in the y-z plane, and "axial rotation" refers to rotation
of the tibia about its longitudinal (z) axis. Thus, in Figure
l, flexion is represented by arrow "a" and axial rotation
is represented by arrow "b". As the tibia of a natural knee
is flexed through 90 degrees from the fully extended knee
position shown in Figure l, it undergoes a rotation about
its longitudinal axis of approximately lO degrees. The pros-
thesis embodying the present invention enables the tibia to
execute the aforementioned axial rotation while the tibia
executes the transverse rotation.
Figure 2 shows an exploded perspective of a pros-
thesis 10 embodying the present invention. Prosthesis lO
comprises a femoral component 12 having a fixation shank 14
adapted to be implanted in femur "F" and a pair of spaced
apart bearing surfaces 16, which simulate the condyloid ele-
mentc of a natural knee, each of which has holes 17 there-
1102051
through, said holes 17 defining a bearing surface. Each ofholes 17 contains a sleeve element 33. ~earing surfaces 16
comprise a convex surface of revolution having an axis ap-
proximately perpendicular to the axis of the femur, and are
adapted to be seated in, and bear upon, a bearing shoe 20
which has concave upper surfaces 50 and which is adapted to
be seated on a plateau surface 22 of flange portion 23 of
tibia stem 24. The bearing surfaces 16 and the bearing sur-
faces defining the holes 17 are concentric and part of the
same unitary mechanical element and are adapted to support
weight and be subject to force while experiencing movement.
Upper part 30 of tibia stem 24 is attached at one end to
plateau 22 and defines a hole 32 therethrough near the other
end together with holes 31 which exit transverse to hole 32.
A transverse support shaft 34 provided with a transverse hole
37 fits through hole 32 and sleeve elements 33 to attach femoral
component 12 to tibia stem 24. Pin 35 is inserted through
holes 31 and 37 to retain shaft 34. It is here noted that
the coupling means and bearing surfaces include the elements
which attach the femoral component to the tibia stem, and
a coupling means suitable for use with the present prosthetic
joint is fully disclosed in the aforementioned U.S. Patent
No. 3,996,624. A lower rod 38 depends from the lower surface
of the flange portion 23 and is received in a hole 42 of a
tibia sleeve 26 in a manner which permits free rotation of
the tibia stem within the sleeve about the longitudinal axis
of the rod. The tibia sleeve 26 is adapted to be implanted
in a tibia. Furthermore, tibia sleeve 26 has an upper thrust
bearing surface 52 on a flange portion 53 which is adapted
to engage a thrust bearing surface 40 located on the lower
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surface of the stem flange portion 23.
secause the knee is subject to large compressive
loads which manifest themselves as thrust forces acting parallel
to the axis of rod 38, the cooperating thrust bearing surfaces
40 and 52 are made generously large. Thus, the bearing load
per unit area is made sufficiently low to provide long bearing
life with low wear rate. The interface of surfaces 40 and
52 is shown as being flat or planar but may be a surface of
revolution.
To permit the artificial joint to distract under
conditions which may exist at the limit of maximum flexion,
the stem 24 is free to move out of sleeve 26. Thrust bearing
surfaces 40 and 52 are free to separate one from the other.
Rod 38 is free to slide upward in hole 42. Rod 38 serves
to impart guidance and lateral stability of stem 24 relative
to the sleeve 26. Because the created implant cavity and
the tibia bone T are tapered with the largest dimension nearer
~ the joint, it is structurally advantageous to make rod 38,
; hole 42, and sleeve 44 tapered as shown in the drawings.
While the feature of the tibia stem 24 as being
freely movable axially in tibia sleeve 26 is based on allowing
the artificial joint to be extensible for functional reasons,
it provides another very practical advantage. This advantage
presents itself during the surgical implant procedure for
the following reasons. In conventional hinged knee prostheses,
there are generally two main components to the prosthetic
joint: the femoral component and the tibial component. Im-
mediately after these components are fixed to the bone by
cementing, the access to the posterior joint space is blocked
b~ the presence of the two components. The surgical opening
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'l': `))
11~2~51
is at the anterior of the knee and yet excess cement must
be removed from behlnd the prosthesis where vision is often
blocked and access is limited. There can also be stray bone
chips and occasionally even fixed parts of the bone which
interfere with the free function of the hinge. In the case
of the inventive prosthesis, the femoral component 12 is
cemented in place and the tibia sleeve 26 is cemented in place,
subsequently the tibial stem 24 can be inserted and removed
repeatedly at the will of the surgeon while he tries the
function of the joint and examines the posterior joint space
with clear view and access. The surgeon can therefore be
certain that there are no extraneous pieces of cement or bone
in the joint space and that the joint flexes freely through
the entire desired range of motion. This very practical con-
sideration is of importance to the surgeon.
The bearing surface 52 has stops 54 which correspond
to the shape of the flange portion 23 and coact with corres-
ponding wing elements 94 formed on the flange portion 23.
The coupling means permits flexion of the tibia with respect
to the femur, and the tibia stem 24 undergoes axial rotation
relative to the tibia sleeve 26 which is fixed in the tibia
"T". Therefore, the fcmur "F" and tibia "T" are permitted
to rotate in two planes with respect to each other. The axial
rotation is limited by stops 54 and wing elements 94. Figures
3 and 4 show the assembled prosthesis 10 installed in the
femur "F" and tibia "T" to form the prosthetic knee joint
of the invention.
With reference to Figures 2, 3, 4 and 5, tibia sleeve
26 is shown as comprising a portion 60 depending from flange
portion 53 having grooves 62 on the end thereof which is remote
l~OZ~51
from the head portion. Portion 60 is adapted to be cemented
into the medullary canal of the tibia and the grooves 62 ensure
a secure connection between the sleeve and the tibia. Likewise,
lower surface 64 of flange portion 53 has a plurality of grooves
66 therein for ensuring a secure connection between the flange
portion 53 and the upper bearing surfaces of the tibia.
As shown in Figure 2, depending portion 60 is tapered
and has a cross-section ad~acent grooves 62 which is smaller
than the cross-section thereof adjacent the flange portion
53. The taper of approximately 2 degrees per side is pre-
ferred by reason of the taper in the tibia bone.
As shown in Figure 2, flange portion 53 of tibia
component 26 is essentially oval-shaped, and the stops 54
are positioned on the marginal edges thereof. The stops 54
extend radially inward from the marginal edge of the sleeve
flange portion 53. Ribs 74 are defined on the periphery of
flange portion 53 and have a shape which conforms to the
generally arcuate shape of the flange portion outer periphery.
The stops 54 are defined by surfaces which join with ribs
74 on one side and concave surfaces 78 on the other side.
The flange portion 53 is recessed at 82 to provide a slot
to accommodate blood vessels and the like. The hole 42 may
be offset from the center of the flange portion 53.
The grooves 66 in flange portion 53 may be essentially
parallel to each other, or any other suitable configuration.
The tibia stem 24 is shown in Figures 2, 3, 4 and
includes tapered rod 38 which moves freely in the taper of
the hole 42 in tibia sleeve 26. Fillets ~0 at the juncture
of lower rod 38 and the surface 40 are shaped to reduce stress
concentrations.
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The perimeter of flange portion 23 is shown in Figure
2 as having a circular sector 92 which fits within the concave
surfaces 78. The circular sector 92 on each end joins abutment
shoulders 94 which intersect the circular sector 92 at ap-
propriate points to permit a limited degree of relative rota-
tion of the tibia sleeve 26 about its longitudinal axis with
respect to the stem 24 before one of the shoulders 94 abuts
a corresponding shoulder 54. Therefore, this angle is chosen
to correspond to the desired amount of rotation of a tibia
about its longitudinal axis relative to the femur. The angle
therefore is preferably in the order of 10 degrees in each
direction, however, other extents of rotation are possible
depending upon specific circumstances. Ultimate practical
experience may show that in some cases the amount of rotation
need not be limited by the prosthesis. That is, the patient's
anatomy remaining outside the prosthesis, tendons, ligaments
and scar tissue may provide enough restraint to rotation.
As both shoulders 94 are displaced from shoulders 54 when
the tibia sleeve 26 is in the repose or neutral position,
the tibia sleeve 26 can rotate about its longitudinal axis
in either a clockwise or a counterclockwise direction.
Bearing shoe 20 is shown in Figures 2, 3, 4 and
6, and is seen to be U-shaped having a pair of legs 100 and
a central slot 102 having a cross-section which corresponds
to the transverse cross-section of upstanding post 30. The
bearing shoe is adapted to be seated on the surface 22 and
be engaged by bearing elements 16. As shown in Figure 6,
the shoe defines lips 112 on its lower surface 110 which depend
from each of the legs 100. Each lip is adapted to engage
an edge of the stem flange portion 23 as is shown in Figure
.: I )
1~205:1
4, when the shoe is seated on surface 22.
As shown in Figure 6, the shoe has an upper surface
114 which is divided into two sections, a concave section
formed by surfaces 50 and a tapered section 124. When the
shoe is positioned in the prosthesis 10, each of the arcuate
surfaces 50 is adapted to receive one of the bearing elements
16. When assembled and implanted, the bearing elements 16
are adapted to be in weight bearing relationship with the
bearing shoe 20 which has concave upper surfaces 50. See
U.S. Patent No. 3,996,624 for a description of the advantageous-
ly large bearing area of the hinge joint provided by redundant
bearings 16 in 50 and 33 in 17.
With reference to the Figures, the compressive force
on the prosthesis generated by the function and mechanics
of the knee is distributed over the relatively large surface
area of the thrust bearing surfaces 40 and 52. Thus, these
surfaces will not wear as rapidly as would surfaces with smaller
areas of contact. Therefore, the device embodying the present
invention permits rotation in two planes while having ade~uate
bearing configurations to ensure a long lasting prosthesis.
The bearing surfaces 40 and 52 are adapted to distribute weight
and transmit force uniformly while at the same time able to
undergo relative rotation.
The elements of the prosthesis 10 must be fabricated
from biologically compatible, surgically implantable materials.
For example, the tibia sleeve 26, the bearing shoe 20, and
a sleeve element 33 can be a plastic, such as ultrahigh
molecular weight polyethylene, and the remaining elements
are preferably of surgically implantable structural metal.
The femoral component and the tibia stem can both be of grade
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316 surglcal stainless steel, or they can both be of an ac-
ceptable cast chromium, molybdenum, cobalt alloy (such as
ASTM F75-74~ or -the femoral component can be of cast chromium,
molybdenum, cobalt alloy while the tibial stem can be of
titanium-6 aluminum-4 vanadium alloy.
The device of the present invention is adaptable
to various sizes and either a right or left leg.
Figures 9 and 10 show an alternative form of the
prosthesis which includes a prosthesis 10' comprising a tibial
component 150 implanted directl~ in a tibia "T", a femoral
component 152 having condyloid elements 154, and an intermediate
bearing shoe 156 seated on surface 157 of head 158 of the
tibial element. The condyloid components 154 engage seats
168 in the shoe 156. The tibial component150 comprises a
rod 155 and projection elements 160 depending from surface
162 of the head 158, all of which can be attached to the upper
end of the tibia to secure the tibial components 150 thereto.
Bearing shoe 156 is to a degree similar to the bearing shoe
20 of the preferred embodiment. Head 158 comprises a pair
of guide elements 164 located on the marginal edge thereof
and a slot 166 in which the knee cruciate ligaments are loca-
ted. Bearing shoe 156 is U-shaped and comprises a pair of
legs 167, each having bearing element seats 168. A slot 169
in the shoe 156 corresponds to the slot 166 of the tibial
component head 158. The outer surface 170 of the bearing
shoe 156 and the inner surface 172 of the guide elements 164
are curved along similar radii and cooperate to guide the
rotation of the tibia with respect to the longitudinal axis of
tibia. Condyloid elements 154 are preferablv surfaces of
revolution as are bearing element seats 168, and each element
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11~2~51
154 has a contour which is congruent to the seat 168 in which
it moves. This geometry provides adequate bearing area for
the motion of flexion. Axial rotation of the tibia is permitted
by the rotation of bearing shoe 156 on the weight bearing
or thrust bearing surface 157 of tibial component 150. This
interface is between congruent bearing surfaces of generous
area. The tibial component rod 155 is shown offset from the
center of the head, however, other suitable positions for
these elements can be selected.
The prosthesis of Figures 9 and 10 differs from
the preferred embodiment in that for stability it relies,
to a large extent, on functioning knee joint ligaments. That
is, it is adapted to implantation in patients with a lesser
degree of degradation of the knee joint than is the preferred
embodiment. The same is true for the prosthesis shown in
Figures 7 and 8.
Figures 7 and 8 show still another embodiment of
the prosthesis embodying the present invention. The prosthesis
10" comprises a tibial component 180 similar to the tibial
component 150, and a femoral component 190. The femoral com-
ponent 190 comprises a pair of arcuate bearing elements 192
attachable to the lower end of a femur by suitable means,
such as upstanding stems 194 which are implanted in the femur
using polymethyl methacrylate cement. The tibial component
180 supports a bearing shoe 196 which engages with the bearing
elements 192. The bearing shoe 196 is seated on the upper
surface 198 of tibia head 199 of the tibial component 180.
The function of bearing shoe 196 is similar to that described
for the shoe 156, and is guided by ribs 200 formed on the
periphery of head 199. Stems 202 and 204 provide a means
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to attach the tibial component 180 to the tibia.
The prosthesis 10" has the advantage of being im-
plan-table while requiring the removal of minimal amount of
bone.
As a further alternative, a locking means can be
used in conjunction with any embodiment of the prostheses
of this invention. One type of locking means, shown in Figure
11, is a pin 210 used in conjunction with the tibial component
of the prosthesis 10. After prosthesis 10 is implanted, a
rotation of the tibia to the desired axial alignment can be
selected by the surgeon. A hole for locking pin 210 is drilled
through the bone, the sleeve 44, and the rod 38. As is shown
in Figure 11, a pin 210 can be inserted in a transverse
direction into the tibia. The pin passes through sleeve 44
and is driven through tibia rod 38 of the prosthesis 10. Thus,
the sleeve will be locked against rotation with respect to
the tibia rod, and the prosthesis will be allowed to rotate
in only one plane. Another suitable locking means is a locking
pin 212 extending downwardly through the bearing shoe, the
tibia stem flange portion, and the sleeve flange portion and
into the top end of the tibia.
Obviously, numerous modifications and variations
of the present invention are possible in light of the above
teachings. It is therefore understood that, within the scope
of the apended claims, the invention may be practiced otherwise
than as specifically described.
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SUPPLEMENTARY DISCLOSURE
The interface of surfaces 40 and 52 is shown in
the drawings as being flat or planar. However, any relatively
shallow surface of revolution whose axis is approximately
parallel to the axis of the tibia such as a cone, part of
a sphere or other configuration with equivalent projected
bearing area to the flat interface can be used to define the
congruent thrust bearing interface between flange 23 of stem
24 and surface 52 of sleeve 26 so long as the congruent thrust
bearing surfaces forming the interface are able to function
as a low friction thrust bearing capable of freely rotating
under the full range of loads that the knee experiences. It
is known, for example, that conical mating metal surfaces
where the apex angle of the cone is less than 15 will either
lock when subjected to axial thrust load, or the interface
friction will increase to such an extent as to become wholly
impractical in a knee joint. Therefore, it is necessary that
the conical mating surfaces have an apex angle sufficiently
great to permit low friction axial rotation while subjected
to axial thrust load. It is mandatory, therefore, that the
conical apex angle be greater than 15, and preferably greater
than 30. As a practical design matter, it is necessary in
order to achieve the best results than an apex angle greater
than 120 be used. A flat surface (i.e., a conical apex angle
of 180) is being used commercially with excellent results.
Whereas the above discussion considers a conical
bearing surface by way of example only, it should be appreciated
that other surface of revolution configurations can be employed
in the invention. The mean curvature of such other surfaces
of revolution or the defined apex angle of such other surfaces
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of revolution must meet the above cri-teria, i.e., must be
shallower than or greater than, respectively, an approximate
designated apex angle of an approximately similar conical
surface of revolution, the smallest angle being 15 for the
reasons explained, 30 being the preferable lower limit, and
120 being the practical design limitation.
As indicated hereinbefore, in order to permit the
artificial joint to distract under conditions which may exist
at the limit of maximum flexion, the stem 24 is free to move
10 out of sleeve 26. Thrust bearing surfaces 40 and 52 are free
to separate one from the other. Rod 38 is free to slide up-
ward in hole 42. The maximum separation anticipated in use
is estimated to be one half inch. Rod 38 serves to impart
guidance and lateral stability of stem 24 relative to the
sleeve 26. In order to maintain stability of alignment to
insure proper re-engagement when coming together the rod 38
should be at least 1.5 inches long. Rod 38 and hole 42 can
be straight or tapered, so long as there is clearance between
them when fully engaged.
This advantage presents itself during the surgical
implant procedure for the following reasons. In conventional
hinged knee prostheses, there are generally two main com-
ponents to the prosthetic joint: the femoral component and
the tibial component. Immediately after these components
are fixed to the bone by cementing, the access to the posterior
joint space is blocked by the presence of the two components.
The surgical opening is at the anterior of the knee and yet
excess cement must be removed from behind the prosthesis where
vision is often blocked and access is limited. There can
also be stray bone chips and occasionally even fixed parts
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of the bone which interfere with the free function of the
hinge. Also, when the femoral component 12 is cemented in
place and the tibia sleeve 26 is cemented in place, subse-
quently the tibial stem 24 can be inserted and removed re-
peatedly at the will of the surgeon while he tries the function
of the joint and examines the posterior joint space with clear
view and access. The surgeon can therefore be certain that
there are no extraneous pieces of cement or bone in the joint
space and that the joint flexes freely through the entire
desired range of motion.
Figure 10 depicts a space between the outer surface
170 of the bearing shoe 156 and the inner surface 172 of the
guide elements 164. This space may be so proportioned as
to allow bearing shoe 156 to move in a transverse forward
and rearward direction that may result due to the action of
the cruciate ligaments. As a conse~uence, the bearing shoe
156 is adapted to move forward and backward, as well as ro-
tationally, relative to the head 158.
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