Note: Descriptions are shown in the official language in which they were submitted.
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LINKED CONDYLAR TOTAL KNEE REPLACEMENT
Background
This invention relates to a linked condylar knee prosthesis.
There are several different types of knee replacement in use today, for the
treatment of arthritis of the knee and other conditions. The most commonly
used
design is the total condylar replacement. In this design, the prosthesis
replaces the
femoral-tibial and the patello-femoral bearing surfaces of the natural knee.
The tibial
component consists of two parts, a metal tray to cover the upper tibia, with a
plastic
bearing surface, articulating with the Femoral Component, fixed to the metal
tray. A
plastic component is frequently used to resurface the patella. In all versions
of this
design, the anterior cruciate is not retained, its function being replaced by
dishing of
the plastic bearing surface. In one version, the posterior cruciate is
retained. In
another version, ' the posterior cruciate is resected and its function
replaced by
increased dishing or by an intercondylar cam. The latter design is usually
referred to
as a posterior stabilised, or posterior cruciate substituting. Most cases of
arthritis of
the knee can be treated with one of these types of condylar replacement, and
the
results have a high success rate at up to 10 years and even beyond.
When the arthritis is limited to only one of the pair of bearing surfaces,
such
as the medial side in varus osteoarthritis, and both of the cruciate ligaments
are
viable, it is sometimes considered preferable to use a compartmental knee
replacement, generally called a "uni". When inserted correctly, the functional
results
closely resemble normal. Such knees are often used in the younger and more
active
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patients because of the very limited amount of bone and tissue removal, and
the
incisions can be much smaller than for conventiunal total condylar
replacement.
Another consideration for the younger active patients is the wear of the
bearing
surfaces. In order to reduce the wear of the bearing surfaces, the femoral-
tibial
bearing can be made to be conforming which minimises the contact stresses.
However, in order to allow the rolling and sliding movements, the plastic
bearing
needs to be mobile, such that it slides and rotates on a polished upper
surface of the
tibial tray. Such designs are called mobile bearing knees. These can be made
in
either the compartmental form, or as a total condylar type.
At the other extreme, there is a requirement for the treatment of knees
involving one or more of the following conditions: severe bone loss of
deformity,
severe instability, gross deformity with non-viable soft tissues, bone tumour
where
the distal femur 'or proximal tibia requires resection, failed knee
replacements of
various types. In general, it can be said that this category includes those
knees for
which a total condylar replacement would be inadequate, particularly in terms
of
providing the required stability. In this case, a knee replacement which
provides the
appropriate amount of constraint is required. One type of knee which has been
used
for such cases is a variant of a total condylar called the constrained
condylar. As the
name implies, the design incorporates all of the design characteristics of a
total
condylar, but the Femoral Component includes a housing located in the
intercondylar
region, into which a post projecting from the centre of the tibial bearing
component is
located. This combination provides varus-valgus stability, partial anterior-
posterior
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stability, and partial rotational stability. An advantage of the design is
that the bone
resection required for insertion is little greater than that for a total
condylar, while the
intercondylar region only requires squaring off. However, the limitations are
that
there is no limit to hyperextension, there can be inadequate anterior-
posterior stability
in some cases, and the varus-valgus stability is limited in some designs by
the strength
and stiffness of the tibial plastic post. When greater stability and strength
is required,
some type of linked total knee is required.
The simplest type of linked total knee is a fixed hinge, and indeed this type
of
knee was already being used in many cases as early as the 1950's. These early
designs were unsatisfactory for a number of reasons. However, more recently,
the
design and the surgical technique have improved to the extent that the success
rate of
hinges approaches that of total condylars, even taking into consideration the
more
severe cases which are dealt with. There are three disadvantages of fixed
hinges
however. Firstly, the degree of bone resection required is large, about 25 mm
from
the distal femur and 10 mm from the proximal tibia. Secondly, intramedullary
stems
of length about 150 mm, rigidly fixed to the bones, are required for fixation.
Thirdly, the motion is restricted to that of a hinge with no provision for
internal-
external rotation or other movements.
However, it would be an advantage if the bone-preserving characteristics of
the constrained condylar type could be combined with the stability
characteristics of a
rotating hinge. This invention discloses a solution which seeks to accomplish
that.
Such a design would have a much wider application than either type as it could
be
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used for all of the indications combined. Another advantage of the concept is
that it
would remove the uncertainty as to which type of knee to use, a constrained
condylar
type or a linked type. The proposed design could be used in all circumstances.
According to one aspect of the present invention there is provided an
intercondylar knee prosthesis which comprises a Femoral Component having
condylar bearing surfaces and a tibial component having bearing surfaces for
receiving the condylar bearing surfaces, the femoral and tibial components
being
linked together by a post which is secured at a first end to the tibial
component and at
the opposite end is received in an intercondylar housing of the femoral
component,
said opposite end of the post having surfaces which permit relative rotational
movement of the post within the housing, but prevents the femoral component
separating from the tibial component.
The opposite end of the post may be a sphere or a cylinder or other shaped
surface which permits the femoral component to undergo at least a limited
degree of
lateral-medial rotation, e.g. up to about ~ 10 -~ 15°, e.g. about
12° to each side of
the anterior-posterior center-line.
Preferably, the post comprises a cylindrical post fixed to the tibial
component
mounted on the post. The plastic pivot component is fixed against rotation on
the
cylindrical post and is formed with spherical or cylindrical surfaces for
engaging in
and rotating within recesses in the intercondylar housing.
The invention will be illustrated by the following specific embodiments shown
in the accompanying drawings, in which:-
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Figure 1 is an 'exploded' perspective view of one embodiment of knee
prosthesis in accordance with the invention;
Figure 2 is a perspective view of the embodiment shown in Figure 1 after
assembly;
Figure 3A, B & C show the stages of assembly of the prosthesis; and
Figures 4A & B show posterior and lateral views of a modified prosthesis in
accordance with the invention.
Description of Specific Embodiments
The components of the prosthesis are shown individually in Figure 1. The
femoral component (1) is made from a metal such as cast cobalt-chrome or
titanium
alloy. In use, this component is affixed to the distal end of the femur which
is
shaped during surgery with rectangular and angular cuts so that the Femoral
Component is a close fit. Component (1) replaces the normal bearing surfaces
ofthe
distal femur. The main bearing surfaces of component ( 1 ) are the lateral and
medial
condyles (2 & 3), which extend distally to posteriorly. These bearing surfaces
transmit the forces between the femur and the tibia. At the anterior of
component
(1) is a patella bearing surface (4), consisting of a groove along which the
patella
slides as the knee is flexed and extended. In the centre of the femoral
component
(1) is an intercondylar housing (5), which fits within the intercondylar
region of the
femur, requiring only squaring off of this area in the femur in order to
locate and
provide space for the housing (5). The posterior region of the housing is
hollowed
with parallel sides. In line with the centre of curvature of the femoral
condyles,
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spherical surfaces are machined into the housing so that each side of the
sphere
creates spherical saucer-like recesses (6) on either side of the anterior-
posterior (A-P)
centre-line of the housing, with a spherical surface also being formed at the
top of the
housing. The anterior part of the housing is solid, but has a hole (7) bored
into it
from the top for the fitting of a femoral stem, to facilitate fixation of the
femoral
component into the femur. Preferably, the hole is tapered to allow for modular
stems of different lengths and diameters to be fitted to the femoral component
( 1 ).
The tibial base plate (10) is made from a metal such as cast cobalt-chrome or
titanium alloy. This component is affixed during surgery to the upper end of
the
tibia, which has been resected horizontally to accommodate the component. A
spigot ( 11 ) is attached to the lower surface of the baseplate in line with
the
intramedullary canal of the tibia, for the purpose of attaching a stem, along
similar
lines to that on the femoral side. A metal post (12), preferably cylindrical,
projects
from the upper surface of the baseplate. This may be formed integrally with
the
baseplate or rigidly fixed thereto. The lower part (13) of the post is
preferably
conical and radiused at ( 14), in order to maximise its strength for
horizontally
directed forces applied at the top of the post. A cylindrical hole (15) passes
through
the top of the post in an anterior-posterior direction, the purpose being
described
below.
A plastic tibial bearing component (16), made from a material with high wear
resistance such as ultra-high molecular weight polyethylene (UHMWPE), locates
onto the tibial base plate. The bearing component is intended to lock tightly
to the
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base plate, e.g. using cooperating rims and undercuts on the tibial base plate
and
meniscal component. Post (12) projects through a hole (26) in the bearing
component ( 16).
Mounted on the top of the post ( 12) is a plastics pivot component ( 17) made
from UHMWPE. It comprises a cylindrical part (18) integral with a spherical
top
(19). The spherical top has flats (20) on opposite sides, such that the width
across
the flats is substantially the same as the diameter of the cylindrical part (
18). There
is a vertically oriented hole (21) along the centre of the pivot component
(17) in line
with the centreline of the cylindrical part, the hole being substantially the
same shape
and diameter as the upper part of the post on the tibial base plate. This is
so that the
pivot component (17) fits closely over the cylindrical upper part of post
(12). Pivot
component (17) has a second hole (22) for receiving and locking pin (23)
extending
in an anterior-posterior direction through the component, in line with the
centre of
the spherical surface (19).
Locking pin (23) is made from forged cobalt-chrome or titanium alloy. The
material needs to be such that the two ends of the pin can be compressed,
closing the
slot, without permanent deformation or fracturing of the pin. Pin (23) is
formed
with a slot (24) and tangs (25) to snap outwardly after insertion. The
diameters of
the tangs and pin head of the locking pin are such that they are a clearance
fit in the
hole in the pivot component (17). The main cylindrical part of the pin is a
sliding fit
in the hole in the post ( 12) of the tibial base plate ( 10).
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Assembly of components into the knee
Assembled condition of the components shown in Figure I is shown in Figure
2. As noted, the ends ofthe bones are shaped to receive the femoral component
(I)
and the tibial base plate (10). The plastics tibial bearing component (17) is
fixed to
the top of the tibial base plate. The plastic pivot component (17) is located
onto the
post (12) projecting from the tibial base plate. The orientation of the pivot
component ( 17) in the fully assembled position is as in Figure 1. The pivot
component (17) is held in place on the post by the locking pin (23). The
spherical
sides of the plastic pivot component are located within the spherical saucer-
like
recesses (6) in the housing of the femoral component. Ldcewise, the spherical
top of
the pivot component (17) engages against the spherical top of the housing. In
the
assembly, the lateral and medial metal femoral condylar bearing surfaces (2,3)
locate
on to the corresponding plastic tibial dished lateral and medial bearing
surfaces
(31,32).
It may not be immediately obvious how the assembly can be accomplished
within the confines of the knee. Once the femoral and tibial components (1) &
(10)
are affixed to the respective bones, it may be difficult to assemble the
plastic pivot
component (17) on to its post and then into the femoral housing. The assembly
process is described with the aid of Figures 3A, 3B & 3C. The femoral
component
(1) and the tibial base plate (10) are affi~ced to their respective bones, and
the plastic
tibial bearing component (16) is fixed to the tibial base plate. The knee is
flexed to
about 120 degrees (Figure 3A) and the tibia is pulled anteriorly. The tibial
post (12)
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is now anterior to the femoral component ( 1 ). The plastic pivot component (
17) is
sud on top of the post, in an orientation such that the plane of the flats
(20) is parallel
to the sagittal plane of the knee. In this orientation, when the tibia is
moved
posteriorly towards the femoral component, the plastic pivot component (20)
slides
into the slot (33) of the housing (5). It is noted that the width of the
plastic pivot
component across the flats is substantially the same as the width of the
housing
(Figure 3B) so that the pivot component (17) is a sliding fit into the slot
(33).
When the spherical part of the pivot component is in line with the spherical
saucer-
like recesses (6) in the housing, the pivot component is rotated by 90 degrees
about
its vertical axis. The spherical surfaces are now in engagement (Figure 3C).
In
order to fix the pivot component (17) into this orientation, the locking pin
(23) is
snapped into place. When the tangs of the pin spring outwards again, the Pin
cannot
disengage. T'he pin has a secondary purpose of fixing the pivot component (17)
on
the post (12) in a vertical direction.
Alternate configurations can be specified for engaging a spherical surface
into
a spherical recess in the housing, such that assembly is possible but
disengagement is
prevented. For example, in a modified embodiment as shown in Figures 4A & 4B,
the top of the plastic pivot component (20) does not have the flats but has a
complete
spherical surface on top of the cylindrical part ( 12). The inside of the
housing (5) in
the femoral component (1), rather than having spherical recesses, has a
cylindrical
hole which is substantially the same diameter as the spherical top (19)
drilled in from
the distal end of the component to enter the spherical recesses. With the
femoral
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component at 90 degrees of flexion, this cylindrical hole lies horizontally.
The
plastic pivot component ( 17) can now enter from the anterior until it reaches
its
proper location. In order to prevent it from escaping, an arcuate-shaped
component
(35) made from plastic or metal is pressed into the cylindrical hole. A small
shelf in
the cylindrical surface in the femoral component allows this arcuate-shaped
component to snap into and be held in place. The component essentially
restores the
shape of the femoral component to the shape in the original embodiment shown
in the
earlier Figures. One advantage of using an annulus is that a complete sphere
can be
used on the pivot component ( 17) which may have slightly greater strength. On
the
other hand, it requires an extra part (35) for which the tolerances are
stringent.
Another method for locating the pivot component (17) is shown in Figure 4A.
This component has a boss (36) at the lower end. The component is first
assembled
on the post (12).' The plastic tibial bearing component (16), in lateral and
medial
halves, is fixed into the tibial base plate. This interaction prevents the
pivot
component ( 17) from migrating upwards, thus making it more rigid in relation
to
varus-valgus and hyperextension movements. The prevention of rotation could be
with a locking pin through the anterior of the bearing component and into the
boss.
Function of the assembled knee
When the linked knee is assembled as described above, the femoral and tibial
components cannot disengage. The joint acts as a hinge point, pivoting about a
traverse line through the centre of the sphere. This line is also the centre
of the
sagittal radius of the lateral and medial femoral condyles, so that in flexion-
extension,
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the condyles remain in contact and transmit the axial forces through the knee.
For
this to occur, it is necessary that the femoral bearing surfaces have the same
radius in
the sagittal plane from the distalmost point to the posterior upper limit.
Another aspect of the design is the capability for internal-external rotation
about a vertical axis through the centre of the tibial post (12), that axis
passing
through the centre of the sphere (19). Because the surfaces in the housing are
spherical, it is possible for the femoral component ( 1 ) to rotate about this
vertical axis
relative to the tibial component. This provides for internal-external
rotation, which
is an important aspect of the motion of the natural knee. Not only does it
allow for
more natural function, but it reduces the stresses on the fixation of the
components to
their respective bones. Internal-external rotation may be restricted by
providing
stops. However, the interaction of the condylar bearing surfaces in the dished
recesses in the plastic meniscal bearing component exerts a restraining effect
on such
rotation and tends to bring the femoral component back to the position shown
in
Figure 2 in relation to the meniseal component ( 16), after a force tending to
cause
rotation is removed.
The posterior part of the femoral housing is designed to allow for a high
range of flexion, such as 130 degrees. It is unlikely that a knee will reach
this angle
and hence the linked knee joint will not restrict the motion of any knee into
which it
is inserted. When the knee is brought to full extension, the larger anterior
radius of
the femoral condylars are designed to locate on the anterior of the plastic
tibial
bearing component. Anterior to the distalmost point, the radius is larger,
such that a
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restraint is provided to further extension. This provides that motion beyond
full
extension is prevented by the inwraction between the femoral-tibial contact
points
and the action of the sphere in the housing. A few degrees of hyperextension
is
provided by allowing clearance at the anterior surface of the housing. In this
way,
the knee is brought to a stop without a sudden impact. In cases where the
condylar
surfaces and the corresponding dished recesses in the plastic bearing
component are
closely conforming (which is the preferred arrangement), the condylar surfaces
may
be relieved to provide recesses. This allows for maacimum extension as
described in
WO 94/26212, the disclosure of which is specifically incorporated herein by
reference.
Varus-Valgus movement between the femoral and tibial components is
prevented because of the close fit between the plastic pivot component and the
housing in the femoral component: this stability is a very important goal of
the knee
prosthesis of this invention.
It will be appreciated that in order not to cause a kinematic mismatch between
the femoral-tibial bearing surfaces and the spherical surfaces in the femoral
housing,
when the knee undergoes the various movements described above, appropriate
tolerances will need to be provided. In practice, the sphere is allowed a
small
amount of upward-downward movement on the post, for example, by making the
hole in the pivot component slightly larger than the diameter of the diameter
of the
tangs and the pin head. Other methods may be adopted for providing such
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tolerance, e.g., the recess (6) in the intercondylar housing can be made
slightly larger
than the spherical or cylindrical surface ( 19).
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