U.S. patent number 3,837,009 [Application Number 05/312,850] was granted by the patent office on 1974-09-24 for knee prosthesis.
This patent grant is currently assigned to New York Society for the Relief of the Ruptured and Crippled. Invention is credited to Peter S. Walker.
United States Patent |
3,837,009 |
Walker |
September 24, 1974 |
**Please see images for:
( Certificate of Correction ) ** |
KNEE PROSTHESIS
Abstract
A knee joint prosthesis comprises a femoral component having a
pair of laterally spaced-apart condylar portions received on the
femoral condyles and having surfaces shaped substantially to match
the shapes of the condylar surfaces of the femur and an
intercondylar portion connecting the femoral components together.
The intercondylar portion has a medial slot that opens toward the
tibial plateau. A tibial component in the form of a generally
plate-like platform is placed on the tibial plateau and has
laterally spaced-apart tibial surfaces disposed opposite from and
supporting the condylar surfaces of the femoral component. A post
extends upwardly from the tibial component into the slot in the
femoral component, and an axle extends transversely through
openings in the post and the intercondylar portion of the femoral
component to connect the tibial and femoral components together in
hinged relation for relative pivotal movement of the components
about a transverse pivot axis to afford flexion of the leg.
Inventors: |
Walker; Peter S. (New York,
NY) |
Assignee: |
New York Society for the Relief of
the Ruptured and Crippled (New York, NY)
|
Family
ID: |
23213288 |
Appl.
No.: |
05/312,850 |
Filed: |
December 7, 1972 |
Current U.S.
Class: |
623/20.26 |
Current CPC
Class: |
A61F
2/385 (20130101) |
Current International
Class: |
A61F
2/38 (20060101); A61f 001/24 () |
Field of
Search: |
;3/1
;128/92R,92C,92CA |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Vitallium Surgical Appliance Catalog, Austenal Medical Division,
Howmet Corp., New York, N.Y., 1964, M.G.H. Femoral Condyle
Replacement (No. 6662), page 62..
|
Primary Examiner: Gaudet; Richard A.
Assistant Examiner: Frinks; Ronald L.
Attorney, Agent or Firm: Brumbaugh, Graves, Donohue &
Raymond
Claims
I claim:
1. A knee joint prosthesis comprising a femoral component having a
pair of condylar portions laterally spaced-apart from each other
and having outwardly facing surfaces generally matching in shape
the anatomical surfaces of the femoral condyles (the condylar
surfaces of the femur) and an intercondylar portion interconnecting
the femoral condylar portions, the intercondylar portion being
generally U-shaped and defining a slot located between the femoral
condylar portions and adapted to open toward the tibial plateau; a
tibial component in the form of a plate-like platform adapted to be
positioned on the tibial plateau and having laterally spaced-apart
outwardly facing condylar surfaces disposed opposite the femoral
condylar portions of the femoral component for support thereof and
a post extending upwardly into the slot in the femoral component;
and an axle extending transversely across the slot in the femoral
component through a hole in the post of the tibial component and
into aligned holes in leg portions of the intercondylar portion on
either side of the slot in the femoral component and connecting the
tibial and femoral components together in hinged relation for
relative pivotal movement of the components about a transverse
pivot axis to afford flexion of the leg at the joint, each hole
into which the axle extends in at least one of the components of
the prosthesis having a clearance from the axle in a selected zone,
the clearance being shaped, dimensioned and positioned to afford
relative rotation between the tibial and femoral components about
an axis parallel to the axis of the tibia, i.e., to provide
rotational laxity in the joint, the rotational laxity being at a
maximum when the prosthesis is in a position corresponding
generally to full leg flexion and the outwardly facing surfaces of
the condylar portions of the femoral component are in contact with
the outwardly facing condylar surfaces of the tibial component, the
rotational laxity being at a minimum when the prosthesis is in a
position corresponding to full leg extension and said surfaces are
in contact.
2. A knee joint prosthesis according to claim 1 wherein there are
clearances between lateral faces of the post of the tibial
component and the faces of the intercondylar portion defining the
slot in the femoral component.
3. A knee joint prosthesis according to claim 1 and further
comprising a tibial pin extending through an opening in the tibial
component and positioned in the component to extend through the
tibial epiphysis.
4. A knee joint prosthesis according to claim 1 and further
comprising a pin connected to the femoral component and positioned
on the component to extend through the femoral epiphysis.
5. A knee joint prosthesis according to claim 4 wherein the pin of
the femoral component is unitary with the intercondylar portion of
the femoral component and is an extended curved portion of reduced
transverse cross-section merging smoothly with the intercondylar
portion of the femoral component.
6. A knee joint prosthesis according to claim 1 wherein each of the
femoral condylar portions of the femoral component is an elongated
doubly curved member shaped and dimensioned to extend from
generally the anterior to generally the posterior of the femoral
condyle and having (a) a varying curvature in the
anterior-posterior direction closely matching the anatomical shape
of the femoral condyle and (b) in transverse cross-sections along
its length a laterally outwardly located essentially flat zone and
a laterally inwardly located curved zone that is externally convex,
and wherein each condylar surface of the tibial component has in
transverse cross-section a shape matching the cross-sectional shape
of the condylar surface of the corresponding femoral condylar
portion of the femoral component.
7. A knee joint prosthesis according to claim 6 wherein each
condylar surface of the tibial component is a surface generated by
a substantially straight anterior-posterior line line moved along a
path conforming substantially to the transverse shape of the
corresponding condylar surface of the femoral condylar portions of
the femoral component.
8. A knee joint prosthesis according to claim 1 wherein the axis of
the axle is positioned to be spaced posteriorally from the axis of
the tibia.
9. A knee joint prosthesis according to claim 1 wherein the
distance between the axis of the axle and an imaginary surface
matching the lateral profiles of the femoral condylar surfaces of
the femoral component is at or near a maximum at the anterior
extremity of such surface and generally diminishes at successive
points on such surface moving from the anterior extremity toward
the posterior extremity of such surface.
10. A knee joint prosthesis comprising a femoral component having a
pin portion positioned on the femoral component to extend through
the femoral epiphysis, a pair of laterally spaced-apart femoral
condylar portions in the form of anteriorally-posteriorally
elongated, doubly curved plate-like members having a varying
curvature in the anterior-posterior direction and having outwardly
facing condylar surfaces generally matching the anatomical shapes
of the respective condylar surfaces of the femoral condyles and an
intercondylar portion connected to the inner edges of the
respective femoral condylar portions and to the pin, the
intercondylar portion having an outwardly open slot located
generally between the femoral condylar portions of the femoral
component and adapted to face the tibial plateau; a tibial
component in the form of a plate-like platform adapted to be
positioned on the tibial plateau and including in each of the
laterally outward zones of the surfaces thereof facing the condylar
surfaces of the femoral condylar portions of the femoral component
a condylar surface positioned in facing relation to the
corresponding condylar surface of the femoral component, each
condylar surface of the tibial component having a shape closely
matching in transverse cross-sections along its length the
transverse shape of the corresponding condylar surface of the
femoral component and being a surface generated by a generally
straight anterior-posterior line moved along a path conforming to
said transverse shape of said corresponding condylar surface of the
femoral component, the tibial component further including a post
projecting upwardly from the upper surface thereof and received in
the slot in close clearance relation to the walls of the slot;
means adapted for securing the tibial component to the tibia; and
an axle coupling the tibial component and femoral component
together in hinged relation for relative pivotal movement of the
components about a transverse pivot axis to afford flexion of the
leg at the joint, the axle extending through aligned transverse
openings in the post of the tibial component and the intercondylar
portion of the femoral component, each axle opening in at least one
of the components having, with the leg at or near full extension,
the following clearance relationships with the axle;
1. a clearance in the anterior-posterior direction to afford
relative rotation between the tibial and femoral components about
an axis parallel to the axis of the tibia, i.e., to provide
rotational and anterior-posterior translational laxity in the
joint, the rotational laxity being at a minimum when the leg is at
or near full extension and the outwardly facing condylar surfaces
of the condylar portions of the femoral component are in contact
with the condylar surfaces of the tibial component, the rotational
laxity being at a maximum when the leg is at or near full flexion
and said surfaces are in contact;
2. a very small clearance between the top of the axle and the top
of the opening to afford a slight upward displacement of the
femoral component in a direction away from the tibial component in
conjunction with the relative rotation between the components about
an axis parallel to the axis of the tibia;
3. a clearance between the lower portion of the axle and the lower
portion of the opening to enable angular displacement of the
femoral component in an anterior-posterior direction relative to
the tibial component upon flexion of the joint, i.e., flexural
laxity.
11. A knee joint prosthesis according to claim 10 wherein the axis
of the axle is positioned to be spaced posteriorally from the axis
of the tibia and the distance between the axis of the axle and an
imaginary surface defined by the extremities of the condylar
surfaces of the femoral condylar portions of the femoral component
is at or near maximum at the anterior extremity of said imaginary
surface and generally diminished at successive points moving from
the anterior extremity toward the posterior extremity and wherein
the axle engages the top of the opening when the leg reaches full
extension so that flexion substantially beyond full extension is
precluded.
12. A knee joint prosthesis according to claim 10, wherein there is
a clearance of outwardly increasing magnitude between the surfaces
defining the slot in the intercondylar portion of the femoral
component and the lateral faces of the tibial post, moving along an
anterior-posterior axis in at least one direction from the zone of
a plane parallel to the axis of the tibia and including the axis of
the axle.
13. A knee prosthesis according to claim 12 wherein the surfaces
defining the slot in the femoral component are planar and parallel
and are positioned on the femoral component to be oriented
substantially anteriorally posteriorally of the femur and wherein
portions of the lateral faces of the tibial post on at least one
side of said plane converge outwardly.
14. A knee joint prosthesis according to claim 13 wherein said
portions of the lateral faces of the tibial post are planar and the
planes thereof intersect at an angle equal to the desired maximum
degree of rotational laxity permitted by the prothesis.
15. A knee joint prosthesis according to claim 14 wherein said
angle is on the order of 12.degree..
16. A knee joint prosthesis according to claim 10 wherein the axle
is affixed to the intercondylar portion of the femoral component
and all clearances are between the axle and the opening in the
tibial post, the opening in the post, as viewed axially, being
defined by a substantially arcuate upper wall in relatively closer
clearance with the axle, a substantially arcuate lower wall in
relatively larger clearance with the axle, and substantially
straight side walls connecting and merging smoothly with the upper
and lower walls, whereby the rotational laxity afforded by the
prosthesis is at a maximum when the axle resides in the power
portion of the opening (its position at full leg flexion) and is at
a minimum when the axle resides in the upper portion of the opening
(its position at full leg extension).
17. A knee joint prosthesis according to claim 10 wherein the under
surface of the tibial component has at least one slot therein for
enhancing the fixation of the tibial component to the tibia against
movement out of its position as placed.
18. A knee joint prosthesis according to claim 10 wherein the
tibial component has a recess at the medial anterior portion of the
upper surface thereof and where the tibial pin includes an enlarged
head portion received in said recess.
19. A knee joint prosthesis according to claim 10, wherein the
under surface of the tibial component has at least one boss
projecting therefrom for enhancing the fixation of the tibial
component to the tibia against movement out of its position as
placed.
Description
BACKGROUND OF THE INVENTION
The knee joint is the largest joint of the body, is subject to the
greatest stress of any joint in the body and is required to sustain
the high stresses while permitting not only flexural movement of
the leg in a posterior-anterior direction but a significant amount
of relative rotation or swivelling between the femur and the tibia
about the axis of the tibia (or the axis of the femur, depending
upon what the leg is doing or one's point of view) an
anterior-posterior relative translation between the femur and
tibia. Moreover, the bone structure and soft tissue structure
associated with it must provide a high degree of stability in
various positions. For example, the leg in the extended position
involves a locking of the joint against flexure of the leg beyond
the normal extended position. Various injuries and diseases of the
knee joint are frequently of sufficient severity to partially or
totally prevent functioning of the joint and can be extremely
painful.
U.S. Pat. application Ser. No. 224,479, filed Feb. 8, 1972, now
U.S. Pat. No. 3,774,244 by the present inventor, for "Knee Joint
Prosthesis" (assigned to the assignee of the present invention)
describes and illustrates a knee joint prosthesis that provides for
either partial or total replacement of the interengaging surfaces
of the femoral and tibial condyles. The prosthesis of that
application affords relief from pain and restores function and
load-bearing capability to a knee-joint that is, as a result of
disease or injury, rendered partially or fully incapable of
function, principally as a result of failure of the femoral and
tibial condyles but which, nonetheless, retains most of its
stability. As described in more detail below, certain features of
the knee joint prosthesis described in the aforementioned prior
application are, preferably, included in the knee joint prosthesis
described and illustrated herein, and reference may be made to that
prior application for a complete description of such features.
SUMMARY OF THE INVENTION
There is provided, in accordance with the present invention, a knee
joint prosthesis intended for use in knees that as a result of
injury or disease are not only partially or totally incapable of
normal function, but have been damaged to the point of being
markedly unstable; in other words, not only are the condyles
incapable of functioning properly to transfer loads from the femur
to the tibia and to afford proper articulation, but other
components such as ligaments and other soft tissues of the knee
joint have been damaged or destroyed or there has been substantial
destruction or erosion of bone so that replacement of the condylar
surfaces does not restore function and stability.
More particularly, a knee joint prosthesis, according to the
present invention, includes a femoral component having a pair of
laterally spaced-apart condylar portions placed at the femoral
condyles and a generally U-shaped intercondylar portion that
interconnects the femoral components. The cavity or opening between
the legs of the U opens generally toward the tibia and
posteriorally and provides a slot in the intercondylar portion of
the femoral component. A tibial component in the form of a
plate-like platform having laterally spaced-apart femoral surfaces
at the lateral portions of its outwardly facing surfaces opposite
the femoral condylar portions of the femoral component is placed on
the tibial plateau. A post extends upwardly from the medial portion
of the upper surface of the tibial component and into the slot
between the legs of the U-shaped intercondylar portion of the
femoral component. An axle extends through holes in the post of the
tibial component and the legs of the intercondylar portion of the
femoral component and couples the tibial and femoral components to
each other in hinged relation for relative pivotal movement of the
components about a transverse pivot axis, thereby to afford flexion
of the leg.
A preferred form of prosthesis according to the present invention
embodies femoral and tibial condylar portions of the femoral and
tibial components that are shaped in the manner described in the
prior application of the present inventor referred to above, and
reference may be made to that application for a full description
and illustration of the condylar surfaces of the two components.
Briefly, however, the femoral condylar portions are anteriorally
posteriorally elongated, doubly curved, plate-like members having a
varying curvature in the anterior-posterior direction that conforms
generally to the anterior-posterior anatomical shape of the femoral
condyles. The condylar surface of each femoral condylar portion is
of uniform transverse shape throughout its length and includes a
substantially flat portion at the laterally outward portion and
convexly curved portions at the laterally inward portions. Each
condylar surface of the tibial component is a surface generated by
moving an anteriorally posteriorally oriented line along a path
conforming to the transverse shape of the corresponding condylar
surface of the femoral component. In other words, in transverse
cross-section, each tibial condylar surface has a shape matching
the transverse shape of the femoral condylar portion of the femoral
component and is essentially uniform in all transverse sections
along its anterior-posterior length. The matching transverse
curvatures of the interengaging condylar surfaces of the two
components of the prosthesis provide lateral stability in the joint
in that they restrict relative lateral displacement between the
components, but articulation of the joint in a manner closely
resembling the normal anatomical articulation or flexure of the
joint is provided.
The condylar surfaces of the tibial component (that is, those
surfaces of the tibial component that are engaged by the femoral
component) are preferably generated by straight lines and thus are
straight in all lateral cross-sections taken in anteriorally
posteriorally extending planes. Accordingly, anterior-posterior
displacement of the femoral component relative to the tibial
component, a function that is afforded by the anatomical knee
joint, is permitted by the prosthesis.
The hinge connection between the femoral and tibial component of
the prosthesis of the present invention provides enhanced stability
of the knee joint, as compared with the prosthesis described and
illustrated in the prior application referred to above.
Nonetheless, the form of the prosthesis of the present invention
provides flexibility or laxity closely resembling that of the
anatomical knee joint, thereby facilitating an approximation of
normal knee joint function. In particular, rotational laxity, that
is, the ability of the tibia to rotate about its longitudinal axis
relative to the femur is afforded by providing the following
geometrical and dimensional characteristics:
1. a predetermined clearance between the axle and the hole in the
intercondylar portion of the femoral component in which the axle is
received;
2. predetermined clearances between the lateral surfaces of the
post of the tibial component and the faces of the slot in the
intercondylar portion of the femoral component which receives the
post, and
3. the cross-sectional shapes of the slot and post are so related
as to afford limited pivoting of the post in the slot.
Preferably, the clearance between the axle and the hole through
which it passes varies, depending upon the degree of flexure of the
knee, the degree of rotational laxity increasing from a relatively
small amount of a few degrees of relative rotation when the leg is
extended to a somewhat larger degree of rotation, say, somewhere on
the order of 9.degree. to 15.degree. of relative rotation, when the
leg is substantially flexed. The clearance between the axle and the
hole or holes through which it passes in the hinge connection and
the lateral clearances between the lateral faces of the post and
the faces of the slot allow the desired amount of relative rotation
of the prosthesis components but restrict rotation beyond the
desired value.
In a preferred form, the post of the tibial component has, in cross
section (in planes generally perpendicular to the longitudinal axis
of the tibia) lateral surfaces that converge outwardly, relative to
a plane passing through the axis of the axle and substantially
perpendicular to the principal plane of the outwardly facing
surface of the tibial component. Such convergence may be either in
the form of convexly curved lateral surfaces or flat, convergent
faces.
The clearance relationships described above (and described in more
detail below) permit the prosthesis to function in a manner closely
approximating normal anatomical function. Preferably, however,
inasmuch as flexibility in the joint is somewhat inconsistent with
stability, the flexibility provided by the geometry of the
prosthesis and, in particular, by the various clearances
intentionally provided to produce the desired flexibility, is
somewhat less than that provided by the anatomical knee joint. In
any case, the degree of flexibility is a matter of the precise
design of the prosthesis and can be varied in accordance with the
opinions of the doctors in the circumstances of a particular
patient or type of patient.
The components of the prosthesis are implanted in the knee joint in
assembled relation, thus facilitating the surgical procedure and
ensuring accuracy of location of the parts relative to one another,
and are secured by an appropriate cement, such as a cement based on
polymethyl methacrylate. Firm fixation of the femoral component in
the femur is enhanced by providing, in the femoral component, a pin
that extends through the epiphysis of the femur, preferably within
the cortex, and a short distance into the lower portion of the
femoral diaphysis. Some bone removal on the epiphysis is necessary
to accommodate the intercondylar portion of the femoral component
and a small amount of resecting of the femoral condyles in regions
that will underlie the femoral condylar portions of the femoral
component of the prosthesis may be appropriate. In addition, the
internal surfaces of the femoral condylar portions of the femoral
component may be provided with a series of small bosses or pins
that project into a body of cement installed in the resected
portions of the femoral condyles.
The tibial component is implaced on the tibial plateau, also using
an appropriate cement for securement, by resecting a small amount
of the cortex of the tibial epiphysis in the region that will
underlie the tibial component. Fixation of the tibial component is
enhanced by providing undercut or dovetail-shaped grooves in the
undersurface of the tibial component and providing ribs or fins
that depend downwardly and extend some distance into the tibia. A
pin is installed through a hole through the anterior portion of the
tibial component, the pin extending through the tibial epiphysis
within the cortex and into the upper end of the tibial diaphysis
and being cemented in place.
Although various materials may be used for the prosthesis, it is
preferable to employ a surgical metal, such as a surgical
cobalt-chrome alloy. The interengaging surfaces of the components
should be highly polished to reduce friction to a minimum and to
minimize wear and thus prolong useful life.
A knee joint prosthesis, in accordance with the present invention,
provides various important advantages. The fact that the two
components are physically interconnected and are geometrically and
dimensionally formed to restrict relative movements other than
those intentionally provided to accommodate anatomical knee
function ensures stability of a corrected joint in which the
prosthesis is implaced. Thus, the ability to restore stability to
an unstable anatomical knee joint is, of course, the most important
advantage of this invention. The prosthesis also permits limited
relative movements, thus retaining the laxity of the anatomical
knee joint and providing restoration of function approximating
anatomical function. The laxity afforded by the prosthesis also
reduces the forces imposed on the fixation so that loosening is
less likely to occur. The prosthesis requires a minimum amount of
bone cutting and resection before placement, and yet the components
are securely fixed to the bones and are not likely to require
replacement. On the other hand, if there is a failure of the
prosthesis, there will still be adequate bone available for another
implacement or for an alternate surgical procedure. The components
are strong, durable and long wearing, thus making a failure of the
prosthesis very unlikely in any case.
DESCRIPTION OF THE DRAWINGS
For a better understanding of the invention, reference may be made
to the following description of an exemplary embodiment, taken in
conjunction with the figures of the accompanying drawings in
which:
FIG. 1 is a side view from the external side of a right knee joint
having the prosthesis implaced therein, the bones being shown in
outline only and the prosthesis being shown in full elevation;
FIG. 2 is a front view, partly in elevation and partly in section,
as represented by the lines 2--2 of FIG. 1 and the direction of the
arrows, of the prosthesis of FIG. 1;
FIG. 3 is a top view of the tibial component of the prosthesis of
FIGS. 1 and 2;
FIG. 4 is a top view in cross-section of a portion of the
prosthesis, the cross-section being taken generally along the lines
4--4 of FIG. 2 and in the direction of the arrows, and the view
being on a larger scale than FIG. 2;
FIG. 5 is a side elevational view showing the axle and the hole in
the tibial post on a larger scale than FIG. 1; and
FIG. 6 is a side view of the external side of the prosthesis (as in
FIG. 1) illustrating the prosthesis in the position it assumes with
the knee flexed to 90.degree..
DESCRIPTION OF EXEMPLARY EMBODIMENT
FIGS. 1, 2 and 6 of the drawings illustrate an exemplary embodiment
of the prosthesis, according to the invention, in place at the knee
joint of a human leg. The femur (thighbone) 10 and the tibia
(legbone) 12 are shown in dotted outline. The prosthesis includes a
femoral component 14, a tibial component 16, a tibial pin 18 by
which the tibial component is more firmly secured to the tibia and
a hinge pin or axle 20 by which the femoral component and tibial
component are connected together for relative pivotal movement to
afford flexure of the knee joint.
The femoral component 14 consists of a pair of laterally
spaced-apart femoral condylar portions 22, each of which is a
doubly curved, plate-like element having an anterior-posterior
curvature and a length generally conforming to the corresponding
condylar surface of the femoral condyle and having in transverse
cross-sections (FIG. 2) along its length, a relatively flat outer
portion 24 along the lateral extremities and a curved inner portion
26. Each femoral condylar portion of the femoral component is of
generally uniform cross-section throughout its length. Projections
in the form of small pins 27 on the internal surfaces of the
femoral condylar portions 22 extend into a layer of cement placed
in a resected portion of the femoral epiphysis underyling the
femoral condylar portions 22 and assist in fixation of the femoral
component.
The femoral component further includes a generally U-shaped
intercondylar portion 28 that interconnects the femoral condylar
portions 22 and defines between laterally spaced-apart legs of the
U a slot 30. The slot 30 is open throughout the portion of the
femoral component that faces generally towards the tibia in all
positions of the leg. (As described below, the slot 30 receives a
post on the tibial component and the slot is therefore necessarily
open to allow full flexural movement of the joint). A curved
femoral pin portion 32 extends from the upper end of the
intercondylar portion 28, and the intercondylar portion and pin
portion together thus extend through the femoral epiphysis,
preferably within the cortex, and terminate at generally the area
where the epiphysis meets the diaphysis of the femur.
The tibial component 16 is a generally flat, plate-like piece
having, in plan (see FIG. 3), a shape appropriate to permit it to
be mounted on the tibial plateau and having in the lateral portions
of the outwardly facing surface laterally concavely curved portions
34 and 36 that receive and support the femoral condylar portions 22
of the femoral component 14 and function, in the prosthesis, as the
condylar surfaces of the tibial plateau. In transverse
cross-section (see FIG. 2), the condylar surfaces 34 and 36 of the
tibial component are shaped to match the transverse shapes of the
corresponding femoral condylar portions of the femoral component.
In particular, the surfaces 34 and 36 are surfaces generated by
moving a straight line along a path conforming to the transverse
shape of the outwardly facing surfaces of the femoral condylar
portions 22 of the femoral component, such outwardly facing
surfaces constituting the condylar surfaces of the femoral
component and matching the condylar surfaces of the femur. The
configuration of the femoral condylar portions of the femoral
component and the femoral surfaces 34 and 36 of the tibial
component are substantially the same as those described and
illustrated in U.S. Pat. Application Ser. No. 224,479, filed Feb.
8, 1972, for "Knee Joint Prosthesis" (referred to above), and
reference may be made to that application for a more complete
description of the femoral condylar portions of the femoral
component and the condylar surfaces of the tibial component of the
prosthesis of the present invention.
The tibial component 16 also has an upwardly extending post 38 that
is received in the slot 30 of the femoral component. The hinge pin
or axle 20 is fixed in holes in the legs of the intercondylar
portion 28 of the femoral component and passes through a hole 40 in
the post 38 of the tibial component. The tibial pin 18, which has a
slight taper, passes through a hole in the anterior portion of the
tibial component. An enlarged head 18a on the pin resides in a
recess 42 formed in the upper medial anterior portion of the tibial
component. Firm fixation of the tibial component to the tibia is
further assured by a transversely extending, undercut groove 46
formed under the anterior portion of the undersurface of the tibial
component and a pair of laterally spaced, depending ribs or bosses
48. A body of cement placed under the tibial component protrudes
into the undercut slot 46 and provides a keyway lock while the ribs
48 extend down into adhesive-filled slots cut into the bone. The
slot 46 and the ribs 48 ensure fixation of the tibial component
against rotational or translational shifting of the tibial
component on the tibial plateau.
Referring to FIGS. 4 and 5 of the drawings, the clearances provided
in the hinge system of the prosthesis are of considerable
importance in providing for controlled laxity in the joint, such
laxity being necessary to reproduce in the prosthetic joint the
capability of movements approximating the movements afforded by the
anatomical knee joint. The flexibility or laxity afforded by the
prosthesis is a result of several geometrical relationships and
predetermined clearances established in the prosthesis, as
discussed in the following paragraphs.
The location of the axis of the axle on the femoral component,
relative to the condylar surfaces of the femoral component (that
is, the outwardly facing surfaces of the femoral condylar portions
22 of the femoral component) is established to provide pivotal
movement of the joint in a manner closely simulating the pivotal or
flexural movement of the anatomical knee joint. Moreover, the
configuration of the opening 40, relative to the axle 20, provides
a locking function that restricts flexure of the joint in a
direction beyond the normally extended position of the leg. More
particularly, the axis AA of the axle 20 is positioned, relative to
an imaginary surface SF defined by the extremities of the condylar
surfaces of the femoral component such that the distance DA between
the axis AA and the imaginary surface SF is at or near a minimum at
the posterior extremity PE of the tibial surface of the femoral
component and is at or near a maximum at the anterior extremity of
the surface SF, the dimension DA varying somewhat at various points
along the surface SF but, in general, increasing moving from the
posterior extremity PE to the anterior extremity AE. With the leg
in fully extended position, as illustrated in FIGS. 1 and 2 of the
drawings, the axle resides near the top of the opening 40 in the
tibial post 38. Controlled rotational laxity in the knee joint is
afforded by providing, in the hinge connection between the tibial
and femoral components of the prosthesis, for limited rotation of
the tibial component relative to the femoral component. The maximum
control over the amount of such relative rotation is a result of
(1) the configuration and relative dimensions of the axle 20 and
the axle opening 40 in the post 38 of the tibial component and (2)
the geometry and dimensions of the slot 30 in the femoral component
and the post 38. FIGS. 4 and 5 of the drawings illustrate the
geometrical and dimensional relationships involved.
In the anatomical knee joint, the ability of the tibia to rotate
about its longitudinal axis relative to the femur is, in general,
at a maximum with the leg at maximum flexion and diminishes to a
minimum with the leg fully extended. This anatomical function is
simulated in the prosthesis by providing a variable clearance
between the axle 20 and the axle opening 40 in the post 38 of the
tibial component. As discussed above, the distance between the axis
of the axle 20 and an imaginary surface defined by the extremities
of the femoral condylar portions 24 of the femoral component
increases for approximately a minimum at the posterior extremity of
the femoral condylar portion to a near maximum at the anterior
extremity. This dimensional relationship requires that the hinge
afford movement of the axis of the pin relative to one of the two
components of the prosthesis, and this in turn means that there
must be a clearance between the axle and the holes that receive it
in one of the two components.
In a preferred embodiment of the prosthesis, the axle, which is
preferably of circular cross-section, extends into holes bored in
the legs of the intercondylar portion of the femoral component and
is tightly fitted within such holes against movement. All clearance
in the hinge connection is provided in an enlarged hole of a
carefully established shape in the post of the tibial component. It
is important to point out at this juncture, however, that a
reversal of this particular arrangement is entirely possible; that
is, the axle can be rigidly connected to the post of the tibial
component and received in specially shaped oversized holes in the
legs of the femoral component, such modification representing
merely a reversal of parts but no essential difference in function.
Assuming, merely for purposes of simplifying this description, that
the axle is affixed to the femoral component and received in an
oversized, specially shaped hole in the post of the tibial
component, the clearances between the axle and the hole in the
post, considering the prosthesis in lateral profile with the leg in
fully or near fully extended position, include the following
clearance relationships (see FIG. 5):
1. There is a small clearance t between the top surface of the axle
and the top of the hole in the post to allow the femoral component
to displace a small distance vertically away from the tibial
component upon slight rotation of the tibia relative to the femur.
Such relative rotation requires a slight cocking of the femoral
component relative to the tibial component which, in turn, means
that the interengaging condylar portions of the two components, by
reason of their shape in transverse cross-section, requires a small
amount of vertical movement of the femur relative to the tibia;
2. There is a small clearance l between the axle of the anterior
and posterior surfaces in the upper portion of the hole for the
same reason as the clearance described in subparagraph 1 above. The
desirable small rotational laxity in the prosthesis requires that
the axle cock or rotate slightly in the hole in conjunction with
the relative cocking or rotation of the femoral component relative
to the tibial component. The magnitude of the anterior-posterior
clearance increases to a maximum in the bottom portion of the hole
and establishes the degree of rotational laxity with the leg in the
various positions, since the degree of which the axle cocks in the
hole is limited by engagement of the axle with the walls of the
axle hole;
3. There is a relatively large clearance b between the bottom
portion of the axle and the bottom portion of the hole to allow for
relative downward displacement resulting from the variable
dimension between the above-mentioned imaginary plane defined by
the extremities of the femoral condylar portions of the femoral
component and the axis AA of the axle.
By comparing FIGS. 1 and 6 of the drawings and, of course, in view
of the variable dimension between the axis of the axle and the
plane defined by extremities of the femoral condylar portions of
the femoral component, it is evident that the axle 20 occupies a
position near the top of the opening 40 when the leg is at maximum
extension, occupies a position near the bottom of the opening when
the leg is at full flexure and occupies variable intermediate
positions between the top and bottom at various degrees of leg
flexure. Inasmuch as the clearance in the anterior-posterior
direction between the axle 20 and the opening 40 is greater in
approximate proportion to the degree of flexure of the leg and
inasmuch as the cocking of the axle in the opening 40 is limited by
the clearance in the anterior-posterior direction, the hinge
arrangement in the prosthesis provides a minimum amount of
rotational laxity in the joint with the leg at full extension and
increasing degrees of rotational laxity in approximate proportion
to the degree of flexure of the leg.
Rotational laxity in the prosthesis is also afforded by providing
geometrical and dimensional relationships between the slot 30 in
the femoral component and the lateral faces of the post 38 of the
tibial component that permit rotational laxity. Preferably,
however, such relationships also restrict lateral laxity, i.e.,
relative translation in the lateral direction of the femoral and
tibial components. The geometrical and dimensional relationships
between the slot 30 and the post 38 in an exemplary embodiment of
the prosthesis are best seen in FIG. 4 of the drawings.
The faces defining the slot 30 are planar and parallel and are
spaced apart a distance designated S. The line XX in FIG. 4
represents a plane through the axle that is perpendicular to the
major surface of the tibial component. The lines YY in FIG. 4
represents a plane through the axis of the axle opening in the post
38 and perpendicular to the major plane of the tibial
component.
In cross section, along at least a portion that is received within
the slot 30, the post 38 includes a center portion that is
rectangular and has a dimension d approximately equal to the
maximum dimension of the opening 40 and a dimension A that is
somewhat less than the dimension S between the faces of the slot.
The dimension A is chosen so that when the post 38 (and thus the
tibial component) rotates or cocks relative to the femoral
component in either direction through an angle equal to one-half or
any other desired fraction of the total desired maximum rotation,
the corners of the rectangular portion of the post 38 engage the
faces of the slot 30.
The portions of the lateral faces of the post 38 outwardly from the
cross-sectionally rectangular portion of the post converge
outwardly, and the included angle between the planes of the
convergent portions is equal to the desired amount of rotational
laxity in the prosthesis. Accordingly, when the tibial component
rotates relative to the femoral component to an extent equal to
one-half or any other selected fraction in either direction, the
convergent faces engage corresponding portions of the walls of the
slot 30. FIG. 4 illustrates such engagement and therefore
illustrates the manner in which the dimensional and geometrical
relationships between the post and the slot limit the amount of
maximum rotation of the tibial component relative to the femoral
component.
It is evident from FIG. 4 and from appropriate trigonometric
calculations of the dimensions and angles illustrated in FIG. 4
that only a relatively small lateral clearance between the lateral
faces of the post 38 in the rectangular medial portion and the
walls of the slot 30 (S minus A) is necessary to permit a
substantial angular rotation. Accordingly, rotational laxity is
afforded without introducing any substantial lateral laxity.
Moreover, the matching shapes in transverse cross-section of the
condylar surface of the femoral component and the condylar surface
of the tibial component restricts, in any event, relative lateral
translation.
The geometrical and dimensional relationships between the post and
the slot described above and illustrated in FIG. 4 are merely
exemplary. A similar result of providing rotational laxity in the
prosthetic joint may be achieved by providing an appropriate
clearance between a planar face or by providing outwardly convexly
curved lateral faces on the post. Moreover, the cross-sectional
shapes of the walls of the slot may be altered, rather than or in
addition to altering the cross-sectional shape of the post.
In addition to affording rotational laxity as well as controlling
the degree of rotational laxity as a function of the degree of
flexure of the leg, the construction of the hinge in the embodiment
of the prosthesis shown in the drawings affords anterior-posterior
laxity, which is also a characteristic of the anatomical knee
joint. Thus, the prosthesis of the invention permits restoration of
function in a manner reasonably duplicating the functions of the
anatomical knee joint. Nonetheless, the prosthesis restores
stability by controlling and limiting lateral, rotational and
anterior-posterior laxity.
The above-described embodiments of the invention are intended to be
merely exemplary, and those skilled in the art will be able to make
numerous variations and modifications of them without departing
from the spirit and scope of the invention. All such variations and
modifications are intended to be included within the scope of the
invention as defined in the appended claims.
* * * * *