U.S. patent number RE37,277 [Application Number 08/529,671] was granted by the patent office on 2001-07-10 for tibial prosthetic implant with offset stem.
This patent grant is currently assigned to Hayes Medical, Inc.. Invention is credited to James L. Baldwin, Steven I. Whitlock.
United States Patent |
RE37,277 |
Baldwin , et al. |
July 10, 2001 |
Tibial prosthetic implant with offset stem
Abstract
A tibial prosthetic implant includes a base or base plate with
an offset tibial stem. The base includes an inferior surface
adapted to abut a resected surface of a patient's tibia and forms a
base for articulating surfaces adapted to articulate with the
patient's femoral condyles. The longitudinal center axis of the
tibial stem extends from the inferior surface of the base and is
offset from a center of the base. The offset places the stem in
position to extend into the central canal of the tibia so that it
does not substantially interfere with the cortical bone when the
inferior surface of the base substantially abuts and aligns with
the resected surface of the tibia.
Inventors: |
Baldwin; James L. (Portland,
OR), Whitlock; Steven I. (Austin, TX) |
Assignee: |
Hayes Medical, Inc.
(Sacramento, CA)
|
Family
ID: |
25474847 |
Appl.
No.: |
08/529,671 |
Filed: |
September 18, 1995 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
940441 |
Sep 4, 1992 |
05271737 |
Dec 21, 1993 |
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Current U.S.
Class: |
623/20.32 |
Current CPC
Class: |
A61F
2/389 (20130101) |
Current International
Class: |
A61F
2/38 (20060101); A61F 002/38 () |
Field of
Search: |
;623/16,18,20 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Drawing of knee implant (Prouty); date unknown (1 page). .
Drawing of Intermedics Natural Knee System Device; dated 1987 (1
page). .
Drawing of Johnson & Johnson Orthopaedics' P.F.C.; dated 1990
(2 pages)..
|
Primary Examiner: Isabella; David
Attorney, Agent or Firm: Shaffer & Culbertson
Culbertson; Russell D.
Claims
We claim:
1. A tibial prosthetic implant .Iadd.for patients having normally
shaped tibias, the implant .Iaddend.comprising:
(a) a base having an inferior surface adapted to substantially abut
a resected surface of a patient's tibia and a transverse
cross-sectional shape adapted to approximate a peripheral shape of
the resected surface of the tibia;
(b) an articulating portion connected to the base, the articulating
portion of the implant having articulating surfaces opposite the
inferior surface of the base for articulating with a patient's
femoral condyles;
(c) attachment means for securing the base to the resected surface
of the tibia so that the inferior surface of the base substantially
abuts and aligns with the resected surface of the tibia;
.[.and.].
(d) a tibial stem extending from the inferior surface of the base,
the tibial stem having a longitudinal center axis that is offset
anteriorly and medially from a center of the base such that the
stem is in position to extend into the central canal of the
patient's tibia without substantially interfering with the cortical
bone of the tibia when the inferior surface of the base abuts and
aligns with the resected surface of the tibia.[...]. .Iadd.;
and
(e) wherein the longitudinal center axis of the tibial stem is
offset anteriorly from the center of the base a distance that is
between 59% to 68% of a total anterior-posterior medial condyle
depth dimension of the base as measured from an outermost posterior
edge of the base, and wherein the longitudinal center axis of the
tibial stem is offset medially from the center of the base a
distance that is between 52% to 55% of a total medial-lateral width
dimension of the base as measured from an outermost lateral edge of
the base..Iaddend.
2. The tibial prosthetic implant of claim 1 wherein the tibial stem
includes:
(a) a central root portion extending an entire length of the tibial
stem; and
(b) a medial-posterior web extending generally medially and
posteriorly from the stem central root portion, a lateral-posterior
web extending generally laterally and posteriorly from the stem
central root portion, a medial-anterior web extending substantially
medially and anteriorly from the stem central root portion, and a
lateral-anterior web extending generally laterally and anteriorly
from the stem central root portion, each web having an enlarged
portion at an end of the stem proximal to the inferior surface of
the base and a relatively narrower portion at an end of the stem
distal of the base. .[.
3. A tibial prosthetic implant comprising:
(a) a base having an inferior surface adapted to substantially abut
a resected surface of a patient's tibia and a transverse
cross-sectional shape adapted to approximate a peripheral shape of
the resected surface of the tibia;
(b) an articulating portion connected to the base, the articulating
portion of the implant having articulating surfaces opposite the
inferior surface of the base for articulating with a patient's
femoral condyles;
(c) attachment means for securing the base to the resected surface
of the tibia so that the inferior surface of the base substantially
abuts and aligns with the resected surface of the tibia; and
(d) a tibia stem extending from the inferior surface of the base,
the tibial stem having a longitudinal center axis that is offset
medially from a center of the base such that the stem is in
position to extend into the central canal of the patient's tibia
without substantially interfering with the cortical bone of the
tibia when the inferior surface of the base abuts and aligns with
the resected surface of the tibia..]. .[.
4. The tibial prosthetic implant of claim 3 wherein:
(a) the longitudinal center axis of the tibial stem is offset
medially from the center of the base a distance that is between 52%
and 55% of a total medial-lateral width dimension of the base as
measured from an outermost lateral edge of the base..].
5. A base plate for a modular tibial prosthetic implant .Iadd.for
patients having normally shaped tibias.Iaddend., the base plate
comprising:
(a) a plate of material having an inferior surface adapted to
substantially abut a resected surface of a patient's tibia;
.[.and.].
(b) a tibial stem extending from the inferior surface of the plate,
the tibial stem having a longitudinal center axis that is offset
anteriorly and medially from a center of the plate such that the
stem is in position to extend into the central canal of the
patient's tibia without substantially interfering with the cortical
bone of the tibia when the inferior surface of the plate
substantially abuts the resected surface of the tibia and the
center of the plate aligns with a center of the resected surface of
the tibia.[...]. .Iadd.and
(c) wherein the longitudinal center axis of the tibial stem of
offset anteriorly from the center of the plate a distance that is
between 59% to 68% of a total anterior-posterior medial condyle
depth dimension of the plate as measured from an outermost
posterior edge of the plate, and wherein the longitudinal center
axis of the tibial stem is offset medially from the center of the
plate a distance that is between 52% to 55% of a total
medial-lateral width dimension of the plate as measured from an
outermost lateral edge of the plate..Iaddend..[.
6. The tibial prosthetic implant of claim 5 wherein:
(a) the longitudinal center axis of the tibial stem is offset
anteriorly from the center axis of the plate a distance that is
between 59% to 68% of a total anterior-posterior medial condyle
depth dimension of the plate as measured from an outermost
posterior edge of the plate; and
(b) the longitudinal center axis of the tibial stem is offset
medially from the center of the plate a distance that is between
52% to 55% of a total medial-lateral width dimension of the plate
as measured from an outermost lateral edge of the plate..]. .[.
7. A base plate for a modular tibial prosthetic implant, the base
plate comprising:
(a) a plate of material having an inferior surface adapted to
substantially abut a resected surface of a patient's tibia; and
(b) a tibial stem extending from the inferior surface of the plate,
the tibial stem having a longitudinal center axis that is offset
medially from a center of the plate, such that the stem is in
position to extend into the central canal of the patient's tibia
without substantially interfering with the cortical bone of the
tibia when the inferior surface of the plate substantially abuts
the resected surface of the tibia and the center of the plate
aligns with a center of the resected surface of the tibia..].
.[.
8. The base plate of claim 7 wherein:
(a) a longitudinal center axis of the tibial stem is offset
medially from the center of the plate a distance that is between
52% to 55% of a total medial-lateral width dimension of the plate
as measured from an outermost lateral edge of the plate..].
9. A tibial prosthetic implant of the type having a base that
includes an inferior surface adapted to substantially abut a
resected surface of a patient's tibia so as to support articulating
surfaces that are adapted to articulate with the patient's femoral
condyles, wherein the improvement comprises:
(a) a tibial stem extending from the inferior surface of the base,
the tibial stem having a longitudinal center axis that is offset
anteriorly and medially from a center of the base such that the
stem is in position to extend into the central canal of the
patient's tibia without substantially interfering with the cortical
bone of the tibia when the inferior surface of the base abuts the
resected surface of the tibia and the center of the base aligns
with a center of the resected surface of the tibia.[...].
.Iadd.and
(b) wherein the longitudinal center axis of the tibial stem is
offset anteriorly from the center of the base a distance that is
between 59% to 68% of a total anterior-posterior medial condyle
depth dimension of the base as measured from an outermost posterior
edge of the base, and wherein the longitudinal center axis of the
tibial stem is offset medially from the center of the base a
distance that is between 52% to 55% of a total medial-lateral width
dimension of the base as measured from an outermost lateral edge of
the base..Iaddend..[.
10. The tibial prosthetic implant of claim 9 wherein:
(a) a longitudinal center axis of the tibial stem is offset
anteriorly from the center of the base a distance that is between
59% to 68% of a total anterior-posterior medial condyle depth
dimension of the base as measured from an outermost posterior edge
of the base; and
(b) the longitudinal center axis of the tibial stem is offset
medially from the center of the base a distance that is between 52%
to 55% of a total medial-lateral width dimension of the base as
measured from any outermost lateral edge of the base..]. .[.
11. A tibial prosthetic implant of the type having a base that
includes an inferior surface adapted to substantially abut a
resected surface of a patient's tibia so as to support articulating
surfaces that are adapted to articulate with the patient's femoral
condyles, wherein the improvement comprises:
(a) a tibial stem extending from the inferior surface of the base,
the tibial stem having a longitudinal center axis that is offset
medially from a center of the base such that the stem is in
position to extend into a central canal of the patient's tibia when
the inferior surface of the base abuts the resected surface of the
tibia and the center of the base aligns with a center of the
resected surface of the tibia..]. .[.
12. The tibial prosthetic implant of claim 11 wherein:
(a) the longitudinal center axis of the tibial stem is offset
medially from the center of the base a distance that is between 52%
to 55% of a total medial-lateral width dimension of the base as
measured from an outermost lateral edge of the base..].
Description
BACKGROUND OF THE INVENTION
This invention relates to implants for total knee arthroplasty, and
more particularly, to an improved stemmed tibial prosthetic
implant.
Total knee arthroplasty involves providing new articulating
surfaces for the tibia, femur, and patella. The most common
technique for providing new articulating surfaces for the tibia
involves resecting an upper portion of the tibia and then attaching
a prosthetic implant to the tibia over the resected surface. The
tibial implant includes a base adapted to abut the resected surface
of the tibia and an articulating portion which includes the new
articulating surfaces. The base of the implant has a shape that is
adapted to generally approximate the shape of the resected surface
of the tibia so that the implant generally aligns with .Iadd.and
completely covers .Iaddend.the resected surface when the base is
properly attached to the tibia.
The implant may also include a plurality of pegs extending from the
lower or inferior surface of the base. The pegs are adapted to
extend into the bone of the tibia when the implant is secured to
the resected surface and provide enhanced torsional stability about
the longitudinal axis of the tibia. In addition or alternatively to
pegs, a stem may extend from the inferior surface of the base so as
to extend a substantial distance into the tibia when the implant is
attached over the resected surface. Tibial stems provide further
torsional stability and axial strength by providing more surface
area for contact with the bone. The stem also provides a "keel"
effect to prevent the tibial implant from toggling or wobbling.
The tibia itself comprises an outer layer of hard cortical bone and
a central canal of relatively soft cancellous bone. Much of the
strength of the tibia is provided by the cortical bone. In a
normally shaped tibia, the central canal is offset from the center
of the tibial articulating surfaces or the center of the tibial
plateau. The stems of prior tibial implants .Iadd.used for normally
shaped tibias .Iaddend.were, however, positioned centrally to the
implant base. Although the central location of the stem allowed a
particular implant to be used for either the right or left knee,
this stem position resulted in serious drawbacks. The primary
drawback was that the centrally located stem was substantially
offset from the center of the tibial canal itself when the base of
the implant was aligned with the resected tibial surface. In fact,
stems located centrally to the base occasionally contacted the
posterior .Iadd.and/or lateral .Iaddend.cortical bone of the tibia.
This interference with the cortical bone sometimes prevented the
base of the implant from seating flush against the resected surface
of the tibia, thereby inhibiting adequate initial stability and
fixation which is essential for successful knee arthroplasty.
SUMMARY OF THE INVENTION
It is a broad object of the invention to provide a stemmed tibial
implant that overcomes the above-described problems and others
associated with prior tibial implants .Iadd.used for patients
having normally shaped tibias.Iaddend..
In order to accomplish this object, a tibial implant .[.according
to the invention.]. .Iadd.for use in a normally shaped tibia
.Iaddend.includes a stem that is offset from the center of the
implant base. .Iadd.The offset is within a particular range for use
in a normally shaped tibia. .Iaddend.Similarly to prior tibial
implants, the present implant includes a base, an articulating
portion, and attachment means for securing the implant to a
patient's tibia during the implantation procedure. However, the
tibial implant according to the invention also includes a stem
extending from a point on the inferior surface of the base that is
offset from a center point of the base. This offset enables the
stem to extend into the central tibial canal when the implant is
attached to the .Iadd.normally shaped .Iaddend.tibia with the
inferior surface of the base abutting and aligning with the
resected surface. The offset stem does not extend into or otherwise
interfere with the cortical bone of the tibia. Also, optimal keel
effect is achieved when the stem is central to the condylar weight
bearing surface, providing equally dense bone on either side of the
stem.
.[.The optimum stem offset varies from patient to patient.
Normally, however, the stem is offset anteriorly or forward of the
center point of the base and also medially or to the inside with
respect to the center point of the base..]. .Iadd.Although the
optimum stem offset varies from patient to patient, normally shaped
tibias will accept in the central tibial canal a stem that is
offset anteriorly or forward of the center point of the base and
also medially or to the inside with respect to the center point of
the base. .Iaddend.The anterior offset is preferably in the range
of approximately 59% to 68% of the total anterior-posterior medial
condyle depth dimension as measured from the outermost posterior
edge of the base. The medial offset is preferably in the range of
approximately 52% to 55% of the total base medial-lateral width
dimension, as measured from the outermost lateral edge of the
base.
The stem may be cylindrical in shape, square, or any other shape
desired for the particular application. The preferred stem,
however, has a cruciate transverse cross-sectional shape and
comprises a central root portion with four webs extending at
different angular orientations from the root portion. Each web
extends generally toward a different corner of the base and
includes an enlarged portion proximal to the base and a relatively
narrow portion at the end of the stem distal to the base.
These and other objects, advantages, and features of the invention
will be apparent from the following description of the preferred
embodiments, considered along with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an exploded view in perspective of a stemmed tibial
prosthetic implant embodying the principles of the invention.
FIG. 2 is a top plan view of the prosthetic implant shown in FIG. 1
as attached to a patient's tibia.
FIG. 3 is a front view of the attached prosthetic implant shown in
FIG. 2 with the tibia shown in phantom.
FIG. 4 is a right side view of the attached implant shown in FIG. 2
with the tibia shown in phantom.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring particularly to FIG. 1 a tibial implant 10 embodying the
principles of the invention includes a base 12 and an articulating
portion 14. A stem 16, and preferably, a plurality of pegs 18 are
connected to the base 12 of the implant. As shown in FIGS. 2 and 3,
the base 12 also includes two openings 24 for receiving bone screws
26. The bone screws 26 along with the openings 24 form attachment
means for attaching the implant 10 to the patient's tibia as
described below.
Although the base 12 and articulating portion 14 may be integrally
formed, the preferred form of the invention shown in the figures is
modular with a base plate forming the base 12 and a separate insert
comprising the articulating portion 14. The base plate and insert
are adapted to connect together by suitable means to form the
implant 10. The articulating portion or insert 14 is preferably
formed from a suitable plastic material and includes articulating
surfaces 20 adapted to articulate with the femoral condyles (not
shown) of the patient's knee. The preferred base or base plate 12
comprises a planar plate of a biologically compatible material and
includes a lower or inferior surface 22 to which the stem 16 and
pegs 18 are connected. The stem 16 and pegs 18 may be integrally
formed with the base 12 or may be formed separately and connected
by suitable means.
Referring to FIGS. 2 through 4, the transverse cross-sectional
shape of the base 12 and the shape of the inferior surface 22 are
both adapted to approximate the shape of the tibial plateau 28, and
particularly, a resected surface 30 through the tibial plateau. As
used herein a transverse cross-section through the base 12 is along
a plane extending parallel to the plane of the inferior surface 22.
Also, in order to better approximate the shape of the resected
surface 30, the base 12 is asymmetrical about a medial-lateral
center line. That is, base 12 includes a lateral portion 32 adapted
to approximate the shape of the lateral tibial plateau and a larger
medial portion 34 adapted to approximate the shape of the medial
tibial plateau. This asymmetrical base shape serves to maximize the
base contact or overlap with the cortical bone 36 of the tibia 38
thereby providing strength while minimizing any overhang over the
cortical bone which could interfere with soft tissue extending to
or through the knee joint (not shown). Regardless of the shape of
the base 12, the base has a center 40 that is defined by the
intersection of the medial-lateral center line 42 and the
anterior-posterior center line 44 as shown in FIG. 2.
Referring still to FIGS. 2 through 4, the longitudinal axis 48 of
the stem 16 is offset from the center 40 of the base 12. The offset
enables the stem 16 to extend into the central canal 50 of the
tibia 38 when the base 12 is positioned over the resected surface
30 with the inferior surface 22 abutting and generally aligned with
the resected surface. As shown particularly in FIGS. 3 and 4, the
stem 16 does not extend into the cortical bone 36 of the tibia 38.
Thus, the cortical bone 36 below the resected surface 30 is left
intact with its full natural strength. Also, since the stem 16 does
not extend into the cortical bone 36, the implantation procedure
does not require boring or broaching the hard cortical bone and the
trauma to the area that such boring or broaching necessitates.
Furthermore, optimal keel effect is achieved when the stem is
central to the condylar weight bearing surface, providing equally
dense bone on either side of the stem.
As shown best in FIG. 2, the offset required for the desired stem
position is both medial and anterior from the center 40 of the base
12 for normally shaped tibias. Although the exact offset will vary
from patient to patient, the preferred offset in the medial
direction is between 52% to 55% of the total medial-lateral width
dimension as measured from the outermost lateral edge 54 of the
base 12 along the anterior-posterior center line 42. The preferred
offset in the anterior direction is in a range between 59% to 68%
of the total anterior/posterior medial condyle depth dimension as
measured from the outermost posterior edge 56 of the base 12 along
the medial-lateral center line 44. .Iadd.This medial offset in the
range of 52% to 55% and anterior offset in the range between 59% to
68% produces the desired stem position in the cancellous bone of
the tibial central canal for most normally shaped tibias such as
the tibia illustrated in FIGS. 3 and 4..Iaddend.
Although any stem shape may be employed in a tibial implant
embodying the principles of the invention, the preferred stem 16
forms a cruciate shape. The cruciate-shaped stem 16 includes a
center root portion 60 and a plurality of longitudinally aligned
webs extending from the root at different angular orientations
about the stem longitudinal axis 48. In particular, the stem 16
includes a medial-posterior web 62, a lateral-posterior web 64, a
medial-anterior web 66, and a lateral-anterior web 68. Each web
includes an enlarged portion 70 at the end proximal to the base 12,
narrowing down continuously to a relative narrower portion 72 at
the end of the stem distal to the base. Also, as shown best in FIG.
2, the enlarged portions 70 of the lateral-posterior and
medial-posterior webs 64 and 62 extend further from the center root
60 of the stem 16 than the enlarged portions of the
lateral-anterior and medial-anterior webs 68 and 66. These web
shapes generally follow the internal contour of the central canal
50 of the tibia to maximize contact with the cancellous bone in the
canal.
Implanting the stemmed tibial implant 10 embodying the principles
of the invention includes first resecting an upper portion of the
patient's tibia 38 to remove the natural articulating surfaces and
form a suitable resected surface 30. The resected surface 30 is
preferably planar and extends substantially transversely to the
longitudinal axis of the tibia 38 in the anterior-posterior view
and parallel to the anatomic posterior slope of the tibia in the
medial-lateral view of the tibia.
Once the desired resected surface 30 is produced, the base 12 of
the implant 10 shown in FIGS. 1 through 4 is positioned with the
inferior surface 22 abutting and generally aligned with the
resected surface. The base 12 is fixed in place with bone screws 26
extending through the screw openings 24 in the base. With the
inferior surface 22 of the base 12 abutting the resected surface 30
of the patient's tibia 38 and with the base properly aligned, the
stem 16 extends into the central canal 50 of the tibia and does not
interfere with the cortical bone 36. Although some broaching will
be required to produce an opening for the stem 16, the offset
position of the stem will necessitate little, if any, broaching of
the cortical bone 36, only broaching of the much softer cancellous
bone in the central tibial canal 50.
The above described preferred embodiments are intended to
illustrate the principles of the invention, but not to limit the
scope of the invention. Various other embodiments and modifications
to these preferred embodiments may be made by those skilled in the
art without departing from the scope of the following claims. For
example, although the embodiment shown in the drawings includes a
plurality of pegs 18, an implant embodying the principles of the
invention may include only the offset stem 16. Also, although not
shown specifically in the drawings, the inferior surface 22 of the
base 12, the pegs 18 if present, and the stem 16 may all include a
layer of porous material adapted to provide enhanced bonding to the
bone.
* * * * *