Title: Junction for a modular implant
Abstract: The present invention provides an improved junction for modular implant components.
Patent Number: 6,905,515 Issued on 06/14/2005 to Gilbertson
| Inventors:
|
Gilbertson; Leslie N. (Warsaw, IN)
|
| Assignee:
|
Zimmer Technology, Inc. (Chicago, IL)
|
| Appl. No.:
|
748824 |
| Filed:
|
December 27, 2003 |
| Current U.S. Class: |
623/22.4; 623/22.42; 623/23.46 |
| Intern'l Class: |
A61F 002/32 |
| Field of Search: |
623/2211,224,224.1,224.2,224.3,231.4,231.5,233.5,234.6,234.4,234.3
|
References Cited [Referenced By]
U.S. Patent Documents
Primary Examiner: Snow; Bruce E
Attorney, Agent or Firm: Reeves; Cary, Feuchtwang; Jonathan
Claims
1. A modular implant for insertion into a femur adjacent a hip joint, the implant comprising:
a proximal body component having a top end for engaging the hip joint, a bottom
end for insertion into the femur, a medial side, a lateral side, a neck formed
adjacent the top end and a bore formed into the bottom end, the bore having a bore
opening and an interior surface forming a female side of a male/female junction,
the bore having a longitudinal junction axis;
a stem component having a first end for engaging the proximal body component
a second end for insertion into the femur, and a projection formed adjacent the
first end, the projection having an exterior surface forming a male side of the
male/female junction, the projection being engageable with the bore in male/female
seating arrangement along the junction axis, the male and female sides contacting
one another adjacent the bore opening, the contact between the male and female
sides adjacent to the bore opening on the lateral side being offset longitudinally
toward the top end relative to the contact between the male and female sides adjacent
the bore opening on the medial side.
2. The implant of claim 1 wherein the bore and the projection form complimentary
tapers, the tapers narrowing from the bottom end toward the top end and from the
second end toward the first end.
3. The implant of claim 1 wherein the proximal body component has an exterior
surface spaced from the interior surface of the bore, the exterior and interior
surfaces defining a wall between them, the wall having a wall thickness that increases
over a portion of the wall between the bottom end and the top end as the bore taper
diverges inwardly from the exterior wall in a direction generally parallel to the
junction axis such that the contact between the male and female sides adjacent
the bore opening on the lateral side is offset in the direction of increasing wall thickness.
4. The implant of claim 1 further comprising a femoral head component supported
on the neck of the proximal body component and an acetabular component engageable
with the femoral head component.
5. The implant of claim 1 wherein the bore opening is transverse to the junction axis.
6. The implant of claim 1 wherein the implant further includes a joint load receiving
head and further wherein the medial side is generally in compression in use and
the lateral side is generally in tension in use such that the contact between the
male and female sides adjacent the bore opening is offset longitudinally toward
the top end on the tensile side of the implant.
7. The implant of claim 1 wherein the proximal body component has an exterior
surface spaced from the interior surface of the bore, the exterior and interior
surfaces defining a wall between them, the wall having a stiffness that increases
over a portion of the wall between the bottom end and the top end in a direction
generally parallel to the junction axis, the projection further having a stiffness,
such that the contact between the male and female sides adjacent to the bore opening
on the lateral side is offset in the direction of increasing wall stiffness.
8. The implant of claim 1 wherein the bore opening adjacent the lateral side
is offset radially away from the projection.
Description
BACKGROUND
Medical implants to replace or augment various parts of the mammalian body
have been successfully used to reduce pain and improve function. For example, orthopaedic
implants for replacing portions of bones and joints damaged by disease and/or trauma
often eliminate pain and/or increase mobility. Orthopaedic implants for hips, knees,
shoulders, ankles, elbows, wrists, the digits of the hands and feet, vertebral
bodies, spinal discs, and other bones and joints have been developed. Many medical
implants are made more versatile by providing them as separate modular components
that can be combined to form an implant suited to a particular patient's condition.
Where such modular components are supplied, a means for attaching them to one another
is provided.
SUMMARY
The present invention provides a junction for modular implant components.
In one aspect of the invention, a modular joint implant includes a male/female
junction between first and second joint components. The first component includes
a bore having a longitudinal junction axis and a bore opening. The second component
includes a projection engageable with the bore in male/female seating arrangement.
A first portion of the bore opening is offset axially relative to a second portion
of the bore opening.
In another aspect of the invention, the first portion is offset in a direction
of increasing stiffness of the wall surrounding the bore.
In another aspect of the invention, the first portion is offset in a direction
of increasing wall thickness.
In another aspect of the invention, the first portion is offset on a side of
the
implant that is generally in tension when the implant is loaded.
In another aspect of the invention, a modular joint implant includes a male/female
junction having a side that is predominately in compression in use and a side that
is predominately in tension in use. The implant includes a first component including
a bore having a bore opening and an interior surface forming a female side of the
male/female junction. The first component further has an exterior surface. The
interior and exterior surfaces define a wall between them having a wall thickness.
The wall thickness on the tensile side of the implant being greater than the wall
thickness on the compressive side of the implant.
In another aspect of the invention, a modular joint implant includes a male/female
junction having a side that is predominately in compression in use and a side that
is predominately in tension in use. The implant includes a first component including
a bore having a bore opening and an interior surface forming a female side of the
male/female junction. The bore has a side on the tensile side of the implant that
is shifted axially relative to a side of the bore on the compressive side of the implant.
These and other aspects of the invention will be described in reference to
the appended drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Various embodiments of the present invention will be discussed with reference
to the appended drawings. These drawings depict only illustrative embodiments of
the invention and are not to be considered limiting of its scope.
FIG. 1 is an exploded perspective view of a modular implant junction according
to the present invention;
FIG. 2 is a side section view of the modular implant junction of FIG. 1; and
FIG. 3 is a side section view of an optional configuration of the lateral side
of the modular implant junction of the present invention.
DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS
Embodiments of a junction for a modular implant are applicable to a variety
of implants for use throughout the body. A femoral hip stem has been used to illustrate
the invention. However, the invention may also be applied to various other implants
including orthopaedic implants for hips, knees, shoulders, ankles, elbows, wrists,
the digits of the hands and feet, vertebral bodies, spinal discs, and other suitable implants.
FIGS. 1–2 depict a modular femoral hip implant
10 for replacing
the proximal head and neck of a femur of a hip joint that has been damaged due
to injury or disease. In use, the proximal head and neck are surgically removed
and the femoral hip implant
10 is inserted into the proximal femur. The
femoral hip implant
10 supports a femoral head
12 that may be a modular
and separate component as shown. Optionally, the femoral head
12 may be
integral to the femoral hip implant
10. An acetabular component
14
may be implanted in the acetabulum of the pelvis to articulate with the femoral
head
12. Optionally, the femoral head
12 may articulate with the
natural acetabulum. The femoral hip implant
10 has a medial aspect
16
and a lateral aspect
18. When the patient loads the joint, such as by standing,
walking, or other activities, forces are transmitted to the femoral hip implant
10 through the head
12. These forces tend to create a bending moment
that places the medial aspect
16 of the femoral hip implant in compression
and the lateral aspect
18 in tension.
The femoral hip implant
10 may include modular components such as a proximal
body
20 and a stem
22. The proximal body
20 has a top end
24 and a bottom end
26. A neck
28 extends upwardly and medially
from the top end
24 to support the femoral head
12 for articulation
with the acetabular component
14. The proximal body
20 and stem
22
include a male/female junction for holding them together. In the illustrative embodiment,
the female side of the junction is depicted in the proximal body
20 and
the male side of the junction is depicted on the stem
22. It is contemplated
that the male/female portions may be reversed and still fall within the scope of
the invention. The proximal body
20 includes a bore
30 (FIG. 2) having
a bore opening
32 and an interior surface
34 forming the female side
of the male/female junction. The bore
30 has a longitudinal junction axis
36. The proximal body
20 has an exterior surface
38 spaced
from the interior surface
34 of the bore
30. The exterior
38
and interior
34 surfaces define a wall
40 between them. The wall
40 has a wall thickness
42 that may be constant or that may increase
from the bore opening
32 toward the top end
24 as shown in FIG. 2.
An increasing wall thickness may be accomplished by tapering the bore such that
it narrows from the bore opening
32 toward the top end
24 as in the
illustrative embodiment. A tapered bore can be made self-locking as is known in
the art.
The stem
22 includes a bottom end
44 and a top end
46. The
bottom end
44 is configured for insertion into the intramedullary canal
of the patient's femur. The top end
46 includes a projection
48 having
an exterior surface
50 forming the male side of the male/female junction.
The projection
48 is engageable with the bore
30 in male/female seating
arrangement along the junction axis
36. A threaded stud
33 extends
from the projection
48 and is received by a counter bore
35 formed
in the proximal body
20. A nut
37 threads onto the stud
33
to secure the male/female junction.
When the femoral hip implant
10 is loaded, the medial aspects of the
proximal body
20 and stem
22 are placed in compression and the lateral
aspects of the proximal body
20 and stem
22 are placed in tension.
Due to differences in the bending stiffness of the proximal body
20 and
stem
22 in the region of the male/female junction, the bore wall
40
may move relative to the exterior surface
50 of the projection
48.
Cyclic loading can lead to fretting between the interior surface
34 of the
bore
30 and exterior surface
50 of the projection
48. This
may be more problematic on the tension side since tensile forces may initiate and
propagate fatigue cracks.
The magnitude of the fretting motion is related to the relative stiffness of
the male and female parts of the junction. The relative motion at the opening
32
of the bore
30 is an accumulation of the relative motion along the entire
length of the junction. This accumulated relative motion may be decreased by decreasing
the length along which the relative motion accumulates on the tensile side of the
junction. However, it is ineffective to simply make the junction shorter, because
this will create higher stresses on the smaller diameter male cross section where
such a shortened junction would end. However, by shortening only the tensile side,
the compressive side is still supported and the tensile stress at the end of the
tensile side increases only slightly while the relative fretting motion decreases
significantly. In the illustrative embodiment, a lateral portion
52 of the
bore opening
32 is offset axially upwardly relative to a medial portion
54. This offset, or relieved, portion
52 can be created by stepping
up the lateral portion
52, sloping up the lateral side such that the bore
opening
32 is transverse to the junction axis
36, or by forming the
bore opening in some other suitable shape.
Fretting in the male/female junction can also be reduced by better matching
the stiffness of the male and female sides of the male/female junction. The present
investigators have found that one way to better match the stiffness of the male
and female sides is to increase the stiffness of the bore wall
40 adjacent
the bore opening
32 on the tensile side of the implant
10. Increasing
the stiffness can be accomplished by increasing the outer diameter of the proximal
body
20 adjacent the bore opening
32 to move material radially away
from the junction axis
36 such that the bending moment of inertia is increased.
Increasing this stiffness can also be accomplished by increasing the wall thickness
of the proximal body
20 at the junction of the proximal body
20 and
projection
48. In the illustrative embodiment, the bore
30 is tapered
so that it narrows proximally and the wall thickness
42 increases proximally.
By offsetting a lateral portion
52 of the bore opening
32 axially
upwardly relative to a medial portion
54 the wall thickness on the lateral
side
18 of the bore opening
32 is increased. This offset, or relieved,
portion
52 can be created by sloping the lateral side such that the bore
opening
32 is transverse to the junction axis
36 as shown, by stepping
up the lateral portion
52, or by forming the bore opening in some other
suitable shape. In the illustrative embodiment, the projection
48 forming
the male side of the junction narrows upwardly. Thus, moving the lateral portion
52 upwardly also moves the lateral interface to an area of decreased stiffness
of the projection
48 to further match the stiffness of the male and female
portions laterally. Finally, by moving the lateral side upwardly while having the
medial, anterior, and posterior sides of the junction extend further downwardly,
the fatigue properties of the lateral side are improved while the overall interface
of the male and female parts is kept relatively large to facilitate secure engagement
between them.
Another way increase the stiffness of the bore wall
40 is to enlarge
the bore
30 on just the tensile side of the junction as shown in FIG. 3
such that the bore wall is offset away from the projection
48. This has
the same effect as moving the tensile side axially upwardly as described above.
Other ways of better matching the stiffness of the male and female sides of the
junction may also be used and are considered within the scope of this invention.
It will be understood by those skilled in the art that the foregoing has described
illustrative embodiments of the present invention and that variations may be made
to these embodiments without departing from the spirit and scope of the invention
defined by the appended claims.
*
