U.S. patent application number 16/299911 was filed with the patent office on 2019-07-04 for compliant anti-resorption implant.
The applicant listed for this patent is Smith & Nephew, Inc.. Invention is credited to Ryan L. Landon.
Application Number | 20190201205 16/299911 |
Document ID | / |
Family ID | 48799588 |
Filed Date | 2019-07-04 |
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United States Patent
Application |
20190201205 |
Kind Code |
A1 |
Landon; Ryan L. |
July 4, 2019 |
COMPLIANT ANTI-RESORPTION IMPLANT
Abstract
Systems, devices, and methods for providing orthopedic implants
that reduce the negative effects of stress shielding on surrounding
bone structure. The orthopedic implants include two portions, a
shell portion that forms an articulation interfaces and an
intermediate portion that forms a bone interface. The shell portion
is designed to reduce absorption of Carticulation forces and evenly
distribute incident forces to the intermediate portion. The
intermediate portion is designed to form a strong interface with
native bone and transmit forces from the shell into the bone.
Inventors: |
Landon; Ryan L.; (Olive
Branch, MS) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Smith & Nephew, Inc. |
Memphis |
TN |
US |
|
|
Family ID: |
48799588 |
Appl. No.: |
16/299911 |
Filed: |
March 12, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14368341 |
Jun 24, 2014 |
10226347 |
|
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PCT/US13/20468 |
Jan 7, 2013 |
|
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16299911 |
|
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|
61587854 |
Jan 18, 2012 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A61F 2002/30971
20130101; A61F 2/3094 20130101; A61F 2002/3082 20130101; A61F
2002/30014 20130101; A61F 2002/30985 20130101; A61F 2002/30006
20130101; A61F 2002/30011 20130101; A61F 2/3859 20130101; A61F
2002/30879 20130101 |
International
Class: |
A61F 2/38 20060101
A61F002/38; A61F 2/30 20060101 A61F002/30 |
Claims
1.-18. (canceled)
19. A method of manufacturing an orthopedic implant, comprising:
forming a shell region of the orthopedic implant; and forming an
intermediate region of the orthopedic implant, wherein the
intermediate region has different mechanical properties than the
shell region.
20. The method of claim 19, wherein the shell region and
intermediate region are formed by rapid manufacturing.
21. The method of claim 19, wherein the shell region is formed
using a different material than the intermediate region.
22. The method of claim 19, wherein the intermediate region is
formed by depositing material at a different density than the shell
region.
23. The method of claim 19, wherein the intermediate region is
formed as a porous region, and the shell region is formed as a
solid region.
24. The method of claim 19, wherein forming the shell region
comprises forming an outer articulation surface; wherein the shell
region has a plurality of cross-sections; and wherein each
cross-section is taken perpendicular to the outer articulation
surface.
25. The method of claim 24, wherein each cross-section comprises a
cross-sectional profile and a cross-sectional area; and wherein the
method further comprises varying the cross-sectional profiles among
the plurality of cross-sections while maintaining uniformity among
the cross-sectional areas.
26. The method of claim 24, further comprising: selecting a
cross-sectional area; and selecting a plurality of cross-sectional
profiles, wherein each cross-sectional profile has the selected
cross-sectional area, and wherein the plurality of cross-sectional
profiles have different shapes; and wherein forming the shell
region comprises forming each of the cross-sections with a
corresponding and respective cross-sectional profile and the
selected cross-sectional area.
27. The method of claim 24, wherein the plurality of cross-sections
comprises a first cross-section and a second cross-section; wherein
the first cross-section comprises a first cross-sectional profile
and a first cross-sectional area; wherein the second cross-section
comprises a second cross-sectional profile and a second
cross-sectional area; wherein the first cross-sectional profile is
different from the second cross-sectional profile; and wherein the
first cross-sectional area is equal to the second cross-sectional
area.
28. The method of claim 27, wherein the first cross-sectional
profile has a first thickness dimension and a first width
dimension; wherein the second cross-sectional profile has a second
thickness dimension and a second width dimension; wherein the first
thickness dimension is greater than the second thickness dimension;
and wherein the first width dimension is less than the second width
dimension.
29. The method of claim 24, wherein the plurality of cross-sections
comprises: a first cross-section of an anterior portion of the
shell region, the first cross-section having a first
cross-sectional area; a second cross-section of a medial condyle
portion of the shell region, the second cross-section having a
second cross-sectional area; and a third cross-section of a lateral
condyle portion of the shell region, the third cross-section having
a third cross-sectional area; and wherein the first cross-sectional
area is equal to the sum of the second cross-sectional area and the
third cross-sectional area.
30. A method of manufacturing an orthopedic implant, comprising:
selecting a uniform cross-sectional area for a shell region of the
implant; selecting a plurality of different cross-sectional
profiles for the shell region, wherein each of the cross-sectional
profiles has the selected uniform cross-sectional area; and forming
the shell region, wherein the shell region includes an outer
articulation surface and a plurality of cross-sections taken
perpendicular to the outer articulation surface, and wherein each
of the cross-sections has the uniform cross-sectional area and a
corresponding and respective one of the plurality of different
cross-sectional profiles.
31. The method of claim 30, further comprising forming an
intermediate region of the orthopedic implant, the intermediate
region comprising a shell interface and a bone interface opposite
the shell interface.
32. The method of claim 31, wherein forming the shell region and
forming the intermediate region comprise forming the intermediate
region with different mechanical properties than the shell
region.
33. The method of claim 30, wherein a first of the cross-sections
extends through an anterior portion of the shell such that the
first of the cross-sections is contiguous, and wherein a second of
the cross-sections extends through a lateral condyle of the shell
and a medial condyle of the shell such that the second of the
cross-sections is non-contiguous.
34. The method of claim 30, wherein selecting the plurality of
different cross-sectional profiles comprises: selecting a first
cross-sectional profile of the plurality of different
cross-sectional profiles with a first width dimension and a first
thickness dimension; and selecting a second cross-sectional profile
of the plurality of different cross-sectional profiles with a
second width dimension and a second thickness dimension; and
wherein the first width dimension is greater than the second width
dimension; and wherein the first thickness dimension is less than
the second thickness dimension.
35. A method of manufacturing an orthopedic implant, comprising:
forming a shell region comprising an outer articulation surface, a
first condylar region, a second condylar region, and a junction of
the first condylar region and the second condylar region; and
forming an intermediate region having a bone interface; wherein
forming the shell region comprises forming a strengthening rib; and
wherein forming the strengthening rib comprises: forming a first
portion of the strengthening rib such that the first portion
extends in a medial-lateral direction at the junction of the first
condylar region and the second condylar region; and forming a
second portion of the strengthening rib such that the second
portion extends in an inferior-superior direction at the first
condylar region.
36. The method of claim 35, wherein forming the strengthening rib
further comprises forming a connecting portion connecting the first
portion and the second portion.
37. The method of claim 36, wherein the connecting portion is
curved.
38. The method of claim 35 further comprising forming at least one
additional strengthening rib, and wherein the at least one
additional strengthening rib comprises at least one of a
medial-lateral strengthening rib and an inferior-superior
strengthening rib.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to U.S. Provisional
Application No. 61/587,854, file Jan. 18, 2012, which is hereby
incorporated by reference herein in its entirety.
BACKGROUND
[0002] Orthopedic implants are used to strengthen or replace joints
that typically experience high levels of stress and wear. In a
primary replacement surgery, joints that have experienced trauma or
have been worn to a degree that inhibits normal functioning of the
joint are replaced or reinforced with stronger wear-resistant
implants. In a revision surgery, primary implants that have either
been unsuccessful or have worn to a degree that inhibits their
function are supplemented or replaced with revision implants. The
constant daily stress and wear at the replacement joints,
especially for weight-bearing knee replacements, require both
primary and revision implants to be strong enough to withstand
significant abuse.
[0003] Conventional knew implants are made of rigid, high-modulus
metals used to provide ample support and to withstand the high
stresses typically present at the knee joint. High-strength metals
such as titanium, stainless steel, zirconium, cobalt-chrome alloy,
and other metal alloys are often used in femoral and tibial
implants to achieve the strength and stability required. These
metal components exhibit higher rigidity and higher modulii of
elasticity than the bone structure into which they are implanted in
order to withstand the significant stresses that are incident on
the knee joint during everyday activity. The size of the implants
and the metal materials used to make them results in a heavy
implanted component that a patient must adjust to, and patients may
experience discomfort from "feeling the weight" of the metal
component after implantation.
[0004] With the rigid materials and the large implant designs of
traditional knee implant components, the knee replacements bear a
majority of stresses incident on a patient's joint after
implantation. As a result, the bone surrounding the implant often
experiences lower stresses than normal. The decreased stresses
cause bond resorption as the bone breaks down to adjust and
accommodate the decreased need for support from the bone at the
joint. In response to the changes in normal loading, a bone
remodels itself to either build up more mass to strengthen the bone
or break down bone mass to weaken the bone. This process is known
as Wolff's low, and it causes a normal bone to become stronger if
loading on the bone increases or weaker if loading on the bone
decreases.
[0005] Bone resorption around an implant can have significant
negative effects, as it decreases the integrity of the bone and its
ability to hold the implant solidly in place. This complication is
known as stress shielding as the high-strength implant "shields"
the surrounding bone from stress and loading that is necessary to
cause the bone to maintain its strength. The resulting bone
resorption causes patients to experience pain and feel the weight
of their knee implants during their everyday activities. In some
cases, the bone resorption resulting from stress shielding can even
cause an implant to fail completely, necessitating a second
revision knee replacement surgery.
SUMMARY
[0006] Disclosed herein are systems, devices, and methods for
providing an orthopedic implant that reduces the negative effects
of stress shielding on surrounding bone structure. In particular,
the systems, devices, and methods provide orthopedic implants that
transmit forces incident on the implant to surrounding bone
structure. The systems, devices, and methods reduce points of
device stress concentration and provide uniform distribution of the
stress transmitted to the surrounding bone.
[0007] In certain embodiments, an orthopedic implant includes a
shell region having an outer articulation surface and an
intermediate region having a bone interface and different
mechanical properties than the shell region. In certain
implementations, the intermediate region has a different modulus of
elasticity, density, or porosity than the shell region. In certain
implementations, the intermediate region is comprised of a
different material than the shell region.
[0008] In certain implementations, the shell region includes
strengthening ribs. The strengthening ribs may be disposed at an
anterior portion of the shell, on condylar portions of the shell,
at a junction between condylar portions of the shell, or a
combination thereof. The ribs may be disposed on a surface of the
shell region, or may be disposed in an internal portion of the
shell region.
[0009] In certain implementations, the cross-sectional area of the
shell region is substantially uniform. In such implementations, the
thickness of the shell region is varied throughout the shell. In
other implementations, the thickness of the shell region is
substantial uniform.
[0010] In certain implementations, the intermediate region is
movable relative to the shell region. The intermediate region may
also be porous, with the bone interface forming a porous bone
ingrowth surface. The bone interlace is shaped to accommodate a
standard resected bone geometry, and contours of the bone interface
are different than an inner contour of the shell.
[0011] In certain embodiments, a method of manufacturing an
orthopedic implant includes forming a shell region of the
orthopedic implant and forming an intermediate region of the
orthopedic implants, the intermediate region having different
mechanical properties than the shell region. In certain
implementations, the shell region and intermediate region are
formed by rapid manufacturing, and the intermediate region may be
formed of a different material or may have a different density or
porosity than the shell region.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The foregoing and other objects and advantages will be
appreciated more fully from the following further description
thereof, with reference to the accompanying drawings. These
depicted embodiments are to be understood as illustrative and not
as limiting in any way:
[0013] FIG. 1 shows a perspective view of an illustrative femoral
implant;
[0014] FIG. 2 shows a perspective view of a cross-section of the
femoral implant shown in FIG. 1;
[0015] FIG. 3 shows a perspective view of an illustrative femoral
implant having internal channels;
[0016] FIGS. 4A and 4B show a finite element analysts of two
illustrative implant components;
[0017] FIG. 5 shows a perspective view of a shell portion of the
femoral implant shown in FIG. 1;
[0018] FIGS. 6A-C show cross-section views of an illustrative
femoral implant shell having a substantially uniform
cross-sectional area;
[0019] FIGS. 7A-C show cross-section views of an illustrative
femoral implant shell having a substantially uniform thickness;
[0020] FIG. 8 shows a perspective view of an illustrative femoral
implant shell having anterior reinforcing ribs; and
[0021] FIG. 9 shows a perspective view of an illustrative femoral
implant shell having condylar reinforcing ribs.
DETAILED DESCRIPTION
[0022] To provide an overall understanding of the systems, devices
and methods described herein, certain illustrative embodiments will
now be described. For the purpose of clarity in illustration, the
systems and methods will be described with respect to orthopedic
knee implants. It will be understood by one of ordinary skill in
the art that the systems, devices and methods described herein may
be adapted and modified as is appropriate and that the systems,
devices and methods described herein may be employed in other
suitable applications, such as for other types of joints and
orthopedic implants, and that other such additions and
modifications will not depart from the scope hereof.
[0023] FIG. 1 shows a femoral component 100 of a knee replacement
implant. In contrast to conventional implants that are made of a
solid block of metal, the femoral component 100 is composed of two
portions: a shell 102 and an intermediate portion 104. Together
these two portions form a single implant component with an
articulation surface 106 that contacts and articulates against a
tibial implant component and a bone interface that contacts and
affixes to a patient's femoral bone.
[0024] In particular, the shell 102 forms the outer portion of the
femoral component 100 that interfaces with a tibial implant
component. The outer articulation surface 106 is shaped to allow
for natural movement of the femur during flexion and extension of
the patient's femur and tibia after implantation. The inner portion
of the shell 102 is designed to reduce the stress-shielding
complications that can be caused by conventional knee replacements.
In particular, the shell 102 has a degree of compliance and
flexibility to allow (or transmittal of forces incident on the
femoral component 100 to the bone into winch the femoral component
is implanted, thus decreasing bone resorption and encouraging the
maintenance of strong bone around the implant. The contours of the
inner surface of the shell 102 are designed such that stress
concentration points are reduced in order to more evenly spread the
stresses and loads incident on the knee to the bone surrounding the
implant. Use shell 102, while being compliant to a degree, is also
rigid and strong enough to maintain its integrity and keep the
anatomical shape of the distal end of the femur under the wear and
stresses caused by normal flexion and extension of the knee.
[0025] The intermediate portion 104 of the femoral component 100
forms an interface between the shell 102 and a patient's bone into
which the femoral component 100 is implanted. The intermediate
portion 104 may be integrally formed with the shell 102 or may be
formed as a separate component that is bonded to an interior
surface of the shell 102. In certain implementations, the
intermediate portion 104 is a separate component that is bonded
with the shell 102 in a manner that allows a small amount of
relative movement between the intermediate portion 104 and the
shell 102. For example, the intermediate portion 104 may be
intermittently bonded to the shell 102 to allow for a small amount
of flexing or movement between bonded areas. The intermediate
portion 104 may also include channels or other cutout areas at an
interface with the shell 102 that create small gaps, for example at
areas of high bending, to allow the shell and intermediate portion
to flex and move relative to each other.
[0026] In addition to the shapes of the shell 102 and intermediate
portion 104 and the interface between the two portions of the
component 100, the materials used for these two portions are
selected to reduce stress shielding effects. In certain
implementations, the materials used are selected such that the
overall modulus of the femoral component 100 is near the modulus of
the native bone into which the component is implanted. The shell
102 is made of a material that is able to maintain integrity under
articulation forces while still transmitting those forces into the
intermediate portion. For example, the shell 102 may be made of
titanium, titanium alloy, stainless steel, cobalt-chromium,
tantalum, zirconium alloy, other metallic alloys, or any other
suitable material. The intermediate portion 104 is made of a
material that does not significantly absorb forces transmitted
through the shell and passes these forces on into the surrounding
bone. The material also has strength to maintain its integrity and
provide a solid interface with the bone. For example, the
intermediate portion 104 may be made of polyethylene,
polyetheretherketone (PEEK), polyurethane, porous metals such as
tantalum, titanium, cobalt-chromium, stainless steel, starch,
ceramic, hydroxyapatite, glass, or any other suitable material. A
polymer intermediate portion may be made of polyetheretherketones
(PEEK), also referred to as polyketones, poly-alpha-hydroxy acids,
polycapropactones, polydioxanones, polyesters, polyglycolic acid,
polyglycols, polylactides, polylactic acid, poly-D,L-lactic acid,
poly-L,L-lactic acid, polyorthoesters, polyphosphates,
polyphosphoesters, polyphosphonates, polysaccarides,
polycaprolactone, polypropylene, fumarate, polytyrosine carbonates,
or polyurethanes. In some instances, it may be desirable for the
bone to biologically affix directly to the implant without the use
of bone cement. In these cases, it may be desirable to form the
intermediate portion from metal powder or wires that are bonded or
sintered together to form a suitable ingrowth surface.
[0027] The materials selected for the shell 102 and intermediate
portion 104 can provide an implant that has mechanical properties
that are substantially similar to the properties of the native bone
that the implant replaces. The native bone is made up of two types
of bone, a dense outer cortical bone and a spongy inner cancellous
bone. The materials used to make the shell 102 and intermediate
portion 104 are selected to match the mechanical properties of the
cortical bone and cancellous bone, respectively, to provide an
implant that mimics the properties of the replaced bone more
closely than a conventional implant. In addition to matching the
mechanical properties, the materials used can also provides an
implant that is lighter than a large metal conventional implant and
can reduce a patient's discomfort, especially in the early stages
following implantation surgery.
[0028] In some embodiments, the shell 102 and the intermediate
portion 104 are manufactured as two separate components that are
bonded together to form the femoral component 100. In sme
embodiments, the femoral component 100 is formed as on unitary
component having two distinct regions forming the shell 102 and the
intermediate portion 104. For example, rapid manufacturing
processes can be used to form the femoral component 100 as a
unitary component with distinct portions that have different
properties and may be made of different materials. Rapid
manufacturing machinery can create the distinct portions by
changing the density of a material or type of material deposited to
create the shell 102 and the intermediate portion 104. In some
embodiments, a stronger material is deposited in the shell region,
and a different, weaker material is deposited in the intermediate
portion. In some embodiments, the same material is used but is
deposited more densely in the shell region than in the intermediate
portion. In either of these approaches, the shell 102 may be formed
as solid region while the intermediate portion 104 is formed as a
weaker porous region that encourages bone ingrowth.
[0029] One or both of the shell 102 and intermediate portion 104
can also be formed with powdered metal by laser sintering or a
similar process. Laser sintering can produce a shell or
intermediate portion with multiple powdered metal materials or with
powdered metal deposited at different densities in different areas
of the implant. This approach allows for implant designs that are
customized to provide varying local mechanical properties on a
micro scale, for example by depositing higher modulus or denser
materials in areas of a shell that experience higher stresses,
while maintaining the macro-scale design and stress transmission
properties of the implant.
[0030] The intermediate portion 104 includes a bone interface
formed by faces 108a-c. As shown in FIG. 1, this bone interface is
shaped to accommodate a standard shape of a patient's resected
femoral bone following preparation of the bone using common
techniques for knee replacement. In contrast to the shape of the
prepared bone, the inner contours of the shell 102 are smooth and
continuous, and thus the intermediate portion 104 serves as an
intermediary that facilitates connection of the smooth contoured
shell 102 to a patient's bone that is resected during surgery. The
femoral component 100 may be cemented onto the patient's bone or
may be implanted in a cementless procedure. In cementless
implantations, the intermediate portion 104 is porous and forms an
ingrowth surface that allows the surrounding bone to grow into the
intermediate portion 104 to form a solid fixation of the femoral
component 100 and surrounding bone.
[0031] In addition to providing desirable stress transmission
properties, the shell 102 and intermediate portion 104 form an
implant that creates a solid fixation between the faces 108a-e and
a patient's bone during implantation. Because the modulus of the
femoral component 100 is lower than a conventional solid implant,
the component 100 is more compliant than the conventional solid
implant. The compliance of the component 100 allows the implant to
flex more easily when it is press-fit onto a patient's prepared
bone. This flexibility allows for looser tolerances on the
precision of bone cuts, as the implant can flex to accommodate a
patient's bone without requiring a high press-fit force that may be
required with a conventional implant. As a result of accommodating
the bone and providing a better initial fit, the component 100
provides a strong permanent fixation between the bone and the
component, for example by providing a "squeezing" pressure to the
native bone that encourages ingrowth of the bone into the
implant.
[0032] The two portion 102 and 104 of the femoral component 100
shown in FIG. 1 help reduce the effects of stress shielding and
distribute loading forces and stresses over the femoral component
100. In particular, the shape and contours of the interior surface
of the flexible shell 102 are designed to avoid sharp transitions
and points where stress can be concentrated, leading to uneven
distribution of incident forces over surrounding bone. The smooth,
continuous contour of the shell 102 is shown in FIG. 2, which
depicts a cross-section of the femoral component 100. The interior
of the shell 102 and the intermediate portion 104 are shown,
exposing the interface between the surface 110 of the intermediate
portion 104 and the surface 112 of the shell 102. In certain
implementations, the shell 102 and the intermediate portion 104 are
integrally formed, for example by the use of rapid manufacturing
techniques, and the interface between surface 110 and surface 112
is a transition from one type or design of material to another. In
certain implementations, the shell 102 and the intermediate portion
104 are not integrally formed and are two separate components that
are bonded together at the interface of these surfaces 110 and
112.
[0033] In some implementations, the interface between the shell 102
and the intermediate portion 104 allows for a small amount of
relative movement between the two portions of the component 100.
The relative movement can be accommodated by the inclusion of open
channels at the interface of the shell 102 and the intermediate
portion 104, as shown in FIG. 3. FIG. 3 depicts a cross-section of
the component 100 that reveals optional internal channels 103a-f at
the interface of the shell 102 and the intermediate portion 104.
The channels 103a-f are disposed near the areas of the interior
corners of the intermediate portion 104, but may be located at any
suitable location along the interface between the shell 102 and the
intermediate portion 104.
[0034] The channels 103a-f allow the component 100 to absorb
relative movements between the shell and intermediate portion
without interfering with bonding between the two components or
creating significant wear particles that can compromise the
implant. The intermediate portion 104 is able to flex slightly in
the areas of the channels 103a-f without applying a force to or
bending the shell 102 in the area of the flexing. Each of the
channels 103a-f can be compressed by forces applied to the interior
surface of the intermediate portion 104 until the channels are
closed and the material of the intermediate portion 104 around the
channels 103a-f contacts the shell 102. Because the channels 103a-f
absorb this flexing of the intermediate portion 104 and shell 102,
it may be preferable to locate the channels at areas of the
interface between the shell and intermediate portion that
experience the highest stresses or highest bending during normal
use.
[0035] The surface 112 of the shell 102 contributes to the ability
of the shell to spread loading stresses over bone surrounding the
implant and to reduce concentration of these forces in one or more
locations along the shell 102. The surface 112 is a smooth
continuous surface that does not have sharp points or corners. The
lack of sharp points and corners allows incident stresses to be
spread over a wide area of the shell 102 rather than concentrating
those forces at points or corners.
[0036] The stress distributions and transmission properties of the
component 100 are described with respect to finite element analyses
shown in FIGS. 4A and 4B. The analyses compare the properties of a
conventional implant 105a, shown in FIG. 4A, made of cobalt
chromium steel and a two-region implant 150b, shown in FIG. 4B,
that is similar to component 100 and has a cobalt chromium shell
and a polymeric intermediate portion, shown in FIG. 4B. A
displacement of 5 mm was applied to both implants, displacing the
posterior end of the solid steel implant 105a from initial position
152a to displaced position 152a' and displacing the posterior end
of the two-region implant 150b from initial position 152b to
displaced position 152b'. The spheres in FIGS. 4A and 4B indicate
the internal stress created in the implants during displacement,
and the size of the spheres is proportional to the magnitude of the
stress. The solid implant 150a and two-region implant 150b were
anchored at anterior points 153a and 153b, respectively, for the
displacement, and the stress present at the outer surface in these
areas is a result of the anchoring rather than an internal stress
in the components generated during displacement.
[0037] The location and size of the stress spheres in FIGS. 4A and
4B highlight the different stress concentration and transmission
properties of the two implants 150a and 150b. Implant 150a has a
higher overall internal stress indicated by the number and larger
sizes of the stress spheres in FIG. 4A. The internal stress is
absorbed by the implant itself and would not be transmitted into
surrounding bone during normal use, thereby causing the negative
effects of stress shielding. Because of the rigid metal that makes
up the implant 150a and the sharp corner transitions in the
implant, the internal stress is not uniformly distributed, and the
stress is focused at the corners 154a, 156a, and 158a. The
concentrated forces are greater near the areas of greater
displacement, as the stress increases from corner 154a to the
posterior corners 156a and 158a. There is also a substantial amount
of internal stress along the outer surface 160a of the implant
150a, shown by the stress spheres spread over the surface. The
distribution of the stresses at the outer surface highlight the
difference between the inner and outer contours, as the stresses
are spread out over the outer surface 160a on the exterior side of
the implant 150a but are concentrated at corners 154a, 156a, and
158a on the interior side.
[0038] Implant 150b exhibits a lower amount of internal stress than
the implant 150a, as shown by the fewer spheres and smaller sphere
sizes in FIG. 4B. Implant 150b also does not exhibit the stress
concentration seen in implant 150a, as the internal corners 154b,
156b, and 158b on the polymeric intermediate portion do not have
any appreciable concentrated stress in the deformed state. There is
internal stress in the shell region of the implant 150b shown by
the spheres on the outer surface 160b of the implant, but these
spheres, and the stresses they represent, are lower than the outer
stresses in implant 150a and are also substantially uniformly
spread over the outer surface 160b. The substantially lower
internal stress highlights the ability of the implant 150b to
transmit forces into surrounding bone and reduce internal
absorption of the forces, and thus reduce the negative effects of
stress shielding that can be cause by a solid steel implant such as
implant 150a.
[0039] The stress transmission and distribution properties of the
implant shown in FIG. 4B are created by the shape and design of the
shell component, particularly the contours of the inner surface of
the shell that interfaces with the intermediate portion, and the
material used for both the shell and the intermediate portion. The
shell 102 of implant component 100 is shown in FIG. 5 with the
intermediate portion 104 removed from the component to expose the
interior surface 114 of the shell 102. This configuration shows the
substantially continuous and smooth contours of the interior
surface 114. The interior surface 114 does not have sharp
transitions and corners that can concentrate stresses during
articulation of the shell 102, thereby allowing the knee implant
shell 102 to evenly distribute the forces incident on the shell 102
during articulation. In particular, the interior surface 114 of the
shell 102 exhibits smooth transitions between the posterior,
inferior, and anterior sections of the shell 102, rather than sharp
corners between these sections.
[0040] In addition to the continuous contours of the interior
surface 114, the thickness of different areas of the shell 102
affects the distribution of stresses incident on the shell 102 and
maintenance of the integrity of the shell material. The
cross-sectional area of the shell can be configured to provide the
desired stress distribution. In certain embodiments, the thickness
of the shell 102 is maintained uniform throughout the shell from
the posterior end 113 to the anterior end 115 to reduce uneven
distribution of incident stresses. In certain implementations, the
thickness of the shell 102 is varied from the anterior end 113 to
the posterior end 115 to maintain a uniform cross-sectional area of
different locations along the shell 102, helping to maintain
strength and rigidity of the shell.
[0041] Certain embodiments of shells having substantially uniform
cross-sectional area or substantially uniform thickness are
illustrated by viewing the cross-sectional profile of a femoral
implant component shell at different locations along the shell,
such as at the locations indicated by lines A-A, B-B, and C-C shown
in FIG. 5. FIGS. 6A-C illustrate one approach in which the
cross-sectional area of a shell 202 is maintained substantially
uniform by varying the thickness of the shell. The substantially
uniform cross-sectional area of the shell 202 contributes to the
ability of the shell to maintain its shape and material integrity,
as it affects the second moment of inertia resistance of the shell,
which is the resistance to bending under forces applied to the
shell. Specially, maintaining a substantially uniform
cross-sectional area in a shell can help create substantially the
same resistance to bending under force at different areas of the
shell that have different external geometries, such as different
widths. The uniform cross-sectional area and resistance thus
provides a shell that does not have points that are significantly
more compliant and susceptible to unwanted bending than other
points within the shell. In addition to the cross-sectional area,
the thickness of the shell can be varied to maintain uniformity for
another suitable mechanical property, such as moment of inertia and
section modulus.
[0042] To maintain a uniform cross-sectional area, the thickness of
an implant shell can be varied throughout the implant, and wider
areas of the component are thinner than narrower areas of the
component. FIG. 6A shows a cross-sectional profile taken at a
location on the shell 202 that corresponds to the line A-A of shell
102 in FIG. 5. The cross-sectional profile has a face 204 that
includes the thickness "A" of the shell 202 at this location. The
area of the face 204 is substantially uniform compared to
cross-sections taken at other areas of the shell 202.
[0043] FIG. 6B shows a cross-sectional profile of the shell 202
taken at a second location that corresponds to the line B-B of
shell 102 in FIG. 5. The cross-sectional profile includes a face
206 having a thickness "B" of the shell 202 at this location.
Because the shell 202 is wider at this location, the thickness B is
smaller than the thickness A shown on face 204 in FIG. 6A. The
smaller thickness at this location maintains the substantially
uniform area of the face 206 and the face 204.
[0044] FIG. 6C shows a third cross-sectional profile of the shell
202 taken at a location of the shell 202 that corresponds to the
line C-C of shell 102 in FIG. 5. The cross-sectional profile shown
includes two faces, face 208 on a condylar portion corresponding to
condylar portion 116 of shell 102 and face 210 on a condylar
portion corresponding to condylar portion 118 of shell 102. The two
faces 208 and 210 have the same thickness "C", which is greater
than both of the thicknesses A and B shown on faces 204 and 206,
respectively. The thickness C is selected such that the sum of the
areas of the faces 208 and 210 substantially equal to the
cross-sectional area of face 206 and the area of face 204.
[0045] In certain implementations, it may be preferable to maintain
a uniform thickness throughout an implant shell rather than a
uniform cross-sectional area. Because the width and other
dimensions of the shell differ, a uniform thickness creates a
varied cross-sectional area and, potentially, a varied resistance
to bending forces. Some implant materials, however, are strong
enough that there is not a significant resistance to bending forces
with minor changes in cross-sectional area, and thus a uniform
thickness does not result in a significant bending risk. The
uniform thickness may be preferable for such an implant to maintain
uniform transmission of forces incident on the implant into
surrounding bone structure. Forces on the implant are absorbed to
some degree fey the implant material, and thicker areas of an
implant may absorb more force than thinner areas due to the
increased material in the thicker areas. Maintaining a uniform
thickness of an implant keeps the absorbed forces substantially the
same throughout an implant shell, and preferably low enough to
transmit most of the normal functioning forces into surrounding
bone to maintain its integrity.
[0046] FIGS. 7A-C illustrate this approach in shell 302, which has
a substantially uniform thickness. FIG. 7A shows a cross-sectional
profile of a shell 302 taken at a location corresponding to the
line A-A shown in FIG. 5. The cross-sectional area includes a face
304 having a thickness "D", and this thickness is substantially
constant throughout the shell.
[0047] FIG. 7B shows a second cross-sectional profile of shell 302
taken at a location on the shell corresponding to the line B-B and
shown in FIG. 5. The cross-sectional profile includes a face 306
and the same thickness D as face 304 shown in FIG. 7A. In this
implementation, while the thickness of the implant is uniform, the
cross-sectional is not uniform, as the face 306 has an area larger
than the face 304. However, the uniform thickness keeps the
absorbance of incident forces substantially constant, as there is
no significant difference in the amount of shell material between
the external surface 312 and the internal surface 314 along either
cross-sectional face 304 or 306.
[0048] FIG. 7C shows a third cross-sectional profile of the shell
302 taken at a location corresponding to the location shown by line
C-C in FIG. 5. Again, the cross-sectional profile shows the
thickness D on the faces 308 and 310 of condylar portions of the
shell 302. Each of these faces 308 and 310 has an area that is
different from the areas of faces 304 and 306, but the uniform
thickness again allows for transmittal of forces through the faces
308 and 310 similar to the faces 304 and 306.
[0049] The illustrated approaches of maintaining either a uniform
cross-sectional area or uniform thickness of an implant shell help
reduce stress risers and provide uniform transmission of incident
stresses over an area of bone. Depending on the type of implant,
material, or incident forces in a particular application, both the
cross-sectional area and thickness may be designed to accommodate
specific needs for the implant. The designs face a trade-off
between providing a thick implant shell that resists bending and a
thin implant shell that reduces absorbance of incident forces. In
certain implementations, the requirements for a particular implant
are met by varying both the cross-sectional area and thickness
slightly throughout an implant shell to provide adequate bending
resistance and uniform force transmission in different areas of the
shell.
[0050] The external stresses which are incident on an implant
shell, such as shell 102, during articulation of a knee may not be
uniformly distributed over the external surface of the shell due to
the anatomic motion of the knee. For example, the anterior area
117, the condylar portions 116 and 118, and the junction 119
between the two condylar portions of shell 102 shown in FIG. 5 may
experience higher stress at various points during articulation of
the shell. In order to maintain the integrity and shape of the
shell 102 under these increased forces while still maintaining the
substantially uniform cross-sectional area or thickness of the
shell 102, it may be desirable in certain implementations to
provide reinforcement to these areas that experience higher
stresses.
[0051] FIG. 8 shows a shell 402 that includes reinforcements in
areas of higher stress caused by non-uniform normal stresses
incident on the shell. The shell 402 has a interior surface 404
that is substantially continuous and smooth, as discussed above
with respect to the inner surface 114 of the shell 102. The
interior surface 404 has an anterior portion 417 on which
heightened stress is incident during normal articulation of the
shell 402. The heightened stress may be present at anterior portion
417 as a result of normal motion of the knee, for example from
contact forces when a patient's foot strikes the ground while
walking. To address this heightened stress, the anterior portion
417 has reinforcing ribs 406a-c to provide added support to this
portion of the interior surface 404. These reinforcements allow the
shell 402 to articulate under non-uniform forces without bending or
deforming in the area of anterior portion 417.
[0052] The reinforcing ribs 406a-c are shown FIG. 8 extending along
the interior surface 404 of the shell 402. In such an
implementation, an intermediate portion, which may be substantially
similar to the intermediate portion 104 shown in FIG. 1, is bonded
to the shell 402 and has shell interface surface that accommodates
the reinforcing ribs 406a-c to maintain close contact with the
interior surface 404. Because the ribs cover only the anterior
portion 417, the substantially smooth and continuous contour of the
shell 402 is mostly maintained over the full surface 404. The
reinforcing ribs 406a-c may be designed with curved edges to avoid
introducing the sharp transitions that could potentially act as
stress concentration points. In certain implementations, the
reinforcing ribs are disposed in an internal portion of the shell
402, for example by depositing the material of shell 402 more
densely in the interior, or by using a stronger material for
interior portions, without introducing any change to the smooth
interior surface 404.
[0053] In addition to the ribs 406a-c, the shell 402 may contain
additional reinforcements to provide added support at the posterior
condylar portions 408 and 410 and the junction 412 between the two
condylar portions. The additional support at condylar portions may
be desired due to the decreased width and material mass of the
shell 402 in those areas. Instead of providing an implant shell
having thicker material throughout these portions, the reinforcing
ribs may be desirable, to provide the added support without
significantly affecting the transmission of forces into bone.
[0054] FIG. 9 shows a perspective view of a shell 502 having a
series of reinforcements on a posterior portion 517 of the shell.
The reinforcements include condylar ribs 508a-b and 510a-b, side
ribs 516a-b and a junction rib 514. The condylar ribs 508a-b on
condylar portion 504 and the condylar ribs 510a-b on the condylar
portion 506 give added support to these condylar portions to resist
deformation or twisting of the condylar portions as a knee
approaches full flexion. The longitudinal ribs 516a-b resist
bending and deformation of the posterior portion 517 of the shell
502 during flexion of the implant. The junction rib 514 complements
this support and provides additional reinforcement at the junction
512 between the condylar portions 504 and 506 to resist twisting or
deformation of one of the condylar portions 504 and 506 relative to
the other.
[0055] It is to be understood that the foregoing description is
merely illustrative and is not to be limited to the details given
herein. While several embodiments have been provided in the present
disclosure, it should be understood that the disclosed systems,
devices, and methods, and their components, may be embodied in many
other specific forms without departing from the scope of the
disclosure.
[0056] Variations and modifications will occur to those of skill in
the art after reviewing this disclosure. The disclosed features may
be implemented, in any combination and subcombinations (including
multiple dependent combinations and subcombinations), with one or
more other features described herein. The various features
described or illustrated above, including any components thereof,
may be combined or integrated in other systems. Moreover, certain
features may be omitted or not implemented.
[0057] Examples of changes, substitutions, and alterations are
ascertainable by one skilled in the art and could be made without
departing from the scope of the information disclosed herein. All
references cited herein are incorporated by reference in their
entirety and made part of this application.
* * * * *