U.S. patent application number 12/714288 was filed with the patent office on 2010-06-24 for prosthetic joint.
Invention is credited to Harold Lloyd Crowder, Franz W. Kellar.
Application Number | 20100161064 12/714288 |
Document ID | / |
Family ID | 44507589 |
Filed Date | 2010-06-24 |
United States Patent
Application |
20100161064 |
Kind Code |
A1 |
Kellar; Franz W. ; et
al. |
June 24, 2010 |
Prosthetic joint
Abstract
A prosthetic joint includes: (a) a bone-implantable first member
comprising a rigid material and including a body having an annular
flange extending outward at a first end thereof, the first end of
the body and the flange cooperatively defining a wear-resistant
concave first contact surface having a protruding rim and a
recessed central portion; and (b) a bone-implantable second member
comprising a rigid material with a wear-resistant convex second
contact surface; (c) where the first and second contact surfaces
bear directly against each other so as to transfer axial and
lateral loads from one of the members to the other, while allowing
pivoting motion between the two members; (d) wherein the flange is
shaped so as to deform elastically and permit the first contact
surface to conform to the second contact surface when the joint is
placed under a predetermined load.
Inventors: |
Kellar; Franz W.; (Gastonia,
NC) ; Crowder; Harold Lloyd; (Concord, NC) |
Correspondence
Address: |
TREGO, HINES & LADENHEIM, PLLC
9300 HARRIS CORNERS PARKWAY, SUITE 210
CHARLOTTE
NC
28269-3797
US
|
Family ID: |
44507589 |
Appl. No.: |
12/714288 |
Filed: |
February 26, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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11936601 |
Nov 7, 2007 |
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12714288 |
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Current U.S.
Class: |
623/18.11 |
Current CPC
Class: |
A61F 2/4425 20130101;
A61F 2002/3066 20130101; A61F 2002/30682 20130101; A61F 2/38
20130101; A61F 2002/30673 20130101; A61F 2/3859 20130101; A61F
2002/30322 20130101; A61F 2002/3092 20130101; A61F 2002/30153
20130101; A61F 2002/3611 20130101; A61F 2/36 20130101; A61F 2/30767
20130101; A61F 2250/0036 20130101; A61F 2002/3895 20130101; A61F
2310/0058 20130101; A61F 2/34 20130101; A61F 2002/3446 20130101;
A61F 2/367 20130101; A61F 2310/00239 20130101; A61F 2/389 20130101;
A61F 2/3094 20130101; A61F 2002/30968 20130101; A61F 2/32 20130101;
A61F 2002/30922 20130101; A61F 2002/30654 20130101; A61F 2002/30563
20130101; A61F 2002/30878 20130101; A61F 2250/0026 20130101; A61F
2002/30663 20130101; A61F 2002/30934 20130101; A61F 2/3676
20130101; A61F 2002/30589 20130101; A61F 2310/00179 20130101; A61F
2002/30014 20130101; A61F 2250/0018 20130101; A61F 2002/30112
20130101; A61F 2002/30324 20130101; A61F 2002/3495 20130101; A61F
2002/30937 20130101; A61F 2002/443 20130101; C23C 30/00 20130101;
A61F 2310/00011 20130101; A61F 2002/30955 20130101; A61F 2002/30929
20130101; A61F 2002/30675 20130101; A61F 2230/0004 20130101; A61F
2230/0019 20130101 |
Class at
Publication: |
623/18.11 |
International
Class: |
A61F 2/30 20060101
A61F002/30 |
Claims
1. A prosthetic joint, comprising: (a) a bone-implantable first
member comprising a rigid material and including a body having an
annular flange extending outward at a first end thereof; the first
end of the body and the flange cooperatively defining a
wear-resistant, concave first contact surface having a protruding
rim and a recessed central portion; and (b) a bone-implantable
second member comprising a rigid material with a wear-resistant,
convex second contact surface; (c) where the first and second
contact surfaces bear directly against each other so as to transfer
axial and lateral loads from one of the members to the other, while
allowing pivoting motion between the two members; and (d) wherein
the flange is shaped and sized so as to deform elastically and
permit the first contact surface to conform in an irregular shape
to the second contact surface when the joint is placed under a
predetermined load.
2. The prosthetic joint of claim 1 wherein the rim has a curved
cross-sectional shape.
3. The prosthetic joint of claim 1, wherein the rim has a
substantially conical surface.
4. The prosthetic joint of claim 3 wherein the rim has a free shape
defining a first contact area with the second contact surface and a
loaded shape defining a second contact area with the second contact
surface which is substantially larger than the first contact
area.
5. The prosthetic joint of claim 1 wherein: (a) the first member
further includes a disk-like base disposed at a second end of the
body opposite the first end, such that a circumferential gap is
defined between the base and the flange; and (b) the
circumferential gap filled with a resilient nonmetallic
material.
6. The prosthetic joint of claim 1, wherein a resilient sealing lip
protrudes from the rim and engages the second contact surface in a
sealing relationship.
7. The prosthetic joint of claim 1 wherein the contact surfaces
comprise a ceramic material, a metal, or a combination thereof.
8. The prosthetic joint of claim 1 wherein the first contact
surface includes more than one protruding rim, with a
circumferential relief area defined between adjacent rims.
9. The prosthetic joint of claim 1, wherein a thickness of the
flange, at a root where it joins the neck, is about 0.04 mm to
about 5.1 mm.
10. The prosthetic joint of claim 1, where the flange is sized so
as to permit elastic deflection of the flange while limiting
stresses in the flange to less than the endurance limit of the
material from which the flange is constructed, when an external
load in the range of about 0 to 300 lbs. is applied to the
joint.
11. The prosthetic joint of claim 1, wherein the first contact
surface has a pattern of grooves formed therein, comprising: (a)
one or more circular grooves; and (b) one or more generally
radially-aligned grooves which intersect the circular grooves.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Continuation-in-Part of application
Ser. No. 11/936,601, filed Nov. 7, 2007, which is currently
pending.
BACKGROUND OF THE INVENTION
[0002] This invention relates generally to medical implants, and
more particularly to prosthetic joints having conformal geometries
and wear resistant properties.
[0003] Medical implants, such as knee, hip, and spine orthopedic
replacement joints and other joints and implants have previously
consisted primarily of a hard metal motion element that engages a
polymer contact pad. This has usually been a high density high wear
resistant polymer, for example Ultra-High Molecular Weight
Polyethylene (UHMWPE), or other resilient material. The problem
with this type of configuration is the polymer eventually begins to
degrade due to the caustic nature of blood, the high impact load,
and high number of load cycles. As the resilient member degrades,
pieces of polymer may be liberated into the joint area, often
causing accelerated wear, implant damage, and tissue inflammation
and harm.
[0004] It is desirable to employ a design using a hard member on a
hard member (e.g. metals or ceramics), thus eliminating the
polymer. Such a design is expected to have a longer service life.
Extended implant life is important as it is now often required to
revise or replace implants. Implant replacement is undesirable from
a cost, inconvenience, patient health, and resource consumption
standpoint.
[0005] Implants using two hard elements of conventional design will
be, however, subject to rapid wear. First, a joint having one hard,
rigid element on another will not be perfectly shaped to a nominal
geometry. Such imperfections will result in points of high stress,
thus causing localized wear. Furthermore, two hard elements would
lack the resilient nature of a natural joint. Natural cartilage has
a definite resilient property, absorbing shock and distributing
periodic elevated loads. This in turn extends the life of a natural
joint and reduces stress on neighboring support bone and tissue. If
two rigid members are used, this ability to absorb the shock of an
active lifestyle could be diminished. The rigid members would
transmit the excessive shock to the implant to bone interface. Some
cyclical load in these areas stimulates bone growth and strength;
however, excessive loads or shock stress or impulse loading the
bone-to-implant interface will result in localized bone mass loss,
inflammation, and reduced support.
BRIEF SUMMARY OF THE INVENTION
[0006] These and other shortcomings of the prior art are addressed
by the present invention, which provides a prosthetic joint having
wear-resistant contacting surfaces with conformal properties.
[0007] According to one aspect of the invention, a prosthetic joint
includes: (a) a first bone-implantable member comprising a rigid
material and including a body having an annular flange extending
outward at a first end thereof, the first end of the body and the
flange cooperatively defining a wear-resistant, concave first
contact surface having a protruding rim and a recessed central
portion; and (b) a second bone-implantable member comprising a
rigid material with a wear-resistant, convex second contact
surface; (c) where the first and second contact surfaces bear
directly against each other so as to transfer axial and lateral
loads from one of the members to the other, while allowing pivoting
motion between the two members; and (d) wherein the flange is
shaped so as to deform elastically and permit the first contact
surface to conform to the second contact surface when the joint is
placed under a predetermined load.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] The invention may be best understood by reference to the
following description taken in conjunction with the accompanying
drawing figures in which:
[0009] FIG. 1 is a cross-sectional view of a portion of a resilient
contact member constructed in accordance with the present
invention;
[0010] FIG. 2 is an enlarged view of the contact member of FIG. 1
in contact with a mating joint member;
[0011] FIG. 3 is a side view of a resilient contact member in
contact with a mating joint member;
[0012] FIG. 4 is a cross-sectional view of a cup for an implant
according to an alternate embodiment of the invention;
[0013] FIG. 5 is an enlarged view of a portion of the cup of FIG.
4;
[0014] FIG. 6 is a perspective view of a finite element model of a
joint member;
[0015] FIG. 7 is a cross-sectional view of an implant joint
including a flexible seal;
[0016] FIG. 8 is an enlarged view of a portion of FIG. 7;
[0017] FIG. 9 is a side view of a prosthetic joint constructed in
accordance with an aspect of the present invention;
[0018] FIG. 10 is a cross-sectional view of the prosthetic joint of
FIG. 9 in an unloaded condition;
[0019] FIG. 11 is a cross-sectional view of one of the members of
the prosthetic joint of FIG. 9;
[0020] FIG. 12 is an enlarged view of a portion of FIG. 10;
[0021] FIG. 13 is a cross-sectional view of the prosthetic joint of
FIG. 9 in a loaded condition;
[0022] FIG. 14 is an enlarged view of a portion of FIG. 13;
[0023] FIG. 15 is a cross-sectional view of an alternative joint
member;
[0024] FIG. 16 is an enlarged view of a portion of FIG. 15;
[0025] FIG. 17 is a cross-sectional view of another alternative
joint member;
[0026] FIG. 18 is a cross-sectional view of another alternative
joint member including a filler material;
[0027] FIG. 19 is a cross-sectional view of another alternative
joint member including a wiper seal;
[0028] FIG. 20 is a cross-sectional view of another alternative
prosthetic joint;
[0029] FIG. 21 is a cross-sectional view of a prosthetic joint
constructed in accordance with another aspect of the present
invention;
[0030] FIG. 22 is a cross-sectional view of a prosthetic joint
constructed in accordance with yet another aspect of the present
invention; and
[0031] FIG. 23 is a perspective view of a joint member having a
grooved surface.
[0032] FIG. 24 is a exploded perspective view of two mating joint
members;
[0033] FIG. 25 is a top plan view of one of the joint members shown
in FIG. 24;
[0034] FIG. 26 is a cross-sectional view of one of the joint
members shown in FIG. 24;
[0035] FIG. 27 is a contact stress plot of the joint member shown
in FIG. 26;
[0036] FIG. 28 is a perspective view of a rigid joint member used
for comparison purposes;
[0037] FIG. 29 is a cross-sectional view of the joint member shown
in FIG. 28; and
[0038] FIG. 30 is a contact stress plot of the joint member shown
in FIG. 29.
DETAILED DESCRIPTION OF THE INVENTION
[0039] The present invention provides a specialized implant contact
interface (implant geometry). In this geometry, an implanted joint
includes two typically hard (i.e. metal or ceramic) members;
however, at least one of the members is formed such that it has the
characteristics of a resilient member, such as: the ability to
absorb an impact load; the ability to absorb high cycle loading
(high endurance limit); the ability to be self cleaning; and the
ability to function as a hydrodynamic and/or hydrostatic
bearing.
[0040] Generally, the contact resilient member is flexible enough
to allow elastic deformation and avoid localized load increases,
but not so flexible as to risk plastic deformation, cracking and
failure. In particular, the resilient member is designed such that
the stress levels therein will be below the high-cycle fatigue
endurance limit. As an example, the resilient member might be only
about 10% to about 20% as stiff as a comparable solid member. It is
also possible to construct the resilient member geometry with a
variable stiffness, i.e. having a low effective spring rate for
small deflections and a higher rate as the deflections increase, to
avoid failure under sudden heavy loads.
[0041] FIG. 1 illustrates an exemplary contact member 34 including
a basic resilient interface geometry. The contact member 34 is
representative of a portion of a medical implant and is made of one
or more metals or ceramics (for example, partially stabilized
Zirconia). It may be coated as described below. The geometry
includes a lead-in shape, Z1 and Z2, a contact shape, Z3 and Z4, a
lead-out shape, Z5 and Z6, and a relieved shape, Z7. It may be
desired to vary the cross-sectional thickness to achieve a desired
mechanical stiffness to substrate resilience characteristic. The
presence of the relieved region Z7 introduces flexibility into the
contact member 34, reduces the potential for concentrated point
contact with a mating curved member, and provides a reservoir for a
working fluid.
[0042] The Z7 region may be local to the contact member 34 or may
be one of several. In any case, it may contain a means of providing
fluid pressure to the internal contact cavity to produce a
hydrostatic interface. A passive (powered by the regular motion of
the patient) or active (powered by micro components and a dedicated
subsystem) pumping means and optional filtration may be employed to
provide the desired fluid interaction.
[0043] A hydrodynamic interface is desirable as, by definition, it
means the contact member 34 is not actually touching the mating
joint member. The lead-in and lead-out shapes Z1, Z2, Z5, Z6 are
configured to generate a shear stress in the working fluid so as to
create the fluid "wedge" of a hydrodynamic support.
[0044] FIG. 2 shows a closer view of the contact member 34. It may
be desirable to make the contact radius (Z3 and Z4) larger or
smaller, depending on the application requirement and flexural
requirement. For example, FIG. 3 illustrates the contact member 34
in contact with a mating joint member 38 having a substantially
larger radius than the contact member 34. The radius ratio between
the two joint members is not particularly critical, so long as one
of the members exhibits the resilient properties described
herein.
[0045] The contact member 34 includes an osseointegration surface
"S", which is a surface designed to be infiltrated by bone growth
to improve the connection between the implant and the bone.
Osseointegration surfaces may be made from materials such as
TRABECULAR METAL, textured metal, or sintered or extruded implant
integration textures. TRABECULAR METAL is an open metal structure
with a high porosity (e.g. about 80%) and is available from Zimmer,
Inc., Warsaw, Indiana 46580 USA.
[0046] FIGS. 4 and 5 illustrate a cup 48 of metal or ceramic with
two integrally-formed contact rings 50. More contact rings may be
added if needed. As shown in FIG. 5, the volume behind the contact
rings 50 may be relieved. This relieved area 52 may be shaped so as
to produce a desired balance between resilience and stiffness. A
varying cross-section geometry defined by varying inner and outer
spline shapes may be desired. In other words, a constant thickness
is not required. A material such as a gel or non-Newtonian fluid
(not shown) may be disposed in the relieved area 52 to modify the
stiffness and damping characteristics of the contact rings 50 as
needed for a particular application. The cup 48 could be used as a
stand-alone portion of a joint, or it could be positioned as a
liner within a conventional liner. The contact ring 50 is shown
under load in FIG. 6, which depicts contour lines of highest
compressive stress at "C1". This is the portion of the contact ring
50 that would be expected to undergo bending first. The bearing
interface portion of the resilient contact member could be
constructed as a bridge cross-section supported on both sides as
shown or as a cantilevered cross-section depending on the desired
static and dynamic characteristics.
[0047] FIGS. 7 and 8 illustrate an implant 56 of rigid material
which includes a wiper seal 58. The wiper seal 58 keeps particles
out of the contact area (seal void) 60 of the implant 58, and
working fluid (natural or synthetic) in. The seal geometry is
intended to be representative and a variety of seal characteristics
may be employed; such as a single lip seal, a double or multiple
lip seal, a pad or wiper seal made from a variety of material
options. Different seal mounting options may be used, for example a
lobe in a shaped groove as shown in FIGS. 7 and 8, a retaining ring
or clamp, or an adhesive. The wiper seal 58 may also be integrated
into the contact face of the interface zone.
[0048] It may be desirable to create a return passage 62 from the
seal void region 60 back into the internal zone 64 in order to
stabilize the pressure between the two and to allow for retention
of the internal zone fluid if desired. This is especially relevant
when the hydrostatic configuration is considered.
[0049] FIGS. 9-14 illustrate a prosthetic joint 100 comprising
first and second members 102 and 104. The illustrated prosthetic
joint 100 is particularly adapted for a spinal application, but it
will be understood that the principles described herein may be
applied to any type of prosthetic joint. Both of the members 102
and 104 are bone-implantable, meaning they include osseointegration
surfaces, labeled "S", which are surfaces designed to be
infiltrated by bone growth to improve the connection between the
implant and the bone. Osseointegration surfaces may be made from
materials such as TRABECULAR METAL, textured metal, or sintered or
extruded implant integration textures, as described above. As shown
in FIG. 10, a central axis "A" passes through the centers of the
first and second members 102 and 104 and is generally
representative of the direction in which external loads are applied
to the joint 100 in use. In the illustrated examples, the first and
second joint members are bodies of revolution about this axis, but
the principles of the present invention also extend to shapes that
are not bodies of revolution.
[0050] The first member 102 includes a body 106 with a perimeter
flange 116 extending in a generally radially outward direction at
one end. Optionally, a disk-like base 108 may be disposed at the
end of the body 106 opposite the flange 116, in which case a
circumferential gap 111 will be defined between the base 106 and
the flange 116. The first member 102 is constructed from a rigid
material. As used here, the term "rigid" refers to a material which
has a high stiffness or modulus of elasticity. Nonlimiting examples
of rigid materials having appropriate stiffness for the purpose of
the present invention include stainless steels, cobalt-chrome
alloys, titanium, aluminum, and ceramics. By way of further
example, materials such as polymers would generally not be
considered "rigid" for the purposes of the present invention.
Generally, a rigid material should have a modulus of elasticity of
about 0.5.times.10.sup.6 psi or greater. Collectively, one end of
the body 106 and the flange 116 define a wear-resistant, concave
first contact surface 118. As used herein, the term
"wear-resistant" refers to a material which is resistant to surface
material loss when placed under load. Generally the wear rate
should be no more than about 0.5 .mu.m (0.000020 in.) to about 1.0
.mu.m (0.000040 in.) per million cycles when tested in accordance
with ASTM Guide F2423. As a point of reference, it is noted that
any of the natural joints in a human body can easily experience one
million operating cycles per year. Nonlimiting examples of
wear-resistant materials include solid metals and ceramics. Known
coatings such as titanium nitride, chrome plating, carbon thin
films, and/or diamond-like carbon coatings may be used to impart
wear resistance to the first contact surface 118. Optionally, the
first contact surface 118 could comprise a separate face layer (not
shown) of a wear-resistant material such as ultra-high molecular
weight (UHMW) polyurethane.
[0051] The first contact surface 118 includes a protruding
peripheral rim 120 (see FIG. 11), and a recessed central portion
122, which may also be considered a "pocket" or a "relief". As used
herein, the term "recessed" as applied to the central portion 122
means that the central portion 122 lies outside of the nominal
exterior surface of the second member 104 when the joint 100 is
assembled. In one configuration, shown in FIGS. 9-14, and best seen
in FIG. 11, the rim 120 is concave, with the radius of curvature
being quite high, such that the cross-sectional shape of the
surface of the rim 120 approaches a straight line. FIGS. 15 and 16
show another configuration of a joint member 102' in which the rim
120' has a convex-curved cross-sectional shape. The cross-sectional
shape of the rim may be flat or curved as necessary to suit a
particular application.
[0052] The annular configuration of first contact surface 118 with
the protruding rim 120 results in a configuration which permits
only pivoting and rotational motion, and is statically and
dynamically determinate for the life of the joint 100. In contrast,
prior art designs employing mating spherical shapes, even very
accurate shapes, quickly reach a statically and dynamically
indeterminate condition after use and wear. This condition
accelerates wear, contributes to the fretting corrosion wear
mechanism, and permits undesired lateral translation between the
joint members.
[0053] The second member 104 is also made from a rigid material and
has a wear-resistant, convex second contact surface 124. The first
and second contact surfaces 118 and 124 bear directly against each
other so as to transfer axial and lateral loads from one member to
the other while allowing pivoting motion between the two members
102 and 104.
[0054] Nominally the first and second members 102 and 104 define a
"ring" or "band" contact interface therebetween. In practice it is
impossible to achieve surface profiles completely free of minor
imperfections and variations. If the first and second members 102
and 104 were both completely rigid, this would cause high Hertzian
contact stresses and rapid wear. Accordingly, an important feature
of the illustrated joint 100 is that the flange 116 (and thus the
first contact surface 118) of the first member 102 is conformable
to the second contact surface 124 when the joint is placed under
load.
[0055] FIGS. 10 and 12 show a cross-sectional view of the flange
116 in an unloaded condition or free shape. It can be seen that the
distal end of the rim 120 contacts the second contact surface 124,
while the inboard end of the rim 120 (i.e. near where the flange
116 joins the body 106) does not. FIGS. 13 and 14 show the flange
116 in a deflected position or loaded shape, where substantially
the entire section width of the rim 120 contacts the second contact
surface 124, resulting in a substantially increased contact surface
area between the two members 102 and 104, relative to the free
shape. The rim 120' of the joint member 102' (see FIG. 16) is
similarly conformable; however, given the curved cross-sectional
shape, the total amount of surface contact area remains
substantially constant in both loaded and unloaded conditions, with
the rim 120' undergoing a "rolling" or "rocking" motion as the
loading changes.
[0056] The conformable nature of the flange 116 is explained in
more detail with reference to FIGS. 24 through 30. As noted above,
the first member 102 has a flange 116 and a concave first contact
surface 118. The second member 104 has a convex second contact
surface 124. When assembled and in use the joint 100 is subject,
among other loads, to axial loading in the direction of the arrows
labeled "F" in FIG. 24 (i.e. along axis "A" of FIG. 10). As
previously stated, it is impossible in practice for either of the
contact surfaces 118 or 124 to be perfect surfaces (i.e. a perfect
sphere or other curve or collection of curves). It is believed that
in most cases that a defect such as a protrusion from the nominal
contact surface of just 0.00127 mm (0.00005 in.), that is, 50
millionths of a inch, or larger, would be sufficient to cause
fretting corrosion and failure of a metal-on-metal joint
constructed to prior art standards. A defect may include a variance
from a nominal surface shape as well as a discontinuity in the
contact surface. Defects may arise through a variety of sources
such as manufacturing, installation, and/or operating loads in the
implanted joint.
[0057] FIG. 25 shows the second member 104 which in this particular
example varies from a nominal shape in that it is elliptical rather
than circular in plan view. The elliptical shape is grossly
exaggerated for illustrative purposes. For reference, the
dimensions of the second member 104 along the major axis labeled
"X" is about 0.0064 mm (0.00025 in.) larger than its dimension
along the minor axis labeled "Y". When assembled and loaded, the
flange 116 conforms to the imperfect second contact surface 124 and
deflects in an irregular shape. In other words, in addition to any
uniform deflection which may be present, the deflected shape of the
flange 116 includes one or more specific locations or portions that
are deflected towards or away from the nominal free shape to a
greater or lesser degree than the remainder of the flange 116. Most
typically the deflected shape would be expected to be
non-axisymmetric. For example, the deflection of the flange 116 at
points located at approximately the three o'clock and nine o' clock
positions is substantially greater than the deflection of the
remainder of the flange 116. As a result, the contact stress in
that portion of the first contact surface 118 is relieved. FIG. 27
is a plan view plot (the orientation of which is shown by arrow in
FIG. 26) which graphically illustrates the expected contact
stresses in the first contact surface 118 as determined by
analytical methods. The first contour line "C2" shows that a very
low level of contract stress is present around the entire perimeter
of the first contact surface 118. This is because the entire first
contact surface 118 is in contact with the second contact surface
124. Another contour line "C3" represents the areas of maximum
contact stress corresponding to the protruding portions of the
elliptical second contact surface 124.
[0058] For comparative purposes, FIGS. 28 and 29 depict a member
902 constructed according to prior art principles. The member 902
has a contact surface 918 with an identical profile and dimensions
of the first contact surface 118 of the first member 102. However,
consistent with the prior art, the member 902 has a massive body
920 behind the entire contact surface 918, rendering the entire
member 902 substantially rigid. FIG. 30 graphically illustrates the
expected contact stresses in the contact surface 918 as determined
by analytical methods, when the member 902 is assembled and placed
in contact with the second member 104, using the same applied load
as depicted in FIG. 27. Because of the rigidity of the member 902,
a "bridging" effect is present wherein contact between the contact
surfaces (one of which is circular in plan view, and the other of
which is elliptical) effectively occurs at only two points, located
at approximately the three o'clock and nine o'clock positions. A
first contour line "C4" shows two discrete areas where the lowest
level of contract stress is present. These lines are not contiguous
because there is no contact in the remaining area of the contact
surfaces (for example at the six o'clock and twelve o'clock
positions). Another contour line "C5" represents the areas of
maximum contact stress. Analysis shows a peak contact stress having
a magnitude of two to twenty times (or more) the peak contact
stress of the inventive joint as shown in FIG. 27.
[0059] To achieve this controlled deflection, the flange 116 is
thin enough to permit bending under working loads, but not so thin
as to allow material yield or fatigue cracking. The deflection is
opposed by the elasticity of the flange 116 in bending, as well as
the hoop stresses in the flange 116. To achieve long life, the
first member 102 is sized so that stresses in the flange 116 will
be less than the endurance limit of the material, when a selected
external load is applied. In this particular example, the joint 100
is intended for use between two spinal vertebrae, and the design
average axial working load is in the range of about 0 N (0 lbs) to
about 1300 N (300 lbs.). These design working loads are derived
from FDA-referenced ASTM and ISO standards for spinal disc
prostheses. In this example, the thickness of the flange 116, at a
root 126 where it joins the body 106 (see FIG. 12) is about 0.04 mm
(0.015 in.) to about 5.1 mm (0.200 in.), where the outside diameter
of the flange 116 is about 6.4 mm (0.25 in.) to about 7.6 cm (3.0
in.).
[0060] The joint members may include multiple rims. For example,
FIG. 17 illustrates a joint member 202 where the first contact
surface 218 includes two protruding rims 220, with a
circumferential groove or relief area 228 therebetween. The
presence of multiple rims increases the contact surface areas
between the two joint members.
[0061] If present, the circumferential gap between the flange and
the base of the joint member may be filled with resilient
nonmetallic material to provide damping and/or additional spring
restoring force to the flange. FIG. 18 illustrates a joint member
302 with a filler 304 of this type. Examples of suitable resilient
materials include polymers, natural or synthetic rubbers, and the
like.
[0062] As discussed above, the joint may incorporate a wiper seal.
For example, FIG. 19 illustrates a joint member 402 with a
resilient wiper seal 404 protruding from the rim 420 of the first
contact surface 418. The wiper seal 404 keeps particles out of the
contact area (seal void), while containing working fluid (natural
or synthetic). The seal geometry is intended to be representative
and a variety of seal characteristics may be employed; such as a
single lip seal, a double or multiple lip seal. A pad or wiper seal
may be made from a variety of material options. Different seal
mounting options may be used, for example a lobe in shaped groove
as shown in FIG. 18, a retaining ring or clamp, adhesion substance.
The seal may also be incorporated into the contact face of the
interface zone.
[0063] The joint construction described above can be extended into
a three-part configuration. For example, FIG. 20 illustrates a
prosthetic joint 500 having first, second, and third members 502,
504, and 506. The first and second members 502 and 504 are similar
in construction to the first member 102 described above, and each
includes a body 508, an optional disk-like base 510, and a flange
512. The flanges 512 define wear-resistant concave first and second
contact surfaces 514 and 516, each of which includes a protruding
peripheral rim, and a recessed central portion as described above.
The third member 506 has a double-convex shape defining opposed
wear-resistant, convex third and fourth contact surfaces 524 and
526. The first and second 514 and 516 bear against the third and
fourth contact surfaces 524 and 526, respectively, so as to
transfer axial (i.e. compression) and lateral loads between the
first and second members 502 and 504 through the third member 506,
while allowing pivoting motion between the members 502, 504, and
506. The first and second contact surfaces 514 and 516 are
conformal to the third and fourth contact surfaces 524 and 526 as
described in more detail above.
[0064] FIG. 21 illustrates an alternative prosthetic joint 600
comprising first and second members 602 and 604 constructed from
rigid materials. Both of the members 602 and 604 are
bone-implantable, meaning they include osseointegration surfaces,
labeled "S", as described in more detail above.
[0065] The first member 602 is hollow and includes a disk-like base
606 and a cup 608, interconnected by a peripheral wall 610. An
interior cavity 612 is defined between the base 606 and the cup
608. The cup 608 is constructed from a rigid material and defines a
wear-resistant, concave first contact surface 614. The first
contact surface 614 includes a protruding peripheral rim 616, and a
recessed central portion 618, which may also be considered a
"pocket" or a "relief". The rim 616 may have a conical or curved
cross-sectional shape.
[0066] The second member 604 is constructed from a rigid material
and has a wear-resistant, convex second contact surface 620. The
first and second contact surfaces 614 and 616 bear directly against
each other so as to transfer axial and laterals loads from one
member to the other while allowing pivoting motion between the two
members 602 and 604.
[0067] As described above with reference to the prosthetic joint
100, the cup 606 of the first member 602 is thin enough to permit
bending under working loads, but not so thin as to allow material
yield or fatigue cracking. The first contact surface 614 is thus
conformable to the second contact surface 620 when the prosthetic
joint 600 is placed under external load.
[0068] An inverted configuration of hollow members is also
possible. For example, FIG. 22 illustrates a prosthetic joint 700
comprising first and second members 702 and 704, both constructed
of rigid materials. The first member 702 is solid and includes a
wear-resistant, concave first contact surface 708. The first
contact surface 708 includes a protruding peripheral rim 710, and a
recessed central portion 712, which may also be considered a
"pocket" or a "relief".
[0069] The second member 704 is hollow and includes a dome 714
connected to a peripheral wall 716. An interior cavity 718 is
defined behind the dome 714. The dome 714 defines a wear-resistant,
convex second contact surface 720, which is shaped and sized enough
to permit bending under working loads, but not so as to allow
material yield or fatigue cracking. The second contact surface 720
is thus conformable to the first contact surface 708 when the
prosthetic joint 700 is placed under external load.
[0070] The first and second contact surfaces 708 and 720 bear
directly against each other so as to transfer axial and lateral
loads from one member to the other while allowing pivoting motion
between the two members 702 and 704.
[0071] Any of the contact surfaces described above may be provided
with one or more grooves formed therein to facilitate flow of fluid
or debris. For example, FIG. 23 illustrates a joint member 800
including a concave contact surface 802. The contact surface 802
includes a circular groove 804, and plurality of generally
radially-extending grooves 806 which terminate at the center of the
contact surface 802 and intersect the circular groove 804.
[0072] As noted above, known coatings such as titanium nitride,
chrome plating, carbon thin films, and/or diamond-like carbon
coatings may be used to impart wear resistance or augment the wear
resistance of any of the first contact surfaces described above. To
the same end, it may be desirable to surface treat either or both
interfaces of any of the above-described implants or joints with a
laser, shot peen, burnishing, or water shock process, to impart
residual compressive stresses and reduce wear. The benefit could be
as much from surface annealing and microstructure and microfracture
elimination as smoothing itself.
[0073] The foregoing has described medical implants and prosthetic
joints with wear-resistant properties and conformal geometries.
While specific embodiments of the present invention have been
described, it will be apparent to those skilled in the art that
various modifications thereto can be made without departing from
the spirit and scope of the invention. Accordingly, the foregoing
description of the preferred embodiment of the invention and the
best mode for practicing the invention are provided for the purpose
of illustration only and not for the purpose of limitation.
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