Note: Descriptions are shown in the official language in which they were submitted.
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TST3r~
METAL/COMPOSITE HYBRID ORTHOPEDIC IMPLANTS
FIELD OF THE INVENTION
The present invention relates to orthopedic
implants, and more particularly to load bearing
prosthetic devices including both metal and composite
components.
BACKGROUND OF THE INVENTION
Orthopedic implants include a wide variety of
devices, each suited to fulfill particular medical
needs. Examples of such devices are hip joint
replacement devices, knee joint replacement devices,
shoulder joint replacement devices, and pins, braces and
plates used to set fractured bones. Particular emphasis
has been recently placed on hip joint prosthetic
equipment. A typical configuration for a hip joint
prosthetic includes a proximal region and a distal
region. The proximal region has a ball attached thereto
and adapted to engage a cup portion (an artificial
socket embedded in the pelvis). The ball is attached
via an extension piece called the neck to the body of
the proximal region. The body is joined to a distal
region, which both extend into the femur.
Contemporary orthopedic implants, including hip and
knee components, use high performance metals such as
cobalt-chrome and titanium alloy to achieve high
strength. These materials are readily fabricated into
the complex shapes typical of these devices using mature
metal working techniques including casting and
machining. Yet, these metals are characterized by high,
fixed moduli of elasticities which makes it difficult to
achieve optimal device stiffness within a given
anatomical geometric envelope. In particular, in
regions in which metal implants share load with
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surrounding bone, e.g. the medullary canal of the femur,
the stress in the bone is substantially reduced versus
the normal physiological level. This "stress-shielding"
effect often leads to bone remodeling and may be
implicated in clinical problems such as asceptic
loosening and pain. Stress shielding is particularly
acute in large metal implant systems. Further, large
metal implants require more bone cement and are more
susceptible to loosening than smaller implants.
Composite materials offer the potential to achieve
high strength in orthopedic devices while permitting the
control of stiffness for enhanced load transfer to bone.
In particular, the implant designer can control modulus
by varying reinforcement type, orientation and amount.
Such a device is revealed in PCT patent application
WO/85/04323. The device is formed from a composite
material of continuous filament carbon fibers embedded
within a polymer matrix. The carbon fibers in the
composite material are at specific orientations relative
to a specific dimension of the orthopedic device. The
angularity of the carbon fibers modifies the modulus of
the device. To effect fiber orientation, uniplanar
sheets of carbon fibers are formed and cut into coupons.
The coupons are then stacked into blocks or rolled into
cylinders, to be fashioned into the final device. The
manner in which the sheets or coupons are oriented will
affect final mechanical properties. However, the
prosthetic device according to this invention is limited
in that the orientation of the carbon fibers cannot be
varied along the formed elongated body.
European Patent Publication 0277 727 discloses an
orthopedic device of a biocompatible polymer with
oriented fiber reinforcement. Prostheses of this
reference are formed from plies of continuous filament
fibers that are curvilinearly disposed within a body.
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The plies may have a balanced orientation; that is, for each sheet having
fibers offset at a positive angle there is essentially a sheet having fibers
offset
at about the same negative angle. However, the prosthetic device of this
variety is limited in that the orientation of the carbon fibers cannot be
varied
along the formed elongate body.
U. S. Patent 4,750,905 reveals a prosthesis construction including an
elongate, tapered polymer core containing continuous-filament fibers oriented
substantially along the length of the core. The core includes an elongate
distal
stem. A braided sheath encases the stem. The filaments in the braid encircle
the core in a helical pattern. However, devices according to this reference
cannot be formed in a flexible laydown pattern as in the present invention.
Moreover, the device does not reveal the unique means to fasten the
proximal body portion to the distal composite portion of the orthopedic device
as in the present invention.
It is an object of an aspect of the present invention to provide a hybrid
orthopedic implant wherein the stresses in the surrounding bone are more
nearly equal to their normal physiological level than achieved in an all metal
system. It is a feature of an aspect of the present invention to provide a
variety of means to secure the intraosseous metal portion to the intraosseous
composite portion. It is an advantage of an aspect of the present invention
that the subject orthopedic implants may have a variable modulus along their
lengths due to the use of filament winding and braiding techniques.
These and other objects, features and advantages of the present
invention will become more readily apparent with reference to the following
description of the invention.
SUMMARY OF THE INVENTION
The present invention provides an orthopedic device for human
implantation. The device comprises an intraosseous metal portion and an
intraosseous composite portion attached thereto. The composite portion
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comprises one or more filaments disposed about a longitudinal axis and
within a polymer matrix.
In the orthopedic device the composite portion may be received within
the metal portion and secured thereto. The portions may be secured by a
taper lock, an adhesive joint, or a shrink fit joint. Alternatively, in the
orthopedic device, the metal portion may be received within the composite
portion and secured thereto. In such a case, the metal portion comprises a
first extension that is received within the composite portion and a second
extension positioned outside the composite portion. This first extension may
be secured to the composite portion by a plurality of pins extending radially
from the extension. In another embodiment, the second extension is gfooved
to accommodate a threaded compression nut. A still further means of
securing the metal portion to the composite portion is by a shrink fit joint.
The intraosseous composite portion of the orthopedic device may be
prepared by several processes of the invention. One such process comprises
winding or braiding fiber into a preform and placing the preform into a mold.
Thermoset resin is injected and cured.
Further aspects of the invention are as follows:
An orthopedic device for human implantation comprising:
an intraosseous metal portion; and
an intraosseous composite portion attached thereto, said composite portion
comprising one or more filaments disposed about a longitudinal axis and
within a polymer matrix.
An orthopedic device for human implantation comprising:
an intraosseous metal portion;
an intraosseous composite portion; and
an intraosseous metal insert interposed there between and connecting said
metal portion to said composite portion, said metal insert being received
within said metal portion and frictionally secured thereto,
said metal insert further forming a taper such that the cross-sectional
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area of said metal insert increases from the end received within said metal
portion to the end connected to said composite portion, said metal insert
further
being connected to said composite portion by an adhesive joint or a shrink fit
joint,
said composite portion comprising one or more filaments disposed about a
longitudinal axis and within a polymer matrix.
An orthopedic device for human implantation comprising an intraosseous
metal portion and an intraosseous composite portion, said composite portion
adapted to be received within said metal portion and prepared by fitting the
composite portion together with the metal portion at a suitable temperature,
the
composite portion and the metal portion acting in engagement at another
temperature.
A load bearing orthopedic device for human implantation comprising:
a proximal metal body; and
a distal composite stem attached thereto, said stem comprising one or
more filaments disposed about a longitudinal axis and within a polymer matrix.
An orthopedic device for human implantation comprising:
an intraosseous composite portion comprising one or more filaments
disposed about a longitudinal axis and within a polymer matrix; and
an intraosseous metal portion comprising a first extension received within
said composite portion and secured thereto and a second extension positioned
outside said composite portion, wherein said first extension contains a
plurality of
pins extending radially therefrom.
In accordance with an aspect of the present invention, there is provided an
orthopaedic device for human implantation comprising:
an intraosseous composite portion comprising one or more filaments
disposed about a longitudinal axis and within a polymer matrix;
an intraosseous metal portion comprising a first extension received within
said composite portion and secured thereto and a second extension positioned
outside said composite portion, wherein said second extension includes a
grooved
portion; and
a threaded compression nut configured to engage said grooved portion of
said second extension.
4a
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An orthopedic device for human implantation comprising:
an intraosseous metal portion; and
an intraosseous composite portion attached thereto, said composite
portion comprising one or more filaments disposed about a longitudinal axis
and
within a polymer matrix, said composite portion further having a gradient in
modulus along the length thereof.
An orthopedic device for human implantation comprising:
an intraosseous metal portion; and
an intraosseous composite portion attached thereto, said composite
portion comprising one or more filaments disposed about a longitudinal axis
and
within a polymer matrix; said composite portion having an equivalent flexural
modulus, Ec, selected according to the criteria:
E,, _< E, where 6b/6b" = 1 when E,=E,
and
Ec> E2 where S/S" = 1 when E,,=E2
wherein Ec is the,modulus of elasticity of the extension of the composite
portion
positioned outside of the metal portion; 6b is the maximum bending stress in
the
bone adjacent to the implanted device at the extension of the composite
portion
positioned outside of the metal portion; ab" is the maximum bending stress in
the
bone adjacent to the implanted orthopedic device wherein the intraosseous
portion is entirely metal affixed in place by polymethylmethacrylate bone
cement;
S is the rotatory stiffness of the implanted device including the composite
portion
and the metal portion; and S" is the rotatory stiffness of the implanted
device of
metal affixed with bone cement.
A process for preparing an intraosseous composite for an orthopedic
device for human implantation, comprising:
winding or braiding fiber into a preform;
placing the preform into a mold;
injecting a thermoset resin therein; and
curing the preform and the thermoset resin.
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A process for preparing an intraosseous composite for an orthopedic
device for human implantation, comprising:
impregnating fiber with a polymer matrix;
winding or braiding the impregnated fiber into a preform; and
molding the preform.
A process for preparing in intraosseous composite for an orthopedic
device for human implantation, comprising:
impregnating fiber with a polymer matrix;
shaping the impregnated fiber into one or more laminates;
orienting the laminates one on top of the other in a manner to give a
desired angularity of fibers within the collective laminate;
molding the collective laminate;
and optionally, machining the molded laminate.
An orthopedic device adapted for implantation within a body, the device
comprising:
an intraosseous metal portion; and
an intraosseous composite portion of one or more layers and comprising
(i) a first extension having a modulus and that is received within said metal
portion and secured thereto and (ii) a second extension positioned outside
said
metal portion;
said composite portion having a length and comprising one or more
filaments disposed about a longitudinal axis by winding or braiding each
filament
along said length at winding or braiding angles with respect to the
longitudinal
axis selected to provide the second extension with a modulus that is less than
the modulus ofthe first extension, the winding or braiding angle in some of
the
layers of the first extension being less than the winding or braiding angle in
the
same layers of the second extension, said filaments further being disposed
within
a polymer matrix.
An orthopedic device adapted for implantation within a body, the device
comprising:
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an intraosseous metal portion;
an intraosseous composite portion of one or more layers and compfising a
first extension having a modulus and that is received within said metal
position
and secured thereto and a second extension positioned outside said metal
portion; and
an intraosseous metal insert interposed there between and connecting
said metal portion to said composite portion,
said metal insert being received within said metal portion and frictionally
secured thereto, said metal insert further forming a taper such that the cross-
sectional area of said metal insert increases from a first end received within
said
metal portion and a second end connected to said composite portion, said metal
insert further being connected to said composite portion by an adhesive joint
or a
shrink fit joint,
said composite portion having a length and comprising one or more
filaments disposed about a longitudinal axis by winding or braiding the
filaments
at winding or braiding angles with respect to the longitudinal axis selected
to
provide the second extension with a modulus that is less than the modulus of
the
first extension the winding or braiding angles in some of the layers of the
first
extension being less than the winding or braiding angle in the same layers of
the
second extension, said filaments further being disposed within a polymer
matrix;
and
said intraosseous metal portion and said intraosseous composite portion
adapted to contact the body.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig 1 is a side view of a metal/composite hybrid hip implant in which the
metal body is received within the composite stem.
Fig 2 is an exploded view of a modular hip implant with a composite stem.
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rig 3 is an exploded view of metal/composite hybrid
tibial knee component.
Fig 4 is a side view of a portion of a metal body
joined to a portion of a composite stem by a particular
5 fastening means.
Fig 5 is a side view of a portion of a metal body
joined to a portion of a composite stem by another
particular fastening means.
Fig 6 is a side view of a portion of a metal body
joined to a portion of a composite stem by another
particular fastening means.
Fig 7 is a side view of a mechanical idealization
of a metal composite hybrid hip implant.
Fig 8 is a side view of a mechanical idealization
of a press fit metal hip implant.
Fig 9 is a side view of a mechanical idealization
of a PMMA grouted metal hip implant.
Fig 10 is a side view of another mechanical
idealizati'on of a metal composite hybrid hip implant.
Fig 11 is a side view of another mechanical
idealization of a press fit metal hip implant.
Fig 12 is a side view of another mechanical
idealization of a PtrIIrlA grouted metal hip implant.
DETAIL D DESCRIPTION OF THE ZNVENTION
The orthopedic devices of this invention are
considered to have a wide range of applicability
throughout the human body. Thus, the intraosseous metal
and composite portions relate generally to any portion
of the body where they may be implanted into bone and
further where prosthetics are desirable. For example,
the devices may be implanted to support rotational
movement at the shoulder, knee, hip, and the like. Much
attention is focused herein on the relation of the
orthopedic device to a hip implant. For this use, the
intraosseous metal portion is considered a proximal
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metal body and the intraosseous composite portion is a
distal composite stem. While many of the features of
the invention are discussed in the context of a hip
implant system, it is intended that the many components
of the invention be given the wider applicability to
implants throughout the body.
Having reference of Figure 1, a basic design of a
metal/composite hip implant is illustrated at 10. A
proximal metal body 12 is attached to a distal composite
stem 14. The proximal metal body 12 is connected to a
ball 16, via neck 18. The ball 16 is rotatably engaged
within a cup of the pelvis (not shown), while the
proximal metal body 12 and distal composite stem 14 are
positioned within an orifice in the femoral canal (not
shown).
The design depicted in Figure 1 shows the
connection between proximal metal body 12 and distal
composite stem 14 as comprising an aperture in the stem
14 which receives the metal body 12. Alternatively, an
aperture in the metal body 12 may receive the composite
stem 14.
The modular hip of Figure 2 is illustrative of one
embodiment of the present invention. The distal
composite stem 14 contains a first extension (in this
case taper 22) that is received within the proximal
metal body 12 and secured by friction (or "press fit"),
and a second extension (in this case the untapered
region 24) that is located outside of the proximal metal
body. Dimensionally, the cross-sectional area of the
first extension is different from or equal to that of
the second extension. In one embodiment, both
extensions are cylindrical. More commoDly and as
illustrated, the first region forms a taper such that
the cross-sectional area of the taper decreases
proximally. The untapered region 24 is typically
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cylindrical in shape with a rounded end. The tapered
region 22 is mated to the untapered region 24 by a
bevelled transition portion 30. This is because the
maximum cross-sectional area of the tapered region 22
may be different from the cross-sectional area of ~he
untapered region 24. The bevelled transition portion 30
has a cross-sectional area equal to that of the.tapered
region 22 and equal to that of the untapered region 24
at their respective interfaces. The tapered region 22
is received within aperture 26 of the body 20. The
aperture 26 is designed to closely follow the contours
of the tapered region 22 of stem 14. Thus, tapered
region 22 and aperture 26 both form conical patterns
with a s0-all diameter at the top increasing
progressively toward the bottom. The composite stem 14
is received within the body 20 and the two parts are
joined by friction between taper 22 and aperture 26 in a
manner commonly called a taper lock joint. The stem 14
and body'20 are secondarily joined by fastening means
28.
Other transition region shapes 30 are possible.
The selection of shape is governed by the physical
requirements of space and geometry of the device as well
as the desired stress concentration for a given
composite material.
The modular knee component of Figure 3 is
illustrative of another embodiment of the present
invention. The distal composite stem 14 is used with an
existing tibial component of an artificial knee. The
stem 14 contains an externally tapered region 24 made to
be positioned within an orifice in the medullary canal
of the tibia. The stem includes a second portion 22
which contains tapered orifice 32 which receives taper
26 in the metal body 20. The stem 14 and proximal metal
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body 20 are joined by friction between taper 26 and
aperture 32.
A key issue in metal-composite hybrid systems is
the transitive region which joins the two dissimilar
materials. Several approaches to the metal-composite
joint have been discovered to be useful, and are
depicted in various figures. In Figures 2 and 3, the
metal body 20 is secured to the composite stem 14 via a
frictional taper lock joint. Other means are also
possible as revealed in Figures 4-6. In one design
(Figure 4), body 20 contains an extension 34 including
pins 36 emanating radially therefrom. Composite 14 is
formed by winding or braiding filaments embedded in a
polymer matrix along its longitudinal axis; these
filaments envelop the extension 34 and pins 36. In
another version (Figure 5), the body 20 contains a
sleeve 36 and the composite stem 14 is shaped to be
adhesively received within the sleeve 36. Yet another
fastenincj means (Figure 6) requires the formation of
extension 34 on body 20 to be received within aperture
32 of stem 14 and to have a threaded portion 38 which
accommodates compression nut 40.
The shrink fit joint mentioned previously is yet
another means to secure the intraosseous metal portion
to the intraosseous composite portion. The shrink fit
joint takes advantage of differences in the coefficients
of expansion of the metal and composite selected. The
components are assembled at a temperature which is
different from the temperature at which the orthopedic
device will be employed. At a suitable assembly
temperature, there is clearance between the metal
portion and the composite portion and the two portions
are fitted together. At the temperature at which the
assembled orthopedic device is used, the dimensionai
characteristics of the metal portion and the composite
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portion have changed relative to one another, causing a
dimensional interference to secure the portions
together.
Another feature of the invention concerns the
placement of a metal insert between the intraosseous
metal portion and the intraosseous composite portion,
thus connecting the two portions. The metal insert is
tapered and received within the metal portion and
secured frictionally in the same fashion as the
composite insert described previously. The metal insert
is connected to the composite portion by either an
adhesive joint or a shrink fit joint. The use of a
metal insert enables one skilled in the art to introduce
a third material with its own stiffness characteristics
into the metal/composite system, to further customize
the treatment of stress concentrations and micromotions
to fit a particular need.
The proximal region 12 is fabricated by
conventional metal working techniques. It may consist
of any of a wide variety of metals, the most preferred
being stainless steel, cobalt-chrome alloy, and titanium
alloy.
The composite stem consists of filaments embedded
in a polymer matrix. The filaments are selected from
any of a wide variety of candidates, the criteria of
selection being ease of manipulation, and compatibility
with the polymer matrix. Preferred filaments include
carbon, graphite, glass and aramid fiber. The organic
matrix is selected according to its compatibility with
both the wound filaments and the tissue and other
materials with which it comes into contact. The matrix
is preferably selected from polysulfone, polyether-
ether-ketone, polyether-ketone-ketone, polyimide, epoxy
and polycyanate.
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An important feature of this invention is the
variable modulus along with the length of the composite
stem.. The equivalent flexural modulus of the portion of
the distal stem which interfaces with bone is up to 16
5 million psi. A preferred range of this modulus is V,
million to 8 million psi.
Thus, according to the invention as described
herein, the intraosseous composite portion may further
have a gradient in modulus along the length thereof.
10 The composite portion can be described as having a first
region that interfaces with the metal portion and a
second region that is distal to the metal portion, with
the modulus of the first region greater than the modulus
of the second region. In a preferred embodiment, the
modulus of the first region of the composite portion is
greater than or equal to the modulus of the metal
portion. In still another preferred embodiment, the
composite,portion has a gradient in modulus along the
length thereof. The incorporates of a gradient modulus
within the composite portion finds particular
application in hip implant systems.
One method of making an orthopedic device according
to the invention comprises the steps of first filament
winding or braiding filaments about a longitudinal axis
to form a preform comprising one or more layers. Each
=layer may contain fibers oriented at a constant angle
along the longitudinal axis or fibers oriented at a
changing angle with the longitudinal axis. The angles
used are selected to give desired mechanical properties
~ both globally and locally in the structure. A winding or
braiding process which results in a constant angle along
the axis is called a linear winding or braiding process.
One which results in a changing angle along the axis is
called a nonlinear winding or braiding process. The
preform is then placed in a mold cavity and a
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thermosetting resin is injected into the cavity. The
preform and the resin are cured to form a distal
composite stem, which is removed from the mold.
' A second method for making an orthopedic device
according to the invention comprises the steps of ~irst
coating filaments of a reinforcing fiber with a polymer
matrix, preferably a thermoplastic polymer. Then, the
coated filaments are wound or braided about a
longitudinal axis so as to produce a part comprising
layers using a system which welds the coated filaments
to the previously wound or braided layers, for example,
by application of heat and pressure. Linear or
nonlinear winding or braiding processes may be used to
create the layers so that the the fibers comprising the
layers may lie at a constant or changing angle with
respect to the longitudinal axis to give desired global
and local mechanical properties. The part formed by
this process may be further consolidated in a subsequent
process such as molding or autoclaving.
A third method for making an orthopedic device
according to the invention comprises the steps of first
cutting sheets of reinforced fiber preimpregnated with a
polymer matrix, preferably a thermoplastic polymer, such
that fiber in each cut sheet is oriented in a prescribed
manner. The cut sheets are then stacked in a particular
order to give a desired angle pattern throughout the
structure which in turn determines the global mechanical
properties of the device. A typical ordering of the
angles would be designated [0, a, a901s, where the 0
denotes orientation parallel to the axis of the part,
Ct denotes orientation at an angle alpha to the axis
(alternating between positive and negative angle) and 90
denotes orientation perpendicular to the axis of the
part and s denotes the repetition of the pattern to give
mirror image symmetry. It is known to those skilled in
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the art that other ordering of the orientations is
possible; for example, one may exclude the 0 or oc or
90 components. The resultant stack is then molded using
heat and pressure to form a consolidated laminate which
is then machined to the final shape of the stem.
Detailed design of the intraosseous composite
portion can be based on an analysis of the mechanical
loading conditions in the composite portion and the
surrounding bone. It is an objective of the present
invention to enhance load transfer to the surrounding
bone by using a composite vs. a metal distal stem. In
particular, the present invention achieves a stress
level in the bone closer to the normal physiological
level than achieved with conventional all-metal
implants.
Figure 7 shows a mechanical idealization of a hip
implant according to the invention in which distal
composite stem 14 and proximal metal body 20 are modeled
as cylindrical entities fixed within a hollow cylinder
of bone 40 representative of the shaft of the femur.
For comparison Figure 8 shows an analogous idealization
for an all-metal system 50 press fit into bone 40 and
Figure 9 is are analogous idealization for an all-metal
system 50 grouted into bone 40 using polymethyl-
methacrylate bone cement 60. In all three figures the
idealized structure is subjected to bending moment M;
bending being the principle mode of loading of hip
implant systems.
The following nomenclature is used throughout this
discussion:
Do: outer diameter of bone 40
Di: inner diameter of bone 40, and outer diameter
of distal portion of stem 14 and outer =
diameter of stem 50
Di": outer diameter of PMNSA grouted stem 50
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Ib: moment of inertia of bone 40 cross-section
IC: moment of inertia of distal portion of
composite stem 14
Ic2: moment of inertia of proximal portion of
composite stem 14
Im: moment of inertia of metal body 20
Im': moment of inertia of press fit stem 50
Im": moment of inertia of PMMA grouted stem 50
Ip: moment of inertia of PMMA cross section
Eb: modulus of elasticity of bone 40
Ec: modulus of elasticity of distal portion of
composite stem 14
Ec2: modulus of elasticity of proximal portion of
- . composite stem 14
Em: modulus of elasticity of metal
Ep: modulus of elasticity of PMMA
Ll, L2, L3, L4: lengths in f igures
The maximum bending stress in bone 40 at section
1-1 for the system shown in Figure 7 is found using
mechanics of materials analysis to be
MEbOo/2
Qb = Eblb + ECIC
This can be compared to the maximum bending stress in
the bone without the implant in place
IrIDo/ 2
060 lb It is an objective of the current invention to maximize
the ratio Qb/6bo by modification of the modulus of
elasticity of the composite, Ec.
For comparison, the maximum bending stress in bone
at section 1' - 1' for the press fit metal system of
Figure 8 is
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MEbDO/2
b EbIb + EmIm'
and the maximum bending stress in bone 40 at section 1"
- 1' ' for the PMMA grouted system of Figure 9 is
is MEbDp/2
~b EbIb + EmIm'' + EpIp
By forming the ratios ab/ab' and Qb/b " one can
quantify the improvement in load transfer with a
composite stem vs. the press fit metal stem and PMMA
grouted metal stem. In particular, it is an objective
of the...current invention that the modulus of the distal
composite stem be selected such that these ratios are
both greater than 1 signifying that the stress in the
bone is greater than that achieved for either the press
fit metal stem or the PMMA grouted metal stem. To
better define this criteria we consider the following
typical values for the parameters defining the
mechanical idealizations:
Do = 25 to 35 (mm) bone outer diameter'
Di = 12 to 22 (mm) bone inner diameter and
composite stem and press
fit metal stem diameter
Di" (Di - 4) (mm) Grouted metal stem diameter
Eb m 2.5 million psi
Em - 16 million psi for Ti-6A1-4V alloy
Ep = .33 million psi
The ratio ab/Qb' was computed as a function of
com osite modulus up to 16 million
p psi for each of two
bone outer diameters. For all values of Ec less than
the modulus of the metal stem, the ratio 6b/Qb' is
greater than 1; i.e., the bone stress is always higher
in the composite implant system than in the press fit
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metal system if Ec < E.. Thus, the modulus of a low
modulus metal, titanium alloy, is one upper limit for
the composite modulus of the invention.
The ratio ab/Qb " was computed as a function of
5 composite modulus up to a value of 16 million psi. It
is apparent that for each stem diameter there is a
modulus E1 lower than the modulus of the metal at which
the ratio ab/Qb " becomes equal to one. At values of
composite modulus lower than E1, the ratio ab/ab " is
10 greater than 1. This value of modulus, thus, becomes a
more preferred upper limit for the modulus of the
composite stem. We state this criteria
Ec < E. where Qb/ab " 1 when Ee = E1,
Those skilled in the art will recognize that there
are other constraints on a hip implant system which may
limit the maximum value of ob/abo which can be attained
in practice. The stem must, for example, be stiff
enough to resist rotatory motion if proximal bone
support is lost as modeled in Figure 10. In this figure
the distal stem 14 remains well fixed to bone 40 but the
proximal body 20 no longer makes intimate contact with
bone. Physiologically, this lack of proximal bone
support may typify the immediate post operative period
prior to tissue ingrowth into proximal porous fixation
means or be representative of the state of the implant
years after implantation where bone remodeling has
caused loss of bone support. In either case the distal
stem 14 must have sufficient rigidity to resist rotatory
motion caused by moment M. The rotatory stiffness of
the structure in Figure 10 is given by:
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S =
L, Ly L4
EbIb) + EbZb+EcIc) + EcIc + Ec2Ic2+Em1m
Again, for comparison Figures 11 and 12 presenta
idealized mechanical models for rotatory stiffness for a
press-fit all metal system and a PMMA grouted system
respectively. The rotatory stiffness for these
structures are given respectively as:
S' _ 1
LL L2 L3+L4
~EbIb) + ~EbIb+EmIm' + IEmIml
Ste = 1
L3 L2 L3+L4
Eblb) + ~EbIb+Emlm''+EpIp) + EmIml+EPIP)
It is apparent that the ratio S/S' will always be
less than 1 when Ec < Em. However, it is known that
grouted metal stems provide adequate rotatory stability;
thus, it is another objective of the present invention
to have the ratio S/S " as high as possible and
preferably greater than 1; i.e., the rotatory stiffness
of the metal composite system should be preferably at
least as stiff as the all-metal system which is grouted
in place with PMMA. To better define this criteria we
consider the following typical values for the parameters
defining the mechanical idealizations:
Do = 25 to 35 (mm) bone outer diameter
Di = 12 to 22 (mm) bone inner diameter and
Composite stem and Press
fit metal stem diameter
Di" _ (D;. - 4) (mm) Grouted metal stem diameter
Z,1 a 25 mm
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L2 m 60 mm
L3 s 50 mm
Lq = 75 mm
Eb = 2.5 million psi
En, = 16 million psi for Ti-6A1-4V alloy
Ep s .33 million psi
The ratio S/S " was computed as a function =of
composite modulus up to 16 million psi, the modulus of
Ti alloy, for a 25 mm and 35 mm bone outer diameter
respectively. For each stem diameter there is a modulus
E2 such that the ratio S/S " is greater than 1 if Ec is
greater than E2. We specify this criteria for the
preferred lower limit on the modulus Ec as:
Ec E2 where S/S " s 1 when Ec E2.
The computed values of El and EZ were plotted as a
function of stem diameter. The most preferred
embodiments of the current invention have composite
moduli which fall between these two curves at the given
stem diameter. It is apparent that all the values in
this most preferred range fall in the range 1 to 8
million psi so this forms a preferred range for the
invention.
It will be apparent to those skilled in the art
that more exact mechanical idealizations, e.g. those
using three dimensional finite element analysis, can be
used to define the most preferred range for composite
modulus even more exactly than in the approximate
analysis disclosed above.
Ultimately, the fatigue strength of the composite
distal stem and the transition 30 will further constrain
the exact details of the composite construction. Often,
strength correlates positively with modulus; strength
considerations may impose higher values for the
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composite modulus than specified in the preferred or
most preferred range.
There are many ways to achieve a composite modulus
in the preferred ranges of the invention. For example,
the axial modulus of polyether-ether-ketone/graphite
composites was computed as a function of angle for
[ t=] constructions. It is apparent that composite
modulus in the range 0 to 16 million psi can be achieved
for values of alpha greater than approximately 15
degrees. Modulus in the preferred range 1 to 8 million
psi can be achieved for values of alpha greater than
approximately 30 degrees. Other methods for achieving
specific modulus values include changing the volume
fraction of fiber reinforcement in the composite or
changing the type of reinforcement, e.g., aramid instead
of graphite.
The method for determining the preferred composite
modulus above refers specifically to that part of the
stem 14 which is exposed to bone. In certain
embodiments, only the region 24 is exposed to bone. The
region 22 interfaces with the metal body 20. In order
to minimize the potential for wear between the region 22
of stem 14 and aperture 26 of body 20, the modulus of
the composite comprising region 22 should be made as
high as possible. One can, for example reduce the fiber
angle alpha in the region 22 to increase the modulus.
This may be accomplished by utilizing nonlinear winding
or braiding paths.
EXAMPLE 1
In this example, preforms of Hercules Magnamitelb
Type IM6 dry fiber were braided so as to produce a stem.
The braid design was such as to introduce a gradient in
the modulus of the composite along the stem length. At
the untapered region 24, which is adjacent to bone, a
low modulus was formed for enhanced load transfer while
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at the tapered region 22, which is adjacent to the metal
proximal body of the implant, a higher modulus was
formed to minimize relative motion between the composite
and the metal. In particular, the braid comprised eight
layers with the following construction:
# Braider Braid AnQle
Lmyer Carriers Braid Type Taoer Distal
1 16 Biaxial 18 45
2 32 Biaxial 15 45
3 32 Biaxial 12 45
4 32 Triaxial 12 45
5 32 Triaxial 15 45
6 32 Triaxial 13 45
7 32 Triaxial 15 45
8 64 Triaxial 15 45
After braiding the preform was inserted in a mold
and a thermosetting resin (Dow Tactixn'' 138 epoxy) was
injected and then cured to produce the finished
composite stem. Inspection of the dimensions of the
structure showed that the process yields a true net
shape part without need for finish machining. The
distal stem diameter was 16 mm.
Strain gages were applied to one sample which was
tested in a distally fixed loading configuration. The
equivalent modulus of the distal portion of the stem was
determined to be 4.7 million psi.
EXAMP LE 2
A composite structure was filament wound with a
right circular cylindrical distal region and a male
taper proximal region to be used in a modular femoral
hip system where the tapered region 22 forms the metal
to composite joint and the untapered region 24 resides
in the femoral canal. A thermoplastic filament winding
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system was used; specifically, parts were wound on a
McCleap-Anderson W60 winder outfitted with a welding
head. Parts were wound using preimpregnated Hercules
Magnamite(& Type AS4 graphite fiber. The composite
5 matrix was Du Pont J2 polyamide.
A filament winding program was developed to produce
a higher modulus in the proximal tapered region vs. the
distal region as well as to insure complete coverage of
the part shape. The higher modulus in the taper region
10 is aimed at reducing metal-to-composite relative motion
while the reduced modulus in the distal region is aimed
at enhancing load transfer to surrounding bone.
Eleven filament wound layers comprise the part per
the following table. Layers 1, 3, 6, 9 and 11 comprise
15 90 degree oriented fiber in both the tapered and distal
regions. Layers 4, 7 and 10 comprise 10 degree
oriented fibers in both the tapered and distal regions.
Layers 2, 5 and 8 are nonlinear winding layers
generating 20 angles in the taper region and 55
20 degree angles in the distal region.
Wind An_C_le
3i3YeZ T3iJeZ T)3SLd1
1 9-0 90
2 20 55
3 90 90
4 10 10
5 20 55
6 90 90
30, 7 10 10
8 20 55
9 90 90
10 10 10
11 90 90
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Filament wound structures were finished on a
precision grinding lathe to achieve the desired
tolerance on the external taper. The diameter of the
distal stem was 14.8 mm. Strain gages were applied to
one sample which was tested in a distally fixed loading
configuration. The equivalent modulus of the distal
portion of the stem was determined to be 6.8 million
psi.
EXANtP LE 3
A (0, 30,90] laminate comprising Hercules
Magnamite Type AS4 graphite.fiber and Amoco's UDEL
1700 polysulfone was formed by compression molding. The
laminate was machined to a stem shape. The diameter of
the distal stem was 16 mm. Strain gages were applied to
one sample which was tested in a distally fixed loading
ccnfiguration. The equivalent modulus of the distal
portion of the stem was determined to be 9.5 million
psi.
EXAMPLE 4
A composite structure was filament wound with a
tapered exterior shape and an internattapered region.to
be used in a modular tibial knee system as shown
schematically in Figure 3 where the internal tapered
region 32 is integral to the metal to composite joint
and the exteriorly tapered regions 22 and 24 reside in
the tibial canal. A thermoplastic filament winding
system was used; specifically, parts were wound on a
McClean-Anderson W60 winder outfitted with a welding
head. Parts were wound using preimpregnated Hercules
Magnamite Type AS4 graphite fiber. The composite
matrix was UDELS 1700 polysulfone.
The winding program comprised 9 aonlinear layers._in
which wind angle varied from 75 to 25'degrees moving
from the larger to the smaller diameter of the external
taper.
