Composite orthopedic implant with modulus variations

Novel composite orthopedic devices with modulus variations are disclosed. The device contains an extraosseous portion attached to a first intraosseous portion which is secured to a second intraosseous portion; the intraosseous portions may interface with the intramedullary canal of the human femur. The modulus varies along the length of the device, decreasing from the extraosseous portion to the second intraosseous portion. The variations in modulus are achieved by forming the device from a composite comprising filaments nonlinearly disposed about the longitudinal axes of the device and within a polymer matrix. The method of making the present orthopedic device is also disclosed.

FIELD OF THE INVENTION 
The present invention relates to orthopedic implants, and more particularly 
to load bearing prosthetic devices that exhibit along the length modulus 
variations, and methods of preparation thereof. 
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, and pins, braces and 
plates used to set fractured bones. Particular emphasis has been recently 
placed on hip joint prosthetic equipment. 
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 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. 
Since metals are characterized by a single, high modulus of elasticity (16 
million psi for titanium alloy and 31 million psi for cobalt-chrome alloy) 
it is apparent that optimal design of metallic devices must focus on the 
geometric part of the rigidity without regard to the material parameter. 
Geometric design has several constraints. For example, it is generally 
agreed that good bone apposition is necessary for bone ingrowth into 
proximal porous coatings and that close distal stem fit is necessary for 
rotatory stability. 
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, this device is limited in 
that the orientation of the carbon fibers cannot be systematically 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. 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. Pat. No. 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. 
It is an object of the present invention to provide an orthopedic implant 
with variable modulus wherein the stresses in the surrounding bone are 
more nearly equal to their normal physiological level than achieved in a 
system without modulus variations. It is a feature of the present 
invention to provide a variety of composite materials to design an 
orthopedic implant with particular properties. It is an advantage of the 
present invention that the subject orthopedic implants 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 
comprising an extraosseous portion, a first intraosseous portion attached 
thereto and a second intraosseous portion attached to said first 
intraosseous portion. The orthopedic device is made of composite material, 
with the extraosseous portion having a modulus greater than the modulus of 
the second intraosseous portion. 
In another embodiment according to the invention, the modulus of the first 
intraosseous portion is greater than the second intraosseous portion and 
less than the modulus of the extraosseous portion. In general, the modulus 
of the orthopedic device may vary along the length thereof. This includes 
continuous variation; different regions having particular moduli, and 
combinations thereof. 
The modulus variation may be accomplished according to another embodiment 
of the invention, wherein the orthopedic device is made of composite 
material comprising a plurality of filaments disposed within a polymer 
matrix. The filaments may be linearly or nonlinearly disposed about the 
longitudinal axes of the extraosseous portion and the first and second 
intraosseous portions. The filaments may also be disposed in either a 
helical or a braided pattern, and further other filaments may be 
incorporated therein oriented axially along the longitudinal axes of the 
extraosseous portion and the first intraosseous portion. 
The orthopedic device of the invention may be prepared according to a 
process of the invention. The process comprises shaping a mandrel in the 
configuration of the device, and winding or braiding filaments around the 
mandrel and applying a polymer matrix to form a layer. Additional layers 
may be disposed independently about the various portions thereof. The 
mandrel may be removed.

DETAILED DESCRIPTION OF THE INVENTION 
The orthopedic devices of this invention are considered to have a wide 
range of applicability throughout the human body. Thus, the extraosseous 
and intraosseous 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 extraosseous portion is considered a neck, the first 
intraosseous portion is considered a proximal body and the second 
intraosseous portion is a distal 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 to FIG. 1, a basic design of a hip implant is illustrated 
at 10. A neck 11 is secured to proximal body 12 which in turn is attached 
to a distal stem 14. The neck 11 engages a ball 16, which is rotatably 
engaged within an artificial cup attached to the pelvis. The proximal body 
12 and distal stem 14 are positioned within an orifice in the femoral 
canal. 
Note that the neck 11 of the femoral component is not surrounded by 
bone--the natural femoral neck is removed in conventional hip surgery 
along with the natural head. As such, the prosthetic neck 11 must bear the 
resultant loads and moments transmitted to the femur from the joint and 
surrounding muscles without load sharing. So, the neck 11 must be 
adequately strong to avoid structural failure and adequately stiff to 
avoid excessive deflections which would compromise joint kinematics. There 
are constraints on the diameter of the neck region imposed by the need to 
maintain adequate range of motion of the joint. 
The proximal body 12 of the femoral component provides the primary 
interface of the prosthesis with the femur in the trochanteric region. The 
proximal body 12 is designed for a tight interference fit (or, `press 
fit`) to the proximal femur. Such a fitting is assisted by the ingrowth of 
bioactive surfaces. Ideally, the proximal body 12 is given an anatomical 
shape to follow the contours of the intramedullary canal of the natural 
femur, thus, minimizing the removal of natural bone and maximizing the 
structural capacity of the compound bone-implant system. Alternatively, 
the proximal body 12 is undersized to permit grouting with 
polymethyl-methacrylate (PMMA) bone cement. 
The distal stem 14 serves to provide a secondary interface of the device to 
bone and, in particular, resist rotatory or `toggling`motions which might 
compromise proximal fixation and long term biological fixation. The distal 
stem 14 may be press fit (attempting to follow the natural anatomy) or 
undersized and grouted with PMMA. 
For each functional zone of the device (neck, proximal body and distal 
stem) one can define a set of rigidities and strengths which characterize 
the mechanical response of that region. Generally, the rigidities comprise 
the product of two quantities: a geometric parameter and a material 
parameter. In particular, we can define three rigidities as follows: 
Axial Rigidity=Area X Modulus 
Flexural Rigidity=Moment-of-Inertia X Modulus 
Torsional Rigidity=Polar Moment-of-Inertia X Shear Modulus 
It is these rigidities which determine the response of the system to 
imposed loads and, in particular, control the extent to which the device 
shares load with any surrounding bone. Generally, load transfer will 
increase as device rigidity is reduced. Devices which maximize the amount 
of load transfer to surrounding bone are to be preferred since they will 
result in reduced bone remodeling consequent to "stress shielding". 
However, regional rigidities must be high enough to achieve the functional 
requirements of the device. As noted above, the neck 11 must be stiff 
enough to minimize deflections which would otherwise destroy the proper 
kinematics of the joint. The distal stem 14 must have adequate flexural 
rigidity to resist `rocking` or `toggling` instability. In the present 
invention the distal stem is designed to optimize rigidity to meet the 
competing requirements of load transfer and rotatory stability. In 
addition, regional properties must be adequate to avoid structural failure 
of the device. Material strength usually correlates with material modulus 
which, thus, also sets constraints on the degree to which rigidity can be 
reduced to enhance load transfer. 
Orthopedic implants of composite materials including fibers embedded within 
a polymer matrix offer the ability to control modulus of the implant by 
selectively orienting the fiber patterns with respect to the longitudinal 
or load axis of the device. In one composite system wherein a hip implant 
as in FIG. 1 was constructed of AS4 graphite fibers within a 
polyether-ether-ketone matrix, the modulus of the implant was computed as 
the angle of the fibers formed relative of the load axis was allowed to 
vary. A tenfold variation in modulus is observed by varying the fiber 
orientation from 0.degree. to 75.degree. from the load axis. In 
particular, the use of axially oriented fibers (considered 
0.degree.-5.degree. offset from the load axis) markedly increases modulus 
of the implant. Note further that for this system a fiber orientation of 
from 0.degree. to 75.degree. corresponds to a modulus range of about 1 
million to about 19 million psi. The modulus of cortical bone (2.5 million 
psi) as well as the modulus of titanium/aluminum/vanadium alloy (16 
million psi) are included within this range. In composite systems offering 
more than a single orientation of fiber, it is apparent that increasing 
the amounts of axially oriented fibers will increase the modulus. 
Variations in modulus can also be effected by changing fiber type (e.g., 
aramid v. graphite) and fiber concentration. 
In selecting the percentages of fibers of particular orientations for a 
given composite system, one skilled in the art also examines the strength 
response of the system as a function of fiber orientation. Thus, in the 
system developed previously with graphite fibers in polyether-ether-ketone 
polymer matrix, a system with 100% axial fiber orientation exhibited a 
compressive strength of 160 ksi. A similar system containing 60% axial 
fiber orientation and 40% fiber orientation varied from 5.degree. to 
75.degree. exhibited a varied compressive strength from about 150 to about 
70 ksi. It is important to note that compressive strength generally (but 
not necessarily) decreases as the percentage of axial fiber orientation 
decreases or as the load angle of the nonaxial fiber increases For a 
desired composite system composite strength must be measured by the 
designer relative to fiber orientation. However, it is clear that for 
regions of a device where high strength is prerequisite (e.g., the neck of 
a typical hip implant) low angle constructions are required relative to 
the directions of principal stress. 
The present invention is directed to orthopedic implants, made from 
advanced composite materials and having an intentional variation in 
modulus along the length of the device. In particular, a femoral component 
for a hip implant is defined which has high modulus in the proximal region 
(the neck and it's vicinity) and a lower modulus in the distal stem 
region. Specifically, the neck region has a modulus equal to or higher 
than typical metal alloys used in current orthopedic devices. This insures 
that the neck region does not undergo excessive deflections which would 
compromise functionality of the joint. The distal stem region comprises a 
modulus which optimizes load transfer to surrounding bone and rotatory 
stability of the structure. In particular, a preferred range for the 
modulus in the distal stem is 1 million to 8 million psi. The most 
preferred modulus will vary with the geometry of the system (e.g., the 
stem diameter). 
Detailed design of the second intraosseous portion can be based on an 
analysis of the mechanical loading conditions in this 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. 
FIG. 3 shows a mechanical idealization of a hip implant according to the 
invention in which distal composite stem 14 and proximal body 12 are 
modeled as cylindrical entities fixed within a hollow cylinder of bone 40 
representative of the shaft of the femur. Note that regions 12 and 14 need 
not have the same modulus of elasticity. For comparison, FIG. 4 shows an 
analogous idealization for an all-metal system 50 press fit into bone 40 
and FIG. 5 is of an analogous idealization for an all-metal system 50 
grouted into bone 40 using polymethylmethacrylate 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: 
D.sub.o : outer diameter of bone 40 
D.sub.i : inner diameter of bone 40, and, outer diameter of distal portion 
of stem 14, and, outer diameter of stem 50 
D.sub.i ": outer diameter of PMMA grouted stem 50 
I.sub.b : moment of inertia of bone cross section 40 
I.sub.c : moment of inertia of distal portion of composite stem 14 
I.sub.c2 : moment of inertia of proximal portion of composite stem 12 
I.sub.m ': moment of inertia of press fit stem 50 I.sub.m ": moment of 
inertia of PMMA grouted stem 50 I.sub.p : moment of inertia of PMMA cross 
section E.sub.b : modulus of elasticity of bone 40 E.sub.c : modulus of 
elasticity of distal portion of composite stem 14 
E.sub.c2 : modulus of elasticity of proximal portion of composite stem 12 
E.sub.m : modulus of elasticity of metal 
E.sub.p : modulus of elasticity of PMMA 
L.sub.1, L.sub.2, L.sub.3, L.sub.4 : lengths in figures 
The maximum bending stress in bone 40 at section 1--1 for the system shown 
in FIG. 3 is found using mechanics of materials analysis to be 
##EQU1## 
This can be compared to the maximum bending stress in the bone without the 
implant in place 
##EQU2## 
It is an objective of the current invention to maximize the ratio 
.sigma..sub.b /.sigma..sub.bo by modification of the modulus of elasticity 
of the composite, E.sub.c, 
For comparison, the maximum bending stress in bone 40 at section 1'--1' for 
the press fit metal system of FIG. 4 is 
##EQU3## 
and the maximum bending stress in bone 40 at section 1"--1" for the PMMA 
grouted system of FIG. 5 is 
##EQU4## 
By forming the ratios .sigma..sub.b /.sigma..sub.b ' and .sigma..sub.b 
/.sigma..sub.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 I 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: 
D.sub.o =25 to 35 (mm) Bone outer diameter 
D.sub.i =12 to 22 (mm) Bone inner diameter and Composite stem and Press fit 
Metal stem diameter 
D.sub.i "=(D.sub.i - 4) (mm) Grouted metal stem diameter 
E.sub.b =2.5 million psi 
E.sub.m =16 million psi for Ti-6A1-4V alloy 
E.sub.p =0.33 million psi 
The ratio .sigma..sub.b /.sigma..sub.b ' was computed as a function of 
composite modulus up to 16 million psi for each of two bone outer 
diameters. For all values of E.sub.c less than the modulus of the metal 
stem, the ratio .sigma..sub.b /.sub.b " is greater than 1; i.e., the bone 
stress is always higher in the composite implant system than in the press 
fit metal system if E.sub.c &lt;E.sub.m. Thus, the modulus of a low modulus 
metal, titanium alloy, is one upper limit for the composite modulus of the 
invention. 
The ratio .sigma..sub.b /.sigma..sub.b " was computed as a function of 
composite modulus up to a value of 16 million psi. It is apparent that for 
each stem diameter there is a modulus E.sub.1 lower than the modulus of 
the metal at which the ratio .sigma..sub.b /.sigma..sub.b " becomes equal 
to one. At values of composite modulus lower than E.sub.1 the ratio 
.sigma..sub.b /.sigma..sub.b " is 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 is 
EQU E.sub.c &lt;E.sub.1 where .sigma..sub.b /.sigma..sub.b "=1 when E.sub.c 
=E.sub.1. 
Those skilled in the art will recognize that there are other constraints on 
a hip implant system which may limit the maximum value of .sigma..sub.b 
/.sigma..sub.bo 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 FIG. 6. The distal stem 14 remains well 
fixed to bone 40 but the proximal region 12 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 FIG. 6 is given by: 
##EQU5## 
Again, for comparison FIGS. 7 and 8 present 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: 
##EQU6## 
It is apparent that the ratio S/S" will always be less than 1 when E.sub.c 
&lt;E.sub.m. 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 that 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: 
D.sub.o =25 to 35 (mm) Bone outer diameter 
D.sub.i =12 to 22 (mm) Bone inner diameter and Composite stem and Press fit 
Metal stem diameter 
D.sub.i =(D.sub.i - 4) (mm) Grouted metal stem diameter 
L.sub.1 =25 mm 
L.sub.2 =60 mm 
L.sub.3 =50 mm 
L.sub.4 =75 mm 
E.sub.b =2.5 million psi 
E.sub.m =16 million psi for Ti-6A1-4V alloy 
E.sub.p =0.33 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 E.sub.2 
such that the ratio S/S" is greater than 1 if E.sub.c is greater than 
E.sub.2. We specify this criteria for the preferred lower limit on the 
modulus E.sub.c as: 
EQU E.sub.c .gtoreq.E.sub.2 where S/S"=1 when E.sub.c =E.sub.2 
The computed values of E.sub.1 and E.sub.2 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 will further 
constrain the exact details of the composite construction. Often, strength 
correlates positively with modulus; strength considerations may impose 
higher values for the composite modulus than specified in the preferred or 
most preferred range. 
The region between the neck and distal stem, the proximal body, represents 
the most desirable region for load transfer to bone. Local modulus in this 
region could be optimized to direct load at the medial (inner) calcar 
region of the femur to avoid the stress protection and bone remodeling 
often seen there. In the simplest embodiment of the invention the modulus 
is made to vary continuously from that prescribed in the neck region to 
that prescribed in the distal stem region so as to avoid stress 
concentrations which might otherwise compromise device durability. 
An orthopedic implant is disclosed, which is fabricated from composite 
materials such that the extraosseous portion has an equivalent flexural 
modulus greater than 8 million psi and preferably greater than 16 million 
psi and the second intraosseous portion has an equivalent flexural modulus 
of up to 16 million psi, more preferably in the range of 1 million to 8 
million psi. The first intraosseous portion of the component has an 
equivalent flexural modulus intermediate between them and which preferably 
varies continuously. 
The present orthopedic device may be fabricated by filament winding or 
braiding in which the modulus variation is accomplished by a continuous 
variation in the winding or braiding angle. A winding or braiding process 
which results in a constant angle along the axis is known as a linear 
winding or braiding process. A process which results in a changing angle 
along the axis is known as a nonlinear process. 
Devices according to this invention may be made by first fabricating a 
mandrel 20 such as in FIG. 2 or as in FIG. 9A bent to give an angle beta 
equal to the stem-neck angle desired in the finished part. This mandrel is 
fed into a braiding machine (such as manufactured by Mosberg Corporation) 
which applies bundles of reinforcing filaments at a prescribed orientation 
by controlling the rate of movement of the mandrel relative to the motion 
of the bobbins 30 feeding the filament bundles. The mandrel 20 is shown 
being fed into the braiding machine distal end first. As the mandrel 
advances such that the filament bundles are applied to the proximal region 
of the mandrel, the relative motion of the mandrel and bobbins is adjusted 
to reduce the angle which the filaments make with the mandrel axis, thus 
producing a higher modulus in this region. The resulting fiber orientation 
pattern after a single pass through the braiding machine is shown 
schematically in FIG. 9B. The part may pass through the braiding machine 
several times to apply a number of layers of filaments so as to produce a 
final part of the correct thickness. Fiber angles in each layer may be 
different from those in other layers. Furthermore, all layers need not run 
the full length of the mandrel. This allows variations in part thickness 
along the length. 
The local modulus of the device may be further varied by introducing 
axially oriented fibers along all or a portion of the length of the 
device. These axially oriented fibers may comprise a reinforcing fiber 
which is the same as or different from the angled fibers. A braiding 
machine is configured to introduce axially oriented filaments among the 
angled braiding filaments fed from bobbins 30. FIG. 10 shows schematically 
a layer of axial fibers 31 applied by this process in which axially 
oriented fibers are incorporated in the proximal body 12 of the device, 
thus, increasing the modulus in only that region. 
Filament bundles of varying orientation may also be applied using a 
filament winding machine instead of a braiding machine. Orientation of the 
filament bundles typically fed from a package is controlled in this 
process by the motion of a payout eye, through which filaments pass, 
relative to the rotation of a chuck which holds the mandrel. By increasing 
the speed of the payout eye relative to the chuck rotation, the fiber 
orientation angle is reduced as the proximal body of the mandrel is 
covered, thus, increasing its modulus relative to that in the distal stem. 
Note that curvature of a mandrel may be accomodated by moving the payout 
eye along a second axis normal to the first axis of motion so as to 
maintain close proximity of the payout eye and mandrel. The complex motion 
of the payout eye is best controlled by use of a computer. The motion of 
the filament bundle from a starting point on the mandrel to the end of 
travel along the mandrel and back to the starting point is called a 
circuit. One or more circuits is needed to complete a single layer of 
filaments. Furthermore, multiple layers of filaments may be needed to 
cover the entire surface of the part. The filament orientation within a 
layer and the extent of a layer along the mandrel length may vary from 
layer to layer. 
The orthopedic device may hence be made of a plurality of layers composite 
material comprising filaments within a polymer matrix. These layers are 
independently arranged about the extraosseous portion and the first and 
second intraosseous portions. By "independent arrangement" it is meant 
that each layer may contain any filament type, laid down in any fashion 
(linear, nonlinear, wound, braided, containing axial windings or angular 
windings, and the like) and at any density, irrespective of other layers. 
It is further meant that the various portions of the device may contain 
different numbers of layers. 
It is appreciated that there is an endless set of combinations of numbers 
of layers of composite with selected orientations of fibers. Without 
intending to limit the generality of the foregoing, a preferred device is 
a hip implant wherein the neck and proximal body contain layers of 
composite material with the filaments of each layer independently disposed 
at 5.degree.-45.degree. from the longitudinal axes of these regions, while 
the distal stem contains layers of composite material with the filaments 
of each layer independently disposed at 30.degree.-90.degree. from its 
longitudinal axis. By "independently disposed" it is meant that between 
layers and within each layer the fibers are laid down in any fashion and 
are of any type irrespective of other fibers. The neck and proximal body 
may contain an additional plurality of layers, with the filaments 
independently disposed at 0.degree.-5.degree. from their longitudinal 
axes. 
Alternatively the filament bundles may be applied to the mandrel 20 by use 
of a robot. In particular, a robotic winding system such as that disclosed 
in U.S. Pat. No. 4,750,960 incorporated by reference herein is ideal for 
this application. In this system the mandrel remains stationary and the 
various degrees of freedom of the robot allow application of the filament 
bundles to the mandrel at any orientation. It is apparent that fiber 
orientation may be easily achieved with this technique. Furthermore, use 
of a robot facilitates large changes in orientation angle within a given 
winding circuit. In particular, orientation can be changed such that 
individual circuits have a typical helical geometry in the distal region 
of the structure and become axial in the proximal region. 
Braiding or winding onto a curved mandrel makes removal of the entire 
mandrel difficult. It may be desirable to remove all or part of the 
mandrel to achieve specific mechanical or biocompatibility objectives. A 
multi part structure may be used to facilitate removal of one section of 
the mandrel, e.g., the distal section, while another section, e.g., the 
proximal section is captured in the finished part. For this purpose the 
mandrel is any number of parts joined so that all or some components can 
be removed. 
In an alternative system filament bundles are wound about the distal stem 
in the normal helical manner. However, rather than winding around the 
proximal body of the mandrel they are laid upon a tool or plate configured 
to support the windings in the exterior shape of the proximal body. After 
winding the appropriate number of circuits to build up the desired part 
thickness, the mandrel part together with the plate may be removed leaving 
the all composite part. The robotic winding system disclosed in U.S. Pat. 
No. 4,750,960 is especially well suited for creating this type of 
structure. This structure may itself serve as a captive mandrel for 
further winding or braiding forming a sheath-core structure. This 
particular approach is especially useful for forming hip implants with 
complex outer geometry designed to match the normal anatomy of the 
intramedullary canal of the femur. In this case the core of the implant 
can be designed primarily to achieve the structural requirements of the 
device, including the intended variation in composite modulus along the 
length, while the sheath can be designed primarily to achieve the complex 
anatomical shape. 
The sheath therefore forms an interior portion shaped substantially as the 
exterior surface of the core (which can be the distal stem and the 
proximal body, for example) and an exterior portion shaped substantially 
as the interior surface of the aperture into which it is inserted. 
Those skilled in the art will recognize that the matrix of the composite 
may be introduced prior to braiding or winding by precoating the 
reinforcing filaments, during braiding or winding, or after braiding or 
winding by processes such as resin transfer molding. It is also noted that 
voids within the part and imperfections in the surface of the part may be 
corrected by finishing operations such as molding or autoclaving. 
In an alternative embodiment according to the invention, a sheath/core 
structure is formed such that the core of the structure largely achieves 
the mechanical objectives of the device including the modulus variation 
and the sheath largely achieves the desired shape of the device. 
The composite material may comprise wound 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. 
The nature of the variable modulus and methods of making the orthopedic 
devices of the invention, will be readily understood by having reference 
to the examples that follow herein. 
EXAMPLE 
Example 1 
A multilayer, circular braid of Kevlar 49.RTM. (a registered trademark of 
E. I. du Pont de Nemours and Company) tow was fabricated on a wire mandrel 
which was bent to give the curvature typical of the stem-neck angle of a 
hip implant. Three plies were first applied with a braid angle of 
50.degree. to 60.degree. at all points along the length of the device. 
Four plies of braid with the same angle were then applied to the proximal 
region to build thickness. Finally, an additional four plies were braided 
along the entire length of the device such that the distal region of the 
stem had a braid angle of 50.degree. to 60.degree. while the proximal 
region had a-braid angle of 35.degree.. An epoxy resin was applied to each 
ply after braiding. The entire part was cured as a unit after all layers 
were braided. 
Examples 2 
The braid was formed on a wire mandrel bent to the shape typical of the 
neck-stem geometry of a femoral hip component. Three plies of Kevlar 
49.RTM. tow were braided along the entire length of the structure at a 
50.degree. to 60.degree. angle with respect to the axis of the structure. 
Four additional plies of Kevlar 49.RTM. were braided in the proximal 
region of the structure to build up thickness. Two additional full length 
plies of Kevlar 49.RTM. were then applied. Finally, two plies were braided 
to generate the along-the-length modulus gradient. Specifically, 16 
axially oriented graphite yarns ("pass through" yarns) were introduced 
amongst the 32 braiding yarns forming a triaxial braid in the proximal 
region of the device. The axial graphite yarns were cut as the braider 
moved into the distal region of the device, yielding a gradient stiffness 
structure. The polymer matrix was applied to each ply after braiding. The 
entire structure was cured after all plies were braided. 
Example 3 
A graphite/epoxy prepreg tow (graphite fibers pretreated with epoxy resin) 
was wound onto a shaped mandrel using a robot and computer controls. Wind 
angle was varied such that the distal region of the stem had a wind angle 
from 45.degree. to 55.degree. while the proximal region of the stem had a 
wind angle of 25.degree. to 35.degree., thus, introducing a gradient in 
modulus such that the proximal region was stiffer than the distal region. 
The parts were cured in an oven at 350.degree. F. after winding. 
Example 4 
Using a robotic winding system and a winding circuit including primarily 
nonlinear windings for the distal stem and primarily axial windings for 
the proximal body, a tow of graphite fiber precoated with a thermoplastic 
resin was wound into the shape of a hip implant. The thermoplastic coated 
tow was consolidated during winding by the application of elevated 
temperature and pressure created by tensioning the tow and by use of a 
heated shoe which presses the tow against the mandrel of previously wound 
fibers. Several layers of material were applied in this fashion to achieve 
the desired part thickness. Layers which included the helical/axial 
circuit were used as well as layers which were purely helical in geometry 
but with lower angle in the proximal region of the structure. 
Example 5 
Using a robotic winding system and tooling and winding circuits, the core 
of sheath/core hip implant structure was wound from graphite tow precoated 
with a thermoplastic matrix resin. The thermoplastic coated tow was 
consolidated during winding by the application of elevated temperature and 
pressure created by tensioning the tow and by use of a heated shoe which 
presses the tow against the mandrel or previously wound fibers. 
After producing the core structure, additional helical windings were 
applied using the same robotic winding system in a manner which produced 
an outer shape of complex shape aimed at closely matching the anatomy of 
the medullary canal of the femur.