Method of making an endoprosthesis of compact thermoplastic composite material

An endoprosthesis of compact thermoplastics including composite materials is disclosed. The endoprosthesis includes a plurality of prefabricated fibers such as carbon or aramid. Thermoplastics such as polyacrylates, polyaryl ether ketones, polycarbonates, polyether sulphones, polyethylenes or polyproplyenes are used as a binding agent. The prefabricated fibers have an internal core with predominately longitudinally oriented fibers and an enclosing covering with mutually intersecting fibers. The prefabricated fibers are cut to a length which is preferably five times their minimum diameter. The fibers are oriented substantially parallel to the major axis of the endoprosthesis. During molding and even in shallow cross- sections relative to the major axis of the endoprosthesis, each prefabricated fiber is at least partially overlapped by an adjoining prefabricated fiber. It is preferred that the prefabricated fibers at their ends take various forms including tapering, bevelled, and may have scooped, spherical or dished areas. A compression mold is used. The compression mold defines a female cavity which contains the elongate male profile of the endoprosthesis. Preheated prefabricated fibers are placed into the compression mold of the endoprosthesis. The fibers are all generally parallel to the major axis of the mold. Relative to cross-sections taken across the major axis of the mold, the number and array of the fibers varies as the particular cross-section varies. Specifically, at large cross-sections many fibers are found; at small cross-sections, the fibers are reduced in number.

BACKGROUND OF THE INVENTION 
The problem underlying the present invention is to create an endoprosthesis 
with a structure which is simple and reliable to produce, and which 
exhibits throughout high specific mechanical resistance to tension, 
bending and torsion and very good long term rupture strength. Above all, 
mechanical weak points should also be eliminated. This problem is solved 
by an endoprosthesis according to the invention which utilizes 
thermoplastic mold in combination with composite materials in the form of 
fibers cured within a mold. 
The problem with long fibers in a thermoplastic mold is that it is very 
difficult to obtain uniform distribution of the fibers when filling a 
mold. This is especially true where the mold has varying cross-sectional 
areas over a major axis of the formed article. 
Two technical solutions have been offered. In a first known method, parts 
of constant cross-sectional area having a uniform distribution of fibers 
are produced. Afterwards, the formed material is machined to the desired 
shape. 
In a second known method, a mold is filled in a layered manner--either with 
discrete fibers or material having fibers woven and/or unidirectionally 
laid in discrete layers. Thereafter, the discrete layers are pressed and 
heated effecting curing of the finished article. 
SUMMARY OF THE INVENTION 
An endoprosthesis of compact thermoplastics including composite materials 
is disclosed. The endoprosthesis includes a plurality of prefabricated 
fibers such as carbon or aramid. Thermoplastics such as polyacrylates, 
polyaryl ether ketones, polycarbonates, polyether sulphones, polyethylenes 
or polyproplyenes are used as a binding agent. The prefabricated fibers 
have an internal core with predominately longitudinally oriented fibers 
and an enclosing covering with mutually intersecting fibers. The 
prefabricated fibers are cut to a length which is preferably five times 
their minimum diameter. The fibers are oriented substantially parallel to 
the major axis of the endoprosthesis. During molding and even in shallow 
cross-sections relative to the major axis of the endoprosthesis, each 
prefabricated fiber is at least partially overlapped by an adjoining 
prefabricated fiber. It is preferred that the prefabricated fibers at 
their ends take various forms including tapering, bevelled, and may have 
scooped, spherical or dished areas. 
A compression mold is used. The compression mold defines a female cavity 
which contains the elongate male profile of the endoprosthesis. Preheated 
prefabricated fibers are placed into the compression mold of the 
endoprosthesis. The fibers are all generally parallel to the major axis of 
the mold. Relative to cross-sections taken across the major axis of the 
mold, the number and array of the fibers varies as the particular 
cross-section varies. Specifically, at large cross-sections, many fibers 
are found; at small cross-sections, the fibers are reduced in number. 
Once the mold is filled, the fibers are compacted under heated compression 
within the mold. The placed fibers move and conform relative to one 
another and the walls of the mold. Such movement and conformation occurs 
until all areas of the mold are filled. There results a solid cured 
unitary article with aligned fibers as a result of this process. 
It is important to note that the article can be fully formed by this 
process to its desired shape. It is not necessary to machine or otherwise 
process that article to obtain the desired shape. 
This technique can be advantageously used for forming the stem of a femur 
prothesis. The conforming movement of the fibers during compression 
molding and the finish of the ultimately produced endoprosthesis become 
better when the respective fibers are cut at their respective ends with a 
bevel relative to their major axis as distinguished from parallel to their 
major axis. Although the respective fiber pieces have a length and bevel 
cut which is essentially uniform, it has been found that the varying 
cross-sections required in the endoprosthesis are easily formed. Further, 
even though the cross-section of the prothesis may vary widely along the 
major axis, uniform strength results. This uniform strength results from 
the substantially uniform distribution of fibers and thermoplastic along 
the formed endoprosthesis. 
The inherently stable elongated elements, their intimate connection to one 
another and their alignment along the preferred directions produce 
structures with particularly good specific resistance values for tension, 
compression and torsion. In addition local weak points are thereby largely 
eliminated, even with complicated shapes. 
Advantageous developments of the structures in accordance with the 
invention are also disclosed. For example, the elongated elements may have 
a unidirectional core or a core comprising an elongated braid with a small 
fibre angle. The enclosing cover may be formed in a simple manner from 
mutually intersecting layers or a braid. Particularly satisfactory 
mechanical properties can be obtained with highly compacted functional 
parts having a proportion of voids of at most 1%. Particularly suitable 
thermoplastics may be: polyacrylates, polyaryl ether ketones, 
polycarbonates, polyether sulphones, polyethylenes or polypropylenes. 
Suitable fibers may be of carbon or aramid. 
Depending on the shape of the endoprosthesis it may be possible to obtain 
particularly satisfactory mechanical properties with elongated elements of 
which the length is at least five times their minimum diameter, or with 
elongated elements of which the ends finish obliquely and so produce a 
more intimate connection with adjoining elongated elements. The elongated 
elements may be continuous through the endoprosthesis along their entire 
length, that is, may end only at its surfaces. It is preferred that the 
length of the fibers be less than half the length of the main dimension of 
the product along the major axis. Advantageously all elongated elements, 
even in flatter parts of the endoprosthesis, are at least partly 
overlapped by an adjoining elongated element, and at least 6 elongated 
elements may lie within a primary cross-section. The functional parts 
according to the invention may take various forms: they may dwindle to 
nothing at one end or be bevelled, and they may have scooped, spherical or 
dished areas. 
With the structure according to the invention it is also possible, above 
all, to form endoprosthesis which are exposed to high mechanical stresses. 
Particularly stable endoprostheses may have a stem for implantation in a 
tubular bone, or they may form a stem which ends in a cup or a rounded 
head. By thermoplastically installing a metal mesh on the surface, and by 
anchoring in the surface, it is possible to achieve a particularly 
satisfactory connection to the surrounding bone material or the bone 
cement. In addition radiation markers may be built into the endoprosthesis 
of composite material at predetermined points for the purpose of 
post-operative monitoring of the position of the prosthesis relative to 
the bone.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1A shows an endoprosthesis with a tapering end 26. The endoprosthesis 
comprises compacted elongated elements 2 which at all points are oriented 
along a preferred direction 10 and are mutually connected 
thermoplastically over their entire length. The preferred directions 10 
here run from one end surface 12 to the end 26. The shape and size of the 
cross-section changes in the preferred direction 10 or over the functional 
part 1 from a small cross-section 24 on the tapering end 26 to a larger, 
altered cross-section 23 in the upper part of the endoprosthesis. The 
elongated elements 2 (shown in FIG. 1B, or in cross-section in FIG. 1C) 
have an internal core 3 with predominantly longitudinally oriented fibers 
and an enclosing covering 6 with mutually intersecting fibers. FIG. 1C 
illustrates an oval cross-section with a minimum diameter D. The elongated 
element 2 in the example in FIG. 2 has a unidirectional core 4 surrounded 
by two crossing superjacent layers 7. In FIG. 3 the core comprises an 
elongated braid 5 with a maximum fibre angle W1 of 20.degree. to the 
element axis A. The enclosing covering 6 comprises a braid 8 with a 
relatively large fibre angle of, for example, 35 to 45.degree.. In the 
case of the layer 7 this fibre angle may be even greater, for example 
45.degree. to 55.degree.. With the structure according to the invention 
comprising elongated elements 2 with a longitudinally oriented core and 
highly crossing covering fibers it is possible to produce in a simple 
manner endoprosthesis with an extremely wide range of shapes and with 
excellent mechanical strength and torsion properties throughout the volume 
of the endoprosthesis, and without weak points. The elimination of weak 
points is particularly important in endoprostheses of composite materials, 
and is also correspondingly difficult to achieve. 
FIG. 4 show an elongated element 2 of which the length L is more than five 
times its minimum diameter d, and advantageously probably even ten or more 
times this diameter. The ends of the elongated element 2 are cut off 
obliquely or finish obliquely an angle W2 of, for example, 30.degree.to 
60.degree. to the element axis A, with the result that the thermoplastic 
connection between different elongated elements in the endoprosthesis, and 
their compactness, can be further improved. 
The endoprosthesis structure in accordance with the invention makes it 
possible to produce a great multiplicity of shapes, as the examples 
illustrate. FIG. 5 shows an example of an endoprosthesis with a stem 50 
which is implanted in a tubular bone 55. In the upper portion of the stem 
50 a metal mesh 56, for example a Sulmesh titanium mesh, is anchored 
thermoplastically in the surface 11 of the thermal composite material. 
This permits a durable and strong connection between the bone and 
endoprosthesis, because the bone substance grows into the metal mesh 56. 
At the upper end 12 a ball joint 51, for example of metal or ceramic, is 
put on and connected to the endoprosthesis 1. At predetermined points 
radiography markers 59 are incorporated in the stem 50. This makes it 
possible to precisely determine or monitor the position of the 
endoprosthesis relative to the bone 55 even after the operation. FIG. 6 
shows an example with a rounded head 52 belonging to an endoprosthesis. 
The preferred directions 10 and consequently the elongated elements 2 here 
run tangentially relative to the surface 11 of the head. This produces a 
particularly stable surface. 
Referring to FIG. 7A, the preferred method of manufacture of the 
endoprosthesis of this invention is illustrated. Mold M includes sidewalls 
60 and is shown in cross-section along the major axis of the 
endoprosthesis to be formed. Mold M at tapering wall 64, enlarged section 
or head surface 65 and end section 68 is shown. Mold M has been filled 
with fiber sections 2, preferably pre-impregnated having tapered cut ends 
26. The mold as shown is in readiness for the compression molding step 
shown in FIG. 7B. 
Referring to FIG. 7B, compression anvil A is compressed downward relative 
to fiber sections 2. Relative movement between the fiber sections 2 from 
the disposition shown in FIG. 7A occurs. Likewise, curing and pressing 
occurs. 
Some specific examples can be helpful. In all cases uniform chopped fibers 
as previously described have been utilized. These fibers have lengths 
which are at least 5times the dimension of their respective diameters. 
In a first example, material having 60% by volume carbon filament was 
pre-impregnated with 40% by volume PEEK. The chopped pre-impregnated 
fibers are heated up to a temperature between 380.degree. C and 
410.degree. C. They are introduced into a mold with the general alignment 
previously described (fibers generally parallel to the major axis of the 
mold). The mold is maintained at a temperature of 220.degree. C. 
Immediately upon the filling of the mold M, pressing takes place at a 
pressure of 800 bar for several minutes in the mold. When curing has 
occurred, the mold is opened and the finished endoprosthesis pushed out. 
In a second example, material having 50% by volume carbon filament and 50% 
by volume polyamide is utilized. The chopped pre-impregnated fibers are 
heated up to a temperature of 220.degree. C. The mold is maintained at 
100.degree. C. Again, compression molding occurs within a few seconds 
after filling of mold M at a pressure of 500 bar. The compression molding 
continues until curing the finished piece is removed from the mold. 
It will be appreciated that the length of the pre- impregnated fibers can 
vary. For example, they can range from five times their respective 
diameters to half the length of the article formed--preferably as measured 
along the major axis. The fiber mixture can be uniform or varied so long 
as the length and diameter limitations are generally followed. For 
example, a mixture of uniform chopped pre-impregnated fibers could be 
utilized which have lengths larger than five diameters of the discrete 
fibers but additionally include lengths which are less than half of the 
main dimension.