The invention relates to a bone cement based on physiologically tolerated polymers, the incorporation of acoustic perturbation elements having a sound resistance differing from that of the polymer making it possible, in the case of a necessary re-operation, to disrupt the bone cement by the application of acoustic waves to such an extent that it can easily be removed from the implant location.

BACKGROUND OF THE INVENTION 
The invention relates to a bone cement based on physiologically tolerated 
polymers, in particular on polyacrylates and/or polymethacrylates prepared 
from prepolymers and monomers, and to a process for mechanically 
disrupting fully polymerized plastics, in particular these bone cements. 
A central problem in the implantation of joint endoprostheses is the 
anchorage of the prosthesis in the bone substrate. For this purpose, it is 
known to use bone cements which are based on acrylate and which are 
prepared by mixing and fully polymerizing pulverulent prepolymers with 
liquid monomers containing initiators and accelerators for the 
polymerization. These are used in orthopaedic surgery for the reliable and 
rapid primary fixing of the joint implant to the bone, thus enabling the 
patient to put a load on the joint at an early stage. The bone cement, 
which initially is still pasty in the mixing phase and cures slowly, fills 
the space between the bone and the joint implant almost without any gaps, 
but very rapidly gains its full strength as a result of the continuing 
polymerization process. 
This type of fixing joint endoprostheses to the bone by means of fully 
polymerized bone cement is nowadays a surgical technique which is used and 
recognized throughout the world in orthopaedic surgery. Problems such as 
excessive liberation of energy and hence an unduly high temperature during 
the polymerization, whch leads to necrotization of the tissue, have been 
solved in principle, as have the questions of adequate biocampatibility of 
the bone cement. 
In the long term, however, loosening of the metal implant or of the plastic 
implant in the bone cement substrate can occur, so that prosthesis 
replacements with removal of the bone cement become necessary. The removal 
of the bone cement from tubular bones is technically very difficult and 
involves a long operating time in the case of re-operations of joint 
endoprostheses, so that the predominantly old patients are put at 
considerable risk by long operating times, long bleeding times of the 
exposed soft tissues and an increased danger of infection due to long 
times with an open wound. The importance of this problem can be gauged by 
the fact that up to 20% of all joint prosthesis operations carried out 
nowadays are re-operations. 
SUMMARY OF THE INVENTION 
It was therefore the object to provide a bone cement which meets all the 
requirements with respect to stability and processability, but which can 
easily and quickly by removed from the bone in the case that a 
re-operation becomes necessary. 
This object has been achieved by the present invention. In fact, it has 
been found that perturbation elements which have been incorporated into 
the polymer matrix and the acoustic resistance of which differs from that 
of the polymer matrix can be excited by means of acoustic waves in such a 
way that tensile and compressive stresses, which lead to disintegration of 
the polymer matrix, are generated at the acoustic interfaces. 
The invention therefore relates to a bone cement based on physiologically 
tolerated polymers, in particular on polyacrylates and/or 
polymethacrylates prepared from prepolymers and monomers, wherein acoustic 
perturbation elements having an acoustic resistance differing from that of 
the polymer are incorporated into the polymer. 
The invention also relates to a process for mechanically disrupting fully 
polymerized plastics, in particular bone cements based on physiologically 
tolerated polymers, in particular on polyacrylates and/or 
polymethacrylates prepared from prepolymers and monomers, which comprises 
incorporating acoustic perturbation elements having an acoustic resistance 
differing from that of the polymer, before the polymer is fully cured, and 
destroying the polymer matrix by selective excitation of the perturbation 
elements by means of acoustic waves. 
The advantage of the present invention is that the bone cement according to 
the invention can be used by the surgeon in the same way as the hitherto 
known bone cements. With respect to short-term and long-term stability, 
the incorporation of the perturbation elements does not lead to any 
significant deterioration. An advantageous effect with respect to the 
stability of the bone cement is obtained when the perturbation elements 
are as similar as possible to the polymer with regard to the quasi-static 
mechanical properties, that is to say the parameters such as, for example, 
elasticity and compressibility which determine the stresses arising under 
normal loading (walking, running). Especially, however, when these 
properties differ, it is advantageous when the perturbation elements do 
not possess any sharp edges or points which can lead to undesired local 
stress concentrations and hence to cracks forming in the polymer matrix. 
Perturbation elements with rounded corners and edges, such as, for 
example, ellipsoids of revolution, cylindrical discs with rounded edges, 
or spheres, are therefore preferred. 
In place of individual particles, however, it is also possible to use 
perturbation elements in the form of threads, braidings, fabrics, foils or 
plates. A nettype fabric, for example, can be used to particular 
advantage, since in this way, on the one hand, a considerable 
stabilization of the bone cement can be achieved and, on the other hand, 
in the case of a re-operation, the cement can be very effectively detached 
along the bone/cement contact layer, if the net is arranged in this 
boundary region. 
The size and shape of the perturbation elements should be such that, on the 
one hand, optimum energy absorption from the extracorporeally generated 
acoustic waves is possible and that, on the other hand, there is no 
substantial interference with the processing of the bone cement. 
Perturbation elements of a size or thickness of about 0.01 to 2 mm, in 
particular a thickness of about 0.1 to about 1 mm, are therefore 
preferred. 
In order to obtain effective disruption, a large number of perturbation 
elements, distributed as homogeneously as possible in the polymer matrix, 
is of course necessary. The more perturbation elements ae present, the 
more effective is the disruption. On the other hand, an excessive content 
of perturbation elements impairs the mechanical properties of the bone 
cement. A content of about 0.5 to about 20% by volume, in particular of 
about 3 to about 10% by volume, is therefore preferred. 
The material used for the perturbation elements is a biocompatible material 
of which the acoustic resistance, that is to say the product of density 
and sound velocity, differs markedly from that of the polymer matrix. In 
particular, materials are used of which the acoustic resistance differs 
from that of the polymer by a factor of at least 1.5, preferably a factor 
of at least 1-10. The acoustic resistance of the perturbation elements can 
here by either greater or smaller than that of the polymer. Materials 
which can be used are therefore either bodytolerated metals or alloys, 
such as, for example, TiAl.sub.5 Fe.sub.2.5, CoCrMo or tantalum, or oxidic 
materials, in particular in the form of burned oxide ceramics such as, for 
example, Al.sub.2 O.sub.3, or glass or carbon. In place of these dense 
perturbation elements, however, it is also possible to use perturbation 
elements of a very low density, such as, for example, air bubbles or gas 
bubbles, which can be stirred in during mixing of the cement in a 
homogeneous distribution, or which can be liberated from suitable 
substances, for example by thermal decomposition during setting of the 
cement. 
In order to obtain the greatest possible disruption of the bone cement 
throughout its entire volume, if individual particles are used, these 
perturbation elements should be distributed as homogeneously as possible 
in the bone cement. In the case of very heavy, especially metallic 
perturbation elements, it can therefore by advantageous to embed them in 
an envelope of polymer, in order to improve the dispersibility and to 
reduce sedimentation. 
The bone cement according to the invention is prepared analogously to the 
known bone cements. These are prepared in such a way that approximately 
two parts of a finely particulate prepolymer containing a polymerization 
catalyst (for example dibenzoyl peroxide), in particular polymethyl 
methacrylate or a copolymer of methyl acrylate and methyl methacrylate, 
are mixed with approximately one part of the liquid monomer, for example 
methyl acrylate or methacrylate or mixtures thereof, containing an 
accelerator (for example dimethyl-p-toluidine), to give a formable 
composition which is implanted into the body and cures therein. Such bone 
cements are commercially available, for example under the trade mark 
Palacos.RTM.. In addition, pharmacologically active substances such as, 
for example, antibiotics or materials which facilitate anchorage of the 
bone cement in the body such as, for example, absorbable 
tricalciumphosphate can also be incorporated therein. To prepare the bone 
cement according to the invention, the required quantity of perturbation 
elements is additionally incorporated into such a known bone cement. 
In processing and also in the mechanical properties obtained after curing, 
the bone cement according to the invention is equivalent to the known bone 
cements. The advantage manifests itself, however, when a re-operation 
should become necessary. In this case, tensile and compressive stresses, 
which lead to breaking of the bond and to disintegration of the polymer 
matrix, can be generated according to the invention by means of 
extracorporeal acoustic waves at the acoustic interfaces between the 
perturbation elements and the polymer matrix. 
The acoustic waves used here can be either shock waves generated 
extracorporeally and focused upon the bone cement plug or ultrasonic waves 
having a frequency tuned to the characteristic frequency of the 
perturbation elements. 
Shock waves are already used in medical therapy for the crushing of urinary 
stones. The equipment used in this case for generating and focusing the 
shock waves can in principle also be used for the disruption of the bone 
cements according to the invention. By this means, shock waves in the 
pressure range from about 0.2 to about 5 kbar, in particular from about 
0.5 to about 2 kbar, are generated. The profile of these shock waves, 
which have half widths of about 10.sup.-8 to about 10.sup.-6 seconds, is 
matched to the geometry of the perturbation elements in such a way that 
the half width of the shock wave is smaller than the thickness of the 
perturbation element and the pressure rise time is smaller than the half 
width by a factor of about 10. Preferably, shock waves of square profile 
which have pressure rise times of less than 10.sup.-7 seconds are used. 
In contrast to the equipment used for the crushing of urinary stones, no 
involved and expensive stereo radiographic location systems and 
patient-positioning systems are required for the disruption of the bone 
cement according to the invention; instead, an optical-mechanical 
positioning system is provided for the task according to the invention. 
Using the X-ray and ultrasonic diagnostic equipment available in 
orthopaedic hospitals, the position of the bone cement plug in the tubular 
bone is determined and marked on the skin surface. The volumes to be swept 
by the focal point of the shock waves are unambiguously fixed 
geometrically relative to the skin surface by means of a measurement of 
the thickness of the connective tissue and bone tissue between the skin 
surface and the bone cement around the entire circumference of the joint 
endoprosthesis. By means of an optical system which is either integrated 
into the shock wave-generating system or is rigidly connected thereto on 
an adjustment rail, the focal point of the shock wave can be positioned 
exactly onto the bone cement which is to be destroyed. 
A likewise inexpensive alternative to the optical-mechanical positioning 
system is an ultrasonic location system integrated into the shock wave 
generator. 
An extracorporeal shock wave treatment of the bone cement of a patient just 
before the re-operation of his implant guarantees easy and reliable 
removal of the bone cement. 
In place of shock waves, however, sonic waves in the ultrasonic region, 
that is to say in the MHz frequency range, can also be used. The 
ultrasonic frequency must be tuned to the characteristic frequency of the 
acoustic perturbation elements. The disruption of the bone cement proceeds 
via resonance effect of the perturbation elements. As a rule, frequencies 
from about 1 to 100 MHz are used, with an amplitude from about 0.1 to 10 
bar. 
The use of ulatrasonics has the advantage that ultrasonic equipment is very 
much less expensive than shock wave equipment. Electrodes as an expensive 
consumable material are completely absent, and the pressure amplitudes are 
smaller by a factor of about 1,000. Moreover, the ultrasonic equipment for 
the destruction of the bone cement could be used at the same time for 
locating the bone cement plug. However, it is a precondition for the use 
of ultrasonics that all the perturbation elements have substantially the 
same resonance frequency and hence the same geometry. 
In any case, however, the removal of the bone cement is considerably 
facilitated by using the appropriate method just before the necessary 
re-operation.

EXAMPLE 1 
Polymer matrix: Polymethyl methacrylate with added antibiotics 
Perturbation element: Ellipsoid of revolution, produced by revolution of an 
ellipse with semiaxes of 1 mm and 2 mm about the smaller axis and 
consisting of the material TiAl.sub.5 Fe.sub.2.5 
Shock wave: 
Pressure range &gt;1 kbar 
Pressure rise time &lt;10.sup.-7 seconds 
Half width &lt;10.sup.-6 seconds 
EXAMPLE 2 
Polymer matrix: Polymethyl methacrylate with 1% by weight of gentamycin 
Perturbation element: Surgical metal thread in the form of a net having a 
mesh width of about 5 mm and a thread thickness of about 200 .mu.m. 
Shock wave: 
Pressure range .about.1 kbar 
Pressure rise time .about.10.sup.-8 seconds 
Half width .about.10.sup.-7 seconds