1. The prior art
The U.S. Pat. Nos. 3,387,880, 3,820,534 and 3,918,440 and the corresponding U.K. Pat. No. 1,311,519, 1,393,701 and 1,393,703 describe arrangements for the healing of compound bone fractures, comminuted fractures in conjunction with damage to soft parts of the body, pathological fractures, spontaneous fractures and for overcoming difficulties in healing due to pseudoarthroses, osteomyelitides and the like, in the case of which a magnetic field is induced to produce bioelectric potentials and currents in the defective tissue and as a result to accelerate regeneration and to enable healing to take place in cases which are resistant to therapy.
For this treatment a coil is implanted in the vicinity of the fracture which represents the secondary inductance of a transformer and it is associated with a osteosynthetic part of a conventional type as for example a support plate, a pin driven into the marrow of the bone or even a joint prosthesis, or a coil can be incorporated in such an osteosynthetic part of a joint prosthesis. The inducing magnetic field is excited in a coil surrounding the diseased tissue region. The coil is supplied from a low frequency generator with current and it constitutes the primary inductance of the transformer. The low frequency magnetic field of the coil pervades the whole tissue region, which comprises the bone defect and induces an electrical voltage in the implanted coil, the voltage being caused to act in the tissue via electrodes in the form of bone screws, wires or via metallic foil arranged at the surface of an implantate in an insulated manner.
Experiments on animals and numerous clinical applications of this technique show that in the tissue pervaded by the magnetic field the bone formation prompted by the low frequency alternating electric potentials begins in the immediate neighborhood of the electrodes and continues in the tissue region between the electrodes. As has been shown by histological analysis, the stimulated growth of the bone follows the gradients of the electric potential. If the electrodes extend through the gap of a fracture, generally perpendicular to the fracture surfaces, "bone bridges" form under the influence of the induced potentials and they can quickly take over the supporting fuction of the osteosynthetic elements. As soon as the load carrying capacity of the newly formed bone is sufficient, the whole osteosynthetic part can be removed from the tissue together with the implanted coil and the electrodes.
For bone fracture treatment and bone regeneration experience has shown that a few discrete electrodes, for example two to four electrodes can be used which owing to locally concentrated electric potentials stimulate growth which starts and accelerates regeneration of the tissue. The purpose of the electrodes is a formation of a load carrying bone bridge where the bone is defective. The further conduct of regeneration and consolidation of the bone then takes place under the influence of functional loading in accordance with Wolff's Law.
As regards an implant which is to remain in the tissue for extended periods of time, if possible for the whole of the life of the patient, the requirements applying are in many ways different and they are in effect very much more exacting; the implant is to take over the functions of missing bone components, the support and bearing of an artificial joint or the anchoring of a tooth in the alveolus of the jaw bone.
Metallic implants have not been found suitable for this purpose, since the metal is attacked and not accepted by the tissue. Furthermore ceramic materials developed in more recent times for implantation purposes on the basis of aluminum oxide, quartz, silicon nitride and ceramic compound materials still leave much to be desired. While they have a sufficient mechanical strength and there is a freedom from any substantial attack by body fluids ceramic implant materials have not successfully overcome the problem, as was the case of previously employed metallic implants, of the still insufficient degree of biological tolerance. Even in the case of experiments on animals the process of embedding of, for example, a ceramic joint prosthesis lasts approximately three months despite the high regeneration capabilities of animals. During this time the joint must not be used, that is to say the extremity must be kept still. In the case of accident casualities or aged persons the implant does not become embedded or only does so in a substantially longer period of time. However even periods of one quarter of a year in which the patient must keep the fracture still lead to substantial additional health problems.
Attempts have been made to shorten these times in which the patient must keep still by a special shaping of the implants, for example by the use of a threaded or support rib design, which anchors the implant in the bone with or without the use of a cement. While implants of this type are capable of taking up loads sooner, a functional contact of the implant with the vital tissue is restricted in the case of ceramic prosthesis designed upon these lines to the times applying for metal prosthesis, that is to say on average to approximately 6 to 7 years. An insiduous progressive encapsulation by connective tissue of the implant which is experienced as a foreign body thus leads in the course of time to a painful loosening of the prosthesis. It then becomes necessary to remove the prosthesis and carry out a new implantation, whose life expectancy will, however, be substantially reduced. With the speedy increase in degenerative and accident damage to the human skeleton, which has to be repaired by the implantation of bone prosthesis (at a rate of presently approximately 1500 cases daily) a medicinal and economic problem has been developing on a substantial scale.