The present invention relates to an implant, more particularly, to a dental implant or an artificial radix dentis (root of a tooth) that is to be buried in the natural gums and alveolar bone.
While the artificial root of a tooth is required to possess various characteristics, the most important are the following three:
1) it does not do any harm to the body by releasing ions or some other deleterious substances (i.e., it has biochemical safety); 2) it has good affinity for body tissues including bone (i.e., it has biocomptability); and 3) it maintains balance in strength with other body tissues (i.e., it has mechanical balance).
Dental implant materials known today include ceramics based on apatite [Ca.sub.5 (PO.sub.4).sub.3.(OH,F,Cl)], metals, alumina or zirconia, which are used on their own, and substrates such as metals, alumina or zirconia, which are coated with a calcium phosphate type film. Metals conventionally used as dental implant materials include titanium, cobalt, chromium, tungsten and molybdenum alloys. Alumina used as a dental implant material has been in the form of a single crystal, a combination of single-crystal and polycrystalline structures, and a combination of single-crystal, porous and polycrystalline structures. These and other prior art techniques are described in many patents including JP-A-55-140756 (the term "JPA" as used herein means "an unexamined published Japanese patent application"), JP-A-59-11843 and JP-A-59-82849. JP-A 55-140756 describes a calcium phosphate type sinter, and JP-A-59-11843 and JP-A-59-82849 describe dental implants that are composed of stainless steel and other substrates that are coated with a calcium phosphate type film.
These conventional dental implants, however, have had the following defects. Metals exhibit satisfactory mechanical strength but being foreign to bones, they are not highly biocompatible. In addition, some metals are ionized and will dissolve out to do harm to body tissues. Thus, it is not preferable to use metals alone in an uncovered state. Alumina and zirconia will do no harm to body tissues even if they are used on their own. However, they are too hard compared to natural bone and are not highly compatible with the latter. Thus, during prolonged use, gaps will form between the implant and an adjacent bone, thereby injuring the maxilla or mandible in the joined area. In addition, alumina and zirconia involve cumbersome procedures in handling artificial radix dentis since they cannot be fitted into the maxilla or mandible without being supported by bridges. Metal, alumina or zirconia substrates coated with a calcium phosphate type film have the problem of insufficient adhesion between the substrate and the coating. A problem with apatite-based ceramics is that they must have a considerable thickness in order to maintain a satisfactory level of strength for practical purposes. Furthermore, the bite, or the pressure developed in closing the jaws, will be transmitted directly to the alveolar bone, imparting unpleasant feeling to the user. Thus the artificial radix dentis made of ceramics is not completely satisfactory, especially in terms of biocompatibility and mechanical balance.