1. Field of the Invention
The present invention relates to improvements for an endosseous implant which is applied to dental use and maxillofacial prosthetics.
2. Related Background Art
Endosseous implants (hereinafter referred as simply "implants") currently available for dental use are classified into two types: those of blade type and those of cylinder type.
A blade-type implant which is formed in the shape of a blade or a plate is used to repair tooth deficiency of a in any part in the jaw bone, but especially to repair free end deficiency of a dens premolaris or a dens molaris. Inside the bone surface, beneath dentes premolaris and dentes molaris, there are inferior alveolar canals. As a result, long implants can not be buried therein and wide blade-type implants are usually used. A perforated site (a prepared hole for implantation) is formed with a drill whose diameter coincides with the width of the blade-type implant employed. As the perforated site is formed by moving the drill in the mesiodistal directions, the shape of the perforation may often be deformed.
A cylinder-type implant which is formed in the shape of a column can also be buried of any part in the jaw bone. The perforated site for this cylinder type implant is formed as a cylindrical hole having more exact dimensions corresponding to the diameter of the implant than the perforated site for the blade-type implant. The diameter of a cylinder-type implant (including thread ridges) is limited from about 3 to 5 mm depending on the width of the jaw bone.
The method of operation employed for implantation of the blade-type implant is the push-in method, while either push-in method or thread-fastening method can be employed for implantation of the cylinder-type implant.
Generally speaking, there are two types of implants with respect to fixation to the bone surface. That is, some implants are fixed to the bone surface, several months after implantation, with fibrous tissue intervening between the implants and the bone, while the other type of implants are made of materials which are directly attached and/or bonded to the bone. The latter has been in the main current because stability can be secured over a long period of time.
The surfaces of an implant which comes in direct contact with the bone when buried may be mechanically worked and be surfaced relatively smoothly. Or the implant may be subjected to physical treatment (typical examples include sand blasting finish, plasma spray, and so on) and/or chemical treatment (such as etching with an acid, anodic oxidation, and so on) to have rough surfaces, which improves affinity with the bone.
When such rough surfaces of the implant are buried in and come into contact with the bone, however, food and various germs in the oral cavity may sometimes enter the interface between the implant and the bone, and the gingiva in the vicinity of the dental neck (the upper portion of the main body beneath the collar) may infected with such germs and suffer from inflammation. In this case, bone resorption further advances, and sometimes the implant comes out. On the other hand, when the implant is smoothly surfaced, food and various germs in the oral cavity which enter the interface between the implant and the bone can be easily removed with a tooth brush. Thus, prevention of infection with germs and inflammation caused by such germs is easy.
Accordingly, only the main body of a typical implant of direct contact type has rough surfaces, while the collar thereof has smooth surfaces.
An implant consists of two parts. The upper part smoothly surfaced is generally called "collar". The lower part, which is the main body of the implant, is buried in the bone and support the whole implant against various stress generated in the oral cavity so that the implant may not be unsteady nor come out. The collar is smoothly surfaced so as to prevent various germs in the oral cavity from entering inward. But the collar, unlike the main body, does not contribute to fixation of the implant in the bone.
An implant is buried in the bone according to either non-submerged implant procedure or the submerged implant procedure. According to non-submerged implant procedure, the upper end of the buried implant projects through gingiva and is exposed in the oral cavity. As a result, a super structure can be mounted over the upper end of the implant without incising gingiva. According to the submerged implant procedure, however, the implant is completely buried in the bone. Thus, in order to mount the super structure over the upper end of the implant, several months after implantation, gingiva should be incised. The collar of the implant buried according to non-submerged implant procedure is from 5 to 10 mm long, while the collar of the implant buried according to the sub-merged implant procedure is shorter; about 0.5 to 2 mm.
The main body is supported in the bone generally with the help of mechanical anchoring of screw treads and/or rough surfaces. Recently, in addition, the surfaces of the main body of the implant are sometimes coated with biologically active material(s) so that the implant is chemically bonded to the bone and more stably supported.
Materials used for the implants may be selected from: metals such as titanium, titanium alloy, stainless steel, and so on; and ceramics such as alumina, zirconia, and so on.
The biologically active materials described above may be selected from biologically active glass, biologically active glass ceramics, hydroxyapatite, and so on. The surfaces of the main body are coated with said biologically active material(s) by binding with an adhesive, the fusion method, the thermal spray method, ion plating, and so on. Otherwise, the main body itself is made of such biologically active material(s).
A cylinder-type endosseous implant may have screw threads around the main body. In this case, the diameter of the collar serving as the head of the screw is designed to be greater than that of the screw threads.
The cylinder-type implant having screw threads can be buried according to one of two methods: the non-self-tapping implantation method (hereinafter referred to as implantation method A) and self-tapping implantation method (hereinafter referred as implantation method B). According to the implantation method A, a threaded hole (perforated site) is tapped and the implant is thread-fastened with the threaded hole. According to the implantation method B, a cylindrical or conical hole is drilled in the jaw bone and the implant which is provided with a self-tapping mechanism (such as cutting blades) screws into the perforated site.
An implant whose main body is roughly surfaced and, at the same time, can be buried according to the self-tapping implantation method B has not yet been put into practical use.