The invention relates to a method of manufacturing a prosthesis to be fixed to implants in the jawbone of a patient.
More particularly, the invention relates to a method for enabling stress-free fixation of such prosthesis to implants.
The invention further relates to a system for manufacturing such prostheses. It is known in dentistry to fix prostheses onto implants; these are of cylindrical or helical shape and mostly made from an indifferent metal such as titanium or titanium compounds, and are preferably fitted in the toothless jaw An implant has an internal screw thread by means of which the superstructure (bride or prosthesis) is subsequently affixed with interposition of an insert (an intermediate ring).
After these dental implants have been placed in the jaw, an impression should be made for the finish and build-up of the prosthesis to be fitted. According to a conventional method, this impression is subsequently cast in plaster and with the aid of attachments a wax model is made. With this wax model, casting takes place in noble metal. The crown or bridge is then fitted in the mouth and placed.
Numerous drawbacks are inherent in this method. For instance, the many steps in the process give rise to inaccuracies in the dimensioning. This leas to stresses in the prosthesis during fixation, which gives rise to undesired forces acting on the implants. The impossibility of making a construction which is completely free of stress used to be less of a drawback in the application to natural elements, since a natural tooth or molar is able to adjust to the situation to a certain extent because it is connected to the jawbone through a root membrane which allows some play. In the case of implants, by contrast, a rigid joint (ankylosis) is involved. There has been much improvement in implantology over the last decade. However, the fabrication of stress-free superstructures remains one of the major problems. No really effective solution to this problem has been found to date.
In patients with implants, the stresses are transmitted through the implants to the surrounding jawbone. This can lead to microfractures and loss of the implant.
A crosspiece (which is in fact a real between a number of implants) which has been screwed tight under tension also leads to substantial overloading and this may even lead to the implant being dislodged. In this connection it cannot be excluded that this is accompanied by damage to the jawbone, with all the attendant problems for the patient.
Apart from the foregoing, the risk is larger particularly with complex superstructures, which often rest on five to eight implants. In addition, placing implants with the superstructures resting thereon is a very costly affair.
Now, in the situation where in a patient a structure is used which rests on two or more implants, it is very important that the implantologist can be assured of complete success. If for instance, in such a situation an implant comes loose, with all the attendant bone damage, the entire construction might be lost.
By means of X-ray photographs taken beforehand, the implantologist can determine the most suitable position for providing the cylindrical mortise holes for receiving the intraosteal implants, but because the arch of the jaw is not equally thick throughout, the possibility cannot be ruled out that upon subsequent placement of the prosthesis the implants introduced are not located equally high and do not run parallel. This can also be a source of stresses.
The article "State of the Art of the CEREC-Method" gives a summary of the systems known in 1991, in which a recording of the mouth or an impression of the mouth is digitized and fed to a computer, whereafter the computer controls a tool for making a prosthesis. It is clearly indicated at page 37 that photogrammetry is one of the possibilities of obtaining a three-dimensional image. For this purpose, for instance two cameras are used. One possible application is to make an image of a prepared tooth or molar, for the purpose of making a crown. Further, at page 41, section 3.1, the manufacture of a frame is mentioned; at page 42, paragraph 3.4, the manufacture of an inlay and veneers is mentioned; and at page 44, sections 3.5 and 3.6 the manufacture of crowns and bridges is mentioned. At page 46, section 4, however, it is clearly stated that designing a prostheses by means of a CAD/CAM system is still a fiction. It is now known, however, to make such images of implants for the purpose of making a prosthesis which is to be fixed to these implants. The present invention, however, surprisingly does allow the fabrication of a prosthesis which is to be fixed to implants. This is a definitive invalidation of the assumption generally accepted heretofore, that is not properly possible to make prostheses by means of such a method. The invention provides the insight that this is quite possible, precisely for implants.
International patent application WO-90/14803 describes a method in which a three-dimensional recording is made and printed using photogrammetry. However, no recording is made in the patient's mouth. Instead, a model of the jaw of the patient is placed on a reference tray. Photographs are taken from different positions to enable the photographs to be processed in combination to obtain a 3D picture. The use of photogrammetry for the purpose of implants is not mentioned.
European patent application 0,040,165 describes in very general terms a process in which a 3D recording of a treated tooth is made. This recording is digitized and fed to a computer. The computer then controls a milling machine for manufacturing a crown. European patent application 0,054,785 describes in very general terms a comparable process as described above. Accordingly, it does not involve any prosthesis which is to be fixed to an implant.
European patent application 0,025,911 describes a process in which a copy of an object can be produced on the basis of a 3D recording of the object. The object in question can be a tooth but also a hand-made prosthesis. The recording is made using photogrammetry. All this means that no prosthesis fabricated by the use a CAD/CAM system is involved, where a 3D recording is made of a prepared tooth or molar to which this prosthesis is to be fixed.
European patent application 0,250,993 relates to the making of recordings by means of a video camera. On the basis of a predetermined algorithm, the video image is frozen at a particular moment. On the basis of the still picture thus obtained, a further analysis is performed.
German patent application 33 20 395 describes a recording device which is placed over a tooth or a treated tooth for obtaining a 3D recording. If desired, the two recordings are compared and processed in combination by a computer for the purpose of fabricating a prosthesis.
German patent 282,615 of former East-Germany describes in very general terms a method for fabricating crowns on the basis of a 3d recording by means of a computer-controlled tool. However, this method involves the making of a replica of an object to be copied, on the basis of a 3D recording of that object.
In summary, it can be stated that the use of photogrammetry in a method for fixing dental prostheses onto implants in the jawbone of a patient is not known from any of the above-discussed publications.
Also known are methods and systems in which the object to be recorded is actively irradiated with electromagnetic waves.
Such methods and systems utilize, for instance, laser beams which are directed to the object under examination and the reflections of which are subsequently analyzed. According to another active method, a predetermined pattern, for instance a rectanglular grating, is projected on the object in question. The pattern will be distorted depending on the shape of the object. Then an image of the object in digitized form is fed to a computer. By comparing the distorted pattern with the original pattern, an idea of the three-dimensional shape of the object can ben obtained.
The following references relate to such active systems; FR-2,635,965; FR-2,682,473; FR-2,690; WO-91/03980; WO-91/18356; WO-91/05520; WO-94/00074; EP-0,299,490; EP-0,091,876; DE-4,034,007; DE-3,541,891 and U.S. Pat. No. 4,663,720.
The system according to the present invention, however, does not necessarily utilize active electromagnetic radiation sources that are part of the system.