Dentistry – Prosthodontics – Preliminary casting – model – or trial denture
Patent
1996-01-26
1999-01-12
Lewis, Ralph A.
Dentistry
Prosthodontics
Preliminary casting, model, or trial denture
433223, 433173, 433 68, A61C 1300
Patent
active
058578536
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
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 makin
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Strid, K.-G., "On the application of photogrammetry to the fitting of jawbone-anchored bridges," Prosthetic reconstructions on osseointegrated implants: Proceedings of the Gothenburg Conference, Sep. 1983, Albrektsson, T., et al., eds., Swedish Dental Journal (Stockholm 1985), pp. 93-105.
Quaak Johannis Adriaan
van Nifterick Willem Frederick
Alten Brett G.
Ingerman Jeffrey H.
Lewis Ralph A.
Nobel Biocare AB
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