In the field of oral implantology, the exact positioning of the dental implant in the jaw bone of the patient is crucial to the success of the treatment. This is especially so when the amount of available bone for anchoring the dental implant is limited.
The problem of exact positioning can be addressed, for example, by using a drilling jig in which the at least one opening serves as a drill guide and exactly predetermines the position of the respective dental implant and of the hole drilled for the latter. A suitable technique is described in WO 03/003933, for example, which relates to the production of a super-structure provided with tooth prostheses. Specifically, the method in question comprises the steps of fixing a reference element (hereinafter also called “reference implant”) in the jaw bone, then taking an impression of the jaw bone with the reference element fixed therein. A temporary model of the superstructure is then prepared on the basis of the impression. The model is then placed onto the reference elements, after which a computed tomogram of the jaw and of the model is prepared. The information obtained from the computed tomogram concerning the anatomical structures in the jaw, for example the position of nerve strands and blood vessels, in spatial relationship to the superstructure, allows a suitable choice to be made as regards the position of the holes drilled for the implants. On the basis of the chosen positions, a drilling jig is then prepared which has means for fixing to the reference elements and in which the openings exactly predetermine the position of the respective drilled hole.
A step that is of critical importance involves taking the impression of the jaw with the reference elements, which impression serves as a basis for the preparation, first, of the temporary model and, second, of the drilling jig.
Methods of taking impressions for dental implants have been described many times in the prior art, for example in Schroeder, A. et al., “Orale Implantologie” [Oral implantology], 2nd edition, Georg Thieme Verlag Stuttgart, 1994, page 202 et seq. Reference is also made, by way of example, to EP-A-1274365, EP-A-1274366, WO2007/093648, U.S. Pat. No. 6,213,773, U.S. Pat. No. 6,379,148, U.S. Pat. No. 6,524,106, U.S. Pat. No. 6,508,650, EP-A-0747017 and U.S. Pat. No. 6,045,361. Basically analogous methods are available for taking impressions of reference elements anchored in the jaw bone.
In the customary methods, the impression is generally taken using an impression coping, which usually has an impression cap and, connected to the latter, a suitable fastening element for fastening the impression cap to the dental implant. Using an impression tray, the space around the impression coping and the adjacent tissue structures in the patient's mouth is filled with an initially plastically deformable impression compound that can harden. The impression compound, after hardening, is removed from the patient's mouth and forms a negative model, on the basis of which the temporary model can be prepared.
In the customary methods of taking impressions, a distinction is made between what are called open-tray impressions and closed-tray impressions. The two methods of taking impressions are described, for example, in U.S. Pat. No. 7,066,736.
When taking open-tray impressions, the connection established, by means of the fastening element, between the impression cap and the dental implant is actively undone before the impression compound is removed from the patient's mouth. Since the fastening element is generally in the form of a screw, the impression tray has, in a corresponding area, an aperture through which the screw can be gripped, for example using a screwdriver, and the screw connection can thus be actively undone.
When the impression tray is removed, the impression cap remains in the impression compound in the open tray technique. This means that the position of the dental implants, in respect of the spatial situation in the patient's mouth, can be transferred exactly to the negative model. Impression copings that can be used for taking open-tray impressions are described, for example, in U.S. Pat. No. 5,213,502 and in US-A-2006/0121416.
By contrast, when taking closed-tray impressions, the connection between impression cap and dental implant is not actively undone before removal of the impression compound, with the result that a corresponding opening does not have to be provided in the impression tray. It is conceivable for the impression cap to remain on the dental implant, as is described in WO 2004/039280, for example. In this case, the impression cap is generally inserted again in a further step into the hardened impression compound, i.e. into the negative model. This can lead to inaccuracies, especially if the necessary care is not taken when reinserting the impression cap into the negative model. Alternatively, the impression cap can be designed in such a way that the connection between impression cap and dental implant is undone during removal. A corresponding impression cap is disclosed in U.S. Pat. No. 6,382,977, for example.
An open-tray or a closed-tray impression may be indicated depending on the situation presented in the patient's mouth. Accordingly, different impression caps are used depending on the chosen type of impression.
Against this background, WO 00/02497 and WO 2004/039280 are concerned, for example, with the problem of making available an impression system which can be used both for the open-tray technique and also the closed-tray technique. However, according to the systems described in these documents, the impression cap in the open-tray technique remains on the dental implant and not in the impression compound removed from the jaw, and this can give rise to the above-described problems concerning the accuracy of the transfer.
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