1. Field of the Invention
The present invention relates to a dental material which is curable in a first stage to an elastic phase, in which the material can be processed mechanically or surpluses can be removed, and in a second step, to its final form.
2. Background Information
Dental materials which have an elastic phase over a practice relevant processing period, which must be neither too long nor too short, are particularly well suited for special fields of use and are therefore preferred over the materials on the market at the present time.
These special fields of use are, on the one hand, the field of temporary crowns and bridges for the provisional treatment of prosthetic works and of temporary fillings for the provisional treatment, especially of several fillings situated next to one another. On the other hand, they are plastic dental cements for the cementing-in of a dental workpiece, the surpluses of which can easily be removed in this elastic phase.
The materials used for temporary crowns and bridges which are on the market, at present, are predominantly self-curing polymethacrylate/methacrylate systems, composites based on methacrylate or other polymer systems such as, for example, epimine systems.
To prepare a temporary or provisional crown or bridge, the mixed temporary crown and bridge material is placed in a suitable mould, with which the material is placed in the predetermined form on the tooth stump or stumps to be treated. The material then hardens in this impression. At a certain time, however, at which the material has still not completely hardened, the impression must be removed, together with the temporary provision. With the materials used up until now, the time period available for this is, however, very short and is often missed.
The reasons for this are that the hardening of these materials in the impression is difficult to control and that it is further influenced by many disruptive factors such as mixing conditions, temperature, moisture, etc. This has the consequence that if the crown is removed too early, the material is either still viscous or not dimensionally stable, i.e., the temporary treatment, namely, the provisional bridge or crown, must be prepared again. If, on the other hand, one waits too long, the temporary crown or bridge can no longer be removed from the stump, especially when decaying areas are present. Added to this issue is the fact that, as a rule, temperature problems are also connected with the use of these materials. The polymerization reaction which, as mentioned, is not controllable, proceeds exothermically. So much thermal energy is liberated that damage to the pulp can occur.
Similar problems occur during use as a temporary filling material. With large cavities situated next to each other (predominantly MOD), which are treated with inlays prepared in the laboratory, the provisional treatment of the cavities is frequently undertaken in such a way that the cavities situated next to each other are treated en bloc. For this purpose, the self-hardening temporary material is introduced into the isolated cavities and removed again from these cavities at a moment when the material still has a certain elasticity. If this moment is not exactly timed, problems arise as have already been described with respect to the use as temporary crown and bridge material. In most cases, hardening has proceeded too far, meaning that the temporary filling can no longer be removed. Furthermore, problems connected with temperature stress are also to be expected here.
After the temporary filling has been removed from the cavity, decaying areas are removed, and then the workpiece is fixed with a temporary cement.
A similar technique has very recently evolved for the production of chairside (i.e. at the dentist's chair) or labside (in the dental laboratory) milled ceramic inlays. As described above, the isolated cavity is filled with a so-called proinlay material and the material modelled as the final inlay fillings is to be shaped. In most cases, one also encounters problems here in removing the proinlay from the cavity, on the one hand because of the decaying areas, and on the other hand because of the need for very precise timing. The proinlay is afterwards transferred with a copying cutter into the definitive ceramic inlay. Chairside, the ceramic inlay is fitted immediately, and with the labside technique, the cavity is treated with a material described above.
It is clear from the problems described that, both for temporary crowns and bridging materials and for temporary filling materials, there is the need for a material which hardens controllably in a first stage of a certain elasticity and which remains stable over a practice-relevant period and can therefore be removed easily from the stump or from the cavity. It can then be worked and finished while still in this form and subsequently hardened in a second stage to its final form.
The other field of use mentioned above is a composite cement with possible surplus removal. When cementing-in dental workpieces such as e.g. ceramic inlays, ceramic or metal crowns with tooth-colored composite cements, it is very difficult to recognize cement surpluses and remove them as gently as possible. If the surpluses are removed in an unhardened state, the cement from the cement joint is taken out also, in most cases, resulting in a cement depletion. If, on the other hand, one tries to remove the cement surplus in the hardened state, the cement as well as the hard tooth substance are also very severely damaged in most cases.
It is clear that for this field of use, also, there is a need for a material which hardens controllably in a first stage to an elastic phase in which the workpiece is already fixed in the correct position but which allows the surpluses of the cement to be cut away with a sharp instrument such as a scalpel.
In EP-A-195 224, applicants describe a dental material for producing artificial teeth or parts such as crowns or inlays from two separate components to be mixed together during use. In a first stage, a polyfunctional isocyanate is condensed with a polyalcohol, with the help of tin catalysts, to a polyurethane which is present as an elastic phase which remains stable over an extended period of time. Then, in a second stage, at least one methacrylate compound contained in the overall system is fully cured by hot, cold, or light polymerization to artificial teeth or tooth parts.
EP-A-410 199 (Bayer) teaches plastics, fully curable in a number of steps, which consist of at least one silicopolyether, at least one radically curing monomer, at least one catalyst for hot, cold, or light polymerization, and at least one catalyst for the condensation of the silicopolyether. Such plastics based on silicopolyethers do, however, have the disadvantage that the silicopolyethers negatively influence the polymerization of the radically curing monomers. The polymerization of the second stage, i.e. of the monomers or the methacrylate, is incomplete which again has a negative effect on the physical properties of the hardened plastics. In particular, the flexural moduli of the cured plastics are unsatisfactory.
Epimines and their use in the dental field are also known. They are used particularly as molding materials and as temporary crowns and bridge materials. Epimines with a molecular weight of circa 6000 are used in molding materials, as described in DE-B-1 544 837. They polymerize to an elastic phase and in doing so serve their purpose. Short-chained epimines with a molecular weight of circa 500 are used in a temporary crown and bridge material (U.S. Pat. No. 3,453,242 and U.S. Pat. No. 4,093,555). These polymerize to a hard composition, whereby an elastic phase is passed through for a short time during the polymerization.
To the best of the knowledge of the applicants, there are no publications in the art in which a product is obtained with epimines alone, or in combination with other polymerization systems, which remains in an elastic phase over a longer period of time and then passes into a hard state after suitable activation.