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with each other. The materials described in these publica› tions comprise, in addition to phosphoric acid, amino acids, complexing agents, inorganic or organic acids, sometimes mixed with metal salts, for the pre-treatment of the tooth surface. 

[0011] Depending on the type of the acid, further addi› tives, concentration and duration of application, these alter› native conditioning agents sometimes have positive effects on the dentin morphology. However, in comparison with phosphoric acid, they usually lead to a poorer conditioning of enamel, which results in a less durable adhesive bond between adhesive and enamel. As is familiar to a person skilled in the art, this is normally accompanied by a reduced marginal seal quality and a reduced longevity of the resto› ration. To achieve a strong enamel adhesion and a high enamel margin quality, the total-etch technique using phos› phoric acid therefore still represents the most effective method (Lopes, G. C. et al.; Enamel acid etching: a review, Compend. Cantin. Educ. Dent. 2007, 28, 662-669). 

[0012] The object of the invention is to provide condition› ing agents for the treatment of teeth which provide results which are comparable to the total-etch method using phos› phoric acid with respect to enamel adhesion and marginal seal quality, but which do not have the above-described disadvantages of said method. In particular, the agents are to yield a high level of adhesion irrespective of the degree of residual wetness of the tooth surface and not to induce the degradation of collagen. In addition, the agents are to be able to be used simultaneously on tooth enamel and on dentin without the need to observe different exposure times. Finally, the agents are not to cause the occurrence of post-operative sensitivity through the over-etching of dentin. 

[0013] This object is achieved according to the present invention by aqueous solutions which comprise at least one salt of a transition metal and/or one salt of a main group metal. According to the present invention, metals of the fourth to sixth periods of the periodic table of elements are preferred. It was surprisingly found that such solutions are suitable for use as conditioning agents in the case of dental restoration therapy. 

[0014] According to the present invention, by a condition› ing agent for dental restoration therapy is meant an agent that can be applied directly to the tooth surface, i.e. to the tooth enamel and/or the dentin, and in particular to the smear layer present after machining of the tooth, and that prepares the tooth surface for the application of subsequent dental restoration materials, such as e.g. adhesives, by improving the adhesion of these materials to tooth enamel and/or dentin. 

[0015] It was found that aqueous solutions of transition metal and main group metal salts are capable of etching the tooth structure and of dissolving the smear layer at least partially, even without the addition of inorganic or organic acids. 

[0016] According to the present invention, solutions of salts of polyvalent metals are preferred, in particular of metal salts in which the metals have a valency of +2 and in particular +3 or +4. Particularly preferred are salts of metals of group 4 (CAS group IV B) as well as of main groups 14 and 15 (CAS groups IV A and VA) of the periodic table of elements, quite particularly preferred are the salts of Zr $^{4}$+, H:t4+, Sn $^{4}$+ and Bi$^{3}$+ and Ti$^{4}$+ and in particular salts of Zr$^{4}$+, Hf4+, Sn$^{4}$+ and Bi$^{3}$+. Of these the salts of Zr$^{4}$+, Hf4+, Bi$^{3}$+ 

and Ti $^{4}$+ and in particular salts of Zr $^{4}$+, Hf4+, and Bi3+ are most preferred. The salts can be used alone or as a mixture of two or more of said salts. 

[0017] In all cases, those metal salt solutions are quite particularly preferred according to the present invention that contain no added acid in addition to the metal salts, wherein by acids is meant Bronsted acids, i.e. proton-donating acids with a pK value <9. The metal salt solutions according to the present invention thus preferably contain no mineral acids, no phosphoric acid and no organic acids such as carboxylic, phosphonic and sulphonic acids. 

[0018] The metal salts used according to the present invention are preferably the salts of strong acids, in particu› lar of mineral acids, i.e. they preferably contain as anions c1-, Br-, 1-, NO$_{3 }$ -, so/-, optionally as mixed salts with oxoamons. 

[0019] In general, those salts are preferred that contain no anions which impede the formation of aqua complexes or the achievement of homogeneous solutions in the desired concentration range or that have a buffering effect. The solutions according to the present invention thus preferably contain no EDTA, no nitrilotriacetic acid (NTA), no formate, acetate, carbonate, hydrogen carbonate and no salts of acids with a pKa value >3. 

[0020] Particularly suitable transition and main group metal salt solutions according to the present invention are solutions of SnC1$_{4 }$ and in particular of ZrOCl$_{2}$ , ZrO(NO$_{3}$ ) 2 , Zr(SO$_{4}$ ) 2 , Hf(SO$_{4}$ ) 2 , HfC1$_{4}$ , Hf(NO$_{3}$ ) 4 , TiOSO$_{4}$ , Bi(NO$_{3}$ ) 3 or mixtures thereof. 

[0021] In addition to the above-named metal salts, the transition and main group metal salt solutions according to the present invention can preferably additionally comprise one or more salts of other main group and/or transition metals, wherein in the case of main group metals salts of metals with a valency of +2 to +3 are particularly preferred and in the case of transition metals metals with a valency of +2 are particularly preferred. Quite particularly preferred are salts of aluminium, copper or zinc, in particular salts of Cu$^{2}$+, Zn$^{2}$+ and Al$^{3}$+, such as e.g. Al(NO$_{3}$ ) 3 , CuSO$_{4 }$ and ZnSO$_{4}$ . It was found that these metals strengthen the effec› tiveness of the solutions against MMPs. The metal salt solutions can advantageously also comprise a tin salt as an additional metal salt, preferably a salt of Sn $^{4}$+, more pref› erably SnC1$_{4}$ . The use of tin salts as additional salts implies that the metal salt solutions must comprise at least one further metal salt. 

[0022] According to a particularly preferred embodiment, the formulation according to the present invention consists substantially of an aqueous or aqueous-organic solution of one or more salts of transition and main group metals and contains no additions of acids, such as organic or inorganic acids. 

[0023] It was found that the metal salt solutions used according to the present invention are capable of etching dentin and tooth enamel and of dissolving the smear layer totally or partially. The exact reason for this effect is not known. It is assumed that the metal salts form so-called aqua complexes in aqueous solution, i.e. complexes which con› tain at least one water molecule as ligand, and that these complexes react acidically by donating H+ in accordance with reaction equation (1): 

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