US 2018/0228700 Al  2  Aug. 16, 2018  [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):  [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.  [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.  [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$^{3}$+  [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.  [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.  [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.  [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.  [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.  [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.  [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.  [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.  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.  [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).  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.  Me(H$_{2}$O)"+H$_{2}$O<=;.Me(OH)(n-l)+ +H$_{3}$O+ 