20 Cytotoxicities of oxides, phosphates and sulphides of metals T. Hanawa, M. Kaga*, Y. Itoh*, T. Echizenya, H. Oguchi* and M. Ota Departments of Dental Materials and Engineering and *Pediatric Dentistry, Hokkaido University School of Dentistry, Kita 13 Nishi 7, Kita-ku, Sapporo 060, Japan The cytotoxicities of oxides, phosphates and sulphides, which possibly form on dental alloys, were tested using the ‘agar method’. The relative magnitudes of the cytotoxicities of these compounds exhibited tendencies similar to those of the metal components in the compounds. In correlation with dental amalgams, zinc oxide was the most cytotoxic, just as with metallic zinc, while tin oxides and mercury oxides exhibited no cytotoxicity. Zinc was the most influential element in the cytotoxicity of dental amalgams. On the other hand, in correlation with Ni-Cr alloys, Co-Cr alloys, and/or stainless steels a chromium oxide and a cobalt oxide showed serious cytotoxicity, whereas the iron oxides were non-cytotoxic. Sulphides, which possibly form on silver alloys, showed cytotoxicity. Oxides and phosphates of titanium and aluminium, and hydroxyapatite, were not cytotoxic, supporting the use of these materials for implants. Not only the valence of an element but also the chemical species combining with the element affect the cytotoxicity of the alloy containing it. Keywords: Dental materials, alloys, zinc, cytotoxicity Received 25 September 1990; revised 6 February 1991; accepted 7 March 1991 The cytotoxicities of biomaterials and dental alloys, and those for the component elements, have been well studied’-16. However, the surface composition of an alloy is different from the bulk composition of the alloy17, even in air, and some component elements of the alloy may be undetected in the surface layer in 1 nm order, for example vanadium in Ti-6A1-4V’8-2*. In addition, the surface composition of a metal or an alloy changes when the metal or alloy is deposited in the oral cavity or tissue. For example, a titanium surface is usually covered by 2-5 nm thick TiO, in air’g-25, whereas the surface is covered by a calcium phosphate in a neutral electrolyte solution”, ” whose composition of inorganic electrolytes is similar to that of most media used for cell culture. The surfaces of Ti-6Al-4V and Ti- 50Ni are covered by complex oxides of TiO,/Al,O, and TiOJNiO/Ni,O,, respectively, whereas the surfaces are additionally covered by calcium phosphates in the electrolyte solution’g-21. Vanadium is not responsible for the formation of the surface film of Ti-6A1-4V’8~21. In addition, stainless steels, cobalt-chromium alloys, and nickel-chromium alloys are covered, in air, by Cr,O,/ Fez0,/Fe30J, Cr,O,/CoOICo,O,, and Cr,O,/NiOINi,O,, respectivelyzo3 ‘l. The surfaces of these alloys are covered by calcium phosphates, phosphates, and/or chloride in the electrolyte solutionzO~“. Since nickel in stainless steel and cobalt-chromium alloy is not responsible for the formation of the surface films20*21, the influence of nickel on toxicity is minor in these alloys when nickel in the film is almost dissolved. On the other hand, the surface layers on Au alloys and Ag alloys possibly contain silver oxide and/or copper oxide in air and also contain chloride from the electrolyte solution20’21. In particular, Ag-In alloys and Ag-Sn-Zn alloy form ZnO/ In,O, and SnO,/ZnO, respectively, in the oral cavityz61 27. In addition, the surface layer of Ag-Pd alloy in the oral cavity contains a sulphidez8. Furthermore, the surfaces of dental amalgams are covered by zinc oxide and/or tin oxide in air”, whereas complex oxides and chlorides are formed in the oral cavity30-35. As described above, the surfaces of dental alloys are covered by more complex compounds than those pre- dicted from the compositions. Therefore, it is important to investigate not only the dental alloy and the component elements of the alloy but also the compounds which form on the alloy in the oral cavity and tissues. In this study, the cytotoxicities of oxides, phosphates, and sulphides, which possibly form on dental alloys, were investigated. It is believed that this information will enhance the understanding of the toxicities of dental alloys. EXPERIMENTAL Correspondence to Dr T. Hanawa Analytical-grade commercial compounds of oxides, phosphates, and sulphides in powder form (Wako Pure Biomaterials 1992, Vol. 13 No. 1 0 1992 Butterworth-Heinemann Ltd 92/010020/05