Corrosion Processes Leading to the Development of Metallic Pigmentation of the Gingiva

Alloys based on gold, platinum, silver and formerly also palladium are along with dental amalgam materials frequently used by stomatologists. During exposure these materials come into contact with medium of the oral cavity (saliva, sulcular fluid) and the exposure of metals in an aqueous medium leads to their corrosion. From the value of the electrochemical potential of the metalic material in the oral medium, or to put it more accurately the free corrosion potential, it is possible to assess to what extent conditions for its corrosion were created. Corrosion losses of dental alloys in the great majority are not a fundamental problem of these metallic materials. Even from material undergoing uniform corrosion at very slow rate (i.e. 1 mmmm/a) corrosion products are released which may cause problems if they accumulatein the organism. In the submitted paper the authors summarize the results of measurement of the free corrosion potential of metal components in the oral environment of patients with metallic pigmentations of the gingiva. These values are compared with data obtained duringelectrochemical laboratory and exposure tests and with the results of metallographics evaluation of extracted implants. The mean value of the free corrosion potential of amalgams in vivo was 0.011 # 0.059 V/SHE, the range of potentials of prosthetic work based on NiCrMo alloy was almost identical with that amalgams. Free corrosion potential of gold alloys had a mean value of 0.096 # 0.055 V/SHE, palladium alloys 0.110 V/SHE. In one of the patients a galvanic effect was recorded (macrocell) between the CoCrMo-based skeleton and a crown made from goldalloy. The potential of all components with the prosthesis in situ was -0.060 to -0.068 V/SHE and the potential of crown sharply increased to -0.033 V/SHE after prosthesis removal with the rising trend. Restoration of the skeleton led to a rapid change of potential of the combined materials back to -0.07 V/SHE. The existence of a macrocell may have a negative effect on subjective sensations of the patient and on the corrosion behavior of the influenced materials. The incidence of gingival deposits at first could not be ascribed to certain type of material; prostheses in the close vincinity of pigmentations were made from all existing types. In previouswork evidence was provided that metallic spots contain most frequently silver. The latter ispart of many alloys of gold, palladium amalgams and other materials. Dental metal components thus in principle can be source of silver. Somewhat more difficult is to explain the presence of metallic pigmentations of the gingiva surrounding teeth with crowns made from NiCrMo-based alloys in the vicinity of which they are found too. According to medical records the prosthetic devices were administered to the investigated subjects in six instances on Ag90Sn9ZnCd alloy and one instance on amalgam. Metallographic analysis provided evidence that under crowns during exposure crevices are formed via which the oral medium can get into contact with the underlaying metallic material and lead to corrosion. This is associated with the release of silver and copper compounds and their possible deposition in tissues. The explanation of the formation of metallic pigments in the vincinity of NiCrMo-based prostheses leads to the hypothesis of acorrosive role of material to which the prosthesis is attached. This must be however taken as a working hypothesis and for its verification it will be necessary to analyze a larger number of samples.

Keywords: corrosion; ion release; spontaneous corrosion potential in vivo