Corrosion ProcessesLeading to the Development of Metallic Pigmentation of the Gingiva

Overview

Alloys based on gold, platinum, silver and formerly also palladium are along withdental amalgam materials frequently used by stomatologists. During exposure these materialscome into contact with medium of the oral cavity (saliva, sulcular fluid) and the exposure ofmetals in an aqueous medium leads to their corrosion. From the value of the electrochemicalpotential of the metalic material in the oral medium, or to put it more accurately the freecorrosion potential, it is possible to assess to what extent conditions for its corrosion werecreated. Corrosion losses of dental alloys in the great majority are not a fundamental problemof these metallic materials. Even from material undergoing uniformcorrosion 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 ofthe free corrosion potential of metal components in the oral environment of patients withmetallic pigmentations of the gingiva. These values are compared with data obtained duringelectrochemical laboratory and exposure tests and with the results of metallographics evaluationof extracted implants. The mean value of the free corrosion potential of amalgams in vivowas 0.011 # 0.059 V/SHE, the range of potentials of prosthetic work based on NiCrMo alloy wasalmost identical with that amalgams. Free corrosion potential of gold alloys had a mean valueof 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/SHEand the potential of crown sharply increased to -0.033 V/SHE after prosthesis removal with therising trend. Restoration of the skeleton led to a rapid change of potential of the combinedmaterials back to -0.07 V/SHE. The existence of a macrocell may have a negative effect onsubjective 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 componentsthus in principle can be source of silver. Somewhat more difficult is to explain the presence ofmetallic pigmentations of the gingiva surrounding teeth with crowns made from NiCrMo-basedalloys in the vicinity of which they are found too. According to medical records the prostheticdevices were administered to the investigated subjects in six instances on Ag90Sn9ZnCd alloyand one instance on amalgam. Metallographic analysis provided evidence that under crownsduring exposure crevices are formed via which the oral medium can get into contact with theunderlaying metallic material and lead to corrosion. This is associated with the release of silverand copper compounds and their possible deposition in tissues.The explanation of the formationof 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 asa working hypothesis and for its verification it will be necessary to analyze a larger number ofsamples

Key words:
corrosion – ion release – spontaneous corrosion potential in vivo