Analyses of rampant corrosion in stainless-steel retainers of orthodontic patients.

Retainers were collected from private, university, and dental labs. After viewing these corroded and control appliances using scanning electron microscopy, corroded maxillary and mandibular retainers were selected along with a control stainless-steel retainer for in-depth chemical analysis. Using electron spectroscopy for chemical analysis, monochromated Al x-rays were rastered over areas 1.5 x 0.3 mm. After survey spectra were acquired, high-resolution multiplex scans were obtained and binding energy shifts were noted. Using Auger electron spectroscopy, a spot size of approximately 30 nm was analyzed. Photos, survey scans, and depth profiles were acquired using a 3.5kV Ar(+) ion beam that was calibrated using a SiO2 standard. Via electron spectroscopy for chemical analysis, the brown stains contained Fe and Cr decomposition products in which three carbon species were present. Proteinaceous N was found as amines or amides. No Ni was present because it had solubilized. The Cr:Fe ratio indicated severe Cr depletion in the stained regions (0.2) versus the control regions (1.3). The stained regions appeared mottled, having both dark and light areas. Via AES, the dark versus light areas of the stained regions indicated that there was an absence versus a presence of both Cr and Ni. In the dark areas corrosion penetrated 700 nm; in the light areas the depth equaled 30 nm. By comparison, the passivated layer of the control retainer was 10-nm thick. After sputtering away the affected areas, all specimens had similar spectra as the control regions. The bacterial environment created the mottled appearance and induced electrochemical potential differences so that, upon reducing the passivated layer, an otherwise corrosion-resistant alloy became susceptible to rampant corrosion. An integrated biological-biomaterial model is presented for the classic case of an orthodontic acrylic-based stainless steel retainer subject to crevice corrosion.