A Novel Model For Monitoring Erosive Mineral Loss In Situ A. Melvan *, C. Ganss, V. Brune, J. Klimek (Dental Clinic , Justus-Liebig University, Giessen, Germany) Up to now in most studies profilometry and microhardness measurements were used for assessment of erosive mineral loss. However, microhardness measurements do not quantify mineral substance loss and profilometry does not register mineral uptake. One reliable method to monitor mineral loss and uptake is the longitudinal microradiography (LMR) which is a non destructive method allowing consecutive measurements in vitro. Since the oral environment can not be simulated sufficiently in vitro, the aim of this study was to adapt the LMR procedure to in situ studies. For this reason a modification of the sample management was introduced.   From impacted third molars 52 enamel specimens were prepared and ground flat to a thickness of 400µm and an experimental area of 2-3mm². Twenty specimens were distributed to the in vitro group and 4¥8 to the in vivo group. At the beginning and immediately after termination of the test period the specimens were subjected to LMR to determine the baseline value and the mineral loss (µm). All specimens were immersed for 6¥5 minutes/day in a 0.05M citric acid solution of pH 2.4. For the test period of 5 days the in vitro specimens were stored in a NaCl solution and the in situ specimens were mounted in the palatal aspect of removable mouth appliances. The participants wore the appliances for the whole test period brushing their teeth with fluoride free toothpaste. To guarantee a reproducible positioning of the in situ specimens in the LMR apparatus, the enamel samples were embedded in resin which fit in the microradiography holders. Significant differences between the groups were found out (p<0.001). The mean mineral loss in the in situ group was -40.68±15.5 (µm) and in the in vitro group -58.41±9.26 (µm). Additionally significant differences between the subjects were observed.   The results indicate that in situ conditions are important for the assessment of the erosive potential of acidic solutions. It had been shown that LMR measurements can be performed in situ.