Surface layers can be assessed using in vitro or in situ models. Methods of analysis can include scanning electron microscopy (SEM) (Figure 16), quantitative light fluorescence or optical coherence tomography which measure changes in the surface zone and are used to determine surface roughness. Additional methods include acid solubility testing and atomic force microscopy (Figure 17). Replica SEMs can be used with in situ research to measure changes in the surfaces of eroded lesions or softened zones of enamel lesions over time. An additional method of analyzing the surface layers, secondary ion mass spectroscopy (SIMS), can be used to measure the composition of the surface layer and to show material firmly deposited at the surface.
SEM showing loss of enamel (a) and, at greater magnification, the softened layer at the advancing front of the lesion (b).
Atomic force micrographs show the decreased loss of structure on the enamel slab treated with stabilized stannous fluoride toothpaste. Stannous fluoride deposits are insoluble in concentrated acid and provide enhanced protection, especially at low pH.
Hardness recovery, hardness loss inhibition, surface composition/fluoridation and solubility reduction measurements can all be used to assess the mechanisms and efficacy of a proposed preventive therapy against dental erosion. Protocols used include surface preparation using an acid challenge and measurement, treatment steps, surface measurements after secondary acid challenges (typically citric acid, or for in situ studies orange juice), and then measuring specimen hardness again after re-immersion in saliva.51-55,58,59