Already two decades ago it has been postulated that site-specific aspects of salivary fluoride clearance may have important implications for the site-specificity of oral diseases . It is now well known that at least three factors are influencing this site-specificity of oral pathobiology: the different local composition and pathogenicity of oral biofilms (local microbiome), the site-specific host response towards bacterial phylotypes as commensals or pathogens (local immunity), and finally, the individual variability of salivary and plaque clearance of fluoride.
Whereas the first two factors are exclusively in the focus of basic research, the kinetics of fluoride in oral fluids are rather well documented. This is the reason why clinical recommendations for the treatment of incipient caries lesions or for the stagnation of lesions can be concluded. Therefore, the bioavailability of fluoride in saliva, and consequently in plaque fluid plays a crucial role in preventing a net mineral deficit in enamel, cementum and dentin due to caries challenge.
Bioavailability of fluoride is dependent upon various factors such as fluoride administration [2–7], fluoride formulation, and salivary secretion rate [8–11]. Fluoride bioavailability in plaque may also be influenced by the compounds of the administered fluoride source. Recently it has been demonstrated, that e. g. sodium lauryl sulphate changes the structure of plaque biofilms which may have an effect on fluoride uptake or release . Several studies have demonstrated, that salivary fluoride concentration increases dramatically after fluoride administration either after tooth brushing or mouth rinsing with fluoridated products, but is back to the baseline level two hours after fluoride administration [4, 9, 13, 14].
Fluoride concentration in saliva is the source for the fluoride delivery to dental plaque. Recently it has been demonstrated that elevated fluoride products like dentifrices with 5000 ppm NaF or amine fluoride [10, 15] or oral hygiene tablets directly dissolved in saliva with 4350 ppm NaF enhance remineralization of advanced enamel lesions  and result in increased bioavailability of fluoride in saliva . Application of high concentrations of fluoride leads to the formation of a CaF2 layer on the enamel surface. It has been reported that this CaF2 layer dissolves rapidly and releases bioavailable fluoride .
No calcium-fluoride-like deposits were detected in plaque shortly after a NaF mouth rinse , and the authors concluded that the inability to form more persistent CaF2 deposits may account for the rapid loss of fluoride in plaque after the use of topical fluoride agents. Concerning the plaque clearance of fluoride representing consequently the F- bioavailability over the day and night time rather controversial results have been reported: An experimental ex-vivo study demonstrated a rapid and very substantial uptake of fluoride by plaque after exposure to 1000 ppm NaF immersion , whereas from an in-vivo study it was concluded that elevated salivary fluoride concentrations were not reflected in dental plaque, measured 6 h after brushing (1400 ppm fluoride) and rinsing (250 ppm fluoride) . Other data demonstrated after one hour post brushing (1074 ppm fluoride) a rapid fluoride uptake and 12 hours later a clearance back to the placebo levels . More detailed fluoride kinetics data in dental plaque are missing.
As fluoride binding to the plaque reservoirs and the release from the reservoir is rather complex, the source of the fluoride may play an important role. It is well known that different fluoride formulations lead to different salivary fluoride concentrations after tooth brushing . NaF is instantly dissociating in saliva. Sodium monofluorphsphate (NaMFP) requires hydrolysis to release free fluoride ions , and amine fluoride may bind to organic constituents in saliva and plaque and releases fluoride slower than the other two. Higher fluoride concentrations may result in the formation of a CaF2 layer on the enamel surface which also may serve as fluoride reservoir . The different dissolution properties may lead to different fluoride concentrations in plaque, consequently affecting the caries protective effect of plaque fluoride content.
It was, therefore, the aim of the present investigation to follow up the fluoride bioavailability in whole saliva and in individual plaque samples from baseline immediately after tooth brushing and up to 360 minutes, to compare a NaF formulation dissolved directly in saliva with an amine fluoride dentifrice formulation. The null hypothesis that there is no difference in the fluoride bioavailability after NaF or amine fluoride application was tested.