Effect of long-term administration of fluoride on physico-chemical properties of the rat incisor enamel

Abstract

Rats were maintained on drinking water containing different amounts of fluoride (0, 9, 23 45, 68, and 113 ppm) for 70 days. Physico-chemical properties of the incisor enamel were examined after fluoride administration, using contact microradiography, histochemistry, and microhardness tests. The tooth enamel formed during high fluoride exposure showed marked hypocalcification. Much of organic substance in the enamel seemed to have been retained. In addition, the microhardness of enamel showed a marked decrease. These changes were most prominent in the outer region of enamel and were proportional to the concentration of fluoride administered. Such changes following fluoride administration indicated inhibition of enamel maturation, i.e., an inhibition of the progressive depposition of minerals and/or in inhibition of organic matrix withdrawal by ameloblasts. Enamel seemed more affected by fluoride than dentine.

The Impact of Fluoride on Ameloblasts and the Mechanisms of Enamel Fluorosis

“…Mineralization defects in rat incisor maturation-stage enamel can develop during prolonged exposure to fluoridated drinking water at levels as low as 9-10 ppm fluoride (Shinoda, 1975;Angmar-Månsson et al, 1976), and are characterized by the development of a generalized hypomineralized porous subsurface area along the entire crown enamel (Fig. 3a) (Shinoda, 1975;Angmar-Månsson et al, 1976;Whitford, 1982, 1984;Richards et al, 1992;Kierdorf et al, 2004). This type of defect correlates to the porous white opacities seen clinically.…”

“…In drinking water, fluoride levels of at least 10-30 ppm are necessary to induce lasting enamel disturbances in rodents, and levels used in most experimental studies are in the range of 25-100 ppm (Shinoda, 1975;Angmar-Månsson et al, 1976;Fejerskov et al, 1979;Ekstrand et al, 1981;Angmar-Månsson and Whitford, 1984;DenBesten, 1986;Kubota et al, 2005). The high fluoride levels in drinking water needed to cause enamel fluorosis in rats (about 10 times the amount of humans) have raised the question as to whether studies in rats have relevance for the development of enamel fluorosis in humans.…”

“…The minimal fluorotic dose also depends on the time of exposure: Exposure to 9 ppm (0.5 mM) fluoride for 70 days significantly decreases the hardness of the outer enamel of erupted rat incisors, indicating that a prolonged exposure to a low fluoride dose already induces functional defects in the enamel (Shinoda, 1975).…”

“…This type of defect correlates to the porous white opacities seen clinically. With increased fluoride exposure, the extent and degree of this hypomineralization area increase inward toward the enamel-dentin junction (Shinoda, 1975;Angmar-Månsson and Whitford, 1985;Milhaud et al, 1992;Fejerskov et al, 1994), particularly in the cervical portion of the teeth (Richards et al, 1985;Suckling et al, 1988;Kierdorf and Kierdorf, 1997).…”

“…Micro-indentation studies have indicated that these subsurface areas (containing the opaque zone) are softer with increasing fluoride doses in drinking water (Shinoda, 1975;Suckling and Thurley, 1984;Suckling et al, 1988;Milhaud et al, 1992). Matrix proteins disappear from non-fluorosed enamel in the maturation phase, but are retained in subsurface enamel of fluorosed teeth at fluoride levels, beginning at 25 ppm F in drinking water (DenBesten, 1986;Zhou et al, 1996).…”