Fluoride therapy has been used clinically for many years, but its use remains controversial and many basic questions remain unanswered. Accordingly, this study returns to an animal model to study the effects of high doses of fluoride on bone mineral in rabbits. Twelve rabbits, aged 3(1/2) months at the start of the study, received drinking water fluoridated at 100 ppm while their 12 control counterparts drank distilled water. All rabbits were sacrificed after 6 months. Fluoride was readily incorporated into femoral cortical bone (7473 +/- 966 ppm F versus 1228 +/- 57 ppm in controls; P < 0.00005). Fluoride therapy led to increased mineralization, as measured by density fractionation (P < 0.0005 for the distributions). The bone mineral itself was altered, with a significant increase in the width of crystals (66.2 +/- 2.0 A versus 61.2 +/- 0.9 A; P < 0.01). The microhardness of both cortical and cancellous bone in the femoral head of fluoride-treated rabbits was greater than that in the controls (P < 0.05). The phosphate, calcium, and carbonate contents in the bone was the same in both groups. Finally, fluoride administration did not affect the architecture or connectivity of cancellous bone in the femoral head. Previously published data indicated that the mechanical properties of bone were adversely affected; this suggests that the effect of high doses of fluoride on the strength and stiffness of bone may be mediated by its effect on bone mineral.
“In this study, despite the observed increased in hardness of both cancellous and cortical bone, the fracture stress and elastic modulus of vertebrae tested in compression and femora tested in three-point bending were decreased by fluoride treatment. The fact that the hardness (which is dependent largely on the mineral content) increases even though the modulus (which depends on both the mineral content and the collagen) decreases suggests that there is a change in the relationship between the bone mineral and the collagen. The mechanical strength of bone is thought to derive mainly from the interface between the collagen and the mineral, so if fluoride administration alters bone mineral, it may affect this interface and therefore result in modified mechanical properties.”