The “in vitro” research on fluoride and bone strength confirms what has repeatedly been found in animal and human studies: the more fluoride a bone has, the weaker the bone becomes. In an in vitro bone study, the researcher directly exposes a human or animal bone to a fluoride solution and then studies the effects of this exposure on the bone’s mechanical properties.

As can be seen in the following studies, fluoride treatment has been consistently found to reduce a bone’s mechanical strength.

In vitro studies

“[T]he elastic modulus measured in cortical bone using the BDI and the Oliver-Parr method decreased significantly after NaF [sodium fluoride] treatment, compared to control measurements prior to NaF treatment . . . . The general finding of the previous papers was that NaF reduces cortical-bone strength and elastic modulus, which is in well agreement with the results presented here.”
SOURCE: Thurner PJ, et al. (2009). The effect of NaF in vitro on the mechanical and material properties of trabecular and cortical bone. Advanced Materials 21:451-57.

“[T]he treatment with high concentrated solutions of NaF [sodium fluoride] results in the apparent failure of the organic-mineral interface, and ultimately in mineral platelet detachment from the underlying collagen fibrils. . . . [O]ur results nicely complement prior work investigating mainly the effects of NaF treatment on bone biomechanics; NaF treatment was previously found to decrease bone strength in vivo, as well as in vitro without significantly affecting bone mineral density. From such experiments it was concluded that NaF weakens the organic=inorganic interface in bone for which we present, for the first time, a direct visual proof.”
SOURCE: Kindt JH, et al. (2007). In situ observation of fluoride-ion induced hydroxyapatite-collagen detachment on bone fracture surfaces by atomic force microscopy. Nanotechnology 18:135102.

“Bovine femur cortical bone specimens were tested in tension after being treated in vitro for 3 days with sodium fluoride solutions of different molarity (0.145, 0.5, and 2.0M). The treatments alter the mechanical properties of the bone samples with different degrees as compared to control samples (untreated). The mechanical properties of the treated samples have lower elastic modulus, yield and ultimate stress, acoustic impedance and hardness, and higher ultimate strain and toughness as compared to control samples. The observed effects were intensified with the increasing molarity of the treatment solutions. This study shows that the fluoride treatment can be used to investigate the composite behavior of bone tissue by altering the structurally important bone mineral content in a controlled manner.”
SOURCE: DePaula CA, et al. (2002). Changing the structurally effective mineral content of bone with in vitro fluoride treatment. J Biomech. 35(3):355-61.

“Fluoride exposure in vivo can reduce the material strength of bone, an effect that has been attributed to a change in mineral structure. An in vitro model of fluoride exposure offers the potential to study directly the effects of fluoride on bone mineral. Previous investigators have reported that soaking bones in sodium fluoride in vitro reduces bone strength. However, long soaking times and the absence of physiological buffering ions from their treatment solutions may have caused mineral dissolution that contributed to the decrease in bone strength. Our objectives were to further characterize the effects of in vitro fluoride exposure on bone mechanical properties and to determine if the changes reported in previous studies of bovine cortical bone would be observed for whole rodent bones. We soaked 60 mouse femora in sodium fluoride solutions, with and without physiological buffering ions, and evaluated their torsional and bending properties. Fluoride soaked bones had a 30-fold increase in fluoride content and a 23% increase in water content compared to controls. These changes were associated with average reductions in ultimate load of 45%, reductions in rigidity of 70%, and increases in deformation to failure of 80%. The effect of fluoride was similar for bones treated in buffered and non-buffered solutions, and was observed in both torsion and bending. Our findings confirm those of previous studies and highlight the strong effect that in vitro fluoride exposure has on bone mechanical properties. The in vitro model of fluoride exposure offers a tool to further study the effects of ion substitution in bone.”
SOURCE: Silva MJ, Ulrich SR. (2000). In vitro sodium fluoride exposure decreases torsional and bending strength and increases ductility of mouse femora. Journal of Biomechics 33(2):231-4.

“The fluoride treatment affects the mechanical properties by decreasing the structurally effective bone mineral content. This permits us to create mechanically different bone samples from the same bone tissue and allows us to investigate the composite behavior of the bone tissue in an effective way.”
SOURCE: Kotha SP, et al. (1998). Varying the mechanical properties of bone tissue by changing the amount of its structurally effective bone mineral content. Biomed Mater Eng. 8(5-6):321-34.

“The mechanical properties of composites are influenced, in part, by the volume fraction, orientation, constituent mechanical properties, and interfacial bonding. Cortical bone tissue represents a short-fibered biological composite where the hydroxyapatite phase is embedded in an organic matrix composed of type I collagen and other noncollagenous proteins. Destructive mechanical testing has revealed that fluoride ion treatment significantly lowers the Z-axis tensile and compressive properties of cortical bone through a constituent interfacial debonding mechanism. The present ultrasonic data indicates that fluoride ion treatment significantly alters the longitudinal velocity in the Z-axis as well as the circumferential and radial axes of cortical bone. This suggests that the distribution of constituents and interfacial bonding amongst them may contribute to the anisotropic nature of bone tissue.”
SOURCE: Walsh WR, et al. (1994). The effect of in vitro fluoride ion treatment on the ultrasonic properties of cortical bone. Ann Biomed Eng. 22(4):404-15.

“Interfacial bonding interactions between the mineral and organic constituents of bone play an important role in the mechanical properties of cortical bone… Under a uniaxial tensile force, modification of interfacial bonding by phosphate and fluoride ions results in a reduction in the ultimate and yield stress and elastic modulus. In tension, phosphate ions effect is reversible upon removal of phosphate ions, while the fluoride ion effect is irreversible. Interestingly, when tested in compression, phosphate ion treatment results in a stiffening effect, while fluoride ions continue to lower the ultimate stress and elastic modulus.”
SOURCE: Walsh WR, Guzelsu N. (1993). The role of ions and mineral-organic interfacial bonding on the compressive properties of cortical bone. Bio-medical materials and engineering 3: 75-8

“The reduction in interfacial bonding due to fluoride action lowers the mechanical properties of bone tissue.”
SOURCE: Walsh WR, Guzelsu N. (1991). Fluoride ion effect on interfacial bonding and mechanical properties of bone. Journal of Biomechanics 24: 237.