Although fluoride has generally been found to reduce the bone density of cortical bone, it is well documented that fluoride can increase the density of trabecular bone (aka cancellous bone). Trabecular  bone is the primary bone of the spine, whereas cortical bone is the primary bone of the legs and arms.

While increases in bone mass are generally associated with increased bone strength, this is not the case with fluoride. As shown below, both human clinical trials and animal studies have found that fluoride-induced increases in trabecular bone density are not accompanied by a corresponding increase in bone strength. This is because fluoride-induced bone mass has an abnormal, inferior quality that fails to provide the protections of normal bone. Fluoride-induced bone mass is also associated with an increased risk for skeletal stiffness and pain.

Fluoride & Bone: Density Vs Strength

“While NaF [sodium fluoride] may increase bone mass, the newly formed bone appears to lack normal structure and strength.”
SOURCE: Everett ET. (2011). Fluoride’s Effects on the Formation of Teeth and Bones, and the Influence of Genetics. Journal of Dental Research 90:552-60.

“The weight of evidence indicates that, although fluoride might increase bone volume, there is less strength per unit volume.”
SOURCE: National Research Council. (2006). Fluoride in Drinking Water: A Scientific Review of EPA’s Standards. National Academies Press, Washington D.C. p5.

“The evidence suggests that NaF-induced BMD increases may be associated with increased fracture risk…”
SOURCE: Gutteridge DH, et al. (2002). A randomized trial of sodium fluoride (60 mg) +/- estrogen in postmenopausal osteoporotic vertebral fractures: increased vertebral fractures and peripheral bone loss with sodium fluoride; concurrent estrogen prevents peripheral loss, but not vertebral fractures. Osteoporosis International 13(2):158-70.

“In this meta-analysis we found that fluoride increased BMD without showing efficacy on the incidence vertebral fracture.”
SOURCE: Haguenauer D, et al. (2000). Fluoride for the treatment of postmenopausal osteoporotic fractures: a meta-analysis. Osteoporosis International 11(9):727-38.

“In conclusion, a BMD increase does not always produce an increase in bone strength in the fluoride-treated bone.”
SOURCE: Giavaresi G, et al. (1999). The mechanical properties of fluoride-treated bone in the ovariectomized rat. Calcified Tissue International 65: 237-241.

“Increases in bone mass resulting from fluoride treatment did not lead to increased bone strength. In fact, many biochemical parameters, namely, vertebral and femoral fracture stress, and femoral elastic modulus, were negatively correlated with bone mass. This seems counterintuitive, however, the bone samples with the highest bone mass also had the highest fluoride content, and the negative effect of increased fluoride content on bone strength far overwhelmed the positive effect of increased bone mass.”
SOURCE: Turner CH, et al. (1997). Fluoride treatment increased serum IGF-1, bone turnover, and bone mass, but not bone strength, in rabbits. Calcified Tissue International 61:77-83.

“The discrepancy that an increase in bone mass and geometric properties in both trabecular and cortical bones by low-dose, long term NaF (sodium fluoride) treatment did not increase vertebral strength nor proportionally improve femoral strength indicated that the bone intrinsic biomechanical properties could be changed by NaF treatment.”
SOURCE: Jiang Y, et al. (1996). Effects of low-dose long-term sodium fluoride preventive treatment on rat bone mass and biomechanical properties. Calcified Tissue International 58:30-9.

In the fluoride-treated animals “bone strength…. did not increase with bone volume, suggesting that for bones with higher volume, there was less strength per unit volume, that is, a deterioration in bone ‘quality.'”
SOURCE: Lafage MH, et al. (1995). Comparison of alendronate and sodium fluoride effects on cancellous and cortical bone in minipigs: a one year study. Journal of Clinical Investigations 95: 2127-2133.

“It is concluded that the increase in bone mass during fluoride treatment does not translate into an improved bone strength and that the bone quality declines.”
SOURCE Sogaard CH, et al. (1995). Effects of fluoride on rat vertebral body biomechanical competence and bone mass. Bone 16: 163-9.

“bone fluoride content was positively correlated with femoral bone density; however, the increased bone density was not associated with increased bone strength.”
SOURCE: :Turner CH, et al. (1995). Fluoride reduces bone strength in older rats. Journal of Dental Research 74: 1475-1481.

“again, if bone formed during fluoride therapy is of poor quality, neither increased mass nor increased trabecular thickness would directly translate into increased strength.”
SOURCE: Sogaard CH, et al. (1994). Marked decrease in trabecular bone quality after five years of sodium fluoride therapy–assessed by biomechanical testing of iliac crest bone biopsies in osteoporotic patients. Bone 15: 393-99.

“fluoride treatment seems to be an intriguing example, in which bone mineral density (BMD) measurements do not at all correlate with the biomechanical properties of bone. Consequently, BMD data from clinical studies without exact analysis of antifracture effectiveness are useless for assessment of the risk-benefit ratio of fluoride treatment.”
SOURCE: Fratzl P, et al. (1994). Abnormal bone mineralization after fluoride treatment in osteoporosis: a small-angle x-ray-scattering study. Journal of Bone and Mineral Research 9:1541-9.

“[T]he large increase in the mineral density of cancellous (trabecular) bone in the vertebrae during fluoride treatment did not result in a significant reduction in the rate of vertebral fracture…”
SOURCE: Riggs BL, et al. (1990). Effect of Fluoride treatment on the Fracture Rates in Postmenopausal Women with Osteoporosis. New England Journal of Medicine 322:802-809.

“[A]lthough the risk of fractures normally rises as the bone mass declines, increases in bone mass with fluoride treatment may not reduce fracture rates.”
SOURCE: Lindsay R. (1990). Fluoride and bone: quantity versus quality (editorial). New England Journal of Medicine 322: 845-846.

“Due to possible adverse influences of fluoride on the mineralized tissue physical characteristics… the increase in bone mass does not necessarily result in an increase in bone strength… [W]e wish to emphasize the fact that it can be very misleading to rely on simple measures of bone volume fracture and/or bone density as indicators of bone strength in fluoride treated patients.”
SOURCE: Carter DR, Beaupre GS. (1990). Effects of fluoride treatment on bone strength. Journal of Bone and Mineral Research 5(Suppl) 1:S177-84.

“It has been proposed that the positive effect of fluoride on bone mass might have a corresponding effect on bone strength. This assumption, however, could not be demonstrated in the present study: the biophysical parameters were found to be reduced (normalized compressive strength) or unchanged (maximum compressive strength, stiffness, and energy absorption capacity) after fluoride treatment in spite of the significant increase in trabecular bone density.”
SOURCE: Mosekilde L, et al. (1987). Compressive strength, ash weight, and volume of vertebral trabecular bone in experimental fluorosis in pigs. Calcified Tissue Research 40: 318-322.

“whereas the bone may appear more dense radiologically, it may, in fact, be more fragile and more susceptible to fracture. Our findings of a highly significant increase in fractures in the high fluoride exposure group suggests that this may indeed be the case.”
SOURCE: Carnow BW, Conibear SA. (1981). Industrial fluorosis. Fluoride 14: 172-181.

“the increase in bone mass that occurrs after therapy cannot necessarily be equated with an increase in bone strength.”
SOURCE: Riggs BL, et al. (1980). Treatment of primary osteoporosis with fluoride and calcium: Clinical tolerance and fracture occurrence. Journal of the American Medical Association 243: 446-449.

“Our observations corroborate the findings that, in general, elevated dietary fluoride results in an acceleration of bone mineralization. Uniquely, however, the increase in mineralization was accompanied by a decrease in bone strength… the changes in bone that occur with prolonged and excessive fluoride ingestion may result in a reduction of bone strength.”
SOURCE: Chan MM, et al. (1973). Effect of Fluoride on Bone Formation and Strength in Japanese Quail. Journal of Nutrition 103: 1431-1440.

“The practical issue one is faced with in attempting to access the therapeutic effect of sodium fluoride is entirely different than that with any other form of treatment of osteoporosis. This is because, in the case of every other currently used method of treatment, there is a high presumption that bone formed during the period of treatment is of good quality and, therefore, that if an increased bone mass were demonstrated, it could be assumed that bone strength would be increased. The problem is that there has not so far been a convincing demonstration that these other agents cause an increased bone mass. In the case of fluoride, there is no reasonable doubt that bone mass can be increased, but the significance of such change is uncertain, since the histological appearance of fluorotic bone is abnormal and its physical strength could not be assumed to be normal. Therefore, it is critical to establish whether the strength of fluorotic bone is normal, decreased or increased.”
SOURCE: Rich C, Feist E. (1970). The action of fluoride on bone. In: TL Vischer, ed. (1970). Fluoride in Medicine. Hans Huber, Bern. pp. 70-87.