The recent paper by Einhorn et al. [1] drew the conclusion that fluoride incorporation into bone does not impair bone’s mechanical properties. This result is in conflict with the results of others concerning fluoride and bone strength. For instance, several investigators–including ourselves–have shown that bone strength decreases as bone fluoride levels in the mineral phase increase to beyond about 4500 ppm [2-7]. When the fluoride content in the mineral phase reaches 10,000 ppm, bone strength is decreased by about 13% if calcium intake is normal [7] or by as much as 45% when fluoride intake is associated with calcium deficiency [2, 5]. Further- more, the conclusion by Einhorn et al. that fluoride’s effects on bone are due to pathological bone remodeling are in conflict with findings showing a bone strength deficit caused by fluoride accumulation in the absence of gross bone pathology (i.e., woven bone formation) [4] and C. H. Turner, unpublished observations). We think some of our recent findings can explain the discrepancy between the results of Einhorn et al. and those of other studies.

For our purposes, it is important to point out the difference between bone tissue strength which is an intrinsic property of bone and the force required to break a bone which is an extrinsic property. That is, tissue strength values are independent of the size and shape of the bone and are reported in pounds/in 2 or newtons/m 2. In contrast, breaking force will vary with bone size and is reported in units of pounds or newtons. It is crucial to keep this distinction in mind because intrinsic strength and breaking force can show different trends in fluoride studies because fluoride affects the size of the bone. We completed a study using the protocol of Einhorn et al., that is, weanling rats were fed various doses of sodium fluoride for 3 months after which ex vivo bone strength measurements were made. The only difference in our protocol was that we used three-point bending tests of femora to measure mechanical properties whereas Einhorn et al. used torsion tests of tibiae. The results were telling; intrinsic bone strength measured using engineering bending equations was negatively correlated with bone fluoride con- tent (Fig. 1). However, bone-breaking force (an extrinsic measure) was not changed even with the highest fluoride dose (Fig. 2). The explanation for this finding is that bone size increased slightly with fluoride dose. This was evident from our finding that the cross-sectional moment of inertia of the femur was positively correlated with bone fluoride con- tent (P < 0.05). These results are consistent with those of Einhorn et al. who reported no fluoride effect on breaking torque–which is an extrinsic measure like breaking force– and therefore would be expected to follow the same trends as breaking force. However, these results are inconsistent with the conclusions of Einhorn et al., namely, that fluoride intake does not affect bone strength.

A more accurate summary of the effects of fluoride intake on bone strength is the following: fluoride intake decreases intrinsic bone strength but also causes a slight increase in bone size which fully compensates for the decreased bone strength in young healthy animals.

References

1. Einhorn TA, Wakley GK, Linkhart S, Rush EB, Maloney S, Faierman E, Baylink DJ (1992) Incorporation of sodium fluoride into cortical bone does not impair the mechanical properties of the appendicular skeleton in rats. Calcif Tissue Int 51:127-131

2. Beary DF (1969) The effects of fluoride and low calcium on the physical properties of the rat femur. Anat Rec 164:305-316

3. Faccini JM (1969) Fluoride and bone. Calcif Tissue Res 3:1-16

4. Mosekilde L, Kragstrup J, Richards A (1987) Compressive Letters to the Editor strength, ash weight, and volume of vertebral trabecular bone in experimental fluorosis in pigs. Calcif Tissue Int 40:318-322

5. Riggins RS, Zeman F, Moon D (1974) The effects of sodium fluoride on bone-breaking strength. Calcif Tissue Res 14:283- 289

6. Wolinsky I, Simkin A, Guggenheim K (1972) Effects of fluoride on metabolism and mechanical properties of rat bone. Am J Physiol 223:46-50

7. Turner CH, Akhter MP, Heaney RP (1992) The effects of fluoridated water on bone strength. J Orthop Res 10:581-587