Response to Letter to the Editor, “The Long-term Effects of Water Fluoridation on the Human Skeleton”

Thank you for allowing us the opportunity to respond to the Letter to the Editor submitted by Phillips et al. in response to our paper in this journal, “The Long-term Effects of Water Fluoridation on the Human Skeleton” (Chachra et al., 2010). We appreciate the points that they raise regarding the confounding effects of age, gender, and disease states on the effect of fluoride on bone quality. In fact, we explicitly address the interaction effect of age and fluoride, under the heading “Variability in the Data” (p. 1221). The reference cited there is the doctoral thesis of the first author (Chachra, 2001), from which the data presented in this paper are excerpted. That thesis includes a detailed analysis of the data by age, gender, and disease state, which were collected for each patient and are presented in Table 1 (p. 1220). These analyses are outside the scope of the work presented here, but briefly, there was no evidence of a gender-specific relationship between the mechanical properties and the fluoride content. The relationship between the ultimate compressive stress and the fluoride content was slightly weaker when the osteoporotic samples were excluded, suggesting that both disease states have a similar effect on mechanical strength of bone. As for the inclusion/exclusion criteria of patients: We apologize, since it was not clear from the brief text that all patients undergoing primary total hip arthroplasties under the care of our collaborating orthopedic surgeons were approached to participate in this study. No other criteria were applied. Finally, we agree with Phillips et al. that the work presented here does not conclusively address the effects of long-term administration of environmental levels of fluoride on the mechanical properties of bone. To do that, we would need to track not only the duration of exposure to fluoridated water, as they point out, but also factors such as diet (Singer et al., 1985) and use of fluoride-containing oral care products and fluoride supplements (Ismail, 1994; Whitford, 1994). Gathering these kinds of detailed data retroactively for the requisite period of many decades is prohibitive. As well, beverages and foods are increasingly prepared with fluoridated water (Clovis and Hargreaves, 1988), so living in a non-fluoridated area does not guarantee the absence of exposure to environmental fluoride. This inability to measure the exposure accurately was the rationale for the comparison of mechanical properties with the fluoride content of the bone tissue, measured directly. Fig. 2 (p. 1222) displays the wide variation in response to fluoride, since only about 5% of the variability in the data could be explained by the fluoride content. As we pointed out, this is consistent with many decades of epidemiological studies that have failed to find a relationship between fluoride exposure and bone quality (McDonagh et al., 2000). In agreement with Phillips et al., we wrote in the conclusions of our paper (p. 1222) that we cannot rule out subtle effects of long-term ingestion of fluoride on the mechanical properties of bone, especially in the case of susceptible individuals. However, at the population level, there is no substantive evidence that fluoridated drinking water alone has an effect on bone quality.