The effects of fluoride are both topical and systemic.  The systemic effects are demonstrated in mild to very mild dental fluorosis optimally fluoridated water.  As peer-reviewed science has demonstrated mildly fluorosed teeth to be more decay resistant, many consider this effect to not even be undesirable, much less adverse.  Dental fluorosis can only occur systemically. 
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—-The Association Between Enamel Fluorosis and Dental Caries in U.S. Schoolchildren Hiroko Iida, DDS, MPH and Jayanth V. Kumar, DDS, MPH
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Additionally, saliva with fluoride incorporated into it provides a constant bathing of the teeth in a low concentration of fluoride all throughout the day, a very effective means of dental decay prevention. Incorporation of fluoride into saliva occurs systemically.
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Recommendations for Using Fluoride to Prevent and Control Dental Caries in the United States
US Centers for Disease Control
Recommendations and Reports
August 17, 2001/50(RR14);1-42

Steven D. Slott, DDS