Declining Trend of Dental Cavity Prevalence  

Regional variations in caries experience within the United States were noticed many years ago, though their documentation came only with the examination of young men in the armed forces during World War II. It is of interest to note that regional differences in caries prevalence among different tribes of Native Americans were demonstrated in the early 1930s, with more severe disease among tribes in the northwest than among those in the southwest. This regional pattern is still seen today in the rest of the American population.

 The World War II surveys were in general agreement that the most severe caries experience was seen in recruits from New England, the Pacific Northwest, and the Great Lakes area, with distinctly less in young men from the south, the southwest, and the mountain states. In the years since World War II, some of these differences have been obscured by the spread of water fluoridation, but they were still quite apparent in the late 1960s.

Regional differences in caries experience are not unique to the United States, for just about every country exhibits similar variations. In Britain, for example, oral health has long been poorer in Scotland and northern England than it has been in southern England. These differences persist to the present day, despite a decline in caries experience among children and improving conditions in adults when compared to data from a generation or more ago.

The Decline in Caries

By the late 1970s and early 1980s, there were published reports to show that the prevalence of caries among children in the United States was declining. Similar information from other parts of the developed world around the same time provided good reason to believe that the reduction in caries experience was a widespread phenomenon.

The decline in caries experience among children was confirmed for the United States with the publication of results from the National Dental Caries Prevalence Survey in US School Children of 1979-1980. This survey showed that decay scores among children aged 5 to 17 were 32% lower than those found in the first National Health and Nutrition Examination Survey of 1971-1974. Even allowing for some methodological differences between these two surveys, it was clear that the decline was real, even if its precise extent was uncertain.

The next national survey of US schoolchildren in 1986-1987 found that the decline was continuing, for decay scores for 5- to 17 year-olds were again 36% lower than those recorded seven years earlier. In the 1986-1987 data, the highest value for decayed surfaces was only 0.76 for the 17-year-olds. The decline has also been documented in primary teeth: mean decay for 6-year-olds in the 1979-1980 survey was 4.76 (181); this was down to 3.73 in 1986-1987.

The caries decline in the developed nations was documented at a conference at the Forsyth Dental Center in Boston in 1982. The proceedings of which were published in a special issue of the Journal of Dental Research for November 1982. At that conference, data from nine different countries all pointed to the same conclusion, namely, that caries experience in children had declined considerably over a generation or so. Since then, further evidence for the caries decline has continued to be published, though by the late 1980s a few British reports suggested that the caries decline may have bottomed out.

There was nothing to suggest that the downward trend in caries experience among North American children had bottomed out in the early 1990s; however, there were also less data collected there than in Britain. The situation must continue to be monitored.

An important aspect of the caries decline is its effect on the different tooth surfaces; the impact on free smooth surfaces and surfaces between the teeth has been more pronounced than on pit-and-fissure surfaces. In a three-year longitudinal study in Michigan in the early 1980s, 81% of all new lesions were on chewing surfaces. The net result is that while the total number of new carious lesions has been dropping, the proportion made up of pit-and-fissure lesions is growing. This trend has enhanced the attractiveness of fissure sealants as a preventive measure.

The decline in cavity scores is naturally less obvious among adults, because many of  adults grew up before the decline became apparent. The data from the National Survey of Oral Health in US Employed Adults and Seniors in 1985-1986 shows the mean decay values for American adults aged 33 to 39 by region. It is interesting to note that the traditional regional differences are apparent in these adults, whereas they are not nearly so evident with the lower caries experience seen in the schoolchildren.

Reasons for the caries decline have been examined without any clear causes emerging. That should not be surprising, for history is full of examples of diseases that have waxed and waned without precise knowledge of why. One of the best recent examples is that of tuberculosis, now an uncommon disease in developed countries, but which was the greatest cause of death in the 19th century in North America and Europe. The rate of steady downward decline in tuherculosis over the last 120 years did not change with the introduction of BCG vaccine or with the development of the antibiotic streptomycin.  This suggests that the decline could not be attributed to those causes. Continuing improvements in sanitation, living conditions, and public health regulation are generally seen as the primary reasons for the decline in tuberculosis.

With the caries decline, fluoride exposure is generally considered the major reason, though not the only one. It is difficult to ascribe the decline to decreasing consumption of sugars, better oral hygiene, changes in the bacteria] ecology of the oral cavity, or to widespread use of antibiotics, whereas the evidence supports an influential role for fluoride. As with the cyclical nature of other diseases; however, it is possible that there are factors operating that have not yet been identified.

Susceptibility of Different Teeth

When caries was more prevalent and severe than it is at present, it was found that teeth attacked by caries were affected within two to four years of eruption. That can no longer be considered a rule. but some teeth still are more susceptible to caries than others. In the pioneering Hagerstown studies, the rank order of susceptibility of teeth to caries was listed as follows:

1. Mandibular first and second molars.

2. Maxillary first and second molars.

3. Mandibular second hicuspids, maxillary first and second bicuspids, maxillary central and lateral incisors.

4. Maxillary canines and mandibular first bicuspids.

5. Mandibular central and lateral incisors. mandibular canines (third molars had not erupted in the children studied).

Although overall caries experience has declined since the Hagerstown studies were carried out in 1937, it is not certain that this rank order of tooth vulnerability has changed. Caries is uncommon in anterior teeth in the 1990s and second molars may be more susceptible to caries than first molars.