| Sign In to gain access to subscriptions and/or personal tools. |
Dental Caries Seen from the Pulpal Side: a Non-traditional ApproachProfessor of Dentistry and founding Dean, Institute of Dentistry, University of Oulu, and Chief Dentist, University Hospital of Oulu, Yliopisto PO Box 5281, FIN-90014 University of Oulu, Finland; markku.larmas{at}oulu.fi
Key Words: dental caries • dentin • pulp • repair • enzymes • sucrose • fluoride
When I had published my first scientific paper (thereby joining the dental research community and IADR) over 35 years ago, I received a letter from Dr. Albert Schatz. He wrote: "In the United States, Miller’s disciples have erected an ’acid curtain’ to insulate caries research against new ideas." As an interesting anecdote, he also informed me that the etiology of dental caries had been "determined" in an IADR meeting in the 1940s, where the acid theory won, by vote, over Schatz’ proteolysis-chelation theory. I asked my supervisor about Dr. Schatz and was told that he is an ’odd’ researcher, who had the privilege, during his doctoral training, of working in the laboratory of a Nobel laureate (Dr. Selman Waksman). Dr. Schatz did not encounter any special obstacles to publishing his papers, but his new ideas of dental caries were believed to be obtained by unscientific methods and therefore were unacceptable in the traditional journals. In a way, this reflects the general situation that prevailed in contemporary dental research. Scientists were able, if they so desired, without any practical copying equipment, to contact all colleagues in the field by personal letters. There was also an active debate going on in dental journals through letters to the editor. I have never since seen a scientific communication to equal the brilliant paper by Jenkins entitled "A critique of the proteolysis-chelation theory of caries" (Jenkins, 1961), in which he reviewed, point by point, 20 pages of text on the aspects of destruction of enamel during caries, refuting much of Schatz’ work. I began studying dental caries, which is only partly the same disease as enamel caries. Therefore, my earliest findings met with strong resistance by the referees of dental journals, reminding me of the ’acid curtain’. Bit by bit, however, I have succeeded in opening that curtain, and this year, the JDR has published several articles from my research group on the problems of dental caries, all with different approaches.
It can be said that one early sign of dental caries is increased synthesis of collagen, followed by its mineralization, which is histologically manifested as reparative dentin formation. The statement that dental caries consists of ’enhanced synthesis of collagen and its mineralization’ certainly irritates all cariologists, although it might be biologically correct. Minerals constitute about 69%, water 10%, and organic matter 21.0% of dry dentin, which is anatomically the mineralized layer between predentin, pulp, and enamel/cementum. Teeth are the only organs of the body whose composition is expressed on a ’dry weight’ basis. (What is the ’dry weight’ of the rest of the human body?) Dentin is penetrated by dentinal tubules filled with dentinal fluid, which is normal interstitial fluid, rich in sodium, and is also saturated with calcium and phosphate ions. Interestingly, calcium and phosphate levels do not increase during caries (Larmas et al., 1986). The rest of ’wet’ dentin consists of cellular processes and bodies of odontoblasts as well as predentin. These unmineralized dentin structures are not normally included in the anatomical or ’dry’ dentin. Therefore, caries in dentin can be described in two different ways: (1) According to the anatomical definition, it is the softened, i.e., decalcified, and destroyed portion of mineralized dentin, i.e., the ’lesion’; or (2) according to the functional definition, it is the outcome in pulp-dentin after an acid attack by plaque microbes. Dentin destruction starts at the dentin-enamel border (the lesion), and reparative dentin formation under the lesion (the response) is an altered steady state of the pulp-dentin complex. Evidently, the pulp-dentin complex regulates and modulates the rate of lesion progression in dentin (for recent reviews, see Larmas, 2001; Smith, 2002) and possibly did so even earlier, when caries progressed in enamel only. When the focus of dental caries research is widened from lesion formation to pulpal responses, the following hypotheses can be presented:
We began to test these hypotheses on caries progression and its regulation, postulating that one of the possible effects of fluoride might be a reducing and stabilizing effect on (the rapid) caries progression during primary dentinogenesis. Consequently, the progression of caries in dentin was thought to be reduced by fluoride at proper concentrations. Clinical studies supported this view. The percentage of caries reduction due to water fluoridation is more marked in dentinal lesions than in all lesions (Backer Dirks et al., 1961). Indeed, our group demonstrated that fluoride reduced the rates of both primary dentin formation and caries progression in dentin during the post-weaning period of experimental rats and submitted to the JDR an article reporting this observation. The reviewer’s evaluation was very short: "The authors are the only persons in the whole world who do not know that fluoride enhances remineralization and reduces demineralization on enamel surface and fluoride does not have any mystic systemic effect." Because that was the only evaluation we received, I found myself wondering why concentrations of fluoride similar to those we used in our caries experiments in dentin were also used in experiments on the prevention of osteoporosis. The connection between enamel surface and osteoporosis remained obscure to me. Later, I had to admit that the reviewer had been partly right. The reducing effect of fluoride on both caries progression in dentin and dentin formation under the lesion was not caused by the fluoride ion alone but was a combined effect of a high-sucrose diet and fluoride in drinking water, which we reported later (Kortelainen and Larmas, 1990). The sucrose effect was more pronounced and was dose-dependent (Huumonen et al., 1997). We also noticed in the literature that all animal experiments on the effects of different ions on caries from the 1960s onward were lacking a non-sucrose control diet, and the recent reports on the effect of any ion are therefore actually reports on a combined ion/high-sucrose effect. This was not the situation in the 1950s, when non-sucrose controls were included in experimental caries studies. We were able to show that the high-sucrose diet is the ’arch criminal’ in odontoblast metabolism that reduces dentin formation during primary dentinogenesis to about half of normal. But the normal rates of both caries progression and dentin formation are at least ten times faster in young rats than in adult rats (Kortelainen and Larmas, 1994). This might explain why cariogenicity depends more on the diet than on the prevailing mutans streptococcal species (for review, see Van Palenstein Helderman et al., 1996). This also led to an analysis of the mechanisms of sugar action. The reducing effect of glucose on collagen synthesis by odontoblasts was established in vitro in a tissue culture system (Välikangas et al., 2001). This ’culture’ is not a real tissue culture, because odontoblasts did not divide, but they were capable of producing collagen and maintained their steady state of metabolism for a few days in culture (Tjäderhane et al., 1998a). In this system, glucose was observed to reduce the rate of collagen synthesis, which effect was independent of the insulin concentration in the culture medium (Välikangas et al., 2001). This in vitro experiment was recently repeated in vivo, and we were able to demonstrate that sugar reduced dentin formation in the rat and that hyperinsulinemia did not enhance that reducing effect (Pekkala et al., 2002). Experiments without insulin are now being conducted on diabetic animals, because it is well-known that wound healing is impaired in diabetes mellitus, and collagen synthesis under the caries lesion in dentin might be a special form of ’wound healing’. Because one of the mechanisms of fluoride is that it inhibits sugar uptake by micro-organisms (Hamilton, 1977), I asked my friends, during the 2002 IADR Meeting in San Diego, whether fluoride works in the same way with mammalian cells. I was advised, and found from the literature, that (very) high levels of fluoride induce hyperglycemia in experimental animals (McGown and Suttie, 1977). Hyperglycemia is a symptom of diabetes and may reduce dentin apposition, thereby possibly affecting caries progression. I therefore have a feeling that, despite the comment by my anonymous reviewer, fluoride may have an as-yet-unknown systemic effect on caries prevention in addition to its well-known effect on the enamel surface.
In addition to the experimental approach, we have also used statistical methods to analyze the rate of caries progression when the process reaches dentin (and a dentist makes a filling). Using life-table methods, Carlos and Gittelsohn (1965) estimated the risk of tooth failure due to caries separately for each tooth as a function of tooth age in several longitudinal cohorts recruited for caries trials in New York State. They reported a maximum caries risk about only two years after tooth eruption in the second permanent molars and one or two years later in all other teeth, including the mandibular incisors. This means, in practice, that caries during primary dentinogenesis was very rapid in every tooth in the USA in the 1940s and 1950s. We found that the first carious attack into dentin was even faster in molar teeth in Finland. We used a survival model and Bayesian inferential methods in our statistical analysis and showed that the risk in molar teeth was highest immediately after eruption in the cohorts born in 1960 and 1970, while in the 1980 cohort, the risks of individual teeth were so low that no such dependencies on tooth age could be established (Härkänen et al., 2002). Carlos and Gittelsohn (1965) excluded all permanent teeth that had erupted before the first examination of the trial. Therefore, their results with regard to molar teeth were biased and not completely comparable with ours. Still, both their observations and ours suggest that the caries rate during primary dentinogenesis has declined from the 1940s up to the present. What has happened with caries during secondary dentinogenesis and/or tertiary dentinogenesis remains to be analyzed in the future. This observation can also be viewed against the progression rate of caries in living vs. extracted teeth. It was possible that more than 50% of molars became carious deep in dentin during the year the tooth began erupting, i.e., a mean of six months after gingival perforation (Larmas et al., 1995). When we made an in vitro experiment by dropping lactate at pH 4.5 on the occlusal ’window’ of an extracted molar which had been washed with water every morning and every night during the working days, we did not see any lesion formation after three years and terminated the experiment at that point. Only slight decalcification of enamel could be seen (Hietala et al., unpublished observations). Caries did not progress in enamel when the dead tooth was located outside the oral cavity, although the acid attack was constant and lasted for more than three consecutive years, and no salivary defense systems were available. This observation reminds me of a quotation of Bonner (in Burnett et al., 1976): "It is paradoxical that the hardest of all tissues is so susceptible to lesions in living humans, while it is the most indestructible in the dead."
Acids are unable to destroy type I collagen in dentin, which can be destroyed only by the action of proteolytic enzymes. Therefore, it is not surprising that the soft caries lesion in dentin exhibits a large range of hydrolytic and other enzymes that were histochemically localized in dentinal tubules. In the studies that we conducted as early as the 1970s (for review, see Larmas, 1986), the proteolytic enzymes degrading the extracellular matrix proteins and peptides and many other individual hydrolytic enzymes were localized histochemically in the dentinal tubules. Also, lactate dehydrogenase (LDH), which catalyzes the production of lactic acid capable of destroying the mineralized part of dentin, was discovered deep in the sterile lesion (Larmas, 1972), which shows that lactate production may take place there even without microbes. Biochemically, enhanced collagenolytic activity was observed in carious dentin, suggesting the presence of collagenase activators or enzymic destruction of the inhibitor in the collagenase inhibitor complex (Dayan et al., 1983). When enzymography and functional activity assays with gelatin as substrate or quantification of the degradation of type I collagen was used, the presence of MMP2, MMP8, and MMP9 was identified in demineralized dentinal lesions in vitro (Tjäderhane et al., 1998b). The latent purified forms of these enzymes were activated at low pH (4.5), followed by neutralization. These enzymes were not found in samples of cariogenic bacteria (Tjäderhane et al., 1998b), which was also the case with amino-peptidases in caries lesions in vitro (Larmas and Mäkinen, 1971). Whether the proteases necessary for the destruction of dentin collagen originate from microbes or host cells, or both, remains uncertain. These observations suggest the possibility that host proteases from either odontoblast and pulp tissue or saliva are activated due to the exposure of pH changes in dentinal fluid during the carious process, resulting in destruction of the collagenous matrix during the carious process in dentin (Tjäderhane et al., 2001). Human dentin-pulp complex cells express several matrix metalloproteases (MMPs), including MMP-2, -8, -9, -14, and -20 (Llano et al., 1997; Tjäderhane et al., 1998b; Palosaari et al., 2000, 2002; Sulkala et al., 2002). They may participate in dental caries progression (destruction and/or responses) (Tjäderhane et al., 1998b; Martin-De Las Heras et al., 2000; Sulkala et al., 2002). DNA microarray demonstrated an extremely high level of MMP-13 (collagenase-3) expression in healthy and carious tooth samples, with less pronounced expression of MMP-16 and specific tissue inhibitor (TIMP-1). Western blot and immunohistochemistry staining confirmed the abundance of MMP-13, with no differences between healthy and carious tooth pulp tissues (Sulkala et al., unpublished observations). Thus, the enzyme spectrum of soft carious dentin is large, indicating that many biological processes are possible there. The presence of many proteolytic enzymes, including different endopeptidases (collagenases and gelatinases) and aminopeptidases, may explain the degradation of the protein matrix. The presence of different enzymes catalyzing the hydrolysis of mucopolysaccharides also suggests the possibility that other dentin connective tissue components may be hydrolyzed in physiological conditions. All these observations indicate that the processes occurring inside the caries lesion in dentin are much more complicated than those occurring in the decalcified portion of the caries lesion in enamel. The pulpal responses to caries in dentin further complicate the picture. The above observations emphasize the roles of biologically active (or less active cells) during health and disease and link the host to the caries process. The connection between osteoblasts and osteoporosis has been widely analyzed in medicine. A search of PubMed (10 December 2002) with these two key words revealed a total of 792 articles, whereas the key words ’odontoblast’ and ’caries’ identified only 99 articles, about 10% of which had been published by our research group. A search with the key words ’osteoblast’ and ’fluoride’ revealed 166 articles, whereas ’odontoblast’ and ’fluoride’ identified only 26 articles. Research on a chronic bone disease, osteoporosis, has been ten times more abundant than research on another chronic disease, caries, in dentin. In contrast, there are one hundred times as many doctors treating dental caries clinically worldwide compared with those treating osteoporosis. Dental caries research has successfully solved the chemistry and crystallography of apatite destruction (in both bones and teeth), and even the submolecular changes are known. What we need now is knowledge of the patholobiology of this important and common disease in dentin-pulp. This article is a public challenge for oral biologists to join the research family interested in dental caries, to broaden the approach from pure chemistry to applied biology of dentin, which is the only way to proceed toward understanding dental caries when pulpal responses are included in it, as they should be. The number of scientists in the field of oral biology is so large today that I am unable to reach them all by personal letters, which is why this "Discovery!" essay is needed. Received for publication November 14, 2002. Accepted for publication January 2, 2003.
Journal of Dental Research, Vol. 82, No. 4,
253-256 (2003) This article has been cited by other articles:
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
 |
|
|
 |
|
Sign In to gain access to subscriptions and/or personal tools.
TOP
INTRODUCTION--A BIT OF HISTORY
DENTIN CARIES VS. CARIES...
years of artificial fluoridation of drinking water in the Netherlands. Arch Oral Biol 5:284–300.
