Enamel Crystal ChemistryWhere Do We Go From Here? G. S. INGRAM Unilever Research, 455 London Road, Isleworth, Middlesex, England J Dent Res 58(B):904-905, March 1979 This section of the meeting has been devoted to accumulated on changes of apatite lattice enamel crystal chemistry and its behavior during dimensions with composition. We would formation and demineralization. In dealing with be very interested to know how the lattice calcified tissues of any type we are looking at dimensions of the early enamel crystallites a crystalline formation with the most diverse compare with those in the later enamel. range of properties which one could choose for There is evidence that the acid phosphate its purpose. The size, shape, and intercrystalline play an binding give mechanical strength, cutting ability, groups in seed apatite crystallites important role in the later crystal growth and flexibility where necessary.
which yields the bulk of enamel and other The crystalline lattice can accommodate calcified tissues. If the precursor of apatite a number of other ions and still maintain its is OCP then it would be of fundamental primary function. The material exists over importance to know the involvement of that a range of compositions and can change its material's acid phosphate group in subsesurface composition to reflect prevailing quent growth and transformation processes solution conditions. Finally, although it is and the OCP-inhibitor interactions which subject to destructive dissolution it can, preserve that structure in the presence of under favorable conditions, undergo sub- hydroxyapatite. Much has been written and even more stantial repair. on the role played by fluoride speculated I make no apology for making reference in this meeting to studies concerned with in the stability of enamel mineral under a bone and dentin, where appropriate, as well wide range of pH conditions. When considering the earliest stage of mineral appearas to enamel. ance during enamel formation we should In considering what we next need to learn about enamel crystals, perhaps we should determine the action of fluoride upon this start where they do: at first inception of mineral, which may be the most calciumcrystallization. Much has appeared on amor- deficient stage of enamel development. Since the previous meeting on tooth phous phases early in calcification and their transformation, usually in vitro, into enamel, a number of workers have published an apatite precursor, probably OCP. The data on the inhibition of apatite crystal kinetics of this process need to be estab- growth by such diverse species as the diphoslished particularly in the presence of various phonates, pyrophosphate, copper, magnesium, ATP and salivary glycoproteins. It is levels of fluoride. Over the years the electron microscope fairly easy in the case of the anions to envisage has shown the favored dissolution of the displacement of surface phosphate, thus centers of crystallites and we have seen altering the apatite surface so that it will quite elegant pictures of this process. There no longer act as a template for apatite crystal is substantial evidence that this is happening growth. It is probably over-simplistic to at crystal dislocations but it is worth asking expect cations to affect the process in the why these dislocations favor dissolution same way and a number of techniques will etch-pits roughly at the centers of the ends need to be combined to answer this quesof enamel crystallites. It seems probable tion. In particular, some of the current that the first nuclei of mineral produced analytical methods are not adequate to detect with certainty the small, but imporcontain so many inclusions such as HP042 differences occurring. tant, crystal for a basis and the 0, that H2 CO32 Very close control will also be needed on lattice dislocation is present at the central core of crystal growth. I cannot envisage the levels of all ions in these inhibitions and ways of identifying this with certainty but in the more sophisticated crystal growth would be interested to know whether it is experiments which will be needed to establish the controlling parameters in mineralizanear the truth. A considerable amount of data has been tion processes and their respective sensi904
Vol. 58(B)
905
be a valuable tool in a study of this type. Many of the questions about the state of combination of carbonate in calcified tissue have been or are in the process of being answered. The fact that it replaces phosphate or HP042- groups has been adequately reported but its exact function needs further elucidation, especially since the presence of carbonate species influences apatite crystal growth properties. There is no reason, on the basis of ionic size, why sulphate should not replace phosphate in hydroxyapatite in much the same way as carbonate but it does not appear to do so. Once again we do not know the reasons. Pyrophosphate has been variously reported in plaque and enamel but we have no knowledge of the state of combination of pyrophosphate in apatite or enamel or of its function. The reactivity of the ion with apatite is itself a sufficient incentive to study the mode of interaction. Pyrophosphate reacts primarily with the enamel apatite crystal surface and this brings me to the closing topic in my suggestions for future studies in enamel crystal chemistry. The nature of that surface has caused a dissolves during caries. Earlier reference has been made to the great deal of speculation but firm evidence exchange of surface and lattice ions in about actual chemistry and stoichiometry hydroxyapatite. The crystal dimensions of the surface is lacking. To provide a point themselves restrict the degree to which some of controversy, I would like to put the view candidate ions exchange. The preferen- that merely being in the surface does not tial uptake of fluoride and exclusion of necessarily alter the properties or composichloride by apatite illustrate this. Strontium tion of the exposed layers of ions. May I can replace calcium to some extent, but the suggest that, to start with, we put together absolute level of selectivity in this process the evidence we have about the nature of the is not yet known. Thermochemistry may enamel crystallite surface and its interface with a steady-state solution of its ions.
tivities. After its formation and eruption into the mouth the tooth enamel is capable of further changes described as post-eruptive maturation. That this is an important process is shown by the change in caries susceptibility of molars at various stages after eruption and the influence of age of the dentition on rat caries. There is a good case for studies of the permeability of these teeth to neutral molecules and its correlation with the kinetics of uptake of calcium and phosphate by different elements of the teeth. It is conceivable that the restricted accessibility of the enamel crystallites in the molar fissure to the calcium and phosphate of the saliva may play a part in the caries susceptibility of that region. The major reason that we are holding the meeting is that enamel is subject to the dissolution processes we see in caries. A great deal of work has been devoted to establishing the thermodynamic solubility parameters of hydroxyapatite and the other calcium phosphates. However, it may be of greater importance to find the stoichiometry, or lack of it, with which the enamel apatite
Session VI Discussion Dr. Nancollas: Three quick points on your paper, Dr. Ingram. With respect to the fluoride effect, we have now used the constant composition method with very low fluoride. You can, in fact, if you control the supersaturation with respect to the various phases, pick up the boundary between HAP and FAP as these phases form on the surface of the enamel.
Dr. Ingram: Can you make some speculation about the energetics that might be involved in that particular system, because going from HAP to FAP involves intermediate phases of fluoridated hydroxyapatite. Dr. Nancollas: This is something that we are currently doing. With respect to the
J Dent Res Special Issue B, 1 979
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effects of anions on crystal growth, I suppose that one of the main influences is binding to calcium on the surface of the apatite. For instance, the phosphonates need to bind to only a very few calcium sites in order to completely stop the growth of the crystals, perhaps only four percent of the total surface. Dr. Ingram: I would agree. It would depend very much upon the nature of the anion. We considered phosphonates, pyrophosphate, and statherin. Statherin is probably like a caterpiflar with all its feet at one end. Dr. Fleisch's group found that if you use quite low concentrations of pyrophosphate, in fact, you could detect an increase in the orthophosphate content in the solution. But then, people have confused that with chelation effects. Dr. Nancollas: And the final thing, you mentioned the use of calorimetry. We used solution calorimetry in some of our systems. The observed heat changes are pretty small, because one is looking at a very small change in the presence of large heats of hydration. Dr. Ingram: Oh, yes. This is quite true. The big nuisance is always the heat of wetting. In our work this heat was of the order of 1.5 Joules. The additional heat of interaction with fluoride was about 0.42 Joules at its plateau value compared with 1.2 Joules with monofluorophosphate. If one calculates the heats liberated from the uptake figures one sees values of 10 Joules per mole of fluoride and 40 Joules per mole of monofluorophosphate. I think it is interesting that, energetically, it is highly desirable to remove those HPO4 groups. However, some HP042- groups are necessary to maintain crystal growth. Dr. Nancollas: One of the problems is that you are always producing water and removing hydrogen with very small change in pH. Dr. Ingram: We have measured a certain small pH change when using the unbuffered system here. Dr. Robinson: I just wanted to raise a point about your removing acid phosphate. You heat it. I presume that produces pyrophosphate. So when you've got limited crystal growth after that treatment, not only have you removed the acid phosphate, but you have also got a crystal poison there, i.e., pyrophosphate. How easy is it to distinguish between the two effects?
Dr. Ingram: You are quite right. We have now tried to reverse this effect. We took the heated material, put it into a Pasteur pipette and passed some very pure monofluorophosphate through the heated hydroxyapatite. We have, in fact, managed to elute some pyrophosphate. We have not yet detected this pyrophosphate but have taken the monofluorophosphate-washed solid and incubated it in the system with the calcifying solution, and, whereas you obtain no crystal growth with the heated material, after monofluorophosphate treatment, it started to grow crystals again. Dr. Gedalia: Is it your opinion that the ion adheres to the outer enamel surface, or is it taken up as a composition of F and P03? Dr. Ingram: Well, it is, of course, a complex anion. If you take pure monofluorophosphate it will not produce a response in a fluoride electrode. We have sufficient evidence showing that it can displace surface acid phosphate groups. The treated solid, with its HP042- groups displaced, cannot then undergo crystal growth in a calcifying medium. However, if one stores the treated solid in water, slow hydrolysis of the monofluorophosphate occurs with release of fluoride into the water. In a calcifying system this leads to rapid crystal growth and incorporation of about 90% of the liberated fluoride into the crystals. Dr. Gedalia: We analyzed enamel that had been treated with FP032- ions. We haven't gotten the same amount of F uptake with an equivalent sodium fluoride solution. Dr. Ingram: This is correct. I think the reason is because the FP032- goes where it is needed. It goes into what Dr. Arends and his colleagues have shown, i.e., the increased level of acid phosphate sites in subsurface caries lesions. I believe that monofluorophosphate is selective where the fluoride can go into any crystallite of hydroxyapatite. Dr. Gray: I think I'll use my privilege as chairman to start off another round of comments. I've been listening to all these talks about how we alter solubility. I maintain that I don't care what you do, I can take my system or Dr. Silverstone's system, and make caries with them. Although I believe solubility differences can account for differences in caries rates such as seen in rampant caries, I don't think they explain
FPOJ
Vol. 58(B)
the caries reduction we see with fluoridation or even with topical application of fluoride. Even if one includes the antienzyme effect of fluoride on bacteria, as Dr. Moreno has discussed, one still comes up short. There is something that fluoride is doing, and I wonder if it really has anything to do with antisolubility. The other side of the coin is that if we could truly make enamel insoluble, we ought to stop the caries process. I throw that out for comments. I believe that Dr. Fejerskov has had the same experience as I. I can take enamel with nice, beautiful crystals, very poor crystals, or from a fluoridated area, and I can make caries in it in vitro. I am running out of ways for explaining low caries rates. Does anyone wish to comment? Dr. Silverstone: We all have the same problems. It may just be in terms of degree. For example, what I call a large lesion, histologically, is one going three quarters of the way through the enamel. What I call even larger, is one that reaches the dentinoenamel junction and goes several hundred microns into the dentin. And, in the enamel, you may have fifty, sixty or seventy percent pore volume. Yet, if you take that tooth with that lesion, put it back in the mouth and take the conventional bite-wing radiograph, it is unlikely that you will even pick up the lesion. So, the sorts of lesions we are working with are well below the clinical level. Fluoride might be working just by maintaining that surface zone, which acts as a rate-restricting membrane, in keeping the lesions within those terms, which we call large, but the clinicians have not, as yet, actually diagnosed. Dr. Gray: Well, if I extrapolate what you are saying, you mean we have to run our systems until they cavitate to be sure. Dr. Silverstone: That is the trouble. Dr. Brown: I feel that the fluoride has to be there when the crystals are growing, because it causes the octacalcium phosphate to hydrolyze to apatite. Topical fluoridation is an entirely different sort of thing, because its purpose is to convert hydroxyapatite to fluorapatite. It is not generally recognized that we cannot get very much of the fluoride into the hydroxyapatite crystals, because this is a solid-state reaction which is extremely slow at body temperature. This is why we have gone to a procedure in which some of the enamel apatite is converted
907
into CaHPO4-2H20 before the topical fluoride treatment. Dr. Moreno: I think that, as time goes by, it becomes increasingly more obvious that fluoride has multiple effects. One of them, for example, is the effect on tooth morphology in the case of systemic fluoride. In children who have taken fluoride supplements from birth you find shallower fissures which are easier to clean thereby preventing accumulation of debris and cariogenic plaque. Also, it appears that incorporation of fluoride into the crystalline structure of apatites brings about marked changes in the electrochemical properties of apatitic surfaces. Thus, the surface pellicles developed by adsorption of salivary constituents onto hydroxyapatite have different properties than those developed onto fluoridated hydroxyapatites; in the latter case, the ionic permselectivity is considerably higher. It is also known that fluoride, within certain concentration ranges, accelerates the rate of crystal growth of calcium apatites. Similarly, as reported by Silverstone and other investigators, fluoride increases the rate of enamel remineralization. Thus, I think that the cariostatic effect of fluoride is related to several mechanisms. It is significant that a certain correspondence exists between the effect of fluoride on the crystal growth rates of synthetic apatite crystals and on the rate of enamel remineralization in vitro. In this latter case, however, systematic investigations should be conducted to define the relationship between the driving forces, i.e., the degree of supersaturation of the remineralizing solution, and the kinetics of the process. This kind of work has to be conducted with enamel itself because the relationship between the degree of supersaturation and the rate of enamel remineralization is probably different from that found in crystal growth experiments in which diffusion is not a limiting process by design. Dr. Fejerskov: I quite agree with Dr. Gray that it is worthwhile to reconsider the effect of fluorides in caries prevention with reference to basic mechanisms. We all agree that there may be several different mechanisms, but Dr. Moreno just mentioned one which I would like to go strictly against and that is the idea that fluoride has the potential to change the morphology of the tooth. This idea is based on clinical examinations, but the examiners knew they were in a socalled optimal fluoridated area, so they
908
expected to find "smaller' fissures. Then you go back to the fluoride experiments. In rats when one injects tremendous amounts of fluoride, one can change the fissures in the molars but I don't think we can draw a comparison between these experiments and the human clinic. Actually, if you compare teeth collected from so-called optimal, fluoridated water areas with those from an area with 0.2 ppm of fluoride you will observe that there is no difference in the fissures at all. Dr. Moreno: My comment was mostly based on reported differences observed between populations in fluoridated and non-
J Dent Res Special Issue B, 19 79
fluoridated areas and in particular on the work of Aasenden and Peebles (Arch. Oral Biol. 19:321-326, 1974) which clearly shows that the pits and fissures in the teeth of children who have taken fluoride supplements from birth are atypically shallow. Dr. Fejerskov: Just a short comment. Yes, I know that paper. I can tell you that if you go, for instance, to Denmark in a 1.4 ppm area you will have the same number of fillings in the occlusal fissures of first molars in this area as in any other area (Moller, Dental fluorose of caries, Rhodos Copenhagen, 1965).
which yields the bulk of enamel and other The crystalline lattice can accommodate calcified tissues. If the precursor of apatite a number of other ions and still maintain its is OCP then it would be of fundamental primary function. The material exists over importance to know the involvement of that a range of compositions and can change its material's acid phosphate group in subsesurface composition to reflect prevailing quent growth and transformation processes solution conditions. Finally, although it is and the OCP-inhibitor interactions which subject to destructive dissolution it can, preserve that structure in the presence of under favorable conditions, undergo sub- hydroxyapatite. Much has been written and even more stantial repair. on the role played by fluoride speculated I make no apology for making reference in this meeting to studies concerned with in the stability of enamel mineral under a bone and dentin, where appropriate, as well wide range of pH conditions. When considering the earliest stage of mineral appearas to enamel. ance during enamel formation we should In considering what we next need to learn about enamel crystals, perhaps we should determine the action of fluoride upon this start where they do: at first inception of mineral, which may be the most calciumcrystallization. Much has appeared on amor- deficient stage of enamel development. Since the previous meeting on tooth phous phases early in calcification and their transformation, usually in vitro, into enamel, a number of workers have published an apatite precursor, probably OCP. The data on the inhibition of apatite crystal kinetics of this process need to be estab- growth by such diverse species as the diphoslished particularly in the presence of various phonates, pyrophosphate, copper, magnesium, ATP and salivary glycoproteins. It is levels of fluoride. Over the years the electron microscope fairly easy in the case of the anions to envisage has shown the favored dissolution of the displacement of surface phosphate, thus centers of crystallites and we have seen altering the apatite surface so that it will quite elegant pictures of this process. There no longer act as a template for apatite crystal is substantial evidence that this is happening growth. It is probably over-simplistic to at crystal dislocations but it is worth asking expect cations to affect the process in the why these dislocations favor dissolution same way and a number of techniques will etch-pits roughly at the centers of the ends need to be combined to answer this quesof enamel crystallites. It seems probable tion. In particular, some of the current that the first nuclei of mineral produced analytical methods are not adequate to detect with certainty the small, but imporcontain so many inclusions such as HP042 differences occurring. tant, crystal for a basis and the 0, that H2 CO32 Very close control will also be needed on lattice dislocation is present at the central core of crystal growth. I cannot envisage the levels of all ions in these inhibitions and ways of identifying this with certainty but in the more sophisticated crystal growth would be interested to know whether it is experiments which will be needed to establish the controlling parameters in mineralizanear the truth. A considerable amount of data has been tion processes and their respective sensi904
Vol. 58(B)
905
be a valuable tool in a study of this type. Many of the questions about the state of combination of carbonate in calcified tissue have been or are in the process of being answered. The fact that it replaces phosphate or HP042- groups has been adequately reported but its exact function needs further elucidation, especially since the presence of carbonate species influences apatite crystal growth properties. There is no reason, on the basis of ionic size, why sulphate should not replace phosphate in hydroxyapatite in much the same way as carbonate but it does not appear to do so. Once again we do not know the reasons. Pyrophosphate has been variously reported in plaque and enamel but we have no knowledge of the state of combination of pyrophosphate in apatite or enamel or of its function. The reactivity of the ion with apatite is itself a sufficient incentive to study the mode of interaction. Pyrophosphate reacts primarily with the enamel apatite crystal surface and this brings me to the closing topic in my suggestions for future studies in enamel crystal chemistry. The nature of that surface has caused a dissolves during caries. Earlier reference has been made to the great deal of speculation but firm evidence exchange of surface and lattice ions in about actual chemistry and stoichiometry hydroxyapatite. The crystal dimensions of the surface is lacking. To provide a point themselves restrict the degree to which some of controversy, I would like to put the view candidate ions exchange. The preferen- that merely being in the surface does not tial uptake of fluoride and exclusion of necessarily alter the properties or composichloride by apatite illustrate this. Strontium tion of the exposed layers of ions. May I can replace calcium to some extent, but the suggest that, to start with, we put together absolute level of selectivity in this process the evidence we have about the nature of the is not yet known. Thermochemistry may enamel crystallite surface and its interface with a steady-state solution of its ions.
tivities. After its formation and eruption into the mouth the tooth enamel is capable of further changes described as post-eruptive maturation. That this is an important process is shown by the change in caries susceptibility of molars at various stages after eruption and the influence of age of the dentition on rat caries. There is a good case for studies of the permeability of these teeth to neutral molecules and its correlation with the kinetics of uptake of calcium and phosphate by different elements of the teeth. It is conceivable that the restricted accessibility of the enamel crystallites in the molar fissure to the calcium and phosphate of the saliva may play a part in the caries susceptibility of that region. The major reason that we are holding the meeting is that enamel is subject to the dissolution processes we see in caries. A great deal of work has been devoted to establishing the thermodynamic solubility parameters of hydroxyapatite and the other calcium phosphates. However, it may be of greater importance to find the stoichiometry, or lack of it, with which the enamel apatite
Session VI Discussion Dr. Nancollas: Three quick points on your paper, Dr. Ingram. With respect to the fluoride effect, we have now used the constant composition method with very low fluoride. You can, in fact, if you control the supersaturation with respect to the various phases, pick up the boundary between HAP and FAP as these phases form on the surface of the enamel.
Dr. Ingram: Can you make some speculation about the energetics that might be involved in that particular system, because going from HAP to FAP involves intermediate phases of fluoridated hydroxyapatite. Dr. Nancollas: This is something that we are currently doing. With respect to the
J Dent Res Special Issue B, 1 979
906
effects of anions on crystal growth, I suppose that one of the main influences is binding to calcium on the surface of the apatite. For instance, the phosphonates need to bind to only a very few calcium sites in order to completely stop the growth of the crystals, perhaps only four percent of the total surface. Dr. Ingram: I would agree. It would depend very much upon the nature of the anion. We considered phosphonates, pyrophosphate, and statherin. Statherin is probably like a caterpiflar with all its feet at one end. Dr. Fleisch's group found that if you use quite low concentrations of pyrophosphate, in fact, you could detect an increase in the orthophosphate content in the solution. But then, people have confused that with chelation effects. Dr. Nancollas: And the final thing, you mentioned the use of calorimetry. We used solution calorimetry in some of our systems. The observed heat changes are pretty small, because one is looking at a very small change in the presence of large heats of hydration. Dr. Ingram: Oh, yes. This is quite true. The big nuisance is always the heat of wetting. In our work this heat was of the order of 1.5 Joules. The additional heat of interaction with fluoride was about 0.42 Joules at its plateau value compared with 1.2 Joules with monofluorophosphate. If one calculates the heats liberated from the uptake figures one sees values of 10 Joules per mole of fluoride and 40 Joules per mole of monofluorophosphate. I think it is interesting that, energetically, it is highly desirable to remove those HPO4 groups. However, some HP042- groups are necessary to maintain crystal growth. Dr. Nancollas: One of the problems is that you are always producing water and removing hydrogen with very small change in pH. Dr. Ingram: We have measured a certain small pH change when using the unbuffered system here. Dr. Robinson: I just wanted to raise a point about your removing acid phosphate. You heat it. I presume that produces pyrophosphate. So when you've got limited crystal growth after that treatment, not only have you removed the acid phosphate, but you have also got a crystal poison there, i.e., pyrophosphate. How easy is it to distinguish between the two effects?
Dr. Ingram: You are quite right. We have now tried to reverse this effect. We took the heated material, put it into a Pasteur pipette and passed some very pure monofluorophosphate through the heated hydroxyapatite. We have, in fact, managed to elute some pyrophosphate. We have not yet detected this pyrophosphate but have taken the monofluorophosphate-washed solid and incubated it in the system with the calcifying solution, and, whereas you obtain no crystal growth with the heated material, after monofluorophosphate treatment, it started to grow crystals again. Dr. Gedalia: Is it your opinion that the ion adheres to the outer enamel surface, or is it taken up as a composition of F and P03? Dr. Ingram: Well, it is, of course, a complex anion. If you take pure monofluorophosphate it will not produce a response in a fluoride electrode. We have sufficient evidence showing that it can displace surface acid phosphate groups. The treated solid, with its HP042- groups displaced, cannot then undergo crystal growth in a calcifying medium. However, if one stores the treated solid in water, slow hydrolysis of the monofluorophosphate occurs with release of fluoride into the water. In a calcifying system this leads to rapid crystal growth and incorporation of about 90% of the liberated fluoride into the crystals. Dr. Gedalia: We analyzed enamel that had been treated with FP032- ions. We haven't gotten the same amount of F uptake with an equivalent sodium fluoride solution. Dr. Ingram: This is correct. I think the reason is because the FP032- goes where it is needed. It goes into what Dr. Arends and his colleagues have shown, i.e., the increased level of acid phosphate sites in subsurface caries lesions. I believe that monofluorophosphate is selective where the fluoride can go into any crystallite of hydroxyapatite. Dr. Gray: I think I'll use my privilege as chairman to start off another round of comments. I've been listening to all these talks about how we alter solubility. I maintain that I don't care what you do, I can take my system or Dr. Silverstone's system, and make caries with them. Although I believe solubility differences can account for differences in caries rates such as seen in rampant caries, I don't think they explain
FPOJ
Vol. 58(B)
the caries reduction we see with fluoridation or even with topical application of fluoride. Even if one includes the antienzyme effect of fluoride on bacteria, as Dr. Moreno has discussed, one still comes up short. There is something that fluoride is doing, and I wonder if it really has anything to do with antisolubility. The other side of the coin is that if we could truly make enamel insoluble, we ought to stop the caries process. I throw that out for comments. I believe that Dr. Fejerskov has had the same experience as I. I can take enamel with nice, beautiful crystals, very poor crystals, or from a fluoridated area, and I can make caries in it in vitro. I am running out of ways for explaining low caries rates. Does anyone wish to comment? Dr. Silverstone: We all have the same problems. It may just be in terms of degree. For example, what I call a large lesion, histologically, is one going three quarters of the way through the enamel. What I call even larger, is one that reaches the dentinoenamel junction and goes several hundred microns into the dentin. And, in the enamel, you may have fifty, sixty or seventy percent pore volume. Yet, if you take that tooth with that lesion, put it back in the mouth and take the conventional bite-wing radiograph, it is unlikely that you will even pick up the lesion. So, the sorts of lesions we are working with are well below the clinical level. Fluoride might be working just by maintaining that surface zone, which acts as a rate-restricting membrane, in keeping the lesions within those terms, which we call large, but the clinicians have not, as yet, actually diagnosed. Dr. Gray: Well, if I extrapolate what you are saying, you mean we have to run our systems until they cavitate to be sure. Dr. Silverstone: That is the trouble. Dr. Brown: I feel that the fluoride has to be there when the crystals are growing, because it causes the octacalcium phosphate to hydrolyze to apatite. Topical fluoridation is an entirely different sort of thing, because its purpose is to convert hydroxyapatite to fluorapatite. It is not generally recognized that we cannot get very much of the fluoride into the hydroxyapatite crystals, because this is a solid-state reaction which is extremely slow at body temperature. This is why we have gone to a procedure in which some of the enamel apatite is converted
907
into CaHPO4-2H20 before the topical fluoride treatment. Dr. Moreno: I think that, as time goes by, it becomes increasingly more obvious that fluoride has multiple effects. One of them, for example, is the effect on tooth morphology in the case of systemic fluoride. In children who have taken fluoride supplements from birth you find shallower fissures which are easier to clean thereby preventing accumulation of debris and cariogenic plaque. Also, it appears that incorporation of fluoride into the crystalline structure of apatites brings about marked changes in the electrochemical properties of apatitic surfaces. Thus, the surface pellicles developed by adsorption of salivary constituents onto hydroxyapatite have different properties than those developed onto fluoridated hydroxyapatites; in the latter case, the ionic permselectivity is considerably higher. It is also known that fluoride, within certain concentration ranges, accelerates the rate of crystal growth of calcium apatites. Similarly, as reported by Silverstone and other investigators, fluoride increases the rate of enamel remineralization. Thus, I think that the cariostatic effect of fluoride is related to several mechanisms. It is significant that a certain correspondence exists between the effect of fluoride on the crystal growth rates of synthetic apatite crystals and on the rate of enamel remineralization in vitro. In this latter case, however, systematic investigations should be conducted to define the relationship between the driving forces, i.e., the degree of supersaturation of the remineralizing solution, and the kinetics of the process. This kind of work has to be conducted with enamel itself because the relationship between the degree of supersaturation and the rate of enamel remineralization is probably different from that found in crystal growth experiments in which diffusion is not a limiting process by design. Dr. Fejerskov: I quite agree with Dr. Gray that it is worthwhile to reconsider the effect of fluorides in caries prevention with reference to basic mechanisms. We all agree that there may be several different mechanisms, but Dr. Moreno just mentioned one which I would like to go strictly against and that is the idea that fluoride has the potential to change the morphology of the tooth. This idea is based on clinical examinations, but the examiners knew they were in a socalled optimal fluoridated area, so they
908
expected to find "smaller' fissures. Then you go back to the fluoride experiments. In rats when one injects tremendous amounts of fluoride, one can change the fissures in the molars but I don't think we can draw a comparison between these experiments and the human clinic. Actually, if you compare teeth collected from so-called optimal, fluoridated water areas with those from an area with 0.2 ppm of fluoride you will observe that there is no difference in the fissures at all. Dr. Moreno: My comment was mostly based on reported differences observed between populations in fluoridated and non-
J Dent Res Special Issue B, 19 79
fluoridated areas and in particular on the work of Aasenden and Peebles (Arch. Oral Biol. 19:321-326, 1974) which clearly shows that the pits and fissures in the teeth of children who have taken fluoride supplements from birth are atypically shallow. Dr. Fejerskov: Just a short comment. Yes, I know that paper. I can tell you that if you go, for instance, to Denmark in a 1.4 ppm area you will have the same number of fillings in the occlusal fissures of first molars in this area as in any other area (Moller, Dental fluorose of caries, Rhodos Copenhagen, 1965).
