Differences in susceptibility of deciduous and permanent teeth to erosion exist, albeit depending on protocol design and method of assessment

This study tested deciduous and permanent teeth considering different protocol designs and methods of assessment, to establish an array of demineralization conditions, ranging from milder erosive conditions (shorter exposure times to acid, higher pH, and gentle shaking) to more severe conditions (longer exposure times, lower pH, and vigorous shaking). We detected significant differences between deciduous and permanent teeth, but previous studies have reported conflicting results^21,22. These discrepancies have been attributed to the different sample sizes used in the experiments, or the different conditions^12,20. In clinical studies, however, the rate of demineralization seem to be faster in deciduous teeth than in permanent teeth²¹. Although in clinical situations abrasion (from tooth brushing) and attrition (from grinding teeth) also play relevant roles in erosive tooth wear, the present study now partly clarifies the divergence in susceptibility between deciduous and permanent teeth.

The most obvious differences between deciduous and permanent teeth are in their anatomy, where deciduous teeth are smaller and have a thinner and more porous enamel layer⁶. Although both kinds of teeth have similar enamel prisms^23,24, the prisms in deciduous teeth are smaller, more curved, and more widely spread^7,8. Moreover, their hydroxyapatite crystals are considerably different. The crystals found in both permanent and deciduous enamel are imperfect forms of hydroxyapatite, basically made up of calcium (Ca²⁺), phosphate (PO₄³⁻) and hydroxyl (OH⁻) ions with some ‘impurity’ ions, such as fluoride (F⁻), carbonate (CO₃²⁻) and sodium (Na⁺)²⁵, in a crystalline structure with the simplified chemical formula: Ca_10–xNa_x(PO₄)_6–y(CO₃)_z(OH)_2–uF_u²⁵. An important impurity ion in the difference between deciduous and permanent enamel is carbonate (CO₃²⁻). When CO₃²⁻ is present in the apatite crystal, this carbonated hydroxyapatite causes a distorted lattice conformation and the crystals are more soluble than the stoichiometric hydroxyapatite. Since deciduous enamel contains greater total CO₃²⁻ content²⁶, it is therefore more susceptible to dissolution. Additionally, deciduous enamel is less mineralized than its permanent counterpart⁵, it presents greater enamel porosity, greater organic content in the enamel structure²⁷, and, consequently, lower elasticity and lower surface microhardness¹⁵. Therefore, chemically and histologically, deciduous teeth have all the attributes for a greater rate of demineralization than permanent teeth.

Indeed, our results generally corroborated this fact. We further observed that, at milder conditions, these differences were more noticeable with the surface microhardness measurements, whereas in more severe conditions, the differences were observed in the calcium results. This reflects the histopathology of erosive demineralization. At milder conditions, the demineralization occurs at a slower rate and the partial loss of mineral causes an initial softening to the enamel surface. Consequently, rSMH detected the differences between the two kinds of teeth at milder conditions, but the amount of calcium released was still too low, and the variations were high enough to hinder the detection of any significant differences between both kinds of teeth. As the severity of the conditions intensified, the softened layer on both deciduous and permanent enamel reached a saturation point²⁸, with no further change to their surface hardness. This can be observed in Fig. 1, especially at pH 3, where the rSMH tends to level off at later erosion times (especially with deciduous teeth), and there is no difference between gentle and vigorous shaking. Therefore, rSMH can no longer detect the differences at more severe conditions, but calcium continues to be released to the acid, and the differences between the two kinds of teeth can be detected with this method of assessment. Despite the greater calcium release from permanent teeth at pH 4 vigorous shaking (to which we currently have no explanation), our results suggest that the lack of differences between the two kinds of teeth observed in the above-mentioned studies^{14,15,16,17,18,19} could actually be due to the method of assessment adopted with respect to the conditions used. This means that some methods can better detect the differences at milder demineralization conditions, while other methods are better for more severe conditions^29,30.

The different conditions used in our study were reached by varying the pH, one of the most important chemical factors in dental erosion²⁵, and the agitation, one of the most important physical factors³¹. Both these factors played major roles in enamel dissolution in our study; furthermore, the interaction of both is of particular relevance as shown by the regression analysis. During enamel dissolution, the H⁺ ions (determined by the pH) will drive the outflow of Ca²⁺, PO₄³⁻, OH⁻ and CO₃²⁻ ions from the enamel to the acid. If there is no agitation, this acid remains almost stagnant in the system, and the layer of acid in the immediate vicinity of the enamel (the Nernst layer) remains semi-static. This allows for the slow neutralization of the H⁺ ions within the Nernst layer by the PO₄³⁻, OH⁻ and CO₃²⁻ released from the demineralized enamel³². This slight rise in the local pH will reduce the demineralization rate. However, during more vigorous agitation, the Nernst layer will be dispersed and renewed, leading to a constant exchange of Ca²⁺, PO₄³⁻, OH⁻ and CO₃²⁻ ions with the medium, and new H⁺ ions from the acid will be available to drive the process forward, generating more demineralization. This process is intensified, if the pH of the solution is lowered, leading to an absolute increase in H⁺ ions.

Drawing a parallel to clinical situations, the more severe conditions used in the present study could represent patients who swish drinks in their mouths (vigorous agitation) or who hold drinks in their mouths (longer time of acid exposure)³¹ consuming either drinks with higher pH such as for e.g. orange juice or with lower pH such as soft drinks or energy drinks. A clinical study has already observed that patients generating these severe conditions with their drinking behavior also presented more erosive tooth wear³³. In the present study, however, we did not include other important variables, such as the type of acid in the acidic substance^34,35, presence of calcium and phosphate ions in the substance^34,36,37, or viscosity of the substance^20,38,39. It would be virtually impossible to design a study controlling and comparing all these variables at once.

Other important clinical factors affecting erosive tooth wear are saliva^10,40,41 and toothbrushing^13,42. Saliva of children contains significant lower calcium⁴³ and protein concentrations⁴⁴, with an apparent linear increase in total protein concentrations with age⁴⁵. This is probably related to the thinner salivary pellicle formed on deciduous teeth⁴¹, which contains less than half of the proteins in common with the pellicle formed on permanent teeth⁴⁰. Despite these differences, adult saliva better protects permanent teeth, whereas saliva from children better protects deciduous teeth¹⁰. Regarding tooth wear, toothbrushing is a substantial factor that could accelerate the wear of both kinds of teeth. If deciduous teeth are more susceptible to erosive demineralization, this could also mean that they present faster wear rate than its permanent pendant. This was observed in some studies, where erosive tooth wear in deciduous teeth was greater than in permanent teeth^13,46, but another study observed no differences between both kinds of teeth¹⁴. Again, different study designs were used, which could account for the difference in the results.

Hunter and coworkers¹¹ once suggested that the differences between deciduous and permanent teeth might rather appear over time; and we had similarly proposed that, given the specific circumstances, these differences would be more distinguishable²⁰. Now, our results show that the differences exist, but are observable depending on the conditions used and the methods of assessment. If the conditions are milder, where only initial changes to the enamel surface occur, surface analyses, such as hardness or roughness, might be more suitable. However, as the severity intensifies, the softened layer will reach a steady state, and the differences will no longer be perceptible. Therefore, at more severe conditions, other methods will be more appropriate, such as calcium analyses or profilometry. If, however, the latter methods are used at milder conditions, or the former methods used in severe conditions, the noise in the results might mask the differences between the two groups. Furthermore, if other factors are introduced into the protocol design, such as saliva or toothbrushing, they will have different effects on the different enamel substrates, thereby affecting their demineralization rates distinctively. Hence, the conditions used will also play a role.

Given the innumerous variables influencing erosive demineralization, it is virtually impossible to make a direct comparison between the different studies, especially when the methodological details are not explicit in the publications. The different models and methodologies used in such studies have already been previously discussed³, but no specific guidelines were proposed for designing “the most appropriate experimental model”. In fact, any methodological condition will be highly dependent on the objectives of each individual study. What is most important is that authors are urged to spare no methodological details when reporting their methods. Although these minutiae might seem trivial (like acid pH, concentration, agitation methods, temperatures, durations, etc.), they will greatly influence the results and might even explain the divergences across the different studies. Moreover, the ideal method for measurement will greatly depend on the characteristics of the tooth surface, on the stage of the erosive lesion, as well as on the expected changes to the structure of the lesion³⁰. So, in-depth understanding of the histopathological aspects of the demineralization process is the decisive factor when planning an experiment.

We conclude that differences in the susceptibility to erosion between deciduous and permanent teeth do exist, but they are only distinguishable provided that the methods of assessment are appropriate for the conditions used, taking into consideration the protocol design.