In the mouth, active enamel lesions occur under undisturbed demineralizing conditions and show progressive lesion dynamics, whereas inactive lesions refer to the process of stagnation of progression due to lesion remineralization and brushing of the surface (removal of biofilm, polishing). To our best knowledge, this is the first study to use specular surface reflection intensity to discriminate caries lesion activity on both native and ground/polished enamel. Our results show that the handheld reflectometer was able to measure a loss of surface reflectivity after caries induction on both native and polished specimens. However, specific differences regarding caries severity (time of caries induction) were not observed on native specimens, only on polished specimens.
Caries induction caused a decrease in %SRI to different degrees on native and polished specimens. Generally, SRI measurements are easier to accomplish on flat and polished enamel, since the flat, smoother surface allows for a more regular reflection of the laser (4), thus yielding higher SRI values as native enamel; whereas, the rougher native enamel surfaces tend to scatter the laser beam, but they are also less susceptible to acid demineralisation than polished enamel (12, 17), and this may explain why %SRI values did not decrease more than 20%, levelling out at a plateau of ±80%, even in groups with longer caries induction times of 8 or 10 weeks. Also, native enamel may have a natural hypermineralized layer on its surface, and this may have also protected the surface against demineralization, leading to a slower progression of the lesion, which could, in part, also explain the differences in the results between native and polished surfaces. Glossiness of polished specimens also decreased during caries development, leading to a decrease in %SRI by more than 50% after 6 weeks, and over 80% after 10 weeks. Surprisingly, we were not able to detect a clear change of %SRI values after the remineralization and tooth brushing period on these specimens.
During the remineralization period, we expected that the combination of the abrasive forces of the toothbrush together with the mineral deposition in the white spot lesions would lead to an increase in enamel surface glossiness (polishing) and, thus, an increase in the %SRI values. On the contrary, toothbrushing actually caused a further decrease in %SRI in all polished specimens. An explanation for this significant decrease in %SRI after the remineralization period is that the initial caries induction period caused a demineralization of the enamel. Since polished enamel is more susceptible to demineralisation than native enamel, the caries induction weakened the enamel surface and caused an initial surface roughening. Subsequently, the brushing with toothpaste, during the remineralisation phase, actually removed part of this weakened surface, thus causing a further roughening of the enamel surface. This is supported by further experiments made in our laboratory. In the present experimental model, we brushed the specimens for a total of 1000 s (10 s twice daily, 5 days per week, for a total of 10 weeks), and when we brushed a sound (caries-free) polished tooth surface for 1000 s, it already caused some roughening of the surface, with an average decrease in %SRI of ± 7 %. So, the act of brushing a polished specimen, even when not weakened by caries, already leads to a detectable roughening of the surface, which is exacerbated when the surface is demineralised during caries induction.
Contrary to the polished specimens, the remineralization period promoted remarkably positive results on native enamel with early caries lesions (2, 4, and 6 weeks of caries induction), where tooth-brushing caused an increase in surface glossiness and an increase in %SRI. So, the native enamel specimens with early caries lesions (2, 4, and 6 weeks of caries induction) behaved according to our expectations and clinical experience, where the caries lesions are apparently shinier (more reflective) when they are inactive (8). Interestingly, similarly to polished specimens, we also observed a decrease in %SRI after brushing/remineralization that had undergone caries induction for 8–10 weeks (where more established enamel lesions were formed – ICDAS score 2). In this case, we hypothesize that the early caries lesions (induced for 2, 4, or 6 weeks) on native specimens were able to decrease the glossiness of the native enamel surface, thus causing a small, albeit distinct, decrease in %SRI, but this early demineralization was not able to weaken the enamel surface enough to allow a further roughening from tooth-brushing. On the other hand, longer caries induction (8–10 weeks) not only caused a decrease in glossiness, but it also caused a weakening of the enamel surface, which was later roughened by the brushing. Furthermore, our limited remineralisation phase (only 10 weeks) produced only a finite toothbrushing period. This was not enough to effectively smoothen the weakened enamel surface, accounting for the lower %SRI values. We can, therefore, also speculate that, in the clinical setting, the recurrent brushing periods that last several months could probably, in time, remove a great deal of the weakened outermost layer of demineralised enamel, thus leading to a smoother surface, with the traditional shiny/glossy appearance of inactive lesions (8). Unfortunately, because we used unpolished, native enamel specimens in this study, we were not able to precisely measure either surface hardness or roughness in the present experiment. Further studies are still necessary to verify our hypothesis, and to assess the relationship between these parameters and %SRI in a caries induction model.
Our caries model mimicking active and inactive enamel lesions worked, and it produced typical white spot lesions. One limitation, however, is that we did not use any histological or non-destructive methods to measure the depth of these lesions, or methods such as TMR to measure the remineralizing effect after toothbrushing. On the other hand, some of these methods are not easily carried out in native enamel surfaces, which we used in the present study. We did, however, use the ICDAS and the Andersson scores to confirm the presence and changes to the white spot lesions. ICDAS is a validated system that links visual appearance to lesion depth (18), while the Andersson score assessed the severity of ICDAS 2 lesions (14). A bacterial biofilm under cariogenic conditions not only leads to superficial enamel erosion, but also, and more importantly, to a subsurface demineralization. The longer and the more aggressive the cariogenic challenge, the deeper the demineralization, as could be confirmed by our results: ICDAS and Andersson in this study were sensitive enough to measure an increase of lesion depth and lesion severity during the caries formation stage. After the remineralization/brushing period of the experiment, the visual assessments were sensitive enough to detect a decrease of severity and, partially, lesion depth. Subsurface demineralization is characterized by a pseudo-intact surface that allows bacterial acids to penetrate the enamel through focal holes (Fig. 4A). Only at an advanced stage of caries does the net demineralization eventually lead to a breakdown of the surface. This was found in some of the native enamel specimens after 10 weeks of caries formation (Fig. 2A). Thus, the enamel specimens, both native and polished, behaved as expected under long-term cariogenic/demineralizing conditions and under remineralizing/abrasive conditions. In theory and from our clinical experience, enamel lesions under an undisturbed biofilm challenged by cariogenic conditions are ‘active’ lesions, while regular removal of biofilm by tooth-brushing, abrasion of the surface by toothpaste, and remineralization of the enamel by fluoride, calcium and phosphorous lead to characteristic surface changes (increase of glossiness, less chalky appearance) that are attributed to ‘inactive’ lesions (9). However, although we observed a change in lesion appearance mirrored in the ICDAS and Andersson scores, we were not able to detect changes of glossiness or mattness in our specimens throughout the duration of the study. This could be because the caries model chosen was rather mild, or that we needed longer brushing periods to allow for further tooth surface polishing.
The visual appearance of enamel lesions is dependent on many factors, e.g. the level of magnification (19), the amount of light that is used during caries diagnosis (20), or on the examiner him/herself: In an ex vivo study on 104 extracted teeth with white spot lesions, only about half of the lesions were unanimously rated “matte” or “shiny” by 4 examiners (21). Therefore, an objective method would be desirable to measure the reflective light and thus to quantify mattness of a lesion. A first promising step was described some time ago using a chromatic confocal white light sensor that measured the perpendicular reflection intensity (PRI) with an angle of 0° between incoming and reflected light (10). Due to the nature of the confocal sensor, the wavelength of the incident light was not important for the measurements, but only its reflection intensity. In an ex vivo study, 43 white spot lesions were visually judged as being ‘active’ or ‘inactive’, and the PRI method was correlated well to these visual judgements. A true validation however was not possible, because it is error-prone to judge the status of lesion activity on extracted teeth without the necessary specific clinical information. Furthermore, some lesions can also be regarded as ‘mixed’ lesions, with surface characteristics of both active and inactive lesions. Therefore, instead of using extracted teeth with caries lesions, we have opted for a bacterial caries model to reliably simulate clinical conditions for caries development, thus standardizing the cariogenic impact. We set up a rather mild caries model, with a total fluoride content of 1 ppmF in the demineralization solution, in order to be able to better display a time-related relationship between caries formation and loss of SRI. Our caries model allowed for distinct differences in %SRI on the polished specimens, where we observe a gradual decrease in %SRI as the duration of caries induction increased.
Different to PRI, the Optipen device operates on SRI (specular reflection intensity), using a red laser light (635 nm) with entrance angle and reflection angle both set at 23°. Therefore, it could be speculated that PRI and SRI measure different reflection features. It was shown earlier that light absorption into human enamel increases with decreasing wavelengths (22). Therefore, absorption would not interfere at the chosen wavelength. In a former study by (23) the wavelength of 633 nm was reported to almost perfectly fit to a Monte Carlo curve. In this curve the scattering maximum is at 0°, while for angles >20° the curve reaches a plateau at log (Fract. scattered energy) of 10−1, indicating that scattering at the surface in enamel is almost negligible (23). So, under the parameters chosen in our experiment, the light beam of the Optipen purportedly is transduced into the enamel and is scattered at the body of the white spot lesion. The formerly established method of laser fluorescence for caries detection operates on a quite similar wavelength of 655 nm and uses the backscattered fluorescence signal to measure lesion depth (24). Thus, while PRI measures only reflection intensity of the surface, the Optipen probably also excites fluorophores of bacterial porphyrins that are located in the subsurface lesion (25). This also explains why the SRI measurements in our experiment remained stable in the native enamel group. PRI was found to correspond well with lesion activity in enamel caries lesions that were induced for 3,6 and 9 days, respectively (26). However, in the latter study only flat polished enamel was used, and the results cannot be transferred to native enamel.
Because lesion activity is not only characterized by optical appearance (shiny or matte), but also by tactile surface features (rough or smooth), it would have been desirable to also measure roughness of our specimens. Although this was not possible in this paper, previous erosion studies using the SRI device have shown a strong correlation between surface roughness and SRI (27), so the decrease in %SRI observed in our specimens is most probably related to an increase in enamel surface roughness. Moreover, there is a strong correlation between SRI and surface hardness, where a decrease in %SRI is also related to a weakening of the enamel surface (3). This corroborates our initial hypothesis, that enamel specimens submitted to our caries model presented a decrease in %SRI, which is associated to a rougher and weaker enamel surface, and less gloss; while the subsequent remineralization period (tooth-brushing) further roughened the enamel surface, provoking an even greater decrease in %SRI values.
Despite the interesting positive results with the Optipen for early enamel lesions on native enamel, there are two main disadvantages to this device that could hinder its use in the clinical setting. The first is that the Optipen is highly surface-dependent, where the SRI values will depend on the surface micro-morphology of each tooth surface. This means that different teeth, depending on their surface micro-morphology and roughness, will yield very different SRI values. It follows that, we cannot generate a universal SRI range for “sound” and “demineralised” tooth surfaces, instead, each tooth surface will have its own “sound” and “demineralised” values. Therefore, the Optipen should rather not be used as a diagnosis tool, but as a monitoring device. The other disadvantage is that the Optipen is also, to some extent, rater-dependent, where different individuals using the device on the same tooth surface will obtain slightly different SRI values. This, however, can be counterbalanced by training, where trained individuals have obtained good agreements, with intraclass correlation of ICC = 0.77 and 0.86 for deciduous and permanent teeth, respectively (3).