A Modified Silver Fluoride Application Technique
All lesions in this study were open and had extensive dentine involvement. Whether or not the treatment with silver fluoride followed by stannous fluoride had an effect in slowing or arresting these lesions is not known, as the investigation was not designed to evaluate this aspect. However, the possibility does exist as recent reviews have attested to the efficacy of silver fluoride preparations, in particular 38% silver diammine fluoride, in arresting carious lesions in primary teeth. The mode of action has been attributed to an inhibition of the demineralization process and a protective effect against collagen degradation. In addition, 38% silver diammine fluoride has demonstrated anti-microbial activity against organisms in an in vitro biofilm prepared using multiple cariogenic bacteria.
The retention of an uninterrupted black surface on open carious lesions 6 months after the application of silver fluoride followed by stannous fluoride was associated with a statistically significant reduction in lesion progression. It is noteworthy that this effect was seen at the major sites of caries initiation in primary molars and it occurred even though the cohort had a high caries rate as shown by the elevated mean decayed, missing and filled surfaces score at baseline. The usefulness of the black surface color as a predictor of caries activity was shown by the finding that lesions with an interrupted or lost black surface were 4.6 times more likely to have decay progression. Also the sensitivity of the approach in the context that an incomplete or absent black surface was a diagnostic aid for decay progression was 80%, whereas the specificity referring to situations where an uninterrupted black surface was present and decay had not progressed was 81%.
Several studies have reported the black staining of arrested carious dentine lesions in primary teeth at some stage after the application of 38% silver diammine fluoride. How soon after the application these changes occurred is not known because, in all these studies, 6 months elapsed before the treated lesions were re-examined. An exception was one of the three treatment groups in one of the studies. In this group the tannic acid from tea was used as a reducing agent to turn the surface of a lesion dark brown after the silver diammine fluoride application. However, this color change was not used to monitor lesion progression. When a deliberate reducing agent is not used, the rate of development of any color change would depend, amongst possible other factors, on the amount of reducing agents already in the lesion. However, if color is being used as an indicator of lesion progression, this variable is eliminated and a standardized baseline color is obtained by the use of a reducing agent as the second stage of a silver fluoride treatment.
The data in this study were clustered and, because of this, the Datta and Satten Rank Sum Test was used for statistical analysis rather than the generalized estimating equation (GEE) approach that generalizes the T-test. The GEE approach is more sensitive to outliers than non-parametric, rank-based approaches and both groupings in this study had outliers. Because it was desired to compare lesions with and without an interrupted black surface at specific tooth sites, the relatively small number of lesions would have limited the ability to correctly specify a GEE working correlation structure.
The composition of the surface stain and why its loss or break up is associated with lesion progression have not been established. However, it is possible that the underlying lesion becomes softer and more hydrated resulting in a loss of support and a breakup of the surface deposit.
Use of a silver salt solution followed by a reducing agent is a standard technique for the argyrophil staining reaction in histology. In such an application free silver ions bind with various functional groups in the tissue and are reduced to metallic silver. These become catalytic sites for further silver deposition once the reducing agent is applied and a large number of silver ions are reduced (Ag > Ag). Some silver oxide is probably also present. Extrapolating this to the present study, the stannous ions from the stannous fluoride acted as electron donors to reduce the free silver ions to metallic silver. The end result was probably a deposit of which the mineral portion comprised mainly silver and silver oxide.
In this study, the distinctive black surface color led to a high degree of reproducibility when lesions were examined in photographs taken 6 months after the initial treatment (Kappa coefficient = 0.9). Also the clear caries outline in many radiographs facilitated the high degree of reproducibility of duplicate lesion-depth measurements as evidenced by the high Pearson product moment correlation coefficient and Spearman rank correlation coefficient. With the digitized radiographs, adjustments for minor differences in projection angles between radiographs taken at baseline and 6 months were made using Regeemy Software. When using this approach it was found necessary, after placing the corresponding reference points (tie points) on each radiograph, to double check this aspect before producing the corrected image. For some reason the program produced a number of completely unrelated tie points that had to be removed.
Assessments of lesion progression from bitewing radiographs showed that, with some lesions, the distance between the lesion and the pulp actually increased in the 6-month observation period. This occurred even though the lesions were open. A similar phenomenon of an increase in the cavity-pulpal distance with time has been reported previously. It was found that, when lesions in primary molars were first treated with silver fluoride followed by stannous fluoride and later restored with resin composite, in 37% of the lesions the distance between the base of the lesion and the pulp increased by 0.2 mm or more over 18 months. Presumably this was due to the deposition of reactionary (tertiary) dentine.
A limitation of the approach used in this study was that the degree of openness of a lesion was not taken into account and related to the retention or otherwise of the black-mat surface. This aspect is worthy of future investigation as it has long been known that open carious lesions in primary teeth progress less rapidly than closed ones.
The current findings show that the presence of an uninterrupted black surface on open carious lesions in primary molars is a good indicator of caries stasis following the application of silver fluoride then stannous fluoride. In this study it applied even though the children had a high caries rate. A potential advantage of the approach in outreach programs where silver fluoride applications are being applied and assessed by dental auxiliaries is that it should facilitate easy recognition of lesion changes. By not requiring the use of explorers or probes to assess the hardness of a lesion surface it should also reduce the possibility of iatrogenic damage especially when a lesion is deep.
Discussion
All lesions in this study were open and had extensive dentine involvement. Whether or not the treatment with silver fluoride followed by stannous fluoride had an effect in slowing or arresting these lesions is not known, as the investigation was not designed to evaluate this aspect. However, the possibility does exist as recent reviews have attested to the efficacy of silver fluoride preparations, in particular 38% silver diammine fluoride, in arresting carious lesions in primary teeth. The mode of action has been attributed to an inhibition of the demineralization process and a protective effect against collagen degradation. In addition, 38% silver diammine fluoride has demonstrated anti-microbial activity against organisms in an in vitro biofilm prepared using multiple cariogenic bacteria.
The retention of an uninterrupted black surface on open carious lesions 6 months after the application of silver fluoride followed by stannous fluoride was associated with a statistically significant reduction in lesion progression. It is noteworthy that this effect was seen at the major sites of caries initiation in primary molars and it occurred even though the cohort had a high caries rate as shown by the elevated mean decayed, missing and filled surfaces score at baseline. The usefulness of the black surface color as a predictor of caries activity was shown by the finding that lesions with an interrupted or lost black surface were 4.6 times more likely to have decay progression. Also the sensitivity of the approach in the context that an incomplete or absent black surface was a diagnostic aid for decay progression was 80%, whereas the specificity referring to situations where an uninterrupted black surface was present and decay had not progressed was 81%.
Several studies have reported the black staining of arrested carious dentine lesions in primary teeth at some stage after the application of 38% silver diammine fluoride. How soon after the application these changes occurred is not known because, in all these studies, 6 months elapsed before the treated lesions were re-examined. An exception was one of the three treatment groups in one of the studies. In this group the tannic acid from tea was used as a reducing agent to turn the surface of a lesion dark brown after the silver diammine fluoride application. However, this color change was not used to monitor lesion progression. When a deliberate reducing agent is not used, the rate of development of any color change would depend, amongst possible other factors, on the amount of reducing agents already in the lesion. However, if color is being used as an indicator of lesion progression, this variable is eliminated and a standardized baseline color is obtained by the use of a reducing agent as the second stage of a silver fluoride treatment.
The data in this study were clustered and, because of this, the Datta and Satten Rank Sum Test was used for statistical analysis rather than the generalized estimating equation (GEE) approach that generalizes the T-test. The GEE approach is more sensitive to outliers than non-parametric, rank-based approaches and both groupings in this study had outliers. Because it was desired to compare lesions with and without an interrupted black surface at specific tooth sites, the relatively small number of lesions would have limited the ability to correctly specify a GEE working correlation structure.
The composition of the surface stain and why its loss or break up is associated with lesion progression have not been established. However, it is possible that the underlying lesion becomes softer and more hydrated resulting in a loss of support and a breakup of the surface deposit.
Use of a silver salt solution followed by a reducing agent is a standard technique for the argyrophil staining reaction in histology. In such an application free silver ions bind with various functional groups in the tissue and are reduced to metallic silver. These become catalytic sites for further silver deposition once the reducing agent is applied and a large number of silver ions are reduced (Ag > Ag). Some silver oxide is probably also present. Extrapolating this to the present study, the stannous ions from the stannous fluoride acted as electron donors to reduce the free silver ions to metallic silver. The end result was probably a deposit of which the mineral portion comprised mainly silver and silver oxide.
In this study, the distinctive black surface color led to a high degree of reproducibility when lesions were examined in photographs taken 6 months after the initial treatment (Kappa coefficient = 0.9). Also the clear caries outline in many radiographs facilitated the high degree of reproducibility of duplicate lesion-depth measurements as evidenced by the high Pearson product moment correlation coefficient and Spearman rank correlation coefficient. With the digitized radiographs, adjustments for minor differences in projection angles between radiographs taken at baseline and 6 months were made using Regeemy Software. When using this approach it was found necessary, after placing the corresponding reference points (tie points) on each radiograph, to double check this aspect before producing the corrected image. For some reason the program produced a number of completely unrelated tie points that had to be removed.
Assessments of lesion progression from bitewing radiographs showed that, with some lesions, the distance between the lesion and the pulp actually increased in the 6-month observation period. This occurred even though the lesions were open. A similar phenomenon of an increase in the cavity-pulpal distance with time has been reported previously. It was found that, when lesions in primary molars were first treated with silver fluoride followed by stannous fluoride and later restored with resin composite, in 37% of the lesions the distance between the base of the lesion and the pulp increased by 0.2 mm or more over 18 months. Presumably this was due to the deposition of reactionary (tertiary) dentine.
A limitation of the approach used in this study was that the degree of openness of a lesion was not taken into account and related to the retention or otherwise of the black-mat surface. This aspect is worthy of future investigation as it has long been known that open carious lesions in primary teeth progress less rapidly than closed ones.
The current findings show that the presence of an uninterrupted black surface on open carious lesions in primary molars is a good indicator of caries stasis following the application of silver fluoride then stannous fluoride. In this study it applied even though the children had a high caries rate. A potential advantage of the approach in outreach programs where silver fluoride applications are being applied and assessed by dental auxiliaries is that it should facilitate easy recognition of lesion changes. By not requiring the use of explorers or probes to assess the hardness of a lesion surface it should also reduce the possibility of iatrogenic damage especially when a lesion is deep.
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