Elsevier

Acta Biomaterialia

Volume 88, 1 April 2019, Pages 491-502
Acta Biomaterialia

Full length article
Effects of silver diamine fluoride/potassium iodide on artificial root caries lesions with adjunctive application of proanthocyanidin

https://doi.org/10.1016/j.actbio.2019.02.020Get rights and content

Abstract

Treatment of carious root surfaces remains challenging due to the complex pathological processes and difficulty in restoring the original structure of root dentine. Current treatments targeting the de-/re-mineralisation processes are not entirely satisfactory in terms of the protection of the dentinal organic matrix and the highly organised structure of dentine. In this in vitro study, a cross-linking agent – proanthocyanidin (PA) was used in conjunction with a fluoride-based treatment – silver diamine fluoride/potassium iodide (SDF/KI) to putatively stabilise the organic dentinal framework as well as strengthen the collagen-mineral phase interaction. The effectiveness of this strategy was evaluated 24 h after application in terms of the distribution of ion uptake and microstructure of dentine after treatment as well as analysis of the nano-mechanical properties using a dynamic behaviour model. Results showed that individual use of SDF/KI significantly improved the surface microhardness and integrated mineral density (Z) up to 60 µm depth and the recovery of creep behaviour of demineralised dentine in the surface area compared to that treated with deionised distilled water (DDW). The combined treatment of PA and SDF/KI achieved a more homogenous mineral distribution throughout the lesions than SDF/KI alone; a more significant incremental increase in surface microhardness and Z was observed. Specifically, a superior effect on the subsurface area occurred with PA + SDF/KI, with significant improvements in microhardness, elastic modulus and recovery of creep behaviour of the demineralised dentine. Application of SDF/KI induced small discrete crystal formation distributed over the dentine surface and PA contributed to the formation of slit-shaped orifices of the dentinal tubules that were partially occluded.

Statement of Significance

Demographic transitions and improved oral health behaviour have resulted in increased tooth retention in elderly people. As a consequence, the risk of root dentine caries is increasing due to the age-associated gingival recession and the related frequent exposure of cervical root dentine. Root caries is difficult to repair because of the complex aetiology and dentine structure. The recovery of dentine quality depends not only on reincorporation of minerals but also an intact dentinal organic matrix and the organic-inorganic interfacial structure, which contribute to the biomechanics of dentine. With the capability of dentine modification, cross-linking agents were applied with a fluoride regimen, which improved its treatment efficacy of root caries regarding the distribution of ion uptake and recovery of dentine biomechanics.

Introduction

With the change in demography of society and improvement in oral health behaviour, the retention of natural teeth is progressively increasing in developed countries [1]. As a result, root caries, the major cause of oral pain in the elderly, is expected to increase exponentially due to more teeth and therefore cervical root surfaces being present, caused by age-related gingival recession [2], [3], [4], [5]. Treatment of carious root surfaces can be more challenging than those of coronal surfaces due to the complex pathological processes and the difficulty in restoring the root structure, composed of the complex protein assemblies and organised mineral components of root dentine.

Dentine is an hierarchically bio-composite material under multiple template-mediation. The basic building block of dentine is highly mineralised collagen fibrils (∼50–100 nm diameter), where the tablet-shaped dahllite crystallites (∼5 nm thick, 70% by weight) are embedded within the gaps of the fibrils with an orientation preferentially aligned along the long axis of the fibril. Proportionately massive periodic mineralised collagen fibrils are densely staggered and overlapped and form covalent intra- and inter-molecular cross-links [6], [7], [8], [9]. The nanoscale interfacial interaction between collagen fibrils and the embedded mineral phase enables load transference between the organic and inorganic components, which endows on dentine a capacity of load bearing during mastication and recovery after deformation [10], [11]. In a typical root caries process, firstly, exposure to biofilm-derived acid causes mineral to dissolve, leading to the exposure of cleavage sites of collagen fibres and activation of host-derived collagenases (e.g. matrix metalloproteinases (MMPs) and cathepsins). The subsequent collagen proteolysis results in the breakage of high conformational cross-linkages, which contributes to further cleavage events by gelatinases/peptidases and accelerates subsequent mineral loss [12]. Consequently, demineralisation, together with structural damage of the collagen framework, causes localised destruction of dentine, which therefore impairs the biomechanics and biochemistry of the original dentine [13], [14].

The efficacy of currently recommended treatments, e.g. 5 000 ppm fluoride toothpastes, 22 600 ppm fluoride varnish and 38% silver diamine fluoride solution (44 800 ppm fluoride), highlights the efficacy of high concentration fluoride in managing root caries [3], [4], [15]. The cariostatic mechanism of fluoride mainly relies on the inhibition of demineralisation and enhancement of remineralisation [16]. Direct effects of fluoride on preventing breakage of the collagen framework and de-activation of endogenous proteases have not been demonstrated [17]. Thus, an incremental increase of mineral density alone cannot be regarded as an appropriate endpoint for assessing the effectiveness of the treatments with regard to improving dentine quality, which depends upon the sum total of the tissue characteristics including ultrastructure (especially the particular location of the mineral within the organic matrix of the tissue), mineral density and biomechanical characteristics [18], [19]. Failure to protect the organic structure of root dentine may compromise its biomechanical properties [20].

Given the pathological process of root caries, reducing enzyme activity and biomodification of dentine matrices are likely to protect the conformational dentinal organic matrix from degradation in root caries. A series of laboratory studies have reported the effects of MMP inhibitors in reducing the activity of MMPs in partially demineralised human dentine [21], [22], [23], [24], [25], whilst two studies have a direct observation on topical application of two MMP inhibitors, i.e. non-antimicrobial chemically modified tetracyclines and zoledronate (a bisphosphate with MMP-inhibiting activity) in preventing carious lesion progression in rats [26], [27]. However, their inhibitory effects lack selectivity to certain enzymes and the effects may be limited to a certain time period. Thus, biomodification of dentine seems to be a promising approach which enhances the biomechanics and bio-stability of the dentine matrix. Amongst diverse biomodification strategies, application of cross-linking agents is of particular interest. The cross-linkers could induce multiple inter- and intra-molecular cross-links by interacting with various extracellular components of the dentine matrix [28]. As a result, the cross-linked organic matrix may modify or hide the regions in collagen fibrils vulnerable to collagenases and maintain the mineral phase in the gaps of collagen fibrils from further dissolution, thus improving the biomechanical and biochemical properties of the tissue [29].

Proanthocyanidin (PA), a plant-derived cross-linking agent containing polyphenolic compounds, has been investigated extensively for its potential use in reparative caries therapies [30]. It not only interacts with proteins via covalent, ionic, hydrogen bonding or hydrophobic interaction, but also shows direct effects on non-collagenous protein such as proteoglycans [31] and inhibits the activity of endogenous proteases [32], [33]. Dentinal collagen treated with PA showed a significant improvement in mechanical and biological properties [34], [35]. Therefore, PA has the potential to stabilise the partially demineralised organic matrix and maintain the mineral phase in the stabilised collagen framework in its original arrangements, which might be beneficial for ion uptake and reaction as well as the collagen-mineral phase interaction, thus leading to a higher mineral density and improved biomechanics.

In this study, SDF/KI was investigated as a regimen for root caries treatment, and PA was applied as a bio-mediator to protect the dentine organic matrix. An artificial root caries model was used to ensure the consistency of the results. The treated root dentine specimens were analysed regarding their mechanical, chemical and structural properties. Especially, nano-indentation was used extensively to analyse the dynamic mechanical properties of dentine at a nano-scale. The viscoelastic behaviour of the treated dentine was systematically explored using a dynamic behaviour model. The microstructure and mineral uptake and distribution of SDF/KI with or without PA were also observed.

Section snippets

Materials

Proanthocyanidin was obtained from the International Laboratory of USA (>95% oligomeric proanthocyanidins) and SDF/KI (Riva Star®; consisting of 30%-35% silver fluoride, >60% ammonia solution and a saturated solution of potassium iodide) from SDI Limited (Bayswater, Australia). The demineralising solution was prepared using 50 mmol/L acetic acid, 1.5 mmol/L CaCl2 and 0.9 mmol/L KH2PO4 and adjusted to pH 5 with KOH. All the chemicals were purchased from local chemical suppliers.

Preparation of root dentine specimens

Non-carious human

Surface and cross-sectional microhardness test

The surface and cross-sectional microhardness measurements of the demineralised (artificial caries) lesions from different treatment groups are presented in Table 2. Treatment with PA showed slightly lower surface hardness values (40.40 ± 5.78 MPa) than the control group (45.11 ± 3.02 MPa) but was not statistically significant (P = 0.519). Treatment of SDF/KI with or without the application of PA both significantly improved the sample surface hardness compared to the no treatment group

Discussion

The present study evaluated the potential effects of the combined use of PA as a cross-linking agent with a high concentration fluoride agent (SDF/KI) in the treatment of root caries in a laboratory-based study. The results indicated SDF/KI induced small discrete crystal formation that was distributed over the dentine surface and associated with an improvement in the mechanical properties (microhardness, elastic modulus and creep behaviour) of demineralised root dentine in the surface area 24 h

Conclusion

Within the limitations of this study, it was concluded that application of PA on artificial root carious lesions contributed to partial occlusion of dentinal tubules and it did not affect the mechanical properties and mineral density of the dentine (P > 0.05). Treatment of artificial root caries with SDF/KI induced small discrete crystal formation distributed over the dentine surface and the ion uptake and distribution from SDF/KI was up to 60 µm. SDF/KI application resulted in recovery of its

Acknowledgements

This work was supported by Australian Dental Research Foundation (ADRF) Nathan Cochrane Memorial Grant. J.C. was supported by the China Scholarship Council - University of Melbourne PhD Scholarship. The authors would like to thank staffs in Showa University for their help in the nanoindentation testing.

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