Influence of povidone-iodine on micro-tensile bonding strength to dentin under simulated pulpal pressure

A current area of interest in adhesive dentistry is the durability of resin restorations [1]. The generation of resin-dentin bonds entails various challenges, including the preservation of structural integrity and strength. The bonds between resin and dentin using dentin adhesive systems can become damaged over time [2]. Aged composite resin bonded to dentin reportedly exhibited hydrolytic degeneration of collagen [1] without any bacterial enzymes [2].

Matrix metalloproteinases (MMPs) are part of the composition of dentin structure [3]. They are a group of proteases that can destroy the organic matrix of etched dentin [4]. All MMP family members are secreted as inactive proenzymes (Pro-MMPs). Pro-MMPs can be activated by proteases, other members of the MMP family, acids, reactive oxygen, and denaturants [2]. These Pro-MMPs need calcium ion (Ca²⁺) for their activation. Following bacterial demineralization, Ca²⁺ ions are released and act as a cofactor for the Pro-MMPs activation. These MMPs lead to dentin destruction. Similarly, following acid etching demineralization, MMPs are released and activated [2].

Establishing adhesion to mineralized tissue is based on the reaction of biologic apatite with acids. Etching dentin with 37% phosphoric acid lead to hydroxyapatite crystals dissolution, demineralization of the surface of the dentin matrix, exposure of the underlying collagen and creates porosities within that collagen matrix. A demineralized microporous area composed of organic dentin material mainly collagen fibrils is formed. This allows solvated monomers to infiltrate around and into spaces of collagen fibrils to obtain retention for resin-composite materials. However, acid etchings release the Ca²⁺ ions and activate MMPs. These MMPs lead to collagen destruction leading to enzymatic degradation of hybrid layer and subsequently endanger resin-dentin bonding longevity [4]. In order to prevent enzymatic degradation, inhibitors of MMP activity have been applied during adhesive application [5]. One such enzymatic inhibitor is chlorhexidine [2].

Povidone-iodine (PVP-I) was introduced as an antiseptic agent in the 1950s, and is as effective as iodine alone against a broad spectrum of disease-causing microorganisms [6, 7]. It is an organic water-soluble complex that contains molecular iodine and the solubilizing agent polyvinyl pyrrolidone. It is less irritating to the skin than iodine, and unlike iodine it does not require iodides or alcohol to dissolve. Moreover, PVP-I stains are water-soluble.

It has been claimed that PVP-I can induce an endogenous proteinase inhibitor capable of blocking enzymatic activity [8, 9]. Reductions in the collagenolytic activity of MMP-9 by 83% and MMP-2 by 88% were reported after 24 h of PVP-I application on nitrogen and sulfur mustard-induced skin lesions. When skin was analyzed 48 and 72 h after exposure, a similar trend of PVP-I-induced reduction in the two types of collagenase activity was found [8].

The hydrolytic and enzymatic stability of dental adhesives in the oral environment is a concern because the oral environment can severely compromise the durability of resin-dentin bonds. A literature review was performed prior to the current study, to investigate the effects of using chlorhexidine as a MMP inhibitor on dentin substrates, and the influence of chlorhexidine application on the preservation of dentin bond durability. Previous studies have reported that bond strength deteriorated over time following dentin surface pretreatment with chlorhexidine [10, 11]. This suggested that further investigations were needed to evaluate the effects of other materials such as PVP-I on the durability of dentin micro-tensile bond strength (MTBS).

The aim of this study was to evaluate the effects of 10% PVP-I pretreatment on dentin MTBS, and compare it with 2% chlorhexidine pretreatment and no pretreatment in permanent teeth over a period of 2 months.

In the present study, only freshly extracted, non-carious, permanent molars were utilized in an attempt to standardize the dentin substrates used. Intact resin-bonded teeth were stored in distilled water under simulated pulpal pressure. The water exposure of intact resin-bonded teeth may resemble a more realistic clinical situation in terms of hydrolytic degradation than smaller resin-dentin specimens directly exposed to water. Simulated pulpal pressure was used in this in vitro study in an attempt to achieve reliable results that were relevant to real clinical conditions. Simulated pulp pressures ranging from 30 to 37 cm H₂O have been used in many studies investigating the influence of intra-pulpal pressure on the bonding effectiveness of adhesives to dentin [13–15]. In the current study, a lower intra-pulpal pressure was utilized because the intra-pulpal pressure in normal pulp is not as high as previously established [16]. Previous in vivo studies have reported that values of approximately 15 cm H₂O should be used to simulate normal pulp pressure [16, 17].

In the current study, conventional two step etch-and-rinse adhesive was used (Single Bond, 3 M ESPE, St. Paul, MN, USA) which has a pH of 3.6 [18]. Previous studies have suggested that low pH (4.5) acids are capable of activating MMPs [4, 19]. Therefore, it was hypothesized that this adhesive would be capable of activating dentin proteolytic enzymes derived from the underlying partially-demineralized dentin [20].

In the current study, we used 10% PVP-I and 2% chlorhexidine as therapeutic primers of etched dentin. The PVP-I and chlorhexidine were applied to etched dentin before bonding, and the excess was air-dried without rinsing. The technique used in this experiment was a wet bonding technique. PVP-I and chlorhexidine digluconate are soluble in water. The differences in the binding performances of PVP-I and chlorhexidine to collagen and hydroxyl apatite are unknown.

Chlorhexidine has a high affinity to dentin. It can bind electrostatically to the phosphate groups of dentin and to carboxyl groups of collagen fibers. However, chlorhexidine composed of a large water-soluble molecule that might be leaded out of dentin over time [21]. Moreover, the binding mechanism between the PVP-I and the dentinal structure is not clear. However, the application of a miscible (capable of mixing in any ratio without separation of two phases) solution to water-saturated dentin after etching and rinsing should maximize chlorhexidine concentration within the hybrid layer [21]. Without rinsing, excess PVP-I and chlorhexidine may be incorporated into the primer, and released slowly over time [21]. Previous studies have reported that PVP-I has the capacity to be slowly released over time [22–24].

In the present study, the PVP-I group showed no significant reduction in dentin bond strength at the 24 h, 1 week, or 2 months time-points. In contrast, the chlorhexidine and control groups showed significant reductions in dentin bond strength at 1 week and at 2 months. The differences in mean dentin bond strength in the chlorhexidine and control groups between 24 h and 2 months were approximately 6 MPa and 11 MPa, respectively, while the corresponding reduction in the PVP-I group was only ~ 1 MPa. This finding may be explained by the superior water solubility of PVP-I compared to chlorhexidine, and the capacity of PVP-I to slowly release iodine over time [25], which ensures the establishment of a nontoxic, optimal concentration of iodine [26, 27]. This may have resulted in better penetration of PVP-I and inhibition of the MMPs within dentin.

In accordance with the results of previous studies [28–32], in the current study chlorhexidine application after acid etching had no effect on immediate or 1-week resin-dentin bond strength. This is concordant with previous in vitro [33] and in vivo [34] studies using an etch-and-rinse adhesive. Carrilho et al. [33] [34] found that treating etched dentin surfaces of permanent teeth with 2% chlorhexidine did not affect the in vitro or in vivo MTBS of specimens tested at 24 h. Furthermore, a meta-analysis of the effects of 2% chlorhexidine vs. control at baseline (immediate bond strength) revealed no statistically significant difference between groups [35].

In the current study, all the teeth were subjected to pulpal pressure simulation. Campos et al. [36] studied the effects of 0.2% and 2.0% chlorhexidine on dentin bonding durability. Two-step etch-and-rinse (Single-Bond) and all-in-one self-etch adhesive (Clearfil Tri S Bond) were used, and all the teeth were subjected to 30 cm H₂O pulpal pressure and thermo-mechanical stressing. They reported that MTBS was significantly higher in the groups treated with two-step etch-and-rinse adhesive associated with 0.2% and 2.0% chlorhexidine than it was in the control group without chlorhexidine. Additionally, there were no significant differences in MTBS between the group treated with Clearfil Tri S Bond, the control group, and the group treated with 0.2% chlorhexidine after 6 months. Chlorhexidine was reportedly able to reduce the loss of bond strength of single bond adhesive, but not all-in-one self-etch adhesive, after storage for 6 months under simulated pulpal pressure. Those results are concordant with the results of the current study.

The current study had some limitations. The etch-and-rinse system was used in the current study to bond to sound dentin, and thus it may be that the hybrid layer in the current study was infiltrated by the water [37] utilized to exert the pulpal pressure on the dentin surface. However, the presence of an enamel rim sealing the boundaries of the specimens prevented the leaching out of any plasticized adhesive monomer into the storage media [36], and thus diminished the effect of pulpal pressure on the observed bond strength. Additionally, being an in vitro study the results do not directly reflect the clinical conditions of actual teeth, however, simulated pulp pressure was used during bonding and ageing to simulate in vivo conditions. Further clinical studies are needed to confirm the results of the current study.

To the best of our knowledge, this is the first report of data on the effects of PVP-I as a MMP inhibitor on the preservation of dentin bond durability. The results showed that PVP-I, an experimental therapeutic primer, also prevented bond strength deterioration over 2 months of aging. Further studies are needed to explore the reasons behind the preservation of the bond strength. Additionally, long-term studies are needed to evaluate the effects of PVP-I on bond strength. Further studies are also needed to study the inhibitory effects of PVP-I on MMPs directly.

Povidone iodine pretreatment of etched dentin was effective in reducing the loss of bond strength over time, while the chlorhexidine pretreatment and negative control showed significant deterioration in micro-tensile bond strength over time in permanent teeth.