Effect of in vitro chewing and bruxism events on remineralization, at the resin–dentin interface

Abstract

The purpose of this study was to evaluate if different in vitro functional and parafunctional habits promote mineralization at the resin–dentin interface after bonding with three different adhesive approaches. Dentin surfaces were subjected to distinct treatments: demineralization by (1) 37% phosphoric acid (PA) followed by application of an etch-and-rinse dentin adhesive, Single Bond (SB) (PA+SB); (2) 0.5 M ethylenediaminetetraacetic acid (EDTA) followed by SB (EDTA+SB); (3) application of a self-etch dentin adhesive, Clearfil SE Bond (SEB). Different loading waveforms were applied: No cycling (I), cycled in sine (II) or square (III) waves, sustained loading hold for 24 h (IV) or sustained loading hold for 72 h (V). Remineralization at the bonded interfaces was assessed by AFM imaging/nano-indentation, Raman spectroscopy and Masson’s trichrome staining. In general, in vitro chewing and parafunctional habits, promoted an increase of nano-mechanical properties at the resin–dentin interface. Raman spectroscopy through cluster analysis demonstrated an augmentation of the mineral–matrix ratio in loaded specimens. Trichrome staining reflected a narrow demineralized dentin matrix after loading in all groups except in PA+SB and EDTA+SB samples after sustained loading hold for 72 h, which exhibited a strong degree of mineralization. In vitro mechanical loading, produced during chewing and bruxism (square or hold 24 and 72 h waveforms), induced remineralization at the resin–dentin bonded interface.

Introduction

Tooth contact is not a dominant activity over a 24-h cycle. It has been estimated that tooth contact occurs for approximately 17.5 min over a 24-h period; sleep bruxism (Toledano et al., 2014a) related with muscle activity lasts approximately 8 min over a complete sleep period that usually remains between 7 and 9 h (Lavigne et al., 2008, Okeson et al., 1990). The prevalence of bruxism is reported to be 6–20% among the adult population, 20% for the clenching type and it occurs predominantly among females (Lavigne et al., 1996, Lavigne et al., 2008, De Laat and Macaluso, 2002). Most theories adhere to multifactorial etiology and discriminate between peripheral factors (anatomy, dental occlusion, receptor input), central (central nervous system), and psychological factors (De Laat and Macaluso, 2002). Bruxism is commonly considered to be the main contributor to dental attrition, periodontal disease and temporomandibular joint disorders (Nishigawa et al., 2001). Duration of the chewing cycle ranges from 0.7 to 2 s, with a contact time between 0.2 and 0.3 s. During clenching, the occlusal force was observed to be as high as 520–800 N. The duration of bruxim episodes varies from 2 to 375 s (Jantarat et al., 2001, Abbink et al., 1999). Chewing and occlusal trauma can affect restorative strategies involving dentin.

Two main strategies are used to create dentin bonding: (a) etch-and-rinse adhesives which require previous acid etching of the dentin surface, or (b) self-etching adhesives, based on the use of polymerizable acidic monomers that simultaneously condition/prime the dentin. With both strategies, a resin–dentin inter-diffusion zone or hybrid layer (HL) (Nakabayashi et al., 1982) is created, but a volume of demineralized/unprotected collagen remains at the bottom of the hybrid layer (BHL). This unprotected collagen may become the sites for collagen hydrolysis by host-derived matrix metalloproteinases (MMPs) enzymes (Toledano et al., 2012).

It has been stated that dentin is a relatively inert tissue and that mastication or parafunctional habits have limited influence in tissue response. Occlusal loads have been pointed out to be accountable to some degree of irreversible mechanical disruption of the poorly infiltrated collagen fibrils (Tjäderhane et al., 2013). When chewing or bruxing, dental materials used in restorative dentistry not only should be resistant to wear and breakage, but promoters of the protection of resin–dentin interfaces, triggering the bioactive nature of dentin matrix, by releasing bound bioactive molecules and growth factors (Smith et al., 2012). The precise interaction of dentin matrix with these signaling molecules deserves more research.

Information on biomechanical properties of the demineralized dentin and their influence on remineralization are lacking (Xu et al., 2011, Brauer et al., 2011). AFM nano-indentation is the most commonly applied means of testing the mechanical properties of materials or substrates (Poon et al., 2008), and it was deemed to be a suitable method for the determination of the visco-elasticity of the demineralized dentin and its effective remineralization (Balooch et al., 2008, Bar-On and Wagner, 2012). Various modes of micro-Raman multivariate spectra of principal components (PCs) (Almahdy et al., 2012) have been established for analyzing three dimensional data. Finally, Masson’s trichrome stained sections of dentin interfaces may show or not an exposed and unenveloped-stained collagen network at the BHL, confirming any demineralization at this site of the resin–dentin inter-diffusion zone, indicating or not signs of interface degradation (Sauro et al., 2012).

In this study we attempted to clarify the experimental morpho-functional influence of chewing and bruxism events on the resin–dentin interface. The purpose of this study was to evaluate the ability of different in vitro mechanical loading stimuli, as chewing and bruxism, to induce remineralization at the bonded dentin interface created by using different demineralization procedures and two dentin adhesive systems (etch-and-rinse vs self-etch). The tested null hypothesis is that remineralization is not produced at the resin–dentin interface after the application of some in vitro mechanical loading stimuli, reproducing parafunctional habits.