Subsurface degradation of resin-based composites

Abstract

Objectives

To determine the depth of a degraded subsurface layer produced in dental composites as a result of exposure to lactic acid or NaOH, by observing the penetration of AgNO₃ solution.

Methods

Specimens were prepared from four resin composites; Point 4 (Kerr), Premise (Kerr), Filtek Supreme (3M/ESPE), Ceram X (Dentsply), and two polyacid-modified resin composites; Dyract (Dentsply) and F2000 (3M/ESPE). The specimens were immersed in distilled water for 1 week, transferred to one of three aqueous media at 60 °C for 2 weeks; distilled water, 0.01 mol/L lactic acid or 0.1N NaOH, washed and immersed in 50% (w/w) aqueous silver nitrate for 10 days at 60 °C and placed in a photodeveloper solution. After reduction of the silver, specimens were embedded in epoxy resin, sectioned and polished, coated with carbon, and examined by backscattered mode scanning electron microscopy. The depth of silver penetration into the degraded area was measured from the SEM micrographs. Energy dispersive analysis X-ray (EDAX) was used to confirm the presence of silver.

Results

NaOH produced the greatest depth of degradation and lactic acid the least. Premise showed the greatest depth of silver penetration when subjected to NaOH, and Filtek Supreme the second with peeling of the surface and cracking, whereas F2000 and Point 4 showed the least in NaOH and lactic acid.

Significance

ANOVA and Tukey's test showed that the depth of silver penetration was material and solution dependent, and the differences were significant for most of the materials (P < 0.05).

Introduction

The widespread use of resin-based restorative materials and their exposure to the harsh conditions of the oral environment requires that they exhibit significant durability. One of the most important properties determining the durability of dental restorations is their resistance against biodegradation [1], by exposure to plaque acids, foods and enzymes that can cause material softening [2], [3]. It has been assumed that the one of the problems of composite in the oral environment is chemical degradation [4], [5], [6], [7], which may result in a reduction in physico-mechanical properties. To evaluate the subsurface degradation of resin composite by SEM requires staining of the degraded area [8], [9], and fine silver particles can be introduced into a resin composite in order to provide optical contrast between the damaged and undamaged areas.

Different mechanisms of polymer degradation have been demonstrated, such as hydrolytic [10], hydrothermal [11], chemical [2], [3], [4], [5] and chemo-mechanical [12], [13]. Different definitions for degradation have been given; however it has been described by Gopferich [14] as a ‘chain scission process during which polymer chains are cleaved to form oligomers and finally monomers’. Gopferich [14] stated that intrusion of water into the polymer bulk activates the chemical polymer degradation, which leads to the creation of oligomers and monomers.

The ability of resin-based composites to resist degradation against different pH levels for prolonged times has been studied [2], [4], [5], [12], [15], and the influence of polymer matrix, filler proportion, size, type, interparticle spacing, and degree of polymerization related to degradation of resin-based composites has also been examined [16], [17], [18], [19], [20]. A pH of 4.0 has been described [21] as the lowest pH found in dental plaque. Moreover, lactic acid has been cited as one of the main products of bacterial metabolism in human dental plaque in the oral environment [22], [23]. It has been shown that resin-based restorative materials undergo greater micro-morphological damage following a regimen of acid challenge, than after storage in either distilled water or artificial saliva [24].

Immersion in 0.1N sodium hydroxide solution (NaOH) at 60 °C was considered to be an appropriate method to quickly predict the chemical durability of composites [25]. It has been shown [13] that short-term alkaline treatment produces structural damage similar to that detected in restorations recovered from clinical service. Exposing resin-based materials to 0.1N NaOH at pH 13 resulted in hydrolytic degradation, because the hydroxyl (OH⁻) ion concentration is a million times more than in saliva, and can thus accelerate the hydrolytic process [13].

Dental composites based on nanofillers, and nanoceramic fillers have been recently introduced. Filtek Supreme (3M/ESPE) is the first nanofilled commercial product containing a unique combination of nanofillers (5–75 nm) and nanoclusters embedded in an organic polymer matrix. Premis (Kerr), resin composite consists of a “trimodal filler system”; prepolymerized fillers (30–50 μm), barium glass (0.4 μm) and silica nanoparticles (0.02 μm). Ceram X (Dentsply) comprises “organically modified ceramic” nano-particles (∼2.3 μm) with conventional glass fillers of ∼1 μm.

In view of the chemical degradation of resin-based restorative materials, research is required to evaluate the interaction of these materials with their surrounding environment. This laboratory study aims to assess the influence of acidic and alkaline storage media on resin-based restorative materials, including some recent materials, using a silver nitrate staining technique. The null hypothesis is that the acidic and alkaline media have no effect on resin-based restorative materials.