Adhesion of resin-modified glass-ionomer cements may affect the integrity of tooth structure in the open sandwich technique

Abstract

Objective

To study the interfaces between model cavities prepared in teeth and four glass ionomer cements (two conventional and two resin-modified).

Methods

Ten non-cavitated molars and premolars were used and, in each, two 3 mm deep slot preparations were created on opposing sides of the tooth. The teeth were conditioned as appropriate, then restored using the open sandwich technique, using a conventional glass ionomer (Fuji IX, Ketac Molar) or resin modified glass ionomer (Fuji II LC or N100), followed by completion with composite resin. The teeth were then embedded in a transparent acrylic resin and cut parallel to the long axis through both restorations, using a low speed diamond wheel saw. Samples were evaluated using a metallographic light microscope (100×). Three areas were assessed: the axial wall, the axial gingival line angle and the cavo-surface line angle. Bonding was categorized as inadequate or adequate based on the appearance and inadequate bonding was further studied and classified. Data were analysed statistically using the McNamara analysis.

Results

The majority of materials failed to make adequate contact with the axial wall, and there were also flaws at the axial/gingival line angle in several samples. By contrast, the cavo-surface line angle was generally soundly filled and the materials showed intimate contact with the tooth surface in this region. The most serious inadequacy, though, was not lack of intimate contact and/or adhesive bond, but the presence of perpendicular cracks in 30% of the Fuji II LC samples which extended into the underlying dentin.

Significance

The problems of placement and dentin cracking experienced with these materials demonstrate that adhesive bond strength alone cannot be used as the criterion of success for restorative materials. In fact good adhesion can, in certain cases, promote cracking of the dentin due to stresses within the material, an outcome which is undesirable.

Introduction

Glass ionomer (GI) cements were first reported by Wilson and Kent in 1972 and have since become widely used in clinical dentistry . They have many desirable properties, in particular the ability to form satisfactory adhesive bonds with enamel and dentin , and to release of fluoride in a sustained way over a prolonged periods of time . As far as bonding is concerned, they have the additional advantages that no extra preparation of the tooth structure is required for consistent adhesion , and that the bond becomes more durable with time, due to the formation of an ion-exchange layer between the tooth and the cement .

Despite the favorable qualities of conventional glass ionomers, they have some drawbacks. In particular, the slow setting reaction causes the material to be hydrolytically unstable in the early stages of hardening , making it sensitive to early water uptake or loss, depending on circumstances. This may affect the mechanical properties of the material and certainly affects the appearance, which may become chalky due to the formation of micro-cracks in the surface . To overcome these problems, the resin-modified glass ionomers (RMGICs) have been developed . These combine the components of conventional glass ionomers with the monomer 2-hydroxyethyl methacrylate (HEMA), together with appropriate polymerization initiators, so that the material sets by a combination of acid–base reaction and addition polymerization. The majority of brands of resin-modified glass ionomers are light-cured, which gives them the advantage that the clinician has considerable control over the setting reaction, much of which cannot occur until the dental curing lamp is switched on.

Resin-modified glass ionomers retain many of the advantages of conventional glass ionomers, in particular their adhesion to the tooth surface and their ability to release fluoride . However, the monomer HEMA is potentially damaging and its presence reduces the biocompatibility of these materials relative to conventional glass ionomers . Unpolymerized HEMA is known to be released by these materials and HEMA has been shown to be able to penetrate the dentin to reach the pulp , where it proves to be cytotoxic . HEMA monomer is also a well-known allergen .

Conventional and resin modified glass ionomer cements have been employed clinically in the so-called open sandwich technique for restoring Class II cavities . In this technique, glass ionomer cement, or RMGIC, is used for the gingival portion of the restoration, and composite resin is used to complete the repair . Clinical results with this technique are good and the use of a glass ionomer (of either type) has been found to significantly reduce the marginal micro-leakage at the cemento-enamel junction compared with the use of other materials, such as composite resin .

Glass-ionomers of both types have been tested for their adhesion to the tooth surface . Materials have been tested in shear bond mode and, more recently, in microtensile mode . At relatively short times, bond strengths have been found to be acceptable, even without surface pre-treatment . This leads to the suggestion that forming such adhesive bonds is straightforward, and the expectation that the resulting interfaces should be sound and free from flaws. This hypothesis was the original one under test in the current study.

Since the open sandwich technique was pioneered, new versions of glass ionomers, both conventional and resin-modified, have been launched by manufacturers. The current study was initially undertaken with a view to examining the performance in vitro of some of these newer materials in the open sandwich technique, with particular emphasis on their bonding and the interfacial region with the tooth. However, our work has led to some new observations, not previously reported, that raise important questions about the issue of adhesive bonding by glass-ionomers.

Materials and methods

Ten non-cavitated molars and premolars, extracted for orthodontic reasons, were used in this study. Teeth were stored prior to use in physiological saline to which small amounts of thymol had been added for 120 days at a temperature of 8 °C. Materials used in the study are listed in Table 1 , and consisted of two conventional and two resin-modified glass ionomers. One of the materials, N100, is described as “nanoionomer” because it contains nanometer sized filler. It is prepared and dispensed using a double tubed dispenser. Fuji II LC is a capsulated material, and was mixed using a vibratory mixer for 10 s prior to extrusion from the capsule.

Table 1
Glass ionomer cements used.
Brand Manufacturer Type
Fuji IX GC (Japan) Conventional
Fuji II LC GC (Japan) RMGIC
Ketac Molar 3M ESPE (USA & Germany) Conventional
N100 3M ESPE (USA & Germany) RMGIC (nano-ionomer)

In each of the ten teeth, two slot preparations were created on opposing sides of the tooth (see Fig. 1 ), using a fissure bur (ISO 806 314 107 524 012, Lot D-4362 at 300 000 RPM), diamond round bur (ISO 806 314 001 524 016, Lot D-5113 at 280 000 RPM) and a high speed rose bur (ISO 500 314 001 001 016, Lot D-434040 at 280 000 RPM). The depth of each preparation was 3 mm, (the gingival wall of the box is designed so it extends 1 mm below the cervical line of the tooth, into the root). Following cavity preparation, the teeth were stored in saline solution at a temperature of 8 °C for 7 days.

Fig. 1
Slot preparation on opposing sides of the tooth. A = axial wall, B = axial/gingival line angle, C = cavo-surface line angle.

The ten teeth were divided, at random, into two groups: Group A (prepared using the two 3M ESPE products) and Group B (the GC products). In each group, one slot in a particular tooth was filled with conventional glass ionomer cement (Ketac Molar or Fuji IX) and the other slot in the same tooth was filled with resin modified glass ionomer (N100 or Fuji II LC). Materials used are listed in Table 1 .

Having prepared the tooth, the material was placed in a thin layer along the axial wall and gingival floor of the cavity preparation, after which the filling of the cavity was completed by placement of composite material. In this way, four different sample groups were compared for their ability to bind to tooth structure.

The cavities designated for conventional glass ionomer samples were conditioned for 10 s prior to restoration with 20% polyacrylic acid, followed by a 10 s water rinse and drying with cotton pellets. The glass ionomer cements were mixed according to the manufacturer’s instructions, in a 1:1 powder to liquid ratio and condensed into the cavity to a depth of 1 mm on both axial and gingival walls. The cement was allowed to set for 5 min after which the cavity was etched, primed, bonded then restored with composite resin (Filtek Z250, ex 3M; shade A2).

The resin modified glass ionomer slot for Group A (N100) was primed with Ketac N100 primer for 15 s, air thinned for 10 s and light cured for 10 s. The resin modified glass ionomer was dispensed using the Ketac Clicker dispenser and mixed for 20 s, followed by placement into the cavity (using the dispenser provided by 3M), along axial and pulpal walls to a thickness of 1 mm, followed by light curing. For group B, the dentin was conditioned with 20% polyacrylic acid for 10 s, rinsed with water for 10 s and cotton dried. The resin-modified glass ionomer (Fuji II LC) was mixed on a glass slab and placed using a plugger, followed by light curing for 20 s. Lastly, the cavity was etched, prime and bonded and restored with composite resin as for Group A.

Following restoration, each group was stored in saline solution in a tightly sealed container for 1 week, at a temperature of 37 °C. The teeth were then embedded in a transparent acrylic resin (Duracryl) and were cut parallel to the long axis of the tooth, through both restorations, using a low speed diamond wheel saw (model 650). Each tooth yielded two slices, which were polished prior to microscopic evaluation. Samples were then evaluated using a metallographic light microscope (Nikon Eclipse LV 100) under a magnification of 100×. In each tooth, there were three areas of observation: the axial wall (A), the axial gingival line angle (B) and the cavo-surface line angle (C) (see Fig. 1 ).

In the initial assessment, bonding was categorized as inadequate or adequate based on the appearance of the region between the dentin and glass ionomer cement. Inadequate bonding was further studied, and details of the inadequacies recorded. Data were further analysed statistically using the McNamara analysis.

Materials and methods

Ten non-cavitated molars and premolars, extracted for orthodontic reasons, were used in this study. Teeth were stored prior to use in physiological saline to which small amounts of thymol had been added for 120 days at a temperature of 8 °C. Materials used in the study are listed in Table 1 , and consisted of two conventional and two resin-modified glass ionomers. One of the materials, N100, is described as “nanoionomer” because it contains nanometer sized filler. It is prepared and dispensed using a double tubed dispenser. Fuji II LC is a capsulated material, and was mixed using a vibratory mixer for 10 s prior to extrusion from the capsule.

Nov 25, 2017 | Posted by in Dental Materials | Comments Off on Adhesion of resin-modified glass-ionomer cements may affect the integrity of tooth structure in the open sandwich technique
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