The Effect of a Glass Ceramic Insert in Sandwich Technique on Microleakage in Class II Composite Resin Restorations

Introduction

Despite the improvement in the physical properties of composite resins, polymerization shrinkage still remains as a problem in direct restorations, which may affect marginal adaptation (1, 2). Shrinkage is a volume-dependent phenomenon (3). In an attempt to avoid problems, the use of prefabricated ceramic inserts with composite resins has been advocated for the esthetic and cost effective restoration of medium-sized class I and II cavities (4, 5).

Inserts are industrially produced using ceramic of various shapes, sizes, and colors. The insert technique is a dental restorative approach between direct composite restorations and the highly sophisticated, indirect ceramic inlay restoration. The combined advantages create a simple and cost-effective direct method for the esthetic restorations (6).

Besides the lower composite volume, ceramic inserts spreads the occlusal mastication forces over a larger surface. It also reduces peaks, and thus reduces the failure rate of direct composite restorations in the posterior region (7). Kunzelmann et al. (7) demonstrated using of finite element analysis that optimized inserts reduce the failure of posterior composite restorations caused by fatigue. They also showed that an anisotropic E-modulus of the insert minimizes the stress below the insert and its interface. Bowen (8) reported a 42% lesser formation of marginal gaps due to polymerization shrinkage with ceramic inserts with composite resin restorations.

Another approach to overcome shrinkage induced microleakage focuses on using a liner under posterior composite restoration. Some studies suggest the application of a flowable composite or compomers between the floor of the box and the restorative material as a sandwich technique (9, 10).

Flowable composite may exhibit a stress-reduction-by-flow property (9, 11). The use of compomer material is considered to simplify  composite restoration as the same bonding system can be used for the compomer and composite material (12).

Studies pertaining the effect liner on microleackage have given controversial results. (13-17) The purposes of the present in vitro study were:

1- To evaluate the effect of glass ceramic insert and two different sandwich techniques using compomer or flowable composite in gingival microleakage  below the CEJ in class II composite resin restorations.

2- To search if there is any difference in local microleakage between the lateral and medial areas of gingival floor.

Materials and Methods

Sixty freshly extracted sound human upper second premolars with similar size, stored in 0.2% sodium azide solution, were divided into six groups of 10 according to the type of lining material and the restoration technique (with or without the insert).  Teeth were vertically embedded 2 mm below the CEJ in a cylindrical auto-polymerizing acrylic resin (Bosworth, Neocryl, USA).  A box-only cavity was prepared in distal aspect of each tooth. The dimensions of the cavities were 4 mm buccolingual in width and 2 mm in depth at axiopulpal line angle. The gingival margin was located 0.5 mm below the CEJ with nearly 1.5 mm in depth. All preparations were performed with diamond fissure burs (# 878/d2, Teeskavan, Iran) in a high-speed handpiece with water spray. The burs were changed after every five cavity preparations. After cavity preparation, a Tofflemire matrix retainer and soft metal band were used. Then etchant (Ultra Etch, Ultradent, USA) and dentin bonding agent (Excite, Vivadent, Liechtenstein) was applied to the cavity walls according to the manufacturer's instructions. Each group was restored as described in table 1 and the text follows:

Group A (Control):

The cavity was restored with light cured composite resin (Tetric Ceram, Vivadent, Liechtenstein) using gingivocclusal incremental technique. While the first increment was 1 mm thick, the others were placed parallel to the gingival floor up to 2 mm in thickness. Each increment was light cured from occlusal aspect for 40 seconds by QTH light curing unit (UltraLux, DabiAtlante, Ribeirao Preto, SP, Brazil; light irradiance = 450 mW/cm2). After the removal of the metal matrix, the restorations received further light curing from the buccal and lingual aspects, each for 40 seconds. The accuracy of light cure unit was monitored with a radiometer after each five restorations.

Group B:

A thin layer of a compomer (Compoglass F, Vivadent, Liechtenstein) with 1 mm thickness was placed, and light cured from the occlusal aspect for 40 seconds. The rest of the cavity was restored with a hybrid composite (Tetric Ceram) in the same way as group A.

Group C:

A thin layer of a flowable resin composite (Tetric Flow, Vivadent, Liechtenstein) with 1 mm thickness was placed, and light cured from the occlusal aspect for 40 seconds. The rest of the cavity was restored with a hybrid composite (Tetric Ceram) the same as group A.

Group D:

The entire cavity was restored with light cured composite resin (Tetric Ceram) using bulk technique. Right, after the composite placement over the cured bonding agent, a medium sized glass ceramic insert (SonicSys, Vivadent, Liechtenstein) was pressed into the composite at the middle of the restoration until firmly seated.  After seating, the excess composite was removed and the remaining composite was adapted around the insert and against the cavity walls. Light cure was applied on the insert-composite restoration from occlusal aspect for 40 seconds. After the removal of the metal matrix, the restorations received further light curing from the buccal and lingual aspects, each for 40 seconds.

Group E:

A layer of a compomer (Compoglass F) with 1 mm thickness was placed on the gingival floor and the rest of the cavity was filled with hybrid composite (Tetric Ceram). The glass insert (SonicSys) was then pressed into the composite and the following steps were the same as group D.

Group F:

A layer of a flowable composite (Tetric Flow) with 1 mm thickness was placed on the gingival floor and the rest of the cavity was filled with hybrid composite (Tetric Ceram). After composite placement, the glass insert (SonicSys) was pressed into the composite and the following steps were the same as group D.

After 24 hours storing in 37°C and 100% humidity, all restorations were finished (Diamond finishing burs, Diamant, D&Z, Germany) and polished (Soft-lex polishing disks, 3M, USA). Then thermocycling was conducted at 5 and 55°C (± 4°C) bathes for 1,000 cycles. The immersion time for each bath and the interval between them was 30 seconds. Then load cycling was done with a load of 14 N, duration of 0.2 seconds and frequency of 3 Hz for 250,000 cycles. The teeth were then coated with two layers of nail varnish within 1 mm of the margins of the restorations and soaked in a solution of 0.5 % basic fuschin dye at 37°C incubators for 24 hours. The roots were cut off and the crowns were embedded in self- curing epoxy resin (Araldit CY219, Hardener HY5160, Ciba-Geigy, Switzerland). Two cuts were made in a mesio-distal direction 0.5 mm medially from the buccal and lingual margins of the restorations with a water cooled 1.0 mm thick diamond disc (Leitz 1600, Wetzlar, Germany). Therefore, two cuts per tooth provided three sections with four surfaces for gingival microleakage evaluation (Fig. 1).

Dye penetration was evaluated using a stereo-microscope (Blue Light Industry, USA) at x40 magnification. For each surface, the dye penetration at the gingival margin toward and along the gingival and axial walls was recorded separately in a tenth of a millimeter (10^-1 mm). (Fig. 2)  For statistical analysis, the data was categorized according to 4 scores (Table 2). Finally, data were analyzed with Kruskal-Wallis, Mann-Whitney and Friedman tests at α=0.05.

Table 1. Experimental groups, material and the restorative procedures.

Procedure Codes:

a: Enamel etched with 35% phosphoric acid for 30 seconds and dentinal surfaces were simultaneously etched with 35% phosphoric acid for 15 seconds, rinsed for 20 seconds, then air dried gently to keep dentin slightly moist.

b: Dentin bonding agent was applied according to manufacturer's instructions.

c: liner material was applied onto the cavity floor and was light cured according to manufacturer's instructions.

d: Composite resin was incrementally placed into the cavities, and each layer was light cured for 40 seconds.

e: liner material was applied onto the cavity floor

f: Composite resin was bulk placed into cavities

g: A glass ceramic insert, was pressed into the composite at the center of the restoration, and was light cured for 40 seconds

Table 2. Scoring the amount of microleakage

Figure 1. Schematic illustration of two sectioning cuts made four cut surfaces (1, 2, 3 and 4).

Figure 2. Dye penetration in one sample of group B (Compoglass F) at cut surface 2

Results

The results of dye penetration of each group are summarized in Table 3. All testing groups showed some degree of microleakage (Fig. 3). Group D (no liner/ with glass insert) showed the highest amount of microleakage (13.54) and Group A (no liner/ without glass insert) resulted in the lowest amount of total microleakage (6.07).

Insert placement significantly increased microleakage in all cut surfaces. In groups without insert (group A, B, C), there was no significant difference related to the kind of liner.  

Group D which had no liner showed significantly more microleakage than the other groups with insert (group D, E, F) and there was a significant difference between groups D and E and no difference between groups E and F.

Friedman test were performed to detect any existing pattern of microleakage in gingival floor of different cut surfaces in each group. There was no significant difference in local microleakage between four cut surfaces of the restorations in each group (P>0.05). (Fig. 3, 4)

Fig 3. Mean Rank microleakage of each group in different cut surfaces

Fig 4. Mean rank microleakage of each cut surface in different groups

Table 3. Mean Rank of microleakage for testing groups

* Groups with different letters indicate statistical difference at Alpha = 0.05.