Evaluation of Different Dentin Bonding Agents Accompanied with Composite Coronal Barrier

Introduction

Microleakage is a dynamic phenomenon defined as clinically undetectable penetration of fluid, bacteria, molecules and ions between the cavity wall and mounted restorative material )1). Microorganisms have been known as an important factor in the failure of root canal treatment. Enterococcus faecalis among the microorganisms is highly found in primary and secondary infections which results in the failure of the treatments (2). Coronal and apical microbial microleakage along the coronal restoration and root canal filling has been strongly associated with endodontic failure (3, 4). For this reason, endodontic practice and its success have been inextricably tied to the appropriateness and quality of the final restoration (5). On the other hand, the resistance to fracture of endodontically treated teeth was significantly increased when teeth were restored with composite resin in combination with dentin bonding systems (6, 7). Essentially the performance of adhesive systems on the enamel surface is appropriate (8), while the bonding to dentin is barely possible (9). Optimizing bond strength between resin and dentin and resin and root canal posts is of crucial importance. The inadequate bond strength of adhesive systems used for cementing root canal posts can lead to failure of the restoration and an increased possibility of subsequent microleakage and failure of the endodontic treatment (10). In endodontically treated teeth, the higher amount of composite material may increase polymerization shrinkage stress (11). Other problem is the root perforation during the canal preparation (12). The use of Mineral Trioxide Aggregate (MTA) as a canal filling material can reduce these two problems. MTA powder consists of fine hydrophilic particles. The principle compounds present in this material are tricalcium silicate, tricalcium aluminate, tricalcium oxide, and silicate oxide. In addition, small amounts of other mineral oxides exist that are responsible for the chemical and physical properties of this aggregate. The biocompatibility of gray MTA has been reported using cell culture techniques and connective tissue reactions (13). Sarkar et al. reported the propensity of MTA to release Ca and its ability to form hydroxyapatite and concluded that the sealing ability, biocompatibility and dentinogenic activity of MTA is attributed to these physicochemical reactions (14). Mineral trioxide aggregate (MTA) has been regarded as an ideal material for perforation repair, retrograde filling, pulp capping, and apexification since its introduction in 1993 (15, 16).The repair capacity of MTA can in turn be attributed to the antimicrobial properties and high pH (12.5) of MTA, these characteristic of MTA promote growth of the cementum and formation of bone (17, 18).

Korasli et al evaluated the coronal marginal leakage of endodontically treated teeth bonded with four self-etching adhesives and one total-etch adhesive system (19). None of the tested self-etch adhesives completely eliminated microleakage. Dye leakage was restricted to the coronal cavity walls it did not migrate toward the pulp chamber or toward the root canal. Single Bond and Clearfil SE Bond showed significantly lower dye penetration values at occlusal and gingival margins. Er et al compared the sealing ability of different dentin bonding adhesive systems in root-end cavities by the bacterial leakage method (20). They found that there were no statistically significant differences in rate of bacterial leakage among the experimental test materials at 1-4 weeks. Gonzalez-Castillo et al evaluate the capability of Cavit™ G to seal the pulp chamber, used alone, with Clearfil™ S3 Bond or Prime & Bond NT adhesive systems, at 24 hours, 7, 30 or 45 days (4). They concluded that the adhesive systems did not improve the capability of Cavit™ G to seal the pulp chamber over time.

Root perforation is one of the probable problems that may occur during the root canal therapy. Selection of a proper dentin bonding agent for composite filling of such teeth can overcome the leakage problems after restoration. Therefore, the purpose of this study was the evaluation of different dentin bonding agents accompanied with composite coronal barrier

Materials and Methods

A whole number of 45 noncarious human premolar extracted teeth with straight, single root canals were selected for this study. Soft tissues were mechanically removed from the root surfaces. The teeth were placed in a disinfectant (10% formalin) for 48 h in and then stored in normal saline at room temperature until ready to be used. The teeth were decoronated by diamond disk so that the remaining root portion were12 mm in length. The working length was measured by deducting 1 mm from the length recorded when tip of a #10 file (Dentsply/ Maillefer Instruments SA , Ballaigues, Switzerland) was visible at the apical foramen. All teeth were instrumented up to #40 K-file using step-back technique. Irrigation was carried out using 5 mL of a 5.25% NaOCl solution between files. The canals were then dried with sterile paper points. The roots were obturated with gutta-percha (Aria Dent, Tehran, Iran) and AH26 sealer (Dentsply, DeTrey, Konstanz, Germany) by using lateral condensation technique. Then the coronal part of the gutta-percha has been removed by pizorimer No III (Mani, Germany) so that 5mm of apical gutta percha remains within the root canal. Afterward, 3mm of roots were filled with MTA (Dentsply, Tulsa Dental, Tulsa, OK, USA) coronal to gutta-percha. The samples were placed in 100% humidity at 37°C for 24 h to allow the MTA to set.

The samples were divided randomly into three experimental (n=13) groups and two positive and negative control groups (n=3). Excite (2 step etch & rinse), Clearfil SE Bond (2 step self-etch), and iBond (1 step self-etch) adhesive systems were applied for experimental groups according to the manufacturers’ instructions (Table 1). After that, 2mm of canal above the MTA was filled with composite Filtek Z250 (3M ESPE, St. Paul, MN, USA) in two 1mm layers. Each layer has been polymerized for 20 second with a light-curing unit (Hilux LEDMax, Benlioğlu Dental, Ankara, Turkey) with a minimum power density of 400 mW/cm².

Table 1. Adhesive Systems, Batch Number, Composition, and Application Mode

Two coats of nail varnish were applied on the surface of the teeth in the experimental and positive groups, except 2 mm around the apical foramen and coronal surfaces. In the negative control, the surfaces of the specimens were completely covered by two layers of nail varnish. All groups were thermocycled for 500 cycles between 5 °C and 55 °C with a dwell time of 15 seconds.

Then, the roots mounted in plastic caps of glass tubes containing Brain Heart Infusion (BHI) broth. A hole was made through the centre of each cap and the tooth was placed into the hole to the cementoenamel junction. The gap between the tooth and the hole was filled with sticky wax (Fig. 1). The completed apparatus was then sterilized with ethylene dioxide. A 24-h broth culture of Enterococcus faecalis was placed into the 2mm coronal space of the root was suspended in sterile BHI broth to a level sufficient to cover the apical 3mm of the root tip and the fresh 24-h cultures of Enterococcus faecalis were added every two days throughout the study. Tubes were incubated at 37°C until the BHI broth became turbid. The occurrence of turbidity in BHI broth as indicator of bacterial leakage was assessed every 24 h for 40 days. The statistical comparisons among the experimental groups were made by Fisher's exact test for the leakage occurrence, and by Kaplan Meier Survival for the mean time of leakage. Analyses were performed using SPSS software version 17, and P-values

Figure 1. Schematic diagram of the experimental model

Results

During the experiments  all positive controls leaked, based on the results of Fisher's exact test, the positive control group showed significant differences with Excite and Clearfil SE Bond (P=0.036) and no significant differences with iBond (P=0.2). The negative controls do not leak. Based on the results of Fisher's exact test, the negative control group shows no significant differences with Excite and Clearfil SE Bond (P=1.00) as well as iBond (P=0.509). Table 2 illustrates that bacterial microleakage were detected in the test groups: Excite (23.1%), Clearfil SE Bond (23.1%), and iBond (38.5%). Based on the results of Fisher's exact test, there were no significant differences of microleakage among the experimental groups
(P=0.738). The estimated mean times of bacterial microleakage were 8 days in Excite group, 8.67 days in Clearfil SE Bond group, and 7.80 days in iBond group (Table 3). Kaplan-Meier Survival analysis showed no statistically significant difference among experimental groups (P=0.911).

Table 2. Frequency distribution of the samples shows the bacterial microleakage from Fisher's Exact for study groups during 40 days

Table 3. Mean time and confidence interval of bacterial microleakage from Kaplan-Meier for study groups during 40 days

^*log-rank (Mantel Cox)