Effect of MTAD on the Bond Strength to Enamel and Dentin

Journal of Endodontics
Vol. 29, No. 12, December 2003
Tanya K. Machnick, DDS, MS, Mahmoud Torabinejad, DMD, MSD, PhD, Carlos A. Munoz, DDS, MSD, and Shahrokh Shabahang, DDS, MS, PhD

Overview:
Achieving predictable bonding to dentin has long been a challenge in restorative dentistry. The high organic content of dentin, its tubular structure, and the outward flow of fluid make the bonding of resin to dentin difficult to attain. Immediate restoration of endodontically treated teeth is desirable to prevent coronal leakage.

The majority of clinicians use varying concentrations of sodium hypochlorite (NaOCl) to irrigate canals during root canal therapy. However, NaOCl does not remove the smear layer, does not have a complete antibacterial effect, and is toxic, especially at high concentrations. A final flush with EDTA followed by NaOCl appears to remove the smear layer, but EDTA is not antibacterial and erodes the dentin if the exposure time exceeds one minute. Therefore, a new solution with improved capabilities to disinfect the root canal system, remove the smear layer, and prepare the dentin surface for bonding is needed.

In a series of experiments, Torabinejad et al. have shown that MTAD, a mixture of a tetracycline isomer, an acid, and a detergent, is an effective solution for the removal of the smear layer. It appears that MTAD does not significantly change the structure of the dentinal tubules when used as a final irrigant in conjunction with low concentrations of NaOCl as a root canal irrigant.

Purpose:
This study was conducted to compare the effect of MTAD (a mixture of a tetracycline isomer [doxycycline], an acid [citric acid], and a detergent [Tween 80]) and phosphoric acid on the bond strength to enamel and dentin using a conventional OptiBond Solo Plus dentin adhesive system.

Materials and Methods:
One hundred flat surfaces (50 enamel/50 dentin) were prepared for bonding. Specimens were embedded in acrylic with the flat surface exposed. One of the following surface treatments was applied before bonding composite with Optibond Solo Plus:

• 1 minute 5.25% NaOCl/1 minute 17% EDTA
• 1 minute 5.25% NaOCl/1 minute MTAD
• 30-second etch with 35% H3PO4 (positive control)
• 2 minute 0.9% saline (negative control)
• 20 minute 1.3% NaOCl/5 minute MTAD (clinical protocol for MTAD)

Dentin treatment before adhesion of composites is performed both to enhance adhesion and to remove the microbial contents of the smear layer. It is important to eliminate any remaining bacteria that may be present on the cavity walls, in the smear layer, at the enamel-dentin junction, or in the dentinal tubules to halt the deleterious effect of bacteria and their byproducts. In addition, it seems that even the latest adhesive systems are not capable of producing a complete seal in clinical situations, so gaps form and bacteria and their toxic products readily penetrate. Therefore, bonding systems that have intrinsic antimicrobial properties would be beneficial

The standard method to produce the dentin bonding surface is to use the same acid conditioning, 30-70% phosphoric acid, as used in creating the ideal surface for bonding to enamel. EDTA also has been used to prepare the dentin surface for bonding. Some investigators have found that EDTA conditioning in combination with dentin bonding agents results in a significantly greater bond strength to dentin than conventional acid etching.

The use of NaOCl for dentin surface treatment in combination with acid conditioning also has been suggested. Removal of the organic collagen layer following acid conditioning and subsequent bonding directly to the partially demineralized dentin layer may produce more durable adhesion to the hydroxylapatite component of the dentin substrate. It is theorized that dentin substrate conditioning with NaOCl would significantly enhance the long-term bond strength and durability of the resin adhesive system to dentin. If the use of the root canal irrigant could aid in preparing the dentin surface for bonding, this would be an additional positive attribute.

Conclusions:
They investigated the ability of MTAD to disinfect contaminated root canals with whole saliva and compared its efficacy with that of NaOCl. Their results showed that:

• MTAD is significantly more effective than 5.25% NaOCl in eradicating bacteria from infected root canals
• MTAD maintains its efficacy in killing Enterococcus faecalis at 200x dilution, whereas NaOCl ceases to exert its antibacterial activity against this bacteria beyond 32x dilution
• EDTA possesses little or no bactericidal activity when tested using the minimum inhibitory concentration method
• MTAD is a biocompatible material and has similar solubilizing effects on pulp and dentin to those of EDTA

A major difference between MTAD and EDTA is a high binding affinity of doxycycline present in MTAD for the dentin that allows for a prolonged antibacterial effect.

These results are significant in that they demonstrate the efficacy of a single irrigant to remove most of the smear layer and kill a bacterial strain that has been shown to be resistant to many of the commonly used intracanal irrigants and dressings.

Bonding to dentin has not yet achieved the ideal state. Researchers now believe that dentin adhesion relies primarily on the penetration of adhesive monomers into the filigree of collagen fibers left exposed by acid etching. This is the ultimate goal; however, adhesion to dentin remains difficult, and there is some question regarding the optimal treatment to provide an ideal bonding surface.

According to these findings, it appears teeth endodontically treated with the MTAD protocol for clinical use (20 minutes 1.3% NaOCl/5 minutes MTAD) may not need any additional conditioning before the application of the dental adhesive. Future research will focus on the possibility of using MTAD as a pre-bonding treatment for cavity and crown preparations.