Nowadays, MDP is found in every dental adhesive and every component of a resin cement system with adhesive properties from Kuraray Noritake Dental, including those shown in the following figure.

In addition, MDP has become an integral part of most universal adhesives and many adhesive cementation systems made by other manufacturers. However, not all MDP is alike.

MDP’s chemical structure and mechanism of adhesion

The MDP monomer consists of three essential parts: a polymerisable group, a hydrophobic group and a hydrophilic group. The co-polymerisable methacrylate group has a terminal double bond, enabling polymerisation. The large hydrophobic alkylene group—also referred to as the spacer—has the task of maintaining a delicate balance between the hydrophobic and hydrophilic properties of the monomer and offers great resistance to degradation. The hydrophilic phosphate group is responsible for acidic demineralisation, for chemical bonding with calcium in hydroxyapatite and for bonding with zirconia¹ and with metal.

Bonding performance

Many in vitro studies have been carried out to investigate the bonding behaviour of 10-MDP in the context of direct and indirect restorative procedures. No matter whether a cavity is to be filled with resin composite or an indirect restoration is to be placed, a strong and long-lasting bond to tooth structure needs to be established. The critical substrate in this context is dentine, whereas bonding to enamel is less challenging. That is why it is so important that self-etching adhesives containing 10-MDP show an extraordinarily high bond strength to tooth structure, particularly to dentine.² In fact,10-MDP also provides for a high bond stability over time by establishing an acid- and base-resistant zone on the adhesive interface.³ This means that outstanding long-term performance can be expected. Fortunately, the optimal long-term clinical performance of products containing the MDP monomer has been confirmed. In 2015, researchers from KU Leuven in Belgium presented excellent results from a 13-year clinical trial involving the use of CLEARFIL SE BOND.⁴

When bonding to indirect restorations made of zirconia, the surface area of the ceramic should be increased by sandblasting.⁵ If the surface is pretreated in the recommended way, the strength of the zirconia restoration’s bond to the tooth substrate tends to be particularly high when MDP-based resin cement systems are used.⁶ It is thus widely recommended by experts in the field of adhesive dentistry to employ MDP-containing primers or resin cements for the placement of zirconia-based restorations, especially in cases where the tooth preparation does not provide sufficient mechanical retention. The effectiveness of products containing 10-MDP in this context has been confirmed in various clinical studies with observation periods of up to ten years.^7, 8 The products used in these studies were PANAVIA 21, PANAVIA F2.0 and the latest version of the multicomponent cementation system from Kuraray Noritake Dental, PANAVIA V5. PANAVIA V5 was found to perform the best.

Not all MDP monomers are the same

After the basic patent for MDP expired, other manufacturers of dental adhesives and adhesive resin cements started integrating this functional monomer into their own products. However, it has been determined that there are differences in the purity of the MDP monomers synthesised and that these differences have an impact on the long-term bonding performance of products containing these MDP monomers.⁹ According to in vitro test results, the original MDP monomer synthesised by Kuraray Noritake Dental has an unmatched level of purity. This purity has a positive effect on the microstructure and thickness of the hybrid layer formed on dentine, on the degree of nano-layering and on the bond strength measured immediately and after artificial ageing.⁹

Conclusion

This summary of the research to date demonstrates that, after 40 years of clinical use, the original MDP monomer from Kuraray Noritake Dental is still in a class of its own. Because its properties support a strong and long-lasting bond to tooth structure, resin composite and metal oxides, the original MDP monomer is a valuable component of virtually every adhesive system. In order to ensure a high bond quality, it is recommended to use the MDP monomer with a confirmed high purity: the original MDP monomer.

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