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40 YEARS OF MDP WHAT IF KURARAY NORITAKE HAD NOT DEVELOPED THE MDP MONOMER?

24/05/2021
 Adhesion to various substrates is possible thanks to MDP (methacryloyloxydecyl dihydrogen phosphate), which has been shown to create a long term, durable and stable bond to both dental tissue and metal oxides. Research conducted over the past few decades has demonstrated the effectiveness of MDP. The first Kuraray Noritake Dental product to contain MDP was PANAVIA EX. Various other adhesive and cementation systems followed. Our latest products, PANAVIA™ V5 and CLEARFIL™ Universal Bond, also rely to a large extent on the
performance of the original MDP monomer. MDP has the following structure:

  1. terminal double bond group for polymerisation
  2. hydrophobic alkylene group to maintain a delicate balance between hydrophobic and hydrophilic properties and
  3. hydrophilic phosphate group for performance of the acid demineralisation and chemical bonding to tooth structure.

History

In the years between 1978 and the late 1990s, it became possible to increase the bond strength to dentine to more than the intrinsic strength of the dentine itself. MDP played a major role in these developments. The solid ionic bond with calcium from the HAp in dental tissue means that such failures as occur are cohesive in the dentine rather than being failures of the adhesive. Apart from in vitro studies confirming this bond strength, the long term clinical durability recorded over 13 years is also impressive.


Research

Research from Yaun et al., 2007, shows that defects along the interface are largely responsible for degradation of the hybrid layer. CLEARFIL™ SE BOND exhibited no such defects. So, it was
expected that SE BOND would also prove to have a stable connection to dental tissue. Peumans et al. from Leuven University, Belgium found CLEARFILTM SE BOND to still have excellent clinical effectiveness after 13 years. In their publication in the Journal of Dental Research 83, 2004, Yoshida et al. concluded that the choice of the adhesive monomer played a significant role in the effectiveness of the adhesive. They found MDP to be the best and fastest in tests for chemical adhesion (ionic bonding) and stability in a moist environment (insoluble) – better and more stable than 4-MET and Phenyl-P, in that order. Kumiko Yoshihara et al. published ‘Functional monomer impurity affects adhesive performance’, in Dental Materials 31 (2015) 1493-1501. They concluded that the three MDP’s they studied exhibited different levels of purity. Differences in the resultant hybrid layers were observed in all three MDP versions: both impurities and the presence of dimers had an effect on the etching efficacy of the HAp, and also on the intensity of nano-layer presence and the immediate bond strength. After 40 years, MDP is still in a class of its own. To date, no manufacturer has been able to develop an adhesive monomer with better adhesion properties combined with durable bond strength. Many attempts have been made, but none have succeeded. In the meantime, Kuraray’s patent on MDP has expired. This has opened up the market for other manufacturers to synthesise their own MDP. Only a few non-dental chemical companies are supplying MDP to other manufacturers of adhesive systems, and the synthesis of high-purity MDP remains extremely difficult.

Explanation

The three 10-MDP’s studied in this study clearly revealed a different purity. Differences in the ultrastructure of the resultant hybrid layers were observed for the three 10-MDP versions. Both the impurities and the presence of dimers affected the etching efficacy of HAp, the intensity of nanolayering and the ‘immediate’ bond strength.
The purity of 10-MDP present in commercial dental primers, adhesives and cements can be expected to influence bonding perfomance.

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