Many of today’s universal adhesives have moved to MDP in their material makeup. We find out why and what you need to know about it for your restorative technique and outcomes.
When it comes to adhesive systems, many universal adhesives have incorporated methacryloyloxydecyl dihydrogen phosphate (MDP) into their primer formulation. Many clinicians know that MDP in the primer enables bonding to metal and zirconia, but might not know that MDP also forms a chemical bond with calcium phosphate, the mineral phase in tooth structure. Let’s take a closer look at what MDP is and why it does such an excellent job in bonding primers for restorative dentistry.
In 1981, Kuraray introduced the first MDP monomer. From the start, MDP demonstrated effective bonding to enamel, dentin, and metal alloys.1
MDP can also establish an intense and stable chemical interaction with teeth. As a functional monomer, MDP also has acidic molecules that can do a few different things, like:2
Functional monomers have 2 functional groups, 1 that demineralizes the tooth and another that can polymerize. Nathaniel Lawson, DMD, PhD, Associate Professor and Director, Division of Biomaterials UAB School of Dentistry, says MDP is a molecule that has a phosphate-based functional group on one end and a methacrylate-based functional group on the other.
Dr Lawson says MDP can be used for bonding to tooth structure or zirconia. When bonding to the tooth, Dr Lawson says the MDP has the 2 functions of partially demineralizing the tooth and also bonding with calcium in the tooth. MDP is an acidic monomer, similar to other acidic monomers used in self-etch adhesives. Dr Lawson says that acidic monomers partially solubilize the smear layer and remove some of the mineral content of enamel or dentin.
Forming a chemical bond with hydroxyapatite is not business as usual for bonding agents. Many bonding agents infiltrate small surface irregularities in the patient’s enamel or dentin creating a micro-mechanical bond, but can’t react chemically with the tooth surface. Using a universal bonding agent with MDP, clinicians can achieve both a micromechanical and chemical bond.
“The unique aspect of MDP is that it can also bond to the calcium in mineral content of tooth, similar to the bond between calcium and the polyacrylic acid in MDP,” Dr Lawson says.
MDP also has the potential to improve the long-term seal and durability of the bond. In adhesive systems, how the bonding agent interacts with the collagen is critical. The deterioration that can occur with the collagen fibrils in the hybrid layer is what causes the problems with dentin bonding over the long term. The functional monomers like MDP have chemical properties that create the high-strength bond with dentin.3
The same cannot be said of components of other adhesive materials. A common ingredient in dentin bonding agents is 2-hydroxyethyl methyl methacrylate (HEMA) because it is hydrophilic. In other words, HEMA allows the bonding agent to bond and polymerize in the presence of moisture found in enamel and dentin. However, HEMA's high hydrophilicity also makes it prone to water sorption, which can degrade the collagen fibers bond over time, which decreases bond strength and an increase the risk of marginal staining and recurrent caries.4
By contrast, MDP improves shear bond strength and the stability of the margin for restorations, which increases the durability. First, the prevention of water sorption avoids a gradual breakdown of the bonding layer over time. Also, the absence of HEMA helps to preclude moisture from adhering to collagen fibers in the hybrid layer. Moreover, with MDP there is no degradation of those fibers in the hybrid layer either.
In addition, research in Materials suggests that MDP can produce and acid-base resistant zone on the adhesive surface, increasing the response to acid-base challenges. This interface response creates a stable adhesion over time. However, the authors also recommend using a scrubbing technique on dental substrates to promote solvent evaporation and improve the monomers infiltration into the smear layer to create a stable bond between the adhesive and the substrate. Also, they say that giving MDP time to hybridize and form the chemical bond with tooth structure improves bond stability.5
However, in addition to tooth structure, MDP also bonds to zirconia. Having a universal adhesive system with MDP eliminates the need for practices to have a stand-alone or zirconia primer in their inventory. What makes it possible is MDP’s 2 functional molecular groups: the phosphate containing group that bonds with zirconia and the methacrylate end that crosslinks with methacrylate in resin cements, Dr Lawson explains.
“When bonding to zirconia, the phosphate-based functional group bonds to zirconia and the methacrylate end bonds to resin cements. So, MDP are used in primers or universal adhesives that are applied to zirconia crowns before bonding,” Dr Lawson explains.
MDP-based products produce a high bond strength and chemical affinity. Per research in Experimental and Therapeutic Medicine, using MDP primers increased bond strength over those that didn’t use primers. Moreover, when combined with air abrasion techniques, the researchers had the best bond strengths, which they attribute to enhanced surface wettability from the air abrasion technique and the increased bond strength created by primers containing MDP.6 Research in Operative Dentistry reached similar conclusions in 2020, suggesting that MDP-containing silane coupling agent used alone or with universal adhesive solution created durable bonding to zirconia.7
MDP has been around now for 4 decades. Over this time, it has found its way into most of the universal adhesive primers available on the market today, mostly because it creates a strong and reliable bond with tooth structure, metal alloys, and zirconia. Not only does it stop the deterioration of collagen fibrils in the hybrid layer of natural dentition which degrades bond strength over time, but it also forms the same chemical bonds with the chemical makeup of zirconia. These bond strengths are more intense and longer lasting that any of the other materials or techniques used in adhesive systems. For these reasons, you might find that your universal adhesive system containing MDP in the primer is the MVP for your practice, and your restorative outcomes.