E01. Identify the advantages and drawbacks with synthetic resins as restorative materials, and explain why a resin such as bisGMA replaced methylmethacrylate as a resin in some restorative materials.

Advantages:

1. Relatively easy to process

2. Relatively inexpensive

3. Rather biocompatible

4. Relatively insoluble in oral fluids

5. Relatively insensitive to dehydration

6. Can often be colored to match oral tissues  

7. Can be bonded to acid etched enamel and dentin

Drawbacks:

1. Shrink when they cure

2. Can leach components that can trigger biological reactions such as allergic reactions

3. Do not possess the same wear resistance as enamel

4. Can often discolor over time

Methylmethacrylate was introduced to dentistry during the 1930s. From the beginning, it was as a denture base material and it was hardened via heat-curing. During the 1940s, German researchers were able to cure the methacrylates via a so-called "cold-curing" process. The introduction of this curing process made it possible to make plastic restorations that cured directly in the oral cavity. However, a major problem with the methylmethacrylates was that the pure monomer shrunk as much as 21 vol.-% (7 % linearly). To reduce the shrinkage, prepolymerized methylmethacrylate beads were mixed the monomer, making it possible to reduce the shrinkage down to around 3.5 % linearly. However, such shrinkage was still too excessive, which meant that the enthusiasm that had followed the introduction of the self-cured methylmethacrylates was soon replaced by clinical disappointments.

To solve the problem with excessive shrinkage, Dr. Raphael Bowen explored the possibility to mix quartz particles with an epoxy resin. In vitro, this material behaved well, but when tried clinically, the material did not cure. The reason was that the setting reaction of the used epoxy resin was very sensitive for moisture contamination. To solve this problem, Dr. Bowen came up with the idea to replace the epoxy groups with methacrylate groups. By doing so, he produced a dimethacrylate that was called bisGMA or "Bowen's resin."

The advantage with the bisGMA was its higher molecular weight and smaller polymerization shrinkage. The stiff central backbone structure and the pending OH groups made this monomer also quite viscous; a property that has both advantages as well as disadvantages. The advantage with increased viscosity is that the material retains its shape better and that filler particles do not sediment as fast as in a less viscous liquid. The drawback with a viscous resin is that it becomes more difficult to incorporate filler particles into the monomer.

Understanding different dental resins

By starting with ethylene (H₂C=CH₂), one can develop the different dental resins. The first step is to substitute one hydrogen atom with any group, X, and what we have left of the ethylene is the vinyl group (H₂C=CHX). A monomer often used to make different polymers used in society in general is vinyl chloride (H₂C=CHCl). If a carboxyl group (-COOH) substitutes the Cl atom an acrylic acid  (H₂C=CHCOOH) is formed. By polymerizing the above acid, polyacrylic acid (PAA) is formed. Polyacrylic acid is used in materials such as polycarboxylate cements and glass ionomes. By mixing the PAA with different metal oxides (e.g. ZnO), the H-ions of the -COOH groups are released and form water when they react with the oxygen ions of the metal oxide. During this reaction, the positive metal ions form ionic bonds between the now negatively charged PAA chains.  

If the hydrogen atom of the vinyl group of the acrylic acid is substituted with a methyl group (-CH₃), the acrylic acid converts to methacrylic acid (H₂C=CCH₃COOH). The methacrylic acid is fundamental in dentistry, since it forms the central building block of most dental resins.

By allowing the methacrylic acid to react with methanol through a condensation reaction, water and an ester, methylmetacrylate (H₂C=CCH₃COOCH₃)  is formed. Methylmethacrylate is used for making numerous dental constructions, including dentures, trays, removable orthodontic appliances, temporary crowns, etc.

Dimethacrylates (matrix materials in dental composites)

Methacrylic acid can also react with triethylene glycol.(H₂C=CCH₃COOH + HO-CH₂-CH₂-O-CH₂-CH₂-O-CH₂-CH₂-OH + HOOCH₃CC=CH₂). During that reaction, the methacrylate groups bond to each end of the triethylene glycol forming a dimethacrylate called TEGDMA. This dimethacrylate is frequently used as a diluent in dental composites. The backbone structure is flexible, and the polar bond interaction among chains is weak, explaining the fairly low viscosity of this monomer.

The molecule below is the bisGMA, or the so called "Bowen's resin.  

The bisGMA monomer is extensively used in dental composites. The two phenol rings in the central structure makes the backbone structure stiff, and the two -OH groups along the chains form hydrogen bonds between molecules. The molecular stiffness as well as the hydrogen bond formation explains why this monomer is very viscous and needs to be diluted with low viscous monomers such as TEGDMA.  

In order to decrease the viscosity, but at the same time reduce polymerization shrinkage, some manufacturers have substituted some or all of the bisGMA in their composites with UEDMA (shown below). UEDMA is a urethane dimethacrylate. The advantage with UEDMA is its higher flexibility, which increases its likelihood of participating with both methacrylate groups in the polymerization process. The drawback with UEDMA when compared to bisGMA is that the lower viscosity of UEDMA increases the sedimentation rate of filler dispersed in this monomer.