Choosing your chemistry
While there are many different monomers out there, the most common choices for commercial dental composites are bisphenol A glycidyl dimethacrylate (BisGMA), its ethoxylated version (BisEMA), triethylene glycol dimethacrylate (TEGDMA), and urethane dimethacrylate (UDMA). These dental acrylic monomers are usually used in different combinations in different ratios to make use of their individual benefits – and account for any shortcomings.
BisGMA was first incorporated into dental composites in 1962 and continues to be the most commonly used monomer in dentistry today.3 Its high molecular weight and low double bond content provides low polymerization shrinkage, high reactivity, low toxicity and fast setting – and is known for producing strong and stiff polymetric networks.2-6 , 8
However, its high viscosity not only creates handling issues but also decreases filler compatibility and the degree of conversion – which can negatively impact a number of physical properties, including wear and shrinkage. For this reason, BisGMA is often combined with other, lower viscosity monomers (such as TEGMA) to improve handling and physical properties.6, 8
A highly flexible, low-molecular weight, low-viscosity dental monomer, TEGDMA is often used as a diluent for other more viscous monomers to improve handling, as well as increase filler content and copolymerization. Unfortunately, TEGDMA is not without its drawbacks, in that it results in high water sorption, lower mechanical properties and low color stability.2, 3, 5-7, 9
BisEMA is actually a slightly altered version of BisGMA, which features a high molecular weight and stiffness but a lower viscosity, which enables a higher degree of conversion, low polymerization shrinkage, low water sorption and improved mechanical properties. For these reasons, it’s frequently used as an alternative to TEGDMA in composite compositions.2, 3, 5, 6, 9
UDMA was developed to overcome the limitations of BisGMA by presenting a monomer with a high molecular weight but a low viscosity, along with high flexibility and toughness. In practice, formulations based on UDMA have higher flexural strength, elastic modulus and hardness as well as improved monomer conversion when compared to those made with BisGMA.2, 4, 6, 7, 9
However, it’s not without its drawbacks. UDMA features a higher viscosity than TEGDMA and BisEMA, which can limit handling and filler compatibility. And its refractive index is lower than both BisGMA and BisEMA, which can reduce its optical match with common radiopaque filler systems.10
However, not every composite uses these same dental monomers in the same combinations and with so many other options available, it can be difficult to know exactly what you’re using – and whether or not you’re seeing all benefits you’ve been promised. You can think of it as using differently sized stones of different materials to build a wall – without the proper reinforcement, they’re unlikely to produce a consistent, strong structure.