Dental composite resins (better referred to as “resin-based composites” or simply “filled resins”) are dental cements made of synthetic resins.
Synthetic resins evolved as restorative materials since they were insoluble, of good tooth-like appearance, insensitive to dehydration, easy to manipulate and inexpensive.
Composite resins are most commonly composed of Bis-GMA and other dimethacrylate monomers (TEGMA, UDMA, HDDMA), a filler material such as silica and in most applications, a photoinitiator.
Dimethylglyoxime is also commonly added to achieve certain physical properties such as flow-ability. Further tailoring of physical properties is achieved by formulating unique concentrations of each constituent.
Resin-based Composites:
Methacrylate-based monomers:
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BISGMA (Bisphenol A-Glycidyl Methacrylate)
Bis-EMA (Ethoxylated Bisphenol A Dimethacrylate)
UDMA (Urethane Dimethacrylate)
TEGDMA (Triethylene Glycol Dimethacrylate)
HEMA (2-Hydroxyethyl Methacrylate)
Formulation Details:
Resin-based Composites:
Typical composition:
Organic matrix (30-50% by weight): Monomers (e.g., BISGMA, UDMA, TEGDMA)
Inorganic fillers (50-70% by weight): Silica, quartz, zirconia, barium glass
Coupling agents (1-5%): Silane compounds to bond fillers to matrix
Initiator system: Camphorquinone + tertiary amine for light-curing
Inhibitors: Hydroquinone or butylated hydroxytoluene (BHT)
Bisphenol A-Glycidyl Methacrylate (BISGMA):
Chemical structure: Two phenol rings connected by a central carbon atom, with methacrylate groups at both ends.
Properties:
Molecular formula: C29H36O8
Molecular weight: 512.6 g/mol
High viscosity: 500,000-800,000 cP at 23°C
Glass transition temperature (Tg): ~-7°C
Refractive index: 1.55
BISGMA has high molecular weight and rigidity due to its aromatic rings, contributing to its high viscosity and mechanical strength. It’s hydrophobic and has low polymerization shrinkage (5-6%). However, its high viscosity limits filler loading.
Urethane Dimethacrylate (UDMA):
Chemical structure: Aliphatic chains connected by urethane linkages, with methacrylate groups at both ends.
Properties:
Molecular formula: C23H38N2O8 (common variant)
Molecular weight: 470.6 g/mol
Lower viscosity than BISGMA: 5,000-10,000 cP at 23°C
Glass transition temperature (Tg): ~-35°C
Refractive index: 1.48
UDMA offers lower viscosity and higher flexibility compared to BISGMA. It also provides good wear resistance and toughness.
Triethylene Glycol Dimethacrylate (TEGDMA):
Chemical structure: Linear molecule with two methacrylate groups connected by a triethylene glycol chain.
Properties:
Molecular formula: C14H22O6
Molecular weight: 286.3 g/mol
Very low viscosity: 100 cP at 23°C
Glass transition temperature (Tg): -83°C
Refractive index: 1.46
TEGDMA is used as a diluent monomer to reduce overall viscosity. It increases degree of conversion but also increases polymerization shrinkage.
BISGMA (Bisphenol A-Glycidyl Methacrylate)
Pros:
High mechanical strength due to rigid aromatic structure
Low polymerization shrinkage (5-6%)
Good wear resistance
High refractive index (1.55), allowing good color matching
Cons:
Very high viscosity (500,000-800,000 cP at 23°C), limiting filler incorporation
Hydrophobic nature can affect bonding to dentin
Potential concerns about bisphenol A release (though studies show minimal risk)
Relatively high water sorption due to hydroxyl groups
Bis-EMA (Ethoxylated Bisphenol A Dimethacrylate)
Pros:
Lower viscosity than BISGMA (5,000-9,000 cP at 23°C)
Maintains good mechanical properties
Lower water sorption than BISGMA
Allows for higher filler loading
Cons:
Slightly higher polymerization shrinkage than BISGMA
Still contains bisphenol A structure (though modified)
Less reactive than TEGDMA or UDMA
UDMA (Urethane Dimethacrylate)
Pros:
Lower viscosity than BISGMA (5,000-10,000 cP at 23°C)
High flexibility, improving toughness
Good wear resistance
Higher degree of conversion than BISGMA
Cons:
Higher polymerization shrinkage than BISGMA
Lower strength compared to BISGMA
Can increase water sorption in some formulations
TEGDMA (Triethylene Glycol Dimethacrylate)
Pros:
Very low viscosity (100 cP at 23°C), excellent as a diluent
Increases the degree of conversion in composite formulations
Improves handling properties of the composite
Cons:
High polymerization shrinkage (12-14%)
Increases water sorption due to ethylene oxide groups
Can lead to increased cytotoxicity in uncured state
Lower mechanical properties compared to BISGMA or UDMA
HEMA (2-Hydroxyethyl Methacrylate)
Pros:
Hydrophilic nature improves wetting and bonding to dentin
Low viscosity, useful as a diluent
Promotes penetration into demineralized dentin
Cons:
High water sorption, leading to hydrolytic instability
Lower mechanical properties compared to other monomers
Potential for continued release from the polymer network
Higher polymerization shrinkage than BISGMA or UDMA
Product Comparison
Viscosity: TEGDMA < HEMA < Bis-EMA ≈ UDMA < BISGMA
Mechanical strength: BISGMA > Bis-EMA > UDMA > TEGDMA > HEMA
Polymerization shrinkage: TEGDMA > HEMA > UDMA > Bis-EMA > BISGMA
Water sorption: HEMA > TEGDMA > BISGMA > UDMA > Bis-EMA
Degree of conversion: TEGDMA > UDMA > Bis-EMA > HEMA > BISGMA