Introduction
Composite materials combine organic resins with inorganic fillers, which do not dissolve in each other. R. Bowen developed Bis-GMA around 1960. The first self-cured composite appeared in 1962, and blue-light curing followed in 1978. Composites are now the most used filling materials.
Classification by filler
- Nanofilled: very high polish and wear resistance, and they keep their gloss.
- Nanohybrid: the latest generation, universal for anterior and posterior teeth, with excellent esthetics, handling and wear.
Polymerization shrinkage
Only the resin polymerizes, and it shrinks as it does. Stress reaches about 7 MPa and can break the bond, causing gaps, sensitivity and secondary caries. A self-cured composite shrinks toward its center, while a light-cured one shrinks toward the light.
The C-factor is the ratio of bonded to free surfaces of a restoration. The higher it is, the less the material can flow to relieve stress.
Reducing shrinkage stress
- Incremental placement: small layers, each cured through the tooth wall.
- Soft-start curing: for example, 10 seconds at 150 mW/cm², then 700 mW/cm².
- Pulse curing: short, intense pulses of light.
- Flowable liner: a thin layer of low-modulus composite, about 5 GPa, acts as an elastic buffer.
- New chemistry: ormocers shrink about 2%, and siloranes less than 1%.