Composite filling materials, when in a fluid or paste-like state, harden due to the bonding of their molecules (polymerization) when blue light (LED light device) is applied. During this hardening reaction, an inevitable 1.5% to 3% reduction in volume occurs in the material; this is called “polymerization shrinkage”. If the dentist fills the cavity with the filling as a single large block and then irradiates it, the filling will pull the tooth walls inward as it shrinks, creating microscopic gaps (marginal leakage) at the tooth edge; this leads to persistent chewing pain after the filling and rapid decay from underneath. To prevent this, the “Incremental Layering Technique” is applied; the filling is placed in the cavity in inclined layers, no more than 2 mm thick, and each layer is irradiated separately, thus minimizing shrinkage stress.
