Dental filling. A matrix holder is placed around the decayed premolar of a 26 year old male patient in preparation for filling the cavity. A dental restoration, or filling, is carried out when a tooth has been damaged by decay. The affected area is drilled away, cleaned out, and filled with tough amalgam, cement or dental composite. Tooth decay (dental caries) is caused by the acids released by bacteria in plaque, a sticky mixture of food remains that builds up on teeth.