At baseline, ICDAS scores, as well as GI, PlI and MS will be recorded followed by the incorporation of the fixed orthodontic appliances. Subsequently, using a random list participants are provided with a standardized electric tooth brush and a supply of one of the two experimental tooth pastes (hydroxylapatite(HA)/fluoride (FL)) to be used 2 x daily for the next 6 months. HA-tooth paste contains no fluoride, FL-tooth paste contains stannous fluoride and amine fluoride. At days 28, 84 and 168 (end of study) ICDAS, GI, PlI and MS are recorded again. Additionally all study participants will receive a full mouth preventive professional supragingival tooth cleaning followed by the application of a 1% chlorhexidine gel on all tooth surfaces once every 4 weeks during the course of the trial.