Microorganism survived in the root canal after root canal cleaning and shaping procedure is a main cause of root canal treatment failure. There are several mechanisms for the bacteria to survive in the root canal after chemomechanical preparation and root canal irrigation. Bacteria organized as biofilm has been suggested as an etiology of persistent periapical lesion. Recent studies were focus on removal of Enterococcus faecalis biofilm due to the report that the persistence of this bacteria after root canal treatment may be associated with its ability to form biofilm. Several investigations demonstrated that current root canal treatment protocol including use of NaOCl, EDTA and Chlorhexidine as irrigants is quite effective in eliminating E. faecalis biofilm. However, this microorganism still can survive in inaccessible areas of root canal system and evade host immune response, suppress immune activity and produce biofilm. Up to date, there is no possible clinical method to completely get rid of bacteria from the root canal. Once the root canal treatment failure occurred, and conventional treatment incorporating current therapeutic protocol has failed, periapical surgery or extraction should be considered rather than prolong the ineffected retreatment procedure.

This study evaluated the effectiveness and safety of an experimental bleaching strip (Medison dental whitening strip, Samsung medical Co., Anyang, Korea) containing 2.9% hydrogen peroxide. Twenty-three volunteers used the bleaching strips for one and a half hour daily for 2 weeks. As control group, the same strips in which hydrogen peroxide was not included were used by 24 volunteers with the same protocol. The shade change (ΔE*, color difference) of twelve anterior teeth was measured using Shade Vision (X-Rite Inc., S.W. Grandville, MI, USA), Chroma Meter (Minolta Co., Ltd. Osaka, Japan) and Vitapan classical shade guide (Vita Zahnfabrik, Germany). The shade change of overall teeth in the experimental group was significantly greater than that in the control group (p < 0.05) and was easily perceivable. The change resulted from the increase of lightness (CIE L* value) and the decrease of redness (CIE a* value) and yellowness (CIE b* value). The shade change of individual tooth was greatest in canine, and smallest in central incisor. The safety of the bleaching strip was also confirmed.