Frequency and location of diagnosed secondary caries

Since the early days of restorative dentistry, the phenomenon of secondary caries has been known and considered as the basis for the extension-for-prevention concept, the well-known principles of cavity preparation established by G.V. Black in the last century [Black, 1908]. The clinical diagnosed secondary caries has been shown to be principal cause for the replacement of all types of restorations both in permanent and primary teeth, 50%-60% of restorations are replaced as a result of the diagnosis of secondary caries [Mjor and Toffenetti, 2000]. As the development of restorative materials, some literatures regarding secondary caries indicated that the prevalence of secondary caries is associated with the restorative material type, although it may occur with all restorative materials [Burke et al.,

1999b; Forss and Widstrom E, 2004; Mjor, 1997; Mjor and Jokstadt, 1993]. Some published researches showed that compared to amalgam restorations, resin-based composite restorations represented a higher percentage of replacement because of the diagnosed secondary caries [Mjor and Jokstadt, 1993; Bernardo et al., 2007]. On the contrary, others reported that the amalgam was replaced because of the secondary caries more often than composite resin [Wilson et al., 1997; Burke et al., 1999a]. Compared with those studies, which acclaimed that a large proportion of restorations replaced as a result of diagnosis of secondary caries in general dental practice, one controlled clinical trials showed secondary caries represented in less than 1 percent of the restoration failures [Letzel et al., 1989], inversely, another controlled clinical trials by Bernardo et al. reported that secondary caries accounted for 66.7 percent and 87.6 percent of the failures that occurred in amalgam and composite restorations, respectively [Bernardo et al., 2007]. These controversies might be explained that the statistic results could be influenced by many factors, including the age of the population, the status of patients' oral health and dental care, examiner calibration and the duration of the experiment, etc.

Secondary caries, like other dental caries, is initially caused by the activities of microorganisms in dental plaque, so it is possible for any site on the restored teeth where is prone to the bacterial stagnation to develop secondary caries. General practitioners indicated that secondary caries was detected predominately on the gingival margins of Class II and Class I restorations, while seldom on the Class I restorations and the occlusal part of Class II restorations [Mjor, 1998; Mjor and Qvist, 1997]. A number of factors contribute to the more frequent occurrence of secondary caries on the gingival surface. First of all, the gingival aspect of any restorations is more difficult for patient to keep plaque free than any other parts, especially if it is located interproximally, while the occlusal surface is not a generally a plaque stagnation area and toothbrushing can easily reach this area to clean the plaque [Kidd, 2001; Mjor, 2005]. Secondly, during the restorative operation, the gingival surface is prone to contamination by gingival fluid and saliva, which causes the impossible visual inspection of the gingival floor and the deficiencies of insertion of restorative materials. And these deficiencies may lead to secondary caries more easily [Mjor, 2005]. Meanwhile, the less effective bonding of resin composite and the polymerization shrinkage at the gingival cavosurface may also influence the integrity of restoration at the gingival section and result in the development of secondary caries [Mjor, 2005].

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