The specific diagnostic problem and the diagnostic methods

As it was described above, while secondary caries accounts for more than half of replacing restorations regardless of the different materials in the general practice, around 50 percent, this high prevalence is not found in one controlled clinical trial in which only 2 among 2660 Class I or II restorations were replaced due to secondary caries [Letzel et al., 1989]. On the contrary, in another randomized controlled clinical trial 66.7% and 87.6% of the failures that occurred in amalgam and composite restorations because of the diagnosed secondary caries, respectively [Bernardo et al., 2007]. Are they correct or wrong? Why are there are huge differences between these studies? Are the practitioners involved in these studies poorly trained or ignorant about the criteria of secondary caries diagnosis? Indeed, until now it is very difficult to explain the above questions reasonably, however, except the variation between those studies themselves, it should be acknowledged that there are some specific diagnostic problems for secondary caries and it is very crucial to understand secondary caries correctly in order to make an accurate diagnosis.

In 1990 Kidd pointed out that there are several main specific diagnostic problems for secondary caries, including the difficulty of detecting the wall lesion; the relevance of a defective margin(e.g. ditched margin) to the longevity of a restoration and the difficulty of distinguishing secondary from residual caries [Kidd, 1990]. It is suggested that only frankly caries lesion at the margin of the restoration constitutes a dependable diagnosis of secondary caries [Kidd and Bieghton, 1996], whereas it is impossible to detect or see the wall lesion until it is so advanced that the overlying tissue collapses to reveal a large hole or the tooth tissue over it becomes grossly discoloured [Kidd, 1990]. Consequently, dentists often cannot detect or diagnose a secondary caries when a wall lesion is in progress under a sound surface.

Traditionally, the presence of clinically detectable defects in restoration margins has been associated with an increased risk of secondary caries occurring beneath such restorations [Hewlett et al., 1993]. Besides, marginal defects present between a restoration and the cavity wall, such as those occur in occlusal pits and fissures, may act as gathering points for bacterial plaque [Pimenta et al., 1995]. Surveys in which dental practitioners determine reasons for replacing restoration indicate that clinical evidence of defective margins is a commonly used criterion for replacing restorations [Boyd and Richardson, 1985; Qvist et al., 1986]. On the other hand, other studies showed the low relevance of defective margin to restoration replacement and secondary caries which supported the conclusion that the defective margin only can not be the reason to replace a restoration. Söderholm et al. suggested that the use of defected margin as the criterion for restoration replacement would have resulted in the unnecessary treatment of 34% of the teeth examined [Söderholm, 1989]. Kidd and O'Hara reported that caries incidence on the cavity wall adjacent to the margins was the same for both in the intact and defective restoration [Kidd and O'Hara, 1990]. Although, Hewelett et al. found the likelihood of radiographic secondary caries was much higher for defective restorations than for intact restorations through the investigation of radiographic secondary caries prevalence in 6285 teeth clinically defective restorations, it was still suggested that defective restoration status should be combined with radiographic examination [Hewlett et al., 1993]. Therefore, the presence of ditched margins where are plaque stagnation areas which might enhance the prevalence of secondary caries development, however, is not a sufficient factor to determine a possible process of secondary caries formation [Pimenta et al., 1995]. Furthermore, the progression of caries is determined by the dynamic balance between pathological factors that lead to demineralization and protective factors that lead to remineralization. If either the pathological factors are not sufficient or protective factor are present, caries will not develop regardless of tooth morphology [Featherstone, 2004].

According to the definition, secondary caries is a new primary caries and should be differentiated from residual caries. In the past, on the basis of the extension-for-prevention concept, the cavity preparation principles established by G.V. Black, students were taught to prepare the cavity as clean as possible. Nowadays, as the development of conservative dentistry and minimal intervention dentistry and remineralization, it is recommended that dentists should distinguish the affected tissue which could be healed by remineralization and infected tissue, only infected should be removed to preserve more dental tissue and increase the longevity of the teeth [Fusayama, 1988; Kidd, 2010; Massler, 1967; ]. However, it is impossible to predict whether these residual lesions will progress. Thus, it is thought-provoking that the modern dentistry might increase the difficulty of distinguishing the secondary and residual caries. Or it might not be so important to differentiate the secondary and residual caries.

To diagnose the carious lesion, either primary or secondary, the dentists need good lighting, clean teeth, sharp eyes and even good bitewing radiography [kidd, 1984]. Secondary caries develops more frequently at the cervical and interproximal margins [Mjor, 1985; Mjor, 2005], more attention must be paid to find better methods or techniques to detect the secondary caries, despite of those difficulties to make an accurate diagnosis of secondary caries. The conventional visual and tactile methods using a sharp explorer have been advocated in the diagnosis of primary and secondary caries [kidd, 1990]. However, in recent years it has been shown that the sharp explorer seems to be an unwise instrument to detect secondary caries. On one hand, a sharp explorer could cause cavitation of an outer lesion, damage the margin of a restoration, or even become impacted in a marginal discrepancy which might then be misinterpreted as a carious lesion [Bergman and Linden, 1969; Ekstrand et al, 1987]. On the other hand, wall lesions of secondary caries can not easily be detected until they have reached an advanced stage [Kidd, 1990], it is very difficult for explorer to contact the lesion and detect it at the early stage. And it is important to keep in mind that a sharp explorer will stick in any crevice, regardless of whether there is carious lesion [Mjor, 2005]. Additionally, discoloration around dental restorations may be due to the variety of factors such as the physical presence of amalgam, corrosion products, or secondary caries. It could be concluded that colors or stains next to restorations are not always predictive of secondary caries and not useful for the detection of secondary caries [kidd et al., 1995, Rudoolphy,

1995], whereas, it is very difficult to distinguish whether the discoloration originated from the restoration or was to due the demineralization [Ando et al., 2004]. Until now, besides the most common and traditional method of visual examination with a tactile instrument, there are some several other methods available to measure the mineral loss, such as microradiograph [Arends et al., 1987] and CLSM (confocal laser scanning microscopy), which measures the fluorescence area to determine the secondary caries [Fontana et al.,

1996]. It is reported that QLF (light-induced fluorescence) might be a suitable technique for detection of early secondary carious lesions less than 400^m meanwhile LF (infrared laser fluorescence) might be a suitable technique for the detection of secondary caries, especially for lesions over 400^m or dentinal lesions [Ando et al., 2004].

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