**2. Diagnosis of secondary caries**

#### **2.1 Histology of secondary caries**

In the 1970s, Hals et al. did a comprehensive investigation of the secondary caries lesions around various restorative materials both in vitro and in vivo, and natural secondary caries

Secondary Caries 405

1999b; Forss and Widström E, 2004; Mjör, 1997; Mjör 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 [Mjör 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

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 [Mjör, 1998; Mjör 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; Mjör, 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 [Mjör, 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 [Mjör, 2005].

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

**2.3 The specific diagnostic problem and the diagnostic methods** 

the duration of the experiment, etc.

on the extracted restored human teeth [Hals, 1975a, 1975b; Hals et al., 1974; Hals and Nernaes, 1971; Hals and Norderval, 1973; Hals and Simonsen1972]. According to their studies, whatever the restorative material type is, the secondary carious lesion displayed histologically the same basic pattern (Fig. 1): 1) an outer lesion, which is caused by the a new primary attack on the outer surface of the tooth; 2) a wall lesion, might be the consequence of the diffusion of bacteria, fluids or hydrogen ions between the restorations and the cavity wall. It is also supposed in their study that the fluoride released from the silicate material would be taken by both the cavity wall and the tooth surface around the restoration, which might reduce glycolysis and induce the remineralization [Hals, 1975a]. Thus, the individual caries patterns between the teeth with silicate materials and amalgam are different more or less. However, this described pattern of secondary caries including the outer lesion and wall lesion has been confirmed in later experiments [Diercke et al., 2009; Thomas et al., 2007; Totiam et al., 2007].

Fig. 1. A diagrammatic representation of secondary caries

The secondary caries lesion may occur in two parts: an outer lesion, formed on the surface of the tooth as a result of primary attack and a wall lesion formed as a result of diffusion of bacteria, fluids or hydrogen ions between the restorations and the cavity wall (From Kidd, 1990).

#### **2.2 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 wellknown 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 [Mjör 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.,

on the extracted restored human teeth [Hals, 1975a, 1975b; Hals et al., 1974; Hals and Nernaes, 1971; Hals and Norderval, 1973; Hals and Simonsen1972]. According to their studies, whatever the restorative material type is, the secondary carious lesion displayed histologically the same basic pattern (Fig. 1): 1) an outer lesion, which is caused by the a new primary attack on the outer surface of the tooth; 2) a wall lesion, might be the consequence of the diffusion of bacteria, fluids or hydrogen ions between the restorations and the cavity wall. It is also supposed in their study that the fluoride released from the silicate material would be taken by both the cavity wall and the tooth surface around the restoration, which might reduce glycolysis and induce the remineralization [Hals, 1975a]. Thus, the individual caries patterns between the teeth with silicate materials and amalgam are different more or less. However, this described pattern of secondary caries including the outer lesion and wall lesion has been confirmed in later experiments [Diercke et al., 2009; Thomas et al., 2007;

The secondary caries lesion may occur in two parts: an outer lesion, formed on the surface of the tooth as a result of primary attack and a wall lesion formed as a result of diffusion of bacteria, fluids or hydrogen ions between the restorations and the cavity wall (From Kidd,

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 wellknown 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 [Mjör 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.,

Totiam et al., 2007].

1990).

Fig. 1. A diagrammatic representation of secondary caries

**2.2 Frequency and location of diagnosed secondary caries** 

1999b; Forss and Widström E, 2004; Mjör, 1997; Mjör 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 [Mjör 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 [Mjör, 1998; Mjör 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; Mjör, 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 [Mjör, 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 [Mjör, 2005].

#### **2.3 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

Secondary Caries 407

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 thoughtprovoking 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

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 [Mjör, 1985; Mjör, 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 Lindén, 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 [Mjör, 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 400μm meanwhile LF (infrared laser fluorescence) might be a suitable technique for the detection of secondary caries, especially

Dental caries is determined by the dynamic balance between pathological factors that lead to demineralization and protective factors that lead to remineralization [Featherstone, 2004]. As a major pathological factor, oral bacteria, especially acidogenic bacteria, can dissolve the tooth mineral. Those acidogenic bacteria are also aciduric and can live preferentially under

Hitherto, it is unclear about the microbiology of secondary caries yet. Although secondary caries is described alike primary caries in histopathology, whether the etiology of secondary caries is the same as that of primary caries is a matter in dispute. Kidd et al. found no

for lesions over 400μm or dentinal lesions [Ando et al., 2004].

**3. Etiology of secondary caries 3.1 Microbiology of secondary caries** 

acid conditions [Loesche, 1986].

and residual caries.

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

diagnostic problems for secondary caries and it is very crucial to understand secondary

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

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

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

caries correctly in order to make an accurate diagnosis.

regardless of tooth morphology [Featherstone, 2004].

sound surface.

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 thoughtprovoking 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 [Mjör, 1985; Mjör, 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 Lindén, 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 [Mjör, 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 400μm meanwhile LF (infrared laser fluorescence) might be a suitable technique for the detection of secondary caries, especially for lesions over 400μm or dentinal lesions [Ando et al., 2004].
