Invasive Therapy: 18 How Much Caries Do We Have to Remove?
So far, this book has covered the mode of action, effectiveness, and also the implementation, of noninvasive and microinvasive treatment options. As described in Chapters 1 and 2, the carious process is ubiquitous and common to us all; for many, the subclinical changes that occur at the crystal level do not manifest themselves at the clinical level. This is because the carious process is a dynamic process of demineralization, which is capable of arrest and even remineralization, depending upon the environmental conditions (presence of plaque biofilm, sugar substrate, fluoride, etc.). Once a lesion is detectable at its earliest possible stage, the lesion and patient should be managed from a preventive (non-invasive) perspective—this cannot be emphasized enough. Nonetheless, micro-invasive options should be considered when applicable(see Chapters 15–17). But invasive intervention should only be considered if these more conservative options are suspected to have failed and/or a lesion has progressed to such a degree that caries removal and restoration are indicated.
Unfortunately, the painless non- and microinvasive techniques do not work in every patient, for every caries lesion, or for a whole lifetime. However, ironically, dental caries is one of the most common diseases of mankind and yet there is little evidence to suggest where, along the continuum of the disease process from early white spot lesion to frank cavitation, invasive intervention should be considered. This is reflected in the great variation in caries detection, especially on the approximal and occlusal surfaces, the diagnostic threshold for invasive intervention and, in turn, on the number of plans for restorative care among dentists.1–3 While great variation exists, the decision to operatively intervene is critical, as restorations cannot be regarded as permanent, as a large proportion of a dentist′s time is spent re-restoring teeth, and the decision to do so is equally as idiosyncratic as the decision to restore primary caries in the first instance4 (see Chapter 20). The more restorations a patient has, the more re-restoration will take place, and the extent to which carious enamel and dentin is removed varies considerably between dentists, as well. Each time a restoration is replaced the cavity gets larger, compromising the tooth. The decision as to when and how to restore a caries lesion and so enter the restorative cycle is therefore an important one.
This chapter will cover in detail:
Criteria to be used in deciding when to restore a caries lesion
The histopathology of dentin caries and its relationship to appropriate caries removal
The microbiology and pulp–dentin complex reactions associated with dentin caries and how these can be exploited in the future to enable the management of caries to move from a surgical approach to one based upon the microbiology and histopathology of the lesion
The stepwise excavation technique to reduce the risk of pulp exposure
One of the first dentists to comprehensively describe cavity preparation in relation to dental caries was G. V. Black (1836–1915).5 He was able to do this through meticulous study and research of teeth, their morphology, where carious lesions mainly occurred, and the histopathological spread of the disease. G. V. Black also mapped out tooth surfaces into those which were particularly susceptible to caries and those which appeared to be immune. Following caries removal, further preparation of tooth tissue was recommended to extend the cavity margins into immune areas; the so-called G. V. Black′s philosophy of “extension for prevention.”
This had a particular impact on cavity preparation for occlusal and approximal surfaces. On the occlusal surface once all of the carious dentin had been removed at a specific site, it was recommended that the entire fissure system should be prepared with a bur so that the remaining susceptible fissure could also be restored ( Fig. 18.1 ). Fissures were also frequently excised in sound teeth and restored as a caries preventive measure, the so-called prophylactic odontotomy. For the approximal lesion, once caries had been removed, the cavity on the approximal surface was further extended buccally and lingually into the cleansable embrasure space, and subgingivally into a caries immune area. Once the approximal box was complete, extension into the occlusal fissure system was performed not only for retention and resistance form, but also for “prevention” ( Fig. 18.2 ).
With regard to the question of “how much caries do we need to remove?” G. V. Black believed that “generally when the cavity has been cut to form, no carious dentin will remain.” Not only would no carious tooth tissue remain, but considerable sound tooth tissue removal would have taken place, leading to the classic angular and often extensive cavities.
There was little change in cavity preparation until this was reviewed by Elderton in 1984.6 Emphasis was placed on gaining access to caries and its removal, but no longer was extension-for-prevention thought to be necessary, bearing in mind the extensive removal of sound tooth tissue and the short-lived nature of many restorations. Sound pits and fissures could now be protected with fissure sealants and retention to approximal boxes could be achieved with buccal, lingual, and gingival grooves. The advent of adhesive restorative materials also precluded the need for further cavity preparation for retention; more conservative preparations with rounded outlines were the result ( Fig. 18.2 ).
The extension-for-prevention philosophy was established by G. V Black when no sealant procedure and/or adhesive restorations could be performed. This meant that margins of a restoration were placed in areas where no caries should occur.
When to Remove Caries?
An enamel lesion is the reflection of the carious process taking place in the biofilm on the surface of the tooth and therefore should be treated noninvasively, by disrupting the biofilm with regular oral hygiene procedures in the presence of fluoride (Chapters 9–13). Where enamel caries is concerned, G. V. Black wrote that microorganisms from the biofilm “never enter the tissue until the rods are loosened and fall out. The enamel is a solid into which microorganisms cannot penetrate.” More recent evidence suggests that microorganisms can penetrate noncavitated enamel lesions; however, their numbers within the tooth tissue are unlikely to have an impact on caries progression alone.7 A noncavitated enamel lesion therefore does not require caries removal unless it is for esthetic reasons.
The enamel–dentin junction (EDJ) is an important threshold for those investigating caries detection devices and in determining disease prevalence in a population—however, while it would be a convenient threshold upon which to base invasive intervention, it is not a logical one. The majority of coronal lesions extending up to the EDJ and into the outer dentin will be noncavitated and can still be managed non- or microinvasively (see Chapters 9, 17, and 20).
Numbers of microorganisms within noncavitated enamel lesions are low and only play a minor role with respect to caries progression.
Frank cavitation is an obvious disease state in which invasive intervention could be justified, as cariogenic bacteria can readily invade the tooth structure and, hidden from conventional oral hygiene procedures, can proliferate to a critical mass of biofilm to facilitate lesion progression. Apart from the approximal surfaces, cavitation can readily be detected by examination of clean, dry teeth. While the radiograph is important in the detection of approximal caries, it is often not possible to determine whether the lesion is cavitated or not. For occlusal and approximal lesions it is well established that the actual depth of the lesion will be greater than it appears radiographically. Bearing this in mind only 8%–47% of those approximal lesions with a radiolucency up to the EDJ (and deeper clinically) will be cavitated,8,9 and in some studies even as few as 35% with a radiolucency into the outer half of dentin were found to be cavitated.8 A radio-lucency up to the EDJ or just into the dentin alone should not therefore prompt invasive intervention for approximal surfaces on the assumption that they are cavitated.
Predicting whether a radiographically visible approximal lesion will be cavitated or not is difficult. High caries risk and gingival inflammation of the interdental papilla may help, as they are associated with a higher risk of cavitation.10,11 If in doubt the clinician could separate the teeth with orthodontic separators for direct visual–tactile assessment ( Fig. 18.3 ) or use a thin probe (cow-end probe or briault probe) without separation.
Microbial Invasion of Dentin
The invaginated anatomy of the occlusal surface of the tooth means that the initial enamel lesions occur at the entrance to the pits and fissures and within its walls. These lesions spread laterally into dentin and coalesce beneath the base of the fissure, eventually spreading into dentin on a wide front. Some professionals think that fluoride makes the occlusal enamel harder and more resistant to collapse and as such cavitation occurs at a late stage in the carious process ( Fig. 18.4 ). Throughout the carious process demineralization precedes bacterial invasion of the tooth tissues. Therefore at some stage between the occlusal lesion spreading into dentin and becoming frankly cavitated, the lesion in dentin becomes heavily infected and it is thought that this is the factor that should prompt invasive intervention. But what features will predict infection of the dentin?
In a clinical microbiological study, pits and fissures deemed as being carious to various degrees and in need of invasive intervention were investigated by a single operator.12 The visual appearance of the investigation site was scored on a basic visual scoring system that was acceptable at the time, and bitewing radiographs were scored as to whether a radiolucency was present or not, and if so, how deep. Under rubber dam isolation the enamel above the dentin lesion was carefully removed and the dentin assessed for color, consistency, and moisture content. Demineralization of the dentin was determined with a caries detector dye and a dentin sample was taken for microbial analysis. While the visual appearance of the pits and fissures was found not to be a good predictor of infected dentin, lesions visible as a radiolucency in dentin were heavily infected compared with those that were not radiographically visible, and the deeper the lesions appeared on the radiograph the heavier the bacterial load12 ( Fig. 18.5 ). Interestingly, the color of the dentin at cavity preparation had no relationship with the bacterial infection, whereas those lesions that were hard to a probe and dry harbored significantly fewer bacteria than those that were soft to the probe and wet ( Fig. 18.6 ). This finding will have a significant bearing during caries removal.
The appearance of a radiolucency into dentin on the occlusal surface seems to be the single best predictor of the need for operative intervention based upon microbial invasion.
No discussion of how much caries should be removed is possible without reviewing the work by Fusayama and colleagues on dentin caries. If infection of the dentin is a factor that should prompt invasive intervention, it would seem logical that all the infected dentin should be removed during cavity preparation. In the mid-sixties Fusayama et al.13 were concerned about the prolonged viability of organisms left in residual caries beneath restorations. As such they performed an experiment to determine the relationship between several variables and bacterial invasion. They found that softening of the dentin, by bacterial acids diffusing deeper into the dentin, was followed by discoloration of the dentin and then by bacterial invasion. In slowly progressing lesions the discolored area was generally harder than in rapidly progressing lesions.
In a series of further studies two layers of carious dentin were described that could be differentiated by using a basic fuchsin dye.14–18 The first layer was the outermost layer closest to the EDJ. The carious dentin of this layer was severely demineralized and the collagen fibers were denatured with irregular inorganic crystals randomly scattered throughout. This layer was found to be heavily infected with bacteria, hence the frequently quoted term infected zone of carious dentin, and was incapable of remineralization. The second layer of carious dentin was described at the advancing front of the lesion, closest to the pulp and was found to be demineralized, but not to the same extent as the first layer, and the collagen fibers were sound with more cross-linkages and apatite crystals bound to them. This layer, often termed the caries affected zone, was not found to be infected and was capable of remineralization. The basic fuchsin dye (caries detector) was found to stain only the first layer of carious dentin and its use during cavity preparation would ensure that all of the infected dentin caries was removed, in fact “excavation guided by this staining method was always deeper than the bacterial invasion.”17 It was also found that the difference in excavation depth and the depth of bacterial invasion was greater in more active rapidly progressing lesions than less active chronic lesions.
Fusayama et al. distinguished between the infected and affected zones in dentin caries. They promoted a “caries detector” that was supposed to stain the infected zone for clinical caries removal.
Conventional Caries Removal
Conventional cavity preparation has therefore involved gaining access to dentin caries by removing superficial enamel, usually with a high speed bur, and then to render the periphery of the cavity (at the EDJ or outer 1–2mm of dentin if involving the root) completely caries-free by removing all soft and/or stained dentin with a slow round bur. Following on from this, careful removal of pulpal caries is performed with an excavator until hard dentin is reached. It has long been accepted that in deep cavities, hard but stained dentin can be left pulpally, based on the findings that staining precedes bacterial invasion.13 This can then be lined with a calcium hydroxide lining forming the so-called indirect pulp cap (in deep cavities). Some clinicians still advocate the use of caries detector dyes to ensure complete removal of all infected carious dentin. Because of the potential carcinogenicity of basic fuchsin, it is no longer used and has been replaced with 1% acid red in propylene glycol and other protein dyes.
This conventional caries removal and cavity preparation has been performed for more than a century is still done today in dental clinics worldwide; however, there is rapidly growing evidence that questions numerous aspects of this time-honored approach.
Classical caries removal in dentin usually terminates at hard but stained dentin. In addition, a caries detector dye is advocated occasionally to check that infected parts are removed thoroughly.