It should not be underestimated that almost all previous histological studies of the pulp have been based on demineralized histological sections in order to be able to cut thin sections. However, during the decalcification procedure of the tooth specimen, not only the mineral content of the dentine but also the entire enamel is removed. Consequently, important information about the lesion environment has therefore been lost. This may have led to several examples of oversimplification in the understanding of caries pathology:

Consequently, it was taught that even a small enamel lesion without histological contact to the enamel-dentine junction could induce both the translucent zone and the demineralized zone in the dentine [13] and in particular the non-cavitated enamel lesion could hide bacterial invasion [14] as well as undermine sound enamel from the very beginning, even without visible dentine exposure [12–17]. Therefore, many dentists of the last century were trained to both remove the early enamel lesion by operative intervention, as well as to be radical when performing dentine excavation, because carious dentine left over was believed to maintain the pulp inflammation and eventually lead to pulp degradation. The description of the changes in the pulp has often been closely related to scenarios of “points of no returns” in terms of inflammation. Clinically, the so-called deep caries progression should therefore not be treated with an indirect pulp capping [4, 11], as the retained carious tissue would maintain further development of pulp inflammation. In contrast, complete excavation was recommended even if it led to exposure of the pulp, because the pulp was judged to be irreversibly inflamed anyway, even though the patient was not in pain. Moreover, if already an enamel lesion is able to induce inflammation, it would be easy to imagine that a deep lesion would be associated with unwanted inflammation.

However, the advances of pulp biology [18–23] combined with at detailed description of the lesion environment, as well as the clinical evidence of treating deep lesions [24–28], have started to modify the traditional view of pulp inflammation as being “the nonstop train” toward apoptosis and pulp necrosis.

A brief update of principal caries pathology is presented. Detailed histomorphological studies have revealed that the carious enamel-dentine lesion complex is a closed entity as long as there is no destruction and disintegration of the demineralized enamel [8]. The cariogenic biomass is located at the enamel surface only. The enamel lesion contact with the enamel-dentine junction is strictly related to the extent of the demineralized dentine, and no early spread along the enamel-dentine junction can be monitored (Fig. 9.2a). When the established caries remain untreated, they advance in width and depth. The dentine is clinically exposed (Fig. 9.1c, d) and the biofilm is heavily involving the cavity. The microbial ecosystem can be described as being a “closed” environment. At this stage the spread along the enamel-dentine junction is a characteristic feature undermining sound enamel [29] and the lesion becomes wedge shaped, with the base directed toward the surface (Fig. 9.2b). As the deep lesion further progresses, the undermined enamel often breaks off (Fig. 9.1d–f), due to masticatory forces, converting the lesion environment from a “closed” toward a more “open” lesion environment [30]. Clinically and macroscopically the color of the demineralized dentine becomes darker (Fig. 9.2c), and at the central area the cariogenic biofilms overlying the lesion are markedly reduced. Although the lesion has increased in size, the actual activity has temporarily decreased. Even at the macroscopic level, progressive alterations of pulp inflammation can be observed in terms of increasing visualization of the vascular architecture in the pulp (Fig. 9.2a–c).

Studies have shown that the cytoplasm/nucleus ratio of the mature odontoblast cells, subjacent the superficial enamel lesion, is more reduced than unaffected control sites, even before visible alteration in dentine mineralization. Moreover, the subodontoblastic or Höehl’s layer appears indistinct [26]. Enhanced mineralization of the dentine is noted in the central and oldest part of the involved lesion area, as the enamel lesion progresses toward the enamel-dentine junction. At the pulpal site substantial growth of the predentine matrix is noted with bundles of collagen fibers aligned with odontoblasts reduced in size [8]. These first findings of cellular alterations and enhanced mineralization of the dentine are probably not associated with antigen-related pulp reactions as the bacterial-induced dentine demineralization is not yet established.

It is well known that the dentine comprises bioactive extracellular matrix [31–35], and during carious demineralization of the dentine, there is a release of bioactive molecules [22, 36, 37], which can trigger the odontoblasts, members of the first line of host defense (see Chap. 7). However, it is unclear whether this may take place even before bacteria have invaded the demineralized dentine (Fig. 9.2a), also defined as affected dentine [38]. In non-cavitated enamel lesions with subjacent dentine demineralization in humans, observations of a reduced odontoblast layer, as well as the alterations in the subodontoblastic region, may qualitatively reflect an early odontoblast-triggered innate immune response [18]. Moreover, a different pulp response is noted subjacent an active versus a slowly progressing lesion. The cytoplasm/nucleus ratio of the odontoblasts appears markedly reduced in both scenarios, but a maintained indistinct subodontoblastic region is noted only in active sites [31].

In a series of in vitro studies, the odontoblast-like cells were shown to express Toll-like receptors [39–42] making the cells capable to induce mediators known to influence positively or negatively both the inflammatory as well as the immune responses in pathogen-challenged pulp-like tissue. As an example lipoteichoic acid, which is a by-product from Gram-positive bacteria, was able to elicit proinflammatory cytokines by further promoting immature dendritic cell recruitment [42].

The Gram-positive bacteria represent the first members of invading bacteria in demineralized dentine with a clinical dentine exposure (Fig. 9.2a), eventually accounting for 70 % of the cultivable microbiota in lesion sizes involving two-thirds of the dentine and more [43]. During the development of such a “closed” ecosystem, a remarkable homogeneous lactobacilli microflora can be detected. Gram-negative bacteria take over [44] as the lesion advances, and, because of their content of lipopolysaccharide, they are capable of inducing lipopolysaccharide–binding protein which has an even more complex role in terms of triggering the odontoblast-like cells. Recently, it was found that this protein may interact with lipoteichoic acid, hence reducing the receptor-dependent production of inflammatory cytokines by odontoblast-like cells and in this way modulate host defense in human dental pulp [45]. The various laboratory setups [18–22, 32–37, 39–42] with, for example, the odontoblast-like cells have helped gain more and more complex information, eventually leading to an improved understanding of the host defense as well as of tooth repair in the future. However, we are not there yet, in terms of the application of knowledge transformed into treatment modalities in humans.

The established caries lesion with a visible dentine exposure (Fig. 9.1b, c) will comprise all the classical zones of carious dentine: the zone of sclerotic dentine (i.e., increased intratubular mineralization), the dentine demineralization, and, finally, the zone of bacterial invasion and dentine degradation [8]. When the bacteria invade demineralized dentine and it becomes infected [38], the vast majority of the members of the biofilm are Gram-positive bacteria in primary dentine and the innate immune systems are progressively activated. It is not known how the triggered dental pulp immunity is operating during a slow lesion progression. As tested during stepwise excavation of dentin caries, the cultivable microflora becomes markedly reduced in numbers [46], consequently the acidogenic pH levels decrease, which presumably also leads to decreased production of, for example, lipoteichoic acid. Finally, this may temporarily stop the sequence of events leading to a further unwanted inflammatory response. In confirming this, the tertiary dentine appears like a continuous formation of secondary dentine laid down along a slow lesion progression, whereas the tertiary dentine appears partly atubular during ongoing stages of active lesion progress (Fig. 9.3).

Taken together, the odontoblasts are members of the first line of defense responding to various carious stages and activities. How this modulates the role of the odontoblasts during early tertiary dentine formation in humans remains unclear, but it is detailed in Chap. 10.

A definition of deep caries has to be made by the x-ray, because it is within the clinical setting that the general dental practitioners will end up using the findings of laboratory research, and therefore it is of paramount importance to have a reference that can be used in a clinical setting. As the depth of the caries lesion represents a diagnostic problem, it may be relevant to further classify it beyond previous attempts. The deep lesion has previously been defined radiographically as being within the pulpal quarter toward actual contact with the pulp [47]. However, we suggest separating the deep lesion in two scenarios. A deep caries lesion is defined as involving the pulpal quarter of the primary/secondary dentine (Figs. 9.4b. 9.5a, and 9.6a) but still with a radiodense zone separating the demineralized dentine [19]. The extreme deep lesion involves the entire primary/secondary dentine either with no radiodense zone separating the demineralized dentine from the pulp or with a radiodense zone located within the pulp chamber indicative of tertiary dentine (Figs. 9.4a, 9.7a, and 9.8a).