Domenico Ricucci: Biological aspects in conservative dentistry

Endodontics - Interview
2014-05-18

Biological aspects in Conservative Dentistry

As it was anticipated in the previous interview, Dr.Domenico Ricucci starts his co-operation with Style Italiano. The following is the first of a series of monographs on the biological aspects in Conservative Dentistry

Dentin and pulp response to caries. Histologic and histobacteriologic aspects.

This article deals with pulp reactions from initial caries to increasingly extensive caries. Tissue response is studied using conventional light microscopy. For an exhaustive description of the histologic methodology adopted, including fixation, demineralization, microtomy, staining procedures, etc, the readers are referred to other publications (1).

In the past, there has been some disagreement regarding how early a pulpal response to caries can arise (2) mainly because of the limitations of the histologic methods available.
Using conventional light microscopy, after complete demineralization of teeth, in the presence of the so called “white spot” involving varying portions of the enamel thickness (Fig. 1A), the only pathological changes, in an otherwise normal pulp architecture (Fig. 1B), are represented by the presence of some capillaries filled with red blood cells and some inflammatory cells in the subodontoblastic zone (Figs. 1C – 1D) (in the completely normal pulp, capillaries do not contain cells and cannot be seen in tissue sections).

NO TREATMENT IS USUALLY ACCOMPLISHED IN THE PRESENCE OF A WHITE SPOT ONLY.

When the caries lesion reaches dentin, the effects on the pulp become pronounced. The type and intensity of the pulp response to caries usually depends on the depth of the caries lesion in dentin. Therefore, for ease of discussion, the caries lesions will be classified as shallow, medium and deep (3).

Shallow caries

Clinically, the earliest indication of caries is the presence of a discoloration of the fissures as well as a “white spot” on an occlusal fissure or on a proximal surface (Fig. 2A). Cavitation may be absent, but the enamel is already disorganized and unable to resist the diffusion of acids from the bacterial biofilm (Figs. 3A – 3B). As a consequence, the outer dentin may be involved even at an early stage. (3-6). The process of dentin demineralization starts after the enamel demineralization has reached the dentin-enamel junction (DEJ). In these enamel lesions without cavitation, the initial dentin demineralization may occur without the presence of bacteria in the dentin tissue (Fig. 3F). This is because, at an early stage, only acids from the bacterial plaque biofilm can diffuse through the disorganized enamel prisms, but not the whole bacterial cells. When the enamel breaks up at a later stage, bacterial invasion of the dentinal tubules begins.

When bacteria or bacterial by-products reach the peripheral end of dentinal tubules and start to colonize them, the most common reactions observed on the pulpal side consist of an accumulation of inflammatory cells at the point underneath the involved dentinal tubules (Figs. 2B – 2C) and a disruption of the odontoblast layer with a reduction in the number of odontoblasts in the affected area (Fig. 2C). These changes are evident if the affected area is compared with the adjacent unaffected zones. At this stage, bacteria are located only in the most superficial portion of dentinal tubules. Radiographic examination may sometimes reveal a minor area of radiolucency in the enamel, but quite often, the early involvement of the enamel cannot be diagnosed radiographically.

Another aspect is the formation of a certain amount of tertiary dentin in the area where the dentinal tubules involved by the caries process end in the pulp (Figs. 3C – 3E). Tertiary dentin shows less tubules compared to secondary dentin and is lined by a reduced layer of odontoblasts, which have lost their cylindrical shape and appear considerably flattened (Figs. 3D – 3E). This dentin, being less tubular and thus less permeable, has to be considered as an attempt to limit the egression of bacterial toxins into the pulp.

Initial caries is usually not accompanied by any clinical symptoms. THE INITIAL PULP INFLAMMATION PROMPTLY SUBSIDES WHEN THERAPEUTIC MEASURES ARE UNDERTAKEN, NAMELY COMPLETE REMOVAL OF THE CARIES LESION, WHILE TERTIARY DENTIN WILL REMAIN PERMANENTLY IN THAT PULP.

Medium caries

With time and if not treated or controlled, caries progresses in width and depth (Figs. 4A – 4B; 5A). Bacteria progress towards the pulp, affecting a larger area of the dentinal surface (Figs. 4C – 4D) and undermining the enamel. In general, an increase in the number of inflammatory cells is noted only adjacent to the area where the affected dentinal tubules end in the pulp (Fig. 4E), while a few scattered inflammatory cells can be observed at the centre of the pulp tissue, with the rest of the pulp tissue usually showing normality. A common finding is the formation of a certain amount of irritation (tertiary) dentin on the pulp chamber walls underlying the carious process (Figs. 5B -5C). This dentin presented with a few tubules and lined on the pulp side by a thin layer of flattened odontoblasts, which have lost their typical columnar appearance (Fig. 5C). The few tubules present house the cytoplasmic processes of the odontoblasts surviving the insult. In addition, a marked increase of dystrophic calcifications may be observed in the pulp chamber, in the root canals and on the root canal walls. Symptoms are usually absent, but sensitivity to chewing and cold stimuli may be present.

SIMILAR TO THE INITIAL CARIES, COMPLETE REMOVAL OF DEGRADED AND INFECTED DENTINAL TISSUE IN MEDIUM CARIES, FOLLOWED BY THE PLACEMENT OF A RESTORATIVE MATERIAL WITH ADEQUATE SEALING ABILITY SETS THE GROUND FOR THE PULP HEALING PROCESS.

It is important to clarify what is meant by the healing of a pulp that has suffered a carious attack of certain intensity. The complete removal of the degraded and infected tissues will result in a remission of the pulp tissue inflammation, but the return to the normal histological conditions will not occur. It means that the irritation dentin formed at the expense of the pulp in response to the carious attack, and the reduction in the number of odontoblasts in the affected pulp areas will remain definitely. The odontoblast is a terminal cell, which cannot be regenerated by mitosis or differentiation from other cells. (4,7-8) These histological features, however, do not impact on the future viability of the pulp, provided new bacterial aggression does not take place.

Deep caries

When the caries process remains untreated, it advances in width and depth. Both peritubular and intertubular dentin are involved. The lesion is generally wedge-shaped, with the base directed toward the surface (Fig. 6A). Peripherally, most of the dentin mass is infiltrated by bacteria (Figs. 6B – 6C). An important aspect related to pulp response is the formation of a larger amount of tertiary dentin, which is deposited over the pulpal end of the dentinal tubules affected by caries (Fig. 6D). Tertiary dentin may have several synonyms, including irregular dentin, reparative dentin, and irritation dentin. The term irritation dentin seems more appropriate as it indicates that this tissue is formed in response to a pathogenic stimulus in a context of inflammation (8). This dentin is irregular and its quantity is not predictable. It is crossed by a reduced number of tubules with irregular course. Portions of the pulp tissue may remain entrapped in the newly formed calcified tissue, giving rise to finger-like inclusions (Fig. 8C). It should be noted that, contrary to what is commonly believed, irritation dentin offers only a modest resistance to the advance of caries lesions, and in advanced stages of the disease process it is penetrated by bacteria in the same way as the secondary dentin (Fig. 7A). The variability and unpredictability of this situation are so high that in some areas tertiary dentin formation is not observed at all, while in adjacent areas it is abundant and covered only by spindle-shaped fibroblasts arranged obliquely to the tubules.

FROM A CLINICAL POINT OF VIEW, WHEN BACTERIA ARE CLOSE TO THE PULP, BUT ARE STILL CONFINED TO DENTIN, PULP INFLAMMATORY REACTIONS, EVEN SEVERE, MAY REGRESS IF TREATMENT COMPLETELY REMOVES THE INFECTED AND DEGRADED DENTINAL TISSUE. THERE IS, TO A CERTAIN POINT, A DEGREE OF REVERSIBILITY OF PULP INFLAMMATION, WITH IRRITATION DENTIN REMAINING AS A PERMANENT “SCAR” OF PREVIOUS INFLAMMATION. However, diagnosing this borderline histologic condition (reversible/irreversible) by clinical means is very difficult and often misleading.

Caries exposure and irreversibility of pulpal inflammation

Once the pulp is directly exposed by caries and bacteria have invaded the pulp tissue, a focus of necrosis with limited extension is formed in the pulp tissue. It should be emphasized that despite the presence of bacteria in the pulp horn and a severe acute inflammation in the surrounding areas, the rest of the pulp chamber, as well as the radicular pulp, will usually show no signs of inflammation (Fig. 8C). The necrotic area is surrounded by a dense accumulation of polymorphonuclear neutrophils (PMNs) and acellular tissue remnants, indicative of partial liquefaction (Figs. 8D – 8F). Further away from the centre of the destruction there is a typical chronic inflammatory response with a large number of plasma cells, small and large lymphocytes, macrophages, fibroblasts, mast cells and foam cells (4).

FROM A HISTOLOGICAL POINT OF VIEW, THE OCCURRENCE OF AN AREA OF ​​NECROSIS, ALTHOUGH OF LIMITED EXTENSION, CONSTITUTES THE POINT OF TRANSITION FROM A REVERSIBLE TO AN IRREVERSIBLE INFLAMMATORY STATE.

A more advanced stage of the initial pulp degeneration process is depicted in Figs. 9A- 9D). The mandibular third molar of a 45-year old woman had a deep occlusal caries. The symptoms reported were a previous history of spontaneous pain. Percussion was negative. Radiographically an extensive occlusal caries was visible, but there was no evidence of apical periodontitis (Fig. 9A). The tooth was extracted and processed for light microscopy. An overview shows a considerable amount of irritation dentin formed under the pulp chamber roof (Fig. 9B). In some sections the distal portion of the pulp chamber was occupied by a severe accumulation of inflammatory cells, which obscured the pulp architecture, but no necrotic areas could be seen (Fig. 9B). Approximately one hundred sections away an area of coagulation necrosis was present in the distal pulp horn, colonized by bacteria (Fig. 9C). Higher magnification showed that bacteria were arranged in a biofilm structure (Fig. 9D). It is interesting to note that necrosis was limited to the distal portion of the pulp chamber, while the pulp tissue in the mesial half exhibited histologic characteristics of normality. This case confirms the importance of the serial sectioning procedure in the histological analysis of tooth specimens. Observing only the section in Fig. 9B the diagnosis would have been “reversible pulp inflammation”; instead, the section taken 100 sections away (Fig. 9C) allowed the correct diagnosis of “irreversible pulp inflammation”.

It is clear that the treatment of such a condition cannot include measures aiming at conservation of the diseased pulp, and a more aggressive approach has to be adopted, ie, a pulpectomy. Once again, from a clinical point of view, the problem lies in our inability to diagnose by clinical means the actual histologic condition of the pulp. In the everyday clinic, during excavation of carious tissue in a case with such a histologic condition, the clinician can be misled by bleeding from an exposed pulp horn and consider direct pulp capping. It is easy to understand how this treatment measure would “seal” bacteria in the pulp chamber, creating the bases for future problems.

Progression of the pulp degeneration

The initial area of necrosis slowly expands to involve increasing areas of the coronal pulp. It has to be emphasized that the necrotic tissue colonized by bacteria is clearly demarcated from the adjacent tissue, which continues to be vital and relatively normal (Figs. 10A – 10B). An important feature is an increase of dystrophic calcifications in the remaining pulp tissue. Pain may sometimes accompany these processes, but it can be completely absent.

It is important to emphasize that pulp necrosis (and infection) is a slow process that gradually moves in an apical direction. Histologic observation demonstrates that the “strangulation theory” is groundless. This old theory was based on the belief that one artery would enter through the foramen, and two veins would exit. Following the increase in the pulpal pressure caused by inflammation, the veins would be compressed, avoiding the reflux of fluids from the pulp space. At a certain point, the neurovascular bundle would be strangulated and the pulp would undergo necrosis. This theory implied that pulp degeneration is a quick process, caused by the sudden interruption of circulation. Contrary to this, histologic sections of carious teeth demonstrate that pulp degeneration progresses very slowly. In addition, nourishment for the pulp is assured by numerous vessels that enter and exit through a multitude of foramina in the apical third.

The last stage of pulp destruction related to caries begins when necrosis and bacteria penetrate beyond the root canal orifices. These phases of the pulp degeneration process will not be described here. Readers are referred to (1, 3) (Fig. 11).

Fig. 1

Fig. 1A. Maxillary third molar from a 30-year-old woman. The tooth is grinded on a bucco-lingual plane until one or two pulp horns is encountered. This allows proper fixation of the pulp tissue and the right orientation of the specimen in the paraffin block.

Fig. 2

Fig. 1B. Overview of the pulp chamber shows normal characteristics of the pulp tissue (H&E, ×16).

Fig. 3

Fig. 1C. Detail of the area of the dentin-pulp interface indicated by the arrow in Fig. 1B. Some capillaries can be seen, filled with red blood cells (×400).

Fig. 4

Fig. 1D. High power view showing a capillary with red blood cells and an acute inflammatory cell (polymorphonuclear leukocyte) (×1000).

Fig. 5

Fig. 2A. Mandibular third molar extracted for pericoronitis. White spot lesions and discoloration of the occlusal fissures. No cavities at probing.

Fig. 6

Fig. 2B. Histologic section cut on a mesio-distal plane (H&E, ×16).

Fig. 7

Fig. 2C. High power view of the area indicated by the arrow in Fig. 2B. Moderate accumulation of mononuclear inflammatory cells in the subodontoblastic region of a limited area of the pulp (×400).

Fig. 8

Fig. 3A. Maxillary third molar of a 32-year-old woman. The occlusal surface shows extensive discoloration of the occlusal fissures and accumulation of bacterial plaque. No cavitation could be disclosed at probing.

Fig. 9

Fig. 3B. Preparation of a sectioning plane confirms that the enamel is present, although whitish in colour, indicating disorganization of enamel prisms. Note the dark stained area in the subjacent dentin.

Fig. 10

Fig. 3C. Overview of the dentin and the pulp chamber. A band of tertiary dentin is present. The dark dentinal area in Fig. 3B is stained darker with H&E (×16).

Fig. 11

Fig. 3D. Detail of the tertiary dentin. This tissue appears less tubular and with a reduced layer of odontoblasts (×100).

Fig. 12

Fig. 3E. At high power view only a single layer of odontoblasts can be observed. These appear extremely flattened, and can be recognized as odontoblasts only because they send their process (cell body) into a dentinal tubule (×400).

Fig. 13

Fig. 3F. Section proximal to that shown in Fig. 3C. Bacterial staining demonstrates that the dark dentinal area in Fig. 3B is not colonized by bacteria (Taylor’s modified Brown & Brenn, ×16).

Fig. 14

Fig. 4A. Medium caries. Maxillary third molar in a 27-year-old man. The mesial aspect of the crown shows initial cavitation.

Fig. 15

Fig. 4B. The tooth is grinded on a mesio-distal plane.

Fig. 16

Fig. 4C. Sections show that a thick bacterial biofilm is in contact with the dentin on the mesial aspect. Note bacterial invasion of the dentinal tubules (Taylor’s modified Brown & Brenn, ×16).

Fig. 17

Fig. 4D. High power view from the affected dentin surface. Heavy colonization of the superficial dentin tubules. Note that bacteria also colonize lateral branches of dentinal tubules (×400).

Fig. 18

Fig. 4E. Area where the involved dentinal tubules end to the pulp. Only minor accumulation of chronic inflammatory cells can be observed at this stage (×100).

Fig. 19

Fig. 5A. Medium caries. Maxillary second molar in a 50-year-old woman with a mesial caries lesion. Note that the enamel is undermnined.

Fig. 20

Fig. 5B. A layer of tertiary dentin can be observed on the mesial aspect of the pulp chamber, subjacent to the caries (×16).

Fig. 21

Fig. 5C. Detail of the dentin-pulp interface. Tertiary dentin with few remaining tubules and a single layer of flattened odontoblasts (×100).

Fig. 22

Fig. 6A. Deep caries. Maxillary third molar of a 44-year-old man.

Fig. 23

Fig. 6B. Carious dentin (Taylor’s modified Brown & Brenn, ×16).

Fig. 24

Fig. 6C. Detail of the carious dentin. The dentin mass is heavily infiltrated by bacteria (×400).

Fig. 25
Fig. 26

Fig. 7A. Deep caries. In advanced stages tertiary dentin is also penetrated by bacteria (Taylor’s modified Brown & Brenn, ×16).

Fig. 27

Fig. 8A. Irreversibility of pulp inflammation. Maxillary third molar in a 33-year-old woman seeking treatment for severe spontaneous pain. A large occlusal cavity is present.

Fig. 28

Fig. 8B. A periapical radiograph showed that the caries lesion was proximal to the pulp chamber. The tooth was extracted.

Fig. 29

Fig. 8C. Overview of the pulp chamber. A small apparently empty space can be seen in a pulp horn area. The rest of the pulp tissue exhibits a relatively normal appearance. Note the large amount of tertiary dentin, with finger-like inclusions (H&E, ×16).

Fig. 30

Fig. 8D. Section proximal to the one in Fig. 8C. Carious dentin is heavily infiltrated by bacteria (Taylor’s modified Brown & Brenn, ×16).