Learning Objectives
After reading this chapter, the student should be able to:
- 1.
Describe the histology and physiology of the normal dental pulp.
- 2.
Identify etiologic factors causing pulp inflammation.
- 3.
Describe the routes of entry of microorganisms to the pulp and periapical tissues.
- 4.
Classify pulpal diseases and their clinical features.
- 5.
Describe the clinical consequences of the spread of pulpal inflammation into periapical tissues.
- 6.
Describe the histopathological diagnoses of periapical lesions of pulpal origin.
- 7.
Identify clinical signs and symptoms of acute apical periodontitis, chronic apical periodontitis, acute and chronic apical abscesses, and condensing osteitis.
- 8.
Discuss the role of residual microorganisms and host response in the outcome of endodontic treatment.
- 9.
Describe the steps involved in repair of periapical pathosis after successful root canal treatment.
Histology and Physiology of Normal Dental Pulp
The dental pulp is a unique connective tissue with vascular, lymphatic, and nervous elements that originates from neural crest cells. It resides inside the tooth in a chamber with rigid walls.
The pulp contains odontoblasts, highly specialized cells with a secretory function, which not only form dentin, but also interact with dental epithelium early in tooth development to initiate the formation of enamel. The pulp also contains fibroblasts, undifferentiated mesenchymal cells, collagen type I and II, proteoglycans, glycoproteins, and water ( Fig. 1.1 ).
The histologic structure of the pulp is important, because it reflects a unique architecture suited for the formation of dentin and defense against invading pathogens. Odontoblasts form a palisading layer that lines the walls of the pulp space, and their tubules extend about two thirds of the length of the dentinal tubules. The tubules are larger at a young age and eventually become more sclerotic as the peritubular dentin becomes thicker. The odontoblasts are primarily involved in production of mineralized dentin. They are connected by gap junctions that allow them to form a semipermeable membrane. In addition, odontoblasts play an important role in defense as they express Toll-like receptors (see later), cytokines, and defensins, among other immunologic mediators.
Two main types of sensory fibers innervate the pulp: Aδ-fibers in the periphery and C-fibers in the central pulp. The Aδ-fibers are responsible for the sharp response to thermal changes. They extend between the odontoblasts, lose their myelin sheath, and extend to a distance of 100 to 200 μm into the dentinal tubules. The C-fibers are unmyelinated and are responsible for the dull ache that affects patients with symptomatic irreversible pulpitis. The pulp may also have Aβ-fibers and sympathetic fibers in the walls of arterioles.
The pulp vasculature plays a critical role in its response to irritation. When the tooth first erupts into the oral cavity, the root apex is immature, and there is ample blood supply to the pulp. Eventually, the apex matures, and the ability of the pulp to withstand external irritation, such as from trauma or caries, diminishes. However, the pulp of the mature tooth has mechanisms to cope with increased blood flow during inflammation, such as arteriovenous anastomoses and loops that can circulate and increase volume of blood when the need arises. The pulp also contains an elaborate network of arterioles and capillaries around the odontoblasts, which are high-metabolic-rate cells, commonly known as the terminal capillary network .
Etiology of Pulpal and Periapical Diseases
Injury or irritation of pulpal or periapical tissues can result in inflammation. The reactions of the dental pulp to irritants are largely dictated by the type and duration of a stimulus. These irritants can be broadly classified as nonliving (mechanical, thermal, or chemical) or living (microbial) ( 
).
Mechanical Irritants
The potential for pulp irritation increases as more dentin is removed during deep cavity preparations because dentinal permeability is greater closer to the pulp ( Fig. 1.2 ). The removal of tooth structure without proper cooling may also cause pulp inflammation. Deep scaling and curettage may injure apical vessels and nerves, resulting in pulpal damage.
Pulpal damage can occur because of impact injuries. Teeth undergoing mild to moderate trauma and those with immature apices have a better chance of pulpal survival in comparison with those suffering severe injury or those with closed apices. Intrusion injuries are more likely to lead to pulp necrosis than are lateral or extrusion injuries ( Fig.1.3 ).
Periapical tissues can be mechanically irritated and inflamed by impact trauma, hyperocclusion, overinstrumentation of root canals, perforation of the root, and overextension of root canal filling materials ( Fig. 1.4 ). Inaccurate determination of root canal length is usually the cause of overinstrumentation and subsequent inflammation. In addition, lack of an adequate apical resistance form created during cleaning and shaping can cause overextension of filling materials into the periapical tissues, causing physical and chemical damage ( Fig. 1.5 ).
Application of forces beyond the physiologic tolerance of the periodontal ligament (PDL) during orthodontic tooth movement results in disturbance of the blood and nerve supply of the pulp tissue. , In addition, orthodontic movement may initiate resorption of the apex, usually without a change in vitality.
Chemical Irritants
Antibacterial agents, such as silver nitrate, phenol with and without camphor, and eugenol, have been used to “sterilize” dentin after cavity preparations. The effectiveness of many of these products is questionable, and their cytotoxicity can cause inflammatory changes in the underlying dental pulp. Other irritating agents include cavity cleansers, such as alcohol, chloroform, hydrogen peroxide, and various acids; chemicals present in desensitizers, cavity liners and bases; and temporary and permanent restorative materials.
Antibacterial irrigants used during cleaning and shaping of root canals, intracanal medications, and some compounds present in obturating materials are examples of potential chemical irritants to periapical tissues. , When testing the effects of antimicrobial medications on dental pulp cells, researchers showed that calcium hydroxide and lower concentrations of antibiotic pastes are conducive to cell survival and proliferation, but more concentrated forms of antibiotic pastes have detrimental effects.
Microbial Irritants
Although mechanical and chemical irritations are predominantly transient in nature, the most significant cause of inflammation is microbial. Studies have shown that even superficial carious lesions in enamel are capable of attracting inflammatory cells in the pulp. , The initial reaction of the pulp to these irritants is mediated through the innate immune response. This early response to caries results in focal accumulation of chronic inflammatory cells, such as macrophages, lymphocytes, and plasma cells. As caries progresses toward the pulp, the intensity and character of the infiltrate change. Pulpal tissue may remain inflamed for long periods and may undergo eventual or rapid necrosis. This change depends on several factors: (1) the virulence of the microorganisms; (2) the ability to circulate inflammatory fluids to avoid a marked increase in intrapulpal pressure; (3) host resistance, including genetic variations; (4) the amount of circulation and (5) an important factor, lymphatic drainage. Subsequently, microorganisms or their byproducts and other irritants from the necrotic pulp diffuse from the canal to the periapical region, resulting in the development of an inflammatory lesion ( Fig. 1.6 ).
Pulpal and periapical pathoses do not develop without the presence of bacterial contamination. , Kakehashi and collaborators created pulp exposures in conventional and germ-free rats. In the germ-free rats, minimal inflammation only occurred throughout the 72-day observation period. Further, pulpal tissue in these animals was not devitalized but rather showed calcific bridge formation by day 14, with normal tissue apical to the dentin bridge ( Fig. 1.7, A ). In contrast, infection, pulpal necrosis, and abscess formation occurred by the eighth day in conventional rats ( Fig. 1.7, B ). The bacteriological investigation by Sundqvist examining the flora of human necrotic pulps supports the findings of Kakehashi and collaborators and Möller and coworkers. Sundqvist examined previously traumatized intact teeth with necrotic pulps, with and without apical pathosis. The root canals of teeth without apical lesions were aseptic, whereas those with periapical pathosis had positive bacterial cultures.
Several mechanisms have been proposed for identification of microorganisms as irritants by the immune system. Detection of these pathogens can occur via interaction between pathogen-associated molecular patterns (PAMPs) and specific receptors broadly identified as pattern recognition receptors (PRRs). PRRs recognize PAMPs and initiate host defenses. G-protein coupled receptors and Toll-like receptors (TLRs) are part of the innate immune response and activate phagocytic functions to allow microbial ingestion. G-protein coupled receptors bind to chemokines, lipid mediators (e.g., platelet-activating factor, prostaglandin E2, and leukotriene B 4 ) or bacterial proteins, causing extravasation of leukocytes and production of bactericidal substances. TLRs are transmembrane proteins that are expressed by cells of the innate immune system playing a central role in the initiation of cellular innate immune responses. These receptors recognize invading microbes and activate signaling pathways that launch immune and inflammatory responses to destroy the invaders. At least 13 TLRs have been discovered to date with different recognition abilities. Table 1.1 presents some of the currently identified TLRs and their specific interactions.
| PAMP | PRR | Pathogen |
|---|---|---|
| LPS, Lipid A | TLR4 | Gram-negative bacteria |
| Flagellin | TLR5 | Bacteria, flagellum |
| dsRNA | TLR3 | Virus |
| RNA | TLR7,8 | Virus |
| CpG DNA | TLR9 | Bacteria, DNA |
| PAMP | PRR | Pathogen |
Microbiology of Root Canal Infections
Routes of Root Canal Infection
Under normal conditions, the dental pulp and dentin are isolated from oral microorganisms by overlying enamel and cementum. When the integrity of these protective layers is breached (e.g., as a result of caries, trauma-induced fractures and cracks, restorative procedures, congenital anomalies of teeth, scaling and root planing, attrition, or abrasion) or naturally absent (e.g., because of gaps in the cementoenamel junction at the cervical root surface), the dentin-pulp complex becomes exposed to the oral environment. The pulp then becomes at risk of infection by oral microorganisms present in caries, saliva, and dental plaque. The risk increases with the depth of lesions due to the diameter of dentinal tubules increasing as they approach the pulp (see Fig. 1.2 ).
Caries are the most common cause of pulpal exposure ( Fig. 1.8 ). However, microorganisms may also reach the pulp via direct pulpal exposure as a result of iatrogenic restorative procedures, as a result of trauma, and through a periodontal pocket extending to the apical foramen or lateral canal. After pulp necrosis, microorganisms can invade the entire root canal system uninhibited by host defense mechanisms. As a consequence of the interaction between microorganisms and the host defenses, inflammatory changes take place in the periapical tissues and give rise to the development of apical periodontitis.
Endodontic infections can be classified according to their anatomic location as intraradicular or extraradicular. Microorganisms that initially invade and colonize the necrotic pulp tissue cause primary intraradicular infections. Microorganisms that were not present in the primary infection but were introduced into the root canal system during or after initial treatment cause secondary infections. Secondary infections are suspected when a preoperative infection heals after treatment and then recurs at a later time. Persistent infections are caused by microorganisms from a primary infection that resisted intracanal antimicrobial procedures and remained in the prepared root canal system. Persistent and secondary infections are responsible for several clinical problems, including persistent exudation, continuation of symptoms, interappointment flare-ups, and failure of the endodontic treatment.
The goal of root canal treatment is to remove microorganisms from the root canal system. However, in the absence of a strict aseptic technique, microorganisms from caries and dental plaque can be introduced into the canal system during treatment because of lack of rubber dam usage or leakage of the rubber dam. Contaminated endodontic files and instruments, including delivery systems for antimicrobial agents, are additional potential sources for the introduction of microorganisms into the root canal system during treatment.
Microorganisms can enter the root canal system between appointments via loss or leakage of temporary restorative materials, fracture of the tooth structure, and teeth left open for drainage. Entry of microorganisms after root canal filling occurs by loss or leakage of temporary or permanent restorative materials, during preparation of posts or other intracanal restorations without the rubber dam, fracture of the tooth structure, and by recurrent caries that expose the root canal filling material. Leakage after completion of root canal treatment is more likely to occur if the placement of the permanent restoration is delayed.
Extraradicular infection is characterized by microbial invasion and proliferation into the inflamed periapical tissues and is almost invariably a sequel to intraradicular infection. Once the intraradicular infection is properly controlled by root canal treatment or tooth extraction and drainage of pus, the extraradicular infection can be addressed by the host defenses and usually subsides ( Fig. 1.9 ).
Endodontic Infections Are Biofilm Infections
The role of microorganisms as the primary etiologic agents of root canal infections was established in seminal studies published several decades ago. , , In these and subsequent microbiological analyses of samples recovered from root canal infections, species were isolated and identified by using planktonic (liquid) culture techniques. During the past two decades, it became apparent that microorganisms exist in the root canal system not as planktonic cultures, or as single species, but rather as multispecies biofilm communities composed of microcolonies irreversibly attached to a substratum, to an interface like dentin, and to each other. , All anatomic areas of the infected root canal system can harbor microbial cells organized as highly variable biofilm structures , ( Fig. 1.10 ).
Biofilm formation encompasses attachment of microbial cells to a surface, followed by cell proliferation, adherence to other microorganisms, production of matrix, and microcolony maturation. Dispersal of cells allows the formation of new biofilm microcolonies. Microbial cells occupy only a small proportion of the biofilm. Quorum sensing is the expression of specific microbial proteins after the bacterial cells reach a threshold number. It allows the coordinated regulation of expression of these proteins by microorganisms in biofilms to regulate population density and possibly virulence. The majority of the biofilm structure is a highly heterogeneous matrix composed of extracellular polymeric substances (EPSs) produced by cells within the biofilm. The EPS matrix provides multiple functions ( Fig. 1.11 ). From a clinical perspective, the EPS can act as a physical barrier to antimicrobial agents such as antibiotics and disinfectants. Microbial organization into multispecies biofilm communities results in increased pathogenic effects on the host.
The Microbiome of Endodontic Infections
The identity of specific microorganisms in root canal infections has been a major focus of interest for more than a century. Studies using culture-dependent approaches have recovered several species that have been identified as putative endodontic pathogens. The microbiome of carious dentin causing pulpitis and subsequent endodontic infection includes significant numbers of lactobacilli, gram-negative bacteria, and species from the Firmicutes, Actinobacteria, and Proteobacteria phyla. Primary root canal infections harbor a multispecies population of facultative and strict anaerobic gram-positive and gram-negative bacteria, spirochetes, yeasts, Archaea, and other unidentified species. In addition, Epstein-Barr virus may be associated with irreversible pulpitis and apical periodontitis, and papilloma virus and human herpes virus have been found in exudates from acute apical abscesses.
Microorganisms have been traditionally characterized in terms of their morphology (rods, cocci, spirilla), cell wall characteristics (gram-positive and gram-negative), and oxygen tolerance (anaerobic and facultatively anaerobic). Genera cultured from symptomatic and asymptomatic root canal infections include Prevotella, Porphyromonas, Fusobacterium, Peptostreptococcus, Streptococcus, Lactobacillus, Enterococcus, Actinomyces, Propionibacterium, and Candida. , ( Table 1.2 )
| Gram-Negative Bacteria | Gram-Positive Bacteria | ||
|---|---|---|---|
| Anaerobes | Facultatives | Anaerobes | Facultatives |
| Rods | Rods | ||
| Dialister Porphyromonas Tannerella Prevotella Fusobacterium Campylobacter Pyramidobacter Catonella Selenomonas Centipeda |
Capnocytophaga Eikenella Haemophilus |
Actinomyces Pseudoramibacter Filifactor Eubacterium Mogibacterium Propionibacterium Eggerthella Olsenella Bifidobacterium Slackia Atopobium Solobacterium Lactobacillus |
Actinomyces Corynebacterium Lactobacillus |
| Cocci | Cocci | ||
| Veillonella Megasphaera |
Neisseria | Parvimonas Peptostreptococcus Finegoldia Peptoniphilus Anaerococcus Streptococcus Gemella |
Streptococcus Enterococcus Granulicatella |
| Spirilla | |||
| Treponema | |||
More recently, the microbiome of endodontic infections has been redefined using culture-independent molecular biology techniques. These studies have both confirmed the findings from culture studies and greatly expanded knowledge. Many species that had already been considered putative pathogens because of their frequency of detection, as reported by culture-dependent methods, have been found in a similar or even higher prevalence by using molecular approaches, strengthening associations with causation of apical periodontitis. Molecular technology has enabled the recognition of many new putative pathogens that had not previously been found in samples from endodontic infections. , A review of 12 studies that used next-generation DNA sequencing (pyrosequencing) methods to evaluate the microbiome of endodontic infections has corroborated previous multiple reports of microbial diversity. The most abundant phyla were Firmicutes, Actinobacteria, Bacteroidetes, Proteobacteria, and Fusobacteria. The most frequently detected genera were Prevotella, Fusobacterium, Porphyromonas, Parvimonas , and Streptococcus. ( Fig. 1.12 )
Many microorganisms isolated from endodontic infections have also been identified as commensals in the oral cavity. The transition from oral “commensal” to root canal “pathogen” likely reflects an innate ability to switch on genes that enable survival and propagation in a different environment and encode a range of virulence factors ( Fig. 1.13 ). The first reported virulence factor associated with endodontic infections was lipopolysaccharide (“endotoxin”), a virulence factor produced by gram-negative bacteria.
It has been suggested that symptoms increase when certain microbial species are part of the infective endodontic microbiome. Nevertheless, the same species can be found in asymptomatic cases with prevalence comparable to that of symptomatic cases; this apparent discrepancy could be explained in part by the variations in expression of virulence factors by different strains of the same species. Protein analyses (the metaproteome) of endodontic infections, along with the host response, are future steps toward better understanding of interactions between the microbiome of endodontic infections and the host throughout the infection and healing process. ,
Pulpal Diseases
Host Response in the Dental Pulp
The response of the dental pulp to microbial and other physical and chemical irritants is similar to the response in other connective tissues. An inflammatory process starts in the pulp and corresponds to the location where the irritation reaches it. For example, in an incipient carious lesion at the depth of an occlusal fissure, the pulp at the end of the affected dentinal tubules is seen to have a small inflammatory process histologically. This inflammation progresses throughout the coronal pulp as the carious lesion penetrates deeper in dentin, until the microbial irritants eventually invade the pulp in large numbers and cause severe inflammation ( Fig. 1.14 ).
However, unlike other connective tissues, the dental pulp lacks collateral circulation and is confined within rigid dentinal walls. Therefore at a specific point in the disease process, the inflammation changes from reversible (one that would respond favorably to conservative methods of treatment and eventually heal) to irreversible pulpitis. The transition of reversible to irreversible pulpitis is important to identify clinically because it determines the optimal procedure that should be employed to treat it.
Studies have shown that the inflammatory response in the pulp is associated with several cellular and molecular changes ( 
). The degree of irritation appears to trigger a corresponding level of inflammation. This titration of the inflammatory response to the level of irritation is orchestrated by a balance of proinflammatory and antiinflammatory factors in the pulp. The condition of the pulp deteriorates or improves based on the reaction of pulpal factors to the external environment. Cellular responses include the increase in inflammatory cells, most notably neutrophils, lymphocytes, macrophages, plasma cells, mast cells, and dendritic cells ( Figs. 1.15 to 1.17 ). The inflammatory cell response correlates in its intensity with the depth of the carious lesion. Noninflammatory cells, such as odontoblasts and fibroblasts, do contribute to the inflammatory response. Odontoblasts have been shown to express TLRs, cytokines, chemokines, and defensins. However, the degree to which noninflammatory cells contribute to the inflammation is much less than inflammatory cells like neutrophils and macrophages. A recent study showed reasonably good agreement between clinical signs and symptoms, and the histologic condition of the pulp in cases with caries. In this study, microbial ingress into the pulp, which is indicative of severe pathosis, seemed to be confined to cases diagnosed clinically as irreversible pulpitis.
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