Ashraf F. Fouad and Asma A. Khan
3.1 Introduction
This chapter will address the etiological factors and the pathological processes involved in the development of pulpitis and apical periodontitis. The sources of microbial irritation of the dental pulp are diverse, and may arise from the coronal (most common), lateral or apical routes. Microbial irritation of the dental pulp has a profound effect on this tissue, often giving rise to severe symptoms, but frequently leading to the asymptomatic degeneration of the tissue. Likewise, apical periodontitis may be asymptomatic, may be associated with severe symptoms, and may be associated with spreading, occasionally life-threatening infections. The exact reasons for the variation of symptoms among cases with seemingly similar pathological changes, and the lack of correlation between symptoms and histological appearance of the dental pulp, are not fully understood. While many neuropeptides and other inflammatory mediators, which are associated with pain, are found in large concentrations in painful pulpitis and apical periodontitis, it is not clear why they increase in some cases. Frequently, the pulp becomes necrotic without a notable association with symptoms.
Diseases of the pulp and periapical tissues result from an inflammation of the respective tissues, ultimately leading to degeneration and necrosis of the pulp, periapical bone resorption, the development of an inflammatory periapical lesion, possibly cystic formation, and possibly severe infections and/or osteomyelitis. In some situations, root resorption of various types may occur, either directly related to a traumatic injury or orthodontic treatment, or insidiously for known or unknown reasons. It is generally accepted that root resorption follows a similar pathogenesis as bone resorption, after the disruption of the cementum that protects the root surface or the predentine internally. All these pathological entities are variants of inflammatory reactions, and are mediated by a wide variety of cytokines, chemokines, neuropeptides, proteases, and other inflammation-associated molecules. The development of neoplastic changes is not known within the pulp, even in response to meta- static affection of the jawbones. In these cases, the periapical bone may be resorbed, but the pulp generally maintains its vitality. This fact is of importance in the clinical differentiation of pulpal disease from other disease that may mimic endodontic pathosis, but does not originate from the dental pulp. It is interesting why the pulp appears to be immune from developing neoplasms, which if present, would have resulted in cases of spontaneous pulp necrosis or neoplastic disease of pulpal origin.
Animal studies have shown that once the pulp is exposed to oral bacteria, pulp necrosis and the development of apical periodontitis are rapid, and generally occur within a few weeks in different mammalian models. The bacterial population within the pulp space continues to evolve following necrosis, depending on environmental, compositional and nutritional factors. It is important to note that the pathogenesis of disease and response to treatment may depend on the duration of microbial irritation. Prolonged microbial irritation, in primary infections or following incomplete treatment, affects the spatial distribution of the microbial biofilms and their potential expansion into the complex root canal environment, the dentinal tubules, and potentially the periapical region. The effective- ness of endodontic disinfection decreases in these locations, and therefore, the response to treatment may be slower and less robust, than if the tooth were to be extracted.
3.2 Etiology of Pulpitis and Apical Periodontitis
Microbial infections from carious lesions are the most common etiology for pulpitis and apical periodontitis. In normal conditions, the crowns of erupted teeth are covered by biofilms composed of symbiotic microbial communities. In sugar-rich environments, specific bacterial taxa from these communi- ties release acids that demineralize dentin, resulting in the formation of carious lesions. While Streptococcus mutans was considered the singular pathogen in caries for a long time, recent studies suggest that caries is caused by a complex microbiota [39, 128, 149]. Carious lesions limited to the enamel induce subtle changes in the pulp such as accumulation of MHC Class II antigen-expressing cells. Once the caries biofilm destroys enamel and reaches dentin, it induces further inflammatory changes in the pulp [90, 91, 177, 232-234].
The pulp may also be directly exposed to microorganisms and their byproducts by routes other than carious lesions, for example in cases of traumatic injuries resulting in complicated tooth fractures or in situations of iatrogenic pulp exposures. The role of periodontal disease in causing pulpal inflammation and necrosis remains controversial [81, 152]. Some studies suggest that periodontal disease induces the influx of inflammatory cells and pulpal calcifications [115, 184]. However, total necrosis of the pulp was only noted in teeth where all the main apical foramina were infected with bacterial plaque. A preclinical study in which marginal periodontitis was experimentally induced reported mild pathological pulpal changes in 57% of teeth with periodontitis [13]. Pulpal necrosis was noted in only one tooth out of the 92 teeth in the experimental group. Other studies suggest that periodontal disease does not induce changes in the pulp [137].
One of the seminal studies on the role of bacterial infection in pulpal and periapical disease compared the outcomes of pulpal exposures between germ-free and normal rats [98]. In the gnotobiotic rats, the pulps remained vital and the apical tissues remained normal during the study period. In this group of rats, formation of complete dentinal bridges was noted by 28 days after the pulpal expo- sure. In contrast, the normal rats developed pulpal necrosis early in the study period, which was followed by the development of periapical abscesses and granulomas. Another seminal study on infections of the root canal system included one that examined the association between different taxa. In this primate study, the root canal systems of teeth were infected with different combinations of bacterial species. This was the first study to show that taxa form multispecies communities in periapical tissues which elicit stronger inflammatory responses when combined [50]. These findings that pulpal and periapical diseases are caused by polymicrobial infections that form biofilms are supported by several other studies [12, 50, 165, 208].
The development of sophisticated molecular tools have further improved our understanding of the polymicrobial nature of pulpal and periapical infections [59]. For example, a clinical observational study examined samples collected from the deepest layer of the dentinal caries lesions associated with pulpal exposures in teeth with symptomatic irreversible pulpitis (N=10) [170]. Half of the advanced caries lesions were highly dominated by lactobacilli. In the remainder, the most dominant genera were Pseudoramibacter, Olsenella, Streptococcus and Stenotrophomonas. Conversely, other studies have reported Prevotella as the dominant genus in deep dentinal caries [29, 106, 133, 182]. These differences may be due to methodological issues or may be related to geographical differences in the composition of the oral microbiota [11].
The integration of data sets from culture and molecular studies show that over 460 unique bacterial taxa belonging to 100 genera and nine phyla have been identified in endodontic infections. The predominant phyla include Firmicutes, Bacteroidetes, Actinobacteria and Proteobacteria [191]. These studies also show that the microbial diversity in endodontic infections is not limited to bacteria; instead, it also includes fungi, archaea [219, 220], and viruses [28, 47, 82, 120, 158].
While microbial infection is the predominant etiology for pulpal inflammation, procedures such as cavity preparation and restorations can also adversely affect the pulp. Enamel and dentin are among the hardest tissues in the body and the heat generated during cavity preparation can potentially induce inflammation of the pulp [116, 183, 197]. Most current restorative material do not adversely affect the pulp as long as the tissue is not directly exposed to them. However, microleakage around resto- rations can induce pulpal inflammation [73].
3.3 Inflammation Versus Infection of the Pulp and Periapical Tissues
3.3.1 Bacterial Irritation and Invasion of the Dental Pulp
Coronal sources of pulpal irritation such as caries, coronal cracks and fractures, attrition, abrasion erosion, or congenital defects in the crown lead to pulp inflammation. Even early pit and fissure caries is associated with inflammation that corresponds with the pulpal extent of the affected dentinal tubules [20]. Likewise, exposed root surface that does not have cementum coverage allows bacterial invasion of the external surface of dentinal tubules and result in mild inflammation in the corresponding area of the pulp [114]. In these cases, the bacteria may not be physically present in the pulp, but secrete enzymes, toxins or cell wall molecules that travel through the tubules and cause pulpal irritation. As the bacterial irritation approaches the pulp, the inflammation increases in intensity in the zone closest to the irritants, and eventually progresses to distant areas of the pulp. The degree of pulpal inflammation correlates with the amount of bacterial irritants and their proximity to the pulp [140, 165]. Eventually, bacterial infection reaches the pulp and progresses through the vital pulp, leading to pulpal degeneration and initiating periapical inflammation. The same process of inflammation followed by potential infection eventually occurs in the periapical t issues, particularly after complete pulp necrosis. The apical infection may result in an acute or chronic abscess formation, particularly when virulent bacteria reach the periapical region in sufficient numbers. Therefore, both the pulp and the periapical tissues undergo a stage of inflammatory changes, before becoming physically infected by bacteria. One of the main aim of the diagnostic process clinically is to determine whether the diseased pulp and periapex are inflamed or infected, because this would impact anesthetic considerations, emergency and definitive treatment, as well as the determination of prognosis.
Studies have shown that when exposed to oral bacteria, pulpal inflammation and eventual degeneration are generally mediated by innate immunity. The evidence for this comes from murine models, where it was shown that the rate of pulp necrosis is similar in animals deficient in adaptive immune cells, such as T- and B-cells, and normal animals [55, 222]. However, adaptive immunity, particularly B-cells and immunoglobulins, are protective against severe infections caused by large numbers of pathogenic bacteria [212].
Potent bacterial molecules such as bacterial peptidoglycan, lipopolysaccharide (LPS) (also known as endotoxin) from Gram- negative bacteria, and lipoteichoic acid (LTA) from Gram-positive bacteria are capable of inducing significant pulpal and periapical inflammation. For example, animal studies have shown that LPS placement in otherwise sterile root canals induces periapical lesions [34, 45]. LPS is a potent mediator of bone resorption [23], that acts via activation of the arachidonic acid pathway, cytokine production, and complement activation [56]. Moreover, LPS has been shown to activate TLR4 receptors on trigeminal nociceptors [221], and is clinically associated with pain when present in carious lesions [104] and infected pulp [84, 92, 134]. However, LPS is by no means the only bacterial irritant involved in the pathogenesis of pulpitis and apical periodontitis. Studies that examined the progression of pulpal and apical disease in LPS hyporesponsive models showed comparable overall progression of the disease to normal animals, indicating the LPS, peptidoglycan, bacterial proteinases, toxins, and other virulence factors may contribute to the process [60].
Clinically, periapical bone loss is usually visible radiographically when the pulp is necrotic. This is a surprising finding, as periapical inflammation is seen histologically in animal studies during the period of pulp inflammation. Indeed, it has been shown that the clinical phenomenon is related to the low sensitivity of the periapical radiograph in detecting early apical lesions, and that more sensitive techniques, such as cone beam computed tomography (CBCT) is significantly more sensitive than periapical radiography in detecting apical periodontitis in cases with pulpitis [1]. Therefore, the pulp does not have to be totally necrotic for periapical bone resorption, and the formation of a periapical lesion to develop.
3.4 The Dental Pulp
3.4.1 The Defense System in the Dental Pulp
The dental pulp is well equipped to detect invading pathogens and to mount an immune response to them. As with all other tissues in the body, the defense system of the pulp can be classified into the innate and the adaptive immune responses. The innate responses are not antigen-specific and include the outward flow of dentinal fluid and the deposition of intratubular immunoglobulins [130, 136, 155]. It also includes the resident cells (such as odontoblasts and fibroblasts) and innate immune cells (such as dendritic cells, natural killer cells and T cells). Odontoblasts are found at the interface of the pulp-dentin junction and have processes which extent into dentinal tubules. While the primary function of odontoblasts is to produce the predentin matrix during tooth formation and to control its mineralization, they also play an important role in the immune response of the pulp. It has been hypothesized that they represent the first biologically active line of defense for the pulp and, that their role is similar to that of the skin and mucosal epithelial cells. Odontoblasts express Toll-like receptors (TLRs), a class of pattern recognition receptors. Specifically, they express TLRs-1, 2, 4, 6 and 9 [44, 95, 217]. Stimulation of these TLRs activates the MyD88 dependent pathway which triggers NF-kB and MAPK activation, resulting in the expression of pro-inflammatory and chemokine genes such as CCL2, CCL5, CCL7, CXCL8, and CXCL10 [199]. Odontoblasts also express nucleotide-binding-domain leucine-rich repeat (NLR)-family proteins such as NOD2, which recognize peptidoglycan components common to both gram-positive and gram-negative bacteria [198]. Activated odontoblasts release molecules such as nitric oxide (NO), beta defensins (BD), and liposaccharide binding protein (LBP), IL-6, CXCL1, CXCL 2, CXCL 8, CCL2 and IL-10 [41, 42, 44, 111, 154, 217]. Some of these diffuse through the dentin towards the carious lesion to destroy the pathogens (such as NO and BD) or to reduce their pathogenicity (such as LBP). Others diffuse into the pulp where they activate and mobilize immune cells [44] (Figure 3.1).
Fibroblasts also play a role in the immune defense of the pulp, which is in addition to their role in the synthesis and turnover of the extracellular matrix. They express several pattern recognition receptors such as TLRs- 2, 4, 5, and NOD1,2 [83, 103, 126, 199]. Once activated they release mediators such as TNFa, CXCL8, and others [102]. An interesting study compared the responses of fibroblasts and odontoblast-like cells to various TLR agonists [199]. Lipoteichoic acid (LTA), a TLR2 specific agonist, induced upregulation of CXCL-2 and -10 in odontoblast-like cells but not in fibroblasts. While LPS, a TLR4 specific agonist, induced expression of CCL7, CCL26 and CXCL11 in fibroblasts but not in odontoblast- like cells, thus suggesting that odontoblasts and fibroblasts mount specific immune responses by differentially influencing the various immune cells in the pulp.
Resident immune cells in the pulp include leucocytes, mononuclear phagocytes dendritic cells, and natural killer (NK) cells [71]. These cells are continuously sampling their environment to detect invading pathogens. Once a pathogen is detected the number of immune cells increases dramatically. Neutrophils are among the earliest cells, which are recruited followed by monocytes, which differentiate into macrophages [32, 80, 90, 131]. Dendritic cells and NK cells also accumulate at the site of infection where they capture bacterial antigens or destroy the invading pathogen [44, 96, 101, 129, 233]. Interactions between dendritic cells and NK cells result in reciprocal activation and increased cytokine production by both cell types. These innate immune cells also activate the adaptive immune response [107, 224]. Dendritic cells migrate to regional lymph nodes where they present antigens to and activate CD4+ T cells (aka Th0 cells) [17].
In addition to the innate immune responses described above, the nerves innervating the pulp also detect and respond to pathogens. An elegant study using human and rat trigeminal neurons as well as rat dental pulps demonstrated the expression of CD14 and TLR4 in nerves innervating the pulp [221]. Subsequent studies demonstrated that LPS sensitizes the ion channel TRPV1 via activation of TLR4 in trigeminal sensory neurons [40, 52]. These studies along with earlier ones on the neuronal responses to pulpal exposures provide further support of the role of neurogenic inflammation in pulpitis [105, 108, 211].
Figure 3.1 Illustration of the two key aspects of the defense mechanism mounted by odontoblasts. In response to bacteria (B) in carious dentin, odontoblasts (blue) release mediators such as nitric oxide (NO), beta defensins (BDs), liposaccharide binding protein (LBP), IL-6, CXCL1, CXCL 2, CXCL 8, CCL2 and Il-10. Some of these mediators diffuse through the dentin towards the carious lesion to destroy the pathogens or to reduce their pathogenicity. Others diffuse into the pulp where they activate and mobilize immune cells. (Reproduced with permission from [51].)
3.4.1.1 Adaptive Immunity
As mentioned earlier, dendritic cells activate CD4+ T cells in lymph nodes. The latter then differentiate into effector CD+4 helper cells or induced regulatory T cells (Figure 3.2). As the carious lesion progresses toward the pulp, there is an increase in the accumulation of T cells [50, 79, 91]. Healthy pulps contain a small number of B-cells, which increase in number with caries progression and inflammation [71, 79]. The predominant B-cells derived immunoglobulins in inflamed pulps are IgG1 followed by IgA and IgE [30, 80].
3.4.2 Classification of Pulpitis
The most commonly accepted diagnostic classifications are based on our current under- standing on treatment prognosis. Diagnosis is based on the patients' symptoms (presence, duration, severity, and type), the presence of caries or restorations, the response to pulp sensibility tests, and the clinical and radiographic exams. A normal pulp is one in which the tooth is asymptomatic, responds to all clinical tests (palpation, percussion, cold, or electrical stimuli) within normal limits and has a normal radiographic appearance. Normal pulps may contain calcifications.
Inflammation of the pulp is commonly categorized into reversible and irreversible pulpitis. The former refers to a state of mild inflammation that can be “reversed" In other words, the pulp is capable of healing if the appropriate therapy, such as removal of an irritant, is performed. The etiologies for reversible pulpitis include caries, trauma and a recent or defective restoration. Clinical exams reveal absence of palpation and percussion sensitivity, a brief, non-lingering response to thermal stimuli, and occasionally pain on biting. The radiographic appearance of teeth with reversible pulpitis is normal.
When pulpal inflammation is more severe, and the prognosis is that it is unlikely to revert to a normal pulp, the pulpal state is classified as “irreversible pulpitis'! These teeth are often symptomatic and are associated with spontaneous pain and/ or pain that lingers after the removal of a thermal stimulus. The teeth may be sensitive to percussion and often have a normal radiographic appearance.
Figure 3.2 Illustration of the role of dendritic cells (DCs) in activating the adaptive immune response. Immature DCs usually become mature DCs after encountering antigens (Ag). Mature DCs present antigens to naïve CD4+ cells which then clonally expand and differentiate into effector T-helper cells (Thl, Th2 or Th7) or into induced regulatory T cells (iT-reg). Alternatively, immature DC mature partially to form Tolerogenic-DCs (Tol-DC) which in turn induce iT-reg cell differentiation. TGF - Transforming Growth Factor; IL - interleukin; IFN - interferon, Ig - immunoglobulin. (Reproduced with permission from [51].)
Etiological factors include deep caries, deep restorations, and cracks. These pulps are subclassified as “symptomatic irreversible pulpitis'. In contrast, some teeth with carious exposures or trauma present with no symptoms. Based on our current understanding, these inflamed pulps have a poor prognosis and are expected to become necrotic. These pulps are subclassified as “asymptomatic irreversible pulpitis'.
At this point there is inadequate evidence to support that irreversible pulpitis is truly irreversible. Based purely on patients' pain history and the diagnostic tools currently available, it is not possible to determine whether the degenerative inflammatory process involves the entire pulp or whether it is limited to only a part ofthe pulp. As described later in this chapter, the histological evaluation of human pulps clinically diagnosed as “irreversibly' inflamed noted that in some teeth the pulp tissue in one, but not all, pulp horns was inflamed [167]. With the development of new pulp-capping materials, it now appears that partial or full pulpotomy is a viable treatment option and that extirpation of the entire pulp may not be necessary in teeth with irreversible pulpitis [8, 48].
3.4.3 Histological and Molecular Evaluation of Pulpitis
As mentioned earlier, multiple cells in the pulp express pathogen recognition receptors and respond rapidly to microbial infection. A recent study examined the histological and molecular response to pulpal injury and infection by exposing rat pulps to PBS or LPS [166]. The tissues were examined at 3 hours, 9 hours, and 3 days after the exposure. Pulps in intact teeth were used as controls. Infiltration by inflammatory cells was seen as early as 9 hours after exposure and an osteodentine matrix was noted 3 days after the exposure. As compared to pulps exposed to PBS, the area of the pulp infiltrated by inflammatory cells and the osteodentine deposition was larger in pulps exposed to LPS (Figure 3.3). Similar changes were noted with the expression of dentin sialophospho-protein. Flow cytometry analysis revealed an increase of leukocytes and dendritic cells, while the percentage of T cells and NK cells remained unchanged. The same study also examined the expression of inflammatory genes. Treatment with LPS increased expression of IL-6, IL-1 p, IL-10, TNFa, iNOS, CCL2, CXCL1, and CXCL2 at the 3-hour time point. MMP3 was upregulated at 9 hours and 3 days after exposure to LPS.
A clinical observational study examined the histologic status of pulps clinically diagnosed as being irreversibly inflamed (n=32), reversibly inflamed (n=59) and normal (n=4) [167]. A majority (84.4%) of pulps diagnosed as being irreversibly inflamed displayed areas of coagulation or liquefaction necrosis with bacterial colonization and severe infiltration by leucocytes in parts of the coronal pulp. The inflammatory/immune reactions were much less severe in what remained of the coronal pulp and in some teeth; normal uninflamed tissues were noted in the contralateral pulp horns (Figure 3.4).
The remainder of the pulps diagnosed as irreversibly inflamed (15.6%) displayed localized accumulation of inflammatory cells with no tissue necrosis and no bacterial infections - a histological diagnosis consistent with reversible pulpitis. In almost all (96.6%) the pulps diagnosed with reversible pulpitis, the histological diagnosis was consistent with the clinical diagnosis. A mild to moderate accumulation of chronic inflammatory cells and formation of tertiary dentin was noted in these pulps.
Another clinical observational study examined gene expression in pulpitis patients presenting with mild to severe pain as well as in normal teeth [67]. Genes involved in immune response, cytokine- cytokine receptor interaction and signaling, integrin cell surface interactions, and others were expressed at relatively higher levels in the pulpitis group as compared to normal pulps (Figures 3.5 and 3.6).
Moreover, several genes known to modulate pain and inflammation showed differential expression in asymptomatic and mild pain patients (>30mm on the Visual Analog Scale) compared to those with moderate to severe pain (Table 3.1).
Figure 3.3 Masson trichrome staining and immunohistochemical analysis of rat pulps exposed to LPS or PBS. Unexposed pulps were used as controls. The arrow in panel (a) show the site at which the tooth was amputated, and pulp exposed. As compared to PBS (d: low magnification; e: higher magnification), inflammatory exudate was greater after exposure to LPS (g: low magnification; h: higher magnification). Wound healing with deposition of a collagenous matrix (j: low magnification; k higher magnification) was noted by day 3 in the PBS group. Expression of dentin sialophosphoprotein (DSPP) increased at the 3-hour and 9-hour time points (r). (Reproduced with permission from [166].)
Figure 3.4 Histological analysis of a tooth clinically diagnosed with irreversible pulpitis. The patient, a 30-year-old man presented with severe pain. (a) Periapical radiograph of the tooth. (b) Preparation of a bucco-lingual plane for sectioning. (c) An overview of the pulp chamber showing the abscess. Note that the contralateral pulp horn appears normal. (d) Magnified view of the abscess. Note necrotic debris (original magnification 16x). (e) Partial view of the abscess (original magnification 40x). (f) Higher magnification of the rectangular area in E (original magnification 400x). Inset A high power view (original magnification 400x) of the abscess indicated by the arrow in D. Note accumulation of PMNs and bacteria. (Reproduced with permission from [167].)
Figure 3.5 Gene Set Enrichment Analysis results between pulpitis and normal samples. Inflamed pulps were collected from patients diagnosed with irreversible pulpitis (n=20). Normal pulps from teeth extracted for various reasons served as controls (n=20). Genome-wide microarray analysis was performed using Affymetrix GeneTitan Multichannel Instrument. Each bar represents the functional categories and the number of significantly regulated genes between pulpitis and normal groups (q<0.05). (Reproduced with permission from [67].)
Figure 3.6 Gene Set Enrichment Analysis results between groups that reported none to mild pain (>30mm on VAS) and those that reported moderate to severe pain on VAS associated with pulpitis. Samples were collected and analyzed as described in Figure 3.5. Each bar represents the functional categories and the number of significantly regulated genes between none to mild and moderate to severe pain groups (q<0.05). (Reproduced with permission from [67].)
A limited number of clinical studies have examined molecular markers in pulpal blood at the protein levels. The immunoglobulins IgG, IgA, Ig M as well as elastase and PGE2 are elevated in inflamed pulp as compared to healthy pulps [2]. Expression of the cytokine IL-8 is increased in inflamed symptomatic pulps as compared to normal pulps and asymptomatic pulps with caries exposure [49]. The content of the dentinal fluid may also be reflective of molecular changes in the pulp. An interesting clinical study compared the expression of MMP9 in dentinal fluids collected from teeth diagnosed with irreversible pulpitis (n=19) as compared to normal controls (n=12) [235]. MMP9 is a proteolytic enzyme secreted by neutrophils and may be a marker of tissue destruction. The data show a clear increase in levels of MMP9 in the symptomatic teeth as compared to controls.
Table 3.1 Selected up-regulated or down-regulated genes in pulpitis patients. Black - down-regulated; Red - upregulated; Green - no difference.
|
Gene symbol |
Pulpitis vs normal |
None to mild vs moderate to severe pain |
Gene function (genecards.org) |
|
AMELX |
Enamel biomineralization |
||
|
CALCRL |
Bone metabolism |
||
|
CCL20 |
Chemotaxis |
||
|
CD14 |
Innate immune response |
||
|
CD163 |
Acute-phase receptor |
||
|
CD79A |
B-cell receptor function |
||
|
COL10A1 |
Collagen formation |
||
|
COL11A2 |
Collagen formation |
||
|
COL12A1 |
Collagen formation |
||
|
COL14A1 |
Collagen formation |
||
|
COL15A1 |
Collagen formation |
||
|
COL15A1 |
Collagen formation |
||
|
COL18A1 |
Collagen formation |
||
|
COL1A1 |
Collagen formation |
||
|
COL1A2 |
Collagen formation |
||
|
COL21A1 |
Collagen formation |
||
|
COL4A1 |
Collagen formation |
||
|
COL4A2 |
Collagen formation |
||
|
CXCL3 |
Chemotaxis |
||
|
DEFA1B/1A |
Antimicrobial activity |
||
|
DEFA3 |
Antimicrobial activity |
||
|
DSPP |
Dentin formation |
||
|
IL10RA |
IL-10 signaling |
||
|
IL1A |
Inflammatory response |
||
|
IL1B |
Inflammatory response |
||
|
IL6 |
Inflammation, B-cell maturation |
||
|
IL8 |
Chemotaxis |
||
|
LBP |
Innate immune response |
||
|
MMP13 |
Collagen degradation |
||
|
MMP20 |
Amelogenin degradation |
||
|
MMP9 |
Collagen degradation |
||
|
NOD2 |
Innate immune response |
||
|
NR5A2 |
Antiviral activity |
||
|
PTGS2 |
Prostaglandin formation |
||
|
SCN8A |
Sodium ion permeability |
||
|
TLR1 |
Innate immune response |
||
|
TLR2 |
Innate immune response |
||
|
TLR3 |
Innate immune response |
||
|
TLR4 |
Innate immune response |
||
|
TLR6 |
Innate immune response |
||
|
TLR8 |
Innate immune response |
||
|
TLR9 |
Innate immune response |
||
|
TNFA |
Inflammatory response |
The role of epigenetic modifications in pulpitis is only just beginning to be explored. Epigenetic modifications include DNA methylation, modifications of histones and regulation of non-coding RNAs. DNA methylation in pulpal inflammation has only been examined in two studies to date. One of these was on the methylation pattern of the IFN у and noted partial methylation or un- methylation in samples of inflamed human pulps [25]. Another study reported no difference in the methylation of CD14 and TLR2 in normal and inflamed pulps [24]. Recent studies suggest that histone methylation on H3K27 plays a role in pulpal inflammation and reparative processes [89, 230].
MicroRNAs (miRs) are small (18-22 nucleotides), single-stranded, non-coding RNA oligonucleotides. Our lab was the first to report on miR expression in inflamed pulps [236]. Inflamed pulps were from carious teeth diagnosed with symptomatic or asymptomatic irreversible pulpitis (n=18). Normal pulps were extirpated from healthy third molars or teeth extracted for orthodontic purpose (n=12). As compared to normal pulps, 36 human miRs were dysregulated in inflamed pulps. +In a separate in vitro study, we demonstrated that one of the microRNAs differentially expressed in pulpitis- miR181a modulates expression of the cytokine IL-8 [68], thus demonstrating the role of microRNA in modulating the immune response to microbial infections of the pulp.
3.5 The Periapical Tissues
3.5.1 The Defense System in the Periapical Tissues
As noted before, periapical bone resorption and the formation of a periapical lesion start before the pulp is totally necrotic. As the irritants progress within the pulp, it is important that apical bone resorbs and is replaced by a soft tissue, which can mount a formidable immune response. The essential function of the periapical lesion is to defend the body against advancing bacteria from the degenerating pulp. In so doing, the periapical lesion defends against two serious diseases: acute spreading infections and osteomyelitis (Figure 3.7).
While these conditions may occur with some frequency, their incidence and severity are certainly modulated by the apical immune response in most cases. The development of an apical lesion that can be observed radiographically in patients takes a few weeks to months. In animal models the size of the lesion increases steadily and then plateaus as the bacterial irritation and the defense mechanisms reach a stable stage [64]. In some cases, periapical lesions can reach very large sizes (Figure 3.8). It is thought that the size of the lesion may be related to the types and numbers of bacterial irritants, the development of an apical cyst, and the types and concentrations of inflammatory mediators within the lesion. The available evidence suggests that the ultimate size of the lesion is a regulated mechanism that is related to a large number of microbial and host factors [200].
Figure 3.7 Radiograph is of a patient who had non-surgical endodontic treatments on the mandibular left canine and first and second premolars. The molar was normally responsive. The patient started experiencing numbness in the area, and root-end surgery was performed on the premolars. The biopsy report showed intense inflammatory response and multiple bone fragments with empty lacunae. The diagnosis was osteomyelitis.
3.5.1.1 Acute Infections
Acute endodontic infections represent about 56% of all non-traumatic dental emergencies [163]. In the U.S., endodontic infections result in over 400,000 emergency room outpatient visits per year [148]. Endodontic abscesses result in about 8,000 hospitalizations per year [5], was the cause of death for 66 hospitalized patients over a period of 8 years, according to one study [185]. Given that in the U.S., 27-41% of the adult population have at least one tooth with apical periodontitis [22], it is evident that despite the large numbers, severe, life-threatening endodontic infections affect a small number of cases. The more common clinical presentation for symptomatic cases, are ones where there is symptomatic apical periodontitis associated with symptomatic irreversible pulpitis or with necrotic pulp, or acute apical abscesses with localized swelling, lymphadenopathy and minimal constitutional symptoms. According to one study, 57% of all cases of symptomatic irreversible pulpitis are associated with symptomatic apical periodontitis [151]. Conversely, it was shown that about 40% of cases with pulp necrosis and chronic apical periodontitis develop with minimal pain [139].
It is not clear what specific factors are involved in the development of symptomatic apical periodontitis or severe endodontic infections. Numerous studies have explored the possibility that specific virulent endodontic pathogens may be associated with endodontic infections. For example, several studies have shown the association of Fusobacterium nucleatum and Parvimonas micra [27, 61, 178], and black-pigmented bacteria belonging to the genera Prevotella and Porphyromonas [75, 77, 208, 209] with pain. Molecular studies, which employ more sensitive techniques, have shown that spirochetes that are members of the Treponema spp. (mainly T. denticola) are significantly associated with the presence of pain and apical acute infections, particularly in primary infections [117, 176, 189]. The association between this microorganism and acute endodontic infections is an example of the accuracy and sensitivity of molecular micro- biology, because members of the Treponema genus, as are many other spirochetes, are very fastidious and thus difficult to identify by culturing [58]. Molecular studies have also identified other genera of microorganisms that are associated with pain, such as Dialister, Filifactor, Olsenella, Granulicatella, and Synergistes [171, 192]. However, contemporary deep sequencing of bacterial samples from endodontic infections has clearly revealed that the bacterial community harbors hundreds or thousands of bacterial taxa [87, 121, 190]. Moreover, these studies have shown that the relative abundance of the most prevalent individual taxa is very low, and there are no dominant microorganisms present. Therefore, it is not clear how the microbial community profile influences the development of symptoms. It is also likely that these complex microbial communities interact differently with the host response in different patients, making the exact identification or true culprits rather challenging.
Figure 3.8 Teenage patient diagnosed with pulp necrosis and asymptomatic apical periodontitis on the maxillary left lateral incisor. The patient did not return for treatment until a year later. Note the substantial increase in lesion size within that time. (Reproduced with permission from [57].)
Viral infections may exacerbate or potentiate symptoms in endodontic infections. Earlier studies showed that infection with human cytomegalovirus or Epstein-Barr virus was associated with larger and symptomatic periapical lesions [174, 175, 195]. However, a recent systematic review and meta-analysis revealed no significant associations between these viruses and symptoms [93]. It is noteworthy that varicella zoster infection is frequently followed by a post-herpetic pain that could present clinically as dental pain. This viral infection has been associated with the pathogenesis of pulpal and apical pathosis in several case reports, case series, and a recent systematic review [76, 94, 156, 187].
3.5.1.2 Chronic Infections
Apical periodontitis appears to be a very prevalent disease among dental patient and non-patient populations (Table 3.2).
As noted before, this is further evidence that most endodontic pathosis is asymptomatic, and represents a homeostatic relationship between the microbial irritants and the host response. However, chronic or asymptomatic disease may exacerbate, resulting in acute infections. It is thought that this occurs due to changes in the composition of the microbial community, or in bacterial load. It may also be related to changes in the host response, such as if the immune system becomes stressed or compromised. Very little objective data is available in this area from clinical studies, because of the ethical difficulty of studying lesion progression without treatment on patients.
Table 3.2 Prevalence of apical lesions related to at least one tooth in the adult population in selected representative studies. The asterisk denotes dental clinic populations.
|
Bergstrom et al. 1987 [15] |
(Sweden) |
48% |
|
Odesjo et al. 1990 [150] |
(Sweden) |
43% |
|
De Cleen et al. 1993 [35] |
(Netherlands) |
45%* |
|
Saunders et al. 1997 [181] |
(Scotland) |
41%* |
|
Kirkevang et al. 2001 [109] |
(Denmark) |
42% |
|
Boucher et al. 2002 [19] |
(France) |
63%* |
|
Kabak and Abbott 2005 [97] |
(Belarus) |
80%* |
|
Palmqvist 1986 [153] |
(Sweden) (>65y/o) |
72% |
|
Ainamo et al. 1994 [4] |
(Finland) (>75y/o) |
41% |
|
Caplan et al., 2006 [22] |
(USA) (<45y/o) |
27% |
|
Caplan et al., 2006 [22] |
(USA) (>45y/o) |
41% |
A large volume of data is available from animal models or the role of various immunological factors on the size of chronic periapical lesions. Most of these animal models are of chronic periapical lesions, which are the typical lesions that develop upon pulp exposure and extirpation. Animals may develop acute infections in rare situations such as uncontrolled diabetes mellitus [62] or in severe combined immunodeficiency when they are also inoculated with endodontic pathogens [212]. This tendency for a chronic lesion development under normal conditions illustrates the success of the immune responses in preventing severe spreading infections or osteomyelitis due to periapical infections. However, lesions vary in size and rate of progression in these models, depending on a variety of microbial and host response factors.
Animals with deficiency in TLR-4, which recognizes LPS, have smaller periapical lesions at some time points but not others [60, 85]. Interestingly, animals deficient in TLR-2, which recognized lipoteichoic acid of Gram- positive bacteria, have larger, not smaller, induced periapical lesions than controls [33]. It was recently reported that TLR-2 deficiency may enhance CD-14 and TLR-4 signaling, which may explain the increased periapical lesion size in these cases [169]. Furthermore, endodontic pathogens, like F. nucleatum, may cause cytokine production, which is mediated by the p38 mitogen activated protein kinase (MAPK) signaling that is independent of TLRs, nucleotide-binding oligomerization domain-1 (NOD-1), NOD-2 and nuclear factor kappa B (NF-kB) signaling [162]. Taken together, these studies show that periapical lesions result from many diverse bacteria, for which different patterns of recognition are at play.
In the pathogenesis of chronic periapical lesions, in response to normal oral microorganisms, the absence of T- and B-cells [55, 210], complement factor C5 [86], interleukin-4 (IL-4) [38, 180], Interferon (IFN) gamma, IL-12, or IL-18 [179] do not seem to affect the pathogenesis of periapical lesions.
However, periapical lesions are larger in size if there is deficiency of adhesion molecules, like intercellular adhesion molecule-1 (ICAM-1) [38] or P/E selectins [100], IL-6 [9, 88], IL-10 [38, 180] or chemokines like CCR2 [70] and CCR5 [38] which are critical for migration of circulating inflammatory cells to the site of inflammation.
Estrogen appears to be protective against the expansion of periapical lesions [72, 228].
This effect can be modulated by antiestrogen agents, like raloxifene [74]. By contrast, medications like bisphosphonates, for example alendronate [186, 228, 229] and metformin (a hypoglycemic agent) [127], reduce bone resorption in periapical lesions. There are many other medications and host conditions that may influence the expansion rate, ultimate size symptoms in apical periodontitis (and potentially the healing following treatment), and therefore, this is the subject of active research.
3.5.2 Classification of Apical Periodontitis
3.5.2.1 Histopathological Classification of Apical Periodontitis
The histopathological classification of apical periodontitis has historically been of great importance, as it was thought to influence the prognosis of non-surgical root canal treatment. In this regard, apical periodontitis has been classified into three main histopatho-logical conditions: apical granuloma, apical cyst and apical abscess. Other less common apical entities include foreign body reaction (such as to food particles, dental materials or sealers), apical scar (which is formed of fibrotic tissue and may result following surgical treatment of large apical lesions), or cholesteatoma (which is a lesion filled with clefts that result from cholesterol crystals together with giant cells) may be present.
3.5.2.1.1 Apical Granuloma
Apical granuloma assumed this designation due to the presence of inflammatory cells such as neutrophils, lymphocytes, plasma cells, macrophages, and mast cells, which may surround larger multinucleated giant cells or foam cells, giving the appearance of granules. Other granulomatous diseases include tuberculosis, leprosy, and sarcoidosis. Apical granulomas may also include epithelial cell rests of Mallassez. These are the remnants of the epithelial root sheath of Hertwig, which is involved in embryological root development.
Apical granulomas are the primary lesions involved in generating the apical immune response. Even in other histopathological conditions such as cysts or abscesses, a part of the lesion, usually in the periphery, is apical granuloma and is thought to have given rise to the cyst or abscess. The apical granuloma contains the cellular and molecular factors involved in the immune response mentioned above. The differential numbers of inflammatory cells present may depend on the technique used for examination, whether the lesion is from primary or persistent apical periodontitis and whether it is of human or animal model origin. Human studies have shown that macrophages [202, 203] or lymphocytes [14] may dominate the periapical granuloma. Animal studies show that T-helper cells are prevalent in initial phases of the lesion production, and T-suppressor cells dominate later phases of the lesion [201, 225]. Recently, it was shown that T-regulatory (T-reg) cells play an important role in the modulation or control of periapical lesion expansion. T-reg inhibition resulted in a significant increase in periapical lesion severity, which was associated with upregulation of pro-inflammatory, T-helper 1 and T-helper 17 [66].
Periapical granuloma is thought to contain little or no bacteria directly within it. Animal studies show no bacteria in apical granuloma [64, 223], and human lesions (which presumably have a longer induction period by the time they are sampled than in most animal studies) have a low prevalence of bacteria detected histologically [143, 168]. Persistent lesions may have a higher prevalence of bacteria within the lesion [205-207]. When present, bacteria are typically surrounded by intense inflammatory cells, mostly neutrophils and macrophages [64]. Occasionally an area of necrosis is seen within the granuloma that is surrounded by intense inflammatory cellular response, and the lesion is termed an apical abscess. One study showed that 35% of apical lesions that are extracted with the tooth are apical abscesses [146].
3.5.2.1.2 Apical Cyst
Apical cyst results from the proliferation of epithelial rests of Mallassez within a granuloma (Figure 3.9). The epithelium initially forms strands that eventually coalesce. The enlargement of the mass of epithelial cells may create nutritional deficiency in the center of the mass, causing necrosis of tissue and liquefaction to form the cyst with cyst fluid. Alternatively, the epithelial strands may surround a portion of the granulomatous tissue causing the cyst formation [213]. Cyst expansion is thought to be due to pressure from the fluid within the cyst, especially that epithelia cells may secrete bone-resorptive cytokines [123, 214]. Cysts may reach very large sizes clinically, and may cause movement of roots of neighboring teeth.
The diagnosis of an apical cyst is variable. Some pathologists determine that the lesion is a cyst whenever they see evidence of epithelial proliferation within the submitted tissue. Others would only diagnose a cyst when they see intact epithelial lining surrounding a fluid-filled cavity, which may contain cholesterol clefts. This variability in determining the diagnosis has led to wide differences among studies that examined the prevalence of granulomas and cysts (Table 3.3).
A further classification of apical cysts calls for the differentiation of “Bay” [188] or “Pocket” [164] cysts from true apical cysts. In Bay or Pocket cysts, the root apex projects into the cyst, and the root canal appears to open directly into the lumen of the cyst. In a true apical cyst, the cyst's epithelial lining and connective tissue appear to be continuous, and separated from the root apex.
Figure 3.9 Biopsy specimens of tissues recovered during apical surgery on a case with non-healing apical periodontitis and diagnosed by a pathologist as periapical cyst. (a) Epithelial proliferation into strands within the granulomatous tissue (original magnification 100x). (b) Epithelial proliferation (original magnification 200x).
Table 3.3 Prevalence of cysts and granulomas in several frequently cited studies.
|
Sample Size |
Cysts % |
Granulomas % |
|
|
Priebe et al. 1954 [161] |
101 |
54 |
46 |
|
Patterson et al. 1964 [157] |
510 |
14 |
20 |
|
Bhaskar 1966 [16] |
2308 |
42 |
48 |
|
Lalonde and Luebke 1968 [113] |
800 |
44 |
45 |
|
Mortensen et al. 1970 [142] |
396 |
41 |
59 |
|
Block et al. 1976 [18] |
230 |
7 |
93 |
|
Winstock 1980 [227] |
9804 |
8 |
83 |
|
Stockdale and Chandler 1988 [204] |
1108 |
17 |
77 |
|
Spatafore et al. 1990 [196] |
1659 |
42 |
52 |
|
Nair et al. 1996 [164] |
256 |
15 |
50 |
|
Koivisto et al. 2012 [110] |
9723 |
33 |
40 |
The diagnosis of apical cysts versus granuloma can only be made histologically of sub- mitted biopsy material, or with an extracted tooth. Attempts to use radiography [226], even cone beam computed tomography (CBCT) [172], to make this distinction have not been successful. More recent attempts to use specific indices and algorithms for CBCT evaluations may be useful in this respect [78]. There are some general radiographic features of an apical radiolucency that suggest that it is a cyst, such as the large size, being sur- rounded by a well-corticated border, and appearing to push the roots of neighboring teeth. Give the difficulty of clinical diagnosis of a cyst versus a granuloma, it has been difficult to prove or disprove the effectiveness of non-surgical root canal treatment in the treatment of apical cysts. In the 1950s and 1960s, it was thought that surgical enucleation was necessary for the treatment of apical cysts, as is the case with other types of cysts. However, arguments were later made that since cysts were quite prevalent in apical periodontitis diagnosis, the fact that non-surgical root canal treatment of cases with apical periodontitis had higher success than the average prevalence of apical granulomas meant that there must be a reasonable number of cysts that healed with non-surgical treatment. More recently, the case was made that the prevalence of true apical cysts (as opposed to pocket cysts) is small and that these would be resistant to non-surgical treatment [144, 145]. These hypotheses remain to be proven, especially that the diagnosis of a true apical cyst would be impossible to do preoperatively at this time.
The other important issue regarding apical cysts as a pathological entity is that biologically, several studies done to distinguish granulomas and cysts with respect to cellular and molecular content have failed to show notable differences [21, 36, 37, 132, 218]. One recent study examined the literature on gene expression in apical granulomas and radicular cysts more critically using contemporary bioinformatics tools [160]. In this study, it was revealed that in cysts the main genes expressed were TP53 (tumor protein p53) and EP300 (E1A binding protein p300), whereas periapical granulomas were associated with IL2R, CCL2, CCL4, CCL5, CCR1, CCR3, and CCR5 genes [160].
3.5.2.2 Clinical Classification of Apical Periodontitis
The clinical classification of apical periodontitis has changed over the years and in different regions of the world. Fundamentally, a few clinical signs and symptoms warrant the diagnosis of apical periodontitis. In the absence of histopathological diagnosis, the clinical diagnosis is used for determining the treatment plan, the prognosis, the need for further exploration and testing, and the response to treatment.
Patients presenting with apical periodontitis may have no symptoms, may complain of mild to moderate pain with mastication, and may present with severe pain, with or without swelling in relation to the offending tooth. Radiographic features include an area of hypodensity related to the apex (or lateral border) of the offending tooth, widening or expansion of the periodontal ligament space and absence of lamina dura in the area of the radiolucency. The area of radiographic hypodensity may be diffuse in its extent, or well defined and surrounded by corticated border. Occasionally there is an area of increased radiographic density at the border of the radiolucency. Other signs of disease include a sinus tract that leads to the area of apical radiolucency.
The clinical diagnosis of apical periodontitis utilizes these signs and symptoms in a manner that is recognizable and reproducible by the clinician. The exact terminology is further discussed in other chapters. However, with respect to the relationship of pathogenesis and diagnosis of apical periodontitis some known findings have emerged from decades of study. Clinical symptoms may be associated with the presence of certain bacterial species or viruses, as noted before (see also Chapter 4 for more discussion of the micro- biology of endodontic infections). Symptoms may also be associated with the presence of bacterial cell wall components like LPS or endotoxin in the root canal [84, 92]. Periapical pain is likely also associated with several inflammatory mediators that reduce the pain threshold such as neuropeptides, bradykinin, leukotrienes, and eicosanoids [26, 138, 215]. Periapical lesion size may be associated with the levels of bone resorptive cytokines like IL-1beta [112].
3.5.3 Modulation of the Immune Response and Bone Resorption in Apical Periodontitis
Periapical lesions represent a dynamic process of bone resorption and deposition, coupled with the development of the soft tissue lesion that contains a number of immune and structural cells (Figure 3.10). Immune cells include lymphocytes, macrophages, neutrophils, plasma cells, and mast cells. Structural cells include fibroblasts and osteoblasts. In addition, endothelial cells and osteoclasts play an important role in the progression of the inflammatory process. Occasionally, epithelial cells are present, and may give rise to cystic formation as noted previously.
Figure 3.10 Interactions between osteoblastic cells and osteoclast precursors during bone remodeling (reproduced with permission from [59]).
Innate immunological cells and factors appear to play a critical role in initial development of periapical lesions. Animal models of lesion induction show the presence of neutrophils at the site of the apical foramen. They appear to play a significant role in the control of microbial progress into the lesion (Figure 3.11).
Studies have shown that suppression of neutrophils causes significant disruption of lesion formation in animal models [147, 231]. Neutropenic patients may have spontaneous necrosis and abscess formation in relation to mild caries [159]. The role of adaptive immunity becomes evident when the host is expo sed to significant bacterial load of virulent oral bacteria. Thus studies in which animals deficient in T- and B-cells (scid mice) were exposed to 1010 cells/mL of Prevotella intermedia, Fusobacterium nucleatum, Peptostreptococcus micros (now Parvimonas micra), and Streptococcus intermedius [212] or Treponema denticola [54], the compromised animals developed severe swellings and experience disseminated infections.
However, in most cases, the bacterial irritants are not as severe, and the immunological response is capable of controlling it without significant morbidity. A balance between the various immunological factors and the advancing microbial irritants determines the exact size of the periapical lesion. In most animal studies, the size of the lesion reaches a specific radiographic or histologic level within 4-6 weeks that is then stable for the rest of the experiment [10, 55, 60]. This balance is maintained by pro-inflammatory and anti-inflammatory cells and molecular mediators, which act in concert to titrate the resulting response. This is as critical in the case of apical periodontitis as it is in pulpitis and other types of inflammation, limiting the side effects of inflammation. This process assures the protective function of inflammation and limits its destructive effects.
Figure 3.11 Induced periapical lesion in a mouse model. (a) Pulp necrosis and intense inflammatory response at the apical foramen and sporadically within the lesion (original magnification 100x). (b) Higher magnification of the box in (a) showing that most inflammatory cells are neutrophils (original magnification 400x).
As noted before, active bone resorption is associated with developing lesions. Bone resorption involves the kinetics related to nuclear factor kappa B (NF-kB) which is ubiquitously expressed in periapical lesions [173]. Specifically, the receptor activator of NF-kB (RANK) on osteoclasts and their precursors interacts with the RANK ligand (RANKL) on osteoblasts to effect bone resorption. RANKL is so critical for bone modulation that an inactivator of this molecule causes osteonecrosis in an animal model [3]. At the same time, osteoprotegrin (OPG) acts as a decoy to stop the RANK/ RANKL reaction and maintain bone structure (Figure 3.10). This occurs when a balance is reached between the advancing irritants and the host response, by limiting bone resorption, resulting in stable lesion size. The cessation of further lesion development, beyond the needed extent, may be orchestrated by T-reg cells [66] and a variety of cytokines, hormones, and other factors (Figure 3.10).
It follows from this argument that one could potentially assay a periapical lesion (through the root canal of a necrotic pulp or directly) for factors that may indicate whether a lesion is actively developing, stable or healing. Preliminary work in this area has been reported. In one study, active versus inactive lesions were arbitrarily based on a RANKL/ OPG ratio of five-fold or greater [69]. This was correlated with gene expression from an array of 84 wound healing genes for 83 gran- ulomas and 25 control periodontal ligament specimens. It was shown that active lesions were associated with upregulation of TNF (a pro-inflammatory cytokine) and CXCL11 (a chemokine), whereas inactive lesions had upregulated SERPINE 1 (a remodeling enzyme), TIMP1 (a remodeling enzyme), COL1A1 (extracellular matrix component), TGF-beta1 (a growth factor), and ITGA4 (a cellular adhesion molecule) [69]. In another study, the designation of active versus inactive lesions was based on the diagnosis of chronic apical abscess for active lesions or asymptomatic (chronic) apical periodontitis for inactive lesions [119]. The lesions were assayed for matrix metallo-proteinases or their inactivators. Active lesions were found to have upregulated MMP-2, MMP-7, and MMP-9, whereas inactive lesions had upregulated TIMP-1 [119].
3.5.4 Healing of Apical Periodontitis
Once the microbial irritants are eliminated, by tooth extraction, or are substantially reduced, by non-surgical or surgical endodontic treatment, periapical healing ensues. Healing occurs by increased vascularity, formation of collagen, development of osseous islands that eventually coalesce, and deposition of cellular cementum (Figure 3.12) [65]. The periapical lesion contains many of the growth factors needed for wound healing and mineralization. These include transforming growth factors (TGF-alpha and TGF-beta) [69, 216], epidermal growth factor [124], vascular endothelial growth factors [53, 118], and bone morphogenetic protein-2 (BMP-2) [135].
Multiple host-related factors may influence the rate of healing of periapical lesions. For example, diabetes mellitus [7, 63] and smoking [43] were shown to be associated with reduced periapical healing in cohort studies. The study of the role of systemic factors on the healing of periapical lesions is confounded by the fact that many of the patients involved take medications that may alter the bone kinetics within the lesion. For example, it was shown that metformin, which commonly used for the treatment of type 2 diabetes [127], bisphosphonates, which are used for osteoporosis among other conditions [99] and statins [122, 125], which are used by many elderly patients, reduce the size of induced periapical lesions in animal models.
Figure 3.12 Healing four months following non-surgical endodontic treatment of a canine with induced periapical lesion in a ferret. Elements of healing include bone (B), vascular proliferation (V) and cellular cementum (CC). (Reproduced with permission from [65].)
Genetic polymorphism in the IL-1beta allele2 [141] or some of the Fc-gamma receptors [193, 194] may be associated with reduced healing of apical periodontitis. Fc-gamma receptors are located on many inflammatory cells, particularly macrophages, and bind the Fc portion of immunoglobulin to facilitate phagocytosis of the antigens attached to the Fab portion of the antibody. However, it is premature at this stage to blame genetic polymorphism for reduced healing, as some of the findings are not supported by other studies [6].
3.6 Concluding Remarks
It is now thought that a low-grade inflammation of the pulp enables tissue regeneration i.e., tertiary dentinogenesis while a more intense and severe inflammation results in necrosis [31, 91]. In this regard, the mechanisms involved in excessive pulp calcification such as following trauma and in heavily restored dentition are not fully understood. Therapies designed to reduce the pulpal inflammatory response may help maintain the vitality of the pulp, an effective immunocompetent tissue. In a canine model of pulpitis, treatment with MMP-3 reduced the expression of the pro-inflammatory cytokine IL-6 and decreased the number of macrophages and antigen-presenting cells [46]. Thus, reversing the induced pulpal inflammation. Improved understanding to the molecular networks which contribute to tissue regeneration and the maintenance of heathy vital pulps have the potential to significantly benefit dental patients and improve oral health.
Healing of apical periodontitis by non-surgical and surgical endodontic treatment is generally slower than following the extraction of the tooth. It is thought that this may be related to a more effective elimination of microbial factors with extraction. However, this delay in healing with endodontic procedures affect the practitioner's ability to follow up on treatment and render additional treatment when it is indicated. In future, it may become feasible to enhance periapical healing following endodontic treatment by the application of commercially-available growth factors, like Emdogain, or other proteins so that the prognosis can be determined after a shorter period of time. Furthermore, assessment of the level of activity in periapical lesions, as noted previously, may assist with treatment planning and prediction of the development of symptoms. The ultimate understanding of the pathogenesis and healing of the pulp and periapical tissues will involve a better understanding of the endodontic microbiome, as well as the patient's genomic, epigenetic, and medical considerations.
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