It must be noted, however, that although cysts are a direct sequela of periapical inflammation, cyst formation is not inevitable and radicular cysts are in fact quite rare relative to the prevalence of caries and periapical lesions. In an analysis of 256 periapical lesions, Nair et al. (1996 ) showed that only 15% were actually cysts, although a further 37% were granulomas with proliferating epithelium. Their criteria for diagnosis of a cyst was unusually stringent and depended on the ability to examine, in multiple serial sections, the entire specimen and to be able to visualise a distinct epithelial‐lined cavity. Few studies have undertaken such meticulous examination of lesions using serial sections, but those that have have confirmed the findings of Nair et al. (1996 ) that cysts form the minority of periapical lesions. The actual frequencies reported have been 17.1% (Simon 1980 ), 15.2% (Nair et al. 1996 ), 32.0% (Ricucci et al. 2006b ), and 24.2% (Ricucci et al. 2020 ).
Box 3.3 Pathogenesis: Key Facts
Three elements are needed:
A source of epithelium
A stimulus for epithelial proliferation
A mechanism of growth and bone resorption
The cyst develops in three phases:
Phase of initiation – rest cells of Malassez are stimulated to proliferate within a periapical granuloma
Phase of cyst formation – a cavity within the granuloma becomes lined by proliferating epithelium
Phase of growth and enlargement – growth and enlargement are driven by increased osmotic pressure, and are associated with inflammation, cell proliferation and bone resorption
It is convenient first to consider the pathology of periapical periodontitis and then to discuss the pathogenesis of radicular cysts in three phases: the phase of initiation, the phase of cyst formation, and the phase of growth and enlargement.
Pathology of Periapical Periodontitis
Radicular cysts develop within a pre‐existing periapical granuloma where proliferation of the epithelial cell rests of Malassez is initiated by inflammation caused by the necrotic debris and bacterial factors derived from the dead pulp. One can think of this process as part of normal healing, where the host response acts to prevent dissemination of bacteria and stimulates epithelial regeneration. Histologically a periapical granuloma is composed of granulation tissue at various stages of development and maturation. Early lesions may show clear evidence of acute inflammation with polymorphonuclear leukocytes (PMNs), while later lesions become dominated by lymphocytes and plasma cells and may develop a fibrous outer capsule. The process involves many cell types that give rise to a massive array of pro‐inflammatory mediators, cytokines, chemokines, and growth factors, which work together in complex interactions to pursue the final goal of elimination of the cause and healing. However, as well as being protective to the host, these immunomodulatory pathways are also destructive, especially if the bacterial insult persists, as is often the case in periapical periodontitis (Marton and Kiss 2014 ). Many studies have examined periapical lesions to determine which cells and pro‐inflammatory mediators may be important in the development of cysts, and it is difficult and unnecessary to review them all. For example, Bernardi et al. (2015 ) undertook a review of the literature and identified 187 references to epithelium in periapical lesions between 1975 and 2014. After removing duplicates and simple case reports, they found 42 publications that reported the expression of various biological factors that might be involved in the formation of radicular cysts. Because the overall biological process is one of inflammation and the immune response, the factors are not specific to radicular cysts, and the whole field is constantly evolving. Readers interested in the finer points of the associated immunopathological processes should search for relevant up‐to‐date papers. There are a number of detailed reviews that have helped to inform the following discussion of the key points (Nair 2004 ; Silva et al. 2007 ; Lin et al. 2007 ; Graves et al. 2011 ; Graunaite et al. 2012 ; Marton and Kiss 2014 ; Bernardini et al. 2015 ).
It is now well accepted that the whole process is started by bacterial infection in the necrotic pulp, and that the key initiating event is the action of bacterial endotoxins (lipopolysaccharides, LPS) on periodontal ligament fibroblasts, which then secrete a range of pro‐inflammatory cytokines and chemokines. Meghji et al. (1996 ) studied cyst fluids and cultured cyst explants from radicular cysts, keratocysts, and follicular cysts. They showed high levels of endotoxins in radicular cysts compared with negligible levels in the other cyst types. These endotoxins were LPS derived from Actinobacillus actinomycetemcomitans, Porphyromonas gingivalis, and Escherichia coli. Wayman et al. (1992 ) were able to cultivate bacteria from 51 periapical lesions, 23 of which had had previous endodontic therapy. They found a total of 50 different species, of which over 90% were anaerobes. Bacteria were cultured from both granulomas and cysts with equal frequency, but were seen histologically in only 8 (13%) cases. Tek et al. (2013 ) were only able to culture bacteria from about one‐third of periapical lesions. Nair (1987 ), however, in a detailed ultrastructural analysis, found bacteria in 100% of the root canals of teeth affected by periapical lesions, but in only 4 of 31 cases were bacteria found in the extraradicular tissues. In these cases, 1 was infected with Actinomyces and in 3 the bacteria were isolated on the dentinal wall close to the foramen and were associated with abscess formation with accumulations of PMNs. In a later review of the literature, Nair (2004 ) concluded that periapical lesions rarely harbour bacteria unless secondarily infected. This is supported by Ricucci and Siqueira (2010 ), who studied 106 biopsy specimens of tooth roots with apical lesions. They found evidence of bacteria in only 6% of extraradicular lesions. However, bacterial biofilms were found in 77% of the root canals and were more common in untreated (80%) than treated canals (74%). These authors also showed that biofilms were significantly associated with epithelialised lesions, with a frequency of 95%, 83%, and 69.5% in cysts, abscesses, and granulomas, respectively.
Taken together, these data indicate that periapical periodontitis is a result of endodontic infection, following death of the pulp due to caries. Although bacteria proliferate within the root canal, they are rarely encountered in the periapical lesions, confirming the view that cyst formation is initiated by leakage of endotoxins through the apical foramen.
With regard to the host response, all the expected immune cells have been reported in periapical granulomas, with most studies indicating that T lymphocytes predominate and that among these, T‐helper cells (CD4) predominate over cytotoxic/suppressor cells (CD8) (Skaug et al. 1984 ; Nilsen et al. 1984 ; Liapatas et al. 2003 ). However, the proportions of different cell types may vary between sites in the same lesion, or may depend on the stage (or ‘age’) of the lesion (Matthews and Browne 1987 ). Of particular importance, especially in the balance between protective and destructive tissue responses, is the relative number of the T‐helper cell subsets Th1, Th2, Th17, and regulatory T cells (Tregs) (Marton and Kiss 2014 ). Th1 cells, which secrete a range of pro‐inflammatory cytokines, always predominate in periapical granulomas. Th17 cells are the primary source of interleukin (IL)‐17, which is also pro‐inflammatory (Graunaite et al. 2012 ; Marton and Kiss 2014 ), but their action is moderated by Treg cells, which are a major source of anti‐inflammatory transforming growth factor (TGF)‐β. The Th17/Treg ratio seems to be an important regulator of the intensity of the inflammatory response and may thus determine the extent of tissue destruction (Marton and Kiss 2014 ). Kontiainen et al. (1986 ), Babál et al. (1987 ), and Gao et al. (1988a ) found that suppressor/cytotoxic cells