A cyst, derived from the Greek Krystis meaning sac or bladder, is defined as a closed pathological cavity, lined by an epithelium that contains a liquid or semisolid material [1]. Radicular cysts are inflammatory jaw cysts that commonly occur at the peri-apex of a necrotic, infected pulp as a direct sequel to apical granuloma, although a granuloma need not always develop into a cyst. Occasionally they may also be found on the lateral aspects of the root in relation to a lateral accessory canal [2].

Persistent peri-radicular radiolucencies of endodontic origin are commonly due to intra-radicular infection [3], extra-radicular infection [4–6], foreign body reactions [7], true cysts [8, 9] and fibrous scar tissue [10]. Orthograde root canal treatment or re-treatment with the intention to remove/eliminate microorganisms below a critical threshold conducive to healing is the first line of treatment in all cases. Lesions associated with extra-radicular infections, true cysts and foreign body reactions can only be managed by peri-radicular surgery. Peri-apical lesions that heal by fibrous scar tissue require no treatment (Fig. 4.1).

Many lesions that occur in the jaw including lesions of endodontic origin (inflammatory cysts, granulomas, abscesses or fibrous scars) have similar radiological appearances often making it difficult to differentiate among them (Table 4.1 and Fig. 4.2). Although rare, other clinically confusing peri-apical lesions have been documented including lesions of malignancy. It is therefore imperative to have an understanding of the pathogenesis of common endodontic lesions and their management to ensure that misdiagnosis is avoided. Careful history taking and clinical findings should be evaluated taking into consideration radiographic findings to help in narrowing down the differential diagnosis. Follow-up is important not only to ensure that any therapeutic treatment approach has been successful but also to confirm the correct diagnosis has been made [11, 12].

Two distinct categories of radicular cysts have been reported in the literature making a distinction between those cavities containing completely enclosed epithelium (true radicular cyst) and those containing epithelium-lined cavities that are open to the root canals (bay cyst or peri-apical pocket cyst) (Fig. 4.2). The former is an independent entity resulting in a lesion that is self-sustaining and no longer dependent on the presence or absence of root canal infection. As a result surgical excision would have to be performed following conventional root canal treatment to ensure healing. In the latter nonsurgical root canal treatment carried out effectively would eliminate the infection within the root canal space, ensuring peri-apical pocket cysts would heal [8, 9].

Currently the gold standard for diagnosis of a peri-apical lesion is based on clinical and histological findings using serial sectioning of the lesion in its entirety [11]. In the past, assumptions were made based on radiological findings to differentiate between a true radicular cyst and peri-apical granuloma, which were scientifically unsound.

Often lesion size was measured and larger radiographic diameters were more likely thought of as cystic as opposed to granulomatous in nature, which was not always the case [13–15]. Radiological appearances of cysts are often described as round or ovoid radiolucencies surrounded by a narrow radiopaque demarcating margin, which extends from the lamina dura of the involved tooth. In infected or rapidly growing cysts this margin may not be present and these features are therefore not correlative with the definitive diagnosis of a true radicular cyst [16].

Several surgical approaches exist for the management of cystic lesions of the jaws including enucleation, marsupialisation and decompression. Enucleation comprises of complete removal of the cyst lining which is a definitive treatment modality not usually requiring further intervention. The risk of morbidity is higher as nearby structures and teeth may be damaged. Marsupialisation is the conversion of a cyst into a pouch. The cystic roof is removed in its entirety, and the cut edges of the remaining cyst are sutured to the adjacent soft tissue lining the oral cavity, maxillary sinus or nasal cavity into a continuous pouch. Surgical decompression is a minimally invasive technique whereby a large cystic lesion is converted into a small one with the aim of any future surgical intervention having reduced morbidity when considering enucleation. Decompression involves the insertion of a decompression stent/drainage tube into the peri-apical lesion, regular irrigation, periodic length adjustment and maintenance of the drain for varying lengths of time. It is contraindicated in cases of large dental granulomas or any solid cellular lesion for the reason that there is an absence of a fluid-filled cavity to decompress [17–21].

There have been numerous reports of non-odontogenic benign and malignant lesions presenting in the peri-apical area mimicking lesions of endodontic origin. The frequency and distribution of radiolucent jaw lesions in a retrospective study revealed that most non-healing lesions submitted for biopsy were classified as either apical granulomas (40.4 %) or apical cysts (33.1 %), and they were often from the maxillary anterior jaw. Over 20 % of the reported non-healing radiolucent lesions submitted had a more severe pathologic implication, such as odontogenic keratocysts (8.8 %), central giant cell lesions (1.3 %), ameloblastomas (1.2 %) and even the small but important number of metastatic lesions (0.26 %). Most of these lesions were located in the posterior mandible. Granulomas or cysts (73 %) were often from the anterior maxillary jaw. Thus non-healing radiolucent jaw lesions other than granulomas or cysts were reported over 20 % of the time and may have more severe pathological implications, suggesting the value of differential diagnoses [22].

Common cysts that mimic lesions of endodontic origin include odontogenic keratocysts [23], residual cysts [24], lateral periodontal cysts [25] and nasopalatine duct cysts [26].

Benign aggressive lesions that cause locally destructive lesions mimicking peri-apical pathosis include central giant cell granulomas [27, 28], central ossifying fibroma [29], calcifying epithelial odontogenic tumour (Pindborg tumour) [30, 31], osteoblastoma [32] and central odontogenic fibroma [33].

Benign cemento-osseous dysplasia including peri-apical cemental dysplasia can develop around the apices of the teeth, representing a well-recognised diagnostic challenge that is difficult to distinguish from peri-apical granulomas in the early stages [34, 35].

Granulomatous inflammation distinguished from granulation tissue associated with peri-apical granulomas is a distinct entity that has been reported in the literature elicited by foreign material. It has been proposed that foreign body-induced granulomatous inflammation at the peri-apex of the teeth may result in endodontic treatment that fails to heal [7, 36, 37].

A variety of misdiagnosed malignant neoplastic lesions have been reported in the endodontic literature, masquerading at the peri-apex of the teeth similar to lesions of endodontic origin. As a result of clinical and radiographic similarities, these lesions may be mistaken with inflammatory or infectious diseases of the jaws, creating a dilemma for the clinician. Histopathological evaluation is of paramount importance when the lesion fails to respond to routine treatments [12, 31, 38–45].

The most common peri-radicular lesions of the jaws are peri-apical granuloma, peri-apical cyst (radicular and peri-apical pocket cyst) and the chronic dental abscess, which are inflammatory in origin due to the demise of the dental pulp. All three lesions typically present as unilocular, peri-radicular radiolucencies of varying sizes. Unequivocal results of clinical signs and symptoms including pulp vitality testing and radiographic diagnosis often lead to commencement of root canal treatment which tips the balance for healing to occur. The need for follow-up is a prerequisite not only to determine the outcome of treatment but also to ensure that a correct diagnosis was reached. In the cases of lesions that fail to respond, a surgical approach may be necessary to confirm histopathologically the underlying reason for failure and also rule out the possibility of other lesions that mimic endodontic pathology.

A chronic inflammatory reaction that consists of granulomatous tissue predominantly infiltrated with lymphocytes, plasma cells and macrophages. The lesion is maintained by persistent necrotic pulpal contents resulting in a well-circumscribed radiolucent lesion around the peri-apex of a tooth. Occasionally the lesion may present laterally in response to a lateral canal or perforation. Nonsurgical endodontic therapy will eradicate the lesion (Fig. 4.3).

Reactive hyperplasia at the peri-apex of a tooth in response to a low-grade pulpal infection can lead to condensing osteitis. Typically seen in young adults and teenagers, affecting mandibular molars and premolars most commonly. The bone is compact and dense with minimal chronic inflammation appearing more radiopaque at the peri-apex of the tooth. Endodontic treatment is required to resolve the problem.

An abscess is defined as a ‘localised collection of pus’ that can occur around the peri-apex of an endodontically infected tooth. An abscess is a focus of acute inflammation characterised by a distinct collection of polymorphonuclear leukocytes within a pre-existing chronic inflammatory lesion (e.g. granuloma). An acute peri-apical abscess is a very painful condition that is characterised by intense throbbing; extreme pain to touch, biting and percussion; palpation tenderness; and increased mobility of the tooth. An intra-oral or extra-oral swelling may be evident. Radiographically there may not be any signs.

These terms denote a peri-apical abscess that arises from an acute exacerbation of a chronic inflammatory peri-apical lesion (Fig. 4.4).

The lesion appears as a well-demarcated radiolucency at the peri-apex of a tooth. Histopathologically a variable mixture of fibrovascular connective tissue, scar and chronic inflammatory cells is present with foci of neutrophils, oedema and liquefaction necrosis. Often drainage may be evident through a sinus intra-orally with or without symptoms. Nonsurgical endodontic treatment will eradicate the lesion.

An inflammatory lesion that contains a sac-like epithelium-lined cavity that is open to and continuous with the root canal. Pocket cysts are more likely to heal after nonsurgical endodontic therapy of the tooth since the treatment removes the source of irritation. A well-defined radiolucency will be associated either apically or laterally and indistinguishable from peri-apical granuloma or radicular cyst. Nonsurgical treatment will often eradicate the lesion (Fig. 4.5).