Furcation cyst
Tumours in the oral cavity are not uncommon; these are sometimes non-neoplastic lesions such as cysts, which can be large enough to interfere with mastication and the symmetry of the dental arcades. The types of odontogenic cysts include lateral periodontal cysts, radicular (periapical) cysts, dentigerous (follicular) cysts and canine odontogenic parakeratinised cyst. They are considered to be developmental lesions relating to failure of odontogenesis (e.g. dentigerous cysts), reactive hyperplasia of odontogenic epithelium or cell rests (e.g. radicular cysts) or proliferative lesions with a cystic component.
Distinguishing cysts from neoplastic processes relies on clinical findings, radiography and histopathology. In humans, a new odontogenic cyst, named variably as an inflammatory collateral cyst, paradental inflammatory cyst, or buccal bifurcation cyst, is linked to inflammation in the furcation region of a mandibular M1, M2 or M3. Similar cysts that do not match the description of currently recognised canine odontogenic cysts have been described by Soukup and Bell (2020).
The authors report on 20 dogs with an age range of 3.5–13.0 years. All dogs presented with a unilateral cyst, affecting the right maxillary P4 in 13 dogs and the left maxillary P4 in 7. They showed a fluctuant swelling of the buccal alveolar mucosa overlying the furcation area of the affected tooth. No history of trauma, and no other clinical abnormalities were identified; specifically there was no evidence of tooth displacement or developmental defects. Periodontal disease was evident from previous history and radiography in nine dogs limited to gingivitis (stage 1 periodontal disease) in eight dogs.
Radiographically, the cysts were reported as showing a corticated border combined with a central area of increased radiolucency in the furcation region of a tooth root. The cysts tended to be isolated within the interradicular area between the mesial and distal roots of the affected tooth, and did not expand beyond that region. Expansion of cortical bone was not evident radiographically, but tooth resorption in the mid region of the root, not near the tooth root apex, was seen in four dogs. The cysts were managed by extraction of the affected P4 with cyst wall enucleation and curettage in 19 dogs, while in one dog there was en bloc resection of the associated tooth and supporting alveolar bone followed by curettage. One dog showed cyst recurrence, but this was successfully managed after a second treatment of cyst enucleation and curettage.
Histologically, the cyst wall comprised fibrous connective tissue, and some surfaces were bordered by a non-keratinised stratified squamous epithelial lining that ranged from two to several cell layers thick, with an absence of rete pegs. A predominantly lymphoplasmacytic inflammatory infiltrate of varying severity (minimal to moderate) was evident in all cases.
The reason behind the development of canine furcation cysts is unclear. As similar cysts in people often arise in the area of the bifurcation between mesial and distal roots of the mandibular molar teeth, and the mesiobuccal cusp is the first aspect of the tooth to erupt through the gingiva (in humans and dogs), there is an assumption that these are linked. The cysts develop specifically at the furcation of a maxillary P4, and eight dogs had a degree of periodontal disease that the authors consider could have contributed to their development. However, they do not believe that periodontal disease or pericoronitis are major contributing factors, and that there is no suggestion of food impaction in this area, which is a common cause of inflammation. The authors’ prime focus in terms of the aetiology is trauma, as there is a high and repetitive occlusal load to maxillary P4s and they speculate that ‘repetitive trauma of this region triggers proinflammatory tissue repair mechanisms in periodontal tissues and alveolar bone of the furcation region. Such inflammation could stimulate the local epithelial rests to proliferate and form a cyst’.
The authors have identified a benign distinct type of odontogenic cyst that can be differentiated from neoplasia histologically, and from other cysts by their position and/or radiographic features; dentigerous cysts develop at the crown of an unerupted or impacted tooth; radicular cysts often develop at the root apex, but can cause expansion on the lateral aspect of the tooth or in the furcation region of a multirooted tooth, without involvement of the root apex.
Presence of pulpitis or pulp necrosis might be the only features enabling furcation cysts to be differentiated from radicular cyst that occurs in the furcation region of a multirooted tooth. Differentiating a lateral periodontal cyst, a developmental cyst associated with a viable tooth from a furcation cyst is particularly challenging because of their similar clinical presentation. Lateral periodontal cysts often show plaque-like thickening of the epithelium, a feature not seen with furcation cysts.
Major diagnostic criteria used to make the diagnosis of a furcation cyst include no clinical, radiographic, or histological features of endodontic disease in associated teeth, no impacted teeth at the cyst site, a viable tooth without pulpitis adjacent to the cyst, and a simple, non-proliferative cyst lining. The authors recommend that treatment should be restricted to enucleation and curettage of the cystic lining without the need for extraction of healthy adjacent teeth, unless there is substantial destruction of supporting alveolar bone or resorption of adjacent tooth roots.