## 3 Discussion  
The nasopalatine canal constitutes a passage through the hard palate and usually contains the nasopalatine nerve and blood vessels, as well as embryonic remains of the nasopalatine duct, which is obliterated at birth [7,9]. Although the etiology of NPDCs is unknown, they were initially thought to originate from the epithelium trapped during the fusion of the three processes that make up the palate. Some authors proposed that NPDC result from the spontaneous proliferation of embryonic epithelial tissue remnants in the nasopalatine canal [5, 6, 7]. Regarding pathogenesis, several factors have been suspected of promoting this proliferation and the development of an NPDC, such as trauma, infection, genetic predisposition or spontaneous onset, but none could be confirmed with complete certainty [1, 2, 3, 7, 9, 10, 11]. As cysts were found in human fetal nasopalatine ducts, spontaneous cystic degeneration of epithelial rests was also suggested [1,2]. NPDCs are located in the nasopalatine canal projects between the roots of the upper central incisors on radiological examination [12]. The borders of the cysts are generally well-defined and often marked by osteocondensation, unless the cyst is inflamed [3,8,12]. Radiographically, the cysts show as a heart-shaped or pear-shaped image due to the presence of structures more resistant than the spongy bone, such as the nasal spine and the palatine suture. However, the cyst may also appear oval or rounded [3,7,12]. The content of the cyst often produces a homogenous radiotransparent image, although sometimes it can also contain dystrophic calcifications [12]. An NPDC can affect adjacent structures: it often causes the roots of the central incisors to spread [12] or, in rare cases, cause root resorption [3,12]. At the bone level, it can expand the palatine and, less frequently, the vestibular cortices [12]. The lamina dura of the adjacent teeth remains intact [7,9]. It was often stated that NPDCs have limited growth potential. However, some cases of expansive NPDCs are described in the literature. NPDCs can on occasion expand into neighboring anatomic regions, such as the nasal space [13, 14, 15, 16, 17, 18, 19, 20]. A differential diagnosis based on radiologic findings should consider several possibilities. A large nasopalatine canal is the most frequent characteristic of an NPDC. The average diameter of the nasopalatine canal is 3 mm. In the absence of symptomatology, a canal with a diameter not exceeding 6 mm is considered normal [3,8,9,12]. A cyst of the incisive papilla can appear as a radiotransparent oval that is well delimited by a border of peripheral osteocondensation and projects between the apices of the central incisors, but clinical examination enables distinguishing it from an NPDC. A periapical granuloma or a periapical radicular cyst associated with a central incisor may take on the appearance of an NPDC, but they are  
Figure 4: Histopathological examination of the excised specimen showing stratified squamous, cuboidal and ciliated respiratory epithelium (hematoxylin and eosin stain at (**a**) 20\(\times\) and (**b**) 40\(\times\) magnification. The arrow in (**a**) indicates a large nerve bundle. also characterized by an enlarged desmodontal space and a negative vitality test of adjacent teeth. An additional intra-oral apical X-ray with a slightly modified incident angle should demonstrate a lesion centered on the apex of the responsible tooth, if it is a periapical cyst, while the center of the lesion is shifted in the case of an NPDC [12]. A deniferous cyst associated with a mesiodens can give clinical symptoms similar to those of an NPDC, but radiography (discovery of a mesiodens) can guide a diagnosis [9]. Ameloblastomas are rarely present in this localization; a primary bone tumor and an odontogenic keratocyst should also be included among the possible lesions [7]. Other clinical differential diagnosis includes a nasolabial cyst, salivary gland tumor (benign and possibly malignant) and schwanmonia.