The structures of the maxilla and mandible discussed in this section are illustrated in Fig 1-12. The oral vestibule (vestibulum oris) is the space between the cheeks and lips laterally and the teeth, gingiva, and the alveolar region medially. Usually the tissue surfaces lie close together so that the space in between is only potential in nature. An actual space can be produced if the cheeks are puffed out or pressed in when chewing food.
The vestibular fornices (fornix vestibuli superi-oris and inferioris) form the superior and inferior margins of the oral vestibule, where the mucosa of the cheeks and lips reflect back onto the alveolar mucosa. The furrow formed by the vestibular fornices is also known as the vestibular sulcus. It lies roughly at the level of the apices of the roots of the teeth, which is why surgical removal or treatment of tooth roots can be performed via the vestibule (eg, apicoectomy or root end resection).
The fornices can be pushed tangentially up to the alveolar ridge. However, this area can be extremely sensitive if, for example, it is chronically stressed. The loose submucous connective tissue is capable of storing a lot of fluid. Extensive fluid accumulations can cause severe swelling (creating an appearance of "fat" cheeks).
When the mouth is closed, the vestibule communicates with the oral cavity proper through the retromolar space (posterior to the molars). With the mouth wide open, the pterygomandibular raphe (raphe pterygomandibularis) marks the boundary between the oral cavity and the vestibule. The shape of the vestibular sulcus varies from one individual to another, and it is interrupted by various ligamentous attachments and mucosal folds.
The labial frenula or frena (frenula labiorum; frenulum labii superioris and inferioris) are free-edged mucosal folds in the midline that run from the lips to the alveolar mucosa. The vestibule is virtually halved by this frenum, both in the maxilla and in the mandible. A thick superior labial frenum can push apart the two middle incisors to form a median diastema (a gap between the two maxillary central incisors).
The buccal frena (frenula buccales; frenulum buccae superioris and inferioris) are fibrous bundles covered with buccal mucosa that run laterally in varying numbers as part of the tendons of origin of the buccinator muscle (musculus buccinator). They are usually located in the region of the premolars and run from the alveolar ridge dorsally into the fornix; they prevent excessive stretching of the vestibule. Additional mucosal folds may also be present, and these must be taken into account when shaping a denture border.
Directly below the fornix, but slightly above the alveolar ridge, there are muscle attachments that, during movements, alter the vestibular sulcus. It can also be constricted during speaking and chewing.
The pterygomandibular raphe (raphe pterygo-mandibularis; suture of the pharyngeal wall; pterygo = winglike) is a vertical fold of mucosa that limits the vestibule posterior to the molars. This fold is shaped by a strip of tendon that runs from the hamular (hooklike) process of the sphenoid bone (hamulus pterygoideus) to the inside (hylohyoid line) of the mandible.
The buccal vestibular sulcus is narrowest posteriorly where the coronoid process of the mandible reduces the space considerably during lateral movements. The vestibule is also narrowed here by the activity of the masticatory muscles, which is why a denture flange must not be too thick in this region.
After tooth loss, the alveolar processes of the jaw resorb, causing the folds to lose their original height and in some cases completely level out. It is possible to increase fold depth surgically to compensate for badly shrunken alveolar ridges and to create undercut retention areas for the denture base.
The zygomatic crest (crista infrazygomatica) is a bony process of the cheekbone (zygoma) level with the roots of the first molars. Owing to the presence of this crest, the fornix is shallow in this region. The maxillary tuberosity (tuber maxillae) is a robust bony prominence at the posterior end of the maxillary dental arch that is not resorbed after tooth loss.
The mandibular tubercle (tuberculum alveolare mandibulae) is a mucosal elevation over the bony retromolar trigone (trigonum retromolare). It can arise from special muscle attachments. The mandibular tubercle is also frequently known as the retromolar triangle.
The oblique line (linea oblique) is a bony elevation that runs from the retromolar triangle anteriorly and inferiorly to the premolars and into the vestibular sulcus. The attachment of the buccinators extends to this line.
In the animal kingdom, a distinction is made between true teeth and structures that are similar to teeth but are not true teeth. True teeth are made of enamel, dentin, and cementum as well as pulp. They are called dentinal teeth because they are mainly made up of dentin.
Structures that are not true teeth do not have any of the hard substances of the true teeth, ie, enamel, dentin, and cementum. They are mainly the horny teeth found, for example, in the pharynx and esophagus of leatherback sea turtles.