Oral-Facial Complex: Muscles

• The oral-facial complex functions include mastication, swallowing, breathing, speech, and specialized functions like smell, taste, and vision.

Muscles of Mastication and Jaw Movement

• The primary muscles of mastication are the masseter, temporalis, and medial and lateral pterygoid.
• The muscles responsible for opening the jaw are the mylohyoid, digastric, and inferior lateral pterygoid.
• The muscles responsible for closing the mouth are the medial pterygoid, masseter, and temporalis.

Tongue Muscles

• The tongue muscles are divided into intrinsic and extrinsic groups.
• Intrinsic tongue muscles are responsible for changes in tongue shape.
• Extrinsic tongue muscles facilitate protrusion and retrusion of the tongue.
• The intrinsic tongue muscles include the longitudinalis linguae, transversus linguae, and verticalis linguae.
• The cranial nerve that innervates the muscles that anchor and move the tongue is the hypoglossal nerve.
• The extrinsic tongue muscles are the genioglossus, styloglossus, and hypoglossus.

Dental Embrasures

• Dental embrasures are spaces formed when adjacent tooth surfaces flare away from one another.
• Types of dental embrasures include occlusal/incisal, facial, lingual, and gingival.

Dentition

• The primary dentition consists of 20 teeth.
• Primary dentition begins to form in the first trimester of pregnancy.
• Primary dentition is typically complete by age 3.
• The teeth of the primary dentition begin to erupt around 6 months of age.
• The permanent dentition consists of 32 teeth.
• The permanent teeth begin to erupt around age 6.
• By age 12, most permanent teeth have erupted.
• Mixed dentition is the stage in which primary teeth are gradually replaced by permanent teeth.
• Hypodontia is the congenital absence of teeth.
• Anodontia is the complete absence of teeth.

Tooth Function and Anatomy

• Incisors function as cutting or shearing instruments for food.
• Canines have the longest roots of all teeth and are located at the corners of the dental arch.
• Premolars function similarly to canines in tearing food and are like molars in grinding food.
• Molars are located nearest to the temporomandibular joint (TMJ) and serve as the fulcrum during function.
• Main parts of the tooth: anatomic crown, anatomic root, and pulp chamber
• The anatomic crown is covered by enamel.
• The anatomic root accounts for the lower two thirds of the tooth.
• The function of the root is to anchor the tooth in position.
• The four main dental tissues are enamel, dentin, cementum, and dental pulp.

Demineralization and Dentin Types

• Enamel demineralizes at pH 5.5, while dentin demineralizes at pH 6.5.
• Classifications of dentin: primary, secondary, and tertiary
• Primary dentin forms the initial shape of the tooth.
• Secondary dentin is deposited after the formation of primary dentin on all internal aspects of the pulp cavity.
• Tertiary dentin is reparative dentin formed by replacement odontoblasts in response to moderate irritants like attrition, abrasion, or trauma.

Cementum and Periodontal Tissues

• Cementum is a thin layer of mineralized tissue covering the dentin.
• Cementum anchors the tooth to the alveolar bone, along with the periodontal ligament.
• The periodontal tissues include gingiva, alveolar bone, periodontal ligament, and cementum.

Cusps and Ridges

• The four ridges produced by the cuspal gothic pyramid are the mesial cusp ridge, distal cusp ridge, buccal cusp ridge (labial ridge on canines), and triangular ridge on posterior teeth (lingual ridge on canines).
• Mamelons are often found on mandibular incisors.

Maxillary Central Incisor

• Facial view: nearly straight incisal edge, straight mesially, distally curved.
• Lingual aspect: cingulum, mesial and distal marginal ridges
• Incisal aspect: crown roughly triangular, incisal edge nearly straight

Maxillary Lateral Incisor

• Facial aspect: narrower mesiodistally than central incisors, more rounded edges
• Lingual aspect: prominent marginal ridges and cingulum
• Proximal aspect: mesial and distal aspects present a triangular outline

Canine Features

• The maxillary canine has the longest root.
• The maxillary central incisor has the longest crown.
• Canine crowns make a pentagon shape from the facial view.
• The mesial cusp ridge is shorter compared to the distal cusp ridge.
• The labial ridge is unique to canines.
• Canine crowns have a wedge/triangular shape from the proximal view.

Deciduous vs. Permanent Teeth

• Dentin is functionally active in deciduous teeth at 360 and in permanent teeth at 700.
• The occlusal plane is relatively flat in deciduous teeth and more curved in permanent teeth.
• Enamel is thinner in deciduous teeth (about 1mm) and thicker in permanent teeth (about 2-3mm).

Molar Function and Features

• Molars function in mastication and maintaining vertical dimension.
• The cusp of Carabelli is found on the first maxillary molar.
• The maxillary first molar is the widest tooth in the upper arch and has the largest outline of the maxillary teeth.
• Both first and second maxillary molars typically have 4 large cusps.
• The roots of the maxillary molar include the mesiobuccal, distobuccal, and lingual roots.
• The largest and highest cusp on the maxillary molars is the mesiolingual cusp.
• The maxillary second molar may have 3 or 4 cusps.
• The lingual groove separates the mesiolingual and distolingual cusps.
• A 3-cusp maxillary second molar does not have a distolingual cusp, unlike the 4-cusp type.

Maxillary Molars

• The tricuspid form is a three-cusped maxillary second molar.
• The maxillary first molar appears short occlusocervically and broad faciolingually from the proximal view.
• The height of contour of the buccal surface for posterior teeth is typically in the cervical third, usually close to the cervical line.
• The distal marginal ridges of the first and second maxillary molars are described as concave.
• The primary cusp triangle is a triangular shape formed by three cusps on a 3-cusp type maxillary second molar.
• There are 4 fossae on the 4-cusp type maxillary molar.

Mandibular Molars

• The mandibular first molar has the largest mesiodistal dimension.
• The crowns of the mandibular molars are wider mesiodistally than high cervico-occlusally.
• Mandibular molars have 2 roots.
• The mesial root is longer for the mandibular molars
• Mandibular molar roots are nearly twice as long as the crown
• The third molars are the shortest mandibular teeth.
• The roots of the third molars are shorter compared to the first and second molars.
• Teeth generally taper from buccal to lingual.
• The lingual cusps of the mandibular molars are more pointed and longer.
• The crown of the mandibular teeth is narrower in the distal region.

Wisdom Teeth

• The posterior location of wisdom teeth makes them difficult to clean.
• Wisdom teeth are prone to decay because of their wrinkled, fissure occlusal surfaces.
• An operculum is a flap of tissue that may cover the crown of an erupting mandibular molar.
• Pericoronitis is irritation and infection of the operculum.
• The third molar may be bigger than the second molar when it possesses 5 cusps.
• The occlusal surface of the third molar is often wrinkled due to numerous supplemental grooves and ridges.

Deciduous Teeth

• There are 2 incisors, 1 canine, and 2 molars in each quadrant of the deciduous teeth.
• Deciduous teeth have a greater blood supply and a greater apical foramen size, resulting in greater blood supply.
• Neonatal lines are found in deciduous teeth and first permanent molars.
• Incisors can be identified by angle marks: the angle between the incisal edge and mesial surface is sharp, while the distal and incisal angle is more rounded.

Tooth Identification Methods

• Segment mark: mesial cusp ridge is shorter than distal (not for maxillary first premolar).
• Curvature mark: mesial part of crown bigger than distal.
• Root mark: root apex bent distally.

Incisor Functions and Features

• Incisors function to cut food, provide incisal guidance, enable articulate speech, and support lips and tongue.
• From the facial aspect, the maxillary central incisor has a nearly straight incisal edge that is mesially straight and distally rounded.
• The mesial and distal aspects of incisors present a triangular outline.
• The lingual view of incisors shows the lingual fossa, cingulum, and incisal edge.
• The smallest tooth in the mandibular arch is the central incisor.