Mandible

Introduction

• The mandible, or lower jawbone, is the largest, strongest, and lowermost bone of the facial skeleton.
• It is a horseshoe-shaped bone consisting of three parts:
  • Body: It is the horizontal part of the mandible.
  • Two rami: These are vertically placed parts that extend upward from the posterior ends of the body.

BODY OF MANDIBLE

• The body of the mandible forms a flattened, U-shaped arch.
• It presents the following surfaces and borders:
  • Two surfaces: External and internal.
  • Two borders: Superior (alveolar) border and inferior border (base).

External Surface of Body

• The external surface is convex and presents the following features.

1. Symphysis menti

• It is a faint vertical ridge in the midline of the external surface.
• It represents the site of fusion of the right and left halves of the mandible.

2. Mental protuberance

• It is a triangular bony prominence at the lower end of the symphysis menti.
• It forms the prominence of the chin, with its apex directed upward and base downward.

3. Mental tubercles

• These are two rounded elevations situated at the lateral ends of the base of the mental protuberance.

4. Mental foramen

• It is an opening on the external surface of the body, usually below the second premolar tooth or between the two premolars.
• It represents the external opening of the mandibular canal.
• Structures transmitted: It transmits the mental nerve and vessels.

5. Oblique line of mandible

• It is a prominent ridge extending upward and backward from the region of the mental tubercle.
• Posteriorly, it becomes continuous with the anterior border of the ramus.
• Muscular attachments:
  • The buccinator arises from the posterior part of the oblique line in the molar region.
  • The depressor anguli oris arises from its middle part.
  • The depressor labii inferioris arises from its anterior part.

6. Incisive fossa

• It is a shallow depression situated below the incisor teeth, on either side of the symphysis menti.
• Muscular attachments:
  • The mentalis arises from the upper part of the fossa.
  • The incisivus labii inferioris, which blends with the orbicularis oris, arises from this region.

Note: The external and internal surfaces of the mandible immediately below the alveolar border are covered by mucoperiosteum.

Inner Surface of Body

• The internal surface of the body is concave and presents the following features.

1. Mylohyoid line

• It is an oblique ridge extending downward and forward from the region of the third molar tooth towards the lower part of the symphysis menti.
• Attachments:
  • The mylohyoid muscle arises from the mylohyoid line.
  • The superior constrictor of the pharynx arises from the region between its posterior end and the last molar tooth.
  • The pterygomandibular raphe is attached behind the third molar tooth.

2. Mylohyoid groove

• It is a narrow groove that extends downward and forward from the mandibular foramen on the medial surface of the ramus.
• Contents: It transmits the nerve to mylohyoid and accompanying vessels.

3. Submandibular fossa

• It is a shallow depression situated below the posterior part of the mylohyoid line.
• Relation: It accommodates the superficial part of the submandibular salivary gland.

4. Sublingual fossa

• It is a shallow depression situated above the anterior part of the mylohyoid line.
• Relation: It accommodates the sublingual salivary gland.

5. Genial tubercles

• These are small bony projections on the internal surface of the symphysis menti, above the anterior end of the mylohyoid line.
• They are usually arranged as superior and inferior pairs.
• Muscular attachments:
  • The superior genial tubercles give origin to the genioglossus muscles.
  • The inferior genial tubercles give origin to the geniohyoid muscles.

6. Digastric fossa

• It is a small depression near the inferior border of the mandible, lateral to the symphysis menti.
• Muscular attachment: It gives origin to the anterior belly of the digastric muscle.

Borders of Body of Mandible

1. Superior or alveolar border

• It contains sockets for the teeth of the lower jaw.
• Each half of the adult mandible normally accommodates eight teeth, arranged from anterior to posterior as follows:
  1. Central incisor.
  2. Lateral incisor.
  3. Canine.
  4. First premolar.
  5. Second premolar.
  6. First molar.
  7. Second molar.
  8. Third molar.
• The sockets of mandibular molar teeth are generally divided into two compartments by an interradicular bony septum.

2. Inferior border or base of mandible

• It is a thick, rounded border that extends along the lower margin of the body.
• Muscular attachment: The platysma is inserted into the inferior border of the mandible.

RAMUS OF MANDIBLE

• The ramus of the mandible is a quadrilateral bony plate that extends upward from the posterior end of the body.
• Each ramus presents the following features:
  • Two surfaces: Lateral and medial.
  • Four borders: Anterior, posterior, superior, and inferior.
  • Two processes: Coronoid and condylar processes.

Lateral Surface

• The lateral surface is rough, particularly in its lower part, and presents several oblique ridges.
• Muscular attachment: The masseter muscle is inserted into the rough lateral surface, particularly near the angle of the mandible.
• Relation: The parotid gland overlaps the smooth posterosuperior part of the ramus.

Medial Surface

• The medial surface presents the following features.

1. Mandibular foramen

• It is an opening situated slightly above the centre of the medial surface of the ramus.
• It leads into the mandibular canal, which extends through the ramus and body of the mandible.
• Structures transmitted: The inferior alveolar nerve and vessels enter the mandibular foramen.

2. Lingula

• It is a small, tongue-shaped bony projection at the anterosuperior margin of the mandibular foramen.
• Attachment: The sphenomandibular ligament is attached to the lingula.

3. Mylohyoid groove

• It begins below the mandibular foramen and runs downward and forward.
• It gradually becomes indistinct as it approaches the body of the mandible.
• Contents: It transmits the nerve to mylohyoid and accompanying vessels.

4. Pterygoid tuberosity

• It is a rough area on the lower part of the medial surface, near the angle of the mandible.
• Muscular attachment: The medial pterygoid muscle is inserted into this rough area.

Borders of Ramus

1. Anterior border

• It is thin and sharp and continues upward into the anterior border of the coronoid process.
• Muscular attachment: It provides insertion to fibres of the temporalis muscle.

2. Posterior border

• It is thick, rounded, and extends from the neck of the mandible to its angle.

3. Superior border

• It forms a deep concavity called the mandibular notch, situated between the coronoid and condylar processes.
• Structures transmitted: The masseteric nerve and vessels pass through the mandibular notch.

4. Inferior border

• It is short and continuous anteriorly with the inferior border of the body.
• Posteriorly, it meets the posterior border of the ramus to form the angle of the mandible.
• Attachment: The stylomandibular ligament is attached near the angle of the mandible.

Coronoid Process

• The coronoid process is a thin, triangular bony projection that extends upward from the anterior part of the ramus.
• Its anterior border is continuous with the anterior border of the ramus.
• Its posterior border forms the anterior boundary of the mandibular notch.
• Muscular attachment: The temporalis muscle is inserted into the apex and medial surface of the coronoid process. Its insertion extends downward along the anterior border of the ramus.

Condylar Process

• • Condylar process is an upward projection from posterosuperior aspect of ramus. It presents the The condylar process projects upward from the posterosuperior part of the ramus.
• It consists of the head, neck, and pterygoid fovea.

1. Head of mandible

• It is the expanded upper end of the condylar process.
• It is wider mediolaterally than anteroposteriorly, and its articular surface is covered by fibrocartilage.
• Articulation: It articulates with the mandibular fossa and articular tubercle of the temporal bone, with an intervening articular disc, to form the temporomandibular joint.
• A small tubercle may be present near the lateral end of the head.

2. Neck of mandible

• It is the constricted part of the condylar process, immediately below the head.
• Attachment: The lateral ligament of the temporomandibular joint is attached to the lateral aspect of the neck.

3. Pterygoid fovea

• It is a small depression on the anteromedial aspect of the neck, below the head of the mandible.
• Muscular attachment: The inferior head of the lateral pterygoid muscle is inserted mainly into the pterygoid fovea.

Mandibular Canal

• The mandibular canal begins at the mandibular foramen on the medial surface of the ramus.
• It passes downward and forward through the ramus and continues into the body, below the sockets of the teeth.
• In the premolar region, it divides into mental and incisive canals.

1. Mental canal

• It passes upward, backward, and laterally to reach the mental foramen.
• It transmits the mental nerve and vessels.

2. Incisive canal

• It continues forward within the body of the mandible, towards the incisor teeth.
• It carries the incisive branches of the inferior alveolar nerve and vessels.

3. Communications

• Small bony channels connect the mandibular canal with the sockets of the mandibular teeth.
• These channels transmit dental branches of the inferior alveolar nerve and vessels.

4. Contents of mandibular canal

• The mandibular canal contains the inferior alveolar nerve and vessels.
• In the premolar region, these divide into mental and incisive branches that continue through their respective canals.

Ossification of Mandible

·  The mandible develops from the mesenchyme of the first pharyngeal arch, mainly by intramembranous ossification.

·  Ossification begins in the connective tissue lateral to Meckel’s cartilage, near the point where the inferior alveolar nerve divides into its mental and incisive branches.

·  The mandible is the second bone to begin ossification, after the clavicle.

·  Each half of the mandible develops from a primary ossification centre that appears around the 6th week of intrauterine life.

·  Bone formation spreads from this centre to form most of the body and ramus of the mandible.

·  Secondary cartilages develop in certain regions and contribute to endochondral ossification.

Parts Developing Through Cartilage

• The condylar process develops largely through endochondral ossification of secondary condylar cartilage.
• Secondary cartilage also contributes to the development of the coronoid process and parts of the symphyseal region.
• Most of the remaining mandible develops through intramembranous ossification.

Age change in the mandible

• The mandible undergoes several structural changes from birth to old age.
• These changes mainly involve the angle of the mandible, mental foramen, mandibular canal, alveolar process, teeth, and condylar and coronoid processes.
• Growth of the jaws, eruption of teeth, and age-related resorption of alveolar bone are responsible for most of these changes.

1. In Infants and Children

• Angle of mandible: The angle is approximately 150°–160° at birth and gradually decreases to about 140° during early childhood.
• Mental foramen: It lies relatively close to the alveolar margin in newborns. Its position changes as the body of the mandible grows and the teeth erupt.
• Direction of mental foramen: Its opening is generally directed upward and backward, although the orientation varies.
• Alveolar process: It is poorly developed at birth and gradually increases in height with the development and eruption of teeth.
• Teeth: The complete deciduous dentition consists of 20 teeth, with five teeth in each half of the mandible.
• Condylar and coronoid processes: The coronoid process may extend above the level of the condylar process in early life.
• Height of body: The body is relatively shallow at birth and gradually increases in height.
• Mandibular canal: It occupies a relatively low position in the developing mandible.

2. In Adults

• Angle of mandible: The angle decreases to approximately 110°–120°.
• Mental foramen: It is usually situated near the midpoint between the superior and inferior borders of the body.
• Direction of mental foramen: It generally opens backward, upward, and laterally.
• Alveolar process: It is well developed and supports the roots of the permanent teeth.
• Teeth: The complete permanent dentition consists of 32 teeth, with eight teeth in each half of the mandible.
• Condylar and coronoid processes: The condylar process usually extends above the level of the coronoid process.
• Height of body: The body attains its maximum height after the development of the permanent dentition.
• Mandibular canal: It runs through the body approximately parallel to the mylohyoid line.

3. In Old Age

• Angle of mandible: The angle may increase to approximately 140°, particularly in individuals with extensive tooth loss.
• Mental foramen: It appears closer to the superior border because of resorption of the alveolar process.
• Direction of mental foramen: Its orientation may change with age, but considerable individual variation occurs.
• Alveolar process: It undergoes progressive resorption following the loss of teeth.
• Teeth: Partial or complete loss of teeth may occur, resulting in an edentulous mandible in some individuals.
• Condylar and coronoid processes: Their relative heights may change because of age-related remodelling and resorption.
• Height of body: The height decreases, mainly due to resorption of the alveolar bone.
• Mandibular canal: It lies closer to the residual alveolar crest and may become exposed in cases of severe bone resorption.

Main attachments and relations of mandible Muscular Attachments

• Buccinator: It arises from the oblique line of the mandible in the molar region.
• Depressor anguli oris: It originates from the middle part of the oblique line.
• Depressor labii inferioris: It arises from the anterior part of the oblique line.
• Mentalis: It originates from the incisive fossa below the incisor teeth.
• Incisivus labii inferioris: It arises from the incisive region of the mandible and blends with the orbicularis oris.
• Mylohyoid: It originates from the mylohyoid line on the internal surface of the body.
• Superior constrictor of pharynx: It arises partly from the medial surface of the mandible behind the last molar tooth, near the posterior end of the mylohyoid line.
• Genioglossus: It originates from the superior genial tubercles.
• Geniohyoid: It arises from the inferior genial tubercles.
• Anterior belly of digastric: It originates from the digastric fossa near the inferior border of the body.
• Platysma: It is inserted into the inferior border of the mandible.
• Masseter: It is inserted into the lateral surface of the ramus and angle of the mandible.
• Temporalis: It is inserted into the apex and medial surface of the coronoid process, extending along the anterior border of the ramus.
• Lateral pterygoid: Its inferior head is inserted mainly into the pterygoid fovea on the neck of the mandible.
• Medial pterygoid: It is inserted into the rough medial surface of the ramus, near the angle of the mandible.

Ligamentous Attachments

• Stylomandibular ligament: It is attached to the angle and adjacent posterior border of the ramus.
• Lateral (temporomandibular) ligament: It is attached to the lateral aspect of the neck of the mandible.
• Sphenomandibular ligament: It is attached to the lingula, near the mandibular foramen.
• Pterygomandibular raphe: Its inferior end is attached to the mandible behind the last molar tooth, near the posterior end of the mylohyoid line.

Nerves Related to Mandible

• Lingual nerve: It lies close to the medial surface of the mandible, near the roots of the third molar tooth.
• Inferior alveolar nerve: It enters the mandibular foramen and travels through the mandibular canal.
• Nerve to mylohyoid: It passes downward and forward along the mylohyoid groove on the medial surface of the ramus.
• Mental nerve: It emerges through the mental foramen to supply the skin of the chin and lower lip.
• Masseteric nerve: It passes laterally through the mandibular notch to reach the deep surface of the masseter.
• Auriculotemporal nerve: It passes medial and posterior to the neck of the mandible before ascending towards the temporal region.
• Marginal mandibular branch of facial nerve: It runs close to the inferior border of the mandible and may cross below it, particularly near the facial vessels.

Bone Resorption in Mandible

• Loss of teeth leads to progressive resorption (atrophy) of the alveolar process of the mandible.
• Resorption generally occurs in a downward and outward (centrifugal) direction, resulting in a reduction in the height and width of the residual alveolar ridge.
• In contrast, the maxillary alveolar ridge generally undergoes resorption in an upward and inward (centripetal) direction.
• Severe resorption reduces the height of the mandibular body and brings the mental foramen closer to the superior border.
• These changes are particularly prominent in elderly individuals with prolonged tooth loss.