THE MANDIBLE: Anatomy, Foramina, Muscle attachments, Ossification, Age Changes, Blood and Nerve Supply, Lymphatic Drainage

The Mandible

Anatomy, Foramina, Muscle attachments, Ossification, Age Changes, Blood and Nerve Supply, Lymphatic Drainage

The facial skeleton (viscerocranium) comprises 14 facial bones, and the mandible is the largest one. Besides the middle ear bones, the mandible is the only mobile bone in the skull. Unlike other skull bones, the mandible doesn't connect with the surrounding bones via sutures but articulates via a synovial joint called the temporomandibular joint (TMJ). This joint enables the mandible to produce various translatory and rotatory movements and remain attached to the skull. Complex functions like chewing and speaking are feasible due to this capability of movements.

Mandible Anatomy

The mandible has two vertical RAMI and one horizontal BODY. They meet on each side at the angle (GONIAL ANGLE) of the mandible. Let's discuss them one by one.

Body

Body is the anterior portion of the mandible.
On either side where the body ends, the rami begin at the angle of the mandible, also known as the Gonial Angle.
It has two surfaces: Outer and Inner.

The Outer Surface (also known as External Surface) has:

Symphysis Menti / Mandibular Symphysis joins the left and right half of the bone at the midline and is detected as a subtle ridge in the adult.
Mental Protuberance, your chin is a midline depression at the inferior portion of the ridge dividing into two.
Mental Tubercle formed by the elevated edges of the mental protuberance.
Incisive Fossa, depression below the incisor teeth and lateral to the ridge.
Mental Foramen below the second premolar teeth, where the mental nerve and vessels exit.
Oblique Line courses posteriorly from the mental tubercle to the anterior border of the ramus.

The Inner Surface (also known as Internal Surface) has:

The Mylohyoid Line runs obliquely downwards and forwards from below the third molar tooth to the median area below the genial tubercles.
The Sub-mandibular Fossa, which keeps the submandibular gland, is a slightly hollow surface below the mylohyoid line.
The Sublingual Fossa in which the sublingual gland lies, situates above the mylohyoid line.
The Superior and Inferior Genial Tubercles are four small elevations at the posterior surface of the symphysis menti.

It has two borders: Upper and Lower.

The Upper Border (Alveolar Border) consists of sockets for the teeth.
The Lower Border (Inferior Border), also known as the Base, has a fossa at the side of the midline known as the Digastric Fossa.

Rami

The Ramus is the vertical part of the mandible.
It unites with the body at the gonial angle of the mandible. The gonial angle ranges from 110° to 130° depending on the age, sex and ethnicity.
The superior part of the ramus consists of two processes: the Coronoid Process (anterior process) and the Condylar Process (posterior process).
The incisures between these two processes is called the Mandibular Notch, and the masseteric nerve and vessels cross it.
The coronoid process has the temporal crest, which becomes the attachment point for the temporalis muscle.
The mandible articulates with the articular tubercle of the temporal bone via an articular surface on the condylar process, the condyle, to form the temporomandibular joint (TMJ).
The posterolateral side of the ramus has a rough surface, the Masseteric Tuberosity, that serves to insert the masseter muscle.
The medial surface of the ramus has:

The Pterygoid Tuberosity - for the insertion of the medial pterygoid muscle.
The Mandibular Foramen - starting point of the mandibular canal.
The Mylohyoid Sulcus - contains the mylohyoid artery and nerve.

Foramina

A foramen is an opening through which neurovascular structures can travel. The mandible has two foramina.
The mandible has two foramina.

The Mandibular Foramen
Location: on the internal surface of the ramus of the mandible.
Traveling Neurovascular Structures: the inferior alveolar nerve and inferior alveolar artery.
The Mental Foramen
Location: on the external surface of the mandibular body, below the second premolar tooth.
Traveling Neurovascular Structures: the inferior alveolar nerve and artery to exit the mandibular canal.

Muscle Attachments

Muscles that originate from the mandible

Muscles	Point of origin
Buccinator Muscle	Buccinator ridge of mandible
Mentalis Muscle	Incisive fossa of mandible
Depressor Labii Inferioris Muscle	Oblique line of mandible
Depressor Anguli Oris Muscle	Mental tubercle and oblique line of mandible
Anterior Belly of Digastric Muscle	Digastric fossa
Genioglossus Muscle	Superior mental spine
Geniohyoid Muscle	Inferior mental spine
Mylohyoid Muscle	Mylohyoid line

Muscles that insert into the mandible

Muscles	Point of insertion
Lateral Pterygoid Muscle	Pterygoid fossa
Temporalis Muscle	Apex and medial surface of coronoid process of mandible
Medial Pterygoid Muscle	Medial surface of ramus (pterygoid tuberosity) and angle of mandible
Masseter Muscle	Lateral surface of ramus and angle of mandible
Platysma	Lateral surface of ramus and angle of mandible

Ossification

The mandible is the second bone to ossify after the clavicle (collarbone). Each half of the mandible ossifies at the sixth week of life within the uterus in the mesenchymal sheath of Meckel's cartilage. These two halves eventually fuse via fibrocartilage at the mandibular symphysis.
The mandible remains two separate bones at the time of birth. During the first year of a child, the ossification and fusion of the mandibular symphysis occur and result in a single bone. The mandibular symphysis becomes a subtle ridge at the midline of the mandible.
The mandible changes throughout life. In infants and children, the angle of the mandible is obtuse at 140 degrees, whereas in adults, the angle decreases to about 110-120 degrees making changes in the shapes of the face.

How Mandible Changes with Age

At Birth

The body of the mandible contains sockets of the two incisors, the canine and the two deciduous molar teeth only. They are imperfectly partitioned off from one another.
The mandibular canal is large and runs near the lower border of the bone.
The mental foramen opens beneath the socket of the first deciduous molar tooth.
The angle of the mandible is obtuse 170 degrees, and the condyloid portion is almost in line with the body. The coronoid process is large and projects above the level of the condyle.

In Childhood

The two parts of the mandibular join at the symphysis, in the first year, from below upward.
The body becomes elongated in its whole length, especially behind the mental foramen, to provide space for the three other teeth developed in this part.
The depth of the body increases to create room for the roots of the teeth.
The angle decreases to 140 degrees about the fourth year of age.

In Adults

The mental foramen now lies mid-way between the upper and lower border of the bone after the eruption of permanent teeth.
The ramus grows posteriorly to accommodate the eruption of permanent molars and reduces the angle to 110 to 115 degrees and vertically to allow the condylar process to lie higher than the coronoid process.

In Old Age

Teeth fall out, and the alveolar border is absorbed so that the body's height is reduced.
The mental foramen and the mandibular canal are close to the alveolar border.
The angle again becomes obtuse, about 140 degrees, because the ramus is oblique.

Blood Supply of Mandible

The central blood supply happens via The Inferior Alveolar Artery except for the coronoid process, which is supplied by temporalis muscle vessels.
Peripheral blood supply happens via The Periosteum. It runs parallel to the cortical surface of the bone and penetrates cortical bone and anastomose with the branches of the inferior alveolar artery.

Nerve Supply of Mandible

The mandibular branch of the trigeminal nerve (click here to read more about trigeminal nerve) serves the mandible. It gets divided into three nerves:

The Long Buccal Nerve: It is the anterior division of the mandibular nerve and supplies to mucosa opposite to the last three mandibular molars on their buccal aspect.
The Inferior Alveolar Nerve: It is the posterior division of the mandibular nerve and supplies to all the lower jab teeth, lower lip, and buccal mucosa from the incisor to the premolar and the skin over the chin.
The Lingual Nerve: It is also the posterior division of the mandibular nerve and supplies to the anterior two third of the tongue, the mucosa on the lingual aspect of the lower teeth, and the floor of the mouth.

Lymphatic Drainage of Mandible

Most of the mandible and lower teeth drain into the submandibular group of lymph nodes except for a small wedge in the symphysis region and the lower incisor, which drain into the submental group of lymph nodes.
From the submental group, the lymph drains to the submandibular group of nodes.
Most submandibular nodes ultimately drain to the jugulo-omohyoid group of deep cervical lymph nodes.
A few extremely posterior submandibular nodes drain to the jugulo-diagastric group of deep cervical lymph nodes.

Points to Remember

Bony landmarks of the body

Mental protuberance,
Mental tubercle,
Mental foramen,
Oblique line of mandible,
Sublingual fossa,
Mylohyoid line,
Submandibular fossa,
Mental spines,
Digastric fossa,
Alveolar part,
Interalveolar septa

Bony landmarks of the ramus

Coronoid process,
Mandibular notch,
Condylar process,
Head of mandible,
Neck of mandible,
Pterygoid fovea,
Pterygoid tuberosity,
Masseteric tuberosity,
Angle of mandible,
Mandibular foramen,
Lingula,
Mylohyoid groove

References:

1. Human Anatomy By B D Chaurasia's 7th Edition

DrDent_Notes