Anatomy

General considerations
Transoral or transfacial surgical approaches, pin placement for external fixators, reduction and fixation using an internal system can interfere with the following anatomic structures.

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Motor nerve/cranial nerve VII/facial nerve

Main trunk of facial nerve
The main trunk of the facial nerve and its two major divisions (temporofacial and cervicofacial) cross the posterior border of the mandible ramus in the subcondylar neck region.

Marginal mandibular branch of the facial nerve
The nerve is at risk for injury in transfacial approaches to expose the posterior and lateral surfaces of the mandible (submandibular, retromandibular, facelift approaches).

Frontotemporal branches of the facial nerve
The frontotemporal branches of the facial nerve ascend from the main trunk in a diagonal course to the forehead and they are at risk of injury in preauricular or extended coronal approaches.

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Facial vessels

The facial vessels cross the inferior border of the mandible at the level corresponding to the anterior border of the masseter muscle. The vessels are embedded in the lower extensions of the buccal fat pad.

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The artery is always located anterior to the vein. A periosteal layer separates the vascular bundle from the lateral bony surface of the mandible.

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Sensory nerves: branches of the mandibular nerve (CN V3)

Clinically important ramifications of the 3rd division of trigeminal nerve (CN V3):
The third or mandibular division of the trigeminal nerve passes the skull base through the foramen ovale into the infratemporal fossa and divides into a smaller mainly motor portion that gives off branches to the muscles of mastication. There is a larger branch that is predominantly sensory in nature. The sensory buccal nerve arises from the smaller portion.

The larger sensory trunk divides into the auriculotemporal, lingual, and inferior alveolar nerves. They provide sensation to:

• Parts of the external auditory canal
• The parotid
• The temporal region
• The inner cheek
• The anterior two-thirds of the hemitongue
• The floor of the mouth
• The mandibular teeth
• The lower lip/chin area

Auriculotemporal nerve:
The auriculotemporal nerve (mainly sensory) loops around the middle meningeal artery by two roots and reunites laterally. Then it curves posteriorly in a plane on the inner aspect of the medial pterygoid muscle and continues superiorly between the temporomandibular joint and the external acoustic meatus before it enters the upper part of the parotid gland and accompanies the superficial temporal vessels. It has a number of small branches to the temporomandibular joint, to the skin of the external auditory canal and the lateral surface of the tympanic membrane, to the tragus and lower auricular concha and to supply the facial nerve with sensory and parasympathetic fibers for the innervation of the skin over the parotid region and cheek. The upper terminal branch supplies sensation to the lateral side of the head in a zone similar to the vascularization by the superficial temporal artery.

The auriculotemporal nerve and its branches are at risk for injury in external preauricular approaches to the temporomandibular joint and in rhytidectomy incisions.

Lingual nerve:
The lingual nerve lies anterior to the inferior alveolar nerve and first descends deep to the lateral pterygoid muscle and superficial to the medial pterygoid muscle. From the lower border of the lateral pterygoid muscle the nerve passes obliquely forward between the medial pterygoid muscle and the inner aspect of the ascending ramus until it reaches the bottom fold of the pterygomandibular space. At the height of the posterior mylohyoid line it exits the space and curves anteriorly to take a longitudinal direction. For a short distance it lies close to the lingual cortex of the inner mandibular angle (temporal crest), the retromolar trigone and third molar region. Towards the oral cavity it is covered by the mucous membrane and the posterior tail of the submandibular gland, so that it can often be visualized as a pale shade by tautening the soft tissues of the posterior glossoalveolar sulcus. On its course to the dorsum and the tip of the tongue it then turns medially and crosses the lateral side of the superior pharyngeal constrictor, styloglossus and hyoglossus muscles to split into its terminal branches.

Exiting from the petrotympanic fissure the chorda tympani joins the back of the lingual nerve in an acute angle already at the level of the lateral pterygoid muscle.

The lingual nerve provides pain, temperature and touch sensation to the anterior two-thirds of the hemitongue, the glossoalveolar sulcus and the lingual gingiva of the entire lower quadrant. Taste mediating fibers from the anterior two-thirds of the hemitongue are conveyed centrally via the chorda tympani. The chorda tympani also contains visceral efferent fibers travelling to the sublingual and submandibular gland.

The lingual nerve is prone to injury during intraoral approaches exposing the medial side of the retromolar trigone and the inner aspect of the ascending ramus. Sometimes the nerve is hit while drilling or with transverse screw insertion along the upper border (oblique line) of the inner bony angle. The chorda tympani itself is out of reach during fracture repair procedures but its fascicles inside the lingual nerve can be affected.

Inferior Alveolar Nerve:
The inferior alveolar nerve (IAN) is the largest branch of the mandibular nerve and carries sensory and motor fibers. On its way from the infratemporal fossa and mandibular foramen at the medial surface of the ascending ramus it descends in a parallel route (posterior and lateral) to the lingual nerve. The mylohyoid nerve branches off posteriorly just before the main nerve enters the mandibular foramen. The mylohyoid nerve runs inferiorly and anteriorly in the mylohyoid groove and further forward into the digastric triangle. It provides motor innervation to the mylohyoid muscle and the anterior belly of the digastric muscle. It provides sensory fibers that supply a small circular zone of skin above the mental protuberance.
Furthermore the IAN can separate collateral branches in its precanalicular segment, which independently run downward on the bony surface and pierce into the bone through accessory foramina and small vents in the retromolar area. If encountered during exposure at the inner angle it is hard to decide whether they can be transected, because their contribution to the sensory innervation is not to be determined.

The IAN is accompanied by the inferior alveolar vessels in its course through the mandibular canal.

The mandibular canal can be positioned at a variable vertical height that has to be assessed in the preoperative imaging. Within the canal or in their intraosseous course the fascicles of the IAN can either unite into single or double stranded bundles as well as ramify into an ample plexus formation. These neural structures give rise to the inferior dental branches and the inferior gingival branches providing sensation to the teeth and gums. The latter branches often do not directly stem from the inferior alveolar nerve bundle or plexus but arise as a second series from the dental branches.
The IAN bifurcates at the mental foramen with a major portion exiting as the mental nerve to the lateral side. The minor quantity of fibers continues anteriorly inside a canalicular structure as an incisal bundle.

As a matter of principle the intraosseous IAN structures are at potential risk for lesions through insertion of osteosynthesis screws and must be examined on the basis of the preoperative radiographic findings.

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Mental nerve (mental neurovascular bundle)
The mental neurovascular bundle exits the mental foramen, which is the anterior opening of the mandibular canal. The bony foramen is usually positioned at a vertical height midway between the alveolar and basal border of the mandibular body just below the apices of the premolars. In the sagittal plane the mental foramen is located at a level projecting between the tips of the first and the second premolar.

The mental nerve is the major terminal branch of the inferior alveolar nerve. The branches enter the skin of the chin, the vestibular gingiva in the symphyseal region, and the mucosa of the labial sulcus and the lower lip vermillion.

Photographs show the branches of the mental nerve.

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Buccal nerve
The buccal nerve (sensory branch of mandibular nerve) passes medial to the ascending ramus and divides into a multitude of small branches overlying the lateral surface of the buccinator muscle. The terminal branches pierce the muscle for innervation of the buccal gingiva and the buccal mucosa between the pterygomandibular raphe and the corner of the mouth.

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Muscles

Mentalis muscle
The mentalis muscles are paired and function as elevators of the lip and chin. They arise directly from the bony surface of the anterior symphysis in a zone between the labial sulcus and the apices of the lower incisors, located in front of the mental foramina. If the mentalis muscle is cut during an anterior vestibular approach it must be carefully reattached to avoid a sagging or drooping chin.

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Buccinator muscle
The bony attachments of the buccinator muscle run a course below the mucogingival junction opposite to the molars and along the oblique line ascending as the anterolateral rim of the ascending ramus. The attachments extend back into the pterygomandibular raphe. The buccinator is innervated by the buccal branch of the facial nerve. The muscle belongs to the mimetic muscle system and has a unique functional structure allowing for movement comparable to peristaltic motion. Its detachment can result in an impaired bolus transport.
Reminder: The buccinator muscle belongs to the mimic muscle system and has a unique functional structure allowing for a movement comparable to a peristaltic motion. The deep fibers run in parallel bundles from the modiolus to the pterygomandibular raphe at the level of the occlusal plane (intercalar region) and account for the buccinator mechanism building up a ridge towards the occlusal plane. Its detachment can result in an impaired bolus transport out of the buccal space which can be annoying for the patient. The buccinator is innervated by the motor buccal branch of the facial nerve.

Buccal fat pad
The buccal fat pad first described by Marie Bichat in 1801 is located in an extended three-dimensional compartment with its main mass or cheek portion overlying the posterolateral aspect of the buccinator muscle and the maxillary tuberosity, which is partly covered by the anterior border of the masseter muscle. The major processes of the fat pad stretch into deep temporal, pterygomaxillary and pterygoid region. The fat is distinct from facial fat and similar to orbital fat in consistency and its globule-like structure, to which it connects through the infraorbital fissure.
The facial vessels run along the anterior border of the cheek portion.
The fascial layers (capsule) enveloping the buccal fat pad on the medial surface next to the buccinator muscle are rarely ruptured in a subperiosteal dissection along the mandibular body and angle. However, when going up the ramus subsequent to a vertical division of the buccinator muscle anterior to the pterygomandibular raphe in order to reach beyond the coronoid notch the inner capsule of the fat pad can be breached leading to a herniation that will obstruct the vision in the surgical field.

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Bony cross sections and intraosseous structures

• Tooth roots
• Mandibular canal/inferior alveolar nerve/no man’s land

See also:
Heibel H, Alt KW, Wächter R, Bähr W. (2001)
Cortical thickness of the mandible with special reference to miniplate osteosynthesis. Morphometric analysis of autopsy material
Mund Kiefer Gesichtschir; 5(3):180-5.