The Anatomy of the Mandible

The mandible is the large bone that holds the lower teeth in place

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Also known as the lower jawbone, the mandible is the largest and strongest bone of the face. Tasked with holding the lower set of teeth in place, this bone has a symmetrical, horseshoe shape. Not directly connected to other bones of the skull, the mandible is the only moving bone of the skull and is attached to the major muscle groups of mastication (chewing) as well as the ligaments that make up the temporomandibular joint that allows motion. 

Health issues that arise with this bone usually have to do with fracture or dislocation due to trauma. In addition, corrective surgeries may be performed on the mandible to correct misalignment due to improper development of the jaw.

Anatomy

The largest facial bone, the mandible is roughly horseshoe-shaped, defining the lower margins and sides of the face. Anatomically speaking, it’s divided into two major sections: the body and the ramus. 

Structure

The body of the mandible is the almost rectangular front (anterior) portion of the bone, and it is grafted to the ramus (wing-like portion) on each side. In adults, its external surface has a slight ridge at its midline called the mandibular symphysis, which divides and encloses a depression called the mental protuberance as it moves downward. The edges of this part rise up to make up the mental tubercle.

To the side of this, and below the incisive (front) teeth is a depression called the incisive fossa, and there is an opening on each side adjacent to the premolars called the mental foramen. The upper border, also known as the alveolar border, contains hollow spaces for the teeth.  

Representing the “wings” of the mandible, the ramus arises on each side of the body, terminating at two ridges separated by the mandibular notch: the one towards the front called the coronoid process and the other towards the back of the head the condylaris process. These bound the temporomandibular joint, which allows the bone to move.

The lower surfaces of the ramus define the jawline, and the outer sides are connected to the masseter muscle (for chewing). The inner surfaces contain several openings (fossa) that allow important nerves and arteries to access the mouth region.

Location

The mandible’s relations to surrounding structures help determine its function. Notably, the inferior alveolar nerve, a branch of the mandibular nerve, accesses the mandible foramen and runs frontward, providing sensation to the lower set of teeth. At the mental foramen, it branches into the incisive and mental nerves; the latter of these enervates the lower lip, whereas the former processes sensation for the mandibular premolar, canine, as well as lateral and central incisors.

Given that this bone is involved with mouth motions, many important muscle groups also make contact with the mandible. A number of muscles arise from this bone.

  • From the incisive fossa, the mentalis (which allows the lower lip to pout) and orbicularis oris (the muscles surrounding the lips) emerge.
  • The oblique line of the mandible is where the depressor labii inferioris and depressor anguli oris emerge. These are associated with frowning.
  • The alveolar process of the mandible is where the buccinator muscle originates; this muscle assists in chewing.          
  • Running from the mylohyoid line is the mylohyoid muscle, which is the major one of the floor of the mouth. In addition, this section is connected to the superior pharyngeal constrictor, which plays a big role in swallowing.
  • The mental spine gives rise to two important muscles: the geniohyoid (connecting the mylohyoid muscle and the chin) as well as the genioglossus (the fan-shaped muscle that forms a major portion of the tongue).

Furthermore, other muscles link to the mandible, including:

  • The platysma arises from the collarbone and progresses to the underside of the mandible.
  • Inserting into the side surface of the ramus is the superficial masseter, which is a major muscle of chewing and mouth movement.
  • The deep masseter also inserts into the mandible at the outside surface of the ramus and is involved in chewing motion.
  • The medial angle of the mandibular angle (the outer corner of the mandible) and ramus is the site where the medial pterygoid muscle inserts. This thick, roughly rectangular muscle is also involved in chewing function.
  • At the condyloid process, the inferior head of the lateral pterygoid muscle, which moves the jaw downward and from side to side and is, therefore, another important structure for chewing.
  • The temporalis muscle, a broad, fan-shaped structure along the sides of the head that also work to help with chewing, accesses the coronoid process of the mandible.  

Anatomical Variations

Typically, men have more square-shaped mandibles than women, something which arises because their mental protuberances are larger, and they display a mandibular angle that is smaller. 

In rarer instances, however, the alveolar canal may be duplicated or even triplicated. This is usually seen in X-ray and can complicate anesthesia practice in oral or facial surgery as there is a risk of accidentally piercing and damaging the nerves that populate these canals.

In addition, some may have a condition called “micrognathia,” which is an abnormally small mandible; others have the opposite—“prognathia”—which leads to an underbite.

Finally, cleft chin, which is basically an incomplete joining together of the bones of the mandible, can arise during embryonic development. In these cases, there is a Y-shaped dimple in the middle of the chin.

Function

Along with the upper jaw or maxilla, the mandible serves an essential structural and protective function. Not only do important nerves and muscles run through this bone and emerge from it, but it’s also what houses the lower set of teeth.

The mandible is intimately involved with chewing function as well as most any movement of the mouth.

Associated Conditions

Naturally, the most commonly seen issue that arises in the mandible is fracture or dislocation due to an accident or fall. These breaks are most commonly seen in the condyle portion of the bone, though they can arise in other portions, such as the body, the mandibular angle, and other parts of the ramus.

Dislocations can also occur, with the most frequent of these being due to the mandible being pushed back. These can lead to an inability for the patient to close their mouth or a misalignment of the structure.

Other problems, not necessarily related to trauma, may also arise in this part of the body. Misalignment of the jaw—whether due to trauma or arising naturally—can seriously damage teeth and impact other parts of the head and neck.

Furthermore, positioning of the mandible can be implicated in sleep apnea (excessive snoring), cleft palate, and temporomandibular joint disorders (pain right at the juncture of the upper and lower jaw).

A rarer, though no less significant condition is osteomyelitis, which is an infection of the bone. This can lead to bone disintegration within the mandible, which is irreversible. In addition, cysts—sac-like structures—can form in the molars, and if these aren’t treated the jaw bone itself can be damaged.

Rehabilitation

Treatment for mandible fracture depends on the location and scope of the issue. Following CT scan, X-ray, or MRI to assess the break, doctors typically have two choices: reduction or fixation.

Reduction involves approximating the locations of the broken ends and setting the jaw to that position, often with wires wrapped around the teeth. Fixation is similar in nature to reduction but includes the use of an additional arch bar that secures upper and lower teeth to one another for proper positioning. Depending on the injury, soft tissues may also need to be pierced and used as additional support.

Orthognathic surgery treats problems stemming from a misaligned jaw, as well as sleep apnea, cleft palate, and temporomandibular joint disorders. Basically, this is an osteotomy, which is the cutting and shaping of a portion of the bone to manipulate fit. Those with micrognathia may require this type of surgery to correct alignment.

After surgery, a significant amount of rehabilitation will be needed, with emphasis placed on ensuring proper positioning of the mandible with regards to the rest of the skull.

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Article Sources

  1. Breeland G, Patel B. Anatomy, Head and Neck, Mandible. Published 2018.

  2. Mandible. Kenhub. Published 2018.

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