Anatomy, Head and Neck, Mandible

Article Author:
Grant Breeland
Article Editor:
Bhupendra Patel
Updated:
11/14/2018 12:04:31 AM
PubMed Link:
Anatomy, Head and Neck, Mandible

Introduction

The mandible is the largest bone in the human skull. It holds the lower teeth in place, it assists in mastication and forms the lower jawline.  The mandible is composed of the body and the ramus and is located inferior to the maxilla. The body is a horizontally curved portion that creates the lower jawline. The rami are two vertical processes located on either side of the body; they join the body at the angle of the mandible. At the superior aspect of each ramus, the coronoid and condylar processes articulate with the temporal bone to create the temporomandibular joint which permits mobility. Other than the ossicles of the ear, the mandible is the only skull bone that is mobile, allowing the bone to contribute to mastication.

During development, the first pharyngeal arch forms two processes that eventually fuse at the mandibular symphysis to form the mandible. At birth, the mandibular symphysis is composed of fibrocartilage. Within one year of life, the symphysis fuses and a subtle ridge remains at midline on the anterior surface of the body.[1]

Structure and Function

The mandible is made up of the following parts: the body and two rami.

Body

The body is the anterior portion of the mandible and is bound by two surfaces and two borders.  The body ends and the rami begin on either side at the angle of the mandible, also known as the gonial angle. 

  • External surface: The external surface contains the mandibular symphysis at midline, detected as a subtle ridge in the adult. The inferior portion of the ridge divides and encloses a midline depression called the mental protuberance. The edges of the mental protuberance are elevated, forming the mental tubercle. Laterally to the ridge and below the incisive teeth is a depression known as the incisive fossa.  Below the second premolar is the mental foramen, in which the mental nerve and vessels exit. The oblique line courses posteriorly from the mental tubercle to the anterior border of the ramus. 
  • Internal surface: The internal surface contains the median ridge at midline and mental spines, which are just lateral to the ridge. The mylohyoid line begins at midline and courses superiorly and posteriorly to the alveolar border.
  • Alveolar border: The alveolar border, which is the superior border, contains the hollow cavities in which the lower sixteen teeth reside. 
  • Inferior border: The inferior border creates the lower jawline and contains a small groove in which the facial artery passes.

Ramus

The ramus contributes to the lateral portion of the mandible on either side. The coronoid process and condyloid process are located at the superior aspect of the ramus. The coronoid process is anterior and the condyloid process is posterior; the two are separated by the mandibular notch. The ramus is bound by two surfaces and four borders and contains two processes.

  • Lateral surface: The lateral surface contains a portion of the oblique line, which began on the external surface of the body. This surface also provides the origin for the masseter muscle.
  • Medial surface: The medial surface contains the mandibular foramen through which the inferior alveolar nerve and inferior alveolar artery enter and subsequent course the mandibular canal. At the anterosuperior aspect of the mandibular foramen is a sharp process called the lingula of the mandible. At the posteroinferior aspect of the mandibular foramen is the mylohyoid groove, against which the mylohyoid vessels run.
  • Superior border: The superior border which gives rise to the coronoid and condyloid processes.
  • Inferior border: The inferior border is continuous with the inferior border of the mandibular body and contributes to the jawline.
  • Posterior border: The posterior border is continuous with the inferior border of the ramus and is deep to the parotid gland. This border is used in conjunction with the inferior border of the mandibular body to determine the gonial angle.
  • Anterior border: The anterior border is continuous with the oblique line of the external surface of the body.

Coronoid Process

The coronoid process is located at the superior aspect of the ramus. Its anterior border is continuous with that of the ramus, and its posterior border creates the anterior boundary of the mandibular notch. The temporalis muscle and masseter insert on its lateral surface.

Condyloid Process 

The condyloid process is also located at the superior aspect of the ramus and is divided into two parts, the neck and the condyle. The neck is the thinner portion of the condyloid process that projects from the ramus. The condyle is the most superior portion and contributes to the temporomandibular junction by articulating with the articular disk.

Embryology

Occurring during the sixth week of intrauterine development, the mandible is the second bone to ossify, following the clavicle.[1]  The first pharyngeal arch, known as the mandibular arch, gives rise to the Meckel cartilage. This cartilage serves as a template for the development of the mandible. A fibrous membrane covers the left and right Meckel cartilage at their ventral ends, each of which gives rise to a single ossification center. These two halves eventually fuse via fibrocartilage at the mandibular symphysis. Thus, at birth, the mandible is still composed of two separate bones. Ossification and fusion of the mandibular symphysis occur during the first year of life, resulting in a single bone. The remnant of the mandibular symphysis is a subtle ridge at the midline of the mandible.[1]

The mandible constantly changes throughout an individual's life. At birth, the gonial angle is approximately 160 degrees.  By age four, teeth have formed, causing the jaw to elongate and widen; these changes in the mandible's dimensions cause the gonial angle to decrease to approximately 140 degrees. By adulthood, the gonial angle is decreased to approximately 120 degrees.[1]

Blood Supply and Lymphatics

Blood supply to the mandible is via small periosteal and endosteal vessels. The periosteal vessels arise mainly from the inferior alveolar artery and supply the ramus of the mandible. The endosteal vessels arise from the peri-mandibular branches of the maxillary artery, facial artery, external carotid artery, and superficial temporal artery; these supply the body of the mandible.[2] The mandibular teeth are supplied by dental branches from the inferior alveolar artery.

Lymphatic drainage of the mandible and mandibular teeth are primarily via the submandibular lymph nodes; however, the mandibular symphysis region drains into the submental lymph node, which subsequently drains into the submandibular nodes.

Nerves

The main nerve associated with the mandible is the inferior alveolar nerve, which is a branch of the mandibular division of the trigeminal nerve. The inferior alveolar nerve enters the mandibular foramen and courses anteriorly in the mandibular canal where it sends branches to the lower teeth and provides sensation. At the mental foramen, the inferior alveolar nerve branches into the incisive and mental nerve.  The mental nerve exits the mental foramen and courses superiorly to provide sensation to the lower lip. The incisive nerve runs in the incisive canal and provides innervation to the mandibular premolar, canine, and lateral and central incisors.[3]

Muscles

Muscles Originating from the Mandible

  • Mentalis - originates from the incisive fossa
  • Orbicularis oris - originates from the incisive fossa
  • Depressor labii inferioris - originates from the oblique line  
  • Depressor anguli oris - originates from the oblique line
  • Buccinator - originates from the alveolar process
  • Digastric anterior belly - originates from the digastric fossa
  • Mylohyoid - originates from the mylohyoid line
  • Geniohyoid - originates from the inferior portion of the mental spine
  • Genioglossus - originates from the superior portion of the mental spine
  • Superior pharyngeal constrictor - originates partially from the pterygomandibular raphe, which originates from the mylohyoid line

Muscles Inserting on the Mandible

  • Platysma - inserts on the inferior border of the mandible
  • Superficial masseter - inserts on the lateral surface of the ramus and angle of the mandible
  • Deep masseter - inserts on the lateral surface of the ramus and angle of the mandible
  • Medial pterygoid - inserts on the medial surface of the mandibular angle and ramus of the mandible
  • Inferior head of the lateral pterygoid - inserts on the condyloid process
  • Temporalis - inserts on the coronoid process

Physiologic Variants

Males generally have squarer, more prominent mandibles than females. This is due to the larger size of the mental protuberance in males and the decreased gonial angle. The gonial angle is 90 degrees in males, compared to 110 in females.

In rare instances, a bifid or trifid inferior alveolar canal may be present.  This can be detected on X-ray as a second or third mandibular canal. Branches of the inferior alveolar nerve commonly run through these extra foramina and can confer a risk for inadequate anesthesia during surgical procedures involving the mandible.[4]

A cleft chin can result from inadequate or absent fusion of the mandibular symphysis during embryonic development. This often results in a depression of the overlying soft tissue at the midline of the mandible. This is a genetic condition that is inherited in an autosomal dominant fashion and found more frequently in the male population.[5]

Surgical Considerations

Orthognathic surgery, which includes mandible osteotomies and sagittal split osteotomies, is corrective jaw surgery performed to improve bite malalignment, sleep apnea, temporomandibular joint disorders, and structural issues such as cleft palate and micrognathia.

Mandible osteotomy is performed on patients with micrognathia, a condition in which the mandible is undersized. Micrognathia may result in pain and difficulty chewing: correction is often needed.  This procedure is performed by transecting the mandible between the first and second molars, bilaterally; the mandible is extended into its new position and stabilized with hardware.

Sagittal split osteotomy is performed on patients with prognathism, a condition in which the mandible is oversized, causing an underbite.  This procedure is performed by transecting the mandible bilaterally, repositioning it in a more posterior position, and stabilizing it with hardware.

Complications include postoperative facial numbness due to nerve damage. Recovery from nerve damage typically occurs within 3 months of the procedure.

Clinical Significance

Mandibular fractures are most commonly caused by trauma and typically occur in two places. The parasymphysis is especially prone to fracture due to the incisive fossa and mental foramen. A direct blow to the mandible may cause a condylar neck fracture as the articular disk of the temporomandibular joint prevents it from moving posteriorly.[6]

In patients with traumatic mandibular injuries, a standard four-view series of X-ray film may be obtained. In many cases, due to superimposed anatomy, mandibular series do not provide sufficient detail to accurately diagnose condylar fractures. This issue can is solved by using a reversed Towne view for imaging. Newer modalities such as CT scans have proven more sensitive than X-ray and are commonly employed.[7]

Dislocation of the mandible is most frequently in the posterior direction, but anterior and inferior dislocations may be observed. The patient may present with the inability to close his or her mouth or with an asymmetric jawline. Previous dislocation is the greatest risk factor.  Manual reduction is often used to correct the injury. Barton bandages are used after reduction to hold the jaw in place and provide stabilization.

Other Issues

The mandible is a vital bone in terms of forensic evidence. Because the mandible progressively changes over an individual’s life, it is routinely used to determine the age of the deceased.



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      Image courtesy S Bhimji MD