Mandible

Function

The mandible forms the lower jaw and holds the lower teeth in place. It articulates with the left and right temporal bones at the temporomandibular joints. The condyloid process, the superior (upper) and posterior projection from the ramus, makes the temporomandibular joint with the temporal bone. The coronoid process, superior and anterior projection from the ramus. This provides attachment to the temporal muscle.

Teeth sit in the upper part of the body of the mandible. The frontmost part of teeth is more narrow and holds front teeth. The back part holds wider and flatter (albeit grooved) teeth primarily for chewing food. The word mandible derives from the Latin word mandibula 'jawbone' (literally, 'used for chewing'), from mandere 'to chew' and -bula (instrumental suffix). In addition to mastication, the joint of the jawbone enables actions such speech and yawning, while playing a more subtle role in activities such as kissing and breathing.

Phylogeny

The mandible of vertebrates evolved from Meckel's cartilage, left and right segments of cartilage which supported the anterior branchial arch in early fish. Fish jaws surface in species of the large arthrodire genus Dunkleosteus (fl. 382–358 million years ago), which crushed prey with their quickly articulating mouths. The lower jaw of cartilaginous fish, such as sharks, is composed of a cartilagenous structure homologous with Meckel's cartilage. This also remains a significant element of the jaw in some primitive bony fish, such as sturgeons. In reptiles, Meckel's cartilage ossifies into the (multiple) bones of the lower jaw, while mammals of the Cretaceous (145–66 Mya) had both Meckel's cartilage and a mandible.

In lobe-finned fishes and the early fossil tetrapods, the bone homologous to the mandible of mammals is merely the largest of several bones in the lower jaw. In such animals, it is referred to as the dentary bone or os dentale, and forms the body of the outer surface of the jaw. It is bordered below by a number of splenial bones, while the angle of the jaw is formed by a lower angular bone and a suprangular bone just above it. The inner surface of the jaw is lined by a prearticular bone, while the articular bone forms the articulation with the skull proper. A set of three narrow coronoid bones lie above the prearticular bone. As the name implies, the majority of the teeth are attached to the dentary, but there are commonly also teeth on the coronoid bones, and sometimes on the prearticular.

Most vertebrates exhibit a simpler scheme, as bones have either fused or vanished. In teleosts, only the dentary, articular, and angular bones remain, while in living amphibians, the dentary is accompanied only by the prearticular, and, in salamanders, one of the coronoids. The lower jaw of reptiles has only a single coronoid and splenial, but retains all the other primitive bones except the prearticular and the periosteum. In birds, the various bones have fused into a single structure. In mammals, most have disappeared, leaving only the mandible. As a result, there is only articulation between the mandible and temporal bones, as opposed to articulation between articular and quadrate bones. An intermediate stage can be seen in some therapsids, in which both points of articulation are present. Aside from the dentary, only few other bones of the lower jaw remain in mammals; the former articular and quadrate bones survive as the malleus and the incus of the middle ear.

In paleontology, domestic dog bones can be identified via a generally smaller mandible compared to that of the wolf. In recent human evolution, both the oral cavity and jaws have shrunk in correspondence with the Neolithic-era shift from hunter-gatherer lifestyles towards agriculture and settlement, dated to c. 10,000 BCE. This has led to orthodontic malocclusions.

Gallery

Development

The mandible forms as a bone (ossifies) from Meckel's cartilage, which forms the cartilaginous bar of the mandibular arch and, dorsally, parts of the middle ear. The two sides of the jawbone are inferiorly fused at the mandibular symphysis (the chin) during the first year of life. The cartilage of the ramus is replaced by fibrous tissue, which persists to form the sphenomandibular ligament. Between the lingula and the canine tooth the cartilage disappears, while the portion of it below and behind the incisor teeth becomes ossified and incorporated with this part of the mandible.

About the sixth week of fetal life, intramembranous ossification takes place in the membrane covering the outer surface of the ventral end of Meckel's cartilage, and each half of the bone is formed from a single center which appears, near the mental foramen. By the tenth week, the portion of Meckel's cartilage which lies below and behind the incisor teeth is surrounded and invaded by the dermal bone (also known as the membrane bone). Somewhat later, accessory nuclei of cartilage make their appearance, as

• a wedge-shaped nucleus in the condyloid process and extending downward through the ramus;
• a small strip along the anterior border of the coronoid process;
• smaller nuclei in the front part of both alveolar walls and along the front of the lower border of the bone.

These accessory nuclei possess no separate ossific centers but are invaded by the surrounding dermal bone and undergo absorption. The inner alveolar border, usually described as arising from a separate ossific center (splenial center), is formed in the human mandible by an ingrowth from the main mass of the bone.

Embryonic development, right side 24 millimetres (15⁄16 inch) longCartilage in blue95 mm (3+3⁄4 in) long (after ten weeks)Cartilage in blue, accessory cartilage nuclei stippled

Aging

At birth, the body of the bone is a mere shell, containing the sockets of the two incisor, the canine, and the two deciduous molar teeth, imperfectly partitioned off from one another. The mandibular canal is of large size and runs near the lower border of the bone; the mental foramen opens beneath the socket of the first deciduous molar tooth. The angle is obtuse (175°), and the condyloid portion is nearly in line with the body. The coronoid process is of comparatively large size, and projects above the level of the condyle.

After birth, the two segments of the bone become joined at the symphysis, from below upward, in the first year; but a trace of separation may be visible in the beginning of the second year, near the alveolar margin. The body becomes elongated in its whole length, but more especially behind the mental foramen, to provide space for the three additional teeth developed in this part. The depth of the body increases owing to increased growth of the alveolar part, to afford room for the roots of the teeth, and by thickening of the subdental portion which enables the jaw to withstand the powerful action of the masticatory muscles; but, the alveolar portion is the deeper of the two, and, consequently, the chief part of the body lies above the oblique line. The mandibular canal, after the second dentition, is situated just above the level of the mylohyoid line; and the mental foramen occupies the position usual to it in the adult. The angle becomes less obtuse, owing to the separation of the jaws by the teeth; about the fourth year it is 140°. The fibrocartilage of the mandibular symphysis fuses together in early childhood.

In the adult, the alveolar and subdental portions of the body are usually of equal depth. The mental foramen opens midway between the upper and lower borders of the bone, and the mandibular canal runs nearly parallel with the mylohyoid line. The ramus is almost vertical in direction, the angle measuring from 110° to 120°, also the adult condyle is higher than the coronoid process and the sigmoid notch becomes deeper. The adult mandible is the skull's largest and strongest bone.

In old age, the bone can become greatly reduced in volume where there is a loss of teeth, and consequent resorption of the alveolar process and interalveolar septa. Consequently, the chief part of the bone is below the oblique line. The mandibular canal, with the mental foramen opening from it, is closer to the alveolar border. The ramus is oblique in direction, the angle measures about 140°, and the neck of the condyle is more or less bent backward.

Changes in the mandible with age

Clinical significance

In recent centuries, the mandible was sometimes exposed to harmful elements resulting in phossy jaw and radium jaw, until the causes were understood. The mandible is generally difficult to anesthetize due to the depth of soft tissue in the posterior and anatomical variance. American surgeon William Stewart Halsted developed a nerve block technique using a syringe and cocaine to block the IAN, as well as the incisive, mental and lingual nerves. Although some nerve branches were left unblocked, the procedure was performed successfully by 1885 and could be applied in 3–5 minutes with perhaps 80–85% success. Two other, more effective techniques were established by the 1970s.

Fracture

One fifth of facial injuries involve a mandibular fracture. Mandibular fractures are often accompanied by a 'twin fracture' on the opposite side. There is no universally accepted treatment protocol, as there is no consensus on the choice of techniques in a particular anatomical shape of mandibular fracture clinic. A common treatment involves attachment of metal plates to the fracture to assist in healing.

Dislocation and displacement

The mandible may be dislocated anteriorly (to the front) and inferiorly (downwards) but very rarely posteriorly (backwards). The articular disk of the temporomandibular joint prevents the mandible from moving posteriorly, making the condylar neck particularly vulnerable to fractures. Further, various jawbone damage can cause temporomandibular joint dysfunction, with symptoms including pain and inflammation.

The jawbone can also become deviated in mandibular lateral displacement, a condition which can offset facial symmetry and cause posterior crossbite.

Resorption

The mandibular alveolar process can become resorbed when completely edentulous in the mandibular arch (occasionally noted also in partially edentulous cases). This resorption can occur to such an extent that the mental foramen is virtually on the superior border of the mandible, instead of opening on the anterior surface, changing its relative position. However, the more inferior body of the mandible is not affected and remains thick and rounded. With age and tooth loss, the alveolar process is absorbed so that the mandibular canal becomes nearer the superior border. Sometimes with excessive alveolar process absorption, the mandibular canal disappears entirely and deprives the IAN of its bony protection, although soft tissue continues to guard the nerve.

Mandibulectomy

A mandibulectomy is the surgical removal (resection) of all or part of the jawbone. The procedure can be invoked as a response to cancer (i.e. tumor removal), infection, injury, or osteonecrosis. In techniques illustrated in the 19th century, the flesh of the face is incised and peeled back for bone removal. By 1885, cocaine was used as a nerve block in mandibular surgeries. Mandibulectomies are now performed under general anesthesia in the First World, but can be done awake in tandem with a tracheotomy to ensure respiration.

The removal of a portion comprising the mandible's full height is called segmental mandibulectomy and usually necessitates the loss of the mental nerve; it is often accompanied by neck dissection to check for cancer in the lymph nodes. The removal of a shallower portion is known as partial or marginal mandibulectomy; in some cases, the penetrated flesh is limited to the internal mucous membrane of the gums, without significantly disturbing the facial structure.

The removed jawbone fragment can be replaced with metal plating or bone from elsewhere in the body. Oral muscles tend to work differently after the procedure, requiring therapy to relearn operations such as eating and speaking. A feeding tube is utilized during recovery and sometimes also a tracheotomy in case of swollen muscles.

Complications can involve difficulties with free flap transfer and airway management. Additional side effects include pain, infection, numbness, and (rarely, fatal) bleeding. Even successful surgeries can result in deformity, with an extreme version being referred to as the Andy Gump deformity after the comic book character, whose design apparently lacks a jaw; proposed reconstruction methods include implanting synthetic material, potentially involving 3D printing.

Regeneration

Bone loss (as in osteoporosis) can be mitigated in the jawbone via bone grafting, which is sometimes performed to support dental implants (replacing teeth individually or in groups). Mandibular prosthetics date back to ancient Egypt and China, but significant advancements were made in the late 19th century with new techniques for attaching prosthetics to a depreciated jawbone as well as bone grafting.

In 2010, the first successful face transplant was conducted on a Spanish farmer after a self-inflicted gun accident; this included the replacement of the entire mandible.

Forensic medicine

The mandible provides forensic evidence as its form changes over a person's lifetime and thus can be used to determine a deceased person's age. Dental remains of Nazi leader Adolf Hitler, including part of a mandible with teeth (sundered along the alveolar process), are the only part of his body known to have been found.

In culture

In the Hebrew Bible and Christian Old Testament Book of Judges, Samson used a donkey's jawbone to kill a thousand Philistines. As early as 1900, the phrase jaw-dropping was used as an adjective to describe a condition of shock in humans, e.g. when someone's mouth suddenly hangs agape in response to something. The exaggerated visual gag of a jaw dropping to the floor was a trademark of American animation director Tex Avery, who would often employ it when the Big Bad Wolf spies a sexually attractive woman. Gobstoppers, a type of hard candy, are known in North America as jawbreakers due to the fracturing risk they impose on teeth.

Owing in part to the forensic evidence of Hitler's death being limited to his dental remains, a common fringe narrative is that the dictator faked his death, suggesting marginal mandibulectomy and eyewitness deception. In later decades, American real-estate businessman Fred Trump had a mandibulectomy of the right ramus, causing a conspicuous deformity. In his fight against cancer, American film critic Roger Ebert had part of his jawbone removed in 2006, in addition to later surgeries.

In arthropods

Mandibulates are characterized by a head tagma that has antennae and three feeding appendages that make up the arthropod mandible.

Mandibles in insects are pairs of hardened structures that are used to grind, crush and chew food. Mandibles have a broad molar region with ridged structures lined across to assist during chewing the food before it is swallowed. When paired with the maxillae (upper jaw structures), it is referred to as one of the gnathal appendages.

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