Mandible

In humans, the mandible is the largest and lowest bone in the facial skeleton. It is the only movable bone of the skull (discounting the vibrating ossicles of the middle ear). It is connected to the skull's temporal bones by the temporomandibular joints. In addition to simply opening and closing, the jawbone can articulate side to side as well as forward and back. Components The mandible consists of: • The body, curving anteriorly like a horseshoe This ridge divides below and encloses a triangular eminence, the mental protuberance (the chin), the base of which is depressed in the center but raised on both sides to form the mental tubercle. Just above this, on both sides, the mentalis muscles attach to a depression called the incisive foramen. Extending backward and upward on either side from the lower symphysis is a ridge called the mylohyoid line, where the mylohyoid muscle attaches; a small part of the superior pharyngeal constrictor muscle attaches to the posterior ridge, near the alveolar margin. Above the anterior ridge, the sublingual gland rests against a smooth triangular area, and below the posterior ridge, the submandibular gland rests in an oval depression. (right) Borders • The superior or alveolar border is narrower at the curved front than behind. It is hollowed into 16 dental alveoli (tooth sockets) of varying depth and size. To the outer lip of the superior border, on either side, the bucinator muscle is attached as far forward as the first molar. • The inferior border is rounded, longer than the superior, and thicker in front than behind; at the point where it joins the lower border of the ramus is the antegonial notch, where the mylohyoid branch of the inferior alveolar artery exits. Ramus and lateral pterygoid muscles attach to the ramus (partly cut away, along with the cheekbone). The ramus of the human mandible has four sides, two surfaces, four borders, and two processes. On the outside, the ramus is flat and marked by oblique ridges at its lower part. It gives attachment throughout nearly the whole of its extent to the masseter muscle. On the inside at the center there is an oblique mandibular foramen, for the entrance of the inferior alveolar nerve and vessels. Nerves The inferior alveolar nerve (IAN), a branch of the mandibular nerve (itself a major division of the cranium's trigeminal nerve), enters the mandibular foramen and runs forward in the mandibular canal, supplying sensation to the gums and teeth. Before passing through the mental foramen, the nerve divides into two terminal branches: incisive and mental nerves. The incisive nerve runs forward in the mandible and supplies the anterior teeth. The mental nerve exits the mental foramen and supplies sensation to the chin and lower lip. Variation Males generally have squarer, stronger, and larger mandibles than females. The mental protuberance is more pronounced in males but can be visualized and palpated in females. Rarely, a bifid IAN may be present, resulting in a second and more inferiorly placed mandibular foramen. This can be detected by noting a doubled mandibular canal via radiograph. == 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, == 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 (), 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. 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 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 . This has led to orthodontic malocclusions. == 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. 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. 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. == 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. 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. 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. 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. 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; 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. s supporting false teeth 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. ==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, 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. == Additional images ==