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In , the mandible (from the Latin mandibula, 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla).
The jawbone is the skull's only movable, posable bone, sharing joints with the cranium's . The mandible hosts the lower Human tooth (their depth delineated by the alveolar process). Many muscles attach to the bone, which also hosts (some connecting to the teeth) and . Amongst other functions, the jawbone is essential for chewing food.
Owing to the Neolithic advent of agriculture (), human jaws evolved to be smaller. Although it is the strongest bone of the facial skeleton, the mandible tends to deform in old age; it is also subject to fracturing. Surgery allows for the removal of jawbone fragments (or its entirety) as well as regenerative methods. Additionally, the bone is of great forensic significance.
From the inside, the mandible appears concave. On either side of the lower symphysis is the mental spine (which can be faint or fused into one), to which the genioglossus (the inferior muscle of the tongue) attaches; the geniohyoid muscle attaches to the lower mental spine. Above the mental spine, a median foramen and furrow can line the symphysis. Below the mental spine is an oval depression (the digastric fossa of the mandible) where the digastric muscle attaches.Gray's Anatomy: The Anatomical Basis of Clinical Practice (40th ed.), Churchill-Livingstone, Elsevier, 2008, 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.
On the inside at the center there is an oblique mandibular foramen, for the entrance of the inferior alveolar vessels and nerve. The margin of this opening is irregular; it presents in front a prominent ridge, surmounted by a sharp spine, the lingula of the mandible, which gives attachment to the sphenomandibular ligament; at its lower and back part is a notch from which the mylohyoid groove runs obliquely downward and forward, and lodges the mylohyoid vessels and nerve. Behind this groove is a rough surface, for the insertion of the medial pterygoid muscle. The mandibular canal runs obliquely downward and forward in the ramus, and then horizontally forward in the body, where it is placed under the Dental alveolus, with small openings for nerves. On arriving at the Incisor, it turns back to communicate with the mental foramen, giving off two small canals which run to the cavities containing the incisor teeth. In the posterior two-thirds of the bone the canal is situated nearer the internal surface of the mandible; and in the anterior third, nearer its external surface. It contains the inferior alveolar vessels and nerve, from which branches are distributed to the teeth.
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.
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 'to chew' and -bula (instrumental suffix).
In addition to mastication, the joint of the jawbone enables actions such speech and , while playing a more subtle role in activities such as and breathing.
In and the early fossil , 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 , only the dentary, articular, and angular bone bones remain, while in living , the dentary is accompanied only by the prearticular, and, in , 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 . An intermediate stage can be seen in some , 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 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 .Pinhasi, R., Eshed, V., & von Cramon-Taubadel, N. (2015). Incongruity between affinity patterns based on mandibular and lower dental dimensions following the transition to agriculture in the Near East, Anatolia, and Europe. PLOS ONE, 10(2), e0117301.Lieberman, D. E., Krovitz, G. E., Yates, F. W., Devlin, M., & Claire, M. S. (2004). Effects of food processing on masticatory strain and craniofacial growth in a retrognathic face. Journal of human evolution, 46(6), 655–677.Pinhasi, R., Eshed, V., & Shaw, P. (2008). Evolutionary changes in the masticatory complex following the transition to farming in the southern Levant. American Journal of Physical Anthropology: The Official Publication of the American Association of Physical Anthropologists, 135(2), 136–148.Kahn, S., Ehrlich, P., Feldman, M., Sapolsky, R., & Wong, S. (2020). The jaw epidemic: Recognition, origins, cures, and prevention. BioScience, 70(9), 759–771. This has led to orthodontic .
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
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.
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.
The jawbone can also become deviated in mandibular lateral displacement, a condition which can offset facial symmetry and cause posterior crossbite.
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 Gumps, whose design apparently lacks a jaw; proposed reconstruction methods include implanting Artificial bone, potentially involving 3D printing.
Mandibular prosthetics date back to ancient Egypt and Ancient 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.
Dental remains of Nazi leader Adolf Hitler, including part of a mandible with teeth, were the solitary physical evidence used to confirm his death in 1945.
As early as 1900, the phrase 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.
, 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 as the jawbone fragment was ruptured along the alveolar process.
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.
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. 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 , 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 . In , 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. mandible]]
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.
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.
Clinical significance
The posterior of the mandible is notoriously resistant to the full effects of local anesthesia. The IAN provides sensory innervation to much of the mandible and its teeth, making it a target of Nerve block. Injecting the nerve is challenging due to the amount of surrounding soft tissue. American surgeon William Stewart Halsted developed a technique using a syringe and cocaine which was performed successfully by 1885.
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.
+Causes of mandibular fractures
!Cause
!Percentage Motor vehicle accident 40% Assault 10% Fall 10% Sport 5% Other 5%
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.
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
The surgical removal ( resection) of all or part of the jawbone is known as a mandibulectomy. The removal of a portion comprising the mandible's full height is called segmental mandibulectomy and the removal of a shallower portion is known as partial or marginal mandibulectomy. This can be performed in response to cancer (i.e. tumor removal), infection, injury, or osteonecrosis. The removed portion 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. During recovery, a feeding tube is utilized, and sometimes a tracheotomy is also performed to maintain respiration in case of swollen muscles. In a technique illustrated in the 19th century, the flesh of the face is incised along the inferior of the mandible and peeled upward for the bone's removal.
Regeneration
Bone loss (as in osteoporosis) can be mitigated in the jawbone via bone grafting, which is sometimes performed to support (replacing teeth individually or Dentures).
Forensic medicine
The mandible can provide forensic evidence because its form changes over a person's life, and this 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. Judges 15:16 on BibleHub.
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