Generality
The mandible is the lower jaw, it is the bone of the face in which the lower dental arch lodges and constitutes, with the upper jaw, the mouth.
The mandible has a horizontal portion, which takes the name of body or base, and two vertical portions, located on the sides of the body, called branches.
Body and branches have specific anatomical characteristics, which, in the human being, vary (even in a consistent way) in the course of life.
What is the jaw?
The mandible, or lower jaw, is the U-shaped bone of the face that houses the lower dental arch and constitutes, with the upper jaw, the mouth.
Among the various bony structures that make up the face, it is the largest, the most resistant and the only one that has the ability to move.
Anatomy
Uneven and symmetrical bone, the mandible has a horizontal portion, called the body or base, and two vertical portions, on the sides of the body, which are called upright branches or branches.
BODY
The body has the shape of a horseshoe with the concavity facing backwards.
Anatomists recognize 4 regions:
- There outer face
This region has a vertical line in the center, known as the chin symphysis. The chin symphysis is the sign of the union process involving the mandible and which takes place during early childhood.
Just below the chin symphysis, the so-called chin protuberance takes place, that is a triangular-shaped thickening of the mandibular bone.
Immediately at the edges of the chin protuberance, both on the right and on the left, rounded prominences develop, known as mental tubercles.
In an even more lateral position, with respect to the chin protuberance, and always on both sides, there are the external oblique line and the mental foramen.
The external oblique line converges in the so-called coronoid process, which, as will be seen, forms a fundamental part of the branches.
The mental foramen is an orifice, from which the nerve endings and the mental blood vessels emerge. Typically, it occupies the position between the first and second premolars. - There inner face
This region has, in the center, 4 small protrusions, arranged in pairs and called: upper mental spines (or upper genes apophysis) and lower mental spines (or lower genes apophysis). The upper chin spines (superior because in a higher position than the others) represent the anchor point for the genioglossus muscles; the lower chin spines, on the other hand, are the protrusions to which the geniohyoid muscles are hooked.
On each side of the inner face, the following develop: an oblique line (known as the mylohyoid line), the sublingual dimple (which houses the sublingual gland) and the maxillary dimple (which houses the maxillary gland). - The top margin
On the upper edge of the body, cavities take place, called alveoli, which house the roots of the lower teeth. - The bottom margin
On the lower margin, precisely on the sides of the chin symphysis, there is a sort of oval and wrinkled depression, known as the digastric dimple.
Inside the digastric dimple, the anterior digastric muscle is inserted.
BRANCHES
The branches, one on the right and one on the left, represent the vertical continuations of the body; vertical continuations going up and back.
The area where the body begins to bend and form branches is called the mandibular angle. The mandibular angle is comparable to a protrusion; the characteristics of this protrusion vary according to the population to which it belongs, the sex and age of the individuals.
Each branch has two processes: one in the anterior position, called the coronoid process, and one in the posterior position, called the condyle. To separate the aforementioned processes, there is a hollow known by the term mandibular notch (or sigmoid notch).
- The coronoid process it is flattened, has a triangular shape and acts as an attachment point for the temporal muscle.
- The condyle it has two overlapping portions: the neck, at the bottom, and the head, at the top.
The neck has, on the internal side, a hollow, called the pterygoid dimple, which serves for the insertion of the pterygoid muscle.
The head, on the other hand, is the part of the mandible involved in the so-called temporomandibular joint. In fact, its particular globular shape allows it to perfectly fit into the glenoid fossa of the temporal bone.
The temporomandibular joint is the joint element that allows the human being to open, close and move laterally part of his mouth.
On the internal face of each branch, just below the mandibular notch, there is an orifice, called the mandibular hole. The mandibular hole is in communication with the aforementioned mental foramen, through the so-called mandibular canal. Inside the mandibular canal the nerves and nerves run. blood vessels that derive from the trigeminal nerve and constitute the mandibular division of the aforementioned cranial nerve.
INNERVATIONS
The trigeminal nerve - which represents the fifth pair of cranial nerves - has three major branches (or branches): the ophthalmic division, the maxillary division and the mandibular division.
Each division consists of further nerve branches, which neurologists define as minor branches.
Among the various minor branches of the mandibular nerve, there is one, called the inferior alveolar nerve, which has the task of innervating some parts of the mandible.
The inferior alveolar nerve enters the mandibular foramen and runs through the entire mandibular canal until it emerges from the mental foramen.
Along its path inside the mandibular canal, it establishes nerve contacts with the lateral teeth of the lower arch. Almost in correspondence with the chin, on the other hand, it further subdivides, forming the mental nerve and the incisive nerve.
- The mental nerve emerges from the mental foramen and reaches the lower lip, where it has a sensitive function.
- The incisor nerve innervates the incisive nerves (obviously of the lower dental arch). Unlike the previous nerve, it does not protrude from the mental foramen.
It has a sensitive function.
PRENATAL AND IMMEDIATE FORMATION AFTER THE BIRTH OF THE MANDIBULA
The mandible derives from an ossification process involving the so-called Meckel's cartilage and which begins around the sixth week of fetal life.
At the beginning of the process, what will then give rise to the actual mandible consists of two cartilaginous bars. Each bar contains, at the level of where the inferior alveolar nerve forks into the mental nerve and incisor nerve, a center of ossification that generates gradually the various bony parts of the future mandible.
In support of these ossification centers, there are also some accessory ossification nuclei, which, located on the branches, give life to the coronoid process and the condyle on each side of the mandible.
At birth, the human jawbone is a bone that can be divided into two segments, which take the name of right half jaw and left half jaw (NB: the prefix semi- indicates "half", "half"). center of the chin.
During childhood, the two segments merge with each other and the sign of the fusion is the chin symphysis, located in the middle of the outer face of the body of the mandible.
CHANGES OF THE MANDIBULA DURING LIFE WITH AGE €
The jaw changes in appearance over the course of life. In fact, a newborn baby's jaw is not like that of an older baby or adult. The difference is not only in the fusion of the two semi-jaws, but in several other elements.
- Neonatal age. At this stage of life, the mandible is comparable to an envelope containing two incisors, one canine (on each side) and two molars (on each side).
The mandibular canal is very wide, compared to the mandibular bone structure, and runs very close to the lower edge of the body.
The mandibular angle is particularly obtuse and measures almost 175 °.
The condyle has almost the same orientation as the body; on the other hand, the coronoid process has already assumed a vertical position. - Childhood. It is the period in which the fusion of the two semi-jaws takes place.
Beyond this, the mandibular body lengthens, especially posterior to the mental foramen. This lengthening offers space for the growth of three more teeth (per side).
The body also grows in depth, thus also increasing the size of the alveoli, that is, those cavities that contain the roots of the lower teeth.
After the second dentition, the mandibular canal resides above the mylohyoid line, while the mental foramen is in the position it will also have in adulthood (i.e. between the first and second premolars).
The mandibular angle is less obtuse than in the case described above: around the age of 4, it measures about 140 °. - Adulthood. In this phase, the depth of the alveoli and the underlying jaw portion is very similar. The mental foramen is in a median position, that is, it is equally distant from the upper and lower edges of the body. The external oblique line is clearly visible.
On the inner face, the mandibular canal runs parallel to the mylohyoid line.
Finally, the mandibular angle is even less obtuse than previously and, generally, it measures 110-120 °. Such an angle verticalizes the branches, which are definitively developed. - Old age. Old age involves a considerable reduction in the size of the mandibular bone. A gradual process of absorption of the alveoli also contributes to this reduction, leading to the progressive loss of the teeth of the lower arch.
With the advancing age, the mandibular angle becomes, again, particularly obtuse: the typical measurement is around 140 °.
The verticality of the branches varies from subject to subject; in some individuals it remains similar to adulthood, in others it is evidently reduced.
Functions
Thanks to its mobility, the jaw allows the human being to chew food, speak and move the mouth according to the needs of the moment.
Therefore, it allows the execution of fundamental functions such as chewing and phonation.
Diseases of the Mandible
The most common and important problems that can affect the jaw are fractures to its load.
Representing about one fifth of facial injuries, mandibular fractures are often the result of impact trauma involving the face. Among the traumatic circumstances that can cause the jaw to break, the most frequent are: car accidents (40% of cases), assaults (40%), involuntary falls (10%) and blows to the face during a contact sport (5%).
The regions of the mandible most prone to breakage are: the condyle, the mandibular angle and the body.
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