Generality
The mandible, is the lower jaw, is the bone of the face in which the lower dental arch is housed 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 consistently) throughout life.
What is the mandible?
The mandible, or lower jaw, is the U-shaped face bone 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 widest, the most resistant and the only one that has the ability to move.
Anatomy
Unequal and symmetrical bone, the mandible has a horizontal portion, called 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.
The anatomists recognize 4 regions:
- The outer face
In the center, this region has a vertical line, known as a chin symphysis . The chin symphysis is the sign of the union process that involves the jaw and that takes place during the first childhood.
Just below the chin symphysis, the so-called chin protuberance takes place, ie a thickening of the triangular shaped mandibular bone.
Immediately at the margins of the chin protuberance, both right and left, rounded prominences develop, known as mentoring 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 chin hole .
The external oblique line converges in the so-called coronoid process, which, as will be seen, constitutes a fundamental part of the branches.
The chin hole is an orifice, from which the nerve endings and chin-like blood vessels come out. Generally, it occupies the position between the first and second premolar.
- The inner face
This region has, in the center, 4 small protrusions, arranged in pairs and called: upper chin rests (or apophysis superior genes) and lower chin rests (or lower genes apophysis). The upper chin rests (higher because they are higher than the others) represent the anchor point for the genioglossus muscles; the lower chin rests, on the other hand, are the protrusions to which the genioioid muscles are attached.
On each side of the inner face, there 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 upper margin
On the upper edge of the body, there are cavities, called with the term of alveoli, which host the roots of the lower teeth.
- The lower margin
On the lower margin, precisely on the sides of the chin symphysis, there is a sort of oval and wrinkled depression, known as a 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 that go upwards and backwards.
The area where the body begins to bend and form the branches is called the mandibular angle. The mandibular angle is comparable to a protrusion; the features of this projection vary according to the race, sex and age of the individuals.
Each branch has two processes: one in anterior position, called the coronoid process, and one in a posterior position, called condyle . To separate the aforementioned processes, there is a recess known as mandibular incisura (or sigmoidea incisura ).
- The coronoid process is flattened, has a triangular shape and acts as a coupling point for the temporal muscle.
- The condyle has two overlapping portions: the neck, at the bottom, and the head, at the top.
On the inner side, the neck has a hollow, called the pterygoid dimple, which serves to insert the pterygoid muscle.
The head, on the other hand, constitutes the part of the jaw involved in the so-called temporomandibular joint . In fact, its particular globular shape allows it to wedge perfectly in the glenoid fossa of the temporal bone.
The temporomandibular joint is the joint element that allows the human being to open, close and move part of his mouth laterally.
On the inner face of each branch, just below the mandibular notch, lies an orifice, called the mandibular hole . The mandibular hole is in communication with the aforementioned chin-hole, through the so-called mandibular canal . Inside the mandibular canal run nerves and blood vessels that derive from the trigeminal nerve and constitute the mandibular division of the aforementioned cranial nerve.
innervation
The trigeminal nerve - which represents the 5th pair of cranial nerves - has three major branches (or branches): the ophthalmic division, the maxillary division and the mandibular division.
Each division consists of additional nerve branches, which neurologists define by the term 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 hole and runs through the mandibular canal until it emerges from the chin-hole.
Along its path inside the mandibular canal, it establishes nervous contact with the lateral teeth of the lower arch. Almost at the chin, however, it further subdivides, constituting the mental nerve and the incisive nerve .
- The mental nerve emerges from the chin-hole and reaches the lower lip, where it has a sensitive function.
- The incisive nerve innervates the incisive nerves (obviously of the lower dental arch). Unlike the previous nerve, it does not come out of the chin hole.
It has a sensitive function.
PRENATAL FORMATION AND IMMEDIATELY AFTER THE BIRTH OF THE MANDIBLE
The mandible derives from a process of ossification that involves the so-called Meckel cartilage and that starts around the sixth week of fetal life.
At the beginning of the process, what will then give rise to the actual jaw consists of two cartilaginous bars. Each bar contains, at the level of where the inferior alveolar nerve bifurcates in the mental nerve and in the incisive nerve, a center of ossification that gradually generates 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 jaw.
At birth, the mandible of the human being is a bone that can be divided into two segments, which are called the right half-left and the half-left hand (NB: the semi-prefix indicates "half", "half"). The point of separation resides in the center of the chin.
During childhood, the two segments merge with each other and the sign of fusion is the chin symphysis, located in the middle of the outer face of the jaw body.
CHANGES OF THE MANDIBLE DURING LIFE WITH THE AGE?
The jaw changes in appearance over the course of a lifetime. In fact, the jawbone of a newborn child is not like that of an older child or an adult. The difference consists not only in the fusion of the two half-blinds, but in several other elements.- Neonatal age . In this phase of life, the mandible is comparable to a shell containing two incisors, a canine (per side) and two molars (per side).
The mandibular canal is very large, 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; by contrast, the coronoid process has already assumed a vertical position.
- Childhood . It is the period in which the fusion of the two half-blinds takes place.
Beyond this, the mandibular body extends, above all behind the chin-hole. This elongation 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, ie those cavities that contain the roots of the lower teeth.
After the second dentition, the mandibular canal resides above the mylohyoid line, while the chin-hole is in the position it will have also in adult age (ie between the first and second premolar).
The mandibular angle is less obtuse, compared to the case described above: around 4 years of life, it measures about 140 °.
- Adult age . At this stage, the depth of the alveoli and the portion of the lower jaw is very similar. The chin hole is in the middle position, ie 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 before and, generally, measures 110-120 °. Such an angle verticalizes the branches, which are definitively developed.
- Advanced age . Old age involves a considerable reduction in the size of the mandibular bone. To this reduction, a gradual process of absorption of the alveoli also contributes, which leads to the progressive loss of the teeth of the lower arch.
With advancing age, the mandibular angle becomes again, again, particularly dull: 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 clearly 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.
Thus, it allows the execution of fundamental functions such as chewing and phonation .
Diseases of the Mandible
The most common and important issues that can affect the jaw are fractures against it.
| Typical site of mandibular fractures in percentage terms: | |
| condyle | 30% |
| Mandibular angle | 25% |
| Body | 25% |
| Chin symphysis | 15% |
| Branches | 3% |
| Coronoid process | 2% |
Representing about a 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 subject to rupture are: the condyle, the mandibular angle and the body.
