The lower jaw of a person (lat. Mandibula) is an unpaired mobile bone structure of the facial cranial section. It clearly shows the central horizontal part - the body (lat. Basis mandibulae) and two processes running at an upward angle (branches, lat. Ramus mandibulae), extending along the edges of the bone body.
She takes part in the process of chewing food, speech articulation, forms the lower part of the face. Let us consider how the anatomical structure of the lower jaw correlates with the functions performed by this bone.
General plan of the structure of the mandibular bone
During ontogenesis, the structure of the lower jaw of a person changes not only in utero, but also postnatally - after birth. In a newborn, the bone body consists of two mirror halves connected semi-mobile in the center. This middle line is called the chin symphysis (Latin symphysis mentalis) and completely ossifies by the time the child reaches one year old.
The halves of the lower jaw are arcuately curved, they are convex outward. If you outline along the perimeter, the lower border of the body - the base - is smooth, and the upper has alveolar depressions, it is called the alveolar part. There are holes in it where the roots of the teeth are located.
The jaw branches are located with wide bony plates at an angle of over 90 ° C to the plane of the bone body. The place of transition of the body to the jaw branch is called the angle of the mandible (along the lower edge).
Relief of the external surface of the body of the mandibular bone
From the side facing out, the anatomical structure of the lower jaw is as follows:
- the central, forward part - the submental protrusion of the bone (lat. protuberantia mentalis);
- symmetrically on the sides of the center rise chin tubercles (lat. tuberculi mentali);
- upward obliquely from the tubercles (at the level of the second pair of premolars) there are chin holes (lat. forameni mentali) through which the nerve and blood vessels pass;
- behind each hole begins an elongated convex oblique line (lat.linea obliqua), passing into the front border of the mandibular branch.
Such features of the structure of the lower jaw as the size and morphology of the chin protrusion, the degree of curvature of the bone, form the lower part of the oval of the face. If the tubercles protrude strongly, this creates a characteristic relief of the chin with a dimple in the center.
In the photo: the lower jaw affects the shape of the face and the overall impression of it.
Posterior mandibular surface
On the inside, the relief of the mandibular bone (its body) is mainly due to the fixation of the muscles of the bottom of the oral cavity.
The following areas are distinguished on it:
- The chin spine (lat. Spina mentalis) can be whole or bifurcated, located vertically in the central part of the body of the lower jaw. Here the chin-hyoid and chin-lingual muscles begin.
- The double-abdominal fossa (lat. Fossa digastrica) is located at the lower edge of the chin awn, the place of attachment of the bipedal muscle.
- The jaw-hyoid line (lat.linea mylohyoidea) has the appearance of a weakly expressed cushion, runs in the lateral direction from the chin spine to the branches in the middle of the body plate. The maxillofacial part of the upper pharyngeal constrictor is fixed on it, and the maxillary hyoid muscle begins.
- Above this line there is an oblong sublingual fossa (Latin fovea sublingualis), and below and laterally, a submandibular fossa (Latin fovea submandibularis). These are traces of the adhesion of the salivary glands, sublingual and submandibular, respectively.
Alveolar surface
The upper third of the body of the jaw has thin walls restricting the dental alveoli. The border is the alveolar arc, with elevations in the places of the alveoli.
The number of cavities corresponds to the number of teeth of the lower jaw in an adult, including the “wisdom teeth” that appear after all, 8 on each side. The pits are septated, that is, separated from each other by thin-walled partitions. In the area of the alveolar arch, the bone forms protrusions corresponding to the extensions of the tooth holes.
The relief of the surface of the branches of the lower jaw
The bone anatomy in the area of the branches is determined by the muscles attached to them and the movable joint connecting it to the temporal bones.
Outside, in the area of the mandibular angle, there is an area with an uneven surface, the so-called chewing tuberosity (lat. Tuberositas masseterica), on which the chewing muscle is fixed. Parallel to it, on the inner surface of the branches, there is a smaller pterygoid tuberosity (Latin tuberositas pterygoidea) - the place of attachment of the pterygoid medial muscle.
On the central part of the inner surface of the mandibular branch opens the opening of the lower jaw (lat. Foramen mandibulae). In front and medially, it is partially protected by an eminence - the mandibular tongue (lat. Lingula mandibulae). The hole is connected by a channel passing through the thickness of the bone spongy substance to the chin hole on the outside of the mandibular body.
Above the pterygoid tuberosity there is an elongated depression - the maxillohyoid sulcus (lat. Sulcus mylohyoideus). In a living person, nerve bundles and blood vessels pass through it. This furrow can turn into a canal, then it is partially or completely covered by a bone plate.
Along the front border of the inner side of the branches, starting just below the level of the opening of the lower jaw, it descends and continues onto the body of the mandibular roller (lat.torus mandusularis).
Processes of the mandibular bone
At the ends of the branches, two processes are well pronounced:
- Coronoid process (lat. Proc. Coronoideus), anterior. From the inside, it has an area with a rough surface, which serves as a place of attachment of the temporal muscle.
- Condylar process (lat. Proc. Condylaris), posterior. Its upper part, the head of the lower jaw (lat. Caput mandibulae) has an articular surface of an ellipsoidal shape. Below the head is the neck of the mandible (lat. Collum mandibulae), bearing on the inner side a pterygoid fossa (lat. Fovea pterygoidea), where the pterygoid lateral muscle is attached .
Between the processes lies a deep notch - notch (lat.incisura mandibulae).
Mandibular joint
The anatomy of the terminal sections of the branches of the lower jaw ensures its good mobility and articulation with the bones of the facial skull. Movements are possible not only in the vertical plane, the jaw also moves back and forth and from side to side.
The temporomandibular joint is formed, respectively, by two bones: the temporal and lower jaw. The structure (anatomy) of this joint makes it possible to attribute it to the type of complex cylindrical joints.
The jaw joint fossa of the temporal bone is in contact with the anteroposterior portion of the head of the condylar process of the jaw. It is he who should be considered a true articular surface.
The cartilaginous meniscus inside the joint divides it into two “tiers”. Above and below it there are gaps that do not communicate with each other. The main function of the cartilage pad is cushioning when grinding teeth with food.
The temporomandibular joint is strengthened by four ligaments:
- temporomandibular (lat. ligatura laterale);
- maxillary (lat. ligatura spheno-mandibulare);
- pterygo-maxillary (lat. ligatura pterygo-mandibulare);
- awl-maxillary (lat. ligatura stylo-mandibulare).
The first of them is the main one, the rest have an auxiliary supporting function, since they do not directly cover the articular capsule.
How do the lower and upper jaw contact?
The anatomical structure of the teeth of the lower jaw is determined by the need to close and contact with the upper row of teeth. Their specific location and interaction is called a bite, which can be:
- normal or physiological;
- abnormal caused by changes in the development of parts of the oral cavity;
- pathological, when the height of the dentition changes as a result of their abrasion, or the teeth fall out.
Bite changes negatively affect the process of chewing food, provoke speech defects, deform the facial contour.
Normally, the structure and surface relief of the mandibular row of teeth ensure their close contact with the same maxillary teeth. Mandibular incisors and canines partially overlap with similar upper teeth. External tubercles on the chewing surface of the lower molars fit into the fossa of the upper.
Characteristic injuries
The lower jaw is not monolithic. The presence of channels, areas with different density of bone material in it causes typical injuries during injuries.
Common places for mandibular fractures are:
- Wells of fangs or premolars - small indigenous.
- The neck of the posterior (articular) process.
- Mandibular angle.
Since the bone in the area of the chin symphysis is thickened, and at the level of 2 and 3 pairs of molars are strengthened by the internal crest and the external oblique line, the lower jaw breaks extremely rarely in these places.
Another type of damage that affects not the bone itself but the temporomandibular joint is a dislocation. It can be provoked by a sharp movement to the side (from a blow, for example), excessive opening of the mouth or attempt to bite through something solid. The articular surfaces are displaced, which interferes with normal movements in the joint.
A traumatologist must correct the jaw to prevent excessive stretching of the surrounding ligaments. The danger of this injury is that the dislocation can become habitual and recur with a slight effect on the jaw.
The mandibular joint experiences constant stress throughout a person's life. He is involved in eating, talking, important in facial expressions. His condition can be affected by lifestyle, diet, the presence of a systemic disease of the musculoskeletal system. Prevention of injuries and early diagnosis of joint problems is the key to the normal functioning of the lower jaw throughout a person’s life.