March 13, 2010
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Mandible

The mandible, which is the largest and strongest bone of the face, consists of a curved, horizontal body, which is convex forwards, and two broad rami, which project upwards from the posterior ends of the body.

The body of the mandible is curved somewhat like a horseshoe, and possesses an external and an internal surface, separated by upper and lower borders. The external surface is marked in the upper part of the median plane by a faint ridge, often indistinguishable, which indicates the line of fusion of the two halves of the fetal bone. Inferiorly the ridge divides to enclose a triangular raised area, termed the mental protuberance, the base of which is depressed in the centre but raised on each side to form the mental tubercle. Below the interval between the two premolar teeth, or below the second premolar, the bone is pierced by the mental foramen, which gives exit to the mental nerve and vessels. A faint ridge, termed the oblique line, runs upwards and backwards from the mental tubercle, to become salient behind, where it is continuous with the anterior border of the ramus.

The internal surface is divided into two areas by an oblique ridge, termed the mylohyoid line. Sharp and distinct in the region of the molar teeth it becomes almost undiscernible in front. It commences behind the third molar tooth, not quite 1 cm. from the upper border of the bone, and runs forwards and downwards to reach the symphysis menti in, the interval between the two digastric fossa. The area below the mylohyoid line is slightly hollowed out and forms the submandibular fossa for the lodgment of the submandibular (submaxillary) salivary gland. The area above the mylohyoid line widens as it is traced forwards and presents in front a flattened triangular area, termed the sublingual fossa, which lodges the sublingual gland. Above the sublingual fossa and extending backwards to the third molar tooth a strip of the bone is closely covered with the mucous membrane of the lower gum. Above the anterior ends of the mylohyoid lines the posterior aspect of the symphysis menti is marked by a small irregular elevation, which may be divisible into two or more parts and is termed the genial tubercles (mental spine). Posteriorly, a groove, termed the mylohyoid groove, extends downwards and forwards on to the body from the ramus and lies below the posterior end of the mylohyoid line.

The lower border of the body is termed the base of the mandible. It extends backwards and laterally from the symphysis menti, and becomes continuous with the lower border of the ramus behind the third molar tooth. Near the median plane it presents a small, roughened depression, named the digastric fossa. Behind the digastric fossa the base is thick and rounded and presents a slight downward convexity.

Figure 335

Figure 336

The upper border of the body is formed by the alveolar part, which is hollowed into sixteen sockets (alveoli) for the reception of the roots of the teeth. These sockets vary in size and depth, and are single or subdivided by septa according to the teeth which they contain.

The ramus (figs. 335, 336) of the mandible is quadrilateral in shape, and presents two surfaces, four borders and two prominent processes. The lateral surface is flat and marked by oblique ridges in its lower part. The medial surface presents a little above its centre an irregular opening, named the mandibular foramen. This opening leads into the mandibular canal, which curves downwards and forwards to open on the external surface of the body at the mental foramen. It transmits the nerve and vessels which supply the teeth of the mandible. In front and on the medial side the foramen is obscured by a thin triangular process, termed the lingula. Behind the lingula the mylohyoid groove commences and runs downwards and forwards to reach the internal surface of the body. The part of the medial surface which lies behind the groove is marked by a number of short rough ridges. The inferior border of the ramus is continuous in front with the base of the mandible; behind it meets the posterior border at the rounded angle of the mandible. When the angle is prominent the inferior border presents a concavity, directed downwards. Eversion of the angle is characteristic of the male mandible; in the female it is frequently inverted. The upper border is thin and forms a wide notch, termed the mandibular notch. It is surmounted in front by a triangular, flattened projection, termed the coronoid process, and behind by a stout, articular process, named the condyloid process. The posterior border, thick and rounded, extends from the back of the condyloid process to the angle of the mandible. It is gently curved, being convex backwards above and concave below, and is intimately related to the parotid gland. The anterior border is thin above, where it is continuous with the anterior border of the coronoid process, and thicker below, where it is continuous with the oblique line.

The coronoid process is a flattened triangular projection, directed upwards and slightly forwards. Its posterior border is continuous with the mandibular notch and its anterior border with the anterior border of the ramus. Its margins and medial surface provide insertion for most of the fibers of the temporalis muscle. The condyloid process is expanded above to form the head of the mandible, which is covered with fibrocartilage. It articulates with the articular fossa of the temporal bone, an articular disc intervening. It is convex in all directions and its transverse is greater than its anteroposterior measurement. The lateral aspect of the head forms a blunt point, which projects beyond the lateral surface of the rest of the ramus and can be felt in the living subject just in front of the lobule of the auricle. When the mouth is opened the head passes downwards and forwards, and the examining finger sinks into a small depression. The constricted portion below the head is termed the neck of the mandible. It is somewhat flattened from before backwards, and its anterior aspect is Iimited on the lateral side by the backward continuation of the margin of the mandibular notch. Medial to this ridge the anterior surface of the neck presents a rough muscular impression.

The mandibular canal runs from the mandibular foramen obliquely downwards and forwards in the ramus, and then horizontally forwards in the body below the sockets of the teeth, with which it communicates by small openings. It con tains the inferior dental (alveolar) nerve and vessels, from which branches enter the roots of the teeth. Between the roots of the first and second premolars, or below the root of the second premolar tooth, the mandibular canal divides into mental and incisive canals; the mental canal runs upwards, backwards and laterally to reach the mental foramen; the incisive canal is continued forwards below the incisor teeth.

Particular features.-A small shallow fossa marks the bone below the incisor teeth and gives origin to the mentalis and a part of the orbicularis oris muscle. The anterior end of the oblique line gives origin to the depressor labii inferioris (quadratus labii inferioris) and the depressor anguli oris (triangulaxis) muscles. The platysma is inserted into the bone just below these muscles and extends backwards beyond them. The lower margin of the mental foramen is sharp and the mental nerve is directed upwards and backwards as it emerges from the bone. Adjoining the alveolar border the bone is closely covered with the mucous membrane of the gum. Immediately below this area, in the region of the molar teeth, the buccinator muscle has a linear origin, which extends medially behind the last molar tooth to the attachment of the pterrgomandibulax ligament.

The mylohyoid line gives origin to the mylohyoid muscle. Above its posterior end the bone gives origin to fibers of the superior constrictor muscle of the pharynx. and the pterygo mandibular ligament (raphe) is attached just behind the third molar tooth. The lingual nerve gains the tongue by passing above the posterior end of the mylohyoid line. and in this situation is closely- related to the inner surface of the mandible. A strip of bone along the alveolar border is covered by the mucous membrane of the gum, and the sublingual gland lies in contact with the bone anteriorly between this area and the mylohyoid line. The upper genial tubercles give origin to the genioglossi and the lower to the geniohyoid muscles ; both tubercles are placed above the anterior ends of the mylohyoid lines. The submandibular fossa lodges some of the submandibular lymph glands in addition to the salivary gland, and the facial (external maxillary) artery may come into contact with this region as it descends to curl round the base of the mandible, where it sometimes produces a shallow groove. The digastric fossa gives origin to the anterior belly of the digastric and lies below the anterior end of the mylohyoid line.

Figure 337

The ramus and its processes provide insertion for all the principal muscles of mastication. Its lateral surface gives insertion to the masseter muscle, except at its upper and posterior part, where it is covered by the parotid gland.

The medial surface gives insertion to the medial pterygoid muscle at the roughened area , which lies behind and below the mylohyoid groove. The mandibular foramen admits the inferior dental (inferior alveolar) nerve and vessels to the mandibular carnal. Its medial border is formed by the lingula, to which the lower end of the sphenomandibular ligament is attached. Posterior to the lingula the mylohyoid nerve and vessels enter the mylohyoid groove, which may be converted into a bony canal in a part of its extent. The groove reaches the body of the mandible below the posterior end of the mylohyoid line. and the nerve and vessels then pass on to the superficial aspect of the mylohyoid muscle. In front of the mylohyoid groove and below the lingula the medial surface of the ramus is related to the medial pterygoid muscle but the lingual nerve intervenes between the muscle and the bone, as it runs downwards and forwards to reach the tongue. The lowest fibers of insertion of the temporalis muscle descend beyond the coronoid process and are attached to the anterior border of the ramus and the adjoining part of the medial surface. The area above and behind the mandibular foramen is related to the maxillary (internal maxillary) artery and its inferior dental branch, and the part adjoining the mandibular notch is in relation with the lateral pterygoid muscle. The mandibular notch transmits the masseteric nerve and vessels from the infratemporal fossa.

Figure 338

The coronoid process is covered on its lateral aspect by the anterior fibers of. the masseter muscle as they pass downwards and backwards to be inserted into the ramus. Its apex, margins and medial surface receive the insertion of the temporalis muscle, which is continued downwards along the anterior border of the ramus. If the finger is pressed into the yielding part of the cheek below the zygomatic bone, the anterior border of the coronoid process can be identified in the living subject when the mouth is opened. Owing to the way in which it is expanded, the condyloid process projects beyond the surfaces of the ramus, but more soon the medial than on the lateral side. The articular head of the mandible extends only for a short distance down the anterior surface of the process, but it covers the whole of its superior aspect and descends for 5 mm. or more on its posterior aspect. Its superior aspect slopes medially and slightly downwards and backwards. Its projecting lateral part is separated from the cartilaginous part of the external auditory meatus by a portion of the parotid gland. The smooth lateral aspect, of the neck of the mandible gives attachment to the temporomandibular ligament (fig. 508) and is covered by the parotid gland. The rough impression on the front of the neck receives the insertion of the lateral pterygoid muscle. Its medial aspect is related to the auriculotemporal nerve above and to the maxillary (internal maxillary) artery below.

The relation of the parotid gland to the mandible. requires special mention. It occupies the interval below the external auditory meatus, bounded in front by the posterior border of the ramus, behind by the mastoid process and medially by the styloid process; but it extends forwards beyond this area and covers the lateral aspect of the temporomandibular joint and the part of the lateral surface of the ramus behind the masseter muscle. In addition it curls round the posterior border and comes into contact with the medial aspect of the ramus just above the insertion of the medial pterygoid muscle.

Figure 339

Figure 340

Ossification.-The mandible is ossified in the fibrous membrane covering the outer surfaces of Meckel's cartilages. This pair of cartilages form the cartilaginous bars, or skeletal elements of the mandibular arches. Their dorsal or cranial ends are connected with the cartilaginous ear-capsules, and their ventral ends are joined to each other by mesenchymal tissue. They run forwards below the condyloid processes in the 95 mm. embryo and then, bending downwards, lie in a groove near the lower border of the bone ; in front of the canine teeth they incline upwards to the symphysis menti. From the proximal end of each cartilage the malleus and incus, two of the three ossicles of the middle ear, are developed ; the next succeeding portion, as far as the lingula of the mandible, disappears, but its sheath persists to form the sphenomandibular ligament. The part of the cartilage which stretches from the tooth-bud of the second milk molar to a point a little behind the anterior end of the mandible occupies, for a time, a tunnel in the bone, but is ultimately absorbed. The portion below the incisor teeth is ossified and incorporated with the mandible. Ossification takes place in the membrane covering the outer surfaces of Meckel's cartilages (figs. 337 to 340), and each half of the bane is formed from one centre, which appears near the mental foramen about the sixth week of fetal life, i.e, just after the appearance of the primary centers for the clavicle. By the tenth week the portion of Meckel's cartilage which lies below the incisor teeth is surrounded and invaded by the membrane-bone. Somewhat later, accessory pieces of cartilage make their appearance-viz. a wedge shaped piece in the condyloid process and extending downwards through the ramus ; a small patch along the anterior border of the coronoid process ; and smaller nodules in the front part of both alveolar walls and along the front of the base of the bone. These accessory nodules of cartilage are invaded by the surrounding membrane-bone and undergo absorption. The inner alveolar border is formed in the human mandible by an ingrowth from the main mass of the bone.

A number of small ossicles, termed the ossicula mentolia, appear in the fibrosis tissue of the symphysis about the seventh month of fetal life. They enlarge and very soon fuse with one another and with the mandible in no faxed order. Usually two or four in number, they are responsible for the formation of the mental protuberance. Whether they ossify in the remains of the ventral ends of Meckel's cartilages or in the fibrous tissue of the symphysis is uncertain.

CHANGES PRODUCED IN THE MANDIBLE BY AGE

At birth (fig. 341 a, b) the body of the bone is a more shell, enclosing on each side the sockets of the deciduous teeth, imperfectly partitioned off from one another. The mandibular canal runs near the lower border of the bone, and the mental foramen opens below the socket of the first deciduous molar tooth. The angle is obtuse (170), and the condyloid portion is nearly in line with the body. The coronoid process is relatively large and projects above the level of the condyle.

After birth (fig. 341 c, d) the two segments of the bone become joined at the symphysis from below upwards, 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 elongates, 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 by growth of the alveolar part of the bone, to afford room for the roots of the teeth, and by thickening of the subalveolar portion. After the second dentition the mandibular canal is situated just above the level of the mylohyoid line, and the mental foramen occupies the position usual to it in the adult. By the fourth year the angle is reduced to about 140. As the mandible increases in, size, bone is laid down along the posterior borders of the ramus and the coronoid process, while at the same time absorption of bone is occurring along their anterior borders. This process of remodeling goes on continuously until the bone has reached its adult size and it enables the alveolar part to lengthen sufficiently to provide the necessary space for the permanent molar teeth.

In the adult (fig. 341 e) the alveolar and subalveolar portions of the body are of about 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�.

In old age (fig. 341 f) the bone is reduced in size. Following the loss of the teeth the alveolar process is absorbed, and consequently the mandibular canal and the mental foramen, are close to the alveolar border. The ramus is oblique in direction, the angle measures about 140�, and the neck of the mandible is more or less bent backwards. The process of absorption affects chiefly the thinner of the two alveolar walls and, after its completion, a linear alveolar ridge is found on the alveolar border of the bone. In the mandible the labial wall is the thinner in the incisor and canine regions, but it is the lingual wall which is the weaker in the molar region. The alveolar ridge, therefore, lies within the line of the teeth in the incisor region but lies outside that line in the molar region, forming a curve which is wider than the curve of the line of the teeth and intersects it on each side in the premolar region. In the maxilla, however, the labial wall is everywhere the thinner and, after absorption, the alveolar ridge lies wholly within the curve of the line of the teeth.

Figure 341