Mandible
The mandible, or
jawbone, forms the bony framework for the lower part of the oral cavity and the
skeleton of the lower part of the face. It has a U-shaped body, with a broad
flat ramus running superiorly from each end of the body.
The area of fusion of the right and
left halves of the body of the mandible at the anterior midline is the
mandibular symphysis. At the inferior anterior surface of the symphysis is a triangular
elevation called the mental protuberance, the lower outer angles of
which are the mental tubercles. At the inferior part of the inner
surface of the symphysis is a variable elevation, the mental spine or
spines, which may be present as a single eminence or as two eminences, one
superior and the other inferior. These give origin to the geniohyoid and
genioglossus muscles.
Each half of the body of the mandible
has an upper and a lower part, the arch of the lower part being wider than that
of the upper part. The upper part is the alveolar process, so called
because it contains the sockets for the lower arcade of teeth. The lower part,
or body of the mandible, has a much greater proportion of compact bone.
Just lateral to the symphysis on the lower border is an oval depression or
roughened area for the attachment of the anterior belly of the digastric
muscle, the digastric fossa. On the external surface of the mandibular
body, inferior to the second premolar tooth, is the mental foramen, by
which the mental branches of the inferior alveolar nerve and vessels leave the mandibular
canal. Also on the external surface is an ill-defined oblique line that
runs from the mental tubercle to the anterior border of the ramus. Sometimes it
may start from the lower border of the mandible inferior to the molar teeth, or
there may be lines from each of these places that meet as they run
posterosuperiorly. On the internal surface of the mandibular body, a ridge of
bone runs obliquely from the digastric fossa to the level of the socket of the
last molar tooth. This ridge gives attachment to the mylohyoid muscle and is
therefore known as the mylohyoid line. Superior to the mylohyoid line is
a shallow sublingual fossa, in which lies the sub- lingual gland, and inferior to
the mylohyoid line is the fossa for the submandibular gland.
The ramus presents a medial and
a lateral surface as well as anterior, superior, and posterior borders. The
remaining border of the ramus depends on an arbitrary decision as to the
dividing line between the ramus and the body; the angle of the mandible is
the area of junction of the posterior and inferior borders of the mandible. It
is usually slightly obtuse in the young adult and flares slightly laterally. In
the center of the medial surface of the ramus is the mandibular foramen, the
beginning of the mandibular canal that transmits the inferior alveolar nerve,
artery, and vein. The lingula projects partly over the foramen from its
anterior edge, and the mylohyoid groove runs anteroinferiorly from it
for a short distance. Projecting superiorly from the superior border of the
ramus are the triangular coronoid process anteriorly and the condylar
process posteriorly, with the mandibular notch between the two. The
condylar process is subdivided into a head and neck.
The left and right halves typically
fuse by the second year. The position of the mental foramen indicates changes
that occur over time. At birth it is near the mandible’s inferior border
because the alveolar process makes up most of the body. After full
development it is about halfway between the upper and the lower borders. When
an individual becomes edentulous, much of the alveolar process is
resorbed, and the mental foramen comes to lie near or on the superior border.
The angle of the mandible is more obtuse in the infant than after it has become
fully developed. Again, in the edentulous state, it appears more obtuse,
although this may be at least in part due to a backward tilt of the condylar process.
