Page 338 - Clinical Anatomy

ECA5 7/18/06 6:51 PM Page 323

The mandible 323

geniohyoid (assisted by gravity—your jaw drops open when you fall asleep
in a lecture);
•◊◊retraction—posterior fibres of temporalis;
•◊◊protraction—lateral and medial pterygoids together;
•◊◊side to side—lateral and medial pterygoids together, acting alternately on
each side.

Clinical features

Dislocation of the jaw, when uncomplicated, occurs only in a forward direc-
tion. When the mouth is widely open, the condyloid process of the
mandible slides forward on to the articular eminence; from thence, a blow,
or even a yawn, may cause forward dislocation into the infratemporal fossa
on one or both sides. Upward dislocation can occur only in association with
extensive comminution of the skull base, and backward dislocation with
smashing of the bony external auditory canal and tympanic cavity which
lie immediately behind the joint.
Reduction is effected by pressing down on the molar teeth with the
thumbs placed in the mouth, at the same time pulling up the chin; the
former stretches the masseter and temporalis muscles which are in spasm,
the latter levers the mandibular head back into place.

The teeth
There are twenty deciduous or ‘milk’ teeth replaced by thirty-two perma-
nent teeth made up, in each half jaw, thus:
•◊◊Deciduous: 2 incisors, 1 canine, 2 molars;
•◊◊Permanent: 2 incisors, 1 canine, 2 premolars, 3 molars.
The times of eruption of the teeth are useful stepping-stones in a child’s
development as well as being of forensic interest.
As a rough guide, these times can be thought of in multiples of 6, thus:
•◊◊the 1st lower incisor deciduous tooth appears at 6 months
•◊◊all the deciduous teeth have appeared by 24 months
•◊◊the permanent 1st molar }
•◊◊the permanent 1st incisor appear at 6 years
•◊◊the second permanent molar appears at 12 years (approx.)
•◊◊the third permanent molar appears at 18–24 years.
The lower teeth appear somewhat before their corresponding upper
neighbours.
Each tooth is fixed in its socket by the periodontal membrane which is, in
fact, periosteum. This layer is radiotranslucent and is the dark line seen
around the root of each tooth on radiography.

Development
The enamel crown of the tooth develops from a downgrowth of the alveolar
epithelium and represents the toughest tissue in the human body. The rest