Page 125 - Edited - Webster HEAD AND NECK - part 1

HN 103

Normal activity of the temporomandibular joint promotes normal growth of the condyle and of the
mandible as a whole. For example, an untreated fracture of the condyle in a child can lead to ankylosis
(fusion) of the joint, and an underdeveloped mandible (micrognathia).

Wear and tear arthritis (osteoarthritis) is common in the temporomandibular joint. Synovial joints
undergoing such degeneration are prone to noisy function - anything from creaks and clicks to nasty grating.
Such noises from the temporomandibular joint become apparent to the sufferer very early (as, indeed, do noises
from minor perturbations in normal joints and from vigorous chewing), because vibrations are transmitted directly
to the temporal bone(s) which contains the middle and inner ears (q.v.) - i.e. the joint functions as a
peculiar form of hearing aid!

Dislocation of the Temporomandibular Joint is relatively rare. In order to open the mouth, the
mandibular condyle must roll forwards (see above): therefore, the most important ligament (the
temporomandibular ligament) does not resist such movement, and the joint is at its least stable as the jaw is
depressed. Consequently, dislocation always occurs when the mouth is open and the condylar process/head
therefore displaced onto the posterior shoulder of the articular tubercle: the wider open the mouth, the greater
this displacement and the more precarious the joint. Dislocation can therefore occur during dental procedures
or intra-oral surgery, especially if a general anaesthetic is used. Excessive, wide gape yawning (and heaven
knows what else - but I suppose foot sucking must be a risky business) can also dislocate the jaw; and a cross-
punch or other violence to the “slack-jawed” chin can produce the same result. Whatever the cause, condylar
process/head is forced forwards beyond the summit of the articular tubercle and snaps into a false articulation
with the zygomatic arch in front of the tubercle, where it is held by the tone of the jaw-closing muscles. The
dislocation can be uni- or bilateral. Unilateral dislocation twists the mandible to the opposite side. The
condylar process/head in its normal location is palpable just in front of the tragus of the external ear: palpate
your own condylar processes while opening and closing your mouth. (Movements of these parts of mandible
are often readily visible during chewing.) When dislocated, this slight bump is replaced by a hollow, the
“bump” of the condylar process appears instead more anteriorly, under the zygomatic arch.

Reduction of (a) dislocated temporomandibular joint(s) is relatively straightforward. It should be
obvious to you that to negotiate the articular tubercle(s), the mandible must be first pushed downwards and
then backwards. This is done (usually without anaesthetic) by wrapping your thumbs in a good layer of
protecting gauze, and pressing downwards then backwards on the patient's lower molar teeth (if present).
After reduction, the mandible has to be supported by a bandage for~ 3 weeks to allow repair of the disturbed
elements of the joint(s) - especially the capsule and ligaments.
Paralysis of the Mandibular Division of the Trigeminal Nerve This is not common, but normal
biting, chewing and speaking become impaired. The mandibular nerve is easily tested by asking the subject to
clench his/her teeth and palpating the masseter and temporalis muscles on each side: try it on yourself.
Paralysis is readily detectable by this method. Opening the mouth is a surprisingly powerful movement
(almost enough to support body weight for a short period): asking the subject to open his/her mouth against
resistance is therefore another useful test. The movement involves the pterygoid muscles: these are also involved
in medio-lateral shifts of the mandible. If the pterygoid muscles are paralysed on one side, opening the mouth
against resistance activates the intact pterygoid muscles which, in the absence of the counterbalancing
action of their opposite numbers, shift the jaw to the paralysed side (HN 108, Fig. C; & 323). Exactly
the same phenomenon is found following fracture of one neck or condyle (which is quite common), since this
effectively eliminates one lateral pterygoid muscle from participating in movements: the pterygoid
mass thereby operates asymmetrically, and the opening jaw deviates to the side of the fracture.

This last observation underlines the complexity of muscle actions, and the oversimplification involved
in attributing particular movements to individual muscles (HN 96, 99, 100). For example, on HN 108, note that
the medial pterygoid muscles are involved in three actions: closure, protraction, and medio-lateral shifts. Thus,
when only one of these actions is required, the muscles which oppose the unwanted actions of the medial
pterygoids must also be active in order to eliminate the unnecessary displacements. In other words, as is virtually
always the case when studying movements, movements of the mandible are brought about by dynamic
interactions of all the muscles acting on the temporomandibular joint.
K.E.W.
[I am grateful to Dr BKB Berkovitz for reading a draft of this account, and correcting errors and
misconceptions as well as suggesting additional points which might be covered.].

\NewCMedPhysSc\13 HN 101 TemporomJt.