The temporomandibular joint — the TMJ –
is the joint between the lower jawbone –
– the mandible – and the temporal bone of the skull.
The TMJ is responsible for jaw movement and
is the most used joint in the body.
The TMJis essentially the articulation between the
condyle of the mandible and the mandibular fossa –
a socket in the temporal bone.
The unique feature of the TMJ is the articular disc –
a flexible and elastic cartilage that
serves as a cushion between the two bone surfaces.
The disc lacks nerve endings and blood vessels
in its center and therefore is insensitiveto pain.
Anteriorly it attaches to lateral
pterygoid muscle – a muscle of chewing.
Posteriorlyit continues as retrodiscal tissue fully supplied
with blood vessels and nerves.
The mandible is the only bone that moves when
the mouth opens.
The first 20 mm opening involves
only a rotational movement of the condyle
within the socket.
For the mouth to open wider,
the condyle and the disc have to move out
of the socket, forward and down the articular eminence,
a convex bone surface located anteriorly
to the socket. This movement is called translation.
The most common disorder of the TMJ is disc displacement,
and in most of the cases, thedisc is dislocated anteriorly.
As the disc moves forward,
the retrodiscal tissue is pulled in between the two bones.
This can be very painful as this tissue is fully vascular and innervated,
unlike the disc.
The forward dislocated disc forms an obstacle
for the condyle movement when the mouth isopening.
In order to fully open the jaw,
the condyle has to jump over the back end
of the disc and onto its center.
This produces a
clicking or popping sound.
Upon closing, thecondyle slides back out of the disc hence
another”click” or”pop”. This condition iscalled disc displacement with reduction.
In later stage of disc dislocation,
the condyle stays behind the disc all the time, unable
to get back onto the disc,
the clicking sound disappeared but mouth opening is limited.
This is usually the most symptomatic stage- the jaw is said to be”locked” as it is
unable to open wide.
At this stage the conditionis called disc displacement without reduction Fortunately,
in majority of the cases,
the condition resolves by itself after some time.
This is thanks to a process
called natural adaptation of the retrodiscal tissue, which
after a while becomes scar tissue and can functionally replace the disc.
In fact, it
becomes so similar to the disc that it is called a pseudodisc.