By G. Patrick Thomas, Jr., DC, DACBR
A wide variety of conditions affect the
temporomandibular joint (TMJ) including congenital anomalies, ankylosis,
arthritis, and internal disk derangement.  TMJ disease is common,
affecting between 4 and 28 percent of the population. Young females
in particular commonly present with TMJ complaints. 
While standard imaging techniques, such
as plain film radiography, are useful in characterizing many of these abnormalities,
magnetic resonance imaging (MRI) is ideally suited to assess changes of
Brief Review of Anatomy and Function
The TMJ consists not only of the condyle
of the mandible and the articular fossa of the temporal bone, but also
of a small interposed fibrous connective tissue meniscus or disk.
This disk is attached posteriorly to the temporal bone by an elastic band
know as the bilaminar zone. Anteriorly, the superior belly of the
lateral pterygoid muscle fixes the disk into its normal position. 
The disk separates the joint into two compartments.
The upper portion of the joint is capable of only translatory movements;
anterior and posterior motion of the disk in relation to the temporal bone.
The inferior joint space allows for rotation of the mandibular condyle
The normal disk is held on top of the mandibular
condyle by the opposing forces of the superior belly of the lateral pterygoid
muscle and the bilaminar zone. Diseases and conditions that upset
this balance can lead to joint dysfunction and anatomical derangement of
Figure one shows the
articular fossa (F), the mandibular condyle (C), the lateral pterygoid
muscle (L), and the bilaminar zone (B). The disk is black.
Two separate actions occur at the TMJ when
the mouth is opened. The mandibular condyle rotates under the disk,
and the disk translates anteriorly in relation to the articular fossa of
the temporal bone. When the mouth closes this complex motion reverses,
returning the disk to itís resting place under the articular fossa
Causes of Disk Derangement
Chronic spasm of the lateral pterygoid
muscle, trauma and arthritic changes, as well as defects of the bilaminar
zone, can all lead to dysfunction and internal derangement of the TMJ (1).
Two common patterns of derangement are recognized: Anterior disk displacement
with reduction and anterior disk displacement without reduction.
The first pattern is characterized by abnormal disk position with the mouth
closed, and normal relationships of the disk, condyle and fossa with the
mouth open. Anterior displacement without reduction indicates that
the relationships of the three joint components are abnormal with the mouth
both opened and closed. 
MRI Techniques in Evaluation of the TMJ
pulse sequences typically are used for MR imaging of the TMJ. The
inherent contrast of the anatomy in the region makes T1-weighed images
adequate in most cases. T2-weighted images are useful when joint
fluid, tumor, edema or infections are suspected/  Sagittal and
coronal images are acquired with the patientís mouth opened and closed.
Because of the small size of the structures of interest here, surface coils
are essential for adequate signal-to-noise (S/N) ratio.
The important landmarks for assessment
of TMJ function include the articular fossa of the temporal bone, the mandibular
condyle, the disk, and the bilaminar zone.
The closed-mouth scans are examined first.
Damage to the bilaminar zone allows the unopposed lateral pterygoid muscle
to displace the disk anteriorly. This is seen as displacement of
the disk anteriorly in relation to the articular fossa (see
Next assessed is the relationship of the
disk and the mandibular condyle with the patientís mouth opened.
Normally the disk and the condyle move as a unit anteriorly when the mouth
opens. If the disk is displaced with the mouth closed, and resumes
itís normal relationship with the condyle when the mouth is opened, then
anterior displacement with reduction is present (see figure
3). On the other hand, if when the mouth is opened, the condyle
and the disk do not resume their normal relationship, anterior displacement
without reduction is present (see figure 4). 
Unfortunately, non-surgical treatment is
effective in only 25% of patients with anterior displacement with reduction.
Various intra-oral appliances and non-steroidal anti-inflammatory medications
are common treatments.
Patients who suffer from anterior displacement
without reduction are often treated surgically. If the disk is not
fragmented or macerated it can be repositioned. Badly damaged disks
are often removed and replaced with implants or grafts. 
Temporomandibular joint disease is a significant
health problem, both in terms of itís impact to an individual and its incidence.
Conventional imaging methods are often unable to properly detect and diagnose
lesions of this articulation. MRI is ideally suited to assess both
anatomical and functional disturbances
This article first appeared in the February
2002 issue of Missouri Chiropractor.
Berklow R (ed):
The Merck Manual of Diagnosis and Therapy
Rahway NJ, Merck and Co., 1992
Edelman R and Hesselink J:
Clinical Magnetic Resonance Imaging
Philadelphia, WB Saunders, 1990
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