The following description of the TMJ is complements of Dr. Mark Piper:
The condylodiscal ligaments are intracapsular support
structures that are responsible for maintaining the general
posture of the disc superior to the condylar surface. These
ligaments occupy the medial and lateral poles of the
mandibular condyles, and they blend with the fibrous connective tissue of the medial and
lateral portions of the disc. These ligaments are not
weight-bearing structures. However, they play a vital role
in maintaining the disc in proper alignment at both poles.
Furthermore, these internal ligaments must remain tight
enough to tether the disc, but at the same time, they must
have enough laxity to allow the disc to assume a more
posterior relation with respect to the condyle during
forward translation. Like other ligaments, if the joint is
hyperextended, these structures must hold up to these
forces. Finally, although the condylodiscal ligaments function in harmony to hold the disc
in alignment, they are independent structures and are functionally distinct. Hence, they
may break down independently.
The articular disc starts out as a biconcave structure
composed primarily of dense fibrous connective tissue.
The disc generally is divided into thicker posterior and
anterior bands with a thinner mid-portion that will take
the greatest compressive force from the adjacent osseous
surfaces. At the periphery, the disc has some vascular
networking. This anatomic arrangement allows the disc to
lock onto the underlying condyle during compressive
loading. Furthermore, as the condyle translates anteriorly
during normal function, the perimeter of the disc can make immediate adjustments in
relative thickness by minute changes in vascular shunting. This will allow the disc to
remain in a stable relationship over the mandibular condyle while simultaneously
changing its relative thickness on the anterior and posterior bands to match the space
available for it between the condyle and fossa or between the condyle and eminence.
Thus, as the relative space between the bony structures changes, the disc can make
appropriate adjustments in its thickness, at least at the perimeter.
The consistency of the disc is such that it is somewhat
pliable, yet still firm enough to compress against adjacent
articulating structures for participation in normal synovial
nutrition. Furthermore, the disc will be dependent on
synovial fluid compression for its nourishment in the midportion. Because of the relative limitation of the depth that
synovial fluid will penetrate, this mid-portion must remain
between one and three millimeters in thickness. At the
periphery, the disc can receive nutrients both directly and indirectly from the synovial
fluid and directly from its own blood supply.
Discal positioning over the superiorly seated condyle should have the posterior band just
proximal to the mid-fossa. In this relation, any compressive forces from the mandibular
condyle will tend to lock the disc into this posture. Furthermore, the disc must retain this
relative position just proximal to mid-fossa on both the medial and lateral poles. As long
as this relation is maintained, a superiorly compressed condyle cannot slip posterior to the
disc, because of the locking forces generated by the posterior band over the top of the
condyle. During forward translation of the mandibular condyle, the disc will rotate more
posteriorly in relation to the condyle, and the condyle will slide closer to the anterior
band of the disc as these structures reach the level of eminence. At this forward posture,
the shunting of blood out of the anterior band of the disc will allow this area to compress,
and the shunting of blood flow into the posterior band will cause the area to thicken in
order to fill the gap intervening between the posterior slope of the condyle and the
proximal slope of the eminence. Finally, the proper positioning of the disc within the
articular space will allow the adjacent intrarticular hard and soft tissues to perform their
individual physiologic functions, as will be explained subsequently.
The lateral pterygoid muscle has upper and lower bellies that are separated by the
epimysial tissue. The upper head originates from the infratemporal surface of the greater
wing of the spheroid bone. The lower head originates from the lateral surface of the
lateral pterygoid plate. Insertion points for the upper head are both into the disc through
an orifice of the external capsule as well as into the upper part of the fovea of the anterior
part of the condyle. The inferior belly inserts entirely into the anterior neck of the
condyle. The upper and lower bellies have distinctly different electromyographic activity.
On mandibular opening, the inferior head contracts and the superior
head maintains resting activity. On closure of the mandible, the
superior head contracts while the inferior is at rest. Functionally,
then, it is the superior head that may influence discal position.
Hence, because the superior head inserts both into the condyle and
into the disc, it is felt to function by making rapid adjustments in the
relative positioning of the disc and condyle during closure. Thus, the
relative alignment of the disc over the condyle becomes more than
merely mechanically dictated by the shape of the anterior and
posterior bands of the disc, and it may, in fact, be under more
precise neurologic control by the central nervous system.
Muscles function by moving the insertion point closer to the origin.
Therefore, in the TMJ, the condyle and disc will move anteriorly
and medially toward the infratemporal fossa and lateral pterygoid
plate when the lateral pterygoid muscle is in a foreshortened state.
The most common form of muscle foreshortening is through contraction. In fact, all
muscles have underlying electrical activity that allows an average number of myofibrils
to be contractile at any point in time. This defines a "resting length" of the muscle.
However, the resting length of a muscle is also defined by the laxity of its fascial tissues.
The upper and lower bellies of the lateral pterygoid muscle each have an outerconnective tissue layer of epimysium and perimysium, and collagenous septa divide the
muscle fascicles. The reticular layer around the individual muscle fibers makes up the
endomysium. All muscles must on occasion be stretched in order to maintain the laxity of
fibrous connective tissue layers.
Before leaving the muscle, it is important to consider the vascular network that feeds the
lateral pterygoid muscle as well as that blood supply that courses through the muscle. The
maxillary artery arises from the external carotid artery and immediately gives rise to
feeder vessels into the inferior half of the lateral pterygoid muscle. More anteriorly, the
maxillary artery initially lies superficial to the inferior belly; toward the origin of the
muscle, the artery dives between the two bellies. Therefore, anteriorly, the artery will
eventually supply branches to the pterygopalatine fossa. More posteriorly, there are two
important dependent areas of maxillary artery blood flow. Crossing first inferiorly and
then medially to the lateral pterygoid is the middle meningeal artery in its extracranial
course just before it enters the foramen spinosum at the medial part of the articular
eminence of the TMJ. Hence, patency of this vessel may be influenced by the posture of
the TMJ structures. Second, the posterior blood supply from the maxillary artery must
also eventually perfuse at the mandibular condyle. Those branches that perfuse the head
of the condyle course through the superior belly of the lateral pterygoid muscle and
perfuse the cortical bone to supply the underlying marrow. The neck of the condyle
receives its blood supply from vessels that must pass through the upper and lower bellies
of the lateral pterygoid muscles.
The synovial tissue of the TMJ is the internal lining
of the external capsule. Thus, one would normally
expect to find synovial tissue at the periphery of the
joint. Generally, it is accepted that synovial tissue
may be of several types, depending on what intraarticular tissue it covers. For example, most joints
have loose connective tissue, dense fibrous or an
adipose type of synovial tissue. Therefore, that part
of the synovium that rests on the non-articulating
retrodiscal attachment would be of the loose connective type. More fibrous synovial
tissue would be found around the medial and lateral condylodiscal ligaments and the
tendinous insertion of the upper belly of the lateral pterygoid muscle. Where villi of
synovial tissue project into the joint space from the perimeter, there usually is fatty tissue
in the stalks of the villi, and this is classic adipose synovial tissue. Larger villous folds
will be found to contain blood vessels.
The synovial fluid is generally a dialysate of blood that
also contains mucin, lymphocytes, monocytes, and
macrophages. The non-vascularized tissues of the joint
are dependent on synovial fluid for nutrition. Hence, the
thinner mid-portion of the disc and the articular cartilage
covering the condyle, fossa and eminence are dependent
on the pumping of synovial fluid. As in most other
synovial diarthroses, the synovial fluid must be compressed by the articulating surfaces
and thereby be driven into deeper layers of these tissues. Thus, the mechanism of
synovial nutrition in the TMJ is dependent on three factors. First, the disc must be
compressed against the adjacent articulating surfaces in normal juxtaposition between the
condyle and the opposing superior osseous structures. Second, there must be compressive
loading of structures against each other, and the articulating structures must be firm yet
pliable enough to drive the synovial fluid into the tissues effectively. Third, the tissue
being penetrated by synovial fluid must be relatively thin. Thus, the thicker anterior and
posterior bands of the disc will have their own internal blood supply. Additionally, in
most synovial joints, the intra-articular cortical bone is nourished at least in part by
synovial fluid.
The retrodiscal attachment tissues are the intra-articular part of the joint posterior to the
condyle and disc. Functionally, this statement pertains to whether the condyle and disc
are seated in the fossa or whether in fact they are seated more anteriorly. Hence, this
tissue must have a volume that is very strictly defined when the condyle and disc are in
centric relation, and this volume must increase instantaneously when the
condyle translates anteriorly. Thus, there is a rather prominent vascular
shunt in the upper part of the retrodiscal attachment, and this vascular
network is contained within loosely organized fat, collagen and elastin.
Perhaps because the disc tends merely to rotate against the condyle (as
opposed to translating, as the disc does against the upper articular surface), there is a need
for the disc to be tethered to the condyle posteriorly. Hence, there is a stratum at the
interior portion of the retrodiscal attachment that is composed of relatively inelastic and
tightly packed collagen. In fact, this interior stratum of collagen blends medially and
laterally with the condylodiscal ligaments, and functionally, this inferior stratum may be
considered a separate ligament structure that must maintain a certain length to keep the
disc and condyle in proper alignment.
One cannot consider the synovial fluid without also
defining the normal spaces of a joint. An open space
must be maintained in both the upper and lower joint
cavities so that synovial fluid can access the intraarticular structures for nutrition and lubrication.
Ultimately, the superior space will be bounded by the
attachments of the articular capsule medially, and
laterally by the origination of the retrodiscal attachment
posteriorly, and by the blending of epimysium, capsule
and periosteum anteriorly. The capsule is primarily a medial and lateral joint structure.
Medially, it will generally course along an area approximating the squamosphenosal
suture line. Laterally, the capsule will originate from the inferolateral edges of the fossa
and eminence. In the anterior of the upper joint cavity,
the space will be limited generally to the eminence at its
most prominent portion or slightly, onto the anterior slope
of the eminence. This reflection comprises a blending of
epimysium with the periosteum of the anterior eminence
through an orifice of the capsule. As such, this boundary
is usually obliquely oriented medially and anteriorly.
There is no true capsule in the posterior portion of the
TMJ. Therefore, the posterior boundary is limited by the posterior and superior origin of
retrodiscal attachment. This tissue generally originates just anterior to the
squamotympanic and petrotympanic fissures. The inferior joint cavity is bounded
medially and laterally by the insertion of the medial and lateral collateral ligament onto
the condylar surface. Anteriorly, this cavity is limited to the blending of capsule and
tendon between the part of the superior belly of the lateral pterygoid muscle that inserts
into the disc and the part that inserts into the condyle. Posteriorly, the retrodiscal tissues
normally blend with periosteum approximately 10 mm down onto the neck of the
mandibular condyle.
The fibrocartilage covers the intra-articular osseous
surfaces. This tissue is therefore entirely dependent on
synovial fluid for its nourishment. Furthermore, the joint
must be free of adhesions
so that the synovial fluid
will have access to the
articular cartilage. This
tissue is generally thicker
on the loaded portions of the joint surfaces. There is
greater thickness of fibrocartilage on the anterosuperior
condyle and on the proximal slope of the eminence.
Likewise, it is these areas that receive the primary
compression during normal joint movement, and hence as
synovial fluid is driven into these surfaces, a thicker tissue layer can be maintained.
The osseous elements of the TMJ are the condyle, fossa
and articular eminence. The mandibular condyle forms
the inferior articulating structure and is the convex
member of the joint. The condyle receives its
nourishment from three sources. The most superficial part
of the intra-articular cortical bone is nourished by
synovial fluid. The part of the condylar cortex and
marrow that is contained within the joint capsule is
dependent on the perforating blood vessels from the
insertion of the lateral pterygoid muscle for its blood supply. Those parts of the condylar
head and neck that are extra-articular will receive a more direct blood supply from the
periosteum and adjacent muscle tissues. Hence, the parts of the condyle that are most
demanding of the blood supply are posterior and superior. Furthermore, the outflow of
marrow blood is generally to more dependent parts of the cancellous bone, and if there is
to be adequate blood flow into the condyle, there must also be equal flow of blood out
through the condylar neck. The narrower the neck or the tighter the trabecular bone of the
neck, the more vulnerable the condyle may be to venous outflow sludging. This blood
will back into the arterial side, and a diminished condylar blood supply will result.
The superior articulating structures are the fossa and the
eminence. The fossa is the concave portion of the joint,
and functionally, in all synovial joints, the majority of
movements take place with the convex and concave
surfaces in articulation. The fossa generally is wellvascularized extracapsularly by periosteal and muscular
attachments, and blood supply is usually not a problem.
The medial half of the fossa is beneath the middle cranial fossa, and in this area, the bone
is quite thin.
The articular eminence is part of the temporal bone and is convex from anterior to
posterior. As such, this part of the articulation is not structured to function for long
periods with the convex condyle. Even though the disc is normally interposed, the two
convex surfaces will have one point of contact that will overload the joint locally as these
areas. This, in turn, could result in osteochondritis dissecans and localized collapse of
these areas. It is not infrequently found that mastoid air cells extend into the eminence
and as such pose a potential communication between the joint and the mastoid sinuses.
Again, the blood supply to the eminence is generally favorable because of the abundant
adjacent muscle attachments. Of one final note is the potential occlusion of the middle
meningeal artery with antecomedial displacement of TMJ structures.
The last tissue to be considered is the articular
capsule. The attachment points of the capsule have
already been discussed. It is important to realize that
this capsule is an incomplete structure. That is, the
capsule does not extend posterior of the condyle. In
fact, the posterior part of the TMJ is bounded by the
tympanic plate on
the medial twothirds of the joint
and by external ear cartilage on the lateral third. On the
lateral part of the joint, the capsule is a well-defined
structure that functionally limits the forward translation
of the condyle. This capsule is reinforced more laterally
by an external TMJ ligament, which also limits the
distraction and the posterior movement of the condyle. Medially and laterally, the capsule
blends with the condylodiscal ligaments. Anteriorly, the capsule has an orifice through
which the lateral pterygoid tendon must pass. This area of relative weakness in the
capsular lining becomes a source for possible herniation of intra-articular tissues, and this
in part may allow for forward displacement of the disc.