TEMPEROMANDIBULAR JOINT

Introduction

Temporomandibular joint or craniomandibular joint is a form of articulation found

only in mammals. This is called as Temporomandibular joint because this joint is formed

by the articulation of mandibular condyle at the base of the cranium with the squamous

part of temporal bone.

Also known as craniomandibular joint as the mandible is connected to the

cranium through this joint. Temporomandibular is by far the most complex joint in the

body.

As it provides hinging movement in one plane (ie) forward and backward like

hinge of a door it is called as ginglymoid joint. However, at the same time it also provides

gliding movement which classifies it as an arthrodial joint so known as ginglymoarthodial

joint.

It is known as a modified ball socket type of joint as it allows movements in three

planes, sagittal, transverse and coronal. It is also known as compound joint. Compound

joint is the joint formed by these articulations of three bones. As the articular disc

functionally serves as a non-ossified bone that permits the complex movements of the

joints, the joint are called even as a compound joint.

The physiologic activities in which the temporomandibular joint plays a part may

be voluntary or reflex and ranges from mastication, deglutition and phonation, to such

momentary actions such as grasping and yawning.

Development of Temporomandibular Joint

The mammalian craniomandibular articulation develops anterolateral to the otic

capsule from the first branchial arch mesenchyme and is therefore innervated by fifth

cranial nerve. This is the early embryonic joint.

This primary embryonic joint is formed by the joining or is the joint between

malleus and incus which develops from first branchial arch. The malleus and incus are

formed by differentiation of large islands of cartilage, found in the middle ear cavity. This

joint serves as the primary TMJ joint up to 16 weeks of prenatal life. This joint is an

uniaxial hinge joint capable of no lateral motion.

By the end of 7-11 weeks of gestation the secondary TMJ begins to develop. At

about ninth prenatal week a condensation of mesenchyme appears surrounding the

upper posterior surface of rudimentary ramus.

This mass chondrifies at about 10-11 weeks to form cartilaginous mandibular

condyle. With progressive endochondral ossification the cartilage fuses with the posterior

part of the bony mandibular body. At about 9-10 weeks the muscle fibers become more

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differentiated Bloodvessels, nerves etc. can be seen clearly present in the joint region at

about 10 weeks of gestation.

The appearance of mandibular fossa of the temporal bone is some what earlier

than that of the condyle (u) at about 7-8 weeks. Ossification of the fossa is more

prominent at about 10-11 weeks. Ossification continuous in this region and at about 22

weeks the mandibular fossa shows both medial and lateral walls and articular eminence

is evident. The shape of the fossa is concave at about the ninth week and it takes the

definitive concave shape to match the convex condyle. The differentiating mesenchymal

cells interposed between the condyle and mandibular fossa gives rise to the capsular

and intracapsular structures of the TM joints.

Articular Disc

Articular disc is first seen at about seven and one half weeks. By the 10 th week

first signs of collagenous fibers within the articular disc develop and it becomes more

pronounced by 12 weeks. From the 19-20th week the disc increasingly takes on its

definitive fibro cartilaginous composition. At this stage only the disc shows pattern of

differential cell proliferation in which central region becomes thinner than periphery.

Articular Capsule

The articular capsule first appears at about 9-11 weeks. By the 17th week the

capsule is seen as fully formed tissue boundary between intracapsular and extracapsular

components of the TMJ. By the 13th week the lower cavity of the fossa enlarges and the

superior joint cavity becomes more evident. The shapes of the joint cavities are

reciprocal at the time when the upper joint cavity is concave the lower joint cavity is

convex.

Works done by Hooker (1954 and Humphrey (1968) shows that actual mouth

opening actions are observable as early as 7-8 weeks of gestation.

But certain others like Symons (1952), Perry (1985), Moffet (1957) said that only

scattered muscle fibers of lateral pterygoid muscle are clearly discernible at 7-8 weeks.

Therefore, the prenatal jaw opening activity that both Hooker and Humphery observed is

said to have involved the articulations of the primary TMJ.

Anatomy of the TMJ

The temperomandibular joint or craniomandibular articulation is the articulation

between the lower jaw and the cranium. The bony elements of this joint are the

squamous part of the temporal bone above and the mandibular condyles below. This

articulation consists of two synovial joints, the left and right temporomandibular joint.

TMJ is complex both morphologically and functionally. An articular disc

composed of dense fibrous tissue is interposed between the temporal bone and the

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mandible dividing the articular space into an upper and lower compartment, gliding

movement occurs in upper compartment and the lower compartment functions as a

hinge joint. The articulating surface of the TMJ are lined by dense, avascular fibrous

connective tissue.

Relations of TMJ

Laterally 1) Skin, Fasciae.

2) Parotid gland.

3) Temporal branches of the VII nerve.

Medially Tympanic plate separates TMJ from internal carotid artery,

spine of the sphenoid with upper end of sphernomandibular

ligament, Auriculotemporal and chorda tympani. Middle

meningel artery.

Anteriorly Lateral pterygoid muscles.

Massetric nerve and vessels.

Posteriorly The parotid gland separates it from external acoustinc

meatus.

Superiorly Middle cranial fossa

Middle meningel vessels.

Inferiorly Maxillary artery and vein

Blood supply Superficial temporal artery and maxillary artery

Nerve supply Aurientotemporal nerve and massetered nerve.

FUNCTIONAL ANATOMY OF THE TMJ

Mandibular condyle

This is convex in shape and it articulates with the articular fossa which is

separated into the upper and lower compartments by the articular disc. it present as an

ovoid bony knob like process on a narrow mandibular neck. The adult condyle is about

15-20mm mediolaterally and 8-10mms anterio-posteriorly. The articular surface of the

condyle faces upwards and forwards so that in side view the neck of the condyloid

process seems to bend forward. The lateral pole of the condyle extends slightly beyond

the ramus and is roughened for the attachment of articular disc and temporomandibular

ligament.

Articular disc

Each human TMJ is essentially a double joint due to the presence of an intra

articular disc.

The articular surface are of fibrous tissue, condylar perichondrum and temporal

periosteum. Technically classified as a ginglymo arthrodial joint. It adjusts itself to the

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changing contours of the condyle head as it moves in the fossa. This is possible as the

disc is not uniformly thick, but is modified in different regions. The underside of the disc

is concave and fits closely over the condylar head like a cap. This ensures the rotatory

movements of the condylar head in the fossa and the disc moves along with the condyle.

In sagittal section, the disc is divided into three regions according to thickness.

The central area is the thinnest and is called intermediated zone. In a normal condyle is

located In the intermediate zone of the disc, bordered by thicker anterior and

posterior regions. From anterior to posterior the disc shows five zones :

1) Anterior extension

2) Anterior band

3) Intermediate zone

4) Posterior extension

5) Posterior band

Posteriorly the disc is bilaminar. The thickened anterior and posterior bands

forms an ellipsoidal doughnut. This ellipsoidal doughnut functions to stabilize the

condylar head in the glenoid fossa with the jaws at rest. The disc is thus considered as a

flexible, viscoelastic adapter which helps the moving joint surface achieve more off

effective articular surface congruity.

Articular- fossa

This is the concavity within the temporal bone that houses the mandibular

condyle. The anterior wall of the fossa is formed by articular eminence and posterior

wall is formed by the tympanic plate.

The fossa is lined by articular tissue. The posterior part of the fossa elevated to

a ridges called the posterior articular lip.

The posterior articular lip is higher and thicker at its lateral end and is known as

post glenoid process. Medially the articular fossa is bounded by a bony plate that

leans against the spine of sphenoid sometimes drawn into a triangular process

and is known as the temporal spine.

Articular capsule

The capsule forms a thin, fibrous connective tissue sleeve about the joint

which tapers from above down to the condyle neck. It is attached to squamous

temporal bone just peripheral to the margins of the articulating surfaces. They are

vertically oriented and are of such a length so as enable the normal range of joint

movements. All the non articulating surface within the capsule form sunovial membrane,

the surface area of which is increased by the formulation of villi and folds. The sinovial

fluid is a dialysate of plasma with added, mucins and proteins. The cells it contains are

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mainly lymphoid or macrophage in type. The thickened anterolateral and lateral

portions of the capsule which is attached to the articular tubercles is called

temperomandibular ligament.

Ligaments of temperomandibular joint

Ligaments limit the movements of temperomandibular joint. The capsule is too

delicate a structure to support the joint unaided and so joint stability is achieved with

intrinsic and extrinsic ligaments.

Intrinsic ligaments (directly involved with movement of joint and attached in

relation to joint).

The main intrinsic ligament is the temperomandibular ligament or the lateral

ligament. It is located lateral to the capsule. The fibers of the ligament pass obliquely

from its wide origin lateral to the articular tubercle to a narrow insertion in the neck of the

condyle, below and behind the lateral pole of the condyle. Collateral ligaments also act

as intrinsic ligaments. These are rather narrow bands of collagen fibers that run

horizontally backwards on the inner aspect of the capsule from the lateral and medial

aspects of the articular eminence to the respective condyle poles. These restrict the

distal displacement of condyle head. These collateral ligaments along with the

temperomandibular ligaments, helps to attain the clinical ligamentous position.

Extrinsic ligaments

These are not directly involved with the joint, but they modify the range of

movements that are possible.

These are also known as accesory ligaments and they include -

1) Sphenomandibular ligament

2) Stylo mandibular ligament

3) Pterygomandibular raphe

4) Temporomandibular ligament of the opposite side which acts as an extrinsic

medial ligament.

Sphenomandibular ligament

Attached superiorly to the spine of the sphenoid and inferiorly it is attached to

the lingula of the mandibular foramen. It is a remnant of the cephalic end of

meckels cartilage.

Stylomandibular ligament

It is attached above to the lateral surface of styloid process and below to the

angle and posterior border of the ramus of the mandible.

Fibrous capsule and articular disc also serves as the ligaments of TM joint.

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Muscles of mastication

Origin Insertion Action

Masseter a) Superficial layer from

anterior 2/3rd of lower

borer of zygomatic arch

and adjoining zygomatic

process of maxilla.

Lower part of lateral

surface of the ramus

of the mandible

Elevation of mandible.

b) Middle layer anterior

2/3 of deep surface and

posterior 1/3 of lower

border of zygomated

arch

Middle part of ramus

Deeplayer (origin) Insertion

From deep layer of zygomated

arch

With the upper part of ramus and

coronoid process

Temporalis Insertion

1) Temporal fossa

excluding zygomatic bone

2) Temporal fascia

Margins and deep surface of coronoid

process

Anterior border of ramus of mandible

Action

1. Elevates mandible

2. Posterior fibers retrat the protruded mandible

3. Helps in side to grinding movements.

Lateral pterygoid origin Insertion

1. Upperhead from infra temporal

surface and crest of greater wing

of sphenoid.

Pterygoid fovea on the anterior surface of

the neck of the mandible.

2. Lower head from lateral surface of

lateral pterygoid plate.

Anterior margin of articular disc and

capsule of temporomandibular joint.

Actions

1. With the help of suprahyoid muscles helps in depressing mandible to open the

mouth.

2. Helps in protruding mandible along with medial pterygoid.

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3. With medial pterygoid of the same side and alternating with those of the opposite

side brings about side to side grinding movements.

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Medial pterygoid Origin Insertion

1. Superficial head

2. Deep head from medial surface of

lateral pterygoid plate and

adjoining process of palatine bone.

Roughened area on the medial surface of

angle and adjoining ramus of mandible,

below and behind the mandibular foramen

and mylohyoid groove.

Actions

1. Elevates mandible

2. Helps to protrude mandible

3. Brings about side to side grinding movements along with lateral pterygoid.

Movement of the mandible

Both joints always act together, but may differ In movement which include

gliding, spinroll and angulation. The basic movements that occur in TM joint are

rotatory and translatory. Rotatory movements occur in the lower chamber and

translatory movements occur in upper chamber. These movements occur

symmetrically in both joints, when mandible is raised lowered protruded or retruded.

Movements also occur in asymmetrical manner when translation occurs on

one side only to produce lateral jaw positions.

Various movements of the TMJ according to the movement of mandible are -

1) Depression

2) Elevation

3) Protrusion

4) Retraction

5) Lateral chewing movemente and bonnet movement.

Depression of mandibular opening

The opening movement is caused by gravity, relaxation of the elevator

muscles and a combined action of lateral pterygoid, ganiohyoid, mylohyoid and

digastric muscles. Condyles rotate on a common horizontal axis and also glide

forwards and downwards, on the interior surface of the articular disc which slides in

the same direction on the temporal bones due to their attachments to the

mandibular heads and due to the contraction of lateral pterygoide which draw the heads

and discs onto the articular tubercle.

When wide opening occurs the protracting force of the inferior heads of the

lateral pterygoid muscles acting upon the condyles and the disc combines with the

depressing and retracting force of the geniohyoid and degastric muscles acting upon

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the chin and action of mylohyoid muscle on the body of the mandible.These

combined forces produce extensive rotatory and translatory movements.

Elevation or closing movements of the mandible

Closing movement is executed by the elevators of the mandible. Condyles glides

backwards and hinges on its disc and as lateral pterygoid relaxes the disc glides back

and up into the mandibular fossa.

The muscles involved are the temporalis, masetter, and medial ptarygoid of both the

sides. The condyles are retracted by posterior fibers of temporalis during closure. The

disc is pulled backwards by the bilammiar elastic tissue.

Protrusion

In protrusive movements the lower teeth are drawn forward over the upper teeth.

This is primarily as a result of contraction of inferior heads of lateral pterygoid

muscles although there is slight activity of the masseter and medial pterygoid muscles.

The condyle is pulled forward and downward along the articular eminences

while the elevators and depressors apparently stabilize the position of the mandible

related to maxilla.

RETRACTION Of THE MANDIBLE

In this movement the obliquely aligned fibers of the middle temporalis muscle

combine forces with the depressors while the remaining elevators exhibit varying

amount of activity. The articular disc and condyles are pulled backwards into the

mandibular fossa by the contraction of the posterior fibers of temporal is. deep fibers

of the masseter and geniohyoid and digastric play a minor role.

Retrusion is limited to a distance of 1 mm.

Lateral chewing movement

One head with its articular disc glides forwards rotating around a vertical axis

immediately behind the opposite head, then slides backward rotating on the opposite

direction, as the opposite head comes forward In turn. This alternation swings mandible

from side to side muscles involved are medial and lateral pterygoids of each side acting

alternatively.

BENNET MOVEMENT

Definition : The bodily lateral movement or lateral shift of the mandible resulting from

the movements of the condyles along the lateral inclines of the mandibular fossa in

lateral jaw movements.

Bennet angle - The angle formed by the sagittal plane and the path of the advancing

condyle during lateral mandibular movements as viewed in the horizontal plane.

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When the mandible moves to one side or the other either in opening or closing

the condyle on the side to which the mandible is moving rotates minimally and

moves forwards downwards and laterally. For example the mandible moves to the

right, the left condyla moves downwards, forwards and inwards while in contact with

meniscus and eminence. The right condyla is allowed only a small rotatory movement,

because its lateral pole is limited by the temperomandibular ligament and cannot move

backwards for more than 1 mm. It therefore moves laterally and slightly forwards and

downwards due to the combined action of the left lateral and medial pterygoid and to

the contacts that exists between the condyles, menisci and fossa. The force causing the

movement comes from the left side and right condyles moves as it can within the limits

of its ligaments.

Bennet movement consists of an immediate translation which takes place

before the rotation and a progressive translation which accompanies rotation.

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CONTROL OF TMJ MOVEMENTS

The muscle which move the TMJ like the muscles found anywhere In the

body are subject to both reflex controls and controls arising from within the central

nervous system. There are three principal reflexes which control the vertical

relationship between the mandible and maxilla and hence TMJ movements. These are

as follows :

1) Jaw jerk reflexes

2) Jaw opening reflexes

3) Jaw unloading reflexes.

Jaw Jerk Reflexes

The jaw jerk is analogous to the knee jerk and is a stretch reflex whereby

stretching the jaw closing muscles (u) usually by applying a downward tap on the

chin produces a reflex contraction of these muscles. This demonstrates that there is

feedback mechanism from jaw closing muscles to their own motor neurons in the central

nervous system, as one rarely receives downward blows on the chin. This feedback

loop comes from muscle spindles within the muscles which through their primary

afferent nerves make direct connections with the motorneurones in the trigeminal motor

muscles. This feedback mechanism helps with the fine control of TMJ movements

throughout normal function, like taking account of different consistencies of food.

There is no such mechanism for the jaw opening muscles as they contact few or

no muscle spindles.

Jaw opening reflex

These are effected by inhibition of activity in jaw closing muscles, but do not

show any activation of jaw opening muscles. This reflex can be triggered by

stimulating mechanoreceptive nerves from most structures within the mouth or

nociceptive nerves from the mouth or face. The pathway for jaw opening reflex is

polysynaptic with the first synapse in either the trigeminal sensory nuclei or the

adjacent reticular formation and the final one in the trigeminal motor nucleus. The

importance of these reflexes probably lies in their ability to prevent injury when biting or

chewing objects

liable to produce damage.

Jaw Unloading Reflex

This reflex also involves a cessation of activity in jaw closing muscles, together

with an activation of opening muscles.

This reflex is evoked when a hard object which is being bitten breaks suddenly,

thus unloading the jaw closing muscles of the resistance against which they were

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working. The result of which is that opposing teeth do not forcibly hit into one another,

thereby preventing damage. The explanation for this is as follows. When biting on an

object which one knows or suspects might be brittle, one sends exatatory signals

not only to the jaw closing motor neurons but also as a precaution to those of the jaw

opening muscles.

The jaw closing motor neurons also receive positive feedback from their own

muscle spindles and there may be negative feedback to the jaw opener motor

neurones from this same source. This is called as reciprocal inhibition.

When the object breaks the sudden shortening of the muscle would result in

a decrease in spindle activity and hence in the overall excitatory drive to the jaw

closing muscles as well as in a disinhibition of the jaw opening motor neurones. Thus

the decreases and increases in activity in the jaw closing and opening muscles

respectively would be produced.

In addition to the vertical jaw reflexes there are also horizontal jaw reflexes

which involve lateral, protrusive, and may be retrusive movements of the jaw in

response to stimulation of mechanoreceptors in the periodontium and oral mucosa

and TMJ. These may be of great significance in the function and dysfunction of the

TMJ as this may be superimposed upon the normal chewing pattern.

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REFERENCES

1. Anatomy Of Head And Neck

- Chaurasia.

2. Applied Physiology Of Mouth

- Lave I ie

3. Functional Anatomy Of Oral Tissues

- Shaw J. H.

4. The Structure And Function Of Temperomandibular Joint

- G. S. Mackay, R. Yemm.

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