TEMPOROMANDIBULAR JOINT Anatomy & Physiology

CONTENTS

1. Introduction

2. Evolution of TMJ

3. Development of TMJ

4. Anatomy of parts of TMJ

a) Bony structures

i. Condyle

ii. Glenoid fossa

iii. Articular eminence

b) Soft tisse

i. Articular disc

ii. Articular capsule

iii. Ligaments

5. Muscles of TMJ

6. Lever action of TMJ A review of literature

7. TMJ movements

Free movement

Masticatory movement

Reflex control movements

Bennets movement

8.Age changes

TEMPOROMANDIBULAR JOINT

Anatomy & Physiology

Introduction

The temporomandibular joint which is also called as cranimandibular joint is a form of articulation found only in mammals, and is a comparatively modern evolutionary feature being just 70 x 106 years old. Prior to this vertibrates had various types of jaw anticulations. This joint connects mandible to the skull or formed by articulation of the mandibular condyle to the base of the cranium.

Temporomandibular joint takes various names by nature of the joint.

a) Diaorthorodial joint : So called because of its capacity for free movements.

b) Ginglymodiathrodial joint : As both hinge and gliding movements are possible.

c) Compound joint : A compound joint by definition requires 3 or more bones. Though TMJ is made of only 2 bones, it is still considered as compound joint, as disc acts as a third bone.

d) Synovial joint : As the non articulating surface within the joint capsule are lined by a synovial membrane.

e) Modified ball and socket joint: As it allows movement in all three planes. E.g. Sagittal, transverse and coronal.

The physiologic activities of the TMJ may be voluntary or reflex and ranges from mastication degulitation and phonation to gasping and yawning.

Evolution of TMJ:

TMJ is about 70 million years old. Millions of years ago the vertebrates had various types of jaw articulation.

1. Agnatha: The earliest type of vertebrae had its mouth opening on the vertical side anterior by along the vertebral axis. This opening led through an oropharyngeal channel to the gut proper.

2. Gnathostomes : more highly evolved than the agnatha.

3. Osteichthyes : It is a more highly developed movable joint.

4. In amphibians, the dentary bone was seen at the anterior end of the original cartilaginous jaw.

5. In mammals, the dentary bone increased in size. As the function of this dentary bone increased the coronoid process and temporal fossa was formed.

The function and morphology of the TMJ differs from species to species, especially according to eating habits. This can be considered in four groups:

a) Carenivorus: eg. Grizzly Bear.

TMJ is characterized mainly by hinge movement, condyles are cylindrical. Long axis of condyles oriented mesiodistally.

b) Herbivores: e.g Gaint Panda.

Jaws mainly move laterally. Long axis of the condyle oriented anteriomedially.

c) Rodents: e.g Rabbit

Type of chewing movement in rodents is described as a cutting function by continuous erupting central incisor and strong grinding force by molars. Long axis of the condyle anterioposteriorly.

d) Omnivores eg.: Pigs and humans

Adopted to chew every type of food involving cutting, splitting and grinding. The long axis of the condyle runs slightly posterior the horizontal section and lines extended from both sides meet posterior mandible.

Embryology of TMJ:

The mammalian craniomandibular articulation develops anteriolateral to otic capsule from the first branchial arch mesenchyme and hence innervated by fifth cranial nerve. This is early embryonic joint.

This primary embryonic joint formed by joining malleus and incus which develops from first branchial arch. This joint serves as the primary TMJ upto 16 week of prenatal life. This is a uniaxial hinge joint capable of no lateral motion.

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

At about 9th week a condensation of mesenchyme appears surrounding the upper posterior surface of rudimentary ramus. This mass condrifies at about 10-11 week to form cartilaginous mandibular condyle with progressive endochondral ossification. The cartilage fuses with the posterior part of bony mandibular body. At about 9-10 weeks the muscle fibers become more differentiated. Blood vessels, nerve etc can be seen to the joint region at about 10 week of gestation.

The appearance of mandibular fossa of the temporal bone is seen earlier than that of condyle at about 7-8 weeks.

Ossification of fossa is more prominent at about 10-11 weeks. Ossification continues in this region and at about 22 weeks the mandibular fossa shows both medial and lateral walls and the articular eminence is also evident.

The shape of the fossa is concave at about 9th week and it takes a definite concave shape to match convex condyle.

The differentiating mesenchymal cells interposed between the condyle and mandibular fossa gives raise to capsular and inter capsular structures of TMJ.

Articular disc

Articular disc first seen at about 7th week, by the 10th week first sign of collagenous fibers within the articular disc developes and it becomes more prominent by 12 weeks. From 19th 20th week the disc increasingly takes on its definite fibro cartilaginous composition. At this stage only disc shows pattern of differential cell proliferation in which central region becomes thinner than periphery.

Articular capsule:

Articular capsule first appears at about 9-11 weeks. By 17th week the capsule is seen with fully formed tissue boundary between intracapsular and extracapsular components of the TMJ. By then lower cavity of the fossa enlarges and the superior joint cavity are receprocal at the time, the upper joint cavity is concave and lower joint cavity is convex.

Work done by Hooker (1954) and Humphery (1968) shows that actual month opening actions are observable as early as 7-8 weeks of gestation. Others like Symons (1952), Perry (1985), Moffet (1957) said that only scattered muscle fibers of lateral pterygoid muscle are clearly at 7-8 weeks.

Therefore prenatal jaw opening activity that both Hooker and Humphery observed is said to have involved the articulation of the primary TMJ.

Anatomy of temporomandibular joint

TMJ is broadly divided into bony structures of TMJ and soft tissue structures of TMJ.

Bony structures of TMJ:

a) The condylar head of the mandible

The articular surface of the mandible is the upper and anterior surface of the condyle which is found in the posterior of the mandible as 2 processes that project upward on the ramus of the mandible. This articular surface is shaped like a Rugby ball. When viewed from side the articular surface is strongly convex and less so when viewed from the front.

The medial and lateral poles are connected by long axis of the head which makes an angle of 140 with the line joining the external algnostic meatus of both sides in the horizontal plane. Both poles project beyond the surface of ramus and are roughened to receive the attachment of the articular disc. On the anterior surface of the week of the condyle is the shallow depression called the pterygoid fossa, the area of insertion of the lateral pterygoid muscle. Superior surface of the condyle is covered by fibro cartilage, below which is a thin layer of cortical bone which is supported by a small trabeculae. However this cartilage does not form a part of articular surface because it is covered with periosteum derived fibro articular tissue unlike others which is covered by hyaline cartilage. The articulating surfaces are non vascularized and non innervated as a adaptation for load bearing. The adult condyle is about 15-20mm meditalirally and 8-10 mm anterior posteriorly.

The Glenoid fossae:

The glenoid fossa of the TMJ is a shallow, oval depression to the infratemporal area. It is located between base of the zygomatic process anteriorly and external arostic meatus posteriorly. Anterior to the fossa the articular/eminence arises gently and posteriorly, the thin bone of the tympanic area forms the posterior wall of the fossa. The fossa is lined by articular tissue the posterior part of the fossa is elevated to a ridge called the posterior auricular lip. This is higher and thicker at its lateral end and is known as post glenoid process. Medially the articular fossa is bound by a bony plate that leans against the spine of sphenoid called as temporal spine.

Articular eminence:

It is important to understand the difference between articular eminence and articular tubercle. The articular eminence is the transverse bar of dense bone that forms the posterior root of the zygomatic arch and the anterior wall of the articular fossa. It has a large articular surface. The articular tubercule is the small bony projection situated laterally to the articular eminence. It is not a articular surface, instead it serves as the attachment area for the portion of the TMJ. The articular eminence is a saddle shaped elevation at the base of the zygomatic process of the temporal bone.

Soft tissue structures of TMJ

Articular disc

The articular disc is a firm, oval, fibrous plate positioned between the mandibular condyle and articular fossa and eminence. It divides the joint space into upper and lower compartment. It also adapts to the changing contour of the rotating condylar head as it glides to the different parts of the temporal bone, this is possible as the disc is not uniformly thick and the periosteal covering of the temporal bone is padded. The underside of the disc is concave and fits closely over the condylar head like a cap. This enables the rotatory movement of the condyle.

In frontal section the disc is wedge shaped thicker medially and thinner laterally. It is suggested that lateral half of the glenoid fossa is subjected to pressure, since blood vessels are seldom present in the later half of the disc and bony structure of the fossa is thicker laterally than medially.

In sagittal section the disc is divided into three regions, thinnest called as intermediate zone. In normal position the condyle is located in the intermediate zone of the disc, bordered by thicker anterior and posterior region.

From anterior to posterior region, the disc shows 5 zones. Anterior extension, anterior band, posterior extension, posterior band and the intermediate zone in between. Blood vessels are found in the anterior and posterior parts of disc and seldom in intermediate zone, since it is the stress bearing area, structural tissue adaptation so as to avoid necrosis. In case of extra eye stress, it is the site for perforation, displacement or thickening.

Articular capsule or joint capsule:

The capsule is a thin, fibrous connective tissue, sleeve about the joint, tappers above down the neck of the condyle. Its attachment extends from around the glenoid fossa to around the week of the condyle. Inner surface of the capsule is lined by synovial membrane which has a cilia like folds and secrets synovial fluid. Synovial fluid is dialystate of plasma with added mucin and plasma. It acts as a lubricant and as well as nutriative purpose. Since the cells it contains are macrophage in type, this fluid also exhibits phagocytic activity. The thickened anterolateral and lateral portion of the capsule which is attached to the articular tubercule is called as temporomandibular ligament.

3. Ligaments:

The ligaments of the temporomandibular joint are broadly divided into intrinsic ligament and extrinsic ligament.

The articular capsule is too delicate of a structure to support the joint unaided and so stability is actived by intrinsic and extrinsic ligament. Ligaments limit the movement of TMJ.

INTRINSIC LIGAMENT:

a) Lateral ligament or TM ligaments:

This ligament arises from the zygomatic process and articular eminence of the temporal one, runs downwards and posteriorly and consist into area just below the lateral pole of the condylar head and into posterior aspect of neck of condyle. Anatomically, its hard to distinguish this ligament from joint capsule excepting histologically, as there are bloodvessels present between these structures. This ligament is strong and supports the joint preventing lateral displacement of the condyle and restricting its anterior and posterior movement.

b) Collateral ligament:

It is rather a narrow band of collagen fibers that run horizontally backward or the inner aspect of the capsule from lateral and medial aspect of articular eminence, to respective condylar poles. They restricts the displacement of condylar head distally. These collateral ligament along with the temperomandibular ligaments helps to attain the clinical ligamentation position.

EXTRINSIC LIGAMENT:

Though these also aid to restricting jaw movement, less significant functionally due to their distance from the joint and lack of strength.

b) Spenomandibular ligament: Superiorly arise from the new of the sphenoid bone and attached to the lingula of the mandibular foramen inferiorly. It is the remnant of the cephalic end of Meckels cartilage.

c) Stylomandibular ligament: Arises from the styloid process of temporal bone and stylohyoid ligament and inserts into mandibular angle and lower part of the posterior border of the ramus.

Muscles of TMJ:

The muscles inovled in the functioning and stablising of TMJ are

1) The masticatory muscles.

2) Supra hyoid muscles.

1. The masticatory muscles:

These muscles move the mandible during mastication, speech, and other functional and para functional activities of the mandible. They are derived from one muscle mass and are arranged in that order from the superficial to the deep plane. They develop from mesoderm of the first branchial arch and are supplied by the nerve of that arch. Jaw closing is controlled by the temporalis, masseter and medial pterygoid. The lateral pterygoid however aids in opening of the jaws, assisted by suprahyoid and infra hyoid muscles.

a) Masseter:

Origin : This muscles has got two parts. The superficial and deep part. Superficial layer arises from the anterior 2/3 of the zygomatic arch and adjoining zygomatic process of maxilla. Deep layer arises from the deep layer of the zygomatic arch.

Insertion: Superficial layer into the lower part of the lateral surface of the ramus of mandible and the deep part into the upper part of ramus and coronoid process.

Action: equation of the mandible.

Blood supply: a) Massetric branch of the maxillary artery. Facial artery of the carotid and its direct branch.Nerve supply : Massetric nerve.

Temporalis:

Temporalis is the strongest of the masticatory muscle. It is fan shaped.

Origin: Arises from the bony floor of the temporalis fossa and from the deep surface of the temporal fossa.

Insertion: Muscle fibers converge to tendon, which passes deep to the zygomatic arch and is inserted into the coronoid process of mandible and the anterior border of ramus of the mandible.

Action: Anterior and superior fibers elevate, the mandible and the posterior fibers retract the mandible.

Blood supply: Auriculotemporal branch maxillary artry.

Nerve supply: Deep temporal nerve branch of mandibular nerve.

c) Medial pterygoid

Origin: superficial head arises from the tuborosity of the maxilla. Deep head arises from the medial surface of the lateral pterygoid plate.

Insertion: into the medial surface of the angle of the mandible.

Action: Assists in elevating the mandible.

Blood supply: Medial pterygoid branch of maxillary artery.

Nerve supply: medial pterygoid branch of trigeminal nerve.

d) Lateral pterygoid:

Origin: Upper head arises from the infratemporal surface of the greater wing of the sphenoid. Lower head arises from the lateral surface of the lateral pterygoid plate.

Insertion: Two heads converge as they pass backward and are inserted into the front of the neck of the mandible and the articular disc.

Action: Muscle functions by pulling the condyle anteriorly while opening the mouth slightly. Same time the articular disc advances simultaneously along with the condyle. The mandible moves laterally when this muscle contract bilaterally.

Blood supply: pterygoid branch of maxillary artery.

Nerve supply: lateral pterygoid branch of trigeminal nerve.

Suprahyoid muscle:

These muscles are located in the floor of the mouth and act to pull the mouth downward for opening and raise the mouth for swallowing.

They are the digastric, stylohyoid, mylohyoid, geniohyoid.

Lever action of the temporomandibular joint Review of literature

Hylander W.L. in 1975 concluded the mandible acts as a Class III lever, where the condyle acting as the fulcrum. It is said that the condyle might be a stress bearing fulcrum and the muscle force is always divided between the bite force along the tooth row and the reaction force of the condyle.

Hylander W.L. in 1975 carried out experiments in macae monkey in which strain gauges were placed within the bone of the condyle below the joint surface and recorded stains of differencing degrees. The strains were greater on the contialateral side than the epsilateral side.

Taylor in 1980 showed shall no evidence was found of stress in glenoid fossa and it is believed that the articular disc has no protective function. It is however admitted that certain action do occur in the TMJ but these changes do not reflect the lever action.

Therefore it is contended that continued representation of the mandible as a Class III lever was an error as said by Taylor R.M. in 1980.

TMJ movement:

TMJ movements or the mandibular movements can be classified as

1) Free movement

2) Masticatory movements

3) Reflex control of TMJ movements:

Coming back to the TMJ movements

1) Free movements

a) Depression of mandible

When the mouth is opened, the condyle rotate on the undersurface of the articular disc around the horizontal axis. In order to prevent the angle of the mandible from impinging on the parotid gland and sternocleido mastroid muscle, the mandible is pulled forward. This accomplished by the contraction of the lateral pterygoid muscle. This pulls the neck of the mandible and the disc forward and hence the disc moves onto the tubercle.

Depression of mandible is brought about by contraction of digastric, geniotyoid and mylohyoid. The lateral pterygoid play a major role.

b) Elevation of mandible

This movement is reverse of that depression. First the head of the mandible and the disc move backward and then the head rotation on the lower surface of the disc. Elevation is brought about by contraction of temporalis, masseter, and medial pterygoid. Head is pulled back by posterior fibers of temporalis. Disc is pulled back by bilaminar elastic fibers.

Protrusion of mandible:

The articular disc is pulled forward onto the anterior tubercle, carrying the head of the mandible with it. Which the elevators and depressors stabise the position of the mandible in relation to maxilla. In this movement the lower are drawn forward over upper teeth. Protrusion is brought about by contraction of lateral pterygoid of both sides assisted by medial pterygoid.

Retraction of mandible:

The articular disc and head of the mandible are pulled backward into the mandibular fossa.

Retraction is brought about the posterior fibers of the temporalis assisted by deep fibers of messeter and geniohyoid, diagastric play a minor role. Retraction is limited to a distance of 1mm.

Lateral movements:

For this movement to take place, one condyle along with the disc glides forward rotating along vertical axis, while other glides backward rotating in opposite direction. This movement take place alternatively. For this to occurs the muscle of opposite side act alternately and not in unision. Lateral pterygoid and medial pterygoid are the muscles responsible.

Bennet movement: found by Bennet in 1908.

It is defined as the bodily lateral movement of the mandible (or) the lateral shift of the mandible resulting from the movements of the condyle along the lateral thickness of the mandibular fossa in lateral jaw movements.

Bunnet angle:

Angle formed by the sagittal plane and the path of the advancing condyle during lateral mandibular movements as viewed in horizontal plane.

Importance:

1) Its direction and timing influences the freedom of movement to and from the centric and eccentric jaw position.

2) Balanced occlusion will be difficult to achieve if the direction and timing of the Bennet movement is not accurately recorded and transferred to the articulator capable of reproducing those movements.

3) 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 forward, downward and laterally.

Bennet movement consists of the immediate translation which takes place before the rotation and progressive translation which accompanies rotations.

Masticatory movements:

Masticatory movements of the TMJ differs from the free movements due to the fact that these occur will the food in oral cavity and associated with incision and chewing of food. They are of two kinds.

a) Incision (or) cutting movements

b) Mastication (or) brushing and grindig movements

a) Incision

Incision is divisible into 3 parts starting from rest position.

First the mouth is opened by depressing the mandible, extend of opening depends on the dimension of food.

2nd the mandible is elevated to an upward and forward movement of the mandibular incisors and upward and backward movement of the condyle during jaw closure and continuing till the upper and lower incision contact the food.

3rd teeth continues to move upward with the simultaneous application of force on the food, and teeth in contact.

These three phases are refered to as opening, closing and power phase respectively. 1st and 3rd phases are similar to opening and closing of the mandible. Once the teeth contact is reached during incision, the morphology of the anterior teeth guides the mandibular movements to a considerable extend.

b) Mastication:

Mastication can be described as having 3 basic strokes. Opening, closing and power strokes. Chewing cycle associated with the mastication of single piece of food is referred as chewing cycle / sequence. Chewing cycle begins with the opening of the mouth by depression of the mandible, which is accomplished by slight swing to the non chewing side and then back to the chewing side. This is opening stroke.

From the position of the max opening, the mandibular incisors move upward, forward and away from midline. This position of the upward movement is called as closing or fast stroke.

Completion of this leads to power stroke. Which is the forceful contraction between occlusal surface of the molars and premolars. During this stroke the incisions are moved back to midline. When the power stroke ends before the upper and lower teeth make contact is called as Punture stroke.

Reflex control movements:

The muscles of TMJ like other muscles of the body are subjected to both reflex control and control within CNS.

There are 3 principle reflexes which control the vertical relationship between maxilla and mandible and hence TMJ movements, they are

a) Jaw jerk reflex:

It is sometimes refered to the jaw closing reflex. It is analogous to the knew jerk. It is a strech reflex where by stretching the jaw closing muscles usually by applying a down ward tap on the chin, produces a reflex contraction of these muscles. The importance of this reflex is that, it demonstrates the existence of the feedback mechanism from jaw closing muscle to their own motor neuron in CNS. This feedback loop comes from muscle spindle within the muscles. This feedback mechanism helps in fine control of the TMJ movements throughout normal function like taking account of consistency of food. No such mechanism as jaw opening reflex is seen as these opening muscles contain less or no muscle spindles.

ii) Jaw opening reflex:

These are effected inhibition of activity of jaw closing muscles, but do not show any activation of jaw opening muscle. This reflex can be triggered by stimulation of mechano receptive nerves from most structures within the mouth. The importance of these probably lies in their ability to prevent injury while biting or chewing objects liable to produce damage.

iii) Jaw unloading reflex

Jaw unloading reflex also involve a cessation of autivity on jaw closing muscles, together with an activation of jaw opening muscles. This reflex is evoked when a hard object which is being bitten breaks suddently, thus unloading jaw closing muscles of the resistance against which they are working, result of which is that, the opposing teeth do not forcibly hit into one another thereby preventing damage.

Age changes in TMJ

Though variance structures makeup the TMJ, Glenoid fossa and the condyle shows great importance.

Condyle:

The morphology and function of the TMJ is greatly affected by aging of the mandible, especially by the loss of teeth. The convexity of the condyle decreases in edentulous mandible compared with the mandible with teeth and peak of the condyle normally appears in posterior part of head as against the anterior or central in dentulous mandible. Condylar head decreases in height due to loss of teeth much more than coronoid process thus making the littic appear elongated. Some times drastic age changes may result in an almost disappearance of the condylar head. These changes may be the result of resorption or depression on articular surface or resorption on the posterior aspect of head against the glenoid tuberculae.

Resorption occurs more frequently on the lateral aspect of the head than medially and least in region of pterygoid fovea. The edentulous mandible shows decreased density and loss of regularity of the bony trabeculae.

The region of pterygoid forea is most difficult to resorb owing to the great density of the underlying traberculae at the insertion of lateral pterygoid muscle, which contributes to maintenance of bony structure. These trabeculae are arranged in same direction as that of lateral pterygoid muscle.

Age changes in glenoid fossa:

In edentulous mandible the vertical dimension decrease and lateral movements of the jaw changes along with resorption anterior marginal area of the glenoid fossa. Thus the zygmoid curvature from the bottom of the fossa into the eminence becomes more flattened. Changes also occur in medial and lateral marginal areas of the fossa. The distance from the bottom of the fossa to the lateral and medial margin of the fossa decrease in edentulous jaw and it almost becomes flattened. However unlike the condyle, the shape and size of the fossa do not change much.

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