MASTICATION

Contents

1) Introduction

2) Definition

3) Importance of mastication

4) Masticatory apparatus

5) Muscles of mastication

Masseter

Temporalis

Medial pterygoid

Lateral pterygoid

6) T.M.J.

7) Tongue

8) Neural masticatory receptors

9) Major functions of masticatory system

10) Parafunctional movements

Bruxism

Clenching

Nail biting

Pencil chewing etc

11) Clinical implications

T.M.J. referred pain

Orofacial pain

Muscles trismus

High points in restorations

12) References

13) Conclusion

INTRODUCTION:

Feeding or ingestion is the process of transferring food into the gut

for digestion. In many animals the mouth is merely the anterior opening of

the gut. Where food is either swallowed as whole, or in large chunks with

little or no mechanical processing.

However in terrestrial mammals the situation is generally different.

During mammalian evolution, changes occurred in the morphology of skull,

teeth, jaws, and associated orofacial structures that permitted an addition

stage of mechanical processing of food in the mouth, prior to swallowing.

This process of mechanical breakdown of food in the mouth is mastication

or chewing.

Mastication can therefore be regarded as an interruption in the

process of transporting food through oral cavity en route to the gut.

DEFINITION:

L.M. Harrison : defined mastication as a process of chewing food.

IMPORTANCE OF MASTICATION:

1) Increases the surface area of food so that digestive enzymes can act

on a greater area.

2) In case of most fruits and raw vegetables where the surface coating of

the food is made up of indigestible cellulose or hemi cellulose,

mastication causes exposure of inner digestive material.

3) It helps in the flow of saliva.

4) It helps in subsequent deglutition.

Grinding of food to a very find particulate consistency prevents

excoriation of the G.I.T. and increases the ease with which food is emptied

from the stomach into the small intestine and then into all segments of the

gut.

MASTICATORY APPARATUS:

These involve the organs and structures primarily functioning is

mastication viz.

1) Teeth

2) Muscles of mastication

3) T.M.J.

4) Tongue

5) Accessory organs of mastication

Teeth:

Teeth are inarguably the principle organ of mastication and are

generally adopted for the functional requirement of the diet. Mammalian

dentition is heterodont i.e. the teeth in different parts of the mouth differ in

anatomical form and function.

The anterior teeth have sharp edges for grasping, incising or tearing

food while posterior teeth are specialized for cutting flesh of grinding

fibrous plant materials.

The human dentition is made up of 32 teeth. Each tooth can be

divided into two basic parts.

1) Crown and the Root:

The root is attached to the alveolar bone by means of specialized

connective tissue fibers called PDL.

PERIODONTAL LIGAMENT:

The PDL attaches the tooth firmly to its bony socket and also helps to

dissipate the forces supplied to the bone during mastication and acts as a

natural shock absorber.

The 32 teeth are distributed equally in the alveolar bone of the

maxillary and mandibular arches of which 16 teeth are aligned in alveolar

process of maxilla and 16 in alveolar process of mandible.

The maxillary arch is slightly larger than mandibular arch and tooth

sizes are also greater than mandibular teeth.

The permanent teeth can be grouped into 4 classification as follows

(according to morphology of crowns) :

1) Incisors

2) Canines

3) Premolars

4) Molars

Incisors: teeth located in the anterior most region are incisors and are

shovel shaped with an incisal edge. 4 maxillary incisors are larger than

mandibular incisors.

Main function is used for incising or tearing food during mastication.

Canines: Distal to incisors are canines located at the corners of the arches

and are generally the longest teeth with a single cusp and root.

Two maxillary and 2 mandibular canines are present. In humans,

canines usually function and incisors and are used for ripping or tearing of

food.

Premolars: 4 maxillary and 4 mandibular premolars are present.

- Since they have 2 cusps they are called bicuspids.

- The presence of these two cusps greatly increases the biting

surfaces of these teeth.

Their main function is to begin the effective breakdown of food

substances into smaller particles.

Molars: The last class of teeth are the molars. There are 6 maxillary and 6

mandibular molars.

- The crown of each molar has 4-5 cusps.

- This provides for large, broad surface upon which breaking

and grinding of food can occur.

Thus each tooth is highly specialized according to its function. The

exact interarch and intrarch relationships of the teeth are extremely

important and greatly influence the health and function of the masticatory

system.

MUSCLES OF MASTICATION :

Muscles that power the jaw movement during mastication are known

as muscles of mastication.

Other muscles like tongue, muscles in lips and cheeks also aid in

mastication.

Muscles of mastication can be classified and anatomically into 2

categories:

1) Those between the cranium and mandible viz Masseter, Temporalis and

Pterygoids.

2) Those between mandible and hyoid bone viz. Anterior Digastric,

Geniohyoid and Mylohyoid.

Functionally masticatory muscles can be classified as,

i) Jaw elevators: Masseter, temporalis and medial pterygoid.

ii) Jaw depressors: Anterior Digastric, Geniohyoid, Mylohyoid and

Lateral pterygoid.

The muscles Masseter, Temporalis, Medial and Lateral pterygoid are

considered the principal muscles of mastication.

MASSETER:

It is a quadrilateral muscle consisting of 3 layers which blend

anteriorly.

i) The superficial layer

ii) Middle layer

iii) Deep layer

Muscle Origin Insertion

i) Superficial layer It arises by a thick

aponeurosis from the

maxillary process of

zygomatic bone from

anterior 2/3rd of the

inferior border of

zygomatic arch.

Its fibers pass downwards

and backwards to insert

into the angle and lower

posterior half of lateral

surface of mandibular

ramus.

ii) Middle layer Arises from the medial

aspect of anterior 2/3rd of

zygomatic arch and from

the lower border of post

1/3rd of zygomatic arch

Inserts into the central

part of ramus of mandible

iii) Deep layer Arises from deep surface

of zygomatic arch

It inserts into the upper

part of the mandibular

ramus and into the

coronoid process.

Relations:

Superficial: Skin, Platysma, Risorius, Zygomaticus major and Parotid

gland.

Deep: Temporalis and ramus of mandible

Posterior: Margin is overlapped by parotid gland.

Nerve supply: Anterior branch of mandibular nerve.

Actions: Elevates the mandible to occlude the teeth in mastication.

TEMPORALIS :

Origin Insertion

Arises from whole of temporal fossa

(except the part formed by the

zygomatic bone) and from deep

surface of temporal fascia.

Its fibres converge and descend into

a tendon which passes through the

gap between zygomatic arch and

side of skull, and attaches to medial

surface, apex, anterior and posterior

borders of coronoid process and

anterior border of mandibular ramus

of mandible almost to the last molar

tooth.

Relations:

Superficial:

Skin, Auriculars anterior and superior, temporal fascial, superficial

temporal vessels, Auriculotemporal nerves, temporal branches of facial

nerve, zygomatic temporal nerve, epicranial aponeurosis, zygomatic arch

and Masseter.

Deep:

Are femoral fossa, lateral, lateral pterygoid, the superficial head of

medial pterygoid, a small part of buccinator, the maxillary artery, deep

temporal nerves and buccal nerve and vessels.

Nerve supply: Temporalis is supplied by the Deep temporal branches of

anterior trunk of the mandibular nerve

Actions:

1) Elevation: temporalis elevates the mandible and also closes the mouth

and approximates the teeth.

2) Posterior fibres retract the protruded mandible.

3) Also contributes to side-to-side gliding movements.

LATERAL PTERYGOID:

It is a short, thick muscle with two parts or heads.

- Upper head

- Lower head

Muscle Origin Insertion

Upper head It arises from the infratemporal

surface and infratemporal crest of

greater wing of sphenoid bone.

Pterygoid fovea

Lower head It arises from the lateral surface of

lateral pterygoid plate

Anterior margin of

articulating disc and

capsule of TMJ.

Relations :

Superficial: are ramus of mandible, the maxillary artery, tendon of

temporalis, and Masseter.

Deep: are part of medial pterygoid, the sphenomandibular ligament, middle

meningeal artery and mandibular nerve.

Nerve supply: supplied by branch from anterior trunk of mandibular nerve.

Actions:

Upper head: Elevates the mandible and medial movement from laterally

displaced position (Aids mainly in chewing).

Lower head: Depresses the mandible, protrusion of mandible and side-side

movements.

Medial pterygoid:

This is a quadrilateral muscle. It has a small superficial head and a

large deep head and forms the major part of the muscle.

Muscle Origin Insertion

Small head From the tuberosity of

the maxilla and

adjoining bone.

The fibers run downwards and

backwards and laterally insert into

the roughened area on medial

surface of the angle and adjoining

part of ramus of mandible below

and behind the mandibular foramen

and Mylohyoid groove.

Deep head From the medial surface

of lateral pterygoid plate

and adjoining part of

palatine bone.

Nerve supply:

Nerve to medial pterygoid i.e. a branch of the main trunk of

mandibular nerve.

Actions:

1) Elevates the mandible

2) Helps to protrude the mandible

3) Side-to-side movements i.e. chewing movements

Temporomandibular joint:

1) The area where craniomandibular articulation occurs is called the

T.M.J.

2) It provides for hinging movement in one plane, hence can be

considered a ginglymoid joint. At the same time it also provides for

gliding movements, which classifies it as an arthroidal joint. Thus it is

technically considered as a ginglymoarthroidal joint.

3) The T.M.J. is formed by the mandibular condyle fitting into the

mandibular fossa of the temporal bone. separating these two bones

from direct articulation is the articular disc. Functionally, the articular

disc serves as a nonossified bone that permits the complex

movements of the joint.

The T.M.J. can be discussed under the following headings.

1) Articular surface

2) Articular disc

3) Ligaments

Articular surface :

1) The upper articular surface is formed by the following parts of the

temporal bone.

i) Articular eminence

ii) Anterior part of the mandibular fossa.

2) The inferior articular surface is formed by the head of the mandible.

3) The articular surfaces are covered with fibrocartilage.

4) The joint cavity is divided into upper and lower parts by an

intrarticular discs.

Articular disc:

1) The articular disc is an oval fibrous plate that divides the joint into an

upper and a lower compartment.

2) The upper compartment permits gliding movements, and the lower,

rotatory as well as gliding movements.

3) The disc has a concavo convex superior surface and a concave

inferior surface.

4) The periphery of the disc is attached to the fibrous capsule.

Ligaments: these are

i) Fibrous capsule

ii) The lateral ligament

iii) The sphenomandibular ligament

iv) Stylomandibular ligament

i) Fibrous capsule: is attached above to the articular tubercle, the

circumference of the mandibular fossa and the squamotympanic fissure

and below to the neck of the mandible. The capsule is loose above the

intra-articular disc, and tight below it. The synovial membrane lines the

fibrous capsule and the neck of the mandible.

ii) The lateral (temporomandibular) ligament: it reinforces and

strengthens the lateral part of the capsular ligament. Its fibres are

directed downwards and backwards. It is attached above to the auricular

tubercle, and below to the posterolateral aspect of the neck of the

mandible.

iii) The sphenomandibular ligament: it is an accessory ligament, which

lies on a deep plane away from the fibrous capsule. It is attached

superiorly to the spine of the sphenoid, and inferiorly to the lingula of the

mandibular foramen. It is a ruminant of the dorsal part of Meckel’s

cartilage.

The ligament is related laterally to:

- The Lateral pterygoid

- The Auriculotemporal nerve

- Maxillary artery

- Inferior alveolar nerve and vessels

Medially there are

- Medial pterygoid

- The chorda tympani nerve and

- The wall of the pharynx

Near its lower end it is pierced by the Mylohyoid nerve and vessels.

iv) The Stylomandibular ligament: is another accessory ligament of the

joint. It represents a thickened part of the deep cervical fascia which

separates the parotid and sub mandibular salivary glands. It is attached

above to the lateral surface of the styloid process, and below to the angle

and posterior border of the ramus of the mandible.

Relations of T.M.J.:

Lateral : Shin of fascial, parotid gland and temporal branches of the facial

nerve.

Medial: The tympanic plate separates the joint from the internal carotid

artery. Spine of sphenoid, with the upper end of the sphenomandibular

ligament attached to it. The Auriculotemporal and chorda tympani nerves,

Middle meningeal artery.

Anterior: lateral pterygoid, massetric nerve to vessels.

Posterior: the parotid gland separates the joint from the external auditory

meatus. Superficial temporal vessels and Auriculotemporal nerve.

Superior: Middle cranial fossa

Middle meningeal vessels

Inferior: Maxillary artery and vein

Blood supply: Branches from superficial temporal and maxillary arteries

Nerve supply:

- Auriculotemporal nerve

- Massetric nerve

Biomechanics of T.M.J.:

The T.M.J is a compound joint. Its structure and function can be

divided into 2 distinct systems.

1) One joint system is the tissues that surround the inferior synovial cavity

(i.e. the condyle and the articular disc).

Since the disc is tightly bound to the condyle by the lateral and

medial discal ligaments, the only physiologic movement that can occur

between these surfaces is rotation of the disc on the articular surface of the

condyle.

The disc and its attachment to the condyle are called the condyle disc

complex. This joint system is responsible for rotational movement in the

T.M.J.

2) The second system is made up of the condyle discomplex functioning

against the surface of the mandibular fossa.

Since the disc is not tightly attached to the articular fossa free-sliding

movement is possible between these surfaces in the superior cavity. This

movement occurs when the mandible is moved forward (referred to as

translation).

Translation occurs in the superior joint cavity between the superior

surface of the articular disc and the mandibular fossa. Thus the articular disc

as a nonossified bone contributing to both joint systems.

Normal functional movement of the condyle and disc during the full

range of opening and closing. The disc is rotated posteriorly on the condyle

as the condyle is translated out of the fossa. The closing movement is the

exact opposite of opening.

Tongue:

1) The tongue is a highly muscular organ of deglutition, taste and

speech. It plays several key roles in food ingestion and subsequent

intraoral processing.

2) It is partly oral and partly pharyngeal in position, and it is

attached by its muscles to the hyoid bone, mandible, styloid

processes, soft palate and the pharyngeal wall.

3) It has a root, an apex, a curved dorsum and an inferior surface.

Movement of the tongue involves mainly an antero-posteriorly

directed cyclic pattern which is linked with vertical movements of the jaws.

Tongue retraction occurs mainly when the teeth are apart, while tongue is

protruded when the teeth are closer together in the occlusal phase and early

opening phase of chewing cycle. Tongue may also act as an organ of

mastication. Soft foods may be squashed / mushed by the tongue against the

hard palate.

It is divided by the U-shaped sulcus terminalis into an anterior, oral or

presulcal part facing upwards and a posterior, pharyngeal or post sulcal part

facing posteriorly. The anterior part forms about two-thirds of the tongue’s

length.

Oral (Presulcal) part : is located in the floor of the oral cavity, this has an

apex touching the incisor teeth, a margin in contact with the gums and teeth

and a superior surface (dorsum) related to the hard and soft palates. Its

general sensory nerve is the lingual branch of the mandibular, the chorda

tympani branch of facial nerve.

Pharyngeal (postsulcal / part) forms the base of the tongue; it lies

posterior to the palatoglossal arches within the oropharynx, forming its

anterior wall. Devoid of papillae, it is no elevations due to lymphoid

nodules embedded in the submucosa collectively termed the lingual tonsil.

Muscles of tongue:A middle fibrous septum divides the tongue into right and left halves. Each half

contains 4 intrinsic and 4 extrinsic muscles.

Intrinsic muscles Extrinsic muscles

1) Superior longitudinal

2) Inferior longitudinal

3) Transverse and

4) Vertical

1) Genioglossus

2) Hyoglossus

3) Styloglossus

4) Palatoglossus

Blood supply:

Lingual artery, a branch of external carotid artery.

The root is supplied by the tonsillar and ascending pharyngeal

arteries.

Nerve supply: The Lingual nerve is the nerve of general sensation and

Chorda tympani is the nerve for taste in the ant 2/3 rd except for vallate

papilla.

- The Glossopharyngeal nerve is the nerve for both general sensation and

taste for posterior 1/3rd of the tongue including circumvallate papilla.

- The posterior most part of the tongue is supplied by the Vagus nerve

through the Internal laryngeal branch.

Accessory organs of mastication:

1) These play essentially supportive role.

2) Lips can aid in the ingestion of food and provide an anterior oval seal

to prevent spillage of food from mouth.

3) Tongue and cheek combine to direct the bolus on the occlusal surface

of posterior teeth.

4) Salivary glands provide the intra oral lubrication for these activities.

Neural masticatory receptors:

1) The various coordinated masticatory activities of the mandible are

reflected by the approximate muscle function.

2) Each muscle is innervated by (alpha) -efferent motor neurons that

supply the extrafusal muscle film.

3) Where as -efferent supply the intrafusal fibres of the muscle spindle.

4) Each muscle comprises fibres that exhibit rapid twitch contraction or

slow twitch contraction. There are also muscle fibres with

intermediate properties.

5) Contraction of individual muscle fibres is a function of muscle unit.

Muscle unit comprises a single (alpha) motor neurons, its (alpha)

– efferent nerve fibre and number of muscle fibres.

1) Muscle spindle:

1) They comprise stretch sensitive, slowly adopting specialized intrafusal

muscle fibre that are 2) contained with in a capsule laying parallel to the

extrafusal muscle fibres. Spindle generally has a double afferent

innervation.

i) Large – group Ia myelinated afferent fibres terminate in the

central region of each intrafusal fibre called as primary or annual

spiral ending.

ii) Smaller group II myelinated afferent fibre ending on either

side of the central region as spray or secondary endings.

There is a concept that the muscle spindle may be involved in

correcting small errors between the intended and actual mandibular

movements and maintaining a constant posture against the effect of

gravity.

2) Golgi tendon organs:

These are the receptors primarily located at muscle tendon junctions

or TMJ capsule. They are innervated by Ib myelinated afferent fibres. There

is no evidence of such units within the masticatory muscles.

3) Periodontal mechanoreceptors: The periodontal ligament

mechanoreceptors respond to forces applied to the teeth. These

mechanoreceptors have a wide range of properties.

i) Some are excited by just often microns of tooth

displacement.

ii) Some are less sensitive and respond only to much larger

forces.

iii) Some exhibit directional sensitivity, with nerve fibres

responding maximally to forces in one particular direction.

iv) Some are slowly adopting and produce continuous

discharge when constant stimulus is applied.

v) Some adopt more rapidly, producing only a few impulses

immediately when stimulated.

vi) Some are very rapidly adopting units and do not respond

unless a very rapid stimulus is applied.

vii) Some are very slowly adopting units and provide a constant

discharge that can be increased / decreased by applying forces in

specific direction.

Most single fibres respond to mechanical stimulation of just one

teeth, but some also respond to stimulation of upto 3 adjacent teeth. Cell

bodies of these fibres are located in trigeminal ganglion, with some others in

the trigeminal mesencephalic nucleus.

4) Mucous membrane receptors:

There are some cells in the mesencephalic nucleus, main sensory and

spinal trigeminal nuclei that respond to pressure in the palate, particularly in

the region just distal to central incisors.

5) Joint receptors:

Free nerve fibres composite the predominant receptors in TMJ

capsule. The lateral aspect of joint capsule and lateral ligament also contains

Ruffin, Pacinian and Golgi receptors and are supplied by a branch of

Auriculotemporal nerve.

Control of mastication:

Though mastication is a ‘voluntary’ process, little conscious effort is

involved, actually chewing occurs anatomically in much the some way as

walking or breathing. A number of theories have been put forward to

explain how mastication is controlled. Most of these theories include a

contribution from reflex actions.

Jaw reflexes:

Reflex can be defined as an automatic / involuntary activity brought

about by relatively simple circuits without consciousness being necessary

involved.

Jaw reflexes effort the vertical relationship between upper and lower

jaw as well as horizontal relationship which involve lateral and

anteroposterior movement of mandible with respect to the maxilla.

Thus jaw reflexes can be discussed under 2 headings viz.

- Vertical jaw reflexes and

- Horizontal jaw reflexes

Vertical jaw reflexes:

Vertical jaw reflexes can be considered under 2 broad categories.

1) Those evoked by stimulation of receptors with in the muscles

themselves.

i) Jaw jerk reflex

ii) Jaw unloading reflex

2) Those which are responses to stimuli of external origin (eg. food)

i) Jaw opening reflex

ii) Reflexes which involve activation of the jaw elevator muscles.

i) Jaw jerk reflex :

Jaw jerk is the simplest of the jaw reflexes in that if the only one

mediated by a monosynaptic pathway. It is analogous to the knee jerk and is

a stretch reflex whereby stretching the jaw elevator muscles usually by

applying a downward tap on the chin-produces a reflex contraction of these

muscles.

The significance of this reflex lies not in it happening as such during

normal function but in that it demonstrates the existence of feedback

between the jaw elevator muscles and their own motor neurons. This

feedback mechanism helps in the fine control of jaw movements during

normal functions to take account of varying external circumstance. E.g.

change in the consistency of food as it is broken up during mastication.

The reflex arc for the jaw jerk is known to start within the jaw

elevator muscles at the muscle spindle primary ending which via their

primary afferent nerve make direct monosynaptic connections with the

motor neurons in the trigeminal motor nucleus.

ii) Jaw unloading reflex :

This reflex involves some jaw opening but most be distinguished

from those reflexes known as jaw opening reflex. Since its trigger is very

different.

Jaw unloading reflex is evoked when a hard object which is being bit

breaks suddenly thus ‘unloading’ the jaw elevator muscle together with an

activation of jaw depressor muscles. The result is that the opposing teeth do

not come strongly into contact with one another after breaking trough the

hard object and that is this way, potential damage to the masticatory

apparatus is avoided.

This reflex is heavily dependent on receptors in the jaw and muscles.

When one is biting on an object which one knows or suspects to be

brittle, one sends not only powerful excitatory signals to the jaw elevator

motor neurons but also, as a precaution, weaker excitatory signals to the jaw

depressor motor neurons.

Jaw elevator motor neurons receive positive feedback from their own

muscle spindle via jaw jerk pathway.

Signals from jaw elevator muscle spindles produce an inhibitory

effect on the antagonist, jaw depressor motor neurons – this is known to

occur in spinal cord.

Thus while biting on the object there will be 2 excitatory drives to

jaw elevator motor neurons while the depressor motor neurons will be

receiving a mixture of excitatory and inhibitory drives.

When the object breaks, the sudden shortening of elevator muscles

will result in decrease in spindle activity and hence in overall excitatory

drive to the jaw elevator motor neurons and the inhibitory drive to the jaw

depressor motor neurons. In turn this causes the decreased activity in the

jaw elevator muscles and increased activity in the depressor.

iii) Jaw opening reflex :

The term jaw opening reflex can be misleading since there are several

reflexes which can in one or other way, cause jaw opening – including jaw

unloading reflex.

Simplest of there is the disynaptic reflex activation of motor neurons

to the Anterior Digastric muscle in response to the mechanical or noxious

stimulation in or around the mouth.

The 1st synapse is believed to be in the trigeminal sensory nuclear

complex mast probably in nucleus oralis or nucleus interpolasis and 2nd

synapse located in the trigeminal motor nucleus.

Horizontal jaw reflexes:

These reflexes involve lateral, protrusive and passively retrusive

movements of the jaw in response to stimulation of intraoral

mechanoreceptors. These reflexes are for less well understood than the

vertical once mainly because they are more difficult to investigate.

There is a possibility that those reflexes are triggered by horizontal

loading of the teeth and that consequently they might play a role in adjusting

the final closure of the jaws from the moment of 1st tooth contact until the

intercuspal position is reached.

Masticatory mandibular movements:

The range of masticatory mandibular movements were first described

by Ulrich and Bernet at the turn of 20th century. They showed that there was

no fixed axis of mandibular rotation.

Mandibular movements occurs as a complex series of interrelated

three dimensional movements.

They can be broken down into 2 basic components.

Two types of movements occur in the TMJ.

1) Rotational: when the body is turning about axis.

2) Translational: when all the points within a body have identical

motion.

Every possible 3 dimensional movement can be described in terms of

these 2 components.

It is easier to understand mandibular movement when the components

are described as projections in 3 perpendicular planes.

1) Sagittal

2) Horizontal and

3) Frontal (vertical) planes

Reference planes:

Sagittal plane:

In the Sagittal plane, the mandible is capable of a purely rotational

movement as well as translation.

Rotation occurs around the terminal hinge axis, which is an imaginary

horizontal line through the rotational centers of the left and right condylar

processes. The rotational movement is limited to about 12mm of incisor

separation before the T.M. ligaments and structures anterior to the mastoid

process force the mandible to translate the initial rotation or hinging motion

is between the condyle and the articular disc.

During translation, the lateral pterygoid muscle contracts and moves

the condyle disk assembly forward along the posterior incline of the

tubercle. Condylar movement is similar during protrusive mandibular

movement.

Horizontal plane:

In their plane, the mandible is capable of rotation around several

vertical axes e.g. lateral movement consists of rotation around on axis

situated in the working (laterotrusive) Condylar process, with relatively

little concurrent translation.

This slight lateral translation is known as Bennett movement,

mandibular side shift, or laterotrusion. This is frequently present. This may

be slightly forward called lateroprotrusion or slightly backward called

lateroretrusion.

The orbiting (or nonworking) condyle travels forward and medially as

limited by the medial aspect of the mandibular fossa and the

temporomandibular ligament. Finally, the mandible can make a straight

protrusive movement.

Frontal plane:

When a lateral movement occurs in the frontal plane, the

mediotrusive (non-working) condyle moves down and medially, while the

laterotrusive (or working) condyle rotates around the Sagittal axis

perpendicular to this plane.

Due to the anatomy of the medial wall of the mandibular fossa on the

mediotrusive side, transtrusion may be observed.

Due to the anatomy of the mandibular fossa on the laterotrusive side,

this may be lateral and upward or lateral and downward (laterotrusion) and

laterodetrusion. A straight protrusive movement occur in the frontal plane,

with both condylar processes moving downward as they slide along the

tubercular eminences.

Border movements:

Mandibular movements are limited by the T.M.J and ligaments, the

neuromuscular system and the teeth.

Posselt first who described the extremes of the mandibular

movements, which he called as the border movements.

Posselt used a 3-dimensional representation of the extreme

movements the mandible is capable of All possible mandibular movements

occur with its boundaries.

Starting at the intercuspal positions in the protrusive pathway, the

lower incisors are initially guided by the lingual concavity of the maxillary

anterior teeth. This leads to gradual loss of posterior tooth contact as the

incisors reach the edge-to-edge position. This is represented in the Posselt’s

diagram by the initial downward slope. As the mandible moves further

protrusively, the incisors slide over a horizontal trajectory representing the

edge-to-edge position (the flat portion in the diagram), after which the lower

incisors move upward until new posterior tooth contact occurs. Further

protrusive movement of the mandible typically takes place without

significant tooth contact.

The border farthest to the right of Posselt’s solid represents the most

protruded opening and closing stroke. The maximal open position of the

mandible is represented by the lowest point in the diagram.

The left border of the diagram represents the most retruded closing

stroke. This movement occurs in 2 phases.

The lower portion consists of a combined rotation and translation,

until the condylar processes return to the fossae.

The record portion of the most retruded closing stroke is represented

by the top portion of the border that is farthest to the left in Posselt’s

diagram. It is strictly rotational.

Posterior and anterior detriments:

The characteristics of mandibular movement are established

posteriorly by the morphology of the T.M.J’s and anteriorly by the

relationship of the anterior teeth.

The posterior determinants are shape of the articular eminences,

anatomy of the medial walls of the mandibular fossae configuration of the

mandibular condylar processes. Impact of skeletal variables on occlusal form of restorations.

Posterior determinants Variants Impaction restoration

Inclination of articular

eminence

Steeper Posterior cusps may be taller.

Flatter Posterior cusps must be shorter

Medial wall of glenoid

fossa

Allows more lateral

translation.

Posterior cusps must be shorter.

Allows minimal

lateral translation.

Posterior cusps may be taller.

Intercondylar distance Greater Smaller angle between laterotrusive

and mediotrusive movement.

Lesser Increased angle between laterotrusive

and mediotrusive movement.

The anterior determinants: are the vertical and horizontal overlaps and the

maxillary lingual concavities of the anterior teeth.

These can be altered by restorative and orthodontic treatment.

A greater vertical overlap causes the direction of mandibular opening

to be more vertical during the early phase of protrusive movement and

creates a more vertical pathway at the end of the chewing stroke.

Increased horizontal overlap allows a more horizontal jaw movement.

Envelope of motion:

By combining mandibular movements in the three planes (i.e. sagittal,

horizontal, frontal) or 3-dimensional envelope of motion can be produced,

that represents the maximum range of movement of the mandible.

The superior surface of the envelope is determined by tooth contacts,

whereas the other borders are primarily determined by ligaments and joint

anatomy that restrict or limit movement.

Three dimensional movements:

To demonstrate the complexity of mandibular movement, a

seemingly simple right lateral excursion will be used.

As the musculature begins to contract and move the mandible to the

right, the left condyle is propelled out of its centric relation position.

As the left condyle is orbiting anteriorly around the frontal axis of the

right condyle, it encounters the posterior slope of the articular eminence,

which causes an inferior movement of the condyle around the sagittal axis

with resultant tilting of the frontal axis.

Additionally, contact of the anterior teeth produces a slightly greater

inferior movement in the anterior part of the mandible than in the posterior

part, which results in an opening movement around the horizontal axis.

Because the left condyle is moving anteriorly and inferiorly, the horizontal

axis is shifting anteriorly and inferiorly.

This example illustrates that during a simple lateral movement,

motion occurs around each axis (i.e. sagittal, horizontal, vertical) and

simultaneously each axis tilter to accommodate to the movement occurring

around the other axes. All this happens within the envelope o0f motion and

is intricately controlled by the neuromuscular system to avoid injury to any

of the oral structures.

MAJOR FUNCTIONS OF MASTICATORY SYSTEM:

Functional movements:

The three major functions of the masticatory systems are

1) Mastication

2) Swallowing

3) Speech

Most functional movements of the mandible take place inside the

physiologic limits established by the teeth, the T.M.J’s and the muscles and

the ligaments of mastication. Hence these movements are rarely coincident

with border movements.

Mastication:

Mastication is defined as the act of chewing foods.

It represents the initial stage of digestion, when the load is broken

down into small particle sizes for case of swallowing.

It is a complex function that uses the muscles, teeth and periodontal

supportive structures, as well as the lips, cheeks, tongue, palate and salivary

glands.

Chewing stroke:

Mastication is made up of rhythmic and well controlled separation

and closure of the maxillary and mandibular teeth. This activity is under the

control of the Central Pattern Generator (C.P.G) located in the brain stem.

The complete chewing stroke has been described as a tear shaped

movement pattern. It can be divided into an opening movement and a

closing movement.

They closing movement has been further subdivided into the crushing

phase and the grinding phase.

During mastication similar chewing strokes are repeated over and

over as the food is broken down.

Tooth contacts during mastication:

1) When food is initially introduced into the mouth, few contacts occur.

As the bolus is broken down, the frequency of tooth contact increases.

2) In the final stages of mastication, just before swallowing, contacts

occur during every stroke.

3) Two types of contacts have been identified :

i) Gliding contact, which occurs as the cuspal inclines pass by

each other during the opening and grinding phases of mastication.

ii) Single contact, which occurs in the maximum intercuspal

position.

4) The mean percentage of gliding contacts that occur during chewing

has been found to the 60% during the grinding phase and 56% during

the opening phase.

5) The overage length of time for tooth contact during mastication is 194

msec.

6) It has been demonstrated that the occlusal condition can influence the

entire chewing stroke.

7) During mastication the quality and quantity of tooth contacts

constantly relay sensory information lack to the CNS regarding the

character of the chewing stroke.

8) This feed back mechanism allows for alteration in the chewing stroke

according to the particular food being chewed.

9) Generally, take cusps and deep fossae promote predominantly vertical

chewing stroke, whereas flattened or warn teeth encourage a broader

chewing stroke.

When the posterior teeth contact in undesirable lateral movement, the

malocclusion produces on irregular and less repeatable chewing stroke.

1) Normal persons with good occlusion masticate with chewing strokes

that are well rounded, with definite borders and less repeated.

2) The chewing strokes of persons with TMJ pain show a repeat pattern.

The strokes are much shorter and slower and have an irregular

pathway.

The mouth than opens slightly, the tongue pushes the food onto the

occlusal table, and after moving sideways, the mandible classes into the

food until the guiding teeth contact. This cycle is completed as the mandible

returns to its starting position.

This pattern repeats itself until the food bolus has been reduced to

particles that are small enough to be swallowed, at which point the process

can start over.

The directed of the mandibular path of closure is influenced by the

inclination of the occlusal plane with the teeth apart and by the occlusal

guidance as the jaw approaches.

Intercuspal position:

The chewing pattern observed in children differs from that found in

adults. Until about age 10, children begin the chewing stroke with a lateral

movement. After age 10, they start to chew increasingly the adults, with a

more vertical stroke.

Speech occurs when a volume of air is forced from the longs by the

diaphragm through the larynx and oral cavity. Controlled contraction and

relaxation of the vocal cards (we bonds of the larynx) create a sound with

the desired pitch. Once pitch is produced the precise from assumed by the

mouth determines the resonance exact articulation of the sound. Speech

occurs during expiration.

Speaking: The teeth, tongue, lips, floor of the mouth and soft palate form

the resonance chamber that affects pronunciation.

During speech the teeth are generally not in contact, although the

anterior teeth may come very close together during ‘C’ ‘CH’, ‘S’ and ‘Z’

sounds, forming the speaking space.

When pronouncing ‘F’ the inner vermilion border of the lower lip

traps air against the incisal edges of the maxillary incisors.

Parafunctional Movements:

These can be described as sustained activities that occur beyond the

normal functions of mastication, swallowing and speech.

The various parafunctional activities are:

1) Bruxism.

2) Clenching.

3) Nail biting.

4) Pencil chewing etc.

Parafunction is manifested by long periods of increased muscle

contraction and hyperactivity.

Excessive occlusal pressure and prolonged tooth contact occur, which

is inconsistent with the normal chewing cycle. Over a protracted period this

can result in excessive wear, widening of P.D.L and mobility, migration or

fracture of teeth. Muscle dysfunction such as myospasms, myositis, myalgia

and referred pain (borderers) may also occur.

Bruxism: Sustained grinding, rubbing together, or gnashing of teeth with

greater than normal chewing farce is known as Bruxism.

This activity may be diurnal, nocturnal or tooth.

The etiology of bruxism is often unclear. Some theories relate

bruxism to malocclusion, neuromuscular disturbances, responses to

emotional distress, or a combination of these factors.

Clenching: Is defined as forceful clamping together of the jaws in static

relationship.

The pressure thus created can be maintained over a considerable time

with short periods of relaxation in between.

The etiology can be associated with stress, anger, physical exertion,

or intense concentration on a given task, rather than on occlusal disorder.

Effects:

Abfractions i.e. cervical defects at the CEJ may result from sustained

clenching. Also the increased load may result in damage to the

periodontium, temporomandibular joints and muscles of mastication.

The elevator muscles may become over developed. A progression of

muscle splinting, myospasms, and myositis may occur.

Biting force: (Forces of mastication)

The maximum biting force that can be applied to the teeth varies from

individual to individual. Generally males can bite with more force than

females can.

In females maximum biting load ranges from 79-99 pounds (35.8 –

44.9 kg).

A male’s biting load varies from 118-142 pounds (53.6 – 64.4 kg).

The greatest maximum biting force reported is 975 pounds (443 kg).

The biting force also varies from tooth -tooth. The maximum amount

of force applied to a molar is usually several times that which can be applied

to the on incisor.

The range of maximum force applied to the 1st molar is 91-198

pounds (41.3 – 89.8 kg). The maximum force applied to control incisors is

29 – 5’ pounds (13.2 – 23.1 kg).

The maximum biting force appears to increase with age up to

adolescence.

The factors influencing biting force are:

1) Particular tooth.

2) Dietary consistency.

3) Degree of chronic periodontal disease.

4) Jaw separation.

5) Natural / artificial teeth.

6) Biting practice and parafunctional overuse.

7) Craniofacial morphology.

Conclusion

References:

Gray’s Anatomy- 38th Edition

Textbook of Medical Physiology, 9th Edition: Guyton and Hall

Essentials Of Oral Physiology: Bradley

Scientific Basis Of Eating: R.W.A Linden

Human Anatomy Vol 3: B.D Chaurasia

Medical physiology 5th Edition: Sujit K.Chaudhuri