MANDIBULAR MOVEMENTS

CONTENTSIntroductionAnatomy of TMJMuscles of MasticationNeurologic structures & neuromuscular functionsBorder movements of MandibularEccentric Mandibular MovementsMajor Functions of Masticatory SystemMethods Used For Recording Mandibular Movements Clinical Significance of Mandibular MovementsConclusionReferences

INTRODUCTIONThe masticatory system is a complex and highly refined unit.It is the functional unit of the body primarily responsible for chewing speaking and swallowing.The system is made of bone,joints, ligaments,teeth and muscles. and movement is regulated by intricate neurological control systemDuring performance of various functions there is a delicate balance between various components.Precise movement of the mandible is required to move the teeth efficiently across each other during function

ANATOMY OF TMJTMJ is one of the most complex joints in the body.It is called as GINGLYMOARTRODIAL JOINT.TMJ consists of 4 main structures:- Condyle Temporal bone (Squamous part) Articular discLigaments

CONDYLEIt is the portion of the mandible that articulates with the cranium, around which movement occurs.

TEMPORAL BONEThe mandibular condyles articulates at the base of the cranium with the squamous portion of the temporal bone.This portion made up of Concave Mandibular Fossa called as ARTICULAR OR GLENOID FOSSA.SQUAMOTYMPANIC FISSURE Posterior to mandibular fossa.

Anterior to fossa convex bony prominence called ARTICULAR EMINENCE.

TMJ consist of

Upper articular lower articular interarticular disc surface surface

Formed of Formed of

Articular eminence head of the mandibleAnterior part of (condyle)mandibular fossa

TMJ is classified as a COMPOUND JOINT.

Functionally articular disc serves as a non ossified bone.

ARTICULAR DISCComposed of dense fibrous connective tissue, most part of it is devoid of blood vessels and nerves fibers.Extreme periphery of the disc is slightly innervated.In SAGITTAL PLANE it is divide into 3 regions (according to thickness).

ANTERIOR ZONE POSTERIOR ZONE INTERMIDIATE ZONE

Posterior border is slightly thinnest area of the disc thicker than anterior border

SAGITTAL PLANE

ANTERIOR(FRONTAL) PLANE

Attachment of Articular Disc:-

Articular disc is attached to the capsular ligament..It divides the joint cavity into- SUPERIOR INFERIOR JOINT CAVITYTMJ is referred to as SYNOVIAL JOINT

LIGAMENTS:-Muscles move and ligaments limit.Ligaments do not enter actively into joint function, rather they act as passive restraining devices to limit & restrict border movements.3 functional ligaments support the TMJ are:- Collateral ligament Capsular ligament Temporomandibular ligament2 accessory ligaments are:- Sphenomandibular ligament Stylomandibular ligament

COLLATERAL(DISCAL) LIGAMENTS:-They attach the medial & lateral borders of articular disc to the poles of the condyle.Commonly called as DISCAL LIGAMENTS.2 TYPES:- Medial discal ligament Lateral discal ligamentThey are true ligamentsFunction :

CAPSULAR LIGAMENT:-Entire TMJ is surrounded & encompassed by the capsular ligament.Attachment :-SuperiorlyInferiorly

Function :- It resists any medial, lateral or inferior forces that tend to separate or dislocate the articular surfaces.

TEMPOROMANDIBULAR LIGAMENT:- Lateral aspect of the capsular ligament is reinforced by strong, tight fibers that make up the lateral or temporomandibular ligament.The TM ligament is composed of :- Outer oblique portion Inner horizontal portion

FUNCTIONOUTER OBLIQUE PORTION:They resist extensive dropping of he condyle..It also influences the normal opening movement.

INNER HORIZONTAL PORTIONLimits posterior movement of condyleIt also protects lateral pterygoid muscle from overlengthening or extension

ACCESSORY LIGAMENTS Sphenomandibular Ligament Stylomandibular LigamentFunction:Taut - when mandible is protruded Most relaxed when mandible is opened. So, limits excessive protrusive movement of mandible. Shares in activity of the medial pterygoid muscle

MUSCLES OF MASTICATIONThe skeletal muscles provide for the locomotion necessary for the individual to survive.PRIMARY MUSCLES OF MASTICATION Masseter Temporalis Medial Pterygoid Lateral PterygoidSECONDARY MUSCLES OF MASTICATION The suprahyoid group of muscles being used as secondary or supplementary muscles they areDigastricMylohyoidGeniohyoid

MASSETER:-

Quadrilateral muscle and consist of three layers.Origin:Superficial layer:Middle layer:Deep layerInsertion:Superficial layerMiddle and deep fibers pass vertically downward.

Function

Some fibers from inner part of the muscle are inserted horizontally into the capsule and meniscus of mandibular joint exerting a LATERAL PULL on the meniscusDeep segment pulls mandible RETRUED relationMasseter contracts ELEVATES the mandible in the direction of the fibers

MEDIAL PTERYGOID

It is a thick quadrilateral muscleOrigin Insertion

FUNCTION OF MEDIAL PTERIGOID MUSCLEAlong with masseter it forms a MUSCULAR SLING that supports the mandible at mandibular angle.When fibers contract the mandible is ELEVATED.Muscle is active in PROTRUDING the mandible.Unilateral contraction will bring about mediotrusive movement of the mandible.

TEMPORALIS

It is a large, fan shaped muscle.OriginInsertion

It can be divided into 3 distinct portions

consists of fibers fibers run obliquely fibers are alignedthat are directed across the lateral almost horizontallyalmost vertically aspect of the skull coming forward above (forward-downwards) the ear

when it contracts when it contracts it contracts and mandible is raised mandible is elevated retrudes mandiblevertically and retruded {Du Brul-suggested (elevates) that its contraction elevates and slightly retrudesANTERIOR PORTIONMIDDLE PORTIONPOSTERIOR PORTION

ELEVATION OF MANDIBLE POSTERIOR FIBER DRAWS MANDIBLE BACKWARDS

Because angulation of the muscle fibers varies the temporalis is capable of coordinating closing movementsHence it is a significant positioning muscle of the mandible

LATERAL PTERYGOID2 different portions or bellies:- Inferior Superior

Function Superior Lateral Pterygoid:-During opening the superior lateral pterygoid remains inactive, becomes active only in conjunction with elevator muscles. It is active during power stroke & when teeth are held together.

Closing Retracting Lateral movement in ipsilateral directionInferior Lateral Pterygoid:-When right & left ILP contracts simultaneously, the condyles are pulled down the articular eminences & the mandible is protruded.

Unilateral contraction creates a mediotrusive movement of the condyle & causes a lateral movement of the mandible to the opposite side.

opening protracting Lateral movement in contralateral direction

SIDE TO SIDE GRINDING MOVEMENTWhen lateral pterygoid contracts with medial pterygoid of same side, the condyle advances on that side ,while the jaw rotates through the opposite condyle when the medial and lateral pterygoid of the two sides contract alternatively to produce side to side movements of mandible eg chewing

Medial and lateral pterygoid act together to protrude the mandible

DIGASTRICS:-

Not considered a muscle of mastication, but it does have an important influence on the function of the mandible.Divided into 2 portions:- Posterior belly Anterior belly

Function:- When right & left digastrics contract & the suprahyoid & infrahyoid muscles fix the hyoid bone, the mandible is depressed & pulled backward & the teeth are brought out of contact. When mandible is stabilized, the digastric muscles with the suprahyoid & infrahyoid muscles elevate the hyoid bone, which is necessary function for swallowing.

The combinded efforts of the Digastrics and Lateral Pterygoids provide for natural jaw opening.

Other acessory muscles:SurahyoidInfrahyoid muscleSternocleidomastoidPosterior cervical muscles

Woelfel J.B., Hickey J.C., Stacy R.W. & Rinear L. (1960) conducted a study on electromyographic analysis of jaw movements. The objective of the study were-1)To determine the range of variability of muscular activity in jaw movements.2)To determine the range of variability in a series of electromyograms. 3)To provide an analysis of the role played by the external pterygoid muscles in trained (learned) jaw movements.

They concluded that:- 1) The temporal muscle is capable of unilateral and fractional response but does not show increased activity in any part during protrusion or uncontrolled openings. 2)The right and left digastric muscles did not function individually. Their greatest activity was during uncontrolled openings and retrusion of the mandible. 3)The masseter muscle had the greatest activity during clenching into centric occlusion. 4)The external pterygoid muscle was very active during contra lateral excursions, uncontrolled openings, and protrusion but was inactive during hinge openings of approximately 1 cm.

NEUROLOGIC STRUCTURE & NEUROMUSCULAR FUNCTIONFunction of masticatory system is complex. A highly refined neurologic control system regulates & coordinates the activities of entire masticatory systemThe basic component of neuromuscular system is the MOTOR UNIT (which consist of number of muscle fibers that are innervated by motor neuron)MUSCLE FUNCTION:- ISOTONIC CONTRACTION: contraction or an overall shortening.ISOMETRIC CONTRACTION: contraction without shorteningCONTROLLED RELAXATION : stimulation of motor unit is discontinued, fibers of motor unit relax and return to normal length. thus a precise muscle lengthening can occur that allows slow and deliberate movement

NEUROLOGIC STRUCTURES:- The masticatory system consists of following receptors to monitor the status of its components:- 1) MUSCLE SPINDLE- Skeletal muscle consists of two types of muscle fibers a) Extrafusal fibers (contractile) b) Intrafusal fibers (minutely contractile)

A bundle of intrafusal fibers bound by a connective tissue sheath is called muscle spindle. Within each spindle the nuclei of the intrafusal fibers are arranged in 2 distinct fashions:-Chainlike(nuclear chain type)Clumped (nuclear bag type)There are two types of afferent nerves that supply the intrafusal fibers. They are: 1)Primary endings or annulospiral endings 2) Secondary endings or flower spray endings Efferent supply of intrafusal fibers is by fusimotor nerve fibers ( efferent).

When muscle is stretched:

Intrafusal & extrafusal fibers are stretched

Annulospiral & flower spray endings are activated

Afferent neurons carry information to trigeminal mesencephalic nucleus

The CNS then sends back impulse via 2 efferent pathways:-

Fusimotor nerve fibers or efferent motor neurons gamma efferent (for extrafusal fibers) (for intrafusal fibers) Muscle contraction

2) GOLGI TENDON ORGANS- Located in muscle tendon between muscle fibers and their attachment to bone. They are more sensitive than muscle spindles and active in reflex regulation in normal function.

They primarily monitor tension, whereas the muscle spindles primarily monitor muscle length. 3) PACINIAN CORPUSCLES- The pacinian corpuscles are large oval organs made up of concentric lamellae of connective tissue. They are widely distributed. They serve principally for the perception of the movements and firm pressure. These corpuscles are found in the tendons,joints,periosteum,tendinous insertions ,fascia and sub cutaneous tissue.

4) NOCICEPTORS- They are sensory receptors that are stimulated by injury & transmit injury information to CNS by way of afferent nerve fibers.The primary function is to monitor the condition,position and movement of the tissue in the masticatory system.

REFLEX ACTION:- 2 general reflex actions are important in the masticatory system : 1) MYOTACTIC REFLEX or stretch reflex- Is the only monosynaptic jaw reflex. Sudden stretching of skeletal muscle Afferent nerve activity from the spindle Trigeminal mesencephalic nucleus Afferent fiber synapse in trigeminal motor nucleus with - efferent motor neurons Efferent fibers carry information to extrafusal fibers Muscle contraction

Myotactic reflex is an important determinant of rest position of the jaw. It is a principal determinant of muscle tonus in elevator muscles.

2) NOCICEPTIVE REFLEX or flexor reflex-

Polysynaptic reflex to noxious stimuli & hence, considered to be protective. Sudden biting on hard object Noxious stimuli Afferent nerves carry impulse to trigeminal spinal tract nucleus where they synapse with interneurons Excitatory interneuron's inhibitory interneuron's

Synapse with efferent neurons Synapse with efferent neurons in the trigeminal motor nucleus in the trigeminal motor nucleus

they innervte the jaw depressing they innervate the elevator muscles Muscles

Message sent is to contract, that message sent is to discontinue Brings the teeth away contraction

INFLUENCE OF HIGHER CENTERS:-

Although the cortex is the main determinant of action,the brainstem is in charge of maintaining homeostasis and controlling normally subconscious functions. Within brainstem, is a pool of neurons that control rhythmic muscle activity such as breathing, walking & chewing. This pool of neurons is called Central Pattern Generator (CPG)It is responsible for precise timing of activity between antagonistic muscles so that specific functions can be carried out.

CLASSIFICATION:- I) According to Sharry:-

a) According to direction - Opening and closing movements Protrusion and retraction Lateral gliding movements

b) According to tooth contact - Movements with tooth contact Movements without tooth contact

c) Limitation by joint structure - Border movements Intra border movements

d) Functions of masticatory system - Mastication Deglutition Speech Respiration

e) CNS - Innate movements breathing & swallowing Learned movements speech and chewing

II) According to the type of movement occurs in TMJ:- a) Rotational b) TranslationIII) According to the planes of border movements:- a) Sagittal plane border movement b) Horizontal plane border movements c) Frontal plane border movements

MANDIBULAR MOVEMENTSMandibular movements occurs as complex series of 3 dimensional rotational and transitional activities. It is determined by combined and simultaneous activities of both tmjs.2 types of movement occur in tmj:- Rotational Translational

ROTATIONAL MOVEMENT:-

Rotational movement of the mandible occurs in 3 different reference planesHorizontal FrontalSagittal

HORIZONTAL AXIS OF ROTATION:-

An opening and closing motion- hinge movementOnly pure rotational movement in mandibular activityTERMINAL HINGE AXISWhen the condyles are in their most superior position in the articular fossae and the mouth is purely rotated open, the axis around which movement occurs is called the Terminal Hinge Axis.

FRONTAL (VERTICAL) AXIS OF ROTATION:-

Mandibular movement around the frontal axis occurs when one condyle moves anteriorly out of terminal hinge position with the vertical axis of opposite condyle remaining in the terminal hinge position.

SAGITTAL AXIS OF ROTATION:-

TRANSLATIONAL MOVEMENT:-

Translation can be defined as a movement in which every point of the moving object has simultaneously the same velocity and direction. It occurs within the superior cavity of the joint, between the superior surface of the articular disc and the inferior surface of the articular fossa.During normal movements of the mandible both rotation and translation occur simultaneously.This results in a very complex movements.

SINGLE-PLANE BORDER MOVEMENTS:-

Mandibular movements are limited by ligaments and articular surface of TMJs as well as the morphology and alignment of the teeth.When the mandible moves through the outer range of motion, reproducible and describable limits result, which are called BORDER MOVEMENTS.

SAGITTAL PLANE BORDER &FUNCTIONAL MOVEMENTS:-

They have 4 distinct movement components:-1)Posterior opening border determined by ligaments & the morphology of TMJs.2) Anterior opening border3) Superior contact border determined by occlusal & incisal surfaces of teeth.4) Functional determined by conditional responses of neuromuscular system.

Posterior Opening Border Movements:-

Occurs as two stage hinging movements.1st stage:-

2nd Stage:- As the condyle translates the axis of rotation of the mandible shifts into the bodies of rami likely to be the area of attachment of sphenomandibular ligament, resulting in the second stage of the posterior opening border movement.

Anterior Opening Border Movements:-

With the mandible maximally opened, closure accompanied by contraction of inferior lateral pterygoids (which keep the condyles positioned anteriorly) will generate the anterior border movement.

Because the maximum protrusive position is determined in part by stylomandibular ligaments, when closure occurs, tightening of ligaments produces a posterior movement of the condyles.The posterior movement of the condyle from the maximally open position to maximally protruded position produces eccentricity in the anterior border movement. Therefore, it is not a pure hinge movement.

Superior Contact Border Movements:-

This movement is determined bythe characteristics of occluding surfaces of the teeth.through out the movement tooth contact is present.It depends on:- Amount of variation between centric relation and maximum intercuspation. The steepness of the cuspal inclines of the posterior teeth. Amount of vertical and horizontal overlap of anterior teeth Lingual morphology of maxillary anterior teeth. General interarch relationships of the teeth.

In CENTRIC RELATION -tooth contacts are normally found on one or more opposing pair of posterior teeth.-When muscular force is applied to the mandible, a super anterior movement or or shift will occur until the intercuspal position is reached.-The slide from CR to maximum intercuspation, may have a lateral component. -from early 1950s to more recently the distance between MI and centric relation has changed from 1.25 mm by Posselt,1.0mm by Schuyler, 0.8 to 0.5mm by Ramfjord,to 0.2mm by Dawson and Ramfjord

When the mandible is protruded, from maximum intercuspation .

This continues until the maxillary and mandibular anterior teeth are in edge to edge relationship, at which a horizontal pathway is followed. Horizontal movement continues until incisal edges of mandibular teeth pass beyond the edges of maxillary teeth.

Functional Movements:-

Functional movement occurs during functional activity of the mandible. They usually take place within the border movements & therefore, considered as free movements. Most functional movements require maximum intercuspation & therefore typically begin at & below the intercuspal position.

When mandible is at rest, it is found to be located approximately 2 to 4mm below the intercuspal position. This is called the Clinical Rest Position.

Postural position Since, clinical rest position is not a true resting position, the position in which mandible is maintained is termed as postural position.

Chewing Stroke:- If it is examined in sagittal plane, the movement will be seen to begin at the intercuspal position & drop downward & slightly forward to position of desired opening. It then returns in a straighter pathway, slightly posterior to the opening movement.

POSTURAL EFFECT ON FUNCTIONAL MOVEMENT:Head in erect and upright positionHead is directed 45 upward (as assumed during drinking)Head is directed 30 (as assumed during eating) ALERT FEEDING POSITION

HORIZONTAL PLANE BORDER & FUNCTIONAL MOVEMENTS:-When mandibular movements are viewed in the horizontal plane, a rhomboid-shaped pattern can be seen that has a functional component, & 4 distinct movement components:-1) Left lateral border2) Continued left lateral border with protrusion3) Right lateral border4) Continued right lateral border with protrusion

LEFT LATERAL BORDER MOVEMENTS:- With the condyles in the centric relation position, contraction of the right inferior lateral pterygoid move the right condyle - anteriorly and medially. If left inferior pterygoid stays relaxed, with the left condyle still in the CR & result will be left lateral border movement. Left condyle- working or rotatory Right condyle- non-working or orbiting

CONTINUED LEFT LATERAL BORDER MOVEMENTS WITH PROTRUSION:- With the mandible in the left lateral border position, contraction of the left inferior lateral pterygoid along with continued contraction of right inferior lateral pterygoid will cause the left condyle to move anteriorly to the right.

RIGHT LATERAL BORDER MOVEMENTS:-Left condyle- orbiting Right condyle- rotatory

CONTINUED RIGHT LATERAL BORDER MOVEMENTS WITH PROTRUSION:-

FUNCTIONAL MOVEMENTS:-

As in the sagittal plane, functional movement in the horizontal plane most often occur near the intercuspal position. During chewing the range of jaw movements begins some distance from maximum intercuspal position; but as the food is broken down into smaller particles, jaw action moves closer and closer to intercuspal position.

FRONTAL (VERTICAL) BORDER & FUNCTIONAL MOVEMENTS:- A shield-shaped pattern can be seen that has a functional component, & four distinct movement components:- 1.Left lateral superior border. 2.Left lateral opening border. 3.Right lateral superior border. 4.Right lateral opening border.

Left Lateral Superior Border Movements:-With the mandible in maximum intercuspation, lateral movement is made to the left. It discloses a inferiorly concave path being generated .The nature of this path It depends upon morphology and interarch relationships of maxillary and mandibular teeth. The maximum lateral extent of this movement is determined by ligaments of the rotating joint.

Left Lateral Opening Border Movements:- From the maximum left lateral superior border position, an opening movement of the mandible produces a laterally convex path. As maximum opening

Right Lateral Superior Border Movements:-

Right Lateral Opening Border Movements:-

Functional Movements:-

ENVELOPE OF MOTION:-

Given by POSSELTBy combining mandibular border movements in all 3 planes, a 3D envelope of motion is produced.This represents maximum range of movement of the mandible.The superior surface of the envelop is determined by tooth contacts whereas the other borders are primarily determined by ligaments and joint anatomy that limits or restrict movement

ECCENTRIC MANDIBULAR MOVEMENTS

Eccentric mandibular movement can be divided into protrusive and lateral movements which consists mainly of condylar translations.

1) PROTRUSIVE MOVEMENT:-

a)Sagittal Protrusive Condylar Path:-Mandible translates in forward and downward direction during protrusive movement.The right and left muscles do not make simultaneous movements. so pure protrusive movements do not exist in clinical situation(Hobo,Mochizuki,1982)

The orbits produced by the center of the right and left condyle during protrusive movement is referred to as PROTRUSIVE CONDYLAR PATH It forms an angle with horizontal reference plane known as Sagittal inclination of protrusive condylar path.Ranges from 5- 55. (with FH plane as horizontal ref.) Mean 30.4. (Hobo,Mochizuki,1982) 33 when campers plane is used(Gysi,kohler,1929)

b) Sagittal Protrusive Incisal Path:-The orbit of incisal point from maximum intercuspation to edge-to-edge occlusion PROTRUSIVE INCISAL PATHThe mean length of the path is 5 mmAngle formed by protrusive incisal path and horizontal reference plane SAGITTAL INCLINATION OF PROTRUSIVE INCISAL PATH (incisal guidance angle) range between 50-70 degrees. (Gysi,Kohler,1929)

Usually sagittal inclination of protrusive incisal path is steeper than sagittal inclination of protrusive condylar path. (Hobo,1978)

2)LATERAL MOVEMENT:-

Lateral movements are complex activities in most humansLateral movement from occlusal position and back again are assymetric.The right and left condyle carry out different movements.Thus lateral movements:Sagittal planeHorizontal plane

LATERAL MOVEMENTS IN SAGITTAL PLANESagittal Lateral Condylar Path:- When lateral movement is executed the working condyle rotates & moves outward, while the non working condyle translates forward, medially downward orbiting around the rotating working condyle.When the orbit of nonworking condyle is traced in the sagittal plane it is known as Sagittal lateral condylar path.Lateral condylar path is longer & more steep than the protrusive condylar path.

FISCHER ANGLE:- The angle formed between the sagittal protrusive condylar path & sagittal lateral condylar path (approx 5). The angle formed by the sagittal lateral condylar path & horizontal reference plane is known as Sagittal Inclination Of Lateral Condylar Path Angle between sagittal lateral condylar path and FH plane is approx 45-50 (Lundeen,Wirth,1973)

Lateral movement in horizontal planeWorking side lateral movementNonworking side lateral movement

Working side lateral movement

Sir Normal Godfery Bennett(1908) studied working condylar path and called it BENNETT MOVEMENT, now referred to as LATEROTRUSION.Bennett showed that working condyle moves outwards and nonworking condyle moves inwards.Although Bennett has described about the movement which became popularly known as Bennett movement ,the original discovery of this movement should go to BALKWILL,who described the same side shift in 1866. Bennett movement refers to the CONDYLAR MOVEMENT on the working side, were the working condyle rotates and moves slightly outwards. This outward direction of bennett path (laterotrusion) may be combined with anUpward (laterosurtrusion)Downward (laterudetrusion)Forward (lateroprotrusion), orBackward (lateroretrusion) component

Bennett side shift is the bodily side shift of the MANDIBLE on the working side in the horizontal plane. (Mandibular Lateral Translation)

When the mandible is moved laterally to the working side,it rotates on the vertical axis passing through the center of the working condyle.Besides rotation around the vertical axis the working condyle must move laterally (Bennett movement) to accommodate the medial movement of the orbiting nonworking condyle.Therefore the side shift of the working condyle is dependent and is consequent to the medial movement of the orbiting condylar path

Nonworking side lateral movement

During lateral movement the working condyle rotates and moves outwards and the nonworking condyle moves medially and advances in a forward and downward direction.When this path of nonworking condyle is traced on horizontal plane it is known as the HORIZONTAL LATERAL CONDYLAR PATHIt has 2 components:Immediate mandibular lateral translationProgressive mandibular lateral translation

Immediate mandibular lateral translation Occurs when the nonworking condyle moves from the centric relation straight inward or medially,to a distance of approx 1.0mm (Lundeen,Wirth,1973) 0-2.6mm (mean-0.42mm) (Hobo,Mochizuki,1982),as recorded using a electronic mandibular recording deviceBeyond this the condyle moves forward, downward & inwardProgressive mandibular lateral translationIt is the translatory portion of the lateral movement that occur at a rate proportional to forward movement of non working condyle .(GPT 1987)the value of progressive mandibular lateral translation is 7.5 (Lundeen,Wirth,1973) Angle formed by the horizontal condylar path and sagittal plane varies between 2 -44 (mean 16) and is called as BENNETT ANGLE

Bennett movement has 3 components:- Amount Timing Direction

AMOUNT

The amount of medial movement of the orbiting condyle governs the magnitude of lateral shift of the mandible (Bennett shift) IMMEDIATE SIDE SHIFT is the bodily shift of the condyle in horizontal plane. this is regulated by the shape of the glenoid fossa,looseness of the capsular ligaments and contraction of the lateral pterygoids.a mean movement of 1.0 mm (Lundeen,Wirth,1973) Beyond this the condyle moves forward, downward & inward, this is known as PROGRESSIVE SIDE SHIFT.Combined amount of (ISS+PSS) is the Bennett angle, with a mean value of 16

2) TIMING:-The rate or amount of descent of contralateral condyle & the rotation & lateral shift of ipsilateral condyle. Immediate side shift is the 1st movement the mandible makes when initiating lateral excursions. Progressive side shift:- Beyond the immediate side shift the condyles move forward, downward and inward.

3) DIRECTION:- The direction of Bennett movement depends primarily on the direction taken by the rotating condyle during the bodily movement. The direction of the shift of the rotating condyle during Bennett movement is determined by the TM joint undergoing rotation.

LATERAL INCISAL PATH:-

The orbit produced by incisal point during lateral movementis referred to as the lateral incisal path.When the path is traced on a horizontal plane it is called the GOTHIC ARCH tracing.The angle produced by right and left horizontal incisal path is called the gothic arch angle.Mean value - 120

PRACTICAL SIGNIFICANCE:Patients with excessive Bennett movement and little or no anterior guidance present the greatest challenge in occlusal rehabilitation procedures because the cusp movement pathways of there posterior teeth are very shallow. The elimination of eccentric cusp interference can be very difficult. in this study it was shown that increase in anterior guidance to 40 produced only a slight change in the lateral pathways in presence of a 3.5mm Bennett movement. The completely adjustable articulator would be most helpful for such patients.

Patients with very little Bennett movenent,0.75mm or less ,have molar cusp movement pathways that reflect the steepness of the anterior guidance and the non working condylar pathways. The potential for eccentric cusp interference is markedly reduced due to the steep immediate cusp separation seen close to the intercuspal position

A condylar movement screening device that would quickly and simply determine a patients approx bennett movement and the inclination of the nonworking condylar pathway would provide useful diagnosis and treatment information.

MAJOR FUNCTIONS OF MASTICATORY SYSTEMMASTICATION:-It is the act of chewing food. It represents the initial stages of digestionCHEWING STROKE:Mastication is made up of rhythmic & well controlled separation & closure of the maxillary & mandibular teeth.This activity is under control of CPG,located in the brainstem. In frontal plane, it has a tear shaped pattern.

It can be divided into a) Opening Phaseb) Closing Phase i) Crushing Phase ii) Grinding Phase.When the mandible is traced in the frontal plane following sequence occurs-

If the movement of a mandibular incisor is followed in the SAGITTAL PLANE during a typical chewing stroke, it will be seen that during the opening phase the mandible moves slightly anteriorly.

Working sideNonworking side

TOOTH CONTACT DURING MASTICATION:

When food is initially introduced in the mouth,fewer contacts occur.As bolous is broken down frequency of contacts increase.2 types of contacts: -gliding contacts -single contacts

SWALLOWING (DEGLUTITION):-

It is a series of co-coordinated muscular contractions that moves a bolus of food from the oral cavity through the esophagus to the stomach.It consists of voluntary, involuntary and reflex muscular activity.Stabilization of the mandible is an important part of swallowing.The mandible must be fixed so contraction of suprahyoid & infrahyoid muscles can control proper movement of the hyoid bone needed for swallowing. a) Somatic swallow b) Visceral swallow It is believed that when the mandible is braced it is brought into most retruded position.But according to Okeson the quality of intercuspal position will determine the position of the mandible during swallowing and not a retruded relationship with the fossa.

Parafunctional movementsMay be described as sustained activities that occur beyond the normal mastication and speech.It is manifested by long periods of muscle contraction and hyperactivityExcessive occlusal pressure and prolonged tooth contact occur,which is inconsistent with normal chewing cycle. Two most common forms of parafunctional activities are bruxismclenching

CLINICAL SIGNIFICANCEA prosthodontist has to aim to reproduce accurate mandibular movements which allow us to facricate restorations and prostheses in harmony with the patients natural function. Knowledge of the mandibular movements essential, it helps the dentist in: -Selecting and programming of articulators -Treating TMJ disturbances. - Arranging artificial teeth. - Development of occlusal scheme.

Concepts of occlusion differ depending upon whether restoration are fixed or removable .the dentist must have the knowledge of the effect of guiding factors of the mandible

CONDYLAR GUIDANCE Is one of the two end controlling factors not under the control of the dentist.It is determined by the shape of the articular eminence, anatomy of the medial wall of mandibular fossa,and configuration of mandibular condyleEffects of condylar guidance on cusp heighta)The lesser the condylar guidance angle, the shorter the cusps must be.b) The greater the condylar guidance angle, the longer the cusps may be

ANTERIOR GUIDANCE

The anterior determinants are the vertical and horizontal overlaps and lingual concavities on maxillary anterior teeth.These can be altered by restorative and orthodontic treatment.Effects of anterior guidance on cusp heightThe greater the horizontal overlap of the maxillary anterior teeth, the shorter the cusps of the posterior teeth must be.The lesser the horizontal overlap the longer the cusps of the posterior teeth may beThe lesser the vertical overlap, the shorter the cusps of the posterior teeth must be.The greater the vertical overlap, the longer the posterior cusps may be.

Bennetts Movement:- Movement responsible for lateral chewing stroke. -Movement during which the greater lateral force is exerted. - It is extremely important that articulating surfaces are is strict harmony with this side shift. Effect on cusp height: - Greater the side shift of the mandible shorter the cusps must be. - The lesser the side shift of the mandible longer the cusps may be.

Summary

Conclusion nature has blessed us with a marvelously dynamic masticatory system, allowing us to function and therefore exist One has aimed to reproduce accurate mandibular movements, which allow us to fabricate restorations and prostheses in harmony with the patients natural function.