Muscles of masticationMuscles of facial expression

CONTENTSINTODUCTIONDEVELOPEMENTMUSCLES OF MASTICATIONCLINICAL EXAMINATIONCLINICAL CONSIDERATIONMUSCLES OF FACIAL EXPRESSIONAPPLIED ASPECTSCONCLUSIONREFERENCES

“MOTION IS THE CAUSE OF ALL LIFE”

LEONARDO DA VINCI

INTRODUCTION

DEVELOPEMENT

SKETCH OF 20 WEEK FETUS SHOWING MUSCLES DERIVED FROM BRANCHIAL ARCHES

Classically the muscles of mastication areMasseterMedial PterygoidLateral PterygoidTemporalis

Accessory muscles of masticationAnterior belly of digastricMylohyoid muscleGeniohyiod muscleBuccinator

MasseterMasseter

ORIGINA) Superficial layer

LargestFrom anterior 2/3 of the lower border of zygomatic arch and adjoining zygomatic process of maxilla

B) Middle LayerFrom anterior 2/3 of the

deep surface and posterior 1/3 of the lower border of zygomatic arch

C) Deep layerFrom the deep surface of

the zygomatic arch

InsertionA) superficial layer

It passes downward and backwards at 45 degree and inserts into lower part of lateral surface of ramus of the mandible

B) Middle layerIt passes vertically

downward into the middle part of the ramus

C) Deep layerInto upper part of the ramus

and coronoid process of the mandible

Its insertion on the mandible extends from the 2nd molar region at the inferior border posteriorly to include the angle

ACTIONElevation of mandible to close the mouth

Forceful jaw closing

Assist in protrusion of mandible

Temporalis

Fan shaped muscle

Fills the temporal fossa

Bipennate muscle

FIBRES Converge and pass

through gap deep to zygomatic arch

ORIGINTemporal fossa excluding zygomatic bone

INSERTIONMargins and deep surface of coronoid processAnterior border of ramus of mandible

FIBRESAnterior fibers are directed vertically

Middle ones run obliquely across the lateral aspect of the skull

Posterior fibers are aligned almost horizontal,coming forward above the ear to join the other fibers as they pass under the zygomatic arch

Actions

Elevation of the mandible..

Retrusion of mandible

No activity when mandible is elevated very slowly.

Lateral pterygoid Short Conical Has upper and lower head

ORIGIN

A) upper head (small) From infra temporal surface and crest of greater wing of sphenoid bone

B) lower head (larger) From lateral surface of lateral pterygoid plate

INSERTION

Pterygoid fovea on the anterior surface of neck of mandible

Anterior margin of articular disc and capsule of temperomandibular joint

ACTIONSAssists in opening the mouth with

suprahyoid muscle.The combinded efforts of the Digastrics and

Lateral Pterygoids provide for natural jaw opening.

SIDE TO SIDE GRINDING MOVEMENT

medial and lateral pterygoid of the two sides contract alternatively to produce side to side movements of mandible eg chewing).

When the medial and lateral pterygoids of two sides act together they protrude the mandible so that the lower incisors project in front of the other.

Medial pterygoid QuadrilateralHas a small superficial and a large deep head

ORIGIN Superficial head

From Tuberosity of maxilla and adjoining boneDeep head

From medial surface of lateral pterygoid plate and adjoining process of palatine bone

FibresRun downward

backward and laterally

INSERTIONRoughened area on the medial surface of angle and adjoining ramus of mandible below and behind the mandibular foramen and mylohyoid groove

ACTIONElevates mandibleHelps protrude

mandibleRight medial pterygoid with left lateral

pterygoid turn the chin to left side

Anterior belly of digastric It attaches to the lingual aspect of mandible at the parasymphysis

and courses backward to insert in the hyoid bone

ACTIONContraction of this muscle produces a depression and retro positioning of themandible

Mylohyoid

ORIGINFlat triangular sheet of muscle, Originates from mylohyoid line of

mandible

INSERTIONPosterior fibres are inserted into

the body of the hyoid bone, anterior fibres are inserted into the fibrous raphe

ACTIONSupport the tongue and the floor of the mouthStabilises the hyoid bone during mandibular

movementAssists in depressing the mandible and opening

the mouth

GENIOHYOID

ORIGINFrom the inferior mental spine

behind the symphysis menti of the mandible

INSERTION Inserted into the anterior surface of

the body of hyoid bone

ACTIONRetrusion of the mandible

Buccinator It attaches inferiorly along the facial surface of the mandible behind the mental foramenSuperiorly attaches on the alveolar surfaces

behind the zygomatic processesFibres are arranged horizontallyHelps position the cheek during chewing

movements of the mandible

Protrudes: Medial & Lateral pterygoid

Retractors: Posterior fibers of Temporalis, Diagastric & Geniohyoid

Elevators: Superficial & deep fibers of Medial pterygoid & Masseter. Anterior & middle fibers of Temporalis

Depressors: Lateral pterygoid, Diagastric & Mylohyoid

Lateral movers: Medial & Lateral pterygoid on each side

CLINICAL EXAMINATION

MASSETER

TEMPORALIS

FUNCTIONAL MANIPULATION

Clinical considerationMyofacial painTrismusMuscle spasmMuscle hypertrophyBruxisumAnkylosis of TMJTraumaAcute space infections OSMF

Muscles of facial expression

Subcutaneous musclesDevelops from the mesoderm of second branchial archAll of them are inserted into the skinNeed support from teeth for proper function

Orbicularis oris Lip muscle Two parts -intrinsic,deepest stratum,very thin sheet -extrinsic,two strata,formed by converging musclesOrigin-intrinsic part Superior incisivus,from maxilla inferior incisivus from mandible-extrinsic part thickest middle part from buccinator thick superficial strata from elevators and depressors of lips and their angles

Insertion -intrisic part in angle of mouth -extrinsic part in the lips and angle of mouth

The muscles that merge into orbicularis oris zygomaticus,quadratus labii superioris,caninus(levator anguli oris),triangularis(depressor anguli oris)quadratus labii inferioris,mentalis,buccinator and risorius.

MODIOLUSThe insertion of the

group of muscles about the oral cavity, both superficial and deep, partly into the skin and partly into the mucous membrane of the lips and immediate vicinity situated slightly lateral and above the corner of the mouth is called modiolus

Except in instances of excessive ridge resorption,the origins of most of these muscles are removed from the denture bearing area to the extent that their influence on the denture except at the modiolus is negligible.

These muscles can be relaxed with the jaws open while introducing the tray or the imp material in the mouth.

When the lips are tense, a stretching action often results in lacerations at corners of mouth and/or distorted imp material

The labial flanges of the maxillary denture frequently need to be reduced lateromedially in the area of modiolus

If the muscles are not properly supported, none of the facial expressions appear normal.

Incorrectly positioned teeth or an incorrectly contoured denture base will destroy the normal tonicity of the muscles

Lack of support allows sagging; stretching retards the normal contracture of the muscle and results in the loss of tonus.

When the muscles are stretched during mouth opening ,the vestibular space between the bundle in the cheek and the slopes of the residual alveolar ridge are restricted. Reducing the bulk of the flange to accommodate this muscle action helps to prevent dislodgement of the denture when the mouth is opened

With loss of teeth the action is impaired.But when they are correctly supported by

complete dentures, the memory pattern developed within the neuromuscular system when the teeth were present, is restored so the patient’s original appearance is maintained

Three factors affect the face in repositioning the Orbicularis oris with Complete denture

1. thickness of labial flanges of both dentures 2. anteroposterior position of anteriors 3.amount of separation between the jaws

If the jaws are closed too far and the dental arch located too posteriorly,the upward and backward positioning of Orbicularis oris complex will move the insertions of these muscles near to its origin and result in sagging when at rest and to be less effective when contracting.

Many old patients want the nasolabial sulcus to be obliterated because it appears as a wrinkle or skin folds.The sulcus is normal and should not be eliminated.

Repositioning the anterior teeth by protruding or retro positioning to improve appearance is a mistake.

The physiologic length of muscles is determined early.

In fact the muscles of lips,cheeks,tongue,face helped align the teeth

Bring the entire upper arch forward to its original position and maintain the normal arch form of the natural teeth and their supporting structures,

The orbicularis oris and attached muscles contract and force saliva and small particles of food from the vestibule into the oral cavity and seal off the space distal to last molar .The teeth arrangement should allow for this movement to occur

The correct width of the maxillary denture borders play a great part in supporting the muscles and lengthening the distance they must extend to reach their insertion.

If the mouth had been edentulous for long with considerable ridge loss,the borders need to thick to restore the position of these muscles

Buccal frenum in maxilla needs to be relieved coz it has attachments of the following muscles

Levator anguli oris---attaches beneath the frenum Orbicularis oris—pulls the frenum forward buccinator—pulls it in backward

The maxillary labial frenum contains insicivus and Orbicularis oris

Protrusion of the tongue helps in recording the movements of mylohyoid muscle.

The action of superior constrictor is recorded by protruding the tongue.

That of medial pterygoid by asking the patient to close forcefully against resistance

Contraction of mentalis renders the vestibule shallow hence capable of dislodging the mandibular denture specially when the ridge is non existent.

The mylohyoid constitutes the floor of the mouth in the anterior part.

If the denture flange extends below and under the mylohyoid line, it will impinge the muscle and affects its action adversely during swallowing, or the action of the muscle will unseat the denture.

Because the fibers are directed downwards the flange can extend below but not under the mylohyoid line.

It is not always possible to accentuate the flange over the upper molars because the attachment of the buccinator to the hamular process sometimes brings the muscle very close to tuberosity

Action of buccinator does not dislodge the denture because the fibers are parallel to occlusal plane. But again these fibers are perpendicular to masseter, hence when the masseter is activated it pushes the buccinator medial against the border in the area of retromolar pad area. This is a dislodging force and the denture needs to be contoured (massetric notch)

Failure to provide adequate interocclusal distance produces excessive interarch distance when the teeth are in occlusion. This position does not allow the elevators to complete their action, muscles will continue to exert force to overcome this obstacle, and as a result supporting tissues will be resorbed

Excessive interocclusal distance results in a reduced interarch distance when in occlusion. Facial distortion appears more noticeable with over closure than with slightly opened closure, the commisure turns down, and the lips lose their fullness.

The proper contouring of occlusal rims for lip and cheek support allows the muscles of facial expression to act in a normal manner.

The rims should be designed to be within the neutral zone

The best anatomic guides to proper contouring of the anterior section of rims are the nasolabial sulcus, mentolabial sulcus, philtrum and commisures.When support is absent the sulci become more deep,philtrum flattened and commisures droop. When over supported the sulci become distorted and shllow, philtrum partially or totally obliterated, and commisures distorted laterally

NEUTRAL ZONE

While eating the stability of upper denture depends upon the pressure of the tongue against the palate. This presses the palate upwards and outwards, but the cheeks and the lips balance the outward component and the resulting upwards pressure is the main stabilizing factor. This can only be achieved if the teeth are in their correct facio lingual relationship to tongue, cheeks and lips

There is no occlusal scheme that can stabilize teeth if they are in an unbalanced relationship with muscular forces against them

The lower dentures will be unstable,1. If the premolars were set outside the

ridge so that the denture is lifted by the corners of the mouth(modiolus),

2. If the buccal and lingual surfaces of the denture in the molar region were parallel so that tongue and buccinator could not grip the plate and hold it down ,

3. If the edges were not muscle trimmed in the incisor and premolar region and correctly adapted to the muscles in the posterior lingual and buccal regions

If the arch is wide at the lower premolar region ,it will be squeezed in the v shaped muscle band of zygomaticus and caninus and will shoot up out of place. Therefore there should be a sudden narrowing to escape a collision with the modiolus

A little extra width is required in upper premolar region as it will enable the modiolus to grasp the upper denture by the outer cusp of the premolars and hold it up.

Inner cusp of lower premolar can and indeed should be cut off otherwise it will interfere with the movements of the tongue and unstabilise the denture.

The stabilizing or unstabilising force which depends on the polished surface for its application to the denture is the muscular power of the tongue, buccinators, orbicularis oris and other muscles of cheek and lips. It is the shape of this complex surface as a whole, far more than the outline of the muscle trimmed edge of the denture, which determines whether muscle movements will dislodge the piece; while on the other hand if the polished surface is properly modeled so that the tongue, cheeks and lips have complete freedom of movement, the grip which the buccinators and the tongue can exert on the plates will make them wearable long after resorption has occurred and they have ceased to fit the impression surface in the ordinary sense of the term

PTERYGOMANDIBULAR RAPHE

CO-ORDINATED MUSCLE FUNCTION

Occlusal interferences which require displacement of the TMJs to achieve maximum intercuspation of teeth can cause inco-ordination of all the masticatory neuromusculature.This is called occluso-muscle pain

When an occlusal interference is introduced in mouth,it typically evokes a response of hyperactivity and incoordinated contraction in all the muscles that are prevented from functioning in a coordinated pattern of contraction versus release of opposing muscles .

IMPORTANCE OF OCCLUSAL HARMONYWhen closing muscle pull mandible

without interference it is stopped by bone at medial pole

If tooth inclines interfere lateral pterygoid is forced to position the mandible to accommodate to the teeth

There are many variations of timing and degree of muscle contraction to position the mandible for maximum intercuspation of the teeth.

Pattern of deviation is reinforced every time contact is made

Important facet of propioceptive memory is that it fades if reinforcement of pattern ceases.

Elimination of interfering contacts permit an almost immediate return to normal muscle function

posterior tooth interference caused hyperactivity of elevator muscle

But if the anterior guidance was allowed to disclude all posterior teeth from any contact other than CR, elevator muscle stopped active contraction or reduced it.

The reason muscle changes jaw position in the presence of interferences is to protect the interfereing tooth or teeth from absorbing entire occlusal force

Muscles become patterned to the devious closure ,such memorized patterns of muscle activity are called ENGRAMS

Because of engrams it is easy to be fooled by freely hinging jaw that appears to be in correct CR.

When we create an occlusion in centric relation,and disclusion when there is deviation from centric,the anterior teeth (anterior guidance) along with the condylar path take the responsibility of separating the posteriors during all excursions

3 beneficial effects of this are-greatly reduces the horizontal forces against

the anterior teeth,which are the only teeth in contact during excursion

-reduces compressive loading forces on TMJs-makes it impossible to overload or wear the

posterior teeth, even if the patient bruxes We cannot keep teeth in a stable position where

muscle does not want them to be. Muscle is the dominant determinant of both the

horizontal and vertical position of teeth

CURVE OF SPEE

It aligns each tooth for maximum resistance to functional loading

To prevent increase muscle loading of the teeth and the joints during protrusive movement.

If there is any tooth contact posterior to canine during excursion the elevator muscles are triggered into hypercontraction

CURVE OF WILSONResults from inward inclination of

posterior teethIn maxillary arch reverse is there because

of outward inclination of posterior teeth.There are two reasons for this inclination of

posterior teeth 1) one has to do with resistance to

loading 2)second has to do with masticatory

function

Axial alignment of all posterior teeth is nearly parallel with the strong inward pull of the medial pterygoid muscle

Aligning both upper and lower posterior teeth with the principal direction of muscle contraction produce the greatest resistance to masticatory forces, and forms curve of wilson

tongue and buccinator must place food onto occlusal table

there must be easy access for the food to get to the occlusal table

The inward inclination of the lower occlusal table allows for direct access from lingual

The outward inclination of the upper occlusal table provides access from the buccal for the food

When the curve of wilson is made too flat ease of masticatory function may be impaired because of increased activity required to get the food onto the occlusal table

Normal function versus parafunction

The image to the left is demonstrating

normal reciprocal  functioning of the Lateral Pterygoids and

Masseters/Med.Pteygoids/Temporalis'.  The Lateral Pterygoids advance the

condyles, thereby opening the mouth (depressing the mandible), with the

assistance of the Digastric   The oblique orientation of the Masseters

and Medial Pterygoids create a sling.  The non-working side Medial Pterygoid

contracts simultaneously with the opposite side working Masseter. 

It is this oblique orientation of the Med.Pterygoids and Masseters that create the functional "shift" of the mandible, not an unilateral contraction of a Lateral Pterygoid

. 

In the event the Temporalis' do not cease their active contractions, scenarios of varying degrees of parafunction result, as the Lateral Pterygoids encounter resistance to their attempts at condylar advancement, thereby increasing their intensity of contraction and strain on their origins and insertions: the pterygoid plates of the sphenoid bone, and the condylar neck and disc. 

The degree of frequency, duration and intensity of the contractions of a Lateral Pterygoid is a function of the resistance provided by the parafunction ipsilateral and/or contralateral Temporalis. 

The maximum clenching intensity occurs in the musculoskeletally stable position

The mandibular position of the temporalis' most intense contraction is not when the teeth are together, but when they are a particular distance apart, and separated by an object (such as a splint, or food).

REFERENCES Clinical Anatomy. 7th ed Richard S Snell, BD Chaurasia’s ,Human Anatomy volume 3 - 4th edition

Grays Anatomy -36th ed Clinically Oriented Anatomy -4th ed Keith L.Moore, Grants atlas of anatomy -10th ed

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Newman

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