Muscles of mastication/ orthodontics courses in india

Muscles ofMuscles of
MasticationMastication
Anatomy and Physiology.Anatomy and Physiology.
INDIAN DENTAL ACADEMY
Leader in continuing dental education
www.indiandentalacademy.com

•Introduction
•Terms
- Origin
- Insertion
- Action
•Development
•Clinical examination
•Masticatory muscles
- masseter
- temporalis
- medial pterygoid
- lateral pterygoid
•Accessory muscles
- suprahyoid
- infrahyoid
•Applied anatomy
•Reference

Introduction
• In general as one reads about muscles there
are five terms that are constantly seen, they
are
Origin
Insertion
Action
Nerve supply and
Blood supply

Origin: it is generally considered to be the end
of the muscle that is attached to the least
movable structure.
Insertion: it is the other end of the muscle,
which is attached to the more movable
structures.
Action: it is the work that is accomplished when
the muscle fibers contract.
Terms

Masticatory muscles
The muscles most immediately concerned with the
movement of the mandible in mastication and
speech are
Masseter
Temporalis
Medial pterygoid and
Lateral pterygoid

• There are four pairs of muscles attached
to mandible, primarily responsible for
• Elevating
• Depressing
• Protruding
• Retruding
• Lateral movement

Development
• These muscles develop from first
pharyngeal arch [mandibular]
• Thus innervated by the nerve of the
first arch, the fifth cranial nerve
[trigeminal nerve]more specifically,
by the third part of the fifth cranial
nerve; mandibular division.

• Blood supply to all the muscles is
by the maxillary artery, branch of
external carotid artery.

Clinical examination
• Masseter:
• It can be palpated
extra orally by placing fingers
over the lateral surfaces of the
ramus of mandible

• Temporalis:
• Fingers are placed
over the patients temples to
feel the muscle.

• Medial pterygoid:
• Index finger is used to
touch the medial pterygoid
muscle on the inner surface of
the ramus.

• lateral pterygoid:
• The little finger is
inserted facial to the maxillary
teeth and around distal to the
pterygomaxillary, [or] hamular
notch, to palpate the muscle.

• The distance between maxillary
and mandibular incisors is
measured when the patient is
instructed to open ‘all the way’.
• Normal opening in an adult is
about 35-50 mm.
• Normal lateral movement is
about 8-10 mm.

Masseter muscle:
• Masseter means to chew.
• It is a quadrilateral muscle in shape.
• It is one of the outermost muscles in the
skull.
• The muscle consists of three layers,
which blend anteriorly. They are.
The superficial layer
The middle layer
The deep layer

The superficial layer
• It is the largest of the three layers.
• Origin: by a thick
aponeurosis from maxillary process
of zygomatic bone and from the
anterior 2/3’rd of the inferior
border of the zygomatic arch.
• Insertion: fibers pass
downwards and backwards to
insert into the angle and lower
posterior half of the lateral surface
of the ramus

The middle layer:
• Origin: arises from the medial
aspect of the anterior two thirds of
the zygomatic arch and from the
lower border of the posterior third.
• Insertion: inserts into the central
part of the mandibular ramus.

The deep layer
• this layer is also called as
‘zygomaticomandibular muscle’.
• Origin: from the deep
surface of the zygomatic arch.
• Insertion: these fibers travel
downwards, into the upper part of
the mandibular ramus and into its
coronoid process.

• middle and deep layers ofmasseter
referred to as ‘cruciate muscle’.
• The superficial layer is covered
externally by continuous tendinous
tissue
• During clinical palpation the most active
region of the muscle is situated near the
mandibular angle.
• The fibers of deep and superficial layers
diverge from each other at an angle of 50
degrees.

Actions:
• Masseter is considered to supply
‘power’, rather than positioning.
• It elevates the mandible to occlude
the teeth in mastication.
• It has small effect in side to side
movements, protraction and
retraction

Temporalis muscle:
As it is close to temples,thus the name
temporalis.
It is characterized by its fan shape and
fairly thin muscle
It covers a great part of lateral area of the
skull -the temporal fossa.
-Narrow part of parietal bone
-Great part of temporal bone
-Sphenoid bone
‘Temporal fascia’ covers the temporalis
muscle.
Temporalis muscle:

• Above: the fascia is single layered. it is
attached to superior temporal line.
• Below: the fascia is two layered. one attached
to lateral side and the other to the mesial
margin of the upper border of the zygomatic
arch.
• On the lateral portion of the skull, temporalis
inserts at the level of the temporal line and
divides into 3 parts.
1. The anterior bundle
2. The intermediate bundle
3. The posterior bundle

1. The anterior bundle:
• Origin: the upper extremity
attaches to the temporal bone.
• Insertion: the lower extremity
inserts onto the apex of the
coronoid process of the mandible
and to the anterior border of the
ramus.

• This group is active at the closing phase and
inactive at the opening phase.
• No activity is seen during mandibular depression,
expect during maximum opening [or] opening
against resistance.
• This action prevents the accidental dislodgement
of the condyles out of the articular fossa if the
resistance is suddenly removed.
• This bundle is more active in mandibular elevation,
swallowing and rest position.
• This bundle is frequently adapted to crush and
chew food close to centric occlusion. No activity
at normal protrusive movements

2. The intermediate bundle.
• Vigorous action of this bundle
has been observed during
protrusive movements.

3. The posterior bundle.
• Anatomically posterior bundle is
oriented to elevate the mandible
• They function primarily as
mandibular retractors [or]
positioners.
• These remain inactive during
mandibular depression and
protrusion

Nerve supply:
• It is supplied by the deep
temporal branches of the
anterior trunk of the
mandibular nerve.

Action:
• It elevates the mandible and thus
closes the mouth and approximates
the teeth.
• Thus it acts as a postioner.
• This movement requires the
upward pull of the anterior fiber
and backward pull of the posterior
fibers.
• Because the head of the condyles
rests on the articular eminence
when the mouth is open.
• It is active in forcible elevation.

Medial pterygoid:
• Pterygoid means wing like.
• It is a thick, quadrilateral
muscle.
• It is located on the internal
surface of the ramus.
• It lies along the length of the
masseter muscle.
• It is less potent than masseter.

Origin:
• It originates from the medial
surface of the lateral pterygoid
plate and from the grooved surface
of the pyramidal process of the
palatine bone.
• A more superficial slip arises from
the lateral surface of the pyramidal
process and maxillary tuberosity.

Insertion:
• the fibers descend postero
laterally and are attached by a
strong tendinous lamina to
postero inferior part of the
medial surfaces of the ramus
and angle as high as the
mandibular foramen and
almost as far forward as the
mylohyoid groove.

Action:
• It acts as elevator.

Lateral pterygoid:
• It is a short, thick muscle.
• It is also referred to as
“sphenomeniscus muscle”.
• It has two heads
• Upper head- smaller.
• Lower head- larger.
• it is deeply located in the
infratempoal fossa.

Origin
• Lower head: the larger one in the
lower position has its origin in the
external surface of the lateral
pterygoid lamina.
• Upper head: the smaller one in the
upper position originates from the
infratemporal surface of the
sphenoid bone.

Insertion
• The two separate bellies are
oriented posterior where they fuse
at the level of the
temporomandibular joint.
• The most superior fibers of the
upper belly are directly connected
to the anterior surface of the
articular capsule.
• Thus are indirectly attached to
the anterior portion of the condylar
neck.

Action
• It assists in opening the mouth by pulling forward the condyles and the articular disk.
• Slow elongation of the lateral pterygoid is seen during the closure of the mouth,
backward gliding of articular disc and condyle.
• Ipsilateral medial and lateral pterygoid advance the condyle of that side so that jaw
rotates about a vertical axis through opposite condyle.
• When both medial and lateral pterygoid act together they protrude the mandible, so
that the lower incisors project in front of upper.
• Upper head is involved mainly in chewing.
• Lower head is involved in protrusion, but the primary function is to move the disk and
condyle head forward.
• No activity is seen during closing and swallowing.
• The lower belly is synergistic with suprahyoid muscle during protrusive and opening
movements of mandible.
• It is antagonist to suprahyoid muscle during closure movement in mastication and
tooth clenching.
• This bundle acts as a antigravity muscle during certain swallowing movements.
• This muscle is responsible for opening of mandible.
• External pterygoid is active only at uncontrolled opening of mouth.
• This is used to place the condyles in a favorable position as the mandible progresses
in its depressing movement as this muscle is not essential for opening the
mouth,stabalizing force of the external pterygoid is used necessary to prevent the
dislodgement of the mandible during the masticatory function.

THE ACCESSORY MUSCLES
OF MASTICATION
1. Suprahyoid group-
DIGASTRIC
MYLOHYOID
GENIOHYOID
STYLOHYOID
2. Infra hyoid group-
STERNOHYOID
THROHYOID
OMOHYOID
STERNOTHYROID

DIGASTRIC
• Origin: There are muscle fibers at either end
with a collagenous tendon at the middle.
• One end: digastric notch just medial to the
mastoid process behind the ear.
• Other end: digastric fossa on the inferior
surface of the mandible at the midline.
• Action: is two fold.
• By contracting, it can create a backward pull on
the mandible. When the jaws are clenched.
Contraction of the muscle elevates the hyoid
bone, thus helps in the pulling the mandible
downwards.
• Nerve supply
• Anterior belly by the trigeminal nerve.
• Posterior belly by the facial nerve.

MYLOHYOID
• The floor of the mouth is formed by
the mylohyoid muscle.
• Origin: from the mylohyoid line on
the medial surface of the mandible.
• Insertion: to the hyoid bone.
• Nerve supply: mylohyoid branch of
the trigeminal nerve.
• Blood supply: branch of the inferior
alveolar artery.

GENIHYOID
• Origin: inferior genial tubercle
or mental spine on the lingual
surface of the mandible.
• Insertion: hyoid bone.
• Action: depresses the mandible.
• Nerve supply: C1.
• Blood supply: lingual artery.

INFRA HYOID GROUP
• OMOHYOID
• It has an inferior belly, common tendon and
superior belly.
• Arises from the inferior belly and inserted
through superior belly.
• Origin: upper border of the scapula.
• Insertion: lower border of the body of hyoid
bone lateral to sternohyoid.
• Action: depresses hyoid bone ,fixes the hyoid in
mandibular depression.
• Nerve supply: C2 and C3.
• Blood supply: superior thyroid arteries.

STERNOHYOID
• Origin: posterior surface of the
manubrium sterni.
• Insertion: medial part of the lower
border of the hyoid bone.
• Action: depresses the hyoid,fixes
hyoid in mandibular depression.

STERNOTHYROID
• Origin: posterior surface of the
manubrium sterni.
• Insertion: thyroid cartilage.
• Action: depresses the larynx.
• Blood supply: superior thyroid
artery.

THYROHYOID
• Origin: oblique line of the thyroid
cartilage.
• Insertion: hyoid bone.
• Action: depresses the hyoid and
elevates the larynx.
• Nerve supply: C1through
hypoglossal.
• Blood supply: superior thyroid
artery.

BUCCINATOR
• Origin: it is a wide muscle that arises from a horse
shoe shaped line along the outer surfaces of the
maxillary and mandibular alveolar processes in the
area of the molar teeth.
• It originates from pterygomandibular ligament.
• Buccinator fibers arising from the mandible follow
the external oblique line to the mesial aspect of the
first molar and ascend slightly towards the corner
of mouth.

Insertion
• Some fibers insert into the
mucous membrane of cheek and
modiolus.
• Remaining fibers enter the upper
and lower lips to become a part
of the orbicularis oris muscle.

Action
• It pulls the corners of the mouth laterally
and posteriorly.
• Major function is to keep the cheeks taut.
• If it were not so, when jaw closes, the
cheeks collapse and be caught between the
teeth. This is seen in senile patients or
individuals with facial paralysis.
• This muscle helps in deglutition.
• Provides support and mobility for the soft
tissues of cheek.

APPLIED ANATOMY

Masseter muscle:
• The contraction of masseter muscle pushes the buccinator
fibers against the denture border. For this reason, the
borders must converge rapidly towards the retro-molar pad.
• At the time of preliminary impression is made, while the
compound on the borders of the impression tray in this
region is still soft, considerable downward force should be
exerted on the lower jaw by dentist.
• Now the patient is attempting to counteract this downward
pressure which will cause the masseter muscle to contract.
• This forces the softened compound away from impingement
in this region.
• Thus the denture border can also be contoured to
accommodate this action. If this is not done the muscle
interaction will displace the mandibular denture and force it
in an anterior direction

Temporalis muscle
• Temporalis does not participate in biting
force when the mandible is in
protrusion.
• Therefore the action of this muscle is
sometimes used as a test to determine
whether the patient is closing in centric
relation.
• When the mandible is in protrusion, no
bulging can be felt with fingers on the
side of the head.
• This muscle is more sensitive to occlusal
interferences than any muscle.

Mylohyoid muscle
• It elevates the hyoid bone, the tongue and the
membranous floor of the mouth during
swallowing.
• If the denture flange is extended below and
under the mylohyoid line, it will impinge on the
mylohyoid muscle and can affect its action
adversely.
• Thus the action of the muscle can unseat the
denture.
• As the fibers are directed downwards, the
denture flange can extend below but not under
the mylohyoid line. This places the inferior
border of the denture in a compatible position
with tongue.

Buccinator muscle
• The action of buccinator muscle does not
directly dislodge the denture because the muscle
fibers contract in a line parallel to the plane of
occlusion.
• When masseter is activated, it pushes the
buccinator medially against the denture border
in the area of retro molar pad.
• This is a dislodging force and the denture base
should be contoured to accommodate this
interaction between the buccinator and
masseter.
• This is termed as` masseter groove’.
• The position of attachment of buccinator muscle
in the upper jaw determines the vertical height
of the disto buccal flange of the maxillary
denture.

References
• Gray’s anatomy -
• Anatomy and physiology - Rod.R.Steley.
• Anatomy of orofacial structures - Brand – 6th edition.
• Occlusion - Major Ash.
• Occlusion principles and concepts - José dos Santos -
2’nd edition.
• Text book of complete dentures - Rahn. -5’Th edition.
• Boucher’s prosthodontic treatment for edentulous
patients – 10’Th edition.
• Fundamentals fixed prosthodontics -
H.T.Shillingburg.Jr

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