MUSCLES OF MASTICATION

Introduction:

muscles

The muscles of mastication are very important for the functions of the jaws and the supporting structures. These help in various movements of the mandible during different functions and they are mostly chewing, swallowing and sometimes in actions like yawning and coughing. It is difficult to say that any one individual muscle is responsible for any of these actions and it is more so that they act as a group in performing these actions. It is important to have a thorough knowledge of all these muscles to identify any problem related to the said functions identified by the patient.

Components of Mastication

  • The Dentition
  • Periodontal supporting tissues
  • Maxilla and Mandible
  • Temporomandibular Joint
  • Mandibular musculature
  • Muscles of lips, cheeks, and tongue
  • Involving soft tissue
  • Supplying innervation and vasculation

There are basically 2 groups of muscles which help in mastication. They are

  1. The Supra-mandibular Muscles / Elevators of Mandible
  2. The Infra mandibular Muscles / Depressors of Mandible

 

  1. The Supra-mandibular Muscles :

These muscles in conjunction with the suprahyoid musculature work in groups and not as individual units.(Knowledge of the action of individual masticatory muscles, is necessary to  understand  their function during the movements of the mandible).

There are four powerful muscles

  • Masseter
  • Temporalis
  • Medial Pterygoid
  • Lateral pterygoid

These are also called as the “Muscles of Mastication”. The Masseter & Medial pterygoid, are associated as a sling & for the most part they pull in an upward & forward direction. The most massive portion of the temporalis lies anteriorly & it pulls upward in a vertical direction. The pull of the Lateral pterygoid is forward, in a horizontal direction therefore the resultant of the vertical component of the Temporalis & the horizontal component of the Lateral pterygoid is upward & forward, which is in coordination with the masseter-pterygoid sling.

Infra-Mandibular Muscles

The infra-mandibular Muscles are –

  • Digastric
  • Geniohyoid
  • Mylohyoid
  • Stylohyoid.

Development of the muscles :

           The muscles of mastication are developed from the embryonic mesoderm of different pharyngeal arches and are supplied by the nerve of respective arches.

I Arch or mandibular arch

  • Masseter
  • Temporalis
  • Medial pterygoid
  • Lateral pterygoid
  • Mylohyoid
  • Anterior belly of digastric

Supplied by mandibular nerve

2) II arch

  • Posterior belly of digastrics

Supplied by facial nerve

 Masseter:

masseter

Masseter:

This is a quadritateral muscle, has 3 layers blended anteriorly.The Superficial, Middle & Deep layer.

Origin and Insertion

The Superficial is the largest & attached to a thick aponeurosis to the maxillary processes of the zygomatic bone & anterior 2/3rd of the inferior border of the zygomatic arch, it descends backwards to the mandibular angle & lower Posterior half of the lateral surface of its ramus.

masseter partsThe Middle layer extends from the medial aspect of the  anterior 2/3rd of the Zygomatic arch and lower border of its posterior 3rd to the central part of the mandibular  ramus.

The Deep layer extends from the deep surface of the zygomatic arch to the upper part of the mandibular ramus & its coronoid process.

The middle & the deep layers together constitute to form a cruciate muscle.

masseter origin

Relations :

Superficially: are skin, platysma, risorius, zygomaticus major, & parotid gland duct, facial nerve and transverse facial vessels cross the muscle.

Medially : Temporalis & Mandibular ramus. Fat separates it anteriorly from buccinator . The massetric  nerve & artery

Posteriorly: Margin is overlapped by the parotid gland, anterior margin projects over buccinator & is crossed below by facial vein.

MASSETER FIBERS

Never Supply :

A branch of the madibular nerve which enters its deep surface by passing through the mandibular notch immediately anterior to the capsule of the TMJ.

MASSETER NERVE SUPPLY

Blood Supply :

  • Massetric artery (part of maxillary artery)
  • Facial artery
  • Branch of superficial   temporal artery

MASSETER BLOOD SUPPLY

Action  :

  1. Raises the mandible,
  2. Helps occlusion in mastication: its superficial fibres, running obliquely help to protract the mandible and acting alternately, the 2 muscles i.e(right & left) swing the chin from one side to the other producing a grinding movement of the teeth which is assisted by the pterygoid muscles.

Relation :

Superficial: Skin, platysma, zygomaticus major, parotid gland& duct, branches of facial nerve, transverse facial branches of temporal vessels

Deep: Temporalis and the ramus, Massetric nerve and artery

masseter relation

Anterior: margin is separated from Buccinator and buccal branch of mandibular facial nerve by buccal pad of fat.

Posterior: margin is overlapped by Parotid gland.

 

Temporalis

 temporalis

 

Temporalis

This is a fan shaped muscle.

It extends like a fan from most of the temporal fossa (except its zygomatic part), and from the deep surface of the temporal fascia.

temporalis

Origin and Isertion

The fibres converge & descend into a tendon that passes between the zygomatic arch & cranial wall & is then attached to the medial surface, apex, anterior and posterior borders of the coronoid process & anterior border  of the mandibular ramus almost to the third molar tooth.

temporalis fibers

The anterior fibres of temporalis descend vertically, traced posteriorly the fibres are increasingly oblique, while the most posterior fibres are almost horizontal.

Relations :-

Superficial : Skin, auricularis anterior & superior, temporal fascia, superficial temporal vessels, auricular temporal nerve, temporal branches of facial nerve, zygomatico temporal nerve, apicranial aponeuresis, zygomatic arch & masseter.

temporalis relations

Medial : Temporal fossa, lateral pterygoid, superficial head of medial pterygoid, a small part of buccinator, the maxillary artery & its deep temporal branches, deep temporal nerves & buccal nerve & vessels.

Posterior : The tendon, massetric vessels & nerve that traverse the mandibular incisure.

Fat separates its anterior border from the zygomatic bone.

temporalis relation

Nerve Supply :

Deep temporal branches of the mandibular nerve.


temporalis NERVE SUPPLY

Action :

1.Elevation of the mandible to close the mouth and approximate the teeth. This requires elevation by anterior fibres & retraction by posterior fibers, since the mandibular condylar head is on the articular eminence when the mouth is open.

2.Posterior fibers retract the mandible from protrusion. The muscle is concerned in lateral grinding movements.

Blood Supply :

This is furnished by the middle & deep temporal arteries. The middle temporal artery  is a branch of the superficial temporal artery. The deep temporal arteries are branches of the maxillary artery.

temporalis blood supply

Medial Pterygoid Muscle :

medial pterygoid

Medial Pterygoid Muscle :

The Medial Pterygoid muscles,are situated on the medial side of the mandibular ramus is anatomically & functionally a counterpart of the masseter muscle.

It is a rectangular, powerful muscle, although it is not as strong the masseter.

 Origin and Insertion

Its main origin is in the pterygoid fossa. The fibres at its inner surface arise by strong tendons, others arise directly from the medial surface of the lateral pterygoid plate. Anterior fibres arise by strong tendons from the outer & inferior surfaces of the pyramidal  process of the palatine bone & even from the adjacent parts of the maxillary tuberosity.

medialpterygoid muscle origin

. The fibres of the medial pterygoid muscles run downward, backward, & outward & are inserted to the medial surface of the mandibular angle. The field of insertion is approximately triangular. It is bounded by the lower half of the posterior border of the mandibular  ramus and by two lines that start at the mandibular foramen.

 medialpterygoid insertion

Relations :

Laterally is the mandibular ramus, from which the muscle is separated by the lateral pterygoid, sphenomandibular ligament, maxillary artery, inferior alveolar vessels & nerves, lingual nerve & process of parotid gland.

Medially it is related to the tensor veli palatine & separated from the superior constrictor by styloglossus, stylopharyngeus & aerolar tissue.

medialpterygoid relations

Nerve Supply :

The never supplying the medial pterygoid muscles is the medial pterygoid nerve branch of the mandibular nerve. The nerve supplying the medial pterygoid muscles reaches it at its posterior border or slightly in front of it.

Mandibular nerve 

Blood Supply :

The artery supplying the medial pterygoid muscles is a  branch of the maxillary artery.

maxillary artery

 

Action :

  1. It raises the mandible,
  2. Assists protrusion
  3. Slides the chin to the opposite side- the two medial pterygoid muscles alternatively produce a grinding movement similar to the action of the superficial fibres of the masseter.

Lateral Pterygoid  Muscle :-

medial pterygoid

Lateral Pterygoid  Muscle :-

This is a short, thick muscle with an  upper head & lower head.

Origin and Insertion

The larger lower head originates from the  outer surface of the lateral ptergyoid plate, and the smaller, upper (superior) head originates from the infratemporal surface of the greater wing of the sphenoid medial to the infratemporal crest.

The fibres of the upper head at first run downward, then backward and outward in close relation to the cranial base. When they reach the anterior limit of the joint, the fibres bend horizontally back to the neck of the mandible.

medialpterygoid muscle origin

The fibres of the lower head converge upward & outward, the upper fibres running more horizontally, the lower fibres more & more steeply ascending.

The two heads, separated anteriorly by a variably wide gap, fuse in front of the cramiomandibular joint & they can be separated artificially.

Only part of the upper head, namely its upper most & most medial fibres, is attached to the anteromedial surface of the articular capsule & thus indirectly to the anterior border of the articular disc. The majority of the fibres, that is the greater part of the superior head & the entire inferior head, are inserted to a roughened fovea on the anterior surface of the mandibular neck.

 Relations :

Superficially, are the mandibular ramus, maxillary artery crossing deep or superficial to the muscle and the tendon of temporalis & masseter. Medial are the upper part of the medial pterygoid muscles, sphenomandibular ligament, middle meningial artery & mandiular nerve.

 Upper Border is related to the temporal & masseteric branches of mandibular nerve. Lower Border is the lingual & inferior alveolar nerves.

 lateralpterygoid relations

Nerve Supply :-

The nerve to the lateral pterygoid  muscle branches off from the masseteric or buccal nerve, which is the branch of the anterior trunk of the mandibular nerve.

Mandibular nerve

Blood Supply :

Branch of the  maxillary artery.

maxillary artery

 

Action :

  1. Muscle aids opening the mouth by protracting the mandibular condyle & the articular disc, while the mandibular head rotates on the disc.
  2. In closure, backward gliding of the disc & condyle is controlled by slow elongation of the lateral pterygoid & by masseter & temporalis, restoring the teeth to occlusion. Acting with the ipsilateral medial pterygoid, the lateral pterygoid advances its condyle, the mandible rotating on a verticial axis thro the opposite condyle.
  3. Both medial pterygoid & lateral pterygoid together protrude the mandible.

Different actions have been ascribed to the 2 parts of the lateral pterygoid, the superior head being involved in chewing and the inferior head in protrusion.. 

The Infra-Mandibular Muscles :

The infra mandibular muscles are arranged between the cranium and mandible and hyoid bone. Their function is either to elevate the hyoid bone and to depress the mandible. Whether one or the other movement is effected depends on the state of  contraction of other muscles.

anterior view of neck

The infra-mandibular muscles are the

  • Digastric
  • Geniohyoid
  • Mylohyoid
  • Stylohyoid.

Digastric Muscle :

As indicated by its name the diagastric muscle consists of two fleshy parts, a posterior & an anterior belly, which are connected by a strong round tendon.

digastric

Origin and Insertion

The posterior belly arises from the mastoid notch, medial to the mastoid process.

The posteior belly is much longer than the anterior belly, almost circular in cross section, and only slightly flattened in lateromedial direction. Gradully  tapering anteriorly, the posterior belly continues into the round intermediate tendon.

The anterior belly finds its attachment in the digastric fossa of the mandible at its lower border close to the mid line.

The anterior belly, arising from the intermediate tendon, is much shorter than the posterior belly.

digastric origin

Nerve Supply :

The Digastric muscle has a double innervation. The posterior belly is supplied by a branch of the facial nerve, entering the muscle close to its  posterior end. The anterior belly is supplied by a branch of the mylohyoid nerve of the mandibular nerve.

Action :

1.The chief action of both bellies, acting together is to raise the hyoid bone (hence the tongue) in the action of swallowing, an action which cannot be carried out when the mouth is open because the muscle is already shortened.

  1. Acting with the infrahyoid muscles, it fixes the hyoid bone, thus forming a stable platform on which the tongue can move.

digastric muscle relation

 

Geniohyoid Muscle :-

geniohyoid

Geniohyoid Muscle :-

The geniohyoid muscles arise above the anterior end of the mylohyoid line from the inner surface of the mandible, close to the midline & lateral to and including the inferior mental spines, by a short & strong tendon.

The muscle, in contact with that of the other side, proceeds straight posteriorly & slightly downward & is attached to the upper half of the hyoid body.

Posteriorly the muscle gradually widens & assumes a  triangular shape in cross section. 

Nerve Supply :

The muscle is supplied by the 1st & 2nd Cervical nerves, which reach it via the hypoglossal nerve.

 geniohyoid nerve supply

Action :

It pulls the elevated hyoid bone directly forwards, hence increases the anterior posterior diameter of the pharynx to receive the bolus in swallowing.

 

Mylohyoid Muscle :

mylohyoid

Mylohyoid Muscle :

The mylohoid muscle forms, anatomically & functionally the floor of the oral cavity, hence the old term for it was oral diaphragm.

The right & left muscles are united in the mid line between the mandible & hyoid bone by a tendinous slip, the mylohyoid raphe.

 Origin and Insertion     

The muscle arises from the myohyoid line on the inner surface of the mandible.

Posterior fibres take their origin from the region of the alveolus of the lower third molar. The origin of the anterior fibres descend towards the lower border of the mandible.

The posterior fibres of the coarsely bundled muscle run steeply downward, medially, & slightly forward & are attached to the body of the hyoid bone; the majority of the fibres, join those of the contralateral muscle in the mylohyoid raphe

Nerve Supply : The mylohyoid musle is supplied by branches of the mylohyoid nerve of the mandibular nerve.

mylohyoid nerve supply 

Action :

It helps to raise the hyoid bone & tongue in swallowing, and forms the muscular floor of the mouth.

Stylohyoid Muscle :

stylohyoid

Stylohyoid Muscle :

The stylohyoid muscles arise from the lateral and inferior surface of the styloid process.

It is a thin round muscle that converges with the posterior belly of the digatric muscle anteriorly & inferiorly.

It lies first superior & medial to the digastric muscle & then close to its upper border.

Where the stylohyoid muscle reaches the intermediate tendon of the digastric musle, it splits in most individuals into two strips that enclose the tendon of the digastric muscle & insert into the greater horn of the hyoid bone where it joins the body.

Nerve Supply :

The Facial nerve supplies this muscle, the branch supplying the stylohyoid muscle enters the muscle near its anteroinferior end.

stylohyoid nerve supply

Action :

It s function is the elevation & retraction of the hyoid bone during swallowing. 

 

THE BORDER MOVEMENTS

All these muscles of mastication are responsible independently or together in various

movements of the mandible.  Movements of the mandible can be mainly classified as –

  1. Rotational movements
  2. Translational movements

Rotational movements

Rotational movements are when the mandible moves with the tempro mandibular  joint as a central axis.  The rotational movement can be one of them-.

  1. Rotational movement around a horizontal axis with the axis passing through both temporomandibular joints.
  2. Rotational movement around a vertical axis passing through either the right or the left tempromandibular joint.
  3. Rotational movement around the sagittal axis passing through either the right or the left temporomandibular joint.

These three movements can be performed either individually or together.

The muscles responsible for the rotational  movement around the horizontal axis i.e., mainly the opening and closing of the jaw are-

  1. Inferior division of lateral pterygoid.
  2. Digastric
  3. Mylohoid
  4. Geniohyoid

and the second set being-

  1. Masseter
  2. Medial Pterygoid
  3. Superior division of lateral pterygoid, temporalis respectively.

Translational movement (Protrusive )

  •  Bilateral contraction of lateral pterygoid muscles
  • Along with masseter, temporalis and medial pterygoid

Translational movement (Retrusive )

  • Contraction of the Middle and Posterior portion of Temporal muscle
  • Assisted by Masseter, Posterior Digastric and Geniohyoid muscle

 

HORIZONTAL PLANE BORDER MOVEMENT

  • Left lateral border
  • Continued left lateral border with protrusion
  • Right lateral border
  • Continued right lateral border with protrusion
  • Functional movements

 

The movements around the vertical axis and sagittal axis are guided by the contraction of the lateral and medial pterygoid muscle on one side of the mandible and contraction of the temporal muscle on the contra lateral side.

Translational movements

The translational movement which are the protrusive movement are performed by the bilateral contraction of lateral pterygoid muscle along with the temporal muscle contracts and also assisted by masseter, posterior digastric and geniohyoid muscles.

These movements especially the rotational movements are very well recorded by posset in his tracing done on a sagittal plane.  This tracing gives the borders of the movement of the mandible and are mainly-

  1. The posterior open border.
  2. The anterior open border
  3. The superior contact border
  4. Functional movements

The border movement in the horizontal plane was traced and recorded to get one of the movements namely-

  1. Left lateral border movement
  2. Continued left lateral border movement with protrusion
  3. Right lateral border movement
  4. Continued right lateral border movement with protrusion and
  5. An area of functional movements.


Applied Aspects      

Masticatory Muscle Disorders are Classified by American Academy of Oro –Facial Pain as :-

  1. Myofacial Pain
  2. Myositis
  3. Myospasm
  4. Myalgia
  5. Myofibrotic Contracture
  6. Neoplasia

 

  1. Myofacial Pain :- It is characterized by a regional or local dull, aching muscle pain that increases during function. Clinically there are localized, tender sites or trigger points in the muscles, tendon or fascia .Referral pain to teeth, ear, head may or may not be present.
  2. Myositis : It is a true inflammation of the muscle usually due to direct trauma/or infection.
  3. Mycospasm : It is also know as trismus and is an acute muscle disorder characterized by a sudden, involuntary, tonic contraction of a muscle. The acute pain is present at rest as well as during function.
  4. Myalgia : This refers to muscle pain. When including the masticatory muscles and the temporomandibular joint structures, together it is called  Temporomandibular Disorders (TMD). Patients with masticatory myalgia disorders usually present a dull steady pain overlying the muscles in the jaw, head and neck that is sometimes accompanied  by earaches, head aches and shoulder pain.
  5. Myofibrotic Contracture : It is a condition of painless, shortening of a muscle, as a result of fibrosis or scarring of the supporting tendons, ligaments or muscle fibres.
  1. Muscle Neoplasm : It is defined as a new, abnormal, or uncontrolled malignant or benign growth of tissue within the muscle.

Initial diagnosis of these disorders is usually made using signs and symptoms that are characteristic of each disorder. The most common signs include tenderness of the muscles and joints and limitation in the range of motion of mandible. The most common symptoms include jaw pain, fascial pain, earache, headache, and  difficulty in jaw function due to pain.

Other Disorders causing masticatory myalgia :

Specific medical disorders that have a systemic or regional effect on muscles need to be considered in any differential diagnosis of masticatory myalgia. This includes-

  • Thyroid dysfunction
  • Diabetes
  • Neuropathic pain
  • Lupus erythematosus
  • Scleroderma
  • Temporal Arteritis
  • Lyme disease
  • Parkinson’s disease
  • Brain tumour

All these conditions are associated with orofacial myalgia

 

Psychological or behavioral conditions can also accompany and contribute to masticatory myalgia and need to be identified. This includes :-

  • Anxiety and depression

Oral parafunctional activities such as :

  • Bruxism
  • Clenching
  • Jaw bracing
  • Finger or pen biting
  • Lip or cheek biting
  • Postural habits such as telephone cradling or jaw thrust.