Masseter !

The masseter is the strongest chewing muscle, over the angle of the jaw; it closes the jaw and can ache from clenching or grinding the teeth.

Official name: Superficial part of masseter

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What it is

The strongest chewing muscle; it is overworked in clenching and grinding (bruxism) and can ache with TMJ problems.

What it does

The strongest chewing muscle: closes the jaw.

Key facts

Origin: Zygomatic arch (superficial: anterior two thirds; deep: posterior part)
Insertion: Lateral ramus and angle of the mandible
Action: Elevates the mandible (closes the jaw); superficial part protrudes
Nerve: Masseteric nerve (V3)
Blood supply: Masseteric artery (maxillary artery), transverse facial artery

Relations

Superficial: Masseteric fascia, the parotid gland over its back part, the parotid duct, the transverse facial artery, the facial nerve branches, risorius, zygomaticus major and platysma
Deep: Ramus and coronoid process of the mandible, the insertion of the temporalis, and the masseteric nerve and vessels entering through the mandibular notch
In front: Buccinator and buccal fat pad, and the facial artery and vein crossing the mandible
Behind: Parotid gland and the retromandibular vein behind the ramus
Above: Zygomatic arch and zygomatic bone

How to find it

With the fingers on the angle of the jaw and the cheek in front of the ear, the person clenches the teeth: the muscle hardens and bulges under the fingers on both sides.

Examination

Clenching while both sides are palpated: one soft side points to a lesion of the mandibular nerve on that side
Jaw jerk: a tap on a finger on the chin with the mouth slightly open; a brisk reflex suggests bilateral upper motor neuron disease, such as pseudobulbar palsy
Mouth opening measured between the incisors; reduced opening (trismus) suggests muscle spasm, infection of the masticator space or a joint problem

On imaging

Ultrasound and MRI show it as the thick muscle on the outer side of the mandibular ramus, with the parotid duct crossing its surface. Ultrasound compares both sides in benign masseteric hypertrophy and shows a masticator space abscess from a lower molar. Contrast CT or MRI map that abscess or a tumour of the masticator space, and CT shows fractures of the ramus and angle.

Common problems

Clicking or popping when the disc slips forward and back.
Locking (the jaw cannot open fully).
Jaw pain linked to clenching or grinding (bruxism).

How it is diagnosed

Exam of jaw opening and the joint; MRI for the disc in persistent cases.

Treatment

Soft diet, heat, jaw exercises and pain relief.
A night guard (splint) for grinding.
Joint washing (arthrocentesis) or surgery only rarely.

Clinical notes

Benign masseteric hypertrophy, often with bruxism, gives a square lower face and must be told apart from a parotid tumour; botulinum toxin injection is one procedure used to reduce it
Jaw pain on chewing that stops with rest (jaw claudication) is a classic symptom of giant cell arteritis
Mandibular angle fractures are pulled apart or held together by the masseter and medial pterygoid sling, depending on the direction of the fracture line
The masseteric nerve is a common donor for smile reanimation after facial palsy

Development

First pharyngeal arch muscle, so it is supplied by the mandibular nerve (V3), like all the chewing muscles.

Memory aid

Muscles of mastication come from the first arch and the mandibular nerve: «Mastication = Mandibular».

Test yourself

Think of the answer, then tap the question to see it.

What is the action and nerve supply of the masseter?

It elevates the mandible (closes the jaw); the superficial part protrudes it. Masseteric nerve (V3).

What is trismus, and what classic causes involve the masseter?

Limited mouth opening from spasm of the masseter and medial pterygoid: dental infection, pericoronitis of a lower wisdom tooth, or tetanus (lockjaw).

What does a brisk jaw jerk indicate?

An upper motor neuron lesion above the pons, such as pseudobulbar palsy; both arms of the reflex run in the mandibular nerve (V3).

Which duct crosses the outer surface of the masseter?

The parotid duct, which turns around the front edge of the masseter.

Jaw pain on chewing that stops with rest is a classic symptom of what?

Giant cell arteritis (jaw claudication).

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Educational information, not medical advice. For any symptom, see a health professional.

Its names

Official Latin (TA2): pars superficialis masseteris TA2 2106
Official Spanish (DeCS): Músculo Masetero DeCS D008406
English (TA2): Superficial part of masseter

Sources: Terminologia Anatomica (TA2); DeCS; Anatomy, Head and Neck, Masseter Muscle; Anatomy, Head and Neck, Mastication Muscles.

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Representative model, for educational purposes only.

Masseter highlighted on the Durgical 3D model