Muscle:Masseter

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Masseter is a powerful masticatory muscle with two distinct layers — superficial and deep — each producing characteristically different referred pain patterns when trigger points (TrPs) are active. It is one of the most common sources of facial pain, toothache, tinnitus, and TMJ symptoms, and its TrPs are frequently mistaken for dental pathology, TMJ internal derangement, or sinusitis.

Because the mandible spans both sides of the cranium, unilateral masseter dysfunction has biomechanical effects on the contralateral side. Unilateral TrPs tend to deviate the mandible toward the affected side, and this deviation is most apparent when the patient slowly opens and closes the mouth — a presentation similar to unilateral TMJ internal derangement.

Anatomy

The masseter arises from the zygomatic arch and inserts into the lateral surface of the ramus and angle of the mandible. It is organised into two layers:

  • Superficial layer — larger, oblique fibres running inferoposteriorly from the zygomatic process of the maxilla and anterior two-thirds of the zygomatic arch to the angle and lower half of the lateral ramus
  • Deep layer — more vertical fibres from the posterior third of the zygomatic arch to the upper half of the ramus and coronoid process

Primary action: Elevation of the mandible (jaw closing); the superficial layer also contributes to jaw protrusion.

Innervation: Masseteric nerve, a branch of the anterior division of the mandibular nerve (V3).

Main synergists: Temporalis and medial pterygoid.

Referred Pain Patterns

Superficial Layer

TrPs in the superficial layer refer pain primarily to:

  • Anterior-superior TrPs: Upper premolar and molar teeth, adjacent gums, and maxilla — often described by the patient as "sinusitis"
  • Midbelly TrPs: Lower molar teeth and mandible
  • Lower border / gonial angle TrPs: Pain in an arc extending across the temple and over the eyebrow, and to the lower jaw; a TrP at the gonial angle (most likely enthesopathy) may refer pain preauricularly in the region of the TMJ

Referred pain and tenderness from TrPs in the masseter (or temporalis) muscle may cause tooth hypersensitivity to any or all stimuli: occlusal pressure, percussion, heat, and cold.

Deep Layer

TrPs in the deep layer, over the ramus of the mandible, refer pain to:

  • Diffuse midcheek area in the region of the lateral pterygoid muscle
  • TMJ region
  • Upper posterior TrP: Deep into the ear; this TrP may also cause tinnitus of the ipsilateral ear (see Tinnitus below)

Active TrPs in the deep layer can mimic the TMJ pain of rheumatic disease. When pain in the region of the TMJ has been referred from TrPs, the masseter and lateral pterygoid are the muscles most likely to be involved.

Restriction of jaw opening is more severe when TrPs are in the superficial layer than in the deep layer. Patients are often unaware of restricted opening if the jaws open wide enough (approximately 30 mm) to bite a sandwich comfortably.

Tinnitus

Unilateral tinnitus may be associated with TrPs in the upper posterior portion of the deep layer. The tinnitus:

  • May be set off by pressure on the TrP, or may be constant
  • The patient may be unaware of its presence until it stops upon TrP inactivation
  • May be activated or interrupted by stretching the jaws wide open
  • Is usually described as a low roaring
  • Is not associated with the deafness and vertigo common with vestibular or central neurological lesions

The mechanism may be a referred sensory phenomenon, or may be due to referred motor unit activity in the tensor tympani and/or stapedius muscles of the middle ear, which lie within the pain reference zone of masseter TrPs. Spasm of the stapedius could cause oscillation of the middle ear ossicles.

If tinnitus is bilateral, suspect a systemic rather than myofascial cause — however the deep layer can become involved bilaterally, giving rise to bilateral tinnitus with unilateral fluctuation of intensity.

Drug-induced tinnitus (e.g. salicylates) is usually bilateral and dose-dependent.

Complex and Overlapping Patterns

Complex symptoms and overlapping patterns of facial pain may be referred from multiple TrPs in the head and neck muscles. Unilateral or bilateral headache — either migraine or tension-type — can be generated by several different overlapping pain patterns from masticatory and cervical myofascial TrPs. Masseter TrPs play a contributing role in what is often diagnosed as tension headache.

Activation and Perpetuating Factors

Postural Factors

Excessive forward head posture (see SCM — Postural Assessment and Chapter 5 §C) places the mandible in a position that stresses the masseter and can activate or perpetuate TrPs. The mechanism:

  • Forward head posture creates indirect tension in the suprahyoid and infrahyoid muscles (Chapter 12)
  • These in turn pull down and create tension in the mandible
  • The mandibular elevator muscles — masseter, temporalis, medial pterygoid — contract reflexively in response

Chronic mouth breathing (e.g. through a surgical mask or due to nasal obstruction) tends to cause excessive forward head positioning and postural changes that indirectly stress the masticatory muscles.

Acute Overload

  • Sudden forcible contraction — cracking nuts or ice between the teeth
  • Biting off thread (seamstress habit)
  • Direct trauma — blow to the side of the jaw
  • Overload following motor vehicle accident causing flexion-extension injury to suprahyoid or infrahyoid muscles, which in turn produce tension on the jaw

Sustained and Repetitive Habits

  • Teeth clenching or bruxism
  • Gum chewing
  • Nail biting
  • Prolonged clamping on a pipe or cigarette holder mouthpiece
  • Late childhood thumbsucking
  • Significant occlusal disharmony — profound loss of vertical dimension from worn natural teeth, loss of posterior teeth, worn denture teeth, or resorption of alveolar bone

Psychological Factors

The masseter muscles are among the first to contract in persons who are in a state of extreme emotional tension, intense determination, or desperation, and often remain contracted for abnormally long periods.

Other Factors

  • Prolonged over-stretching during a dental procedure
  • Immobilisation of the mandible in the closed position (head halter during continuous neck traction, or jaw wiring)
  • Reflex muscle contraction from any chronic infection or inflammation — chronic pulpal or periodontal inflammation and TMJ arthropathy are frequent causes of masticatory muscle TrP activation that can persist after the inciting cause has resolved. This is frequently overlooked, resulting in unnecessary endodontic treatment or extraction, or a persistent search for why TMJ treatment has failed.
  • Satellite TrP activation: Masseter TrPs are often satellite TrPs activated and perpetuated by key TrPs in the sternocleidomastoid or upper trapezius. Appropriate treatment of the key TrPs often makes direct masseter treatment unnecessary.

Clinical Examination

Postural Assessment

Begin with assessment of anterior head position as described in Pain:TMJ_Screening_Examination and Muscle:Sternocleidomastoid. Forward head posture is a primary perpetuating factor and must be identified and addressed.

Mandibular Range of Motion

Normal interincisal opening is 36–44 mm. A practical bedside test:

  • Patient should be able to pass two or even three of their own knuckles between the upper and lower teeth. Very few patients with active masseter TrPs can pass three knuckles.
  • A more critical test: the distal phalanges of the first three fingers placed between the incisor teeth.

Masseter TrPs can cause significant restriction of vertical opening. Observe the path of opening and closing:

  • Unilateral TrPs tend to deviate the mandible toward the affected side
  • Deviation is most apparent when the patient slowly opens and closes — similar in appearance to unilateral TMJ internal derangement

For the full mandibular opening assessment and TMJ screening protocol see Pain:TMJ_Screening_Examination.

Note on differential for restricted opening:

  • Anterior disc displacement and postoperative trismus can be activated by the medial pterygoid
  • Temporalis TrPs limit opening minimally
  • Masseter TrPs produce the most significant restriction of all masticatory muscles

Trigger Point Examination

Superficial Layer — Pincer Palpation

The preferred technique for the superficial masseter is pincer palpation with one digit inside the mouth and one outside:

  1. Place one gloved finger inside the mouth along the buccal mucosa overlying the masseter
  2. Place the thumb or index finger of the same hand on the outer surface of the cheek
  3. Compress the muscle between the two fingers and roll through the muscle belly

If the muscle is difficult to locate, ask the patient to bite gently on a rubber block or cork — this contracts the masseter, making the muscle belly more palpable.

A taut band with exquisite tenderness identifies TrP sites. Tenderness is enhanced with wider mouth opening to take up any slack in the muscle fibres.

Important distinction:

  • Tenderness identified from the outside finger alone (flat palpation at the attachment) indicates enthesopathy — not primary TrP tenderness. Enthesopathy results from sustained tension at the attachment of muscle fibres.
  • Tenderness at the gonial angle on flat palpation is associated with bruxism.

Deep Layer — Flat Palpation

Deep layer TrPs are located more effectively by external flat palpation:

  • Against the posterior portion of the ramus of the mandible
  • Along the base of the zygomatic buttress

Tinnitus provocation test: Pressure on a TrP in the upper posterior portion of the deep layer may activate unilateral tinnitus. This confirms deep masseter TrP involvement as the tinnitus source.

Vascular Entrapment — Infraorbital Puffiness

The maxillary vein emerges between the masseter and the mandible and may be entrapped by masseter TrPs. The pterygoid venous plexus, which empties primarily into the maxillary vein, lies between the temporalis and lateral pterygoid muscles and between the two pterygoid muscles. It drains:

  • The temporalis muscle via the deep temporal vein
  • The infraorbital region via the orbital vein

The increased firmness of taut bands from masseter TrPs may restrict venous flow from the infraorbital subcutaneous tissues. This engorgement of the orbital vein produces:

  • Puffiness ("bags") beneath the eye on the affected side
  • Narrowing of the palpebral fissure on the affected side

This narrowing can also be caused by satellite TrPs in the orbicularis oculi, which lies within the sternal division SCM TrP referral zone.

Clinical note on injection: The engorgement of the temporal vein and pterygoid plexus that results from this mechanism favours bleeding and ecchymosis after injection of TrPs in the temporalis muscle.

Differential Diagnosis

Condition Distinguishing features
Sinusitis Masseter TrPs refer pain to the maxilla described as "sinus pain" — confirm by palpating TrPs and reproducing the pain; true sinusitis has fever, purulent discharge, and radiographic changes
Odontogenic toothache Masseter or temporalis TrPs can refer pain to teeth mimicking sensitivity to all stimuli; prolonged pain to thermal stimulus suggests pulpitis; sensitivity to percussion and pressure suggests apical inflammation of the periodontal ligament
TMJ internal derangement Deep masseter TrPs mimic TMJ rheumatic disease; mandibular deviation on opening mimics disc displacement — distinguish by TMJ palpation (see Pain:TMJ_Screening_Examination)
Tinnitus of neurological origin Masseter tinnitus is unilateral, low roaring, not associated with hearing loss or vertigo; bilateral tinnitus suggests systemic cause; tinnitus with hearing loss may respond to vitamin B12
Tension headache Masseter TrPs contribute to tension-type headache — often the primary unrecognised source
Trismus from systemic cause Trismus is typical for tetanus or Morgagni syndrome of malignant tumour; cellulitis of adjacent tissues causes masseteric spasm; medial pterygoid spasm from cellulitis of the pterygomandibular space; temporalis spasm from cellulitis in the infratemporal fossa — jaw opening is painful due to these spasms
Orbital puffiness Masseter TrP venous entrapment produces infraorbital puffiness and palpebral fissure narrowing — distinguish from systemic causes of periorbital oedema

Treatment

Trigger Point Release

  • Spray and stretch: Vapocoolant spray applied over the cheek and referred pain zone; passive stretch into contralateral lateral excursion and jaw opening
  • Ischemic compression: Pincer grip sustained pressure on the taut band until release
  • Intraoral massage: Direct intraoral pressure on the superficial layer from inside the cheek
  • Dry needling: Superficial and deep layers approached separately

Occlusal and Habit Modification

Elimination of parafunctional habits is essential for long-term resolution — see Abusive Oral Habits.

Address Key TrPs First

If SCM or upper trapezius TrPs are present, treat these key TrPs before addressing masseter directly — satellite TrP resolution often follows without further intervention.

Satellite Trigger Points

  • Temporalis — primary synergist; commonly co-active
  • Medial pterygoid — primary synergist; commonly co-active
  • Orbicularis oculi — satellite from masseter venous entrapment zone; also satellite of SCM sternal division
  • Sternocleidomastoid — key TrP that activates masseter as satellite

References

  • Travell JG, Simons DG. Myofascial Pain and Dysfunction: The Trigger Point Manual, Volume 1: The Upper Half of Body. 2nd ed. Baltimore: Williams & Wilkins; 1999. Chapter 8.
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