Masseter

1. Overview

The masseter is one of the main muscles of mastication and is among the strongest muscles in the human body relative to its size. Located in the lateral jaw region, it plays a critical role in chewing by elevating the mandible (lower jaw). The masseter works in coordination with the temporalis, medial pterygoid, and lateral pterygoid muscles to facilitate complex mandibular movements.

2. Location

The masseter is located on the lateral aspect of the face and jaw:

• Superficially: Lies just beneath the skin and subcutaneous tissue over the posterior cheek.

• Anteriorly: Bordered by the buccinator muscle and parotid duct.

• Posteriorly: Lies near the parotid gland and the mandibular ramus.

• Deep to: The platysma and skin.

3. Structure

The masseter is a thick, quadrangular muscle composed of superficial and deep parts:

• Origin:

  • Superficial part: Zygomatic process of the maxilla and lower border of the anterior two-thirds of the zygomatic arch.

  • Deep part: Posterior third of the zygomatic arch and deep surface of the arch.

• Insertion:

  • Superficial fibers: Angle and lower lateral surface of the ramus of the mandible.

  • Deep fibers: Upper half of the ramus and the coronoid process of the mandible.

• Innervation: Mandibular division (V3) of the trigeminal nerve via the masseteric nerve.

• Blood supply: Masseteric artery (a branch of the maxillary artery).

4. Function

The primary actions of the masseter include:

• Elevation of the mandible: Closes the jaw during mastication (chewing).

• Protrusion: The superficial part assists in moving the jaw forward.

• Stabilization: Helps maintain tension in the jaw during clenching or holding objects between the teeth.

5. Physiological role(s)

Beyond its role in movement, the masseter contributes to:

• Mastication efficiency: Provides significant bite force, enabling effective food breakdown for digestion.

• Facial contouring: Defines the angle of the jaw and contributes to facial shape and aesthetics.

• Speech and articulation: Stabilizes the mandible during speech and phonation.

• Protective tension: Activates during reflex jaw closure to prevent damage from foreign objects or trauma.

6. Clinical Significance

The masseter is frequently involved in dental, musculoskeletal, and aesthetic conditions:

• Temporomandibular joint disorders (TMD):

  • Masseter hyperactivity or spasm can contribute to jaw pain, clicking, and restricted movement.

• Bruxism (teeth grinding):

  • Involuntary clenching or grinding of teeth, especially at night, can lead to masseter hypertrophy and muscle fatigue.

• Masseteric hypertrophy:

  • Can result in a square jaw appearance; often seen in chronic bruxism or in athletes who use jaw muscles intensively.

  • Botulinum toxin (Botox) is sometimes used to reduce hypertrophy for both functional and cosmetic reasons.

• Mandibular fracture stabilization:

  • The masseter muscle exerts significant force on the mandible and can displace fractures, influencing surgical planning.

• Trigger points and myofascial pain:

  • Chronic tension or overuse may lead to localized muscle knots, pain on palpation, and referral pain patterns.