The masseter is the most powerful jaw-closing muscle per unit cross-sectional area in the body, capable of generating bite forces up to 700 Newtons in the molar region. It is the principal muscle of mastication and its hypertrophy from bruxism or a parafunctional chewing habit produces the characteristic squaring of the lower face that can be addressed medically or surgically. Its surface anatomy defines the posterior boundary of the face anterior to the parotid gland.
| Origin | Lower border of the zygomatic arch and zygomatic process of the maxilla; Inner surface of the zygomatic arch |
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| Insertion | Lateral surface of the ramus and angle of the mandible and the coronoid process |
| Nerve Supply | Masseteric nerve (branch of the mandibular nerve, CN V3) |
| Blood Supply | Masseteric artery from the maxillary artery |
| Actions | Elevation of the mandible (closing the jaw); Assists in protrusion of the mandible (superficial head); Retracts the mandible (deep head) |
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The superficial and deep heads work together to generate the powerful molar bite force needed to grind food, with the superficial head also providing the forward protrusion component important for incisal biting.
Masseter hypertrophy from bruxism is treated with botulinum toxin injection that reduces the bulk and bite force while temporarily relieving temporomandibular joint symptoms. Parotid gland tumours present anteriorly to the masseter in the parotid bed and must be distinguished from masseter hypertrophy by imaging. Trismus following mandibular fracture or third molar extraction is produced by masseter and medial pterygoid spasm and contracture.
The masseter is the prominent muscle palpable over the angle of the jaw that becomes hard as wood during forceful clenching. Its hypertrophy or tenderness during palpation with jaw clenching is a key sign in temporomandibular joint dysfunction assessment.
Bilateral masseter enlargement from bruxism or parafunctional habits producing jaw pain, temporal headaches, and facial widening at the angles, managed with splints, botulinum toxin injection, or surgical reduction.
Restricted jaw opening from masseter and medial pterygoid spasm following trauma, infection, or surgery, managed with physiotherapy, muscle relaxants, and jaw mobilisation.
