Masseter

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Description

The masseter a primary muscle of mastication and it is one of the strongest of human muscles. It is responsible for the elevation of the mandible and some protraction, and also the chewing movement of the mandible at the temporomandibular joint (TMJ). It is a powerful superficial quadrangular muscle that originates from the zygomatic arch and inserts along the angle and lateral surface of the mandibular ramus. In this article, we will explore the anatomy, origin, insertion, and function of the masseter muscle, as well as its role in jaw pain and dysfunction, and therapeutic interventions.

Origin

The masseter muscle originates from the zygomatic arch and maxillary process of the zygomatic bone and it has a superficial and deep part:

• The superficial portion of the masseter muscle originates from a thick aponeurosis on the temporal process of the zygomatic bone and the anterior two-thirds of the inferior border of the zygomatic arch.
• The deep portion of the masseter muscle originates from the posterior third of the inferior border of the zygomatic arch and the zygomatic process of the maxilla.

The fibers of the superficial and deep heads are continuous at their insertion. The masseter muscle has a quadrangular shape appearance on gross examination due to its origins and insertions.

Insertion

The muscle fibers converge inferiorly, forming a tendon that inserts onto the outer surface of the mandibular ramus and coronoid process of the mandible. The intermediate and deep muscle fibers of the masseter muscle function to retract the mandible, while the superficial fibers function to protrude the mandible.

Nerve

The masseter muscle is innervated by the masseteric nerve, which is a branch of the mandibular nerve (V3) of the trigeminal nerve. The mandibular nerve is the third branch of the trigeminal nerve and carries both sensory and motor fibers. The masseteric nerve crosses the mandibular notch to reach the masseter muscle. The motor fibers of the masseteric nerve supply the masseter muscle, allowing it to contract and perform its primary function of elevating the mandible and some protraction. The sensory fibers of the mandibular nerve provide sensation to the lower teeth, gums, and lip, as well as the skin of the chin and lower jaw.

Artery

The masseter muscle primarily receives its vascular supply from the masseteric artery, which is a a small vessel, branch of the maxillary artery. The maxillary artery is a branch of the external carotid artery and supplies deep structures of the face. The masseter muscle also receives blood supply from the facial artery.

Treatment

There are several conventional interventions available for physiotherapy of the masseter muscle dysfunctions, including therapeutic exercise and manual therapy. Therapeutic exercise involves specific exercises that aim to increase the flexibility of the muscles involved in jaw movement; manual therapy aims ischaemic compression by applying pressure to a specific point in the muscle to decrease muscle tension and alleviate pain. In addition to these conventional methods, some unconventional interventions are also possible, such as dry needling. Dry needling involves inserting a thin needle into the trigger point of the muscle to relieve pain and improve function. All techniques can be combined to achieve the best outcomes for patients with masseter muscle dysfunctions.

A randomized controlled trial compared the effects of exercise combined with ischaemic compression and exercise alone on patients with temporomandibular disorders (TMDs). Fifty patients were randomized into two groups, with Group 1 receiving ischaemic compression in addition to exercise, and Group 2 receiving exercise alone. Results showed that both groups had similar effects on range of motion, pain, pain pressure threshold, and functionality at weeks 1 and 4, except for the painless mouth opening and maximum assisted mouth opening values, which were higher in Group 1 at week 1. The study concludes that exercise combined with ischaemic compression and exercise alone have similar effects on patients with TMDs.

A RCT study from 2010 aimed to investigate the effects of dry needling on the masseter muscle trigger points in patients with temporomandibular disorders (TMD). The study involved 12 female patients diagnosed with myofascial TMD, who received either deep dry needling or sham dry needling at the most painful point on the masseter muscle trigger point. The results of the study showed that deep dry needling led to significant improvements in pressure pain threshold, mandibular condyle, and pain-free active jaw opening. The researchers concluded that dry needling can be an effective treatment for myofascial TMD, as it improves the pain threshold and jaw function in patients.