CPD: Understanding Masseter Muscle Anatomy

Understanding the Anatomy of the Muscles of Mastication

Dr Rehanna Beckhurst describes the anatomy of the muscles of mastication, as well as important considerations when administering botulinum toxin injections

With society’s levels of stress and anxiety increasing, the number of stress-related teeth clenching and grinding cases are on the rise.1 However, these symptoms are just clinical manifestations of a more complex problem called temporomandibular disorders (TMDs).2 They are often accompanied by enlargement or hypertrophy of the masseter muscle, giving the lower third of the face a squared appearance. This is an unwanted aesthetic appearance for many female patients, leading to an increased demand for an aesthetic treatment which can aid facial slimming.3 When administering botulinum toxin for facial slimming or treating temporomandibular disorders, knowledge of anatomy and topographic anatomy of the area is paramount. This article will describe the anatomy and function of the muscles of mastication, as well as the anatomical relations of them.

Temporomandibular disorders Aesthetically undesired hypertrophy of the masseter muscle is often caused by temporomandibular disorders which are one of the leading causes of orofacial pain.1 They involve dysfunction of the temporomandibular joint and masticatory muscles, limitations of jaw movement and pain. TMDs are divided into muscle origin and joint origin. Symptoms include muscle and jaw pain, pain when chewing, referred headache, earache and neck pain.4 The aetiology of TMDs is multifactorial and triggers could be of a biologic, environmental, social, emotional and cognitive nature, such as: malocclusion or intra-articular abnormalities, anxiety, stress, diet or parafunctional habits.5

Treatment for TMDs includes conservative treatment, subdivided into pharmacological (anti-inflammatory drugs, muscle relaxants, anxiolytics, and antidepressants in severe cases), non-pharmacological (explanation, self-management, exercise, hot and cold compresses, splint therapy) and surgical. The first report on using botulinum neurotoxin type A (BoNT-A) for masseter hypertrophy was published in 1994, as an alternative treatment option for TMD, which lies on the borderline between conservative and surgical treatments.5,7 BoNT-A is a purified exotoxin of clostridium botulinum and is a neuromodulator, which affects neuromuscular junction by inhibiting production of acetylcholine, disabling muscle contraction. It has been used to treat various neuromuscular disorders since 1980.8 There is a need for more consensus in robust injection protocols and clinical studies to assess the efficacy of this treatment. Knowledge of the anatomy is paramount when injecting this area with multiple advanced physiological functions. Muscles of mastication The complexity of anatomical structures around the jaw and mouth is called the stomatognathic system, and its functions include mastication, deglutition and speech.9 Mastication is the most ancestral function with a higher specialisation.9 Mastication has been defined as a rhythmic, highly coordinated neuromuscular function that involves, in each of its acts, all components of the stomatognathic system, mainly executed by the masticatory muscles.10 This mechanism is achieved by perfect coordination of the masticatory muscles and by nervous control which allows for this exertion of precise and strong occlusal forces.9 Therefore, it is extremely important to understand the anatomy and the relations between the anatomical structures when injecting this area. Also, anatomical variations should be considered, therefore thorough clinical and functional patient assessment is paramount. The primary muscles of mastication are the temporalis, masseter, medial pterygoid and lateral pterygoid muscles.11 The temporalis muscle is located in the temporal fossa, the masseter muscle found in the cheek area, and medial and lateral pterygoid muscles situated in the infratemporal fossa.12 All muscles attach to the mandible and produce movements of it at the temporomandibular joint. Such movements are:13

• Protrusion/protraction (forward movement of the mandible) – mainly lateral pterygoid, with the help of the medial pterygoid • Retraction (backward movement of the mandible) – posterior fibres of temporalis muscle • Elevation or adduction (closing the mouth) – temporalis, masseter and medial pterygoid • Depressions (opening the mouth) – lateral pterygoid and accessory muscles, gravity • Rotation (side-to-side movements of the mandible) – lateral and medial pterygoids

The accessory muscles to aid masticatory function are the buccinator (helps to press cheeks against posterior teeth to keep food within the oral cavity) and strap muscles, which can be subdivided into suprahyoid muscles (digastric muscle, mylohyoid muscle and geniohyoid muscle) and infrahyoid muscles (the sternohyoid, sternothyroid, thyrohyoid and omohyoid muscles).14,15 Their primary function is to raise and depress the hyoid bone and larynx, and also assist in the depression of the mandible against the opposing force.11 Platysma also assists the depression of the mandible against resistance.14 The four main muscles of mastication are supplied by the motor fibres of the mandibular nerve deriving

from the trigeminal nerve (CN V), and the arterial supply derives from the branches of the maxillary artery, a branch of the external carotid artery.11

Temporalis muscle The temporalis muscle is a large flat fan-shaped muscle, originating from the temporal fossa. It is divided into two parts: the anterior fibres run almost vertically, while the posterior fibres run almost horizontally. Both parts then connect closer to form a large tendon as they run down medially from the zygomatic arch to the ascending ramus of the mandible, inserting in its anterior and lateral surfaces, while some fibres may continue into the masseter muscle. It is innervated by the deep temporal branches of the mandibular nerve (CN V3), while blood supply comes from the deep temporal branches of the maxillary artery and middle temporal branches of the superficial temporal artery.12 The main role of the temporalis muscle is the adduction (elevation) of the mandible, but because of radial orientation of its fibres, it contributes to protrusion and even retraction and takes part in the grinding action by moving the mandible from side to side.15

Medial pterygoid muscle The medial pterygoid muscle (also called the internal pterygoid muscle) originates from the pterygoid fossa and from the lateral lamina of the sphenoid process of the sphenoid bone. It runs medially down the ramus of the mandible and inserts into the pterygoid tuberosity on the medial side of the mandibular angle. It has two parts: deep and superficial.15 Together with the masseter muscle on the outside and the medial pterygoid on the inside of the ramus, they form a sling to elevate (adduct) the mandible. The medial pterygoid is also involved in side-to-side movement during chewing and grinding, as well as protrusion of the mandible. It can be palpated medially from the ramus of the mandible both intra and extra orally.15 The innervation of the medial pterygoid muscle is supplied by the medial pterygoid branch of the mandibular nerve (CN V3). Its blood supply stems from the pterygoid branches of the maxillary artery, as well as the buccal and facial arteries.12

Lateral pterygoid muscle The lateral pterygoid muscle (also called the external pterygoid muscle) is located deep from the temporalis and masseter muscles, between the sphenoid bone and temporomandibular joint. This fan-shaped muscle has two heads: the superior, smaller head originates from the greater wing of the sphenoid bone and most of it inserts into the articular disc, with additional insertions often found on the condylar process. The inferior, larger head originates from the external surface of the lateral lamina of the pterygoid process of the sphenoid bone and inserts into the condylar process of the mandible. The muscle lays almost horizontally.15 The lateral pterygoid muscle is the only muscle in the mastication group that opens the mouth; without it, the jaw opening would rely mainly on gravitational forces.16 Due to different innervation between the heads, both parts of the muscle can move independently and perform different functions at the same time. The innervation is supplied by the branches of the mandibular nerve (CN V3). The superior head and lateral fibres of the inferior head are supplied by the buccal nerve, a branch of the mandibular nerve (CN V3), while the medial part of the inferior belly is supplied directly by the mandibular nerve (CN V3).12 Vascularisation is supplied by the pterygoid branches of the maxillary artery and the ascending palatine branch of the facial artery.12 When contracting bilaterally, the lateral pterygoid muscle causes translation and rotation in both temporomandibular joints.17 The inferior head pulls the condyle of the mandible forward, resulting in rotation of the condyle against the inferior surface of the articular disc. Simultaneously, the superior head pulls the articular capsule and disc in the same direction to cushion the movement of the condylar head. The result of these simultaneous movements is an anterior translation of the disc and the condyle, with simultaneous rotation of the condyle, which is clinically seen as protrusion and depression of the mandible.12 The movement of opening the jaw is then completed by alternate contraction of the digastric and the geniohyoid muscles. This movement is limited by the retro-discal fat pad within the temporomandibular joint, to limit protrusion and depression. During the jaw closing, the same translation and rotation occurs within the temporomandibular joint, but only in the opposite direction. The inferior head of the muscle contracts to smoothen the posterior translation of the articular disc and condylar head, counteracting the net pull of the masseter and temporalis muscles which draw the mandible posteriorly when closing the mouth.12 When the inferior head contracts unilaterally, the mandibular condyle rotates anteromedially, moving the jaw side-to-side. This action occurs in synergy with contraction of the medial pterygoid muscle on the same side, and is clinically seen as swinging the jaw to the opposite side when chewing/grinding on one side of the mouth.12 Although palpating the lateral pterygoid muscle is a standard diagnostic procedure for diagnosing TMD, some studies suggest that this muscle is not palpable intraorally or extraorally and should not be performed as it is the medial pterygoid muscle which is palpable instead.12

Superficial temporal a.

Middle meningeal a.

Maxillary a. Buccal a. and n. (CN V3)

Deep temoral aa.

Masseteric a.

Medial pterygoid n. (CN V3)

Digastric posterior belly

Inferior alveolar n. (CN V3)

Digastric anterior belly Figure 1: Innervation, blood supply and accessory muscles of mastication20

Zygomaticofacial n. (CN V2)

Infraorbital n. (CN V2) Infraorbital a. Mandibular n. (CN V3) Mental a. Mental n. (CN V3)

Relations and anatomical structures The lateral pterygoid muscle makes up the medial wall of the infratemporal fossa and is contained within the masticator space, bounded by the deep cervical fascia, which splits into the superficial and deep parts at the level of the mandibular ramus. These two parts of the cervical fascia cover the masseter muscle and the deep part of the medial pterygoid muscle.16 The masticator space also contains the tendon of the temporalis muscle, masseter and medial pterygoid, and the pterygoid venous plexus. Within this space, the lateral pterygoid lies deep to the superficial part of the medial pterygoid and the temporalis tendon, but it lies superficial in relation to the deep part of the medial pterygoid, sphenomandibular ligament, middle meningeal artery and mandibular nerve (CN V3).16 Temporal and masseteric branches of the mandibular nerve (CN V3) cross the upper border of the muscle, and the lingual and inferior alveolar nerve cross the lower border. The split between the superior and inferior heads of the lateral pterygoid muscle is where the maxillary artery and buccal nerve (branch of the mandibular nerve CN V3) pass through.15

Masseter muscle The masseter muscle is a strong bulky muscle composed of two layers: the superficial, large layer originates from the maxillary process of the zygomatic bone and the anterior two-thirds of the zygomatic arch, running down to insert into the masseteric tuberosity on the lateral wall of the angle and the lower part of the ascending ramus of the mandible.12 Its deep part originates from the medial surface and inferior border of the zygomatic arch, then runs vertically down and inserts into the upper part of the ascending ramus and coronoid process of the mandible. A study by Lee et al. in 2017 discovered a deep inferior tendon located in the deep superficial part of the masseter muscle in all 44 cadaver specimens examined.11 This tendon may block toxin diffusion from the deep part to the superficial part, therefore, it may stay unaffected and overcompensate for the affected deep part post-toxin injections.17 The masseter muscle is innervated by the masseteric nerve from the mandibular nerve (CN V3), and vascularisation is derived from the masseteric artery, a branch of the maxillary artery.12 The main function of the masseter muscle is the elevation of the mandible when closing the mouth, with a very small contribution to the protrusion function.16 Parafunctional activity of the masseter muscle often leads to its hypertrophy, however, other factors such as parotid gland disease, dental problems, compensatory-occlusal hypertrophy, masseter tumours and neoplasms must be ruled out. With suspected parafunction accompanying hypertrophy, the patient should be referred to a dentist for occlusion analysis and splint therapy as the first choice of treatment.18

Galea aponeurotica

Superior auricularis

Temporalis

Anterior auricularis

Occipitofrontalis occipital belly

Posterior auricularis Occipitofrontalis frontal belly

Masseter Muscle

Buccinator

Platysma

Risorius Orbicularis oculi

Procerus

Levator labii superioris alaeque nasi muscle

Nasalis

Levator labii superioris

Orbicularis oris

Zygomaticus major and minor

Depressor labii inferioris

Mentalis

Figure 3: Anatomical relations of the masseter muscle20 Although current literature suggests the safety and efficacy of masseter treatment with toxins, there is a lack of treatment protocols and guides for safe injections

Depressor anguli oris

Relations and anatomical structures Masseteric fascia covers the entire superficial aspect of the masseter muscle. This fascia is thin, but very strong. Anterolaterally, below the zygomatic bone, the duct of the parotid gland crosses over the muscle. The superficial part of the parotid gland itself is also located superficially to the masseter, laying posteriorly to the muscle, but covering almost the entire posterior part of it.6 Along with the parotid gland, a few more structures can be found over the lateral side of the muscle:22 • Terminal branches of the facial nerve • Facial vein • Facial artery • Risorius muscle • Zygomaticus major muscle

The inner surface of the masseter muscle is almost completely covered by the mandibular ramus, and they form a space called submasseteric space, part of the facial space. This space is where a submasseteric abscess would form, and it is important to differentiate and diagnose any odontogenetic infections.15 Deep posterior to the inner part of the masseter muscle, the temporal muscle is found. Both muscles partially cross over. Anterior to the muscle, the buccinator muscle is located. The risorius muscle is also located anteriorly, laying superficially over the buccinator, and both muscles insert into the modiolus. These are two important muscles to consider when injecting the masseter muscle.15 The above-mentioned duct of the parotid gland penetrates the fibres of the buccinator muscle to find its way to the inside of the oral cavity and finally open onto the inner side of the cheek.20

Treating with botulinum toxin According to numerous studies, masseter injections with toxin are relatively safe and have a high efficacy profile,however, there are complications associated with these injections.2,7,21-25 It is important to note that only injection site-related complications are listed here to demonstrate the importance of anatomical knowledge.

• Loss of full smile/asymmetric smile can happen due to unilateral toxin diffusion and relaxation of the risorius and zygomatic major muscles.

Injecting too anterior, too high and superficially should be avoided.7

Sunken lateral cheeks (infra-zygomatic hollows) due to toxin affecting the upper parts of the masseter and administering the excess dose, leading to its atrophy. Injecting too high should be avoided7 • Difficulty in opening the mouth due to toxin diffusion into lateral pterygoid muscle. In this case, injecting too high should also be avoided19 • Xerostomia – toxin affecting the parotid gland when injected too posterior10 • Neuropraxia – mandibular nerve damage when injected too inferior. In some anatomical variations, the mandibular nerve may lay too superior on the mandibular margin10 • Paradoxical bulging due to excessive compensation of the superficial layer of the masseter muscle. As described above, the deep inferior tendon may prevent toxin diffusion into the superficial part of the masseter and lead to its overcompensation and bulging. Injecting both deep and superficial parts will help to prevent this complication18

Based on anatomical considerations, for safe performance of injections into the masseter when treating hypertrophy for facial slimming, the injector should stay 1cm from all borders. The following guide may be useful:19

• Superior: Ear lobe to cheilion • Inferior: 1cm above the inferior margin of the mandible • Posterior: 1cm from tragus • Anterior: 1cm behind the anterior border of the masseter muscle; this can be felt by asking the patient to clench and palpating the border7

Know your anatomy When treating muscle hypertrophy, if temporomandibular disorder is suspected, a referral to a dental professional should be made for thorough diagnosis and splint therapy, as a first choice of treatment. Although current literature suggests the safety and efficacy of masseter treatment with toxins, there is a lack of treatment protocols and guides for safe injections. Knowledge of anatomy and its variations is paramount when administering any aesthetic treatment.

Dr Rehanna Beckhurst is a cosmetic dental surgeon with a special interest in aesthetic medicine. Dr Beckhurst has been in aesthetics for more than 10 years. She holds a Level 7 qualification and a postgraduate certificate in Aesthetic Non-Surgical Interventions through City of London Dental School. She is a co-owner of ifab clinic in Barnstaple. Qual: MBA, PgDip, CoLDS, ANSI

Test your knowledge! Complete the multiple-choice questions and go online to receive your CPD certificate!

Questions

1. The masseter is the strongest muscle of mastication. What are the other three main muscles?

2. What are the three muscles that elevate the mandible (close the mouth)?

3. What is the main muscle that depresses the mandible (opens the mouth)?

4. Which artery supplies the arterial blood to the muscles of mastication?

5. Asymmetric smile is one potential complication after injecting the masseter muscle with botulinum toxin type-A. What causes it?

Answers

a. Temporalis, lateral and medial pterygoids b. Temporalis, buccinator and medial pterygoid c. Medial and lateral pterygoid and platysma d. Temporalis, buccinator and platysma

a. Temporalis, masseter and lateral pterygoid b. Temporalis, masseter and medial pterygoid c. Masseter, medial and lateral pterygoid d. Platysma, masseter and temporalis

a. Lateral pterygoid muscle b. Platysma c. Masseter d. Medial pterygoid

a. Superficial temporal artery b. Facial artery c. Zygomaticofacial artery d. Maxillary artery

a. Insufficient dose b. Toxin diffusion into risorius and zygomatic major muscle c. Dose too high d. Toxin diffusion into buccinator muscle

Answers: A,B,A,D,B

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