Masticatory muscles. Anatomy. Diagnostic Methods. Muscle Pathologies.

The maxillofacial region hosts two key muscle groups: the masticatory muscles, which control jaw movement, and the facial muscles, which contribute to expressions. Masticatory muscles are essential not only for chewing but also for speaking, swallowing, and even singing, while shaping the jaw and facial skeleton.

The masticatory muscles fall into three functional categories:

1. Elevators of the mandible, including the masseter, temporalis, and medial pterygoid muscles, which close the jaw with precision and strength.
2. Depressors of the mandible, such as the mylohyoid, geniohyoid, and digastric muscles, uniquely attached to two movable structures—the jaw and the hyoid bone.
3. Lateral pterygoid muscles, which protrude the mandible forward during bilateral contraction and shift it sideways with unilateral activation.

This intricate interplay between muscle groups enables the mandible to perform complex and coordinated movements. Active contractions of one set of muscles are balanced by the controlled relaxation of their antagonists, ensuring smooth and precise jaw mechanics crucial for oral function.

Understanding the role of masticatory muscles in Temporomandibular Disorders (TMD) is critical for delivering effective treatments. Masticatory muscle dysfunction is a key factor in TMD-related pain, occlusal imbalances, and mandibular mechanics – conditions you encounter regularly in dental practice. Join the course "Temporomandibular Disorders And Orofacial Pain: Up-To-Date Treatment Protocols" and be equipped with advanced knowledge and methodologies to address the complexities of TMD and enhance patient outcomes.

Masseter Muscle 

The masseter muscle is a powerful and complex structure vital for mastication. It consists of two layers: the superficial layer, with obliquely aligned fibers, attaches to the lower edge of the zygomatic arch, while the deep layer, with vertically oriented fibers, originates internally and integrates with the temporalis fascia. These layers insert broadly, from the mandibular angle's masseteric tuberosity to the posterior lateral surface of the ramus, extending near the second molar.

The masseter's deep portion, sometimes referred to as the zygomaticomandibular muscle, originates from the inner surface of the zygomatic arch and connects to the coronoid process. This portion may function as an antagonist to the posterior temporalis muscle while synergizing with the lateral pterygoid muscle. The upper and lower regions of the masseter are distinctly separated near the posterior upper section.

Surrounding muscles and tissues play complementary roles. The masseter is partially covered by the platysma and risorius muscles, which contribute to facial expressions. The platysma activates during firm clenching and connects to the orbicularis oris, while the risorius responds to emotions. Additionally, the lower third of the masseter lies approximately 2–3 cm anterior to the sternocleidomastoid muscle, which may also contract during clenching. Part of the masseter is enveloped by the parotid gland tissue.

Functionally, the masseter facilitates strong jaw closure. Bilateral contraction lifts the mandible, while unilateral activation shifts it outward and toward the active side. The masseter also aids in mandibular protrusion and stabilizes movements. Innervated by the masseteric branch of the trigeminal nerve (cranial nerve V), it plays an integral role in mastication and mandibular mechanics.

Temporalis Muscle 

The temporalis muscle, a fan-shaped structure, originates from the temporal fossa and the squamous portion of the temporal bone. Its fibers converge into a strong tendon that passes beneath the zygomatic arch to attach to the anterior border and inner surface of the coronoid process of the mandible, extending along the anterior edge of the ascending ramus nearly to the third molar.

This muscle can be divided into three functional regions based on fiber orientation. The anterior fibers slant forward, primarily aiding in clenching, often working synergistically with the masseter. The middle fibers run vertically and contribute to the elevation of the mandible, while the horizontally aligned posterior fibers retract or reposition a protruded mandible backward, acting as antagonists to the masseter during such movements. When all fibers contract simultaneously, they work together to elevate the mandible, ensuring precise and controlled jaw mechanics.

Innervated by the temporal branches of the mandibular division of the trigeminal nerve (cranial nerve V), the temporalis muscle plays a vital role in mastication and mandibular positioning.

Medial Pterygoid Muscle

The medial pterygoid muscle originates from the medial surface of the lateral pterygoid plate and the pterygoid fossa, extending to the palatine bone. Its insertion spans the medial surface of the mandibular angle and the ramus, reaching the area around the mandibular foramen.

Functionally, the medial pterygoid plays a critical role in elevating the mandible, assisting in protrusion, and facilitating lateral positioning. Bilateral contraction lifts and protrudes the mandible, while unilateral activation shifts it toward the opposite side, enabling precise lateral movements. This muscle is innervated by a branch of the mandibular division of the trigeminal nerve (cranial nerve V), underscoring its importance in coordinated mastication and jaw mechanics.

Through its anatomical structure and functionality, the medial pterygoid muscle operates as a key stabilizer and driver of mandibular dynamics.

Lateral Pterygoid Muscle

The lateral pterygoid muscle, with its two distinct heads, is fundamental to mandibular movements, particularly protrusion and stabilization of the temporomandibular joint (TMJ). The inferior head originates from the outer surface of the lateral pterygoid plate, while the superior head arises from the greater wing of the sphenoid bone. Both heads converge at insertion points on the anterior neck of the mandibular condyle, the TMJ capsule, and the front of the articular disc.

Each head performs specific roles during jaw dynamics. The superior head is primarily active during jaw-closing actions such as chewing, clenching, and swallowing. It ensures proper alignment and stabilization of the condylar head and the articular disc against the articular eminence during mandibular closure. In contrast, the inferior head engages during jaw opening and protrusion, facilitating the condyle’s downward, forward, and contralateral movement. Together, the two heads enable protraction, depression, and contralateral abduction of the mandible, while also contributing to the joint’s overall stabilization.

​​When both lateral pterygoid muscles contract simultaneously, they rotate the condylar heads and propel the mandible forward. Unilateral activation shifts the mandible laterally. This coordinated activity is essential for precise mandibular movements during mastication and speech. The lateral pterygoid muscle is innervated by the mandibular branch of the trigeminal nerve (cranial nerve V).

Mandibular motion relies on the harmonious interplay of masticatory muscles. While one group actively contracts, its antagonists stretch, ensuring balanced and efficient movements essential for mastication.

Diagnostic Methods

There are several diagnostic methods used to evaluate the masticatory muscles, each offering insights into their function and potential issues. These methods help detect conditions like muscle hypertonia, hypotonia, dysfunction, or occlusal imbalances.

Modern diagnostic techniques such as electromyography (EMG), ultrasound diagnostics, and compressed air-based muscle function testing offer valuable insights into the state of the masticatory muscles. These methods can help identify hidden conditions like hypertonia and hypotonia, which may not only arise from specific jaw-related disorders but also as a result of occlusal imbalances or TMJ dysfunction. By examining the anatomy of the masticatory muscles from the perspective of kinematics and dynamics of the lower jaw, we can detect disturbances that could contribute to pathological processes. This enables timely diagnosis, differential diagnosis, and the formulation of a preliminary diagnosis.

Patients often seek treatment at dental clinics for complaints related to morphological and functional changes in the masticatory apparatus. These issues may involve disturbances in the kinematics of the lower jaw, which are often the underlying causes of various TMJ and masticatory muscle disorders. The etiology and pathogenesis of these disorders can be influenced by factors such as articulation and occlusion abnormalities, pathological processes and diseases of other components of the masticatory apparatus, psychological disorders, endocrine imbalances, metabolic and infectious diseases, and trauma (e.g., microtraumas, contusions, or fractures of the lower jaw). It is important to note the interrelationship of these etiological and pathogenetic factors. However, a unified medical-economic standard for outpatient care of patients with TMJ and masticatory muscle diseases is still lacking, leading to justified patient complaints and concerns. Therefore, optimizing the diagnosis and treatment of these conditions remains a significant issue in dental practice.

Neglecting or underestimating the functional aspects of the masticatory apparatus—such as the central relationship, occlusal interactions, and individual dynamic characteristics in complex clinical cases related to TMJ pathology—can result in conflict situations and severe consequences for patients. This often manifests as difficulties in adapting to dental prostheses that do not align with the patient’s dental status or functional needs, ultimately impacting the effectiveness of treatment.

Diagnostic Modalities for Masticatory Muscle Dysfunction: Electromyography and Magnetic Resonance Imaging.
 

Electromyography (EMG)

Electromyography (EMG) is a cornerstone in assessing the neuromotor activity of masticatory muscles. By analyzing bioelectric activity, it provides insights into muscle coordination, symmetry, and functional capacity, aiding in the diagnosis of temporomandibular joint (TMJ) disorders and related dysfunctions.

Procedure and Methodology
EMG employs surface or needle electrodes to measure electrical signals during muscle activity. Surface cup electrodes are typically positioned at motor points to assess paired muscle activity bilaterally, ensuring a comprehensive evaluation of functional symmetry. For specific targets like the lateral pterygoid muscle, needle electrodes may be preferred for precision, despite their limitations during dynamic activities such as chewing.

To enhance diagnostic accuracy, EMG integrates functional tests including:

• Central occlusion testing to measure activity during jaw closure.
• Task-specific mastication to assess real-time functional activity.
• Resting jaw position to identify abnormal baseline muscle tension.

Findings in Dysfunction
Pathological changes observed via EMG in cases of TMJ disorders include:

• Asymmetry in paired muscle activity, indicative of neuromuscular imbalance.
• Altered activity ratios between agonist and antagonist muscles.
• Spontaneous resting activity, suggestive of chronic muscle tension.
  For instance, patients with distal occlusion may exhibit compensatory hyperactivity in suprahyoid muscles, coupled with reduced activity in the masseter and temporalis muscles. Recognizing these patterns aids in tailoring therapeutic strategies.

Magnetic Resonance Imaging (MRI)

MRI is a gold-standard imaging modality for evaluating the structural and functional integrity of masticatory muscles, particularly the medial and lateral pterygoid muscles. These muscles are deeply situated and challenging to assess through conventional methods, making MRI indispensable in diagnosing TMJ dysfunctions.

Role in TMJ Dysfunction Diagnosis
MRI captures detailed structural and pathological changes across the full length of the masticatory muscles, including:

• Morphology and Size: Variations in muscle dimensions linked to occlusal and intra-articular abnormalities.
• Localization of Pathology: Identification of hypointense linear zones correlating with clinical dysfunction.
• Dynamic Associations: Studies have shown the lateral pterygoid muscle’s anatomical variations to influence intra-articular TMJ conditions significantly.

MRI findings, such as those on T1-weighted images (T1WI) and gradient-echo (GRE) sequences, correlate with clinical symptoms and other imaging modalities like computed tomography (CT) and ultrasonography. These observations provide a basis for understanding disease progression and formulating a prognosis.

Muscle Pathologies in Facial Pain: Parafunction, Bruxism, and Musculoskeletal Dysfunction

Facial musculoskeletal pain represents a complex interplay of pathological muscle activity, structural imbalances, and psychogenic factors, often manifesting in conditions like muscle parafunction, bruxism, and associated pain syndromes. Understanding their etiology, pathogenesis, and management is essential for effective diagnosis and treatment.

1. Muscle parafunction

Muscle parafunction refers to abnormal, excessive, or habitual muscle activity unrelated to physiological function, such as clenching or grinding the teeth without mastication. This can overburden the masticatory muscles, leading to hypertonicity and the formation of myofascial trigger points (MTPs). These localized zones of muscle tension can elicit pain radiating across the face, head, and neck. Key symptoms include:

• Facial or jaw pain, particularly during stress or chewing.
• Restricted jaw mobility or deviation during opening and closing.
• Palpable muscle stiffness and tenderness.

Parafunctional behaviors, such as prolonged teeth clenching during concentration or sleep, exacerbate the mechanical strain on the masticatory system, predisposing patients to temporomandibular joint (TMJ) disorders.

2. Bruxism

Bruxism, a specific form of parafunctional activity, is characterized by involuntary grinding or clenching of teeth, often during sleep (sleep bruxism) or wakefulness (awake bruxism). This condition is influenced by stress, anxiety, hereditary predispositions, or sleep disturbances. Its effects include:

• Tooth Wear: Localized or generalized wear, affecting aesthetics and function.
• Muscle Hypertrophy: Particularly in the masseter and temporalis muscles, contributing to facial pain.
• TMJ Overload: Leading to inflammation, joint noises, or osteoarthritis.
• MTP Development: Pain radiating to adjacent areas, including the temples, cheeks, and jaw.

Advanced diagnostic tools, such as electromyography (EMG) and imaging, are vital for detecting bruxism and associated muscle strain.

3. Facial musculoskeletal pain

Facial musculoskeletal pain is often intertwined with parafunction and bruxism, compounded by structural asymmetries or systemic musculoskeletal issues, such as scoliosis or skeletal imbalances. Pathophysiological mechanisms include:

• Trigger Points and Chronic Pain: Chronic muscle overload creates MTPs, leading to pain in specific patterns:
  • Temporal pain is often associated with MTPs in the trapezius, temporalis, and sternocleidomastoid muscles.
  • Pain in the ear and TMJ area may result from dysfunction in the pterygoid and masseter muscles.
  • Orbital pain can be linked to MTPs in the orbicularis oculi, trapezius, masseter, and sternocleidomastoid muscles.
  • Cheek and dental pain in both the upper and lower jaws are commonly caused by MTPs in the masseter, lateral pterygoid, and trapezius muscles.
  • Throat pain is frequently related to MTPs in the digastric, pterygoid, and sternocleidomastoid muscles.
• Restricted Mobility: Functional limitations in jaw opening or deviations due to muscle hypertonicity or joint dysfunction.
• Neurological Interactions: In severe cases, conditions like trigeminal neuralgia may exacerbate musculoskeletal pain.

Management strategies

Orofacial pain and temporomandibular disorders (TMDs) can pose diagnostic and therapeutic challenges. Master the process of distinguishing between various TMDs and related conditions and explore the treatment standards set by leaders like Jeffrey P. Okeson, Gary D. Klasser, and Lawrence Gottesman at our course “American School Of Evidence-Based Treatment Of Temporomandibular Disorders”!

A multidisciplinary approach is crucial for managing these muscle pathologies, targeting both symptom relief and underlying causes:

1.  Pain Management:

• NSAIDs (e.g., lornoxicam) for inflammation and pain relief.
• Trigger point injections with local anesthetics or muscle relaxants.
• Orthopedic splints to reduce stress on the masticatory system.

2.  Addressing Parafunction and Bruxism:

• Behavioral interventions and stress management techniques.
• Custom oral appliances to protect teeth and minimize muscle strain during sleep.

3.  Physiotherapy and Rehabilitation:

• Intraoral laser therapy for deep muscle relaxation.
• Manual therapy targeting the lateral pterygoid and cervical muscles.

4.  Psychological Support:

• Antidepressants or anxiolytics for patients with chronic pain and significant emotional distress.

5.  Long-Term Preventive Measures:

• Orthodontic or prosthetic interventions to correct occlusal imbalances.
• Relaxation exercises to reduce muscle tension.

Muscle parafunction, bruxism, and facial musculoskeletal pain are interconnected conditions with significant implications for dental and overall health. Their management requires an integrated approach combining pain relief, structural correction, and behavioral modification. Advanced diagnostic tools and individualized treatment plans can greatly enhance outcomes, addressing both the symptoms and underlying causes of these challenging disorders.

Muscle dysfunction can significantly impact orthodontic outcomes, potentially aggravating treatment challenges and prolonging care. By  joining our course “Musculoskeletal physiotherapy techniques in Orthodontics: Evidence-based Approach and Clinical Protocols” you will integrate musculoskeletal concepts in your practice, prevent muscle parafunction and learn advanced protocols for diagnosing TMDs, CCMD, and orofacial pain.