Masseter Muscle Hypertrophy and Facial Asymmetry

Overview:

A hypertrophic enlarged masseter muscle is not only the result of certain dysfunctional, idiopathic, congenital, or parafunctional habits, but it also alters the facial cosmetic impact negatively by causing facial asymmetry for many patients.

What Is Masseter?

The masseter is an important facial and jaw muscle involved in mastication, elevation, and protrusion of the mandible. These mandible functions develop it into a powerful muscle. The hyperfunction of the muscle leads to hypertrophy of the muscle. The masseter muscle is essential for adequate mastication and is located laterally to the mandibular ramus and thus plays an important role in facial esthetics.

How Does Hypertrophy of Masseters Take Place?

The masseter, a thick quadrate masticatory muscle, arises from the zygomatic arch and inserts into the inferior lateral aspects and angle area of the mandibular ramus. Masseter hypertrophy is an asymptomatic persistent enlargement of one or both masseter muscles resulting from hypertrophy.

The causes are mainly initiated or attributed to certain parafunctional habits like clenching, bruxing (night grinding), or heavy gum chewing. This occurs most often in younger patients where this condition tends to be more prevalent. However, it can also be observed in older age groups with dental disease or wear out of enamel or tooth structure. In these cases, there is an inability to fully activate the masseter muscle.

Anatomically, most of the masseteric thickness is along the inferior portion of the ramus of the mandible. The facial contour, which should normally taper, is impacted because of this hypertrophy. With masseter hypertrophy, the patient’s face takes on a characteristic rectangular or squarish shape.

What Is Congenital or Idiopathic Masseter Muscle Hypertrophy?

Idiopathic masseter muscle hypertrophy (IMMH) was first described by Legg in 1880. Idiopathic hypertrophy of the masseter muscle is said to be a rare disorder of an unknown cause that might be congenital in origin. Research associates it with defective tooth structure, the habit of gum chewing, temporomandibular joint (TMJ) disorders, congenital and functional hypertrophies, and emotional disorders (stress and nervousness).

What Is the Pathophysiology of Masseter Muscle Hypertrophy?

The etiology of the masseter hypertrophy is not clear and has been accredited to unilateral masticatory efforts due to teeth loss, temporomandibular joint disorders, or parafunctional habits, such as bruxism or prolonged use of chewing gum. It generally affects young adults between the second and third life decades. Also, due to the muscle traction towards the affected side, the mandible can be thickened as well. Though the idiopathic or the congenital cause of hypertrophy remains elusive, some hypotheses suggest the implications of emotional stress, bruxism, or teeth clenching as the main causative elements.

What Are the Clinical Features of Masseter Hypertrophy?

The masseter hypertrophy is normally followed by an osseous spur (it delves posteriorly into the mandibular angle), producing a squarish or rectangular outline of the patient’s face. The clinical features of the disease involve esthetic damages, joint disorders, occlusal or biting malfunction, and lateral deviation of the mandible. The congenital form of this condition induces facial asymmetry.

How Is Masseter Hypertrophy Diagnosed?

The pain of MMH is rarely reported in these individuals, and even if it is present, it is well defined and located. This is an important clinical tool for diagnosis. In the radiographic diagnosis of muscle hypertrophy, though, research states the importance of computed tomography (CT) scan; other accessory modalities for investigation include magnetic resonance imaging (MRI) and ultrasound which are more preferred.

Ultrasound imaging is useful over CT and MRI in diagnosing cases with facial asymmetry. Ultrasound imaging is thus considered reliable and economical in comparison to advanced modalities of CT and MRI. Diagnosis can be produced from clinical examination, directed interview, panoramic X-ray, and muscle palpation. The dental surgeon, upon palpating the muscle with the fingers while the patient clenches his or her teeth, ensures that the muscle is more prominent during contraction. With the muscle relaxed and the patient’s mouth slightly open, extraoral palpation using both hands will help identify specifically the intramuscular location of the hypertrophy.

What Are the Differential Diagnoses of Masseter Hypertrophy?

Emphasis must also be placed on the differential diagnosis of this condition clinically which would include the following:

• Salivary gland.

• Benign and malignant tumors arising from an accessory parotid gland or duct.

• Inflammation and Infection - Abscess or cellulitis (specific or nonspecific infection).

• Lymphatic system tumors.

• Metastatic lymph nodes, lymphadenitis, lymphoma, lymphangioma.

• Connective tissue disorders or tumors - Lipoma, fibroma, or pseudotumor.

• Myopathy - Masseter hypertrophy, myositis ossificans, proliferative periostitis.

• Vascular system disorders - Hemangioma, A-V malformations, or false aneurysms.

How Is Masseter Hypertrophy Treated?

1. The surgical procedures are opted for depending on analyzing the event of muscle hypertrophy. This is the reason why in the diagnosis, it is very important to ask the patient to tighten the teeth and, therefore, during the palpation, to feel the muscle's hypertonicity.

2. Also, through palpation, the dentist can verify the irregularity at the mandibular angle. The presence of the osseous spur, as mentioned before, is commonly present at the inferior portion of the affected side and can be observed by means of an X-ray.

3. Surgical intervention - Mainly in these surgical cases, the excision of the internal layer of the masseter muscle and reduction of the thickness of the bone in the region of the mandibular angle via intraoral approach is the treatment of choice.

4. Non-surgical intervention - This modality of treatment includes reassurance to the patient by the use of a tranquilizer or muscle relaxant and psychiatric care.

5. Small doses of botulinum toxin type can also be used. Injections of botulinum toxin type A into the masseter muscle is also another alternative as a treatment. Botulinum toxin type A injection is reported to be a safe and effective treatment modality in muscle hypertrophies.

6. Tranquilizers and intramuscular administration of botulinum toxin type A are used in the mechanism of muscle detensioning. The substance becomes inactive, and the injection should be repeated after 4 to 6 months.

7. Any dental treatment, removal of occlusal interferences, and use of mouthguards are other important criteria of dental management for balanced occlusion.

8. Other maxillofacial surgical procedures in severe hypertrophy include selective mandible basilar and angle ostectomies, soft tissue excision, orthognathic surgery, and reconstructive procedures.

What Are the Complications of Surgical Management of Masseter Hypertrophy?

Complication from the surgical incision of masseter includes,

• Hematoma.

• Facial nerve paralysis.

• Infection.

• Trismus.

Sequelae from general anesthesia should be avoided as much as possible by the dental or cosmetic, or maxillofacial surgeon.

Conclusion:

In conclusion, masseter muscle hypertrophy though asymptomatic in a large number of cases is a condition that warrants both muscular and dental management by the dental and maxillofacial or the cosmetic surgeon and needs differential diagnosis from life-threatening tumors or other serious facial diseases or disorders.