12 Botulinum Neurotoxin for Temporomandibular Disorders, Masseteric Hypertrophy, and Cosmetic Masseter Reduction
Summary
Temporomandibular disorder (TMD) and associated masticatory muscle hyperfunction often result in a pain syndrome typically treated conservatively with diet modification, massage, and anti-inflammatory medications. For refractory cases, botulinum toxin injection into key muscle groups can offer relief from TMD pain. While botulinum toxin’s analgesic properties remain incompletely understood, its action at the neuromuscular junction directly relieving muscle spasm and its potential inhibition of peripheral inflammatory peptide release are felt to be implicated. Workup focuses on patient selection, making sure to exclude those with arthrogenic forms of TMD and those in which botulinum toxin use might otherwise be contraindicated. Cosmetic masseteric reduction or iatrogenic atrophy can be achieved through targeted botulinum injection utilizing a similar injection technique.
12.1 Introduction
Temporomandibular disorders (TMDs) describe a spectrum of disease affecting the temporomandibular joint and surrounding structures. It is estimated that between 5 and 12% of the U.S. population suffers from TMD-related symptoms. 1 Symptoms commonly include referred ear pain, headache, transmitted neck pain, decreased jaw mobility, pain while chewing, and crepitus with movement. 2 TMD-associated pain may arise from the joint itself (arthrogenic) or may be secondary to hyperfunction of the muscles of mastication (myofascial) resulting in chronic inflammation and pain. 3
The myofascial form comprises the majority of TMD cases and may present by itself or in association with articular derangement. Etiologic factors include muscular spasticity secondary to malocclusion, bruxism, hypermobility, external stressors, and psychomotor behaviors such as excessive gum chewing. 4
First-line pharmacologic treatment of TMD consists of anti-inflammatory agents, muscle relaxants, and tranquilizers. Physical treatments such as orthotic devices, physiotherapy, massage, acupuncture, and others are also often used. Other nonpharmacologic approaches include exercise, dietary adjustment, and biofeedback. Surgical interventions such as arthrocentesis, intra-articular steroid injection, arthroscopy, and open arthrotomy are rarely performed. Despite the myriad of treatment approaches, there is a lack of evidence that any one treatment is superior to another and additionally, a significant proportion of patients continue to have functional limitations and pain. 5
Botulinum neurotoxin (BoNT) injection into the muscles of mastication represents a treatment option for those with TMD symptoms refractory to standard therapies. While the analgesic properties of BoNT are not completely understood, in the case of TMD, it is felt to be due to a combination of its action at the neuromuscular junction relieving masticatory muscle spasm and possibly a reduction in the release of inflammatory mediators such as calcitonin gene-related peptide, substance P, and glutamate, thus altering nociception. 6 , 7
There have been several studies that collectively support the efficacy of BoNT for TMD. Schwartz and Freund 8 treated 46 patients with BoNT-A for TMD symptoms and assessed them at 2- and 8-week intervals for subjective pain, mean maximum voluntary contraction, interincisal oral opening, and tenderness to palpation. All patients demonstrated an improvement in all outcome measures except maximum voluntary contraction. Maximum voluntary contraction was reported to decrease after 2 weeks but then revert to baseline levels at 8 weeks. In an open-label study by Bentsianov et al, 4 the authors discovered a 70% response rate (defined as 50% reduction of severity and/or frequency of pain) when using BoNT injection into the masseter and temporalis muscles for TMD symptoms. Most recently, a randomized controlled pilot study of 20 patients by Patel et al 2 showed that after incobotulinumtoxinA injection, there was a statistically significant reduction in pain and composite masticatory muscle tenderness score when compared to placebo saline injection.
12.2 Masseteric Hypertrophy and Cosmetic Masseter Reduction
Masseteric hypertrophy may occur due to an anatomic asymmetry of the jaw, habitual asymmetric use, clenching during exercise or sleep, or excessive chewing of gum. 4 Hypertrophy can be unilateral, resulting in an asymmetry of lateral face or bilateral, producing a square-jaw appearance.
Asian patients more frequently seek aesthetic alteration of hypertrophic masseter muscles to reduce a prominent mandibular angle. In fact, surgical reduction of the mandible is more common in Asia, despite the fact that botulinum toxin offers a less invasive treatment option. BoNT is ideal for patients with muscular hypertrophy rather than mandibular bony prominence as the source of lower face widening. 9
12.3 Workup
12.3.1 Patient Selection
Patients should be at least 18 years of age and have had TMD symptoms for at least 3 months that have been refractory to conventional treatments for at least 6 weeks. The following patients should not be considered for BoNT therapy: patients with proven arthrogenic TMD, those who have had prior surgery for TMD, and patients using medications that have an effect at the neuromuscular junction or who have another disorder that interferes with neuromuscular function (e.g., myasthenia gravis).
Bentsianov et al 4 suggested using a visual analog scale (VAS) score greater than or equal to 4 but less than or equal to 9 on an 11-point scale as an additional patient selection criterion.
12.3.2 History and Physical Examination
A comprehensive history and physical examination with a focus on the head and neck is mandatory. It is important to include a dental history and examination as well. It is particularly important to ascertain if there is a history of psychological or psychiatric disorder, emotional stress, facial trauma, or poor dental care.
Symptoms associated with TMD include preauricular pain or otalgia, headaches, clicking, popping, or snapping of the jaw; limited range of mandibular movement and locking episodes; changes in occlusion; masticatory difficulty; and neck, shoulder, or back pain.
On physical examination, one should inspect for malocclusion, abnormal dental wear, missing teeth, and visible clenching or spasm of ipsilateral neck muscles. The temporomandibular joint should be palpated just below the zygomatic arch and 1 to 2 cm anterior to the tragus. Palpation should occur in both the open and closed positions. Determine the presence or absence of spasm, tenderness, and joint sound. Palpate the masseter, temporalis, pterygoids, and sternocleidomastoid muscles carefully. 7
12.3.3 Laboratory Studies and Imaging
Blood work is usually unnecessary unless systemic illness is suspected, in which case a complete blood count, erythrocyte sedimentation rate, rheumatoid factor, and antinuclear antibody can be considered.
If the diagnosis is unclear from the history and physical examination alone, or medical management has been unsuccessful, imaging may also be considered. Conventional radiography is usually sufficient and may show erosion, sclerosis, or remodeling. Magnetic resonance imaging (MRI) and computed tomography (CT) are considered complementary modalities. MRI is superior for assessing the articular disk and soft-tissue structures of the temporomandibular joint, whereas CT is most useful for evaluating osseous changes, such as erosions, fracture, or postsurgical deformity. 10 , 11
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