Indications

Square jaw lines and wide mandibular borders are often desirable masculine characteristics but can be unattractive for women and become exaggerated when anatomic hypertrophy exists. Bruxism (teeth grinding), anxiety, and clenching can lead to masseter muscle enlargement and accentuate the horizontal width of the mandibular border. Occasionally patients, especially of East Asian descent, express concern about hypertrophic masseter muscles and request improvement in this area.

Anatomic Considerations

The masseter muscle’s origin is along the inferior aspect of the anterior zygomatic arch, and it inserts into the angle of the mandible along both the horizontal (body) and the vertical portions of the mandible (ramus).

Injection Technique

Two different techniques for injecting the muscle can be attempted: intraoral or transcutaneous. In the intraoral technique, the injector’s thumb is placed inside the mouth along the buccal mucosa until the angle of the mandible is palpated, and the patient is asked to bite down (but not on the injector’s thumb!). The anterior edge of the masseter muscle is palpated between the thumb and fingers of the same hand resting outside on the cheek. A 1-inch (2.5-cm), 30-gauge needle is passed intraorally anterior to the mandibular ramus and into the muscle belly. This will be somewhat uncomfortable for the patient. BoNTA is injected in a retrograde fashion as the needle is withdrawn. Two to four passes are performed in several tangential intramuscular injections for a total of 20 BU (or 60 DU) to the muscle.

In the transcutaneous technique, it is useful to place one finger along the lower border of the mandible, one along the vertical border of the mandible, and one as a reference at the inner aspect of the mandibular angle, with the patient clenching as a way to mark out the perimeter of the muscle. With a half- or ¾-inch (1.3 or 1.9 cm) needle or longer, injection can be performed inside that perimeter down to just above the bone, and depot injections of 4 to 5 units can be placed per area. An average of 20 units should be used depending on the mass of the muscle being treated.