Jaw closing is caused by activation of the masseter, the temporalis and – to a lesser extent – the medial pterygoid muscles.

Anatomy of the Mandible


The muscles of mastication are innervated by the trigeminal nerve and include the temporalis, masseter and medial and lateral pterygoids.

The temporalis muscle originates in the temporalis fossa of the temporal bone and inserts on the coronoid process and anterior surface of the mandible ramus and functions to elevate and retract the mandible.
The masseter originates at the zygomatic arch and attaches to the angle and ramus of the mandible. 
The medial pterygoid originates from the medial surface of the lateral pterygoid plate and the tuberosity of the maxilla and attaches to the medial surface of the mandible angle and ramus. 

The masseter and medial pterygoid together form a U-shaped sling around the inferior border of the mandible. The two muscles elevate and forcibly close the jaw.

Anatomy of Medial Pterygoid


1. Masseter Injection

Figure 1. Anatomy of the masseter and temporalis muscles



Figure 2.  Inferior margin of the mandible shown as dotted horizontal line and the muscle is shown having been divided into three. Injection sites are shown (three): black circles with white central dots.

Draw an imaginary line between the corner of the mouth and the intertragal notch (Figure 2).
Ask the patient to clench their jaw and palpate the anterior border of the masseter muscle which can be felt up to the zygomatic arch
Divide muscle into thirds and inject 1 cm above the line of the mandible at three locations; one in between and slightly above.the other two sites
(this technique avoids the marginal mandibular and buccal branches of the facial nerve)/

12.5-50 u; Max 50 u Botox

2. Temporalis muscle injection

The muscle belly is palpated while the patient clenches their teeth. The needle is placed through the skin overlying the temporalis muscle and position is confirmed with EMG activity with the patient’s teeth clenching. To enhance the accuracy the needle is usually inserted along the anterior or superior border of the temporalis muscle, and subsequently advanced posteriorly or inferiorly in the sagittal plane, once the correct depth of the needle placement is confirmed with the EMG. This allows multiple aliquots to be injected with one pass of the needle, a technique that is impossible if the needle is passed in a lateral to medial direction. 0.1 mL (5 units each) aliquots are delivered at four or five points within the temporalis muscle. These aliquots are spaced at 1-cm intervals, preferably through one to three skin puncture sites.

Anatomy of the Temporalis muscle

3. Medial Pterygoid Injection

The needle is placed through the skin in the submandibular area and directed superiorly deep to the mandible and the U-shaped sling, which the masseter forms with the internal pterygoid, and along the inner aspect of the mandibular ramus, thus entering the muscle. The position is confirmed by brisk EMG activity when the patient is asked to clench, and the muscle is injected at two to three points.
The facial artery, internal maxillary artery, or arterial branches may potentially be encountered during these injections.