Anatomy-Based Botulinum Toxin Injection of the Posterior Belly of Digastric Muscle

The posterior belly of the digastric muscle (PBDM) has recently been suggested as a target for botulinum toxin (BoNT) treatment in facial synkinesis. However, anatomically based injection sites for the PBDM have not been established. This study aimed to identify an ideal BoNT injection site by analyzing the intramuscular nerve distribution of the PBDM and its anatomical relationships with surrounding structures. Twelve PBDM specimens were obtained bilaterally from six embalmed cadavers. Modified Sihler's staining was performed to visualize intramuscular nerve distribution. The PBDM was divided into anterior, middle, and posterior thirds, and the course and terminal branches of the facial nerve were analyzed. In all specimens, the PBDM was innervated by a single branch of the facial nerve. Nerve entry points were located in the middle third in 75% of cases and in the posterior third in 25%. Intramuscular nerve endings were most densely distributed in both the anterior and middle thirds in all specimens, whereas the posterior third showed a lower distribution (25%). The anterior and middle thirds of the PBDM appear to be the most appropriate targets for BoNT injection. The area 1 cm posterior to the angle of the mandible may serve as a practical landmark-guided injection site in clinical practice.