Ultrasound-Guided Botulinum Toxin-A Injections into the Masseter Muscle for Both Medical and Aesthetic Purposes

Abstract

With the increasing use of Botulinum toxin type A (BoNT-A) injections in the masseter muscles for both medical and aesthetic purposes, there is a constant need to continually enhance the efficacy of these treatments and reduce the risk of potential adverse events. This review provides an in-depth analysis of the masseter muscle's anatomical structure and essential landmarks and emphasizes the advantages of ultrasound (US) guidance in improving the precision of BoNT-A injections compared to conventional blind methods. The review is supplemented with comprehensive figures, including graphics, clinical images, and ultrasound visuals, to support the discussion. Potential complications such as paradoxical bulging, inadvertent injections into the risorius muscle or parotid gland, facial paralysis, and the risk of bone resorption are examined. Future research should aim at refining injection techniques and assessing the long-term effects of repeated treatments to ensure optimal patient care and safety.

Keywords: botulinum toxin type A (BoNT-A); masseter muscle hypertrophy; ultrasound-guided injection.

Figure 1

Schematic illustration depicting the three heads of the masseter muscle originating from the zygomatic arch: the deep head, origin, and insertion (A), the intermediate head, origin, and insertion (B), and the superficial head, origin, and insertion (C). The deep inferior tendon (highlighted in white) separates the deep portion from the superficial one (D).

Figure 2

Anatomically guided safe zone for BoNT-A injections in the masseter muscle: schematic representation with three targeted injection points.

Figure 3

Ultrasound imaging of the masseter muscle demonstrating its three distinct heads (GE Venue Ultrasound, 12L-RS transducer, 3.4–12.6 MHz): (A) clinical image illustrating transducer placement in the transverse position; (B) original ultrasound image showing the muscle structure; (C) edited ultrasound image with color highlights distinguishing the deep head (orange), intermediate head (red), and superficial head (purple) for clearer visualization.

Figure 4

Ultrasound images showing the reduction in masseter muscle thickness before (A) and three weeks after (B) BoNT-A injection (GE Venue Ultrasound, 12L-RS transducer, 3.4–12.6 MHz). The arrows indicate the maximum vertical height of the muscle in both images.

Figure 5

Paradoxical bulging of the masseter muscle: (A) pre-treatment clinical presentation of the lower face; (B) post-treatment image showing visible and palpable asymmetric masseter bulging (circled).

Figure 6

Ultrasound image depicting the risorius muscle anterior to the medial compartment of the masseter (GE Venue Ultrasound, 12L-RS transducer, 3.4–12.6 MHz): (A) clinical image illustrating the transducer placement in the transverse position; (B) original ultrasound image showing both the risorius and masseter muscles; (C) edited ultrasound image with the risorius muscle highlighted in blue for clearer identification.

Figure 7

Clinical manifestation of risorius muscle involvement after BoNT-A injection into the masseter muscle. The figure shows a patient attempting a full smile, illustrating asymmetry due to the unintended weakening of the risorius muscle on one side. Additionally, there is a noticeable misalignment where the center of the gingiva does not correspond with the center of the upper lip, highlighting the asymmetry in the lower facial region.

Figure 8

Ultrasound image showing the parotid gland in close contact with the posterior region of the masseter (GE Venue Ultrasound, 12L-RS transducer, 3.4–12.6 MHz): (A) clinical image showing the transducer placement in the transverse position; (B) edited ultrasound image with the parotid gland highlighted (yellow dashed area).