Botulinum toxin in the masseter: indication and care

Botulinum toxin in the masseter is one of the most requested topics in orofacial harmonization (HOF). It comes up in two situations that overlap: the patient who wants a slimmer lower third because the face looks "square," and the patient with bruxism who clenches the teeth and feels the muscle tired or sore. In both, the decision depends on understanding where the concern comes from before thinking about injecting.

This guide is a review. It gives no doses, points or injection technique, which are the decision of the qualified professional, following the product label and the rules of their professional council in Brazil.

What does the masseter look like, and why does it widen the face?

The masseter is the most powerful chewing muscle. It originates at the zygomatic arch and inserts on the ramus and the mandibular angle. It is described with a superficial and a deep portion, which overlap and have fibers running in slightly different directions.

Because it sits on the side of the lower third, the volume of the masseter weighs heavily on the width of the face in a front view. People who clench or grind their teeth, chew mostly on one side or are predisposed can develop hypertrophy, with a wider and more angular lower contour.

Neighboring structures matter for safety:

• Parotid gland and parotid duct, in the area behind and above the muscle.
• Branches of the facial nerve, which cross the face superficial to the masseter.
• Smile muscles, such as the risorius and zygomaticus major, in the anterior area.
• Facial artery and vein, which run along the anterior border of the masseter, over the mandible.

When does toxin in the masseter make sense?

What about bruxism?

Bruxism is muscle activity with varied causes and presentations, and the diagnosis is dental. Pain in the temporomandibular joint, clicking, limited opening and tooth wear call for a specific assessment, which may involve a night guard, adjustments, habit control, physical therapy and other options. The toxin, when indicated, is a complement. Promising a "cure for bruxism" with toxin is a technical and ethical error.

How to assess the patient before recommending

The key is to find out what is widening the lower third. A simple sequence:

1. Palpate at rest and while clenching. A hypertrophic masseter becomes evident and firm on clenching. If the volume does not change with contraction, the cause is probably something else.
2. Observe from the front and in three-quarter view. See where the volume is: at the angle, in the cheek or below the mandible.
3. Look for non-muscular causes. Enlarged parotid, a prominent bony mandibular angle, cheek fat or a double chin call for a different approach and, sometimes, a medical workup.
4. Compare the sides. Record any prior facial asymmetry and ask about chewing on one side only.
5. Assess laxity and age. In lax skin, reducing masseter volume can leave the area more sagging or "hollowed."
6. Ask about previous applications. Short intervals and a history of reduced response help with planning and counseling.

What is the expected timeline?

The onset of toxin effect is generally explained in how long botulinum toxin takes to work. In the masseter, the change in contour usually takes longer than the change in strength, because it depends on the muscle shrinking. Duration and the interval between applications vary with the product, the dose and the patient; see how often to apply it.

What are the risks and unwanted effects?

In general, application in the masseter is described as low risk when well indicated, but there are effects the patient needs to know about:

• Change in the smile. If the toxin reaches the risorius or zygomaticus major, the smile may become asymmetric or more "still" on one side.
• Paradoxical bulging. A prominence when clenching the teeth, when part of the muscle stays active while the rest has been weakened.
• Fatigue when chewing. Common in the first weeks, especially with hard foods.
• Hollowed look or apparent laxity. In people who already have laxity, the volume reduction can make excess skin more evident.
• Asymmetry. A different response between the sides.
• Pain, swelling and bruising at the site.
• Reduced response over time. There are reports of a weaker effect in people who receive frequent applications, associated with antibody formation; respecting intervals is part of prevention.

The literature also discusses possible effects of repeated applications on the jawbone, a topic still under study. It is worth telling the patient that the treatment is maintenance-based and that long-term planning takes intervals and real need into account.

Masseter, jawline and the overall plan

Slimming the masseter changes how the lower third reads, and that interacts with the rest of the plan:

• In someone who wants a defined contour, reducing the masseter while also marking the jawline can seem contradictory. Plan in stages and assess the effect of each one.
• In male patients, a wide and angular lower third is often desired. Reducing the masseter without a clear indication can feminize the face. See male orofacial harmonization.
• The jawline and the chin define the proportion of the lower third together with the masseter. The assessment by thirds helps to see the whole face.

How to explain it to the patient

For those with bruxism, make clear that the toxin does not replace the dental assessment or the night guard, when indicated. Record everything in the patient record and use an informed consent form that mentions the effects above. A template is in informed consent form for botulinum toxin. To price the area, see how to price botulinum toxin by area.

And in Harmy?

Masseter toxin checklist

1. Cause of the widening confirmed: muscle, and not bone, gland or fat.
2. Bruxism and joint pain assessed, with referral when indicated.
3. Asymmetry, laxity and previous applications recorded.
4. Indication checked against the label of the registered product.
5. Photos at rest and while clenching the teeth, front and profile.
6. Patient told about fatigue when chewing, change in the smile and paradoxical bulging.
7. Follow-up scheduled to assess response and symmetry.
8. Intervals between applications planned, with no early reapplication.