In the practice of medical procedures for aesthetic purposes, treatments aimed at improving the definition of the jawline and the lower third of the face are taking an increasingly prominent place. However, two very different medical approaches are often confused : jawline contouring through hyaluronic acid injections on one hand, and botulinum toxin injections at the level of the mandible, often referred to as masseter botox, on the other. These techniques pursue distinct objectives, follow different mechanisms of action, and respond to specific indications.
Jawline contouring is based on the injection of a highly cross-linked and dense hyaluronic acid directly onto the mandibular bone. The goal is to redefine the jawline, enhance the mandibular angles, or correct a loss of definition due to age or genetics.
This is a volumizing and sculpting treatment. Once injected deeply, hyaluronic acid acts as a support that :
The result is visible immediately after the injection and becomes more defined once the swelling subsides. Its duration varies between 10 and 12 months depending on the products used, the patient’s anatomy, and their metabolism. This treatment is suitable for both men and women, young or older.
Botulinum toxin injection in the treatment of the lower third of the face pursues a different objective. It is not about projecting or sculpting, but about reducing the mandibular muscle mass and softening the contours of a face that is too square or too wide. The targeted muscle is the masseter, located on each side of the jaw, between the mandibular angle and the temporomandibular joint. Highly used in some patients (bruxism, repeated tension), it can hypertrophy and give the face a heavy, angular, or even masculine appearance in women. Botulinum toxin injection induces partial relaxation of the masseter, which decreases its tone, reduces its volume, and gradually refines the lower part of the face. The result is not immediate: it appears gradually over 4 to 6 weeks, as the muscle relaxes. The effect is temporary, with an average duration of 4 to 6 months. This treatment is particularly indicated for patients who have a square face of muscular origin, without excess fat or significant skin laxity.
The decision to use one or the other of these treatments always relies on a precise clinical examination of the lower face. The doctor must identify the quality of the bony relief, muscle tone, thickness of the subcutaneous tissue, and the presence or absence of fat or skin laxity.
It is essential not to inject a volumizing hyaluronic acid into a face where the width is due to muscular hypertrophy. Conversely, reducing a masseter that is not hypertrophied may unbalance proportions without real aesthetic benefit.
That is why any injection in the lower third of the face must be preceded by a rigorous morpho-functional diagnosis.
In certain cases, the combination of botulinum toxin and hyaluronic acid at the jaw level proves to be relevant and complementary. This does not mean mixing the products in the same syringe or injecting them in the same place, but using them in a thoughtful sequence, adapted to the patient’s anatomy and aesthetic goals. The idea is as follows: in some patients, the shape of the lower third of the face is masked by a hypertrophied masseter muscle. This excess muscle volume widens the posterior part of the face, gives it a square or heavy appearance, and hides the natural bone structure. Injecting hyaluronic acid directly into a jawline already increased by the muscle could further weigh down the face. In this case, it is preferable to proceed in two stages. First, inject botulinum toxin into the masseter to relax the muscle and gradually reduce its volume. Then, once the muscle is refined, inject hyaluronic acid to redefine the jawline, enhance the angle, or project the chin if necessary. This strategy allows for a harmonious reconstruction of the lower third of the face, respecting natural volumes. It is particularly useful in young Asian adults, whose masseter is sometimes genetically developed, in patients suffering from bruxism, or in those whose face appears “wide” due to excessive muscle tone. The key to a good result lies in the proper sequence of procedures, the patient’s patience — waiting for the full effect of the toxin before the hyaluronic acid injection — and the doctor’s experience.
Botox® and hyaluronic acid do not have the same mechanism of action or the same duration of effect, as they follow fundamentally different biological principles.
Botulinum toxin works by inhibiting the release of acetylcholine at the neuromuscular junction, thereby preventing contraction of the targeted muscle.
Its effect begins between the 5th and 10th day after injection, peaks between 4 and 6 weeks, and then gradually decreases.
The duration of action is 4 to 6 months, depending on several factors: individual metabolism, initial muscle tone, and adherence to post-procedure instructions.
Hyaluronic acid is a volumizing or lifting agent. It works by filling, supporting, or projecting, depending on the area and injection plane. Its resorption is gradual, linked to the patient’s natural enzyme activity (hyaluronidase).
Its duration ranges between 10 and 15 months.
The possibility of simultaneously injecting botulinum toxin and hyaluronic acid in the lower third of the face depends on the injected areas, the chosen strategy, and the clinical indication. In theory, there is no formal contraindication to combining the two products in the same session. However, in practice, a staggered approach is often preferable, particularly for reasons of morphological analysis and clarity of the result.
When the masseter is hypertrophied, the logical first step is to inject botulinum toxin alone. This allows the muscle to gradually relax, to observe the reduction of its volume over the weeks, and to wait for the face to refine. Once this effect is established, the injection of hyaluronic acid becomes more precise: the doctor then works on a relaxed muscle, with a more accurate reading of the bony contours. This allows for more precise dosing, better-controlled symmetry, and a more natural result.
Injecting both products simultaneously presents a challenge: the muscle relaxation induced by botulinum toxin, which develops gradually after the procedure, secondarily alters the structure of the lower face, which can distort the perception of volumes at the time of hyaluronic acid injection and compromise the accuracy of the procedure.
However, in certain carefully selected cases, simultaneous injection is feasible.
In a patient with a moderately hypertrophied masseter and a mild need for mandibular redefinition, both products can be used in combination, provided different injection planes are respected: botulinum toxin deep within the muscle, and hyaluronic acid in contact with the bone and in a deep subcutaneous plane.
In this case, the procedure requires a perfectly mastered technique and a fine understanding of volumes.
Article written by Dr Romano Valeria
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