Muscle or Bone · Palpation · CityLink Mall
A wide lower face is either muscle or skeleton, and the two are told apart by a hand on the jaw during a clench. Botulinum toxin reduces the first and does absolutely nothing to the second.
The masseter is skeletal muscle and it behaves like skeletal muscle anywhere else in the body. Loaded repeatedly, it hypertrophies. Running from the zygomatic arch to the angle and lower border of the mandible, it is one of the strongest muscles by cross-sectional area in the body, and in a face that clenches it becomes visibly and palpably larger over years.
The load usually comes from three sources. Nocturnal bruxism, often unrecognised until a dentist points out the wear pattern. Daytime clenching under stress, which is common and largely unconscious. And habitual chewing of firm foods. None of these are pathological in themselves, but all of them are training a large muscle on the side of the face, and the result is width at the jaw angle and a face that reads square from the front.
Botulinum toxin works here by blocking neuromuscular transmission at the injected sites, which reduces contractile activity. A muscle that is not being loaded atrophies — this is disuse atrophy, the same process that shrinks any unloaded muscle — and the narrowing appears over four to eight weeks rather than immediately. The mechanism explains the timeline, and it explains why the effect is temporary: when transmission returns and loading resumes, bulk returns with it.
Bone does not participate in any of this. The gonial angle, the width of the mandibular body and the flare of the ramus are skeletal dimensions. They do not atrophy with disuse, they are not affected by toxin, and they are unchanged by any energy device or injectable. A patient whose width is skeletal who receives repeated masseter injections will experience a weaker bite and no visible narrowing.
Assessment
Masseter treatment is straightforward when the diagnosis is right and pointless when it is not. Three findings decide whether it is appropriate, and all three are established by examination rather than by photograph.
Toxin cannot narrow bone. Anyone injecting a jaw without palpating it first does not know which one they are treating.
Realistic Expectations
Narrowing is gradual and partial. Visible change begins around the third to fourth week and continues to around eight weeks as atrophy develops. The degree of narrowing depends on how much of the width was muscular in the first place, which is why the examination finding predicts the result more reliably than the dose does. A mixed presentation narrows partially, and that is stated beforehand rather than discovered afterwards.
The effect is temporary and the maintenance interval is individual. Muscle bulk returns as neuromuscular transmission recovers and loading resumes, though patients who continue treatment over several cycles often find the muscle takes longer to return to its original bulk. Treatment is spaced by examination of the muscle rather than by a fixed schedule, and intervals commonly lengthen over time rather than shortening.
Dose is calibrated rather than maximised, because the masseter contributes to the structure of the lower face as well as its width. Over-reduction can flatten the posterior cheek, accentuate jowling in a face that was already descending, and in some patients produce a hollow at the angle that looks older rather than slimmer. Where both width and descent are present, masseter reduction is planned alongside lifting rather than in place of it.
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