Paradoxical masseteric bulging explained — the deep and superficial masseter, the tendon between them, and why the retouch is simple.
A bulge after jaw botulinum toxin? The cause of this side effect and the safe answer
Hello. This is Dr. Lee Yumin, medical director of MeSOnBom Clinic.
Botulinum toxin is the byword for quick aesthetic treatment, and jaw (masseter) treatment in particular is something a great many people have regularly.
Because it is simple and its safety well established, many give little thought to side effects. But this particular one occurs more often than people expect.
So today I want to explain, plainly and calmly, the most representative side effect — a bulge in the jaw — why it happens, and who is more likely to experience it.
A few days ago one of our associate doctors had jaw botulinum toxin for the first time in a year.
Too busy seeing patients to keep it up, her jaw muscle had developed very solidly indeed.
Exactly a week after treatment, an urgent message arrived late at night.
"Doctor, look at my jaw."
In the photograph, the muscle on each side bulged firmly, as though she were holding a boiled sweet in each cheek.
A patient would have been alarmed, but as a doctor, honestly, I was quietly delighted.
A perfect real-world example of a side effect I had long wanted to write about had landed in my lap.
The medical term for this — where the treated area bulges when chewing — is paradoxical masseteric bulging.
Having had the injection to slim the face and then appearing to develop a lump is understandably alarming.
To understand it, you first need to know the shape of the masseter.
It is not a single mass but two layers — a deep layer on the inside and a superficial layer on the outside.
What we target with botulinum toxin is the deep layer inside.
Normally, injecting only that deep layer is enough: over time the solution spreads gradually and the superficial layer relaxes too.
But for those whose jaw muscle is unusually large and developed, it is a different matter.
The larger the muscle, the thicker and tougher the tendon (the deep intramuscular tendon) between the two layers.
Think of the tough sinew you sometimes chew in a piece of meat.
With that thick, firm tendon acting as a barrier between the two layers, the toxin placed in the deep layer cannot pass easily to the superficial layer.
The inner muscle, which received the toxin, relaxes and settles — while the outer muscle, which did not, is still full of strength and bulges on its own with every chew.
So those who habitually clench their teeth, or whose jaw muscle is especially large, are more likely to experience it.
A logical question arises at this point.
"Doctor, couldn't you just inject both the inner and outer muscle from the beginning so it doesn't happen?"
It is possible, of course.
But there is a much more important and careful reason why doctors do not.
Just above and around the superficial layer lie the muscles of the mouth that pull the corners up and create a smile.
If toxin were placed shallowly to target the outer muscle from the start, and any of it spread to those muscles, what would follow?
The mouth could pull to one side when smiling, or one side of the lip could leak when drinking — far more serious and distressing side effects affecting the expression.
So to shut out any possibility of facial asymmetry, we treat carefully and mainly in the safest place, the deep inner layer.
A bulging muscle can be corrected quickly; a mouth pulled to one side causes a patient real distress.
To put the conclusion first: if this bulging appears, there is no need to worry at all. The solution is clear and simple.
Just come in without delay.
At the doctor's judgement, a small amount of toxin is added to the outer muscle alone that is asserting itself.
The muscle then relaxes, usually within one to two weeks, and the smooth jawline returns.
(For our associate doctor mentioned above, the jaw muscle was so robust that it took a full month.)
As botulinum toxin has become commonplace, people come for it lightly — but the muscles of the face are far more complex and finely interwoven than people think.
If you have had treatment with us and a bulge like this appears, please do not worry alone; get in touch.
If it is mild we will reassure you and wait for the toxin to spread a little further; if the muscle is markedly raised, we will carry out a careful retouch and restore a slim jawline quickly.
Thank you for reading to the end. I hope your treatment is safe and leaves you looking well.
Our medical team will advise you based on your skin condition.
