In short. The two masseters are rarely the same size. How an uneven jaw is assessed, when treating a single side is the better plan, and what can be promised about the result.
Key points
- Take a photograph straight on with the head level and the jaw relaxed, taken by somebody else at a sensible distance.
- Then put a fingertip on each jaw corner in turn and clench gently, and notice which muscle rises more firmly.
- A difference in the photograph combined with a difference under the fingers is worth describing at an assessment.
A face is not symmetrical, and the two masseters are almost never the same size. The muscle on one side is usually the larger, and the reason is rarely mysterious: people chew on a preferred side, sleep with the face turned one way, and hold the jaw unevenly under stress. The result is a lower face whose width differs between the sides, which is sometimes the complaint and sometimes a finding the patient had not noticed.
Why the sides differ
A muscle enlarges with the load it carries, so the side that does more work becomes the larger one. Habitual chewing on one side puts more load on that masseter over years, and clenching is usually uneven as well, so the two muscles diverge by small amounts that add up. The difference develops slowly and is often first noticed in a photograph rather than a mirror, because a still image removes the movement the eye uses to correct for small differences.
Comparing your own two sides
Take a photograph straight on with the head level and the jaw relaxed, taken by somebody else at a sensible distance. Then put a fingertip on each jaw corner in turn and clench gently, and notice which muscle rises more firmly. A difference in the photograph combined with a difference under the fingers is worth describing at an assessment. Neither one decides anything on its own.
Treating both sides, or only one
A plan that treats both sides identically assumes the two are the same, which is rarely true. Where one masseter is clearly the larger, treating that side and leaving the other alone is a more accurate answer, and it is a plan this treatment allows because the two sides are assessed separately. Where both contribute to the width being treated, both are treated, and the plan says which is which rather than treating symmetry as the goal.
What can and cannot be promised
The difference between the sides can be reduced. It cannot be promised to disappear, because muscles respond at their own rates, the habit continues on both sides to some degree, and the bone beneath each muscle is not identical either. The honest aim is a face whose two sides are closer in width than they were, judged against the patient's own earlier photographs rather than against symmetry as an idea.
When a difference is not this subject at all
A difference the patient has always had is one thing. A difference that has appeared recently, that is progressing, or that comes with pain, numbness, weakness or a change in the bite is another, and it belongs with an assessment that looks for a cause rather than with a plan for a muscle.
When a difference should be assessed rather than treated
- It has appeared recently, or it is becoming more obvious over months.
- It comes with pain, numbness, weakness or a change in how the teeth meet.
- It was never a concern until somebody else described it to you.
Frequently asked questions
How would I know whether my two sides are uneven?
Compare them deliberately rather than in passing. Photographs taken straight on, in the same light, with the head level and the jaw relaxed, show a difference more reliably than a mirror does. Then place a fingertip on each jaw corner in turn and clench, and compare how firmly each muscle rises. A difference found that way is a finding to bring to an assessment rather than a diagnosis.
Is it safe to treat just one side?
It is done, and it is the appropriate plan when one masseter is clearly larger than the other. The same assessment applies to the side being treated, and the untreated side is left as it is rather than being included for the sake of symmetry. Treating one side is a plan like any other, and it is chosen because the finding calls for it.
Does treating one side make the two match?
It reduces the difference rather than removing it. Muscles do not respond at identical rates, the underlying habit usually involves both sides to some degree, and the bone beneath them may differ as well. A patient who expects a match is setting a standard the treatment cannot meet; a patient expecting the difference to be smaller is setting a reasonable one.
Risks & Limitations
- The effect is temporary by design. The muscle is quietened rather than damaged, so the width returns over months unless the clenching habit itself has changed.
- A change to the smile and tiredness with hard food are the effects most often noticed at this site, and both usually settle as the effect fades.
- The treatment narrows only the width the muscle is producing. A width coming from the bone or from fat is not changed by it.
- No outcome can be promised, and how much of the width the muscle explains is established only at an assessment.
When to Seek Care
- Difficulty swallowing, or food or drink going the wrong way — seek medical assessment the same day.
- A weakness that spreads beyond the jaw, or any change in breathing — seek emergency care immediately.
- A fullness at the jaw that is warm, red or painful, or that is growing steadily.
- Fever with swelling that spreads beyond the jaw, or a smile change that is worsening rather than settling.
References & Further Reading
- American Society of Plastic Surgeons (ASPS) — procedure and safety guidance
- National Institute for Health and Care Excellence (NICE, UK) — relevant guidance
- Cochrane Library — systematic reviews
- Chinese Medical Association Plastic Surgery Branch — clinical guidance
This page is patient education, not medical advice, and it does not cite individual studies. Discuss your own case with a qualified clinician. To arrange an assessment at Chongqing West District Hospital, see Contact & Consultation.