In short. Jaw width can be produced by three different layers: the masseter muscle, the fat over it, and the bone of the mandible, and only the last is changed by removing bone. The examination separates them with the hands before any imaging, and most wide faces are a combination rather than a single layer. The plan addresses the largest contributor.
Key points
- The masseter produces a soft, broad lower face that stands out when the teeth are clenched.
- Fat moves with the skin and contributes to the outline without changing the skeleton.
- Bone does not change with clenching, does not move with the skin, and has no non-surgical treatment.
- Most wide lower faces combine layers, so the plan addresses the largest contributor.
- A bone operation for muscle or fat changes something the patient was not looking at.
What are the three layers?
The same appearance of a wide lower face can be produced by three different layers, and only one of them is treated by removing bone. Separating them is the single most useful thing an assessment does, and it is done with the hands before it is done with any imaging.
The muscle
The masseter runs over the corner of the jaw from the cheekbone down to the angle. Where it is heavy, it produces a broad lower face that is soft rather than angular, and it becomes noticeably more prominent when the teeth are clenched. It is a muscle, so it responds to a treatment that relaxes it, and that treatment's effect is temporary rather than permanent, which makes it useful both as a treatment and as a test of the diagnosis.
The fat
Fat sits over both the muscle and the bone, and it contributes to the outline of the lower face. It is soft, it moves with the skin rather than with the jaw, and it changes when the face is lifted or when the cheeks are drawn back. Removing fat is a different treatment from reducing bone, and it does not narrow the skeleton. Where fat is the main finding, a bone operation changes something the patient was not looking at.
The bone
The bone is what remains when the muscle has been relaxed and the fat has been allowed for. It does not change with clenching, it does not move with the skin, and it does not respond to any non-surgical treatment. Imaging establishes how thick it is along the line the operation would follow and where the nerve runs, which is what makes the surgical plan possible rather than approximate.
The clench and relax test
Put two fingers flat over the corner of your jaw on each side and clench your teeth hard. What rises under your fingers is the masseter. Now relax completely and feel the same place again. If the width you dislike disappears when the muscle is relaxed, the muscle is a large part of the finding. If it is still there, what you are feeling is the bone beneath, and that is a different answer.
| Layer or step | What it contributes | How it is recognised | What follows from it |
|---|---|---|---|
| Masseter muscle | A broad lower face that is soft rather than angular | It rises under two flat fingers when the teeth are clenched | A treatment that relaxes it, with a temporary effect |
| Fat | Part of the outline of the lower face, over both muscle and bone | It is soft and moves with the skin rather than with the jaw | Removal, which does not narrow the skeleton |
| Bone | The width that remains when the muscle is relaxed and the fat allowed for | It does not change with clenching and does not move with the skin | An operation on the bone, planned with imaging |
| Muscle and bone together | A common combination rather than one layer alone | Width that persists when the muscle is relaxed | A plan addressing the largest contributor |
| Imaging | Bone thickness along the operative line, and where the nerve runs | Not a finding of the hands | A surgical plan that is possible rather than approximate |
Why does the wrong layer get treated?
The wrong layer is usually treated because a bone operation was planned without the patient being examined while clenching and relaxing, or because a muscular width was treated with an operation on the bone, which changes the skeleton and not the appearance, or because fat was assumed to be bone without the soft tissue being assessed separately.
What the assessment establishes
The three layers are not competing explanations. Most wide lower faces are a combination, and the assessment establishes the proportion so that the plan addresses the largest contributor rather than the most easily treated one.
Sources
Internal links
Frequently asked questions
How can I tell muscle from bone at home?
Clench your teeth and feel the corner of the jaw. A masseter that stands out prominently under the fingers and softens when you relax is muscle. Bone stays exactly the same whether you clench or not. If the width is still there with the jaw relaxed and the muscle soft, and the corner itself extends outwards, that part is skeletal.
Can more than one layer be responsible?
Yes, and it is common. A wide lower face frequently has a skeletal component and a muscular one, with fat over both. The assessment separates them by proportion rather than by presence, and the plan addresses the layer that contributes most. That is why the plan is a judgement rather than a measurement.
What if it turns out to be fat?
Then the plan does not involve the bone. Fat over the jaw is a different target with a different treatment, and a bone operation would change the skeletal width without changing what the patient is actually looking at. This is a common and entirely reasonable outcome of an assessment for jaw contouring.
Risks & Limitations
- Removing bone is easier than restoring it. Over-reduction is difficult and sometimes impossible to correct, so the amount removed is decided from imaging and treated as fixed.
- The marginal mandibular nerve, which moves the lower lip, runs through the operative field. Weakness after surgery is usually temporary and usually resolves over weeks to months; a lasting weakness is uncommon but possible.
- Swelling hides the result for months. The face reads unevenly at two weeks, is not close to final until around six months, and is judged honestly at around a year.
- Numbness of the chin and the lower lip is expected, is watched rather than treated in the early weeks, and may not return fully in some areas.
- The two sides of a jaw are rarely identical, and reducing both by the same amount can make a pre-existing difference more visible.
- The operation changes width and the border between the jaw angle and the chin. It does not lengthen the face, correct a bite, or change the soft tissue over the new contour.
When to Seek Care
- Swelling that is increasing rather than settling after the first week, or that is much greater on one side — contact the team.
- A wound that is opening, or discharge, spreading redness or warmth around an incision.
- A weakness of the lower lip that is worsening rather than improving, or difficulty breathing or swallowing &mdash — seek emergency care at once.
- Fever, or feeling generally very unwell with a wound or a swollen area.
- Calf pain, swelling or tenderness in one leg, or shortness of breath or chest pain &mdash — seek emergency care at once.
References & Further Reading
- American Society of Plastic Surgeons (ASPS) - facial bone contouring and patient safety guidance
- National Institute for Health and Care Excellence (NICE, UK) - guidance on facial surgery and on venous thromboembolism prevention
- Cochrane Library - systematic reviews in oral and maxillofacial surgery
- British Association of Plastic, Reconstructive and Aesthetic Surgeons (BAPRAS) - patient information on facial surgery
- 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.