In short. Under-reduction can be corrected and over-reduction usually cannot. Why the safe amount is measured from imaging, and why the conservative plan is the honest one.

Key points

  • How much can be taken is set by two findings from imaging: where the canal carrying the nerve to the chin runs within the bone, and how much bone has to remain around it for the mandible to stay strong enough to chew with.
  • Those two findings produce a limit, and the plan works back from it.
  • A plan that starts from a desired width and works forward has the order the wrong way round.

Almost every decision in an angle reduction comes down to one number, and the number is chosen before the operation from imaging rather than during it. The argument for choosing it at the conservative end is not caution for its own sake. It is that the two possible errors are not equally recoverable, and a patient deciding whether to accept a plan should understand which error they are being protected from.

Why taking too little is the lesser error

Where too little bone has been removed, the jaw is narrower than before and less narrow than was hoped for. The contour can be assessed once the swelling has settled, and a further reduction can be planned with the first operation's imaging and result in hand. The second operation is smaller than the first, and it is done on a shape that is already known rather than predicted.

Why taking too much has no good answer

Bone that has been removed cannot be restored to what it was. Material placed later to rebuild a contour does not behave like the mandible it replaces, and it does not move with the jaw the way bone does. A jaw that has been over-reduced can be improved, in some cases considerably, but it cannot be returned. That imbalance between the two errors is the whole argument.

The limit is measured, not chosen

How much can be taken is set by two findings from imaging: where the canal carrying the nerve to the chin runs within the bone, and how much bone has to remain around it for the mandible to stay strong enough to chew with. Those two findings produce a limit, and the plan works back from it. A plan that starts from a desired width and works forward has the order the wrong way round.

What taking less means in practice

It is not only a smaller number. It is keeping the curve of the border rather than cutting the corner away in a straight line, keeping enough of the posterior border that the jaw still reads as a jaw, and leaving the two sides measured separately rather than reduced by the same amount. Those choices are what a conservative plan actually consists of.

The question to ask about the plan

The most useful question at a consultation is not how narrow the jaw can be made. It is what the safe limit is in this case, what has to be left around the nerve, and what would be done if the result turned out to be too little.

Pressure in the wrong direction

  • A plan that offers a target width before any imaging has been reviewed.
  • The suggestion that a more aggressive reduction gives a better result.
  • No stated limit, and no answer to what happens if too much has been taken.

An angle reduction is a decision made once. Treating the amount as a limit rather than a target is what keeps the decision reversible in the only direction that allows it.

Related reading

Frequently asked questions

Why can a second reduction correct too little but not too much?

Because removing bone is a subtraction and the supply does not run the other way. A jaw that is still too wide can be reduced again, using the first operation's imaging and its actual outcome rather than a prediction. A jaw that has been over-reduced has no bone left to work with, and material added later does not behave like the mandible it replaces.

What does the safe limit on removal actually depend on?

Two findings from imaging: where the canal carrying the nerve to the chin runs inside the bone, and how much bone has to remain around it for the mandible to stay strong enough to chew with. Those produce a limit rather than a target, and the plan works back from it. A plan built forward from a desired width has the order the wrong way round.

Is there a way to test how a narrower angle would look before the operation?

Not accurately. Filler adds volume rather than removing it, so it can show a fuller contour and cannot preview the result of taking bone away. Simulated images are illustrations rather than measurements, and they cannot show how the soft tissue settles over a new contour. The most reliable preview is a conservative reduction, which is one reason it is preferred.

Risks & Limitations

  • The change is permanent and the amount removed is treated as fixed. Bone that has been taken away cannot be put back, and material added later does not behave like the mandible it replaces.
  • The canal carrying the nerve to the chin runs inside the bone being cut. Numbness of the chin and lower lip is expected rather than rare, usually improves over months, and may not return fully.
  • The nerve that moves the lower lip runs near the outer surface of the bone. Weakness after surgery is usually temporary and usually resolves; a lasting weakness is uncommon but possible.
  • The vessels supplying the face run close to the surface of the angle. Bleeding afterwards is the complication this operation most wants to avoid, because a collection of blood in the floor of the mouth can compromise the airway.
  • The masseter is detached from the bone it was attached to and reattaches. Width can partly return if the muscle thickens again, which is not the bone growing back.
  • Swelling hides the result for months. The face reads wider than expected at two weeks, unevenly at three months, and the result is judged honestly at around a year.

When to Seek Care

  • Difficulty breathing, a feeling of pressure or swelling in the floor of the mouth, or difficulty swallowing. Seek emergency care at once.
  • Swelling that is increasing rather than settling after the first week, or that is much greater on one side. Contact the team.
  • Weakness of the lower lip that appears after the first week, or that is worsening rather than improving.
  • Numbness that is spreading rather than staying stable, or a wound in the mouth that is opening or discharging.
  • Fever, or feeling generally unwell with a swollen and tender jaw. Calf pain or swelling in one leg, or chest pain and shortness of breath, need emergency care at once.

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

Medically reviewed by Tongchun Mao, MD (毛彤春), Deputy Director & Head of Aesthetic Surgery. Last reviewed: 19 September 2026.

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.