In short. What scans and models are for when a cut in the jaw is planned: finding the tooth roots, following the nerve through the bone, and testing the movement first.
Key points
- It is not a picture of the result.
- It is a map of the constraints: where the roots end, where the nerve runs, and how much bone is available for the segment to be taken from.
- The plan is what the surgeon does within those limits, not what the scan decides.
Planning a cut in the jaw is not a matter of choosing a shape from a catalogue. The surgeon needs to know where the roots of the lower front teeth end, where the nerve runs inside the bone, how thick the bone is along the intended line, and where the segment will sit once it has been moved. Most of that is invisible from the outside.
A plain radiograph of the skull from the side shows the outline of the jaw and the position of the teeth, and it is often the first image taken. It shows the nerve canal only as a faint line, if at all. A three-dimensional scan shows the canal as a channel through the bone, which allows the surgeon to see how much bone lies between the planned cut and the nerve on each side.
Models are used where the movement is complex. A model of the jaw built from the scan can be cut and the segment repositioned on the bench, which lets the surgeon see the result in three dimensions before reproducing it in the operating theatre. For an asymmetry, or for a chin that has been operated on before, a model is often the difference between a plan that can be carried out and one that has to be improvised.
What imaging is for
It is not a picture of the result. It is a map of the constraints: where the roots end, where the nerve runs, and how much bone is available for the segment to be taken from. The plan is what the surgeon does within those limits, not what the scan decides.
Photographs serve a different purpose. Standardised views taken before and after allow the change in the soft tissue to be compared with the change in the bone, which is how a surgeon builds an idea of what a given movement produces in a given face. They are also how the result is reviewed honestly later, because memory is a poor record of a face.
Some of the planning happens with other specialists. Where the bite is part of the problem, the orthodontist's plan for the teeth and the surgeon's plan for the bone have to fit together, and the two are worked out against the same images. Where the jaws themselves are being moved, the chin is planned after them, because its correct position depends on where they end up.
Where the scan changes the plan
- The nerve canal runs closer to the intended line of the cut on one side than the other.
- The roots of the front teeth reach lower than expected, leaving less bone beneath them.
- The bone is thinner on one side, so the same movement cannot be made on both.
The useful question to ask at the assessment is what the imaging showed about your own anatomy, and what it ruled out. A plan described only in terms of the appearance it aims at has not been explained.
Frequently asked questions
Do I need a scan before the operation, or is a plain X-ray enough?
A plain radiograph shows the outline of the jaw and the teeth, and it is often taken first. The nerve canal is difficult to follow on it. Where the planned movement brings the segment near the canal, or where the chin has been operated on before, a three-dimensional scan is what shows the channel through the bone and the thickness of bone on each side of it.
What is a model used for?
It is a physical copy of your jaw built from the scan, on which the cut can be made and the segment repositioned before the operation. It is most useful where the movement is complex, such as an asymmetry or a chin that has been operated on before, because it shows in three dimensions what the plan produces rather than leaving it to be worked out during the operation.
Will the planning be shared with my orthodontist?
Where the bite is part of the treatment it should be, because the teeth and the bone have to fit together in the same plan. The orthodontist positions the teeth for the bone that is coming, and the surgeon positions the bone where those teeth can meet. Planning the two separately is how a bite ends up not closing.
Risks & Limitations
- Numbness of the lower lip and chin is expected after a cut in the jaw; it usually improves over months and may not return completely.
- A second operation in the same bone carries a higher chance of numbness that does not recover fully.
- The change is fixed once the bone heals, and reversing it means cutting again rather than removing something.
- Final results are judged at three months or later, and more fairly at a year, not in the first weeks.
When to Seek Care
- Difficulty breathing or chest pain — seek emergency care immediately.
- Fever with swelling, redness or discharge over the jaw, which may mean infection around the plates.
- A numb area of the lip or chin that is getting larger rather than smaller after the first weeks.
- Loss of movement in the lower lip, or an incision inside the mouth that opens.
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.