Learn Orthodontics

Study models

Study model in orthodontics are taken to record the initial/baseline malocclusion of the patient. They may be used to allow a more accurate assessment of the malocclusion and facilitate measurement.

Good study models will show all the teeth, with well defined sulci. The models are trimmed in the laboratory so that when placed on their 'heel' the teeth are in the patient's intercuspal relationship.

sm1 sm2

Taking Impressions

1. Tray selection

The trays should be selected to extend over all teeth and well into the sulci, if necessary the tray may need modification by the addition of wax to ensure full coverage. In Newcastle we have 4 tray sizes:

Orange (for very small children&#47babies); Yellow (Small); Red (Medium); Blue (Large). Most patients require a yellow lower tray and red upper tray.


'Rolling' the tray into the mouth (as illustrated below) can help to get the correct size of tray in.

lower imp 1 lower imp 2 lower imp 3

2. Patient and operator position

The patient should be placed upright in the dental chair. So that the operator can easily place the trays in the mouth. The lower impression is taken first, often with the operator in front or behind the patient, the upper impression is taken last with the operator behind the patient. Notice the height of patinet relative to the operator in the images below. In both case, she is well placed to take control of any problems if they arise.

imp 1 imp 2

3. Informing the patient

It is important to inform the patient what is about to happen, and to reassure them during the procedural. Some patient may 'gag' - particularity with the upper impression (some even manage to be sick!), that's why it is best to take the upper impression last.

There are a number of techniques that can be used to prevent gaging:

  1. Use a stiff impression mix
  2. Patient to sit upright ' as tall as possible'
  3. Breathe through their nose during impression taking
  4. Distraction - using various methods to distract the patient from the procedure

5. Impression mixing

The impression material (usually alginate) should be mixed to be stiff, and to set quickly, with little or no air bubbles incorporated

6. Loading the tray

The tray is loaded by the operator, with care not to place excess material at the back of the tray

7. Taking the impression

LOWER- Place the tray in the mouth, with care to ensure that impression material flows into the labial sulcus. This usually best achived by seating the back of the tray down first and holding the lower lip out of the way so excess material flows into the labial culcus.

low imp 4 low imp 5 low imp 6

UPPER - Place the tray in the mouth, being careful to avoid excess impression material flowing off the back of the tray.

up imp 3

Usually best achived by seating the front of the tray first and lifting the back of the tray until alginate is seen posteriorly. If the patient has a high palate, a finger may be used to place impression material into the palate before inserting the tray. Excess and/or runny alginate running down the back of the throat is likely to make the patinet gag and feel sick

Bite registration

1. Informing the patient

Inform the patient what is going to happen.

2. Preparing the wax

Orthodontists usually use pink model wax for bite registrations. Soften the wax in hot water and fold it into a thick (3-4layers) strip.

wax bite 2

3. Taking the registration

Get the patient to bite together (without the wax in place) to check the occlusion. Then place the softened wax into the mouth making sure that the wax is on the premolar/molar teeth only and ask the patient to close together. Check that the occlusion is the same as when they closed toothier without the wax in place.

wax bite 1

Remove the wax registration from the mouth and chill the wax with cold water, at this stage you may retry it in the mouth to check the occlusion.

Common problems with impressions