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.
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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/babies); Yellow (Small); Red (Medium); Blue (Large). Most patiients 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.
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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 teh 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.
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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:
Use a stiff impression mix
Patient to sit upright ' as tall as possible'
Breathe through their nose during impression taking
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.
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UPPER - Place the tray in the mouth, being careful to avoid excess impression
material flowing off the back of the tray. 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.
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.
video
of taking wax bite registration
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 are shown here