Restorative Dentistry

Model Practice

A fundamental part of many clinical procedures is the taking (or more correctly making) impressions, and the production of plaster of Paris models from them. Accuracy is paramount. Secondly, like many patients I do not like having impressions taken of my mouth and would like my dentist to limit this, especially maxillary impressions, if at all possible. We need also to take in account the time used, the cost of materials, and patient discomfort when repeat impressions are needlessly required.

When I give courses on occlusion, participants are asked to bring along a set of study models of their own teeth. How informative this is when examined closely! I would have expected to see study models to the standards that our orthodontic colleagues routinely produce well-trimmed, and easy to place on a work-surface and view from any angle. How wrong and disappointed can you be!The norm was models poured up in white plaster, yellow stone, or a mixture of both without rhyme or reason, poor trimming, impossible to place on a workbench, air blows, incorrect tray extensions etc.

The questions arise therefore about trays, impression techniques, and laboratory requests.

Trays

The most commonly used trays are plastic, single use, and disposable. The first test is to hold them one side in each hand and try twisting. If they can be flexed in this way, then they could distort during impression taking. Such trays have no place in modern dentistry. Perforated trays are preferable, as impression materials should be retained without the use of adhesives. So, being careful, you apply glue to a perforated plastic tray for an alginate impression. It’s a hot summer’s day and the alginate sets before you have chance to fully seat it in the patient’s mouth. The next stage is to take time to remove the alginate prior to retaking the impression. This is not easy with the glued tray. How long have you spent? Time yourself, it’s longer than you expect, and think again about opportunity costs. Would it be more effective use of your time to just use another tray?

In my opinion the most reliable trays to use are the metal rimlockdesign. I use perforated for alginate and solid for crown and bridge impressions, having abandoned for the most part special trays. Neither need adhesive. However there is an initial cost, and there is a risk that your trays may get lost in the laboratory. This can be avoided by inscribing your name and a number on each tray. Then record in the notes which number trays have been used and their size. When you have to take another impression, this will save time hunting through your stock for the size you want.

Finally, if your technician loses one of your nice metal trays, consider charging them for this loss. A warning works wonders, and you will only have to do it once!

Impression Technique

The standard technique is to load the chosen tray with impression material and place it in the mouth directly in the case of alginate, or after syringing flowable, low viscosity elastomeric crown and bridge impression material only around prepared teeth. In some cases this will give a perfect impression, but in teeth with deep fissures the chances of a perfect result diminish. The classic imperfection is an air blow on the fissure side of the cusp, and this is reproduced on the model as a pimple or worse a flat area of excess plaster. The pimples are usually easy to find, the flat areas harder. If these excesses are not found, then when the models are mounted the occlusion will be incorrect, your technician will make an appliance that is likely to be ‘high on the bite’, and you will spend time at the chairside correcting the fault. However, this potential problem can be prevented by re-engineering your clinical techniques.

Prevention

The risks of such excrescences can easily be prevented.

1. If you are using alginate, then before seating the loaded tray,pick up some alginate on your index finger and wipe some into the fissures. This takes about 10 seconds, well within the setting time, and significantly improves the quality of the resulting model.
2. If you are using elastomeric impression materials, in addition to syringing flowable material around the prepared teeth, syringe also into the fissures of adjacent teeth, and those of the opposite arch. Again, you are reducing the risk of missed air blows!

If you are unsure about the advantages of (a) above, then try this experiment on your next alginate impression. Wipe alginate into the fissures on one side only, then place the tray as normal. Then compare the resulting surfaces when the model is returned from your technician.

Model Materials

I recommend that you specify to your technician how you wish to have your models prepared. I like:

The A set

The first use of my alginate impressions for study models to have the teeth themselves poured up in hard die stone with the bases in white plaster. This uses a very hard more expensive material where it matters, the tooth surfaces on the model, and a cheaper material to form the base. These are the master models. Check carefully for any air blows and put them carefully to one side

The B set

A second pour in ordinary white plaster and orthodontically trimmed for ease of use. This model is softer so can be worked on to create for example a smile design, make special trays, survey for a partial denture design, passed around colleagues for their opinion etc. It is not vital if small fractures of plaster occur with this set as they will not be used for critical stages of the rehabilitation process.

When you make the definitive restoration, for example crowns on maxillary teeth, use as the opposite model, your mandibular master model from (a) above to establish the occlusion. This saves a further impression, has been checked for accuracy and absence of air blows, and will reduce significantly any time that you might have to spend on appliance adjustments at the fit stage, always assuming that your maxillary impression and cast are fault free.