Simply Crown and Bridge
198 Cambridge Road
Kingston Upon Thames
KT1 3LF

Tel: 0208 5494836

Impression and bite advice

One of the biggest problems when it comes to precision Crown and Bridge, are impressions and bites.

Now is that a dental laboratory trying to allocate blame for problems? Not at all. It is recognition of a couple of simple facts. In the laboratory our conditions are very stable. Working accurately is a question of attention to detail. In the surgery, however, your attention to detail may be perfect but constantly changing biological conditions, bleeding and saliva, not to mention difficult patients, can all conspire to produce impressions that look completely accurate but are invisibly distorted.

Here we have a big problem, because if the impression is not accurate, everything else that follows will be inaccurate. The resulting remakes waste surgery and laboratory time, and profit will be lost.

Is there anything that we can do?

Yes. Here are our recommendations for consistent high quality impression taking.

1. Impressions need to be taken in full arch trays. The trays must be of a high quality and extremely rigid. These can be plastic or metal. The standard blue plastic tray is not structurally solid enough, and regularly distorts.

2. High quality impression materials should be used. Our personal favourite is Honigum available from Henry Schein.

3. Always use a good tray adhesive. So many perfectly good impressions are invisibly distorted because they have pulled slightly away from the inside of the tray. A good tray should also have excellent retentive properties not just holes.

4. Take your time. You shouldn't need to rush. A good quality impression material will always give time to work and will be easy to mix.

5. Retraction cord. This is a must when working privately. You really don't want us guessing where the edge of your preparation is. Remember you can see the contrast of tooth and gum. We get no such help to pick out the margin.

6. We recommend a one stage technique. Two stage techniques allow for the compression of an already set material against a flowing material. If the correct pressure relief has not been provided, distortion will occur. To sum up. Two stage techniques are considerably more technique sensitive. A good one stage technique would be as follows:

A good impression, taken in Honigum, showing very clear margins and no occlusal distortion.
  • Syringe the soft body around the preparation.
  • Place the putty in the tray around the whole arch.
  • Prepare a deep indent in the putty giving space for the teeth. This helps to relieve pressure when the impression is placed.
  • Syringe the remaining soft body into the prepared indent. This must also go around the whole arch.
  • Place the tray and use an even, sustained pressure. Allow no movement while setting.

A good opposing impression, taken in Honigum, showing no occlusal distortion.
7. We always recommend using the same silicon impression materials and techniques for the opposing arch. Alginate distorts over time. It absorbs or loses moisture if not kept in perfect conditions. By the time your alginate has sat in the surgery for half a day, been with a courier or in the post for several hours, then waited to slot into the laboratories work schedule, it will not have the integrity it had when it first came out of the mouth. Remember an accurate opposing impression is just as important as an accurate working impression. Technicians cannot always tell if an opposing arch is distorted. We can be working in the belief that we have a 2mm gap when the reality is we only have 1mm.
A syringed bite in president Jet Bite a material favoured in the Laboratory for its resilient nature
The bite is cut and prepared for use
8. When taking the bite please throw away the wax. It doesn't work. You need to syringe a good quality silicon bite material into the area of the preparation. If you are preparing more than one tooth and are concerned about losing the bite, then we recommend that you prepare a few teeth first, evenly spaced around the arch. Leave enough teeth for the patient to easily find centric and then syringe your bite material into these localised areas. Then continue your preparation. The accuracy of the silicon sections enables us to easily locate the patient's pre operative position.
When cut in this way we have visible proof that there is no distortion.
9. In the lab we carefully trim the silicon bite material cutting through the area of the preparation; this enables us to accurately see if the silicon is in place. There should be no gaps around the preparation or the opposing teeth. When this has been established we know we must have the same space on the model as you had in the mouth. If any gaps are visible we know something is distorted. At least with a bite taken like this, the additional information can help us compensate for this distortion.

 

10. We don't recommend the use of triple trays for private work, but if you do wish to use them, please Download. "Triple Tray Distortion"