Why is the crown fitting on the model but not in the mouth?

This has taken dental technicians many years to figure out! I have the answer, you may not like it!

---

Here is the scenario! We had a dentist who sent us crown and bridge cases, everything visually looked fine on the impressions including the prepped tooth not even a triple tray was used, hallelujah right? Wrong! it fitted on the model but not in the mouth, a typical scenario, dentist blames lab, lab blames dentist etc. We then suggested to use impregum material instead and we made the crown in the usual manner , guess what happened next? It fitted in the mouth as it does on the model.

The dentist told the practice manager that this was going on and that she wanted to use another lab because of that situation and it was costing the dentist a lot of time etc. Which is understandable.

The manager of my crown & bridge department just happened to walk past and speak to the head nurse and told her the situation and then suggested to her that one another dentist is having the same issues which shares the same surgery and one more dentist from the same practice but none of the other 5 associates have these issues.

After a bit of detective work! They found the 3 dentist that was having issues was using the same putty for impression with the same batch number where as the other 5 dentist was using a different batch.

I keep on saying this, you need to know what materials you are using and check the materials! Sometimes it’s not always the labs fault or the dentist, materials play a big part. I know if something is wrong in the lab with a product, we go back to measurements, batch numbers, temperatures, cooling & working times, water/powder ratios of a material, check technical procedures and even go straight to the manufactures and ask them if our procedure is correct!

Materials play a big part in transferring the information from the mouth to the lab, if the material is not correctly mixed or is out of date etc you will not know and neither will the lab if the imp is a true representation of the mouth as visually it will look correct. Although sometimes there is instant red flags in impression e.g a huge drag etc.