Ok, so you have downloaded our FREE eCourse on Smile Designs right? Well, this dentist did, liked it and then asked some great questions. I have put his questions here with my answers.
Q. Can I just confirm that Step 6: Vertical opening is taken from the Cervical Enamel Junction on the lowers to the same point on the uppers.
A. Yes
Q. By this are you talking in ICP?
A. Yes
Q. and do you want the distance for just one set of incisors?
A. Yes, just say which ones you are measuring
Q. After making your provisional restorations. If they are veneer preps how do you suggest we cement the temps in place? Would it be spot etch and bond with small amount of composite?
Absolutely, spot etch and bond. I have seen some dentists take a VERY thin diamond disk and run this between the teeth. This allows the tempbond to flow between and provide a mechanical lock for the temps. It also allows a small amount of movement of the teeth when fitting the veneers, this makes fitting easier and prevents the old problem where you fit 5 out of 10 veneers OK, but by the time you get to veneer number 7 and 8 the contacts are getting tight and by the time you try to fit number 10 they are so tight the veneers won’t seat. This was a very well respected cosmetic dentist that I witnessed using this technique.
Q. although you mention zirconia cores for all porcelain restorations you don't mention the various brands. e.g. Lava,procera,chameleon It would be really useful to have your thoughts on the advantages and disadvantages of these makes
Q. Hhmm, very difficult without being biased. Although the companies wouldn’t agree, they are all basically the same. Yttrium stabilized zirconia cores with feldspar/quartz ceramic layered on top. Some say their cores are better because they are coloured (zirconia is inherently pure white), but for me this is irrelevant as I use a different layering technique that suits a white core better.
The key factor is the working of zirconia; zirconia is inherently unstable and can change phase easily when subjected to heat. The zirconia we use in dentistry is tetragonal (heat it up and apply pressure and you get cubic zirconia.. fake diamond!), when it is heated by grinding for example, it easily reverts to monoclinic. Monoclinic zirconia is weaker and has a slightly different CTE, this results in ceramic chipping and cracks.
Many labs aren’t aware of this and merrily grind zirconia like it is metal… aaahh big problems later on when fitted… chipping, cracking and failure!
So long as the dental laboratory uses a reputable supplier (there are many CHEAP zirconia suppliers out there with copy materials) and the lab follow strict NO HEAT protocols you will be fine. Ask the lab for proof of where their zirconia comes from.. most suppliers supply a card of some form.
It is also worth noting that if you grind zirconia in the surgery USE LOADS OF WATER COOLING. If you grind the fit surface you will phase change the zirconia and again end up with cracking.
The lab must also build up the zirconia to support the ceramic, this is often very difficult and is a stage that cheaper labs miss out. This causes stress weaknesses due to cooling after firing which only manifest after fitting when the ceramic fails. Again ask for evidence from the lab on how they design their cores.
Q. and also to know the suggested preparations for the different materials. I've got myself some digital calipers and I'm not afraid to use them!!
A. Regular metal free preps. Shoulder or chamfer all the way round (no knife edges), avoid sharp internal angles. 0.8mm minimum reduction 1.5mm preferred.
Mark
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