The importance of the underlying tooth shade for dental crowns

This blog post is really a pictorial case study of a metal free crown and shows the importance of communicating the underlying shade to the laboratory.

The patient came to see us like this with an old PFM crown on the UL1 which needed replacing - a metal free crowns was requested by the dentists... but was this indicated cosmetically?

The simplest answer is, we don't know unless we can see what's underneath. So we asked the dentist to take a photo of the preparation and hold a shade guide to it so we could gauge the darkness of the tooth.

Once we knew the underlying tooth shade it is then possible to take a shade match, making an informed decision about the ceramic. For example, if we take a shade for a crown that is going to be really thin, then it may be that there is not enough ceramic material thickness to achieve that shade - so we may decide to make an A2 crown in A3 ceramic if that ceramic is going to be thin.

Also, if the underlying tooth is very dark, then we may decide to mix in a little A1 to the A2 ceramic to lighten it slightly to compensate.

So, we took the shade of the patient's tooth, and drew a detailed plan of the colours. Notice 2 things:

1. We've taken a shade of the lingual as this will inevitably shine through and affect the colour of the labial
2. We have taken a 3D shade, in other words we could see that the shade needed an overlay of enamel (EN2) and ultra clear (WIN)

We then went ahead and finished the crown - this is it in situ.

Overall the result was excellent for the patient, and having the underlying tooth shade helped us work with this in order to choose the correct ceramic to make the crown with.

Dental Case Selection

This was a case that came in to us with a problem! The Upper Left central had been root treated and another dentist and laboratory had made a metal free crown.

In this case the patient was insistent on metal free as they had been researching online (sometime the internet is not helpful!) - and as you can see the root treatment shows through and makes the whole crown look grey - I'm sure it looked great on the model, but in the mouth... not so great, so how do we get around this problem?

As we have discussed in previous blogs, covering a dark tooth with a root treatment and metal free crown is very difficult, so in this case we opted for a totally opaque core.

There aren't many opaque metal free substructures around, so we used Vita Inceram Zirconia - this is a material developed by Vita many years ago before the 'true' zirconia ceramics came out. Vita Inceram Zirconia is a combination of alumina and zirconia which results in high strength and an almost opaque core.

We rarely use this material now as we're not often asked to make an opaque metal free crown (usually a porcelain fused to metal crown is used), however you need to ensure that any laboratory you do decide to use has a detailed knowledge of materials available in order that they can advice as to the best restoration for any case.

Here's the result, still a little greynes showing through, but overall a much improved result.

What experiences do you have of trying to mask dark underlying teeth with metal free? What worked? And what didn't?

Are Zirconia Crowns Better than PFM?

Many dental practices will charge a higher fee for Zirconia crowns to the patients as this is often seen as a 'better' or 'private' option. I disagree.

PFM is a perfectly good option and is the ideal choice in many cases.

Here is an example. This patient wanted a crown on the upper right central to replace an old crown that didn't match the surrounding dentition.

The dentist wanted a metal free crown as this was perceived to be the best, however upon attendance at the surgery I noticed a metal post and core and advices that a PFM would be the best option.

The patient and dentist were concerned, but I insisted and this was the result.

A nice crown that matches.

Metal free are, by their nature, more translucent. In situations with natural teeth underneath this is fine, but with darker teeth or root treated teeth the cores can show through.

So long as the technician uses a ceramic margin that incorporates a fluorescent shoulder porcelain the aesthetics of the crown should be fine. Also required is around 1.5-2mm of labial reduction to ensure the opaque doesn't show through.

So are zirconia crowns better than PFM?, Well yes, sometimes and in with the right cases.

To recognise this fact as a laboratory we charge the SAME for all our crowns, this way there is never a temptation to chose one option over another. We believe this provides the best result for the patient.

When would you use a PFM over a Zirconia crown? 

Ziconia crowns vs e.max HT

Is there a problem with zirconia crowns fracturing? Well possibly, what I present here in this video is an argument rather than a definitive YES or NO - watch the video and see what you think, then comment on the blog and let me know your thoughts.




If you can't see the video click here to watch on You Tube

How to prevent marginal ridge failure on zirconia crowns

Hi.

Many dental professionals are talking about failure rates of zirconia crowns, it's often a topic of discussion at lectures and has been a problem for us in the lab also in the past. In repsonse to this I thought I would share with you a video podcast on a design technique for zirconia frameworks to prevent this problem.... since we have been doing this... 0 failures!




If you can't see the video enclosure you can watch it here on YouTube.

Answers to some tricky dentist questions on Smile Designs

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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