A failed dental bridge and how to avoid it

Here is a case that went wrong, so we thought it would be a good idea to post it to our blog and discuss the learnings that can be had from it.

We made this bridge for a lovely dentist and his patient, however we asked if we could do some diagnostics first - unfortunately the patient wanted the bridge returned quickly so a decision was made to go ahead with no diagnostics.

The first bridge is the one on the right. We followed the original incisal length, however once it was tried in the patient felt they were too short, and we were instructed to lengthen them.

Seems simple right? Add some ceramic and we're done. Well, no actually. When we make a bridge with metal (as this one is) we make the metal accurately to support the ceramic evenly all over, so if we lengthen the ceramic there is a whole load of incisal ceramic with no support (this is why this bridge has a large cut incisally, we were testing to see if we thought we could lengthen without remaking the metal).

We felt in this case that we would remake the metal and start over. So we did and this is the bridge on the left.

According to the patient this is now too long!

So the solution? We highly, HIGH recommend doing a diagnostic wax up with cases like this, this then serves a few purposes.

1. It can act as way for the dentist, technician and patient to visualise the desired results AND for the technician to ascertain whether the desired result is indeed possible with the position of the teeth and gums.
2. It provides a template from which we can make a temporisation stent.

This second point is the crucial one in this case, if we had made a diagnostic we could have made a stent from this (pictured right) this could then have been used to make highly accurate temporaries which the patient could have worn to decide on the length.

If  they were too long, a little reduction is simple - if the patient thought they were too short then addition with composite to achieve the desired result is again easy.

All that is then required is an accurate impression of the agreed temporaries and we can make the bridge to match this in the lab.

A simple process that is so often overlooked to the detriment of patient happiness!

How to accurately raise the patients bite

We do lots of cosmetic smile design reconstructions in the lab, and one frequent request for occlusal rehab cases from our dentists is this "Please open the bite by 1.5mm", sounds simple huh?

Well, the problem is - where do we raise the bite from? Many replies we get to this question say "Raise the bite on the posterior teeth by 1.5 mm", but if we do this, then anterioirly the bite is raised much more.

Some times we are asked to simply raise the bite vertically by 1.5mm, but this again has big problems due to the arc of closure.

Opening the bite on an articulator is never the best solution, here is a diagram which seeks to demonstrate this further.

Notice that if the bite is opened vertically then the upper/lower relationship looks very different to if it is raised on the articulator itself due to the arc of closure... obvious really! However this is simply an exageration of what happens when we open the bite on the articulator... we have NO WAY of knowing if the arc of closure is the same as in the mouth, this means we have no way of knowing if the new opened bite will be the same new opened bite when the restorations are fitted... a recipe for disaster.

To overcome this, we recommend opening the bite in the mouth to the amount that you wish, then recording this increased vertical dimension with a squash bite. This then ensures that the opening we mount to is the same as your patient.

How have you overcome this same problem?

Getting a Smile Design RIGHT

This is a little tip for ensuring smile designs go right first time, and it involves an accurate impression of the palate at pre-op impression stage... let me explain.

When we do diagnostic wax ups for crowns or veneers we make 2 matrices to help with the preps, one shows buccal reduction and one show incisal reduction. The key point here is that the incisal reduction matrix often only locates on the palate, especially when posteriors are also being included in the smile design.

After you have prepared the veneers/crowns and we come to make the definitive restorations we use this matrix to build the restorations in to, this ensures we position the incisal edges in exactly the right place. Once again this matrix often locates on the palate only, so if the impressions of the palates from the original diagnostic and the definitive preps don't match then we are not always able to transfer this matrix.... possibilities for errors creep in!

So, although the impression of the teeth and preps is vital, for complete reliability and to reduce the chance of uncertainty, we need an accurate impression of the palate on both the original diagnostic and the final prep impressions.

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

Should you break the contact points on veneer preparations?

As the demand for minimally invasive dentistry increases and as the demand for great looking teeth increases at a steady rate also, a question we are often asked at the lab is "Should I break the contact points with these preps?"

The answer is YES.

1. When we are trying to realign teeth and need to move mesial or distal line angles
2. When there is a mesial or distal composite filling and the finish line is likely to be in composite
3. When teeth are very crowded and we are unable to get a reasonable thickness to the veneer of around 0.4mm
4. When the veneers are covering vey dark teeth and there is a risk of show through from underneath
   

The answer is NO.

1. No need to do it routinely
2. When small adjutments to shape are required
3. When no adjustment to shape is required and the veneers are for mild-moderate colour purposes
4. On spaced teeth

The simplest way to ascertain this is to do a diagnostic wax up and have the laboratory do an 'ideal preparation' model - this should take in to account the existing teeth position and the ideal position (from the waxups). This prep model should be a 'how to ge there' kind of model, with realistic proposals for preps.

So what do you think?

When would YOU break the contact points?

Speak soon,

Mark

Cosmetic Dental Laboratory Smile Designs

Hi, welcome to our first SBO Blog post!

We are excited to announce the first in a series of eCourses (each worth £100 but at no charge to you) that we are going to be launching this year on the subject of Smile Designs. Other eCourses will follow on dental implants and dentures.

Over the years we have made many of these for ecstatic patients, yet we are often asked to compromise on HOW we do them... well, no more!

Here is our definitive guide to Smile Designs from the UK's premier dental laboratory.

Sign up to your eCourse here.