Reducing Porcelain Fracture Risk in Anterior Dental Crowns and Bridges

Today we have great choice in how we can fabricate dental restorations.  When it comes to artificial dental crowns and bridges- more than one crown linked together- there are quite a few possibilities.  Porcelain to metal, all porcelain, porcelain to zirconia, all zirconia... There's even all metal, which is a choice usually confined to the back of the mouth unless the patient has some really unusual notions about the concept of Bling.

Still the undisputed champ of dental crowns and bridges is the porcelain fused to metal variety.  Perhaps I should say "workhorse", because this design has served us so well for so long.  These restorations consist of substructures or frames of hard metal alloys that are cast or milled to fit very precisely to the tooth, and then beautiful porcelain baked over the metal to create the final colors and contours that we need to recreate the form and function of a human smile.  They are very strong as well as esthetic.  In fact of all the types of crowns and bridges, porcelain fused to metal demonstrates one of the better records on fracture rates.  Many factors have to be correct in order to fabricate an excellent bridge or crown.  Some include proper preparation of the underlying tooth, accurate impressions, careful shipping of impressions (unless they are digital!), superlative lab work, and careful cementation of the final restoration.  Fracture rates vary widely among studies, but when performed properly, 4% over five years is a reasonable number.  This includes everything from small fractures that can be repaired or merely polished all the way up to catastrophic fractures that require replacement of the crown or bridge.

This is kind of weird though, when you think about it.  We're talking two extremely different materials here.  Metals, no matter how hard the alloy, flex a bit under loads; porcelain is brittle and inelastic.  Metals expand and contract with temperature changes, porcelain is an excellent insulator.  And the junction between the two, the chemistry of the actual bond that fuses the porcelain onto the metal, is strange and complex and remarkable in that it even exists at all.

Still, there's no sense in running unecessary risks for our patients and there are some perturbing current trends in dentistry that increase the risk of porcelain fracture in certain situations; I would like patients to be aware of these.

In the back of the mouth, we can leave metal on the biting or occlusal surfaces.  Metals often wear down over time at very similar rates to natural human teeth.  (Porcelain wears much more slowly than teeth do.)  The metal biting surface also doesn't have any real chance of fracturing on a peach pit or a popcorn kernel.  These surfaces look ugly though and we dentists really do feel the pressure to make everything we do look just like (or better than) nature these days, so porcelain it usually is.  I rarely see any true problems with this approach.

Back to the front, though, and things are different.  The insides of the upper front teeth are not visible to much of anyone in normal conversation etc.  Yet there is a trend to place porcelain all the way up to the gums in crowns or bridges in this area of the mouth.  This does have certain positive mechanical effects and saves on the cost of metal alloys, which are more expensive pound for pound than dental porcelains. 

There is a practical side to this, though.  Sometimes in the course of human events it is necessary to treat a root canal underneath a crown or bridge that has already been done.  In these situations we can try to tap off the restoration.  The wisdom of doing this depends on how likely it is to break the tooth underneath.  With some cements and many clinical situations, drilling through the crown or bridge is the least invasive way to get to the root canal, and this is what gets done the most.  After all, who wants to tap off seven or more units of a bridge and risk breaking the whole thing?

Here is a bridge for the front teeth just back from the lab on the model with an elastic replica of the gums perched relatively close to where it should be:

   

And here is a view of the inside or lingual surface:

 

As you can see, I can drill, I can tap.  Specifically, I can drill through any of the teeth if I need to access its root canal system for treatment.  Except for the back tooth on the right, I have metal to deal with which can't break; there's no porcelain in the way.  Some vibration forces still get to the porcelain, but I'm not drilling right on it which provides a great reduction in fracture risk.

I can also tap this bridge off with a tool that hooks under the connections and taps sharply back (unless you've already experienced this procedure, don't ask...) and the fracture risk is still present, but reduced over total porcelain coverage designs because in those designs, you're tapping right onto the porcelain itself- a recipe for disaster.

Some might ask why the premolar on the right of the photo doesn't have a metal surface as well.  First of all, it is opposed by a porcelain lower crown and a matching of materials is preferable.  Second, it has already had root canal treatment and though a future need for retreatment is possible, it is unlikely.

Night guards or anti-bruxing (grinding) mouthguards for night time wear also minimize the repetitive stresses on porcelain.  In some patients who grind these are truly advisable to protect the investment the patient has made in their bridge.

As you can see, many factors must be taken into account when planning and designing dental restorations.  Protecting beautiful dental porcelains from excessive risk of fracture is vitally important and as your dentists we must constantly strive to protect the restorations which we provide to you from early failures.

Comments

[West hills Dental]

Wonderful information you have provided. It do help me a lot. And it has given me satisfaction of everything that i saw in this site. Thanks and be back soon!

[Orthodontist Fort Collins]

This is really great especially those who have crowns and those planning to have. Kudos for the article.