Sunday, 24 April 2016

The Problem with Bonding...

So what's the problem with bonding? The only thing you can guarantee is that it's going to fail. This is something that really came to light when I saw a case that was referred in for a second opinion to our restorative consultant clinic.  



The patient above had a repeated debonding veneer on their UL1 and their dentist was considering prepping for a full coverage crown. As you can see, this poor tooth has been prepped again and again and therefore there was virtually no enamel left to bond on to...so what should we do?

Treatment options


1. Re-attempt the veneer - this is very conservative but as I've said there very little enamel remaining so the predictability of the bond is questionable

2. Direct composite - again this is conservative but again there are questions over bond strengths as well as the fact the the opposing tooth has overerupted slightly so you'll have to either slightly re-prep the UL1 or try to use the Dahl principle

3. Full coverage crown - this eliminates the bonding issue as you can create resistance and retention forms when prepping the tooth but you are sacrificing a lot of healthy tooth tissue


But why is enamel so important?


There are 2 reasons why in this case, the loss of enamel has been so detrimental:
  1. Bonding to enamel is much more predictable - this is due to the homogenous structure of enamel prisms compared to the mess of organic substance clogging up the dentinal tubules (smear layer) which is more difficult to bond to. 
  2. Loss of enamel leads to loss of rigidity - this means that in function, the tooth will flex more. Our ceramics/porcelain do not flex as much so this discrepancy promotes debonding

So what did we decide....

In the end we opted to restore with an E.max veneer, re-prepping the incisal edge to clear 1mm from the overerupted tooth and to use a dentine bonding system to maximise the bond. I've discussed this case with different clinicians and many would have opted for one of the other options. Essentially there is no wrong answer - the patient is aware that if this fails again, it's likely the tooth will have to be prepped for a full coverage crown (few months down the line, the veneer is still there....FINGERS CROSSED!).


Which bonding system?


I used Nexus 3 - a resin cement (light cure) with a TOTAL ETCH bonding mechanism... but why?

Total etch means that I etched both the dentine and the enamel, but due to the different make-up of each (i.e. the high mineral content of enamel vs dentine) you need to treat these areas differently:
  • Etch enamel for 25 seconds
  • BUT only etch dentine for 15 seconds (unless it is sclerotic)
Then on the dentine, I used a 4th generation bonding system (this is the GOLD standard i.e. separate bottles of prime, and bond e.g. Optibond FL). The prime is a wetting agent that prepares the surface of the etched dentine so that the hydrophilic end of the bond will then flow into the open dentinal tubules and form the hybrid layer. The other end of the bond molecule is hydrophobic - this is the end that bonds onto the resin. 


This shows how the bond of dentine and composite (or resin cement) looks like under the microscope - source B Van Meerbank 2008 

How does resin bond to ceramic?


We use a SILANE COUPLING AGENT to bond ceramics to resins. Your lab will usually treat your ceramic fitting surface for you using;
  • Hydrofluoric acid to roughen the surface
  • Silane to bond the organic (i.e. resin), to the inorganic (ceramic)
But you always try in your restorations before cementation don't you? Does this affect the lab treatment....? The answer is YES!

Especially when you try in anteriors with try-in pastes. So what options do you have to re-treat the surface?

  1. If you have an onsite lab you can send back to them for re-treatment
  2. Some companies have specially designed products to re-treat the surface e.g. Ivoclean
  3. Or you can clean with etch (phosphoric acid) and use an ultrasonic bath/cleaner to clean off 


I hope this clears up a few questions about bonding! To see my full presentation of the above case see here. You can also see an awesome video by David Gerdolle showing the cementation process of veneers can be found here



Please feel free to leave comments and suggestions in the box below!





No comments:

Post a Comment

Related Posts Plugin for WordPress, Blogger...