All Ceramic Systems

[Dental Bytes][Feature Byte]
All – Ceramic Systems:
Moldable, Pressable, Castable, Stackable

The mid 1980’s brought much excitement with the promise of stronger esthetic porcelain crown systems. At the time I didn’t thinkmuch about how these “new & improved” porcelain crowns were actually fabricated. For all I knew they were blown into the shape of teeth by glass blowers. What I did know was that these crowns could really look “impressive”. Unfortunately these early systems were not “cracked up” to be as strong as what we had hoped. They did however, pave the way for more recent advances which have lead to stronger ceramic products available today.This dental byte provides an organizing framework for your further exploration into the science of dental ceramics:

Dental Ceramics 101
It’s Not Just a Matter of Strength
How Not to Choose a System
Reference Chart

Dental Ceramics 101:

We all know that ceramics function much better under compressive stresses than under tensile stresses. Stresses can cause micro cracks which in turn weaken the porcelain. These micro cracks can form during:

fabrication of the crown (condensation, melting, sintering, etc.)
occlusal stresses
grinding of the crown

Fracture toughness of porcelain reflects it’s resistance to micro crack propagation. Attempts to increase the fracture toughness of dental porcelains has included modifying existing products as well as developing new products.Unfortunately, the relationship between improved fracture toughness and clinical performance has not been demonstrated yet in research.Strengthening mechanisms have focused on two different approaches:

Dispersion of crystals within glassy matrix of porcelain
Dispersion of alumina particles into a core porcelain

The strengthening effect comes about as the crystal or alumina particles cause an increased resistance to micro crack initiation, propagation, and size.

It’s Not Just a Matter of Strength:

It has been said that complex problems have simple, easy to understand wrong answers. This certainly rings true as one begins to probe the complex issues connected with evaluating the strength of dental ceramics. Even the current research literature produces contradictory information regarding strengths of these systems. This however is not surprising, considering the numerous factors and variables encountered as different researchers collect and analyze data.

Unfortunately, strength related laboratory testing does not translate easily into a clinical scenario. For example, data from strength tests which simply “crunch crowns” does not give us any information about clinical failure modes. Materials scientists don’t even consider strength data meaningful unless it is placed into a deeper context which includes principles of fracture mechanics.

Fracture mechanics, unlike simple strength data, puts an emphasis on design aspects and their relationship to failure. Another way to look at it is – how does the design of a structure influence stress distribution on that structure? For example, the particulars of the tooth preparation, as well as the thickness, geometry , and fit of the restoration have much to do with stress distributions. It has been said that the strength of a porcelain is as much a measure of the dentist’s & the technician’s technical skill as any inherent properties of the material.

How Not to Choose a System

When we hear claims that one commercial system “wears like enamel”, or another has “greater flexural strength” we must analyze these statements further. For example, ask ” what layer” of the ceramic system “wears like enamel” ? Did the testing take into account the necessary application of the shading porcelain? Also, what effect does the required veneering porcelain have on the strength properties of the core porcelain? Simple comparisons between systems is just not possible. Finally, conclusions drawn from laboratory data must be balanced with long-term clinical data. The good news is that all-porcelain systems do work well given the approriate situations. Discover which systems lend themselves better to different clinical scenarios. For example, castable glass systems work great when restoring a patient who requires a combination of crowns and veneers. On the other hand, an alumina slipcast system might be better when restoring a tooth next to a porcelain metal crown. A few rules of thumb for maximum success include:

  1. Don’t push the boundaries of these materials – yet.
  2. Don’t minimize the significance of:
    • Appropriate case selection
    • Tooth preparation
    • Dentist’s and technician’s skill