Layered Zirconia
Since the introduction of Zirconia as a core material, Zirconia has radically increased the use of ceramic restorations in the field of dentistry. Its supreme mechanical properties, esthetic colour and high chemical stability make it a very favoured material for restorations. However, as zirconia lacks the translucency of natural teeth, zirconia-based all-ceramic restorations are becoming the new norm as they’ve shown excellent clinical results and high survival rates.
However, chipping, fracture and delamination of the ceramic veneer are the major troubles in restorations of this kind, with a reported incidence between 0-30%.In orderto utilise the strength of the core material, the core-veneer bond strength must be strong enough to transmit functional stresses from the aesthetic veneer to the underlying framework. Focus onthe factors like framework design can improve clinical performancesas it is an important factor which is technique-sensitive and has a final say on the long-term results of layered zirconia restorations.
It is also found that the different design characteristics of the junction between ceramic and zirconia affect the bond between the zirconia core and ceramic veneer. Zirconia crowns with a ceramic veneer layering only on the buccal surface have the least chances of chipping of the ceramic layer under masticatory loads. Therefore, this design should be preferred in posterior restorations such as in molars due to higher masticatory forces present in this region, and in patients suffering from bruxism. Similarly, zirconia crowns with a ceramic veneer layering on the buccal surface that extends onto the occlusal surface are more prone to chipping of the ceramic layer under occlusal forces.
Advantages
- Suitable for both anterior/posterior crowns and bridges
- High level of aesthetics achieved
- No limit to size or length of restoration to be designed
- Metal free restoration
- Zirconia-ceramic flexure strength and fracture toughness are twice that of full ceramics
Mechanical Properties
- Young’s Modulus of Elasticity – 205000mPa
- Poisson’s Ratio – 16
To summarise, zirconia crowns offer superior esthetics, but the more the focus on esthetics, the lesser will be the strength of the restoration. Having a ceramic layering only on the buccal surfaces in molars is ideal, whereas in cases where estheticsare paramount, like premolars and/or smile designing, full ceramic veneered zirconia restorations are the go-to option.