Zirconia on implants
With the digital revolution comes a real ‘material revolution’, evidenced by the rapidly evolution of dental ceramics. Although zirconia has evolved a great deal, it is still not the ideal material: it has both advantages and disadvantages. In the last few years, partially-translucent and translucent zirconia have been added to the portfolio along with traditional zirconia. These are achieved by increasing the cubic phase. The bad news, however, is that this process decreases the flexural strength of traditional zirconia by up to the half.
There are different possibilities for using zirconia in the field of implant prosthodontics, such as (fig 5):
- zirconia abutments or zirconia cemented on titanium bases
- zirconia crowns or hybrid crowns
- fixed partial restorations with zirconia at the implant level or the abutment level
It is widely recognised that for abutments, it is generally better to have a titanium base rather than a whole zirconia abutment. A recent systematic review (Naveau et al., 2019) reached the following conclusions:
- zirconia abutments show a 96% survival rate
- when peri-implant mucosa is thinner than 2mm, the aesthetic appearance of zirconia abutments is superior to that of titanium abutments
- when angulation is greater than 20º, titanium abutments are safer than zirconia abutments to reduce fracture risk (fig 6)
The problem is not in the frame, but in the ceramic veneering that – according to the literature – is prone to chipping. In general, zirconia crowns on implants show good prosthetic survival rates (Abou-Ayash et al., 2017).
Hybrid zirconia crown (zirconia-abutment-crown)
The weakest point is the bonded interface between titanium base and zirconia. Regarding this type of restoration, there are more questions than answers:
- what is the ideal height/configuration of titanium base?
- what influence does angulation have on resistance?
- what is the ideal luting cement/luting protocol?
- what is the long-term clinical performance?
Zirconia fixed partial dentures (FPDs)
The inherent problem here is that the 5-year fracture rate of veneering ceramics is 31%, twice that of zirconia prostheses on teeth (Saito et al., 2010). This may be due to the absence of damping, and proprioception in the case of implants.
Data about zirconia FPDs are under-reported in the literature. Concerning cantilevers, information about hybrid prostheses, or whether restorations should be at implant- or abutment-level, are not indicated.
Several solutions have been proposed to deal with high rates of fracture complications:
- a more anatomical core design (to better support the veneering ceramic)
- modifying the ceramic used for veneers: CAD-CAM fabrication, slow cooling approach, etc.
- not grinding the ceramics
Without veneers, the fractures disappear. First is the matter of antagonist wear, where the roughness value is the major impact factor, according to an in vitro study (Aldegheishem 2015). The greater the surface roughness, the greater the coefficient friction and the greater amount of wear in the antagonist. Therefore, post-sintering surface modifications of monolithic zirconia restorations should be avoided. Any occlusal adjustments should be followed by a thorough polishing sequence.
Full-arch zirconia fixed prostheses
The speaker presented a clinical case treated using a full digital workflow, involving a definite prosthesis made by monolithic zirconia only in the anterior group; the incisal edges of the buccal veneers were protected by the framework to prevent chipping.
The clinical outcome of full-arch zirconia prostheses was reviewed systematically (Bidra 2017). 42 out of 285 were found to have chipping (14.7%), and 4 had fractured or loose abutments (1.4%). Another study found that the five-year cumulative survival rate of 2,039 fixed full-arch zirconia prostheses was 99.3%, with a minimal technical complication (Bidra et al., 2018). Six fractures were recorded (0.29%) caused by inaccurate casts, reduced prosthetic space and adjacent implants positioned too closely. These results support the view that monolithic zirconia are an excellent choice for full-arch implant rehabilitation.
Another meta-analysis was recently performed comparing complete implant-supported prostheses on metal-ceramic, veneered zirconia and metal-resin compositions. Chipping rate was 8%, 15% and 22% respectively. De-cementation was also found to be an issue in metal-ceramic restorations (rate of 11%), but no abutment fractures were reported (Bagegni 2019).
An overview of the speaker’s clinical recommendations can be seen in figure 7.
- zirconia abutments show high survival rates
- clinical data about zirconia hybrid abutments and hybrid crowns is still lacking
- long-term data about single zirconia crowns and FPDs is limited
- there is also limited data about segmented zirconia full-arch prostheses
- prothesis fractures are mostly attributed to limited prosthetic space
- the main complication is chipping of the veneering ceramic
- monolithic zirconia offers enhanced mechanical properties, but further development is still needed to optimise aesthetics