New perspectives

The introduction of zirconia (ZrO₂) was a milestone in the rapidly growing field of ceramics. The speaker started using zirconia fixed dental prostheses (FDPs) more than 20 years ago, when material was still relatively unknown, and was often mishandled. Infra-designed frameworks not supporting the veneer material often led to ceramic chipping, especially in long-span bridges. In a retrospective study on zirconia, the 10-year survival rate was 95.0% (CI 86–100%); however only 78.8% (CI 62.2–99.7%) were chip-free (Teichmann et al., 2018).

Another step forward was when open-pore pre-sintered zirconia became available for fabricating monolithic zirconia constructions.[1] This zirconia could be dry ground by carbide burs and the sinterisation was shorter, making the total process easier and quicker than before. Monolithic zirconia constructions were initially not particularly aesthetically pleasing, but the new frameworks occupied nearly all of the restoration’s profile, leaving only the minimum space needed to be veneered. This significantly reduced the amount of chipping, but did not totally eliminate it unless monolithic zirconia without veneering were used.

Indeed, the speaker highlighted that mechanical problems persist in veneered zirconia restorations, as shown in a 10-year RCT comparing fixed metal-ceramic prostheses with fixed zirconia prostheses. The study reported similar survival rates, but zirconia bridges were associated with significantly higher rates of framework fracture, de-bonding, major fractures of veneering ceramic and poor marginal adaptation (Sailer et al., 2018).

In an attempt to address these mechanical problems, the speaker described how their group had tried to strengthen the veneering material by replacing it with a CAD-CAM lithium disilicate glass-ceramic superstructure which fit perfectly on a specific designed zirconia framework. Both parts were fused together by sintering ceramic. At a mean follow-up of 13.9 months, this three-part ceramic restoration mostly exhibited just minor chipping compared with manually layered, zirconia veneered ceramic restorations (Grohmann et al., 2015).

Resin-bonded fixed dental prostheses (RBFDPs) are another application of zirconia. The speaker described an ongoing ten-year split-mouth study they are pioneering, comparing two wings of RBFDPs made of non-precious metal with zirconia in the restoration of upper missing laterals. Although the study has not been published yet, the most frequently encountered complication is the fracture between the pontic and the canine wing (Edelhoff et al., 2019B). It is now well known that cantilevered RBFDPs have fewer clinical failures in the anterior region (Wei et al., 2016). Some guidelines are recommended for the preparation of the abutment tooth, such as a slight chamfer finish, a 0.5mm-deep flat box in the connector side and a fossa in the centre (Edelhoff et al., 2019A). The ten-year survival rate of RBFDPs reached 98%, but the success rate is slightly lower, at 92%, due to some cases of de-bonding (Kern et al., 2017).

Adhesion

Proper adhesion of zirconia is a major factor for long-term clinical success. A recent systematic review concluded that physicochemical conditioning of zirconia leads to increased adhesion, and MDP-based[2] resin cements tend to offer better results (Özcan & Bernasconi, 2015). These results align with the protocol recommended by the International Academy for Adhesive Dentistry (Inokoshi & van Meerbeek, 2014).

Polishing

The outer surface should be manually polished. To achieve a smooth surface, a sequential application of all polishing steps should be followed (Preis et al., 2015). Regarding the matter of antagonist enamel wear that polished zirconia produces, in vitro studies show that this can be considered similar to that of natural teeth and less than that of metal-ceramics (Gou 2019). However, a higher number of clinical trials will be necessary to create a better assessment.

Four generations

ZrO₂ has undergone several changes and continuous development over different generations, in attempts to achieve better light transmission to improve its aesthetic appearance. By the increase of light transmission for FDPs, it has lost strength (Zhang & Lawn, 2018; Kwon et al., 2018); this is why connector sizes should be increased (fig 3). The monolithic translucent zirconia of two generations (e.g. Lava^TM Plus) offers an acceptable compromise between aesthetic appearance and strength (fig 4). Gradient technology (e.g. IPS e.max ZirCAD Prime) combines the translucency of 5Y-TZP with the strength of the 3Y-TZP. It should be noted that as there are different methods for measuring its mechanical properties, the information presented through advertising and marketing can be misleading.

Another interesting aspect of new monolithic zirconia is that it is less invasive. Indeed, it requires less tooth substance to be removed than any other all-ceramic crown, according to an in vitro comparative investigation (Schwindling et al., 2019).

Future perspectives

The speaker predicted that new developments in intraoral scanning will soon allow practitioners to look inside the tooth structure, to capture the internal morphology of the dentin core. This information can be built into the zirconia restoration to improve aesthetics. Furthermore, in the future the additive approach may be conducted via 3D printing (LaserCusing Process and 3D Binder Jetting).

Conclusions

• Zirconia is now not just one material, but a family of materials. Different compositions should be associated with different indications
• Today’s long-term clinical studies are based on 3Y-TZP zirconia, and report satisfying results for single, 3- or 4-unit FDPs on teeth

[1] Lava™

[2] MDP Monomer (10-Methacryloyloxydecyl dihydrogen phosphate)