All-ceramic crowns are widely used in clinical dentistry and are one of the best treatment options for tooth restoration and aesthetics. Zirconia is the most common restorative material used in all-ceramic crowns. When fabricating zirconia all-ceramic crowns, they can be made from a single layer of ceramic material or a double layer of ceramic material consisting of a core crown and a veneering ceramic. Typically, the latter provides better restoration results in terms of shade and aesthetics, making it recommended and favored by many clinicians and patients.
However, the combination of double-layer materials may come with stability issues. Nevertheless, thanks to advancements in materials and technology, the occurrence of such adverse events has significantly decreased. However, a recent large-scale systematic review validated the following conclusion: when it comes to all-ceramic crown restorations, the failure rate and complication rate of single-layer zirconia (MZ) restorations are significantly lower than those of porcelain-veneered zirconia (PVZ), especially in terms of ceramic fractures. Want to know how to reduce ceramic fractures? Let’s find out together.
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The retention rate of double-layer all-ceramic crowns is lower than that of single-layer crowns.
A retrospective systematic review from Sweden included 74 studies reporting on 6,370 zirconia all-ceramic crowns (4,264 PVZ and 2,106 MZ) with a follow-up period of up to 152 months. The study excluded cases involving removable partial dentures or implants.
The results showed a total of 216 crown failures, with 117 failures in PVZ (4.2%) and 39 failures in MZ (1.9%). At the 5-year follow-up, the retention rate of PVZ restorations was 88.7%, significantly lower than the 93.3% retention rate of MZ restorations, with a statistically significant difference (P=0.007). In the study, “failure” was defined as crown detachment or replacement, while “retention” was defined as the restoration remaining in use in the mouth, regardless of the occurrence of adverse events.
The average time to restoration failure was 38.6 ± 24.0 months for PVZ and 40.3 ± 27.4 months for MZ, with no significant difference.
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Double-layer all-ceramic crowns are more prone to ceramic fractures.
The study also showed a significantly higher incidence of ceramic fractures in PVZ compared to MZ, which may be due to mismatched thermal expansion between the veneering ceramic and the core crown, leading to the development of residual stresses. The incidence of minor ceramic fractures was 7.5% (298/3,993) for PVZ and 0.3% (7/2,106) for MZ, while the incidence of severe ceramic fractures was 2.6% (102/3,993) for PVZ and 0.1% (2/2,106) for MZ, with both differences being statistically significant (P<0.001 and P=0.002). “Minor ceramic fractures” were defined as those that could be repaired chairside (e.g., grinding and polishing), while “severe ceramic fractures” were defined as those requiring laboratory repair or replacement of the restoration.
It is worth noting that there was a significant difference in the incidence of ceramic fractures when different adhesive systems were used in clinical practice. So, which adhesive system can reduce the occurrence of ceramic fractures? The study analyzed three adhesive systems: glass ionomer cement, adhesive resin cement, and resin-modified glass ionomer cement (RMGIC). For PVZ restorations, the use of glass ionomer cement had the lowest incidence of ceramic fractures, with rates of 3.0% for minor fractures and 1.3% for severe fractures. The use of RMGIC had the highest incidence, with rates of 19.5% for minor fractures and 8.0% for severe fractures. The use of adhesive resin had rates of 6.9% for minor fractures and 2.3% for severe fractures. For MZ restorations, there were no ceramic fractures when using glass ionomer cement for bonding. The incidence of minor fractures was low (0.2%) when using adhesive resin, while RMGIC had an incidence of 1.7% for minor fractures. It is evident that the use of glass ionomer cement for bonding these two types of restorations is beneficial in reducing ceramic fractures. The use of zinc phosphate cement as an adhesive is not recommended.
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The pulp vitality loss of the supporting teeth is more likely to occur with PVZ crowns.
The study results also showed that compared to MZ, there was a significantly higher incidence of pulp vitality loss in supporting teeth after PVZ crown restorations, with rates of 0.3% (6/2,106) and 1.4% (59/3,907), respectively, with a statistically significant difference (P<0.001).
This may be because the double-layer ceramic structure of PVZ restorations is thicker, meaning more tooth structure needs to be removed, which increases the risk of heat damage to the pulp during tooth preparation. It is recommended to preserve a thickness of 2 mm of tooth structure, as it usually provides adequate protection for the pulp.
Regarding other study indicators, there was no significant difference in the incidence of crown detachment between the two types of all-ceramic crowns. The use of different adhesive systems or air abrasion before bonding had no significant impact. The incidence of fracture (0.8% and 0.2%) was also not significantly different between the two types of restorations.
All-ceramic crown restorations are a mature and reliable technique, and both double-layer and single-layer ceramics have their own advantages and disadvantages. What is the preference in clinical practice, and how should the precautions be explained to patients? Feel free to discuss and share in the comments section.
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