Translucent zirconia crowns and adhesive cementation

Translucent zirconia crowns: How can they be cemented and what do the new luting composites really offer? A new in-vitro study investigates factors influencing the retention strength of zirconia crowns – from preparation and cementation to aging (Image source: Kuraray Noritake).

The demands placed on modern dental materials are high: they should be aesthetic, stable, biocompatible, and reliable in the long term. Translucent Zirconium oxide Many of these requirements are met – especially in monolithic applications, which significantly reduce the risk of chipping. But what about the bond strength of these all-ceramic crowns, particularly under functional stress and after thermomechanical aging?

This question is addressed in a recent study from the University of Basel and LMU Munich, published in Clinical Oral Investigations (2025). The team systematically investigated the influence of 

• Tooth preparation,
• Fastening protocols, 
• Aging simulation and 
• Operator variability on the retention force of translucent zirconia crowns.

Study design in detail

A total of 216 monolithic zirconia crowns (material: e.max ZirCAD Prime Esthetic, 5Y/4Y multilayer) were cemented onto extracted human molars. The teeth were prepared separately in Basel and Munich to a standardized stump shape (axial height: 3 mm, convergence angle: 10°, cervical diameter: 6 mm) and randomly assigned to three cementation methods:

1. Panavia V5 (Kuraray Noritake) – classically adhesive, with primer (Clearfil Ceramic Primer Plus)
2. RelyX Universal (3M) with Scotchbond Universal Plus Primer – as a self-etching, adhesive system
3. RelyX Universal (3M) without primer – self-adhesive

The crowns were manufactured using CAD/CAM technology and inserted with a constant force (20 N). Curing was carried out according to the manufacturer's instructions.

The adhesive strength of translucent zirconia crowns depends significantly on the luting system used and the correct application of a suitable primer.

What was tested?

The test specimens were manufactured and tested in two centers:

• University of Basel (CH)
• Ludwig Maximilian University of Munich (Germany)

One-third of the crowns from Basel and one-third from Munich were tested for retention strength without aging (control). Another third were aged in a commercial chewing simulator (SD Mechatronik) at LMU Munich, and the final third in a custom hydraulic chewing simulator in Basel. Both simulation protocols consisted of 1,2 million chewing loads (50 N) and 10.000 thermal cycles (5–55 °C) to simulate five years of clinical use. Subsequently, the crowns were axially removed, and the maximum retention strength (MPa) and fracture patterns were analyzed.

Results at a glance

• Adhesive beats self-adhesive: Panavia V5 and RelyX Universal with primer showed significantly higher adhesion values ​​(~7 MPa) than RelyX without primer (~5 MPa).
  → ConclusionPrimers are crucial for the performance of zirconia crowns.
• Aging reduces adhesive strength – but not always to the same extent: In both age groups, retention decreased compared to the control group – as expected.
  Interestingly, it was found that the individual hydraulic chewing simulator (Basel) caused a greater decrease in adhesion strength than the Munich chewing simulator (SD Mechatronik) – despite identical load parameters.
  → InterpretationThe type of aging influences the material reaction more strongly than previously assumed.
• Manufacturing center as an influencing factor: Despite identical protocols, differences were observed between the centers. The authors suggest operator variability during processing (e.g., tooth preparation, primer application, drying, excess luting material) as a possible cause.
  → implicationIn future, in-vitro studies must more closely control the factor of "manufacturing" or consider it as a variable.

Clinical Relevance

The results underscore the crucial importance of a well-designed bonding protocol for the long-term stability of ceramic restorations – especially with zirconia, which, due to its lower surface energy, does not offer conventional micromechanical properties. Without appropriate pretreatment, premature failure can occur, particularly under functional loading.

Another important conclusion: Not every aging simulation is equally effective – the design of the chewing simulator can influence the validity of the results. For manufacturers, testing institutes, and research teams, this means: More transparency and standardization in the in-vitro-Aging is necessary to more reliably reflect clinical reality.

Conclusion of the authors

• The study provides practically relevant insights for selecting the appropriate cementation system for translucent zirconia crowns. The recommendation is clear: Adhesive bonding with the suitable primer is essential for achieving long-term retention and clinical success.
• The researchers emphasize how sensitive the zirconium oxide bonding system is to the respective processing-related influences and aging processes – and that even small differences in handling can have a major impact on the result.