monolithic translucent zirconia restorationThe newest material currently available for anterior and posterior crown restorations is monolithic translucent zirconia. Most companies that have a zirconia product line are including a translucent zirconia option. 

Introduced approximately seven years ago for posterior monolithic crowns, zirconia had a slight increase in translucency compared to the traditional high-strength zirconia used for crown and bridge frameworks. The increased translucency made the crowns slightly more esthetic, but it was challenging for laboratories to create consistent esthetic outcomes. The flexural strength for these ziorconias range from 1000-1400 MPa (Glidewell BruxZir Solid Zirconia, 1000-1400 MPa; Ivoclar Zenostar T, 1200 MPa ± 200; Katana Zirconia ML and HT, 1100 MPa). These materials are still widely used.

The newest translucent zirconia is certainly more translucent than the original monolithic crown materials, but the flexural strength has decreased, and now ranges from 500 to 800 MPa (Glidewell BruxZir Anterior and Ivoclar Weiland Zenostar MT, and Katana (Noritake) STML, 750 MPa and UTML 560 MPa). The translucency is highly variable based on manufacturer formulation, production process, and laboratory sintering times and temperatures. 

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The preparation design and ultimate thickness of zirconia is dependent on which flexural strength material your laboratory uses, and the manufacturer recommendation for minimal thickness of the zirconia for restorations varies significantly. The recommendations for preparation design and material thickness for posterior crowns with high-strength zirconia are:

  • ‘Feather edge’ to 0.3 mm at the gingival margin
  • 0.5 to 1.0 mm for axial walls, cusp tips, and the central groove

The preparation and material recommendations for posterior crowns made with lower flexural strength (500-800 MPa) zirconia are:

  • 0.8 to 1.0 mm in the gingival third
  • 1.2 to 1.5 mm for axial walls, cusp tips and the central groove

For anterior full coverage crowns the manufacturer preparation and material recommendations for the lower flexural strength (500-800 MPa) zirconia are:

  • 0.8 to 1.0 mm in the gingival third
  • 1.0 to 1.2 mm in the incisal third
  • 1.2 mm palatal

As a comparison, the minimum preparation and material recommendations for Ivoclar's e.max Press or CAD lithium disilicate are:

  • Posterior crowns
    • 1.0 mm in the gingival third
    • 1.5 mm for axial walls, cusp tips and the central groove
  • Anterior crowns
    • 1.0 mm in the gingival third
    • 1.2 to 1.5 mm in the incisal third
    • 1.2 mm palatal

I always recommend to clinicians that they must know exactly what material their laboratory is using for their restorations, and follow the manufacturer’s recommendations for preparation design and material thickness, in order to have predictable esthetic and long-term outcomes. Always keep in mind that the thinner the zirconia restoration, the weaker it will be, and it will become more translucent. Increased translucency can be a benefit or a complication depending on the esthetic appearance of the prepared tooth.

 

Robert Winter, D.D.S., is a member of Spear Resident Faculty.


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