Article by Ivoclar North America

Zirconia, or yttria-stabilized tetragonal zirconia polycrystal (Y-TZP), has gained immense popularity in restorative dentistry due to its high strength, durability, and esthetic qualities. However, despite its impressive mechanical properties, the surface finishing protocol significantly influences surface roughness and the material’s wear behavior _[2].  In this article, we will explore the optimal protocols for polishing zirconia.

Polishing Protocols for Zirconia

To achieve a smooth, esthetic surface on zirconia restorations, various polishing methods are employed. The most common techniques include pre-sinter polishing, post-sinter polishing, and glazing. Let’s examine each approach.

1. Pre-Sinter Polishing

Pre-sinter polishing refers to polishing zirconia in its unsintered, or “green-state,” form, which is softer and allows for easier manipulation and smoothing. A series of polishers—such as the spiral-shaped OptraGloss® Extra Oral Polishers (Ivoclar)—can be used effectively during this stage.  For unsintered oxide ceramics, use light pressure and maintain a speed of <5,000 RPM when pre-polishing and <10,000 RPM when high-gloss polishing.

2. Post-Sinter Polishing

Post-sinter polishing is performed after the zirconia has been fully sintered and is in its final, hardened form. While effective, this process is more technique-sensitive due to zirconia’s increased hardness and resistance to abrasion. As a result, achieving comparable surface smoothness may require increased polishing time relative to pre-sinter methods.  For sintered oxide ceramics, complete both pre-polishing and high-gloss polishing between 10,000 rpm – 15,000 rpm.

3. Glazing

Glazing involves applying a glass-like coating over the sintered zirconia and firing it to produce a smooth, glossy finish. While glazing enhances esthetics, intraoral occlusal adjustments can disrupt the glaze layer, and normal functional wear may result in degradation of the glazed surface over time.

Scientific Insights: SEM Analysis and Scientific Studies

Scanning Electron Microscopy (SEM) provides a detailed analysis of surface topography, revealing micro-level differences in texture not visible clinically.

SEM images comparing pre-sinter polish (60s 2-step polish with OptraGloss®), post-sinter polish (60s 2-step polish with OptraGloss®), and glazed (IPS e.max® CAD Crystall./Glaze Paste) zirconia surfaces show that pre-sinter polishing with systems like OptraGloss for 60 seconds produces the smoothest surface finish. In contrast, both post-sinter polished and glazed zirconia exhibit greater surface roughness, particularly in the difficult-to-reach buccal groove.

Furthermore, clinical literature indicates that glazed zirconia demonstrates higher wear potential against natural enamel, which may contribute to unintended damage to the opposing dentition _[1][2].

Conclusion: Optimal Polishing Protocol for Zirconia Restorations

To achieve the most efficient, esthetic, and functionally sound results, pre-sinter polishing is an effective protocol. Supported by SEM images, this approach yields a smooth zirconia surface which is friendly to the opposing dentition.

While post-sinter polishing and glazing remain clinically acceptable, they may not offer the same level of efficiency.  In addition, glazed zirconia finishing protocols are shown to cause more material and antagonist wear when compared to polished zirconia _[2].  When choosing a glazing protocol, consider polishing the occlusal surface prior to applying glaze to help prevent additional wear to opposing dentition as the glaze degrades.

In the end, clinicians should weigh both esthetic goals and functional demands when selecting the appropriate finishing protocol for each case.

References

1. Mitov G, Heintze SD, Walz S, Woll K, Muecklich F, Pospiech P. Wear behavior of dental Y-TZP ceramic against natural enamel after different finishing procedures. Dent Mater. 2012;28(8):909–918. doi:10.1016/j.dental.2012.04.010
2. Janyavula S, Lawson N, Cakir D, Beck P, Ramp LC, Burgess JO. The wear of polished and glazed zirconia against enamel. J Prosthet Dent. 2013;109(1):22–29. doi:10.1016/S0022-3913(13)60005-0