Look at the image below. What comes to mind?

cemented bridge restoration with red arrow indicating a small hole in the occlusal surface
Figure 1: While completing handwork, a hole was created through the occlusal surface of a pre-sintered, three-unit cemented bridge restoration.

This photo of a pre-sintered zirconia shows what can happen when managing a monolithic zirconia restoration. The hole in the image was created while completing handwork to enhance the occlusal anatomy. Unfortunately, a new bridge would need to be milled at this point.

However, what if the occlusal anatomy was enhanced and finished just short of creating the hole? What would be the impact on the material properties? Would a thin (less than 0.5 mm) area significantly impact the material properties in the marginal ridge area?

Occlusal Reduction and Connector Volumes

Big selling points for full-coverage zirconia restorations are material strength and natural-appearing tooth color. However, modifications made during the handling process can alter the physical properties of zirconia, including material strength.

Zirconia is milled in a pre-sintered state where the material manages like a piece of chalk. The decision to use a pre-sintered zirconia block or the puck must take into consideration the features of the mill that will shape the detail of the restoration as designed (3-axis or 5-axis), the quality of the pre-sintered zirconia (powder, binding agent, coloring agent), and the yttria content (3Y or 5Y).

The 3Y zirconia is well-known for its relatively high flexural strength numbers. The recent addition of 5Y zirconia accepts lower levels of flexural strength for improved outcomes related to color and appearance. 5Y zirconia provides 60-75% of the flexural strength of a 3Y while adding a visible difference in apparent translucency.

The 5Y zirconia is typically selected when creating a monolithic zirconia restoration. More traditional porcelain fused to metal prosthesis would be indicated when planning for a long-span fixed bridge restoration. A porcelain metal restoration consists of a transition area between the metal alloy, the opaquer used to block out the color of the metal alloy, and the feldspathic porcelain layered intentionally to create the color and contour of the definitive restoration. Monolithic zirconia for short-span fixed bridge restorations represents an improvement simply by eliminating the transition areas.

Another area visible in the photo is the connector area, which determines the durability of a fixed bridge restoration based on its design for the pontic and the material that surrounds the abutment tooth preparations. Design software provides precise measuring tools and the ability to set parameters, like the connector's material volume. For this example, it was 12 mm.

With a three-unit bridge milled in the pre-sintered monolithic restoration, it became clear the interproximal contours at the connector area needed handwork enhancements to improve the anatomy and contours of the definitive outcome.

Knowing the vertical height of the connector represents the most significant influence of material strength properties, initial adjustments focused on improving the embrasure on the buccal aspect. It is only natural to create a visible separation between the marginal ridge areas and the gingival embrasure areas to maintain cleanability.

But what is the impact on the structural durability of the connector at this point? All the feedback regarding the connector is found in the design software. Once the restoration is milled, and the connector is modified, the feedback only comes after the definitive restoration is cemented.

cemented bridge restoration close up
Figure 2: Connector modifications to the pre-sintered restoration and coloring agents added to the interproximal at the connector area visible after sintering.

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Monolithic Zirconia and Coloring Agents

There is another variable to consider. The coloring agents for zirconia are metal oxides that can be incorporated into the pre-sintered zirconia, a liquid version can be painted on the pre-sintered zirconia, or the pre-sintered zirconia can be submerged in the coloring liquid.

The next step is to sinter the milled restoration in the furnace where the material compacts together. The metal oxides used as coloring agents remain within the sintered zirconia. Literature shows the coloring agents force the zirconia grain structure to form around the metal oxides, reducing the finished restoration's strength.

Selecting a zirconia block or puck processed with coloring agents incorporated to match a shade provides a specific advantage to a 3-unit cemented bridge. Without the metal oxide added to the pre-sintered block or puck, the pontic has a high probability of being higher in value after the complete sintering process.

Adding coloring agents to the pre-sintered block improves the chances that the abutments and pontic of the three-unit FPD will match. Coloring agents can be applied with a brush, and the pre-sintered zirconia can be dipped into a container of liquid coloring agent either technique requires a more experienced technician to create the intended outcome.

Returning to the photo of the three-unit 5Y zirconia designed as a cemented bridge. It was milled from a zirconia puck labeled “pre-shade A1.” The handwork is completed, the occlusal surface has a thin area (instead of a hole), and the connectors were modified.

Now the goal is to add color to increase the intensity on the occlusal surface, the cervical areas, and the interproximal areas, including the connectors. The metal oxide coloring agent found within the pre-sintered block or puck and applied to the pre-sintered restoration is now bound within the sintered monolithic zirconia restoration. The actual impact on the material properties has not been evaluated in detail, and likely many obstacles must be overcome to create research of this nature based on the goals of each customized restoration. The assumption is the outcome will not be as strong as advertised.

diagram of zirconia pre- and post- sintered with indicators of binding agents, yttrium stabilizers, and stains
Figure 3: A diagram representing the zirconia with a coloring agent in the pre-sintered state and following the sintering process where the zirconia becomes denser while decreasing in volume.

What does this mean? The goal is to provide a practical scenario to encourage working within the manufacturer's recommended guidelines for monolithic zirconia about occlusal reduction and connector volumes. With all the options currently available and more coming to the market, monolithic zirconia will have an opportunity to provide a restorative solution for many patients in the future.

Unlike metal alloys, zirconia does not manage compromise well in terms of material thickness. Adding coloring agents could make this area of weakness more apparent. Other areas to watch with monolithic zirconia restorations include mandibular incisors, mandibular second molars, and the marginal ridge adjacent to abutment screw access openings — all likely to challenge the material thickness recommendations and likely to be enhanced by metal oxide coloring agents.

Douglas G. Benting, D.D.S., M.S., F.A.C.P. is a member of Spear Resident Faculty.