Achieving Highly Esthetic Full-Contour Monolithic Crowns

A combination of CAD/CAM technology and zirconia allow technicians to create exceptional final results

Lee Culp, CDT

Since their introduction almost 50 years ago, porcelain-fused-to-metal (PFM) restorations have remained frequently provided treatments for posterior and high-strength indications.^1,2 Traditionally, clinicians considered PFM crowns the standard choice for strength, longevity, and ease of fabrication and placement, as the metal framework creates a strong crown foundation.^2,3 However, although the long-time standard choice, PFM restorations also present several drawbacks.

The combination of metal and ceramic creates challenges in achieving a harmonious and esthetic appearance with the adjacent natural dentition.¹ With the responsibility for combining these two incompatible materials, laboratory technicians are tasked with synergizing porcelain and metal to achieve natural-looking esthetics and functional predictability. When fabricating PFM restorations, laboratory technicians must carefully control temperatures and bond porcelain to metal with a slow heating and cooling process to increase its resistance to fracture.⁴

Common problems with PFM restorations, such as fractures and/or chips in the porcelain material, can contribute to decreased esthetics and function, and shorten the lifespan of a PFM restoration.^5,6,7 Another challenge with PFM restorations lies in the fact that their fundamental material is metal. Many patients believe there is a biocompatibility problem with metal restorations and refuse this type of treatment. Furthermore, some patients have an allergy to metal materials, and one study suggests that as many as one in six general-practice dentists encounter allergic reactions to metal alloys in their patients.⁸ From a clinical standpoint, metals can hide dental caries underneath the restorations when using radiographs, limiting diagnostic capabilities.⁹

Tooth preparation and gingival adaptation are additional considerations for PFM restorations. Many PFM crowns require significant tooth reduction to achieve mechanical retention and esthetics.⁹ An in vitro study reported that complete PFM crowns required 67.5% to 75% coronal tooth structure removal.¹⁰ Additionally, PFM restorations typically require use of retraction cords and subgingival extension of the margins, which can disturb the surrounding tissue.⁹

The development of high-strength all-ceramics provided an alternative to PFM restorations. Restorations made from a monolithic all-ceramic material eliminated many problems technicians faced. However, in their early existence, these materials (eg, zirconia) lacked natural esthetics, required outsourcing to a milling facility, and proved difficult to stain, glaze, and characterize.

With advanced technology and material developments, zirconia has come a long way. Traditionally used in orthopedic implants, additional non-dental applications use zirconia (zirconium oxide ZrO2) extensively.¹¹ Unlike PFM restorations, today’s zirconia restorations do not require veneering porcelain, eliminating the risk of chips or fractures. A study comparing zirconia and PFM crowns found zirconia to be a cost-effective alternative to PFM restorations, and that their survival times and probabilities—independent of coping systems and restoration location —did not differ.¹² Another study published in 2014 comparing zirconia and PFM crowns demonstrated adequate, similar, and comparable success rates.¹³

Now, full-contour monolithic zirconia restorative materials combine high strength and natural esthetics. Milled in the laboratory from single blocks using CAD/CAM, complete systems include stains, glazes, and colors that simplify the laboratory process while maintaining the demanded strength and life-like esthetics.

Full-Contour Zirconia Restorations

The material the author chose to treat the case presented here was Wieland Zenostar^® zirconia from Ivoclar Vivadent ( www.ivoclarvivadent.com). It demonstrates high strength and a natural appearance.¹⁴ Using CAD/CAM technology, laboratory technicians are able to design the restorations virtually, then mill Zenostar zirconia blanks monolithically into crowns and bridges without the need for applying a veneering ceramic. The monolithic property of Zenostar also contributes to preventing complications associated with bilayered ceramic systems.¹⁵

The zirconia material requires minimal preparations and remains gentle on tooth surfaces. With shorter sintering times and fast shading with stains, Zenostar eliminates unnecessary time constraints. Zenostar also demonstrates increased resistance to hydrothermal aging with an improved raw material formula, optimized manufacturing conditions, and a lower sintering temperature.¹⁴

Wieland Zenostar ZR translucent pre-shaded discs (Ivoclar Vivadent) form the basis of fast and easy reproduction of tooth shades. With high light transmission, additional stains, multiple staining options, and one-layer materials support, Zenostar achieves exceptional esthetics, making it possible to fabricate full-contour restorations with individual characterizations. These full-contour restorations with glaze provide high esthetics comparable to traditionally veneered restorations.¹⁶

High flexural strength and excellent anti-aging properties also contribute to Zenostar zirconia’s long-lasting strength.¹⁴ In a mastication simulation study, Zenostar crowns demonstrated low susceptibility to abrasion of the ceramic material and the opposing teeth compared to other materials.¹⁷ Zenostar zirconia restorations also eliminate the gingival margin discoloration that can be commonly seen in PFM restorations.

Overall, Zenostar enables laboratory ceramists to fabricate metal-free, durable, and esthetic posterior restorations that rival PFMs.¹⁴ The highly translucent ceramic material allows for extensive detail and customization to ensure a perfect fit and enhanced esthetics. One study evaluating the clinical effects of Zenostar zirconia determined that it offered high esthetics, toughness, and anti-wearing.¹⁸ Ideal for monolithic crown and bridgework, Zenostar can also be used for establishing frameworks and cut-back techniques.¹⁴

Case Presentation

A 45-year-old female presented with sensitive and unesthetic teeth (Nos. 18 through 20) due to old amalgam restorations (Figure 1). Following a thorough examination, full-contour zirconia Wieland Zenostar, Ivoclar Vivadent) restorations were chosen based on their high strength and esthetics, making them suitable for treatments in the posterior region. The appropriate shade was determined and photographed for communication with the laboratory.

Clinical Protocol

Axial and occlusal reductions were performed according to the minimum thickness considered ideal for full-contour zirconia restorations and based on the tooth anatomy (Figure 2). A definite finish line (eg, shoulder with rounded internal line angles or chamfer) was created, and all sharp edges and line angles were rounded. A conventional impression was taken and appropriate provisional restorations were placed for the patient (Figure 3). Appropriate provisional restorations were then placed for the patient.

Laboratory Fabrication

At the laboratory, the restoration design process begins with a virtual model used for digitally creating the full-contour restorations. After the full-contour zirconia restorations are designed, the appropriate monolithic zirconia block is milled Wieland Zenotec mini, Ivoclar Vivadent) (Figure 4). All slight adjustments are completed with the restorations in the green state, using only carbide grinding instruments (Figure 5). When grinding, little or no pressure is applied, as the material at this stage is relatively soft.

After sintering, the zirconia surface was sandblasted with alumina no coarser than 50 μm at pressures not exceeding 50 psi while grinding. Sandblasting promotes adhesion of cement to the zirconia restorations for a stronger and longer-lasting bond. Once sandblasting was completed, contaminants were removed from the full-contour zirconia restorations’ surfaces with steam or ultrasonic cleaning of the framework for 15 minutes. It was important that the zirconia surface be free of dirt, milling dust/residue, and oily-greasy elements.

Once the restorations’ surfaces were cleaned, the sprues were trimmed off. The internal shade infiltration technique was used to help ensure high esthetics, proper tooth shades’ reproduction, and natural tooth detail in the monolithic material. Gingiva and dentin color infiltration occurred initially (Figure 6), followed by incisal “grey-blue” (Figure 7), and finally occlusal “orange” (Figure 8).

The ceramist checks the fit of the restorations on the die, as well as the opposing teeth and interproximal contact surfaces. The occlusal surface is then polished by hand using a stiff bristle brush and diamond polishing paste (Wieland Zenostar polish, Ivoclar Vivadent). This imparts a smooth surface and ensures minimum abrasion against the opposing teeth after insertion.

For these restorations, the shade was then adjusted using additional full-contour stains (Wieland Zenostar Art Module, Ivoclar Vivadent) to ensure a polychromatic natural appearance (Figure 9). Determination of the color amount and position on the restoration was facilitated with shade tabs and patient photograph comparison. The preoperative photographs dictated the extent of color applied to the incisal/occlusal third of the crowns.

With this material and process, a single crown can be glazed in less than 10 seconds by holding the restoration with tweezers and rotating it swiftly during spraying, ensuring that the glaze layer appears thin and even. The glaze (Wieland Zenostar Magic Glaze, Ivoclar Vivadent) does not retract from the edges or form beads on polished surfaces, but creates the substrate for the stains (Figure 10).

Once staining and characterization is finalized, the fit and occlusion are checked on the model and the restorations are fired. The full-contour zirconia restorations should appear smooth and glazed, after which they can be polished (Figure 11) and a final inspection performed.

Final Restorations

The patient returned and the provisional restorations were removed. The preparations were cleaned and dried. The final restorations were tried in to verify fit and contacts. Since zirconia materials can be susceptible to salivary contamination, an additional cleaning product was used to remove visible saliva and contaminating glycoproteins. The restorations were rinsed, and the entire bonding surface was coated with the cleaning paste (Ivoclean, Ivoclar Vivadent, www.ivoclarvivadent.com) and allowed to react for 20 seconds and then air-dried. The paste opened phosphate molecules that facilitate bonding, and allowed adhesive primers to chemically bind to the treated surfaces (Figure 12).

After the restorations were sufficiently cleaned, an adhesive primer (Monobond Plus, Ivoclar Vivadent) was used to chemically bind to treated restorative surfaces to facilitate a predictable and long-term bond (Figure 13). The primer was applied to the bonding surface of the restorations, reacted for 60 seconds, and then air-dried. Next, a self-etching primer (Multilink A/B, Ivoclar Vivadent) in a 1:1 mixture was scrubbed into the tooth structure for 30 seconds and then air-dried (Figure 14). A universal resin cement (Multilink Automix, Ivoclar Vivadent) was selected because this material provided easier cleanup of excess material and generated high bond strength to ensure the long-lasting adhesion of the restoration to the tooth structure (Figure 15). The luting composite was dispensed into the restoration and seated (Figure 16). The excess cement was removed from the margins, and the cement was allowed to self-cure (Figure 17). The definite restorations were checked and the occlusion verified.

Conclusion

All-ceramic restorative materials continue to grow in popularity among dentists and patients. With improved esthetics, strength, and functionality, zirconia provides an ideal material to replace PFM restorations that can be adapted into monolithic crowns and bridges. In addition to being high in strength, these restorations can be customized with a complete system of glazes and stains. Eliminating complications that may be encountered by combining metal and porcelain, zirconia proves simple and efficient to fabricate with CAD/CAM milling technology. With zirconia, ceramists and dentists now have the tools to achieve monolithic restorations with high esthetics, function, and patient satisfaction.

Acknowledgement

All clinical dentistry performed by Lida Swann, DDS. Dr. Swann is a Prosthodontic Resident at UNC School of Dentistry.

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