As material science advances, what common “myths” exist regarding porcelain dental restorations?

Gregg A. Helvey, DDS, MAGD, CDT; Edward Lowe, DMD; Nelson Rego, CDT; Bobby Williams, CDT

Dr. Helvey

Esthetically, porcelain has some of the best optical properties to mimic tooth structure. A number of "myths," however, are associated with the handling and placement of porcelain restorations. The first myth is that all shade guides are the same regardless of the manufacturer, allowing clinicians to use just one shade guide for all different manufactured porcelains. This is false, as each manufacturer has its own shade formulations, which do not universally match shades from other manufacturers. For example, an A-2 shade from one manufacturer will not exactly match the A-2 shade from another manufacturer. Therefore, clinicians should always use the manufacturer's specific shade guide to select the correct shade of that manufacturer's porcelain.

Lighting in the operatory is also critical when taking shades. A shade tab viewed under one source of light may appear different under a different light source. This phenomenon is referred to as "metamerism" and has an effect on the final shade of the restoration. In other words, the lighting in the dental operatory or area of the office designated for shade taking and the lighting in the dental laboratory must be, literally, on the same wavelength to match shades.

A second myth is that when selecting a particular type of porcelain the material's flexural strength is the most important physical attribute. Actually, the most important physical property in terms of strength in fracture mechanics of a brittle material is fracture toughness. It can be defined as a measurement of the material's resistance to brittle fracture when a crack or flaw is present. Ceramic restorations can contain flaws that are the source of cracks that propagate a lead-up to a catastrophic failure. Clinically, a porcelain restoration is subject to chewing forces that apply subcritical loads. Over time, a porcelain restoration can fail due to this constant cyclic fatigue by crack propagation. Therefore, porcelain material that has higher fracture toughness will be better suited clinically than a material with a lower fracture toughness.

A third myth is that the time required to etch the porcelain intaglio surface with hydrofluoric acid is the same for all brands of porcelain. Not so; each type of porcelain, be it lithium-disilicate, leucite-reinforced, or feldspathic, should be etched for a time recommended by the manufacturer, and zirconia should not be etched at all. In addition, the clinician should know if the laboratory etched the porcelain restoration before sending it to the dental office, as etching a second time may lead to an over-etched intaglio surface, which will appear very white and chalky. If the restoration was previously etched, the intaglio surface should be cleaned after try-in with either a commercial cleaner (eg, Ivoclean^{^®}, Ivoclar Vivadent, ivoclarvivadent.com), steam cleaner, or phosphoric acid rather than re-etching the surface. After the intaglio surface is cleaned and dried, silane or a 10-methacryloyloxydecyl dihydrogen phosphate (MDP)-containing ceramic primer can then be applied. To avoid confusion the dental office may request that the lab not etch or silanate the restoration and simply condition the intaglio surface after the try-in is confirmed.

Finally, a fourth myth is that bonding procedures/steps for porcelain and zirconia restorations are the same. This, too, is false. The classification of the term "ceramic" includes all-ceramic restorative materials (zirconia and porcelain), whereas the classification of "porcelain" includes only materials that have a glass matrix. It is the glass matrix in porcelain materials that is dissolved by the hydrofluoric acid leaving a rough retentive surface that is necessary for bonding resin to penetrate and provide retention.

The bonding steps for porcelain restorations include etching the intaglio surface for the required time, rinsing, drying, and then applying a chemical connector (silane or MDP-based ceramic primer), followed by application of the bonding adhesive. For zirconia, before cementation and after try-in confirmation, the conditioning process is different from porcelain restorations. The latest consensus for zirconia cementation with a resin-based bonding agent is to treat the intaglio surface tribochemically with silica particles (using, for example, CoJet^™ [3M ESPE, 3m.com] blast coating agent applied with an air-abrasion unit). After the intaglio surface is sandblasted, a MDP-based ceramic primer is applied to the intaglio surface, and the zirconia restoration is then inserted with a resin-based bonding adhesive.

Dr. Lowe

The use of ceramic restorations is increasing in dental practices. Now clinicians can meet patients' esthetic expectations without compromising clinical outcomes by using contemporary restorative materials. However, because of the variety of ceramic materials available, clinicians must be aware of the various "myths" about them. Ceramic materials can be classified in various categories: porcelain (feldspathic, glaze), glass-ceramics (leucite-reinforced feldspathic, lithium disilicate, lithium silicate), alumina, and zirconia.

A common myth today is the thinking that all zirconia materials are the same. This is untrue, as zirconia materials are classified in three categories based on yttrium molar content-3Y, 4Y, or 5Y tetragonal zirconia polycrystalline (TZP)-with strengths of approximately 1200 MPa, 850 MPa, and 650 MPa, respectively. In addition, the processing of these materials also affects their quality. Therefore, clinicians must understand each material's properties and prescribe accordingly. For example, zirconia materials with 650 MPa (5Y) need a minimum thickness of 1.5 mm with a deep chamfer margin, whereas stronger zirconia materials having 1200 MPa (3Y) strength can accommodate a feather edge with a minimum thickness of 0.5 mm.

In addition, translucency is observed differently in zirconia materials than in traditional glass-ceramics and porcelains. Higher-translucent zirconia materials obtain their increase in translucency internally. Surface texture and lighting conditions can greatly affect what the patient sees, sometimes leading to restorations appearing to be "gray."

Another myth is that zirconia restorations cannot be chemically bonded. This has been disproven in the literature.¹ To achieve chemical bonding with zirconia, the restoration must be sandblasted to attain a micro-retentive surface. Secondly, the zirconia surface must be cleaned after try-in. A product such as Ivoclean can be used to achieve a clean zirconia surface after saliva contamination. Finally, a zirconia surface should be conditioned with a MDP-containing primer, such as Monobond Plus (Ivoclar Vivadent). Because there is no glass phase in zirconia, it is difficult to acid-etch a zirconia surface; therefore, sandblasting should be used.

Within the glass-ceramic materials category, a myth is that lithium-disilicate and lithium-silicate materials can be used interchangeably. Although both of these materials have similar chemistries, strength, fatigue, fracture toughness, and clinical studies have shown that lithium-silicate materials behave more like leucite-based materials.^2-4

Layered and monolithic restorations are not the same. As is well-known, feldspathic materials with strengths in the range of 160 MPa are used as layering materials. These strengths are significantly lower than those of monolithic restorative materials, which range from 500 MPa to 1200 MPa. Therefore, layered restorations have a significantly higher chance of chipping compared to monolithic restorations.

In conclusion, a variety of restorative material options are available that allow clinicians to provide the optimum service to their patients. However, it is extremely important to understand a material's properties and read instructions-instead of following myths.

Mr. Rego

One common "myth" regarding porcelain dental restorations is that esthetic zirconia is stronger than lithium disilicate. Clinicians are constantly inundated with marketing claims about the high-strength zirconium that is available in dentistry today. The numbers-ranging from 750 MPa to 950 MPa-are all prominently displayed, giving clinicians the impression that the higher the number the better the material. However, this is not necessarily so. Heretofore, the highest-strength all-ceramic material was lithium disilicate, which has a strength of 400 MPa. At a glance it would seem apparent that the material with the higher MPa was stronger. However, the author has found in ongoing research not yet published that a lithium-disilicate restoration when adhesively bonded is harder to fracture than a thicker high-translucent (HT) zirconia restoration. Thus, more tooth structure would need to be reduced to achieve a zirconia restoration that may still be weaker than a lithium-disilicate restoration, though research still needs to confirm this.

Another myth is that lithium disilicate should be sandblasted prior to cementation in order to remove saliva contamination; however, this reduces strength.⁵ Lithium-disilicate restorations can be cleaned with phosphoric acid or Ivoclean.

Zirconium restorations require a very different treatment protocol, as zirconia attracts and binds to salivary proteins. In fact, treating a zirconium restoration with phosphoric acid before cementation results in a 50% reduction in bond strength.⁶ The best way to remove salivary proteins from zirconium is to use Ivoclean or sandblast with 50-µm aluminum oxide at 2 bars.

It is commonly thought that HT zirconia requires less tooth reduction than lithium disilicate; however, clinicians often are unaware that the preparation guidelines for HT zirconium (5Y TZP) require much more tooth reduction than the more opaque materials (3Y TZP). In fact, reduction guidelines for esthetic zirconium call for 1.5 mm occlusal and 1 mm to 1.5 mm facially. Ironically, the notion that new esthetic zirconium is stronger than the more esthetic lithium disilicate is incorrect, as e.max^® (Ivoclar Vivadent) can be fabricated as thin as 1 mm and when bonded is stronger than esthetic zirconium while requiring less tooth reduction.⁷

Finally, some believe there is no difference between bonding and cementing lithium disilicate. However, this is not so. Clinicians need to carefully consider the effect of bonding and material thickness in relationship to fracture when loading a restoration. If there is 1.5 mm of occlusal reduction then cementing the restoration will have the same strength as a bonded restoration that is 0.8 mm thick.

New materials, digital technology, and patient awareness all bode well for the future of dentistry, but clinicians and dental technicians need to carefully consider all of the options available to them to provide the best and most conservative treatments for their patients.

Mr. Williams

Two myths regarding this topic come to mind: First, zirconia restorations are less esthetic than other all-ceramic restorations. Second, zirconia restorations have a higher failure rate than other all-ceramic restorations.

With the advances in all-ceramic materials, especially zirconia, esthetics have improved dramatically over the past few years. Like anything else, "you get what you pay for," meaning there are many less expensive, less esthetic zirconia products on the market. However, a number of manufacturers produce beautiful, highly esthetic materials, which come in a wide range of opacities, chroma saturation, and strength.

Today, a dental laboratory can obtain a pre-shaded puck, a non-shaded puck that can be characterized by painting the green state in the pre-sintered form, and, finally, a multilayered puck with translucency in the incisal third and chroma in the gingival third. Most of these options are offered at different strengths. Typically, the more translucent the material, the lower the flexural strength will be because of the inclusion of very small amounts of alumina. Even the most translucent zirconia is still around the 500 MPa to 700 MPa range, which, of course, is plenty of strength for single crowns.

The recent 2019 Midwinter Meeting in Chicago saw several material companies introduce new zirconia, and the trend seems to be pre-shaded, gradated pucks with even higher strength than ever before. The dental industry now possesses zirconia that is esthetic enough for milling beautiful monolithic restorations that rival lithium disilicate. However, a dental laboratory needs to fully understand when to use each puck in different situations. While strength may be needed for a full-zirconia hybrid, conversely the correct zirconia may be needed to optimize esthetics. As with lithium disilicate, the dental laboratory must be careful not to use a puck that is too translucent.

With the combination of today's very natural, virtual tooth libraries and the beautiful materials being produced, the author's laboratory has gravitated toward monolithic, zirconia restorations. On most cases, I'll cut back facially leaving all the functional areas in zirconia, even the incisal edge.

The second myth concerning a higher failure rate has long since been remedied. Unfortunately, when zirconia first came to the market in the early 2000s, the material companies that produced veneering porcelain gave the dental laboratories porcelain-fused-to-metal firing cycles. It put laboratories in unchartered waters with no idea that the firing cycles were incorrect.

The rate of climb in the porcelain oven was too fast and effectively only matured the veneering porcelain about 75%. Additionally, the veneering porcelain needed to be slow-cooled because the zirconia substructure was a fantastic insulator of heat. By not slow-cooling, ceramists were unknowingly causing microfractures in the veneering porcelain because it would cool down and contract much faster than the zirconia.

Once the slower rate of climb and the slow cooling was implemented, the veneering porcelain failures virtually disappeared. I've taken it a step further and produce most of my zirconia restorations in either full monolithic or monolithic with facial cutbacks.

Gregg A. Helvey, DDS, MAGD, CDT
Associate Professor, General Dentistry,
Virginia Commonwealth University School of Dentistry, Richmond, Virginia;
Private Practice, Middelburg, Virginia

Nelson Rego, CDT
Co-owner, Smile Designs by Rego,
Santa Fe Springs, California

Edward Lowe, DMD
Private Practice, Vancouver,
British Columbia, Canada

Bobby Williams, CDT
Owner, Synergy Ceramics Dental Studio,
Plano, Texas

References

1. Thammajaruk P, Inokoshi M, Chong S, Guazzato M. Bonding of composite cements to zirconia: a systematic review and meta-analysis of in vitro studies. J Mech Behav Biomed Mater. 2018;80:258-268.

2. Hill T, Tysowsky GW. Fracture toughness of five CAD/CAM glass-ceramics. J Dent Res. 2016;95(spec iss A):Abstract 1672.

3. Randi W, Randi A. Biaxial flexural strength and fracture toughness of dental ceramic. J Dent Res. 2018;97(spec iss A):Abstract 1300.

4. Randi A, Abumelha M. Fatigue resistance of 1 mm thickness CAD/CAM crowns. J Dent Res. 2018;97(spec iss A):Abstract 1301.

5. Menees TS, Lawson NC, Beck PR, Burgess JO. Influence of particle abrasion or hydrofluoric acid etching on lithium disilicate flexural strength. J Prosthet Dent. 2014;112(5):1164-1170.

6. Pathak K, Singhal S, Antonson SA, Antonson DE. Effect of cleaning protocols of saliva-contaminated zirconia restorations: shear bond strength. J Dent Res. 2015;94(spec iss A):Abstract 3656.