Ceramics in dentistry: which material is appropriate for the anterior or posterior Dentition? Part 1: materials science

Material properties

Fracture toughness and flexural strength

Fracture toughness is a measure of the ability of a material to withstand crack propagation. Flexural strength is the resistance of a material to breakage or fracture. It indicates how much force is required to break a test sample with a defined diameter. As soon as this value is exceeded, the test specimen breaks.⁵ Values for these properties for lithium disilicate and zirconia are presented in Table 1.⁶ However, the manufacturers of IPE e.max (Ivoclar Vivadent) claim that ‘after thousands of batches produced and tested, we now confirm that the material demonstrates an average biaxial flexural strength of 500 MPa.’⁷

Table 1. Fracture toughness and flexural strength of lithium disilicate and zirconia.6

Ceramic material	Lithium disilicate	Zirconia
Flexural strength (MPa)	365	1039
Fracture toughness (MPa)	2.8–3.5	8–10

A systematic review by Sailer et al⁸ indicated that the incidence of crown framework fracture is related to the mechanical stability of the ceramic material. Weaker ceramics, such as the early feldspathic ceramics, exhibited a high 5-year framework fracture rate of 6.7%. For leucite or lithium-disilicate reinforced glass ceramics, framework fractures occurred in 2.3% of the crowns, while it is only 0.4% for zirconia-based single crowns. Most reports on zirconia have not demonstrated a problem with the zirconia framework, this being reported as circa 1% per year failure rate for core fracture or framework fracture over 5 years.^2,8

The chipping of veneering porcelain in bi-layered all-ceramic restorations is caused by core–veneer coefficient of thermal expansion (CTE) mismatch, surface grinding, inadequate core design, or overloading. To overcome the mismatch of the veneering and core CTE, McLaren and Phong⁶ suggested that using a slow-cooling protocol at the glaze bake stage to equalize the heat dissipation from the zirconia and veneering ceramic will increase the fracture resistance of the porcelain by 20%. An alternative is to use monolithic zirconia, with recent innovations in this material providing improved aesthetics compared with early types of zirconia (Figure 2).

Tinschert and co-workers⁴ compared the laboratory fracture resistance of all ceramic materials in a three-unit fixed partial denture set up. The results indicated that the flexural strength value of lithium disilicate without the veneering ceramic is not consistent, and thus it is not recommended in high-stress bearing areas such as the molar regions. However, the manufacturer of IPS e.max (Ivoclar Vivadent) has recommended it for use in the premolar region.^5,7

Kwon et al⁹ also studied the flexural strength and translucency parameters for 3Y-TZP, 5Y-ZP and lithium disilicate. It is known from previous work¹⁰ that monolithic zirconia (3Y-TZP) has the highest flexural strength. After increasing the yttria content to 5%, a partially stabilized zirconia, with approximately 50% cubic phase, resulted in a more translucent cubic zirconia. Stabilized cubic zirconia does not transform at room temperature, and cubic zirconia will therefore not undergo transformation toughening or low-temperature degradation. In other words, while it has reduced mechanical properties, it will not transform over time. Even though the mechanical properties of 5Y-TZP are weaker, the flexural strength is still higher than lithium disilicate. In this regard, Spies and co-workers¹¹ investigated the laboratory performance of three grades of zirconia formed into four-unit fixed dental prostheses, concluding that:

Ageing, and a combination of ageing and loading, did not negatively affect the fracture load of monolithic four-unit FDPs made from 3Y-TZP, 4Y-PSZ and 5Y-PSZ;

5Y-PSZ FDPs revealed significantly reduced fracture load compared to 3Y-TZP and 4Y-PSZ;

5Y-PSZ might not be suitable for restorations thinning towards the preparation margin (vertical or ‘knife-edge’ preparation)

Lithium disilicate ceramic has a higher fracture resistance than leucite-reinforced glass ceramic or feldspathic ceramic, as lithium disilicate crystals are more efficient in promoting crack deflection and crack branching. In that regard, a study has demonstrated that the fracture resistance of zirconia is higher than lithium disilicate, as polycrystalline materials are less vulnerable to fatigue degradation than glass ceramics.¹²

In the study by Balasudha and co-workers¹³ on the fracture resistance of ceramic restorations after fatigue loading, repetitive masticatory loading caused cracks to originate from the material flaws and propagate through its bulk. The results of the study indicated that cyclic loading in a wet environment will result in low temperature degradation (LTD) and ageing of the material. Ageing leads to the weakening of the material's mechanical properties. After fatigue loading, the fracture strength of zirconia (monolithic or hand-veneered) is still higher than lithium disilicate (monolithic or bi-layered). LAVA (3M, St Paul, MN, USA) hand-veneered (bilayered) in 1.2 mm occlusal thickness produced 2655 N compared to IPS e.max CAD (Ivoclar Vivadent) (bilayered) 1.5 mm, which produced 1732 N. With similar occlusal reduction for both zirconia and lithium disilicate, the flexural strength of zirconia is calculated to be twice that of lithium disilicate. IPS e.max CAD (Ivoclar Vivadent) (monolithic) with 1.5 mm occlusal reduction produced flexural strength higher than IPS e.max CAD (Ivoclar Vivadent) (monolithic) with 1.2 mm tooth preparation. With an increase in the occlusal reduction, the flexural strength is also increased. LAVA (monolithic) 0.6 mm produced 1669 N, compared to 2027 N by IPS e.max CAD (Ivoclar Vivadent) LT (monolithic) 1.5 mm. To obtain a higher flexural strength by IPS e.max (Ivoclar Vivadent), more tooth preparation is required (which cannot, in the opinion of the authors, be justified). Sorrentino and his team demonstrated that monolithic zirconia crowns, with occlusal thickness of 0.5 mm, exhibit sufficient fracture resistance to withstand occlusal loads in the molar regions.¹⁴ Johansson and his team¹⁵ compared the fracture resistance of the ceramic materials after thermo-cycling and cyclic loading, and reported that with same occlusal thickness, zirconia has higher strength compared to lithium disilicate crowns.

From this review of the material properties of lithium disilicate and zirconia, it may be considered that, in the anterior region for a long-span bridge, metal-ceramic or zirconia may be a better option than lithium disilicate. If aesthetics is the main concern, bi-layered zirconia can be used, although problems with chipping and cracking of the porcelain have been reported.¹⁶ However, monolithic zirconia could be used, for example on the occlusal surface of a crown in a posterior tooth, with veneering ceramic only being added to the labial surface, where aesthetics is most important. The use of layered restorations should be limited to highly aesthetic zones: this obviates the risk of chipping of the veneering ceramic.¹⁶ It is also claimed that, in single crowns or three-unit bridges, zirconia can replace PFM material and performs well.⁶

Aesthetics

The translucency of glass ceramic materials depends largely on the volume of crystals within the glassy matrix and the pore size.² Zirconia has been regarded as an opaque restorative material with optical and aesthetic properties less attractive than glassy ceramics, particularly in terms of translucency.¹⁴ An increase in the crystalline content to achieve greater strength often results in greater opacity. For lithium disilicate, with its higher translucency, used in conjunction with a translucent resin cement, the desired natural appearance may be achieved. Figure 3 presents an example of this material used in a crown in the upper anterior region.

Zirconia can be fabricated in a monolithic or layered configuration. The monolithic material, not veneered with any ceramic layer, may present a less attractive appearance, but, it is not affected by chipping of the ceramic layer.¹⁴ For this use, the minimum suitable thickness suggested for monolithic Y-TZP restorations is 0.7 mm while for layered prostheses, total thickness ranges between 1.0 and 1.5 mm.¹⁷

Recently introduced translucent zirconia is significantly more translucent than conventional zirconia. To enhance the aesthetics of the final restoration, the proportion of yttria is increased. This is characterized by the presence of 30–35% of cubic crystals.¹⁴ Interestingly, this aesthetic advantage is achieved while still maintaining approximately two-thirds more flexural strength than lithium disilicate.¹⁸ 5Y-TZP (translucent zirconia) has flexural strength and translucency parameters between those of 3Y-TZP (conventional zirconia) and lithium disilicate.¹⁴

The L*a*b* translucency of 5Y-ZP is slightly less than that of lithium disilicate (e.max CAD LT, Ivoclar Vivadent), so its use for highly translucent monolithic anterior restorations will still be limited.⁹ In some clinical situations, the opacity of the material may help mask discoloured substructures or cement (ie metal posts/abutments, dark teeth, etc).¹⁴ Thus, translucent monolithic zirconia (5Y-TZP) is a suitable aesthetic and mechanical compromise for use in more anterior areas of the mouth, up to the first premolars (Figure 4).¹⁴ Harianawala et al,¹⁹ however, concluded that high translucency lithium disilicate is the most translucent material among the aesthetic ceramic materials they studied.

From the viewpoint of aesthetics, the authors recommend the use of monolithic zirconia for crowns in molar and premolar teeth, although, for a crown on an upper first premolar tooth, consideration could be given to lithium disilicate if aesthetics is an important consideration and the occlusion and other tooth factors are favourable.

Bonding and cementation protocols

In recent years, the advice has been that ceramic crowns should be adhesively bonded with resin cement rather than luted with conventional cements, given that resin cements have been shown in a systematic review²⁰ to be indicated for all-ceramic restorations. To achieve a high bond strength, micromechanical and chemical bonding mechanisms are used. Micromechanical interlocking is based on the creation of micro-irregularities, pits and roughness on the intaglio surfaces by means of etching with hydrofluoric acid and/or physical treatments like sandblasting with alumina particles or diamond bur grinding.¹⁸ Bonding with Y-TZP is difficult to establish due to its acid-resistant and silica-free surface, which cannot be differentially dissolved using hydrofluoric acid. Currently, zirconia is no longer luted, but bonded, because research has indicated that a bond can be achieved if the luting cement contains 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP).²¹

Lithium disilicate

For lithium disilicate, the presence of silica makes it a hydrofluoric acid-sensitive ceramic. Glass-ceramics should be etched with hydrofluoric acid and a silane applied before bonding,² with the importance of etching on the silica surface prior to cementation having been proven.²² After etching with 9–10% hydrofluoric acid gel, a microporous and micro-retentive surface is produced as the acid dissolves the glassy matrix of the porcelain, forming a soluble hydrofluorosilicic acid (H₂SiF₆), that can be washed out with water. Figure 5 presents a diagrammatic representation of the microstructure of lithium disilicate after etching. These surface irregularities increase the surface area, thus improving the bond strength. Lee et al²² showed that the use of HF, silane and adhesive resin cement provided the highest bond strength when compared with other experimental groups. The recommended hydrofluoric acid concentration for etching is 5% for 20 seconds. Airborne-particle abrasion is not recommended for lithium disilicate owing to a significant reduction in the flexural strength of IPS e.max CAD (Ivoclar Vivadent) when this method is used.²²

Dentine surfaces require treatment with a dentine adhesive or the use of a self-adhesive resin cement, such as Rely X Unicem 2 (3M ESPE, Seefeld, Germany). However, the film thickness of the dentine adhesive must be minimal, lest pooling occurs at line angles, potentially adversely affecting the fit of the crown. An alternative solution is to use a combination of dentine adhesives that can be polymerized by the associated resin cement, for example Scotchbond Universal and Rely X Universal (3M, MN, USA) or G-Premio Bond and Link Force with DCA (GC, Leuven, Belgium).

Zirconia

Previously, zirconia was luted using conventional cements, for example, glass polyalkenoate (ionomer) cements. This was generally the practice until Markus Blatz introduced the APC concept in 2016 (Figure 6) for the bonding of the zirconia.²⁵ The advantages of the air particle abrasion concept include the roughening of the intaglio surfaces, improving the resin-cement flow into the microretentive features, thus increasing micromechanical interlocking between the resin cement and zirconia surface,²⁶ and the generation of hydroxyl groups on the Y-TZP surfaces, thereby facilitating the chemical reaction with the phosphate monomer.²¹

Colour stability of resin cements

With lithium disilicate as the material of choice for anterior teeth, the subsequent consideration of aesthetics relates to the type of cement for bonding with respect to the translucency of lithium disilicate. There are three types of resin-based cements: self-cured, dual-cured and light-cured. Polymerization can be achieved by photo-initiators (eg camphorquinone) found in light- and dual-cured resin cements, or chemical initiators (eg benzoyl peroxide) found in self-cured resin cements. The activator benzoyl peroxide, an aromatic tertiary amine, has a tendency for a yellow hue and is more likely to be oxidized in light-cured cements than aliphatic amines, which have greater colour stability.^28,29 Thus, if self-cured cements are used, the aesthetic outcome after bonding may differ from that at the try-in stage.

Dual-cured cements, as the name suggests, can be activated by both light-and self-curing. They have the advantages of both self- and light-cured cements. They have a controlled working time and short setting time. In deeper locations where light curing is not possible, the chemical initiator will interact to achieve polymerization. When the dual-cured cement is photo-activated, there will be a maximum conversion of free radicals to produce a higher bond strength to the underlying tooth structure. Thus, strength and toughness are not only about having a strong material: the choice of the underlying cement is also important.

Self-adhesive resin cements,³⁰ such as Rely X Unicem (3M) may also be considered appropriate, given that they obviate the need to use an intermediate bonding agent, which may be considered a distinct clinical advantage.

Recent research on zirconia bonding and cementation

da Silva and co-workers,³¹ using 75 3-mm thick, 6-mm diameter samples of zirconia ceramic, examined five different luting/bonding protocols for bonding to resin composite. They used tensile testing and examined the fractured surfaces to classify the type of failure. Although the samples for which silane plus Scotchbond Universal (3M) were used showed the highest mean bond strength, it was not significantly different from samples for which Z-Prime Plus (Bisco, Schaumburg, IL, USA) were used. Z-Prime Plus contains 10-MDP, a carboxylic acid monomer, among other monomers. In this regard, Scotchbond Universal (3M) also contains 10-MDP, along with dimethacrylate resins and Vitrebond co-polymer (a polyalkenoic acid co-polymer). It therefore appears that 10-MDP, which the authors state has a ‘strong affinity’ with zirconia, in combination with a carboxylic acid monomer, co-operates in the bond process with zirconia. These results were confirmed in another study that used Fourier transform infrared microscopy (FTIR) analysis.³²

Other ceramic materials available in contemporary practice

There is another group of products, termed the resin-matrix ceramics, that sits alongside the glass-matrix and polycrystalline (oxide) ceramics seen in Table 1.³³ These ceramic materials have an organic matrix highly filled with fine structure feldspathic ceramic particles. The theory behind the development of these materials was to develop a product with an elastic modulus closer to that of dentine, and that would be easier to mill from CAD/CAM blocks than zirconia, easier to adjust and may be repaired or modified intra-orally with resin composite.³⁴ Examples of these resin-matrix ceramic materials include Lava Ultimate (3M), Vita Enamic (VITA-Zahnfabrik, Bad Sackingen, Germany) and Cerasmart (GC). Lava Ultimate (3M) is a highly cured resin matrix reinforced with at least 80% nano-ceramic particles, namely discrete silica, zirconia and zirconia/silica nanoclusters, which serve to reduce the particles' interstitial spacing, thus increasing filler loading.^33,35 Vita Enamic (VITA-Zahnfabrik), a so-called hybrid ceramic, is composed of a dual network of feldspathic ceramic and polymer (specifically urethane dimethacrylate, UDMA, and triethylene glycol dimethacrylate, TEGDMA). Their applications are onlays, inlays, veneers, single crowns and implant-retained crowns.³⁶ It has been shown that such resin-based ceramics have a flexural strength of 230 MPa and a relatively low Young's modulus.³⁷ Because Lava Ultimate (3M) is less brittle, it is less likely to chip or crack when milled.³⁵ This is also the case for Vita Enamic (VITA-Zahnfabrik) and, because it combines the properties of both resins and ceramics, it displays high resistance to loads, sufficient flexibility and optical properties similar to that of natural teeth.

As noted earlier, in general terms, lithium disilicate offers superior aesthetics while zirconia offers higher mechanical properties. In recent years, attempts have been made to develop new ceramic materials with enhanced properties and to overcome the shortcomings of existing products. These hybrid materials involve the combination of existing materials such as zirconia, lithium disilicate and alumina. The increasing use of digital technology, such as CAD/CAM, has allowed the development and optimization of glass ceramic materials reinforced with polycrystalline ceramics. One such product is Celtra Duo (Dentsply Sirona, York, PA, USA) in which the lithium disilicate crystalline phase has been enriched with 10% zirconia by weight, which prevents cracking, thus reinforcing the ceramic structure. The manufacturer boasts that the product has a high polishability owing to the fine particle size.³⁸

The combination of zirconia and alumina has also been investigated in zirconia-toughened alumina and alumina-toughened zirconia. It has been known for some time that the addition of unstabilized zirconia to alumina increases fracture toughness.³⁹ The relative proportions of each material can be varied by manufacturers according to demand. Attempts have also been made to apply nanoparticles of zirconia to microparticles of alumina through pre-sintering.⁴⁰ These ‘composite’ ceramic materials have been shown to have greatly increased cyclic fatigue strength,⁴¹ higher strength, fracture toughness and an increased resistance to degradation at low temperature compared to yttria-stabilized tetragonal zirconia.^42,43

An interesting area of research is the development of grading the proportions of zirconia and alumina. This allows the material composition to change across an interface to provide a more damage tolerant and aesthetic result. This is achieved by infiltrating the surface of either alumina or zirconia, which provide a core of high stiffness, with a glass of low stiffness. Because the structure is graded, there is no sharp interface between layers and therefore reduces the potential for delamination,³³ a not uncommon occurrence in traditional systems. There is also evidence that water absorption is impeded by the residual glass at the surfaces encapsulating the zirconia, reducing the potential for hydrothermal degradation.⁴⁵ Many of these materials are still in development and it will be interesting to see whether the potential benefits will be realized.

Conclusions

Choosing the right material for indirect restorations in the anterior or posterior depends on multiple factors. Following this narrative review of laboratory studies on lithium disilicate and zirconia, the following conclusions may be drawn: