In spite of high zirconia mechanical strength, chipping and fracture of layering porcelains applied to zirconia frameworks continue to be a problem. The mechanical integrity and the bonding of the layering porcelain to the framework material are key factors in the successful performance of veneer/framework restorations. Another clinical problem with the use of zirconia is the difficulty in achieving suitable adhesion to resin cement. Traditional adhesive techniques used with silica based ceramics do not work effectively with zirconia. This article reviews the literature to find the favored protocol to prevent delamination between veneering materials and zirconia framework from one part; and to focus on currently available techniques for internal surface roughness and preferred approaches for zirconia cementation.
Since zirconium dioxide was introduced in dentistry as a framework material for all ceramic restorations, a particular attraction resulted to its use in prosthodontics, due to its excellent mechanical properties (flexural strength 900-1200Mpa, fracture toughness 9,10 Mpa m½) and improved appearance, having a high degree of crack resistance. Due to suitable additives, e.g. yttrium oxide, the crystal lattice is stabilized in its tetragonal high temperature phase, which avoids further transformation into the monoclinic phase, occurring during the cooling process. When a fault takes place; at the beginning of a crack, zirconia grains are transformed locally from tetragonal to monoclinic form, and accompanied by an increase in volume. This procedure is described as transformation toughening.1,2
When properly veneered, it could turn out to be clinically acceptable regarding its color match and having the capability to mask dark backgrounds such as dark tooth or cast post and core.3
It has promising clinical results and high survival rate. However the mechanical complications reported were chipping of veneering ceramic, framework fracture and loss of retention. Chipping has several causes: inappropriate framework support for the layering porcelain, coefficient of thermal expansion mismatch, firing shrinkage of the ceramic and insufficient bond strength. Finally the structural characteristics of zirconia resist conventional conditioning methods usually employed for bonding traditional ceramic restorations to teeth.1,4
Previously, all-ceramic restorations should be placed on a heavy-chamfer or on a shoulder preparation to ensure that it will withstand the stresses that occur in the oral environment.
For zirconia, a smaller finish line can be used.5 Its classical preparation should provide 0.5mm (in case of a collar) to 1mm at the margin, 1.2 to 1.5mm at the circumferential wall, and 1.5 to 2mm at the occlusal surface for ceramic and framework thicknesses.2,6 But with the monolithic zirconia restorations, gentle preparation of only 0.5 to 1 mm are indicated.7
The finish line design does not influence the fatigue or the fracture resistance of veneered zirconia crowns. Selection of any of the finish line designs should be based on the clinical condition of the restored tooth.8
For optimal success, it is critical that the design of the substructure provides proper support and a uniform thickness of overlay porcelain.
The non-uniform porcelain thickness would heat at different rates which could lead to thicker porcelain areas not being fully processed.9 Therefore a uniform layer thickness between 0.7 and 1.2mm of the veneering ceramic is recommended.2
Using a CAD/CAM, porcelain support begins by applying a full anatomical contour of the crown (Fig. 1). A thickness of 1mm is removed to make space for the layered porcelain (Fig. 2). As presented in (Fig. 3) the final design of the coping leaving proper support for the porcelain. Regarding the connectors dimension, the minimum cross section requirement are 7 mm² for anterior 3 unit bridges, 9 mm² for posterior 3 units bridges or anterior 4 unit bridges and 12 mm² for posteriors 4 units bridges or cantilever bridges. The height of the connector surfaces should be as large as possible.2,6,10
Coefficient of thermal expansion
The ideal coefficient of thermal expansion (CTE) between the zirconia framework and layering porcelain has not been established. The use of layering porcelain with a higher (CTE) than that of the zirconia framework results in veneer delamination and extensive micro crack formation.11 A general trend is to use a slightly lower (CTE) to generate compressive stresses in the weaker veneering ceramic; this technique is used with porcelain fused to metal. In the same manner, to prevent chipping and cracking of the layering porcelain, it is recommended to have CTEs slightly lower than or identical to those of zirconia ceramics.4,12,13
Firing of the ceramic
Zirconia has the lowest thermal conductivity (2 W/ (mk) base metal 40 W/ (mk)). Consequently, zirconia requires longer duration for heat to be transferred within the material in order to insure even heat distribution in the interfacial area between the framework and the veneer, as well as the outer surfaces of the restoration even with different framework thicknesses. It also requires slowing cooling rate to prevent stresses within the porcelain, to reduce the risk of chipping or delamination, and to obtain the desired compression of the overlay porcelain.6,9
Bond strength between zirconia and layering porcelain
To achieve best bond strength between zirconia and layering porcelain, the manufacturer’s instructions should be taken into consideration and applied as per recommended.
E.max ZirCAD and VITA In-Ceram YZ don’t allow sandblasting or grinding the framework, since this can damage the surface leading to undesired phase transformation and jeopardize the bond with the layering ceramic. In case of minor correction after sintering, a thermal treatment (regeneration firing) can reverse any phase transformation.2,6 For E.max ZirCAD Zirliner must always be applied prior to the layering procedure in order to achieve a sound bond as well as an in depth shade effect and fluorescence. Zirliner has four shades for non shaded zirconia framework or clear for shaded framework.6
With the Lava Zirconia coping, sandblasting the outer surface is not necessary as the milling process results in an adequately rough surface to accept the overlay porcelain. Remarkably, very light sandblasting does not appear to have affected the bonding strength and appears to increase wetting of the porcelain.9
Whereas for the Cercon Degudent, it is advised to sandblast the inside and outside of the framework with alumina (110–125 µm, max 0.3 to 0.4Mpa, 45° angle).10 Applying a liner is indicated for the Cercon framework with the layered porcelain,14 but not required with the pressed porcelain.15
Aboushelib et al. evaluated in 6 papers the microtensile bond strength (MTBS) between the zirconia and the veneering ceramic.11,16,17,18,19,20 (MTBS) was chosen because shear bond strength (SBS) test may lead to undesirable stress distribution, causing cohesive failure and wrong interpretation of the data.
When comparing polishing to sandblasting Cercon disc or roughening Vitablocks Mark II, no effect on the core-veneer bond strength was noted. The (MTBS) of the Cercon core-veneer would be weaker if the liner was not applied which is basically used to mask the white color of the zirconia and enhance the bond strength between the core and the veneering porcelain.11
The liner should be used with some layering veneers but not in combination with pressable veneers. The two materials have different structural composition and pressing the veneer ceramic over smooth fired liner material results in poor contact between the two materials. The pressable veneers failed mainly cohesively (within the porcelain) with no liner and interfacially (between the core and veneer) when liner is applied, whereas the layered veneers failed cohesively with liner and interfacially with no liner.16
Using the pressing technique will improve the bond strength while the esthetic outcomes are more difficult to achieve because it depends on the precolored ceramic ingots. Aboushelib et al. introduced a double-veneer technique combining layering porcelain over a previously pressed-on ceramic.17
Hence, the (MTBS) of zirconia and press on ceramic wasn’t affected by the addition of a second layer of veneering esthetic porcelain.
When colored zirconia frameworks were introduced to enhance the final esthetics of the layered all-ceramic restoration, Aboushelib et al.18 found that the bond strength of colored zirconia was significantly weaker than the white zirconia frameworks. It was observed that the concentration of the coloring pigments (ferric oxide) was higher at the grain boundaries, which was at the expense of the concentration of the yttrium stabilizing elements.18 Furthermore, sandblasting these yellow samples decreased the bond strength in comparison to the white samples.
In the other two papers, Aboushelib et al.19,20 used a new CAD/CAM veneering method to fabricate a resin replica of the esthetic ceramic seated on the zirconia framework and then processed using press on technologies. In comparison to conventional layering the CAD veneered specimens failed cohesively, while conventional layered specimens failed interracially.
Whereas Nakamura et al.21 postulated that sandblasting at a pressure 0.4 Mpa developed a strong bond to veneering porcelain compared to no sandblasting or sandblasting at a pressure of 0.2 Mpa, while using a tensile bond strength test. Yet the study didn’t include any fatigue testing.
One study 22 evaluated the influence of cyclic loading and flexural strength on zirconia after being subjected to particle abrasion with either 50 or 110µm, grinding or polished as control. It was interesting to note that the polished specimens demonstrated the least reduction in flexural strength after cyclic loading. Different surface treatment increased the amount of surface damage and the number of already available surface cracks ready to propagate.
The (SBS) between zirconia and veneering ceramics wasn’t affected by thermocycling,23 however, the metal ceramic group in this study exhibited higher (SBS) than zirconia and e max group.
In a recent study by means of fracture mechanics test, the toughness of zirconia veneer interface with no treatment is significantly higher than that of interfaces subjected to airborne particle abrasion.24
Bonding resin cement to zirconia
A major weakness of dental zirconia is its inferior ability to adhere to resin cement. As zirconia has a polycrystalline structure and limited vitreous phase, neither hydrofluoric acid etching nor silanization can achieve durable zirconia-resin bonding.25 To promote micromechanical retention, sandblasting method can be used instead of acid etching, and for chemical bonding, instead of silane coupling agents, adhesive monomer could be applied.26 The surface treatment with primers or resin cement containing phosphate ester monomer such as MDP is recommended to improve the bonding to zirconia.
MDP containing resin cement and airborne particle abrasion resulted in the most initial and durable bond strength after water storage and thermocycling.27,28,29 Still, it wasn’t sufficient to use MDP alone without sandblasting when subjected to thermocycling.26 Airborne particle abrasion with 50-110µm alumina particle at 0.25 Mpa is effective in roughening zirconia surface but it can create sharp crack tips and structural defects. Therefore it is recommended to reduce the pressure and use particles up to 50µm. After airborne particle abrasion (with 50 µm Al2O3 ), water storage for 150 days, and thermocycling, (37500 cycles 5˚/55˚) the MDP containing resin cement (Panavia) showed the highest tensile bond strength30 and the highest shear bond strength after sandblasting and thermocycling (10000 times).31
Tribochemical silica coating air abrades the zirconium surface with alumina particles that have been coated with silica embedding the surface with it. This results in preparing the surface for silanization and micromechanical retention, though there can be a loss in bond strength over a long term.27,29 And simple air abrasion can get the same initial bond strength.28,32 A combination between silica coating and primer application increased the bond strength between zirconia and resin cement but it wasn’t sufficient after artificial aging like water storage and thermocycling (12000 cycles 5˚/55˚) even with the MDP containing system.
When testing the retentive strength (removal force along the path of insertion) of zirconia crowns cemented over extracted molars after airborne particle abrasion (with 50 µm Al2O3)33 or silica coating34 and thermocycling (5000 cycles 5˚/55˚), minor difference was detected between various resin cement self adhesive cement and resin modified glass ionomer cement.
In comparison to other resin cements, MDP monomer resulted in the highest (MTBS) values with zirconia and produced higher bond strength than two phosphate monomers (RelyX UniCem, and Multilink) while microshear test failed to detect such difference.35 (MTBS) is recommended to test the adhesive bonding effectiveness and the interfacial material property as its result is comparable to the micro tensile fatigue resistance which is more labor-intensive and time consuming to do.36 The (MTBS) of the MDP containing resin cement is higher than self adhesive or conventional resinous cement, independently of the surface treatment (sandblasting or silica coating).37,38
Rely X unicem had lower bond strength value but like Panavia wasn’t affected by the water aging38 or thermocycling (10000 times)31 and both cement demonstrated satisfactory performance in media with different pH.39
With RelyX Unicem, air abrasion at 0.25Mpa is recommended to contribute to a durable bond yet it can initiate surface defect. The combination of low pressure air abrasion (0.05 Mpa) and MDP containing primer is useful to achieve durable bond after 37500 thermocycling.40 On a shear bond strength test, the better bond were obtained by dual-polymerizing (Panavia, Variolink II) cements than auto polymerizing (Rely X and Multilink) resins. The better bond is associated with sand¬blasting or silica coating and an adhesive containing MDP.41
Some primers mix MDP with another molecule like Monobond plus (phosphate monomer with silane (MPS) 42 or Z-prime plus (organophosphate and carboxylic acid monomers)43 they also showed high bond strength same as the use of a phosphonic acid monomer (6-MHPA).44
Different alternative methods to treat zirconia surfaces have been evaluated to produce reliable long term adhesion. The most innovative was introduced by Aboushelib et al.45,46 and tested to (MTBS). This method was named selective infiltration etching (SIE). They applied a low temperature molting glass on selected zirconia surfaces which is submitted to a heat-induced maturation by 2 short thermal cycles 650-750˚C which resulted in stressing the grain boundary region and infiltration of the glass materials. Then the glass is removed by hydrofluoric acid solution, leaving porosities for the resin cement. The (SIE) specimens bounded with Panavia had the highest bond strength (49.8 Mpa) in comparison with airborne particle abrasion that bonded with the same cement (33.4 Mpa) or with Rely X (23.3Mpa). Contrarily to other cement with selective infiltration etching the MDP containing cement conserved their bond strength after one month of water storage. MPS also produced greater bond strength, but the use of silanes with SIE didn’t aid in producing a hydrolytically stable bond.47
A large range of mechanical, chemical or both approaches have tried to modify the zirconia surface and improve resin bonding. Nano structured alumina coating48 consists of precipitating aluminum hydroxide originating from alumina nitride powder with subsequent thermal treatment. It can improve the bond strength compared to air abrasion pretreatment. Another technique is the Chloro silane combined with vapor phase allows pretreatment that deposits a silica like layer used to increase adhesion with traditional silane and bonding technique.49 In addition, Hot etching acidic solution (HCl and FeCl3 (100˚C))also increases the surface roughness, enabling optimal bond to resin cement.50 Likewise the application of a glass-ceramic/glaze containing a major lithium disilicate phase subsequently treated with hydrofluoric acid and silane might also be useful in improving the bond strength of zirconia to resin cements.51 Furthermore, laser application that removes particles through ablation process by micro explosions and vaporization, inducing phase transformation. Yet its durable bond strength is contradictory.52,53,54,55 One more is the treatment of zirconia with plasma, which reduces the contact angle, and increases the surface wettability which may also improve its bond strength.56
Finally, a subject that’s worth studying is the aging of zirconia or low temperature degradation. It consists of a spontaneous slow transformation from the tetragonal phase to the more stable monoclinic phase in the absence of any mechanical stresses. This phenomenon decreases the physical properties of the material. Moreover, leaving the zirconia framework without ceramic veneering would expose this framework to salivary environment increasing the potential of plaque retention and reducing resistance to low temperature degradation and service life.1
According to the reviewed literature it is possible to prevent failure of veneered zirconia ceramics, and to conclude that:
– Smaller finish line can be used with zirconia.
– The framework design should provide a uniform layer thickness of the veneering ceramic.
– It is better to have identical CTE or slightly lower between veneering ceramics and zirconia.
– Veneering ceramics need longer firing time and slower cooling rate.
– Further treatment or sandblasting of the external surface before veneering, doesn’t seem to increase the bond strength, yet it can damage the surface and jeopardize the bond with layering ceramic.
– The liner is beneficial for colored zirconia using the layered technique, but not necessary for the pressed on technique.
– The use of MDP containing resin cement on air-abraded zirconia crown with 50 microns alumina particles at low pressure can be recommended as most retentive luting method. Likewise, the selective infiltration etching (SIE) method with MDP is a reliable and promising method for establishing a strong and durable bond. Many sophisticated techniques are developed to increase this bonding, but simple methods are also applicable like glass-ceramic glaze.
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Dr. Ghassan Moustapha
Dr. Elias Smaira
Dr. Mireille Rahi
Dr. Habib Abi Aad