Ceramics are routinely used for dental restorations. The use of ceramic as a restorative material substitution for metal-ceramic has increased substantially, because the translucent properties of ceramics can be affected by the metal core.73 Ceramics as dental materials have excellent properties, such as low thermal conductivity, thermal diffusivity, and electrical conductivity. However they are brittle and prone to fracture/ chipping.80
Resin composite repair is the alternative method for repairing chipped/fractured ceramic restorations because it is conservative, easier to manipulate, economical than replacement of whole restoration and can be achieved at the chairside.22, 58 The properties of resin composite have been continuously improved and can be used for both anterior and posterior restorations in addition to ceramic repair. Treatment recommendation for chipped ceramic and its classification was published by Heintze and Rousson et al31.
A chipping grading scale consists of three grades was established according to the treatment methods that followed the chipping. They are: small chippings which were treated with polishing, moderate chipping, repaired with resin composites, and severe chipping which led to replacement of the entire
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restoration. However, no particular criteria were provided on how the degree of chipping severity was determined.22, 31
Some authors reported that repair of fractured ceramic on the crown by surface treatments has shown a 3-year success rate of up to 89%, making this an acceptable option for suitable cases.22, 34
Repair of composite restorations is preferable to complete replacement in terms of cost, accuracy and preserving the tooth structure.6, 41 On the other hand, age of restoration, composition of the ceramics used and different firing processes used for fabrication of ceramic restoration plays an important role in repair bond strength. 23, 41, 79
Moreover repaired All-ceramic crowns were complex restoration where composite resin which is less stiff material was bonded to stiff material (all- ceramic crowns) in the fracture site. In this complex restorative system, where load was applied, high tensile stresses developed in the ceramic/composite interface. Therefore interfacial stresses are highly sensitive to variations in the elastic modulus of the materials and developed due to strain differences of the ceramic and composite resin.4, 41
It is a known fact that, glass ceramic materials have been widely used in dentistry to restore lost/fractured/decayed teeth, because of their optimum properties, adhesion ability to dental tissues,9, 71 high esthetics, biocompatibility and thermal expansion similar to the tooth structure. They are the key factors for
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their adoption by dental practioners.13, 38, 56, 71 Studies with glass ceramic reinforced by lithium disilicate crystals has shown excellent clinical outcomes with great optical/mechanical properties32, 50, 51, 71 and high survival rates over time.23, 63, 71
Pressable lithium disilicate based ceramic was a partially crystallised glass obtained from the controlled growth and nucleation of crystals in the glassy phase. It can be defined as a ceramic composed of one glassy phase and at least one crystalline phase. It has been shown to possess remarkable mechanical properties and was indicated for use in veneers, inlays, onlays, full crowns and three-unit bridges for the anterior and premolar region.5, 15
In the present study we had used lithium disilicate glass ceramic fabricated by hot press method in its two forms as monolithic and as core with fluorapatite dentin layering glass ceramic, due to its above mentioned properties. Test samples for the present study were customized as mentioned in the methodology section to be compatible with the shear bond strength testing equipment employed in the present study.
Intraoral ceramic-repair system for chipped/fractured layering ceramic rely on strong resin bonds and adequate surface treatments. These systems may increase the longevity of a failing restoration and may be a provisional, cost- effective alternative to immediate replacement.6, 7, 9
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A strong resin bond relies on micromechanical interlocking and chemical bonding to the ceramic surface.9 Studies on the repair of lithium disilicate glass ceramic suggested that before repair with resin composite, the fractured ceramic surface must be prepared with a 2 mm wide bevel around the defective site.7, 21, 22 Later, the surface of ceramic must be prepared with a fine diamond bur22, 37 cured with airborne particle abrasion, 90 and/or etching or acidulation followed by silane treatment. This kind of surface treatment influences the bond strength between the resin composite and ceramic.22, 83
Different repair systems on the dental market are compromises on various conditioning strategies. It becomes arduous for clinicians to choose the best system that gives credible results. With regards to the above mentioned literature information, adhesion seems to depend on the combination of physical and chemical bonding.
The clinical success of either a repaired ceramic restoration or a resin cemented ceramic restoration depends on the quality and durability of the bond between ceramic and the resin. The quality of this bond depends upon the bonding mechanism that were controlled in part by the specific surface treatment used to promote micromechanical and/or chemical retention to the substrate. The bond strength of resin to ceramic increases with increasing ceramic surface roughness caused by acid etching which is chemical treatment.10,12, 36
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Various authors suggested the ideal bonding to lithium disilicate glass ceramic is better achieved with the sum of etching with hydrofluoric acid (HF) followed by a silane coupling agent as a treatment protocol.8, 22, 28, 56, 71 It has also been proven in one study that hydrofluoric acid pretreatment of lithium disilicate ceramics followed by silanization showed favorable results even after long-term thermocycling.8 And this protocol has been recognized as the most accepted surface treatment for glass ceramics in several other studies.38, 56, 66, 71, 86 Etching the surface also cleans and removes debris and impurities.67 Thus in regards with above literature data, present study focused only one surface treatment which was accepted in most literature, that was chemical bonding achieved through hydrofluoric acid etching and silane application.
The etching efficiency of hydrofluoric acid depends on the concentration, etching duration, temperature. But according to the manufacturer instruction for IPS emax Press (EMX; Ivoclar Vivadent, Schaan, Liechtenstein), recommended acid concentration of 5% hydrofluoric acid for 20 seconds is sufficient to etch the surface. However, several clinical reports and in vitro studies, published, have reported different hydrofluoric acid concentrations ranging from 4.8% to 10% and etching periods ranging from 15 seconds to 60 seconds were available. Thus, the most suitable etching concentration and duration for glass ceramics is not clear, especially for lithium disilicate glass ceramics. 2, 5, 17, 27, 28, 38, 46, 47, 48, 50, 51, 55, 61, 71, 77, 91, 93
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According to Rontani et al71, the adequate surface treatment for lithium disilicate glass ceramic was achieved with the hydrofluoric acid concentration of 5% applied for 20 seconds, therefore attesting that it is not necessary to use higher concentrations of hydrofluoric acid and/or increased etching times. So in the present study the same concentration and etching duration of 5% hydrofluoric acid for 20 seconds were followed.
Ramakrishnaiah et al67 in his study found that the etched surface of lithium disilicate showed significant changes in the surface microstructure with an increase in the etching time. The width of the porosities increased at a faster rate than did the depth after having been exposed to longer etching cycles, which resulted in relatively large, wide and shallow pores. The increase in the pore width is because of the dissolution and disintegration of the weaker glassy phase. This significant loss of glassy phase weakens the ceramic and affects the bond strength.11, 67
Literature suggest that hydrofluoric acid also has the ability to condition the ceramic surface by removing the glassy matrix and exposing the lithium disilicate crystals.56, 71, 87, 92 As a consequence, it creates an increased surface area for micromechanical entanglement and improving the interaction between ceramic and resin with increased bond strength.22, 25, 38, 56, 71 Hydrofluoric acid dissolved silica phase in ceramics and caused surface degradation.22 hence in the present study, to analyse the etched surface topography, SEM study was done on the
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lithium disilicate and layering ceramic which prior to repair with repair composite resin. SEM images were obtained at 2000X magnification.
A desirable porous surface for repairing lithium disilicate ceramics was achieved by etching for 20 seconds 67, 71 which had been followed in the present study.
Also, in the present study application of silane coupling agent and bonding agent with repair composite resin were followed according to the manufacturer’s instructions. Application of a silane coupling agent to the pre-treated ceramic surface provides a chemical covalent and hydrogen bond and was a major factor for a sufficient resin bond to silica based ceramics. Silane coupling agent usually contain a silane coupler and a weak acid, which enhances the formation of siloxane bonds. Silanization also increases wettability of the ceramic surface.9, 10, 43, 88
Also, the use of a thin layer of unfilled resin prior to the composite resin improved bond strength and the interfacial quality between lithium disilicate glass ceramic and composite resin as it promotes a better infiltration to the superficial irregularities of the etched ceramic surfaces on application.56, 71
Clinically, when the fractured ceramic restoration repaired and exposed to the oral environment, various factors like mastication load, thermal variations can influence the mechanical and physical properties of the material. 28 In order to simulate oral condition artificial aging process called thermocycling was carried out in the present study.
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Özcan59 stated that thermocycling is more effective than other methods for simulation of aging of composites and creates more challenging conditions for composite restorations. Thermocycling is performed aiming to create thermal strains at the bonding interface by thermal changes in water baths between 5- 55°C.
Repetition of thermal alterations in this process weakens the bond between resin matrix and filling material.41, 70 Several factors in thermocycling can affect the bond strength test result such as temperature setting, dwell time and number of cycles. 3, 41 Thus, in this study, samples were subjected to a short thermocycling exposure simulating 3 months of clinical use and this was employed before shear bond testing.
The present study was to comparatively evaluate the shear bond strength between repair composite resin bonded to bilayered lithium disilicate ceramic with layering and repair composite resin bonded to monolithic lithium disilicate ceramic.
In terms of evaluation method, shear bond strength test was chosen because it is the most common method for investigating the bond strength between various surfaces, luting agents, and ceramics.8, 52, 54, 69, 82 Higher strength values are generally found with shear bond strength test. However, comparison of bond strength values obtained from different test methods would not be appropriate.
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The shear test was the commonly used test for evaluating the composite repair bonding.30
Shear test was also used to assess the bond strength, which is the most commonly used test for assessment of repair bond strength too. This method has advantages such as easy preparation of samples and simple testing protocol.30, 41 On the other hand, shear test simulates the oral clinical setting more efficiently than other tests.41 Shear bond strength test was performed by applying the force parallel to the bonding interface30, 32 and the shear bond strength was calculated by dividing the maximum load (in N) to the surface area (in mm2) of the composite resin. Shear bond strength value (in MPa) is the stress on the unit of area.30
After shear bond testing, tested samples were subjected to SEM analysis to assess the mode of failure. SEM analysis was done on the repair composite resin surface that got sheared from the defect well of test sample during testing. SEM images obtained at 2000X magnification.
For clinical applications, usually 15-25 MPa bond strength for direct composite resin has been reported as an optimal value depending on the composite material and repair method.30, 75, 84, 89 In the present study, Group II (monolithic lithium disilicate ceramic restoration) showed a mean bond strength of 13.88 ±3.00 MPa (ranging between 9.43-19.60 MPa), the results which are in agreement with previous studies.5, 8, 47, 55, 56, 71
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Shear bond strength values of Group I (bilayered lithium disilicate ceramic restoration), showed low bond strength value when compared to Group II (monolithic lithium disilicate ceramic restoration). Studies focusing on shear bond strength between repair resin bonded to lithium disilicate with layering are lacking, hence, shear bond strength values of Group I (bilayered lithium disilicate ceramic restoration) cannot be compared with the previous studies. Also, values obtained in this study were below the clinically accepted limits (5.34±1.37MPa).
SEM photomicrograph (Fig. 71) of pre-repaired etched surface (after 20s etching) of Group I representative sample (nano fluorapatite) sample, under 2000X magnification, revealed presence of both undissolved and dissolved surface topography. Patches of etched ceramic surface showing microporosites with poorly-defined pits were observed. There were significant areas of unetched ceramic surface present throughout the observed field, which could be one of the reason for the low bond strength in first group. SEM photomicrograph (Fig. 72)
of pre-repaired etched surface (after 20s etching) of Group II representative sample (lithium disilicate), under 2000X magnification image revealed significant change in surface microstructure as compared to that observed for Group I etched surface. The etched surface showed, a predominantly irregular surface characterized by numerous microporosities in the form of pits, grooves and few striations, that were present throughout the observed field. Fewer areas of undissolved glassy phase of lesser dimensions were also visible, interspersed
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between the predominantly etched surface. And this image appeared similar to the images obtained by the previous authors in their respective surface topography study.11, 67, 81
Thermal cycling, an artificial aging method of dental materials, which causes thermal strain on the bonding surface by influence of liquids and thereby thermal change is simulated.6, 30 These could be the reasons for low bond strength for Group I (bilayered lithium disilicate ceramic restoration ).
Shear bond values of both groups tabulated in the present study showed that Group II (monolithic lithium disilicate ceramic restoration) bond strength value was approximately 15MPa and consequently could be considered sufficient for clinical application.
The difference between our results of Group II and those of other studies may be due to several factors such as differences in the concentration of HF used, types of composite resins used, different surface treatment methods in repair process and different testing conditions. Based on the results obtained in this study, the null hypotheses was rejected, because there was high statistical significant difference between the two tested groups (p<0.05).
SEM analysis was done on the repair composite resin surface bonded to bilayered and monolithic lithium disilicate ceramic restorations, Group I SEM image (Fig. 73) at 2000X magnification showed, predominantly smoothen resin
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surface with sparsely distributed isolated areas of ceramic material, indicative of a mixed mode of failure that was predominantly adhesive in nature between repair resin and ceramic. The mode of failure pattern observed was indicative of a vulnerable bond at the ceramic-repair resin interface. Group II SEM image at 2000X magnification (Fig. 74) also showed a predominantly irregular surface. There were increased areas of the ceramic material of greater thickness distributed over the resin surface, thoughout the observed field, indicative of a mixed mode of failure that was predominantly cohesive in nature within the ceramic. The mode of failure pattern observed was indicative of improved bonding at the ceramic-repair resin interface.
It appears from the results of the present study that the repair of monolithic lithium disilicate ceramic restorations have a better survival rate as compared to bilayered lithium disilicate ceramic restorations. The impact of only one type of surface treatment protocol of applying hydrofluoric acid and one type of repair resin material was employed in the present study. Samples were subjected to a short thermocycling exposure simulatin 3 months of clinical use.
The impact of different concentrations of hydrofluoric acid coupled with different durations of application, using different repair resin subjected to longer thermocycling periods may yield different results, than those obtained in the present study.
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Further studies employing the above parameters coupled with larger sample sizes are recommended to enhance the results obtained with the present study.
Limitations of the present study in accordance with the above mentioned information includes the use of a single type of core ceramic (lithium disilicate), layering ceramic (IPS Emax ceram) and a single brand of commercially available repair composite resin material. It has been proven that the type of composite resin influences its bond strength to ceramic.30, 33Also different type of core and the layering ceramic can attribute to the bond strength with the composite resin. Future studies with large number of samples, the role of artificial saliva and the effect of longer cycles of thermocycling should be considered.