Material and Method: All-ceramic blocks (IPS Empress II; Ivoclar Vivadent, Schaan, Liechtenstein) sized 4.00 mm in length, 5.4 mm in width, and 3.00 mm in height were fabricated by dental laboratory as thirty specimens. CAD/CAM zirconia blocks (n =40) (Prozir; SeramDent, Turkey) sized 5.00 mm in length, 5.4 mm in width and 13.0 mm in height by CEREC System were fabricated from fully sintered Y-TZP core. Zirconia specimens were randomly divided into seven groups for the following different intraoral repair systems(Clearfil, Cimara Zircon, Bisco) and a control group. Every ten specimens were repaired as same sized. Control group was fabricated by conventional firing as unbroken solid zirconia ceramic samples. Each specimen underwent 5000 cycles of thermocycling. The SBSt (Shear bond strength test) was performed by loading force on the repaired piece to record load-to-failure. Failure mode was evaluated using a digital microscope and SEM. SBSt data were analyzed using one-way ANOVA and Tu-key’s HSD test.

Results: Clearfil and Cimara Zircon systems significantly increased the bond strength for composite resin method when compared with the Bisco system (respectively p <0.001, p =0.001). All-ceramic method significantly increased the bond strength when compared with the composite resin in Bisco system (p <0.001).

Conclusion: Although the composite restoration method is effective for repair, the all-ceramic/zirconia repair method can be an option for repairing layered zirconia restorations.

Despite these advantages in CAD/CAM materials, they can break because of insufficient interconnection or insufficient occlusal arrangement, internal tensions, parafunctional habits, fatigue load, inadequate thick-ness, mismatch of coefficient thermal expansion between core and veneered ceramic, and porosities formed during the manufacturing stage ^6-8. Additio-nally, zirconia sintering process and constructional failures, surface treatment techniques such as sandblas-ting, etching and grinding, stylize of the structure, continuous porcelain firings, finish-line designs, luting operations, and zirconia aging can all cause probable chipping of ZBR ^9-15. Micro-fracture spreads start from these stratums of the restorations and chippings can consist undesirably. Following this, the underlying zirconia core can come into the open or the breakage can keep in the porcelain veneer. Dentists have consis-tently faced this type of failure ⁸.

Broken restorations should be replaced instantly with new restoration or repaired using suitable repair mate-rial ^1,16. Restoration replacement is time-consuming, pricey, and there is also a major risk of damaging the prepared tooth when an enterprise is made to extract the faulty restoration ¹⁷. In addition, ZBR is usually cemented with resin or resin-modified glass ionomer cement, which has the ability to chemi-cally bond to the tooth structure ¹⁸. Furthermore, the removal of the zirconia ceramic substructure will ine-luctably outcome in injury to the underlying abutment tooth. Therefore, replacing restorations in these condi-tions is crucial in terms of the risk to the tooth structu-re, as well as a need for laboratory work, the additional cost of producing an entirely new restoration. Intraoral repair of ZBR is an applicable remedy when there is local damage at the restoration. Repairing broken por-celain in the mouth is a comparatively appropriate alternate to the patient and the treating clinician in terms of more cheaper and time-saving, with the suffi-cient restoration of function and appearance ¹⁹.

Dental aerosol-generating operations create an exces-sive amount of splashes and aerosols that cause a great worry for airborne illness contamination, such as COVID-19 ²⁰. The generation of aerosol and splash constitutes a great risk for airborne transmission in the clinical atmosphere ²¹. Most of routine dental opera-tions are creating a concoction of splashes, droplets, and aerosols that include saliva, blood, irrigant water, and alive microorganisms such as bacteria and viruses. Much more caution has been focussed on dental aero-sol-creating treatments due to the coronavirus disease 2019 epidemic (COVID-19). The airborne contamina-tion of disease via salivary, bioaerosols creates an im-portant risk to health workers that practice near to the face and oral tissues, such as dental clinic staff ²².

Nowadays, intraoral ceramic repairment should be preferred by comparison restoration replacement with a new one due to a large amount of dental aerosol-generating procedures such as removal of restoration with sectioned using a diamond bur and preparation arrangement procedures.

Some authors have recommended the usage of an in-traoral repair kit using a composite resin. Method usag-es a porcelain–resin bonding system to bond composite resin with broken restoration. Various studies have been managing to evaluate the shear bond strength among the ceramic repair kits and framework materials ^23-25. In the literature, studies on the impact of intra-oral repair kits on the bond strength of composite resin to new CAD/CAM ceramics are restricted.

Resin cements maintain to improve with advanced features. With the development of CAD/CAM technol-ogy, the composition of resin cements and upper-durability ceramic materials may be one of the choices for repairing fractured veneer ceramics ²⁶. Neverthe-less, there are not enough studies on the use of this composition as a repair method.

Mechanical or chemical surface conditioning tech-niques have been used to increase the bond strength of resin to the ceramic materials, improve the function of fractured ceramic restorations and protract their time of life. These surface treatment procedures are mainly; grinding with a bur, tribochemical silica coating, laser irradiation, zirconia primers, acid etching (e.g. hydrof-luoric acid (HF), acidified phosphate fluoride, and phosphoric acid (PA)), airborne particle abrasion with aluminum oxide ^27-32. There are different repair systems recently on the market with varied conditio-ning protocols. This makes it difficult for clinicians to choose the most correct system to obtain a reliable result ^33,34.

Based these informations, the aim of this study was to evaluate the repair bond strengths of three commercial-ly available ceramic repair systems that applied with two different repair methods.

The null hypotheses tested were; i) there would be no difference in repair bond strengths between the three ceramic repair systems; ii) there would be no difference in repair bond strengths veneered zirconia restorations repaired with the creation of the fractured part by com-posite resin or bonding of the fractured part by resin cement.

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Table 1: Repair systems and their application procedures.

In our study, the opaquers in repair kits were not used since the zirconia infrastructure does not cause any color reflection. A thin layer was applied to the zirco-nia and all-ceramic surfaces by mixing the resin cement (Panavia F 2.0; Kuraray, Osaka, Japan). The all-ceramic specimens were cemented to zirconia blocks by the resin cement which was polymerized with a LED light curing device (Hılux LED 550; Ankara, Turkey). The light was directed for 20 seconds from 5 different surfaces of bonding areas. After applying the surface treatment processes of the repair systems to the zirconia surfaces it was considered to prepare a trans-parent mold so that composite resins could be restored to the same dimensions of all-ceramic specimens. For this purpose, an impression from zirconia-ceramic samples (control group) is taken by using elastomeric impression material (Zhermack Elite P&P putty and light; Kouigo, Italy) in a plastic tray and a clear im-pression surface is provided. Transparent acrylic casts were created from the taken impressions. Acrylic repli-cas placed in a vacuum machine and covered under heat and pressure with 0.4 mm orthodontic transparent SX plaque. The corners of the transparent plates were notched and aligned for easy placement of zirconia blocks. SX plaques can facilitate manipulation and polymerization during layered composite stacking because they are transparent. In this way, accurate sized, homogeneous, and smooth composite restora-tions can be made. Two mm thickness composite resin was polymerized and layered for all groups.

Samples were left in distilled water for 24 hours after polymerization. The kept samples were taken into the thermal cycle process (SD Mechatronik Thermocycler; Julabo GmbH, FT 200, Seelbach, Germany) (between 5 - 55°C, 30 seconds dwell time, 2 seconds waiting time between baths, 5000 cycles). The specimens were fixed in a steel mold and seated in the shear testing jig. SBSt was implemented in a universal testing machine (Instron; Canton, Norwood, USA), the shear load was performed in a direction parallel to the bonded inter-face at a crosshead speed of 0.5 mm/min. The force was applied onto repaired piece (Figur 1).

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Figur 1: Schmitz Schulmeyer (SBSt) specimen, arrows show that direction of load application during shear bond testing.

Force was applied until the composite and all-ceramic blocks showed separation or fracture from the zirconia block. The failure load was recorded in Newtons (N). Data were calculated in newtons and then converted to megapascals (MPa).

Following SBSt, all specimens were observed under an optical microscope (Leica MZ 12; Leica Microsystems GmbH, Wetzlar, Germany) at 20 X magnification to examine the fracture type. After that, they were classi-fied as an adhesive (failure in the zirconia-ceramic or zirconia-composite interface), composite cohesive (failure within the composite resin), ceramic cohesive (failure within the ceramic), and mixed (both fracture types). The surfaces of the dried samples were sputter-coated with gold-palladium and they were also ob-served under a scanning electron microscope SEM (Zeiss Sigma VP; Carl Zeiss, Oberkochen, Germany).

Post-Hoc Power Analysis: The calculated power (1-beta) based on One-Way ANOVA is 1, considering type I error (alfa) of 0.05, sample size of 10, and effect size of 1.91.

Statistical Analysis: SBSt data were analyzed using Statistical Package for the Social Sciences (SPSS) 15 (IBM, Chicago, IL, USA) statistical package program. Kolmogorov Smirnov test was applied to identify whether the data were normally distributed. In addition, the control of variance homogeneity was applied using Levene’s test. One-Way Analysis of Variance (ANO-VA) was applied, followed by the Tukey HSD test which was used for post hoc comparing of the strength of repair systems. Independent Samples t-test was used for comparing the strength of repair methods. Results were evaluated with confidence interval (95%) and level of significance was determined as (p < 0.005).

Oral cavity warmth changes may cause mechanical stresses and fracturing then their propagation in resin-including materials, especially due to differences in the thermal expansion coefficient of the filler and resin matrix ^36,37. Concomitant use of water storage and thermal cycling are generally utilized to mimic intrao-ral environments. This aging process allows assessment of the bonding steady of the resin-zirconia. Clinically aging will directly affect the mechanical, chemical, and physical features of the material and thus its repairabi-lity. The number of cycles in the literature varies between studies and it has been reported that 5000 cycles correspond to an in vivo aging term of 6 months ³⁸. Thermal cycling has been reported to decrease bond strength in general ³⁹. The test specimens in our study were exposed to 24 hours water storage and 5000 thermal cycles before the SBSt.

Different bond strength measurement techniques are used in in-vitro studies such as shear, tensile, microten-sile, and three-point bending in dentistry. The SBSt is more generally used than the alternatives due to the method’s ease of use, basic, rapid, repeatable, and requirements no furthermore sample processing of the densely sintered zirconia. Most contributors have cho-sen SBSt in their studies that are bound up with intrao-ral ceramic repairment^30,40. SBSt was used in the present study to measure the repair bond strength of the repaired specimens.

The supply of an upper and resistant bond strength between ceramic and repairing material is very im-portant in dental restorations to provide their clinical achievement. Mechanical and chemical retention is necessary to succeed in upper bond strength between ceramic and repairing materials. Mechanical retention can be obtained with acid etching, burs (diamond, stone, etc.), and sandblasting. Chemical retention can be obtained with a primer and silane coupling agent. Acid etching and then primer or silane agent imple-mentation is the most common ceramic surface condi-tioning ²³. The whole ceramic repair systems used in the present study contain acid etching and silane coup-ling agents, outside of the Cimara Zircon repair system. Cimara Zircon repair system does not need acid etc-hing, which is the concern of the producers.

Gul and Altınok-Uygun ⁴¹ applied the surface treat-ments of different repair kits to different cad/cam ce-ramic blocks in their study and then nanohybrid resin composite was layered onto treated blocks surfaces. The samples were subjected to thermal cycling prior to the implementation of the repair systems and after the implementation of the composite resin. After microten-sile bond strength test was applied to the bar-shaped (1 × 1 × 12 mm3) blocks. In their study, the bond strength values of all repair kits were compared. The obtained values are ordered from the highest to the lowest as Cimara Zircon, Clearfil, Bisco repair kits. The other different results may be due to the use of micro-tensile bond strength test (MTBSt).

In the study of Kumchai et al ²⁶. beveled cylindrical shaped (Ø 10.5 mm, height 7.5 mm) veneered Zirconia crowns were repaired with bonded ceramic and restora-tive composite resin, similar to our study. In this study, the ceramic cementation process was applied to the beveled porcelain surface, while in our study it was applied to the fully exposed zirconia surface. In this study, veneered zirconia crowns repaired with cement-ed CAD/CAM ceramic materials had majorly upper bond strength than veneered crowns repaired with resin composite. In our study, the repair procedures per-formed only with the Bisco kit are parallel to the re-sults of this study. The reason for the lower bond strength values in the repair method made with ceramic cementation of Clearfil and Cimara Zircon repair kits in our study may be the differences in the surface treatments with this study or the use of different resin cement and composite resin.

In the study by Cınar and Kırmalı ³⁹ disc shaped veneer ceramic, zirconia, and veneer ceramic-zirconia specimens (7 mm in diameter and 3 mm in height) were bonded to composite resin using clearfil repair kit after different thermal cycles. Similar to our study using the Clearfil repair kit, the bond strength value of the repair performed on the zirconia surface was higher than on the ceramic surface.

In the study performed by Kocaagaoglu et al ⁴² in which the same repair kits used in our study were used, surface treatments applied for each repair system were to disk-shaped zirconia ceramic, alumina ceramic, glass ceramic materials (10 mm in diameter, 2 mm thick), and then the composite resin was incrementally con-densed onto the infrastructure material surfaces. In this study, although the bond strength ranking of the repair kits in the alumina ceramic group was similar to the repair group made with ceramic cementation in our study, the bond strength ranking of the repair kits ap-plied to zirconia was the opposite of our study (Bis-co>Cimara Zircon>Clearfil). The reason for the differ-ent results may be the difference in sample sizes or the application of more thermal cycles in our study.

In another study performed by Kırmalı et al ^40, dif-ferent intra-oral repair systems were applied to the disc-shaped zirconia surfaces (7 mm in diameter and 3 mm in height) and then resin composite built-up. In this study, in which all repair kits used in our study were used, bond strengths were listed as Cimara Zircon (17.31±3.62 MPa) > Clearfil (16.97±2.68 MPa) > Bis-co Z-Prime Plus (14.92±2.78 MPa). Although the bond strength values were lower in our study, the lowest bond strength value was found in the Bisco kit, similar to the study in the repair method performed with com-posite restoration. The reason of these different results may be the thermal aging application in our study.

If there are many cracks on the surface of the coating ceramic remaining after the fracture, due to the attenua-tion in the unity of the construction, fracture creation may happen again after repairs. It has been reported that fractures after intraoral repair with composite are caused by masticating forces, trauma, or wrong bond-ing processes ⁴³. Prior to starting the repair proce-dure, the reason of the fracture such as bruxism and premature occlusal contacts in the lateral movements must be detected and removed in order to refrain from unsuccess. Additionally, suitable ceramic repair mate-rial and surface conditioning are crucial for long-term clinical achievement ⁴⁴.

Limitations of this study:

1. Not using saliva; The bond strength of a resin material is sensitive to mechanical or chemical ef-fects in intraoral circumstances ^17, 2. Not using the chewing simulator; The shearing test was not able to simulate the loading strengths alone owing to formed during chewing non-homogeneous stress dispersion ^44,

3. Not using the micro-tensile bond strength (MTBS); SBS outcomes in upper values of variety according to MTBS because of the wider bonding surface field analyzed in the shear test. This wider bonding surface has more defects than narrower surfaces in MBTS ⁴⁵.

Other directions of the test, such as the influence of repair dimension, loading angulation, and periodic fatigue must be conducted for a more exhaustive as-sessment of repair systems. Further in vitro and in vivo studies should be studied to identify the right repair methods using more compositions of repair systems, testing devices, specimen materials, and sample design ^44,45.

1. Cimara Zircon repair system had the highest bond strength between repair systems for repair method with the cementation of the all-ceramic.

2. Clearfil repair system had the highest bond strength among repair systems for repair method with the restoration of the composite resin.

3. With the exception of the Bisco repair system, the repair method with the restoration of the composite resin showed higher bond strength.

4. In zirconia ceramic fractures; It may be recom-mended to restore the broken part with composite resin in cases where it breaks to pieces and crum-bles, to bond the broken facet or fabricated all-ceramic in cases where the broken part is separated as a facet or broken with a regular margin.

Conflıct of Interest
The authors deny any conflicts of interest related to this study.

Acknowledgements
The authors thanks to Ersin Uysal for his professional assistance in statistical analysis.

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