## 4 Discussion  
The bonding of ceramics to hard tissues relies on micro-retention of irregularities created on their surface through different modification techniques. Acid-etching protocols described in the literature vary in terms of application time, the agents used, and acid concentration. Some authors recommend applying 9.5-10% hydrofluoric acid for 20 s [22; 23] to 60 s [24], while others prefer 4.9% hydrofluoric acid for 2 min [25]. Hydrofluoric acid reacts with silicon dioxide, dissolving the ceramic glassy matrix, and generates micro-irregularities and pits, enhancing roughness and facilitating micromechanical interlocking with the resin cement. A similar effect can be achieved through mechanical treatments, such as sandblasting with alumina particles, creating an irregular way surface without visible cracks [4; 26]. Irregularities can also be produced with diamond burs [2] or laser treatments [27]. In more recent protocols, the application of 37% orthophosphoric acid after hydrofluoric acid treatment is suggested to remove inorganic remnants without affecting surface morphology [1; 13]. A silane coupling agent (3-methacryloxymethyltrimethoxysilane) can then be used to induce chemical bonding between the organic resin matrix and the ceramic inorganic phase [1; 10; 11]. According to [PERSON] et al. [11], optimal adhesion is achieved through airborne-particle abrasion, followed by etching with 9% hydrofluoric acid for 90 s, then applying 37.5% phosphoric acid for 1 min for post-etching cleaning, and finally, applying silane that tries in 5 min at 100 \({}^{\circ}\)C. In this study, two recently introduced materials were tested in various combinations with mechanical and chemical surface modifiers to investigate their influence on bond strength and propose an optimal usage protocol. Monobond Etch & Prime, as per the manufacturer, can simultaneously be used for acid-etching and silanizing the ceramic surface, while Ivoclean is a universal cleaner made by the same manufacturer. Applying Ivoclean has been suggested as a cleaning method for the extraoral application of ceramic and metal surfaces that may be contaminated during intraarol try-ins [28; 29]. It contains an alkaline suspension of zirconium oxide particles (10-15%), water (65-80%), polyethylene glycol (8-10%), sodium hydroxide (<1%), and pigments and additives (4-5%) [29; 30]. The removal of residual organic contaminants from the surface of restorations is achieved through the alkalinity or acidity of cleansing agents. Acidity is provided by the contained hydrofluoric acid (HF) and phosphoric acid (H\({}_{3}\)PO\({}_{4}\)), while alkalinity primarily comes from sodium hydroxide (NaOH) solutions. The alkalinity of Ivoclean results in a strong affinity toward the phosphate group of saliva [21]. Phosphate contaminants are absorbed by Ivoclean, leaving a clean zirconium oxide surface [31]. Despite available data on the immediate bond strength of surfaces treated with Ivoclean, the long-term in vivo effects of this treatment method are still unknown [32]. [PERSON] et al. [32] emphasized the safety of Ivoclean for surface conditioning due to its ease of application and removal, as well as its lack of toxicity. [PERSON] et al. [33] acknowledged its effectiveness in removing saliva contaminants and providing a clean surface for improved resin bonding, while several au  
Figure 2: Representative optical microscope images (15\(\times\) magnification) for each category of failure mode observed in the study. **(a)** Adhesive; **(b)** Cohesive in resin bulk; and **(d)** Mixed. White arrows indicate the adhesive agent and black arrows the resin cement. thors found higher shear bond strength (SBS) when Ivoclean was applied prior to air-water spray and acid treatment [34; 35; 36]. Both Ivoclean application and sandblasting are used to remove contaminants from surfaces before applying bonding agents. Groups B and D in the present study followed the same bonding protocol but differed in their cleaning approach.