Bond strength of a direct composite resin to resin-matrix ceramic materials with four surface treatments

Background Aesthetic demands have popularized the use of dental ceramic and indirect composite resin materials in modern dentistry. The introduction of dental CAD/CAM technology expanded the use of these materials even further. Furthermore, continuous advancements in CAD/CAM technology not only made a production of high-quality single visit indirect restoration possible, but also allowed for the use of high strength ceramics when required. This has made CAD/CAM produced restorations an attractive treatment modality for both the clinician and the patient and encouraged the dental materials manufacturers to produce various ceramic and composite CAD/CAM blocks to utilize this technology. However, both ceramic and composite materials being brittle in nature, are prone to catastrophic failure. It was suggested that amalgamation of the crystalline matrix and the polymeric material results in an increase in strength. Resin-ceramic materials, therefore, were developed and promoted on the bases of combining the positive features of both ceramic and composite resin material, however, clinical fractures might still occur. Intraoral repair for localized restoration fracture could be a more conservative and cost-effective alternative to complete restoration replacement. Different repair treatment approaches for chipped/fractured restorations have been described in the literature for both dental ceramic and composite materials. Resin-matrix ceramic (RMC) materials are a relatively new group of CAD/CAM material, therefore, have not yet been well researched. Proper repair protocols for these materials also still have not been established. Various surface preparation techniques are described in the literature to facilitate bonding between two dental materials. Surface roughening procedures were evaluated and described by researchers to enhance the bond strength of composite resin repair to both ceramic and composite materials. The effect of surface roughening on various RMCs, however, is still not clear. Furthermore, the benefit of different surface roughening techniques on the bond strength of composite resin repair to RMC materials is also to be determined. In addition, the few available in vitro studies that investigated this matter used bond strength testing methods which focused on evaluating the maximum stress at failure rather than the fracture energy needed to cause spontaneous crack propagation. Aim Chapter 2: To evaluate the changes in surface morphology of commercially available (RMC) materials following different surface treatment methods. Chapter 3: To evaluate the bond strength of composite resin to different CAD/CAM RMC materials following different surface treatment methods using a three-point bend fracture toughness test method. Methods Chapter 2: The surface of four RMC blocks (18 mm × 14 mm × 12 mm) - Vita Enamic (Vita Zahnfabrick, Bad Säckingen, Germany), Lava Ultimate (3M ESPE, St. Paul, MN, USA), Cerasmart (GC Dental Products, Kasugai, Aichi, Japan) and Shofu Block HC (Shofu INC., Kyoto, Japan) - were divided into four sections and subjected to four surface treatments: grinding only (G); grinding + 4.5% hydrofluoric acid (GF); grinding + airborne-particle abrasion using 50‎ μm aluminium oxide + 4.5% hydrofluoric acid (GBF); grinding + airborne-particle abrasion 50‎ μm aluminium oxide + 37% phosphoric acid (GBP). The resultant surfaces were then characterised using scanning electron microscopy. Chapter 3: A total of 240 beams shaped specimens (5 mm × 5 mm × 17 mm) were prepared from four types of CAD/ CAM RMCs, Vita Enamic (VE), Lava Ultimate (LU), Cerasmart (CS) and Shofu Block HC (SB). Sixty beam specimens were prepared for each material and then subdivided into four groups (n=15) according to the surface treatment method, group G: grinding only; group GF: grinding + 4.5% hydrofluoric acid; group GBF: grinding + airborne-particle abrasion using 50 ‎μm aluminium oxide + 4.5% hydrofluoric acid; group GBP: grinding + airborne-particle abrasion 50 ‎μm aluminium oxide + 37% phosphoric acid. A nanocomposite material was packed onto the treated surfaces following the application of universal adhesive material according to manufacturers' instructions. The bond strength was tested using a fracture toughness 3-point bend test. Specimens were examined under scanning electron microscopy to determine the mode of failure. Data for groups CS and SB were analysed using a one-way ANOVA to evaluate statistical significance (P<0.05) while groups VE and LU, which showed non-parametric data, were analysed using a Kruskal Wallis test. Results Chapter 2: Grinding followed by airborne abrasion produced greater changes in surface morphology of RMCs than grinding alone. Phosphoric acid had no effect apart from surface cleaning. Hydrofluoric acid produced porosity/ holes on the surface. Hydrofluoric acid effect was more pronounced in Vita Enamic when compared with Lave Ultimate and Cerasmart. Shofu blocks HC was the least affected by hydrofluoric acid. Chapter 3: Statistical analysis showed a significant difference between the different surface treatment within groups VE, CS and SB (P< 0.05). LU groups showed no interaction between surface treatment method and bond strength (P= 0.629). VE-G group showed statistically higher bond strength than LU-G, CS- GSBP and SB- GSBF. Conclusions Chapter 2: All surface treatments showed alteration in the surface morphology of RMCs with hydrofluoric acid producing the greatest effect. The etching response was dependent on the size and elemental content of filler particles. Chapter 3: No single surface treatment method can be generalized for all RMC materials to achieve a reliable bond strength. The process of sequentially increasing the surface roughness of the RMC materials via grinding, air-borne abrasion, hydrofluoric acid etching or combination of these methods produced an increase in the surface area available for bonding. However, this did not necessarily result in an increase in bond strength.