Effect of Different surface treatments and cement types on retention of zirconia crowns

Background: Continued development in ceramic materials extended their clinical indications to be used for single or multiple bonded ceramic restorations. All-ceramic zirconium oxide materials have several properties such as high strength and biocompatibility that permit their use as core materials for all-ceramic crowns and fixed partial dentures. Transformation toughening gives these materials their favorable mechanical properties which increases crack propagation resistance. Cement bonding of oxide ceramics has been extensively studied but there is little information in the literature on their bond strength to zirconia. Zirconia can be retained with non-bonding cements such as zinc-phosphate or bonding cement such as resin-modified glass-ionomer and resin cement that are currently indicated for cementing polycrystalline ceramic restorations thus, improving the clinical retention and marginal fit. Searching for a suitable luting method to achieve durable bonds between resin cements and zirconia ceramic cores is still a matter of concern. Different surface treatments for polycrystalline ceramics have been suggested in order to achieve the micromechanical bond that creates interlocking tags of the resin cement with the previously-treated roughened ceramic surface. Limited experimental studies had been done to investigate the influence of conditioning methods, thermocycling and cement types on crown retention and durability of bond strength of zirconia dental ceramics. Aim of Study: This is an in-vitro study that aims to evaluate zirconia crown retention and durability of bond strength between zirconia crowns and 3 different cements after thermocycling. An optical and electron microscopic study was carried out in order to understand effects of the different ceramic surface treatments. Materials and Methods: One hundred and twenty extracted human premolars were collected and prepared for zirconia crowns (approximately 10-degree taper, approximately 4-mm axial length) and were divided into 4 groups (n=30). Computer-aided designing and computer-aided manufacturing zirconia copings were fabricated. Four surface treatments were applied to the intaglio surface of the copings. The control group was received no treatment, the second group was airborne-particle abraded with 50 μm Al2O3, the third group was treated with 30 μm silica-modified Al2O3 and the last group was performed by exploiting the hydrolysis of aluminum nitride (AlN) powder. Dual-polymerizing RelyX Unicem resin cement, Dual-polymerizing RelyX Ultimate resin cement with methacryloyloxydecyl dihydrogen phosphate (MDP) (Scotchbond™ Universal Adhesive) resins and GIC cement were bonded to the ceramic samples. They were stored for 24 hours at 37°C before being artificially aged with 5000 (5°C-55°C) thermal cycles. Retention was measured on a Universal testing machine under pull-off test, with a crosshead speed of 0.5 mm/min. An optical and electron microscopy study was also carried out in order to understand the effects of the different ceramic surface treatments performed. Result: Mean retention values ranged from (2.9-6.7) MPa. Surface treatment increased crown retention, but the difference was not statistically significant (P>.05), except for the alumina coated group. Analysis of the cements revealed that the GIC cement resulted in significantly lower crown retention (P<.05) than the other 2 cement if alumina coating surface treatment was used. Conclusion: For zirconia crowns retention seems to be dependent on interaction between cement and surface treatment.