Adhesion to Zirconia Used for Dental Restorations: A Systematic Review and Meta-Analysis

Purpose: Currently, no consensus exists regarding the best adhesion protocol for zirconia used in dentistry; this is important particularly for restorations where mechanical retention is deficient. This systematic review analyzed the adhesion potential of resin-based and glass-ionomer luting cements to zirconia and aimed to highlight the possible dominant factors affecting the bond strength results to this substrate. Materials and Methods: Original scientific papers on adhesion to zirconia published in the MEDLINE (PubMed) database between 01/01/1995 and 01/06/2011 were included in this systematic review. The following MeSH terms, search terms, and their combinations were used: "Dental bonding", "Zirconium", "Zirconia", "Y-TZP", "Y-TZP ceramic", "Materials Testing/methods", "Test", "Cement", and "Resin bonding". Two reviewers performed screening and data abstraction. Descriptive statistics were performed and the frequencies of the studied parameters, means, standard deviations, confidence intervals (95% CI; uncorrected and corrected), median values, and interquartile ranges (IQR) were calculated for the bond strength data reported for different factor levels: surface conditioning methods (control, physicochemical, physical, chemical), cements (bis-GMA-, MDP-, and 4-META-based resin cements, self-adhesive cements, glass ionomer), aging with and without thermocycling (TC), and test methods (macroshear, microshear, macrotensile, and microtensile). Results: The final search provided 177 titles with abstracts. Further abstract screening yielded 72 articles, out of which 54 were found potentially appropriate to be included. After full text evaluation, 2 of these were eliminated. The selection process resulted in the final sample of 52 studies. In total, 169 different surface conditioning methods, mainly combinations of air-abrasion protocols and adhesive promoters (primers or silanes), were investigated. Altogether, the use of 5 types of cements and 4 testing methods was reported. While 26 studies were performed without TC as aging, 26 of them employed thermocycling at varying number of cycles. This review highlighted that adhesion of the luting cements is significantly influenced by the surface conditioning method (p = 0.044), cement type (p = 0.018), test method (p = 0.017) and aging condition (p = 0.003). In nonconditioned control groups without thermocycling, mean bond strength values ranged between 1.15 (IQR = 3.54) and 8.93 (IQR = 9), and 6.9 (IQR = 0) and 8.73 (IQR = 13.93) MPa for macroshear and macrotensile tests, respectively. After physical conditioning method, MDP monomer based cement presented the highest bond values compared to those of other resin cements using either the macrotensile (no TC: 34.2; IQR = 24.18 MPa, TC: 42.35; IQR = 0 MPa) or microtensile (no TC: 37.2; IQR = 41.5 MPa, TC: 17.1; IQR = 31.15 MPa) test method. Conclusion: Based on the results of this systematic review, increased adhesion could be expected after physicochemical conditioning of zirconia. MDP-based resin cements tend to present higher results than those of other cements types when tested using macro- and microtensile tests. Adhesion studies on zirconia and reporting of data require more standardization. Keywords: adhesion, bond strength, dental cements, meta-analysis, surface conditioning, systematic review, test methods, zirconia