With the development of digital dental technologies and increased demand for aesthetic dental procedures, metal-free ceramic-based restorations are gradually replacing conventional metal-ceramic restorations [1]. Among all-ceramic restorations, zirconia-based restorations have gained much clinical popularity owing to their excellent mechanical properties and acceptable machinability at the pre-sintering stage [2,3]. Yttria-stabilized tetragonal zirconia polycrystals possess a transformation toughening mechanism and are frequently used for natural tooth and implant restorations [4]. Zirconia restorations have been reported to be stably maintained in full-coverage single crowns, fixed partial dentures, and cantilevered resin-bonded fixed dental prostheses [5], [6], [7], [8].

Metal-based full-coverage restorations can be successfully retained using only luting cement, owing to the mechanical retentive form and sufficient strength of the metal framework [9]. In contrast, silica-based ceramic restorations are easily fractured if not chemically bonded to the tooth structure [9]. Therefore, a bonding procedure performed using resin cement is usually recommended for the cementation of ceramic-based restorations [10]. However, bonding zirconia to abutments has been a subject of debate because of the densely sintered and polycrystalline nature of zirconia, which cannot be easily etched by conventional acid etching [11], [12], [13], [14]. Thus, several attempts have been made to bond zirconia restorations to tooth abutments [15], [16], [17], [18]. Several studies have revealed that enhanced bonding can be achieved through surface pretreatments, such as sand blasting and tribochemical silica coating [15,17]. Application of a 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP) coating after airborne-particle abrasion has been reported to help achieve a robust bond between zirconia and the tooth structure [7,17,18].

Several studies have been conducted in an attempt to obtain higher bonding strengths in zirconia restorations cemented using various materials and methods [16,17,[19], [20], [21], [22], [23], [24], [25], [26], [27], [28]]. However, whether an extremely high bonding strength is required for cementation of full-coverage zirconia restorations in clinical practice should be considered. To use dual-curing resin cements properly, thorough moisture-proofing and light polymerization are required during the procedure [29,30]; in addition, removal of the residual cement is difficult [31]. In contrast, resin-modified glass ionomer cement (RMGIC), which has been successfully used for metal-based restorations, requires relatively simple clinical manipulation and provides reasonable adhesion [9]. Moreover, full-coverage densely sintered polycrystalline ceramic restorations are strong enough to endure masticatory forces and may not require the strong bonds essential for silica-based full-coverage restorations [9]. Several studies have reported that RMGIC can be successfully used for the cementation of densely sintered zirconia-based restorations [8,[32], [33], [34]]. However, studies mainly dealing with the clinical outcomes of zirconia restorations cemented with RMGIC are scarce. A study on the use of RMGIC for full-coverage zirconia restorations in a clinical setting would address a gap in the literature and provide valuable insights for clinicians. Thus, the aim of this retrospective clinical study was to investigate the clinical performance and complications of full-coverage zirconia-based restorations cemented with RMGIC and compare them with restorations cemented with self-adhesive resin cement (SAC). The hypothesis of this study was that there would be no difference in clinical outcomes between restorations cemented with the two materials.