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Retention of Zirconium Oxide Ceramic Crowns With Three Types of Cement
Howard E. Strassler, DMD
Palacios RP, Johnson GH, Phillips KM, Raigrodski AJ. J Prosthet Dent. 2006;96(2): 104-114.
STATEMENT OF PROBLEM: Information about the retentive strength of luting agents for zirconium oxide–based crowns is limited. It is unknown if this type of high-strength ceramic restoration requires adhesive cementation to enhance retention. PURPOSE: The purpose of this in vitro study was to determine the ability of selected luting agents to retain a representative zirconium oxide ceramic crown under clinically simulated conditions. MATERIAL AND METHODS: Recently extracted human molars were prepared with a flat occlusal surface, 20-degree taper, and approximately 4-mm axial length. The axial and occlusal surface areas were determined, and specimens were distributed equally by total surface area into 3 cementation groups (n=12). Zirconium oxide ceramic copings (Procera AllZirkon) with an occlusal bar to facilitate removal were fabricated using computer-aided design/computer-assisted manufacturing (CAD/CAM) technology. All copings were airborne-particle abraded with 50-Ìm Al2O3 and then cleaned in an ultrasonic bath with isopropyl alcohol. Provisional cement was removed from the prepared teeth, followed by a pumice prophy. After trial insertion, the copings were cleaned with phosphoric acid, rinsed, dried, and dehydrated with isopropyl alcohol. They were then cemented with a seating force of 10 kg per tooth, using either a composite resin cement with adhesive agent (Panavia F 2.0 and ED Primer A & B [PAN]), a resin-modified glass ionomer cement (Rely X Luting [RXL]), or a self-adhesive modified composite resin (RelyX Unicem [RXU]). The cemented copings were thermal cycled at 5° C and 55° C for 5,000 cycles with a 15 second dwell time, and then removed along the path of insertion using a universal testing machine at 0.5 mm/min. The removal force was recorded, and the stress of dislodgement was calculated using the surface area of each preparation. A 1-way analysis of variance was used to analyze the data (a= .05). The nature of failure was also recorded. RESULTS: Mean dislodgement stresses were 5.1, 6.1, and 5.0 MPa for PAN, RXL, and RXU, respectively. The 1-way analysis of variance revealed no differences in mean crown removal stress among the 3 cementation groups. The predominant mode of failure was cement remaining principally on the zirconium oxide copings in 46% of the specimens, followed by cement found on the tooth in 25.7% of the specimens. CONCLUSIONS: Within the limitations of this study, the 3 luting agents, with mean removal stresses ranging from 5.0 to 6.1 MPa, were not significantly different. The use of a composite resin cement with a bonding agent did not yield higher coping retention compared to the other 2 cements tested.
As clinicians, the use of adhesive restorative materials is a standard of care. Manufacturers continually promise that the latest materials will exceed our expectations in their clinical success. Unfortunately, in many cases these new materials have not been well tested before they have been made available, and within a short period of time improved versions of the same product supplant the product we had just started using. It leads one to wonder what will happen with the restorations that have been placed with the “first generation” of a given product. While we can have a problem with direct-placement restorations when the latest bonding system does not meet our clinical expectations, it is more of an issue when the new product is a cement for indirect restorations. If the cement does not work as advertised the restoration will fail prematurely. As clinicians we take risks when we use new materials, and we would be well served to seek out clinical studies demonstrating the efficacy of these materials. Currently we are in the era of self-etching, self-adhesive systems. These two abstracts (a sampling of many) explore the evaluation of self-adhesive resin cements. The author chose one research study that was in vitro and one study that was in vivo to compare the results seen both on the bench top and when restorations are placed in patient’s teeth.
Currently the most common cements are resin-modified glass ionomer, etch-and-rinse adhesive dual-cure resin cements, and self-adhesive resin cements. The in vitro study performed at the University of Washington by Palacios and colleagues compared a resin-modified glass-ionomer cement (RelyX Luting, 3M-ESPE, St. Paul, MN), an etch-and-rinse adhesive resin cement (PANAVIA F 2.0 and ED PRIMER A & B, Kuraray America Co, New York, NY) and a self-adhesive resin cement (RelyX Unicem, 3M ESPE) when cementing Procera® (Nobel Biocare, Yorba Linda, CA) crowns to extracted human molars. The study design was well thought out and is relevant to what we see as clinicians. The results demonstrated no significant differences between the three cements. All three luting agents are capable of retaining zirconium-oxide crowns successfully.
The second abstract is a clinical research study evaluating the self-adhesive resin cement, RelyX Unicem, in the placement of indirect restorations including porcelain-fused-to-metal crowns, all-ceramic crowns, and all-metal crowns. This is a good clinical study because the researchers evaluated these restorations at 2 years for retention, marginal adaptation and staining, and postoperative sensitivity. Thirty-one restorations were evaluated, with 97% having acceptable margins. Postoperative sensitivity was not a problem with the restorations at the last recall. The conclusion was that this universal self-adhesive resin cement was performing satisfactorily after at least 20 months.
When cementing restorations, the fewer the steps and the easier a cement is to mix, the higher the expectation would be for improved clinical results in comparison to older cements. For those who remember mixing zinc phosphate cement on a cooled glass slab, incorporating small increments and mixing the cement over a large surface area of the slab to a consistency that was determined by using the cement spatula to determine viscosity, the introduction of easy-to-mix (in most cases self-mixing or mixing in a capsule with an automatic mixer) cements has been a fantastic change. Now that there is adequate research available, the change to the latest generation of cements makes sense. From both of these research studies it can be expected that a self-adhesive resin cement will be clinically successful in the cementation with any laboratory-fabricated restoration.
Howard E. Strassler, DMD
Professor and Director of Operative Dentistry
Department of Endodontics, Prosthodontics, and Operative Dentistry
University of Maryland Dental School, Baltimore, Maryland