Effect of surface conditioning with airborne-particle abrasion on the tensile strength of polymeric CAD/CAM crowns luted with self-adhesive and conventional resin cements.

STATEMENT OF PROBLEM: Adhesively bonded, industrially polymerized resins have been suggested as definitive restorative materials. It is claimed that such resins present similar mechanical properties to glass ceramic.
PURPOSE: The purpose of this study was to assess the tensile strength of polymeric crowns after conditioning with 2 different protocols: luted with self-adhesive or with conventional resin cements to dental abutments.
MATERIAL AND METHODS: Human teeth were prepared for crowns and divided into 13 groups (N=312, n=24 per group). Polymeric crowns were CAD/CAM fabricated and divided into 3 groups depending on different surface conditioning methods: A) No treatment, B) airborne-particle abrasion with 50 μm alumina, and C) airborne-particle abrasion with 110 μm alumina. Thereafter, the crowns were luted on dentin abutments with the following cements: 1) RXU (RelyX Unicem, self-adhesive), 2) GCM (G-Cem, self-adhesive), 3) ACG (artCem GI, conventional), and 4) VAR (Variolink II, conventional). Glass ceramic crowns milled and cemented with dual-polymerized resin cement (Variolink II) served as the control group. The tensile strength was measured initially (n=12) and after aging by mechanical thermocycling loading (1 200 000 cycles, 49 N, 5°C to 50°C) (n=12). The tensile strength (MPa) of all crowns was determined by the pull-off test (Zwick/Roell Z010; Ulm, Germany, 1mm/min). Subsequently, the failure types were classified. Data were analyzed with 2-way and 1-way ANOVA followed by a post hoc Scheffé test and t test (α=.05).
RESULTS: No adhesion of the tested cements was observed on unconditioned polymeric CAD/CAM crowns and those luted with VAR. Among the tested cements, GCM showed significantly higher values after airborne-particle abrasion with 110 μm (initial: 2.8 MPa; after aging: 1 MPa) than 50 μm alumina (initial: 1.4 MPa; after aging: 0 MPa). No significant effect was found between 50 and 110 μm particle size alumina in combination with the other 2 cements. After aging, the tensile strength of the crowns luted with GCM (50 μm: 0 MPa and 110 μm: 1 MPa) and ACG (50 μm: 1 MPa and 110 μm: 1.2 MPa) was significantly lower than those luted with RXU (50 μm: 1.9 MPa and 110 μm: 2 MPa). All airborne particle abraded polymeric CAD/CAM crowns (initial: 1.4-2.8; 0-2 MPa) showed significantly lower tensile strength values than the control group (initial: 7.3 MPa; after aging: 6.4 MPa). Although with all polymeric specimens, failure type was adhesive between the cement and the crowns, the control group showed exclusively cohesive failures within the ceramic.
CONCLUSIONS: Airborne-particle abrasion before cementation of polymeric CAD/CAM crowns minimally improved the tensile strength. Both the failure types and the tensile strength values of adhesively luted glass ceramic crowns showed superior results to adhesively cemented polymeric ones. Although the tensile strength results were low, crowns cemented with RXU showed, after aging, the highest tensile strength of all other tested groups.