Objectives: The purpose of this in-vitro investigation was to evaluate the influence of the design and the loading mode on the fracture strength of all-ceramic resin-bonded fixed partial dentures (FPDs). Methods: Twenty-eight frameworks were copy-milled from prefabricated presintered zirconia-reinforced aluminum-oxide blanks using the Celay-System (Mikrona), glass-infiltrated and circumferentially veneered with feldspathic porcelain (Vitadur-Alpha, Vita, Bad Säckingen, FRG) in order to replace an upper incisor on a study model (edentulous width 8.5 mm). The sandblasted restorations were bonded with a phosphate-monomer containing composite resin (Panavia 21, Kuraray, Osaka, J) to the acid-etched abutment teeth which were supported by an artificial periodontal membrane and embedded in a polyester resin. Fourteen FPDs had a conventional two-retainer design (R2) and 14 FPDs had a cantilevered one-retainer design (R1). Subgroups of 7 samples were subjected to a quasi-static loading in an angle of 45 degrees in direction of the long axis of the abutment teeth (1 mm/min, universal testing machine Zwick No 1442, Ulm, FRG). Subgroups of 7 samples (R2C, R1C) were subjected to dynamic loading with 30 N and synchronised thermal cycling (5°C to 55°C) in an oral artificial environment (Kausimulator, Willytech, Munich, FRG) in a loading direction of 45 degrees before testing in the universal testing machine. Results: The mean fracture strengths under quasi-static loading in N were: R2: 148.13 ± 26.42 N; R1: 80.78 ± 34.01 N; R2C: 157.28 ± 13.16 N; R1C: 92.17 ± 26.15 N. Differences between groups and subgroups were statistically significant as shown by one-way ANOVA. The retainer design had a statistically significant influence (p ≤ 0.001) on the fracture strength of the all-ceramic resin-bonded FPDs while the mode of loading had no significant effect (p > 0.5). Conclusions: Under non-axial loading conditions, resin-bonded single-retainer FPDs showed significantly lower fracture strength than FPDs with two retainers.

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