Objectives: The aim of this in-vitro study was to determine the fracture strength and marginal adaptation of all-ceramic three-unit fixed partial dentures (FPDs).

Methods: Human molars were inserted in PMMA resin to create a three-unit (10mm) oral situation. The roots of the teeth were covered with an about 1mm thick layer of polyether to simulate the periodontium. 8 bridges of each series were made of the computer aided manufactured zirconia (Cercon Base/Cercon Ceram Kiss, DeguDent, G), an electro-layered chip-bridge-system (Inceram Alumina- Wolceram, G) and a layering Al₂O₃ technique (Inceram Alumina, Vita, G; - control). All FPDs were fixed with an adhesive bonding system (Syntac classic/Variolink2; Ivoclar-Vivadent, FL). After thermal cycling and mechanical loading (TCML; 6000 thermal cycles [5°C/55°C] and 1.2x10⁶ mastication cycles [50N]) fracture strength (UTM 1446; Zwick; v=1mm/min) was determined. Failure detection was set to 10% of the maximum force. Marginal adaptation was evaluated at both transitions cement-tooth (CT) and cement-FPD (CF) using scanning electron microscopy (Stereoscan 240, Cambridge Instr., GB) before and after TCML. The criteria "perfect margin" was ranked as a smooth transition without interruptions of continuity. Statistical analysis was performed with the Mann-Whitney U-test (p=0.05).

Results:

Median (25%/75%)	Cercon	Wolceram	Inceram (control)
Fracture force [N]	1331 (1224/1428)	575 (487/735)	334 (315/673)
Marginal adaptation [%]
CT :    before -            after TCML	95.5 (91.5/99.0) - 98.0 (95.5/98.8)	98.7 (96.8/99.3) - 98.6 (96.8/99.6)	91.3 (83.9/93.3) - 86.9 (76.0/96.9)
CF :    before -            after TCML	97.0 (90.3/100.0) 94.5 (87.8/98.0)	98.4 (93.9/99.4) - 97.9 (95.2/99.2)	90.1 (86.3/92.3) - 75.9 (69.1/89.8)

Zirconia CAM FPDs showed statistically significant higher fracture resistance than Al₂O₃ FPDs or electro-layered FPDs. Only the reference showed significant lower marginal adaptation.

Conclusion: Electro-layered Al₂O₃ FPDs may only be used with restriction in posterior areas. The fracture forces of zirconia FPDs were at a level where clinical application seems promising.