Objective:The objective of this study was to evaluate the effects of cyclic loading on the load to failure of CAD-CAM crowns made from Vitadur Alpha Porcelain, Vita In-Ceram Spinell, 3M/ESPE MZ 100, and Vita Mark II porcelain. Methods: A CEREC 2 camera was used to make optical impressions of an unprepared typodont tooth (#19), the same typodont tooth was prepared for a full coverage crown, at 1.5-2.0 mm occlusal thickness, and 1.0-1.5 mm axial wall thickness. A correlation program was then used to create a standard crown using the CEREC 2. A Polyvinyl Siloxane impression was made to duplicate the standard die and epoxy resin material was used to form the positive model. RelyX ARC resin bonding cement was used to bond the fabricated crowns to the epoxy dies. The crowns were polished and kept wet. Axis Dental Cera Glaze porcelain polishing kit from coarse to fine was used on Revolution micro motor at 20,000 RPM. Six crowns were tested in each of the five groups for each material (Base line, 5,000, 15,000, 50,000, and 100,000 cycles), at about 450 N load, and approximately 1.6 Hz. Results: The Alpha material showed the largest reduction in failure load with increasing number of cycles (0.91±0.06 KN base line, and failed before 50,000 cycles). In-Ceram Spinell material showed the least reduction in failure load with increasing number of cycles (0.98±0.33 KN, base line; 0.91±0.15 KN, 100,000 cycles). MZ 100 showed a decreased failure load with increasing number of cycles (1.33±0.16 KN, base line; 1.2±0.38 KN, 100,000 cycles). MarkII showed decreased failure load with increasing number of cycles (0.95±0.09 KN,base line; 0.75±0.11 KN, 100,000 cycles). Conclusions: Under these test conditions, MZ100 and In-Ceram Spinell were more resistant to strength degradation by cyclic loading than a conventional porcelain material.

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