ABSTRACT
BACKGROUND: The authors conducted studies to determine if hygroscopic expansion of resin-modified glass ionomer, or RMGI, cement would cause fracture of the sintered aluminum oxide copings of all-ceramic crowns. MATERIALS AND METHODS: The authors scanned standardized dies using computer-aided design/computer-aided manufacture technology. Aluminum oxide copings were fabricated, inspected and luted onto titanium dies. The copings were seated with 11 pounds of force for 10 minutes. The copings were placed in a container with 100 percent humidity and stored at room temperature. At appropriate intervals, the authors inspected the copings with surgical loupes and under microscopic magnification. The authors also inspected the copings before conducting load testing. RESULTS: After six and 12 weeks, the authors found no signs of fracture in any of the 30 copings cemented with one of two RMGI cements. Seventeen additional samples were inspected four years after cementation, and no fractures were evident. In a follow-up study, the authors cemented 18 copings with RMGI cement one week before testing, 10 weeks before testing or 60 weeks before testing. Using a universal testing machine, they loaded the samples to fracture at 0.5 millimeter/minute. Statistical analysis indicated a significant difference in the mean load-to-fracture values between the samples tested one week after cementation and those tested 60 weeks after cementation. CONCLUSIONS: The results showed no fracture of the all-ceramic copings as a result of expansion of the cement alone. The mean load-to-fracture values were significantly greater for the group tested 60 weeks after cementation. CLINICAL IMPLICATIONS: The all-ceramic crowns examined in this study demonstrated excellent esthetics, incorporated a strong coping system and allowed ease of cement choice.
