A study by Fischer, et al published in the June issue of Journal of Dentistry investigated differences in porcelain-metal bond strength after aging of ceramic-fused-to-metal restorations, based on whether the laboratory air abraded the metal or polished the metal prior to firing the ceramic onto the metal alloy. The study compared the effect of aging of a CFM test system using 4 alloys with 2 metal finishes, either roughened by air abrasion with 50-µm alumina or polished using a 1-µm diamond paste prior to firing. All specimens were cycled in 5°C and 55°C water baths for 20 seconds, with a frequency of one cycle per minute (including transfer time). A total of 6,000 cycles were used to simulate 5 years of clinical wear in the mouth. A mechanical testing device applied a bending force to the porcelain-alloy strips until cracking occurred. Bond strengths for the 4 alloys and 2 metal surfacing techniques were calculated and compared statistically. The study found that specimens with porcelain fired onto air-abraded alloys had insignificant loss of porcelain-metal bond strength and remained at approximately twice the minimum value specified by the International Standards Organization (ISO). For the polished alloy specimens, there was significantly greater loss of bond strength, although strengths still remained above the ISO reference value. The study authors concluded that a roughened/abraded alloy surface results in greater porcelain-metal bond strength than the same alloy with a polished surface, and all bond strengths tend to diminish with aging. Although mathematical projection of outcomes onto aging cycles of greater than 15 years considerably increases the loss of bond strength, indications are that aging is not critical for clinical success of the porcelain-metal bond. The results of this study suggests that clinicians should specify that cast alloys be air abraded rather than polished prior to porcelain firing.
(Source: Oakstone Review, July 2009)
