| Summary: | This study examined indirectly the behaviour of zinc phosphate cement used to lute cast gold crowns. In order to standardise an <i>in vitro</i> method, cast nickel chrome dies were to represent natural molar teeth prepared to receive full gold crowns whilst the cast gold crowns were produced using standardised laboratory techniques. Initial work was undertaken to develop methods to control the distribution and film thickness of the cement beneath the crown. These were investigated together with the effects of preparations geometry and casting rigidity. Cement coverage had a strong influence on strain distribution. Increasing the cement film thickness led to an increase in compressive microstrain in the walls of the crowns, whilst both increasing the convergence angle and reducing the axial wall height of the preparation increased the axial compressive microstrain. However, the influence of axial wall height was greater than that of convergence angle. The influence of the casting rigidity showed an interesting finding. Heat treatment of the crown did not show differences in microstrain compared with the as cast control group, but increasing the occlusal surface thickness by a factor of three doubled the compressive axial microstrain recorded. The strain response curves for uncemented, partially cemented and fully cemented gold crowns to increasing static loads were recorded: these data were used to characterise the microstrain measurements from the later series of dynamic loading experiments. In the dynamic loading series, the crowns were partially cemented on their dies and axial loading was applies between 0-450N for approximately 300,000 cycles with a frequency of about 2Hz. In a series of four experiments, pairs of crowns were tested dry, immersed in water, in water and acid, and in acid alone (pH 2.74). The results were plotted as microstrain against a number of cycles. These showed that the hydration had a strong effect on the strain recorded on the axial surfaces of the cemented crowns compared with the dry samples. Whilst the presence of pure water decreased the microstrain, the presence of acid increased it. The pronounced effects were recorded in the presence of acidic media. Microscopic investigation showed damage to the luting cement was increased by the presence of moisture and further increased by the presence of acid. The conclusions drawn from this study were that the use of axially-placed strain gauges provided a method for monitoring the strain in the axial walls of cast gold crowns cemented with zinc phosphate cement. The strains were indicative of the condition of the bond between the die and crown provided by the cement. This method has further application in the non-destructive monitoring of the bond provided by luting cements between an indirect restoration and the underlying preparation.
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