Effect of Luting Cement Space on the Strain Response of Gold Crowns Under Static Compressive Loading.

The aim the work was to investigate the effect of varying degrees of luting cement thickness on the strain of the cemented gold alloy crowns under compression. Five dies with their corresponding crowns were fabricated using a lost wax technique. Three gold crowns for each die were fabricated under the control of specific die spacer layers to provide a space of 40 µm (10 layers of die-spacer thickness) and 80 µm (20 layers of die-spacer thickness). The crowns were subsequently cemented using zinc phosphate cement. The crowns were subjected to gradual static compressive loading between 10N to 250N (Newton) and the strain measured simultaneously. The results were statistically analysed using Independent t-test for the different die-spacer thickness at the 95% confidence interval (p = 0.05). It was found that a significant relationship in the three thicknesses. It was concluded that the absence of die-spacer significantly reduced strain response, whereas a very little change in the strain recorded as the die spacer layers has increased. Clinically, decreasing the number of die-spacer layers is advantageous as it provides a lower strain response under static compressive loading that would improve the longevity of the cemented full crowns inside the patient's mouth.

Occlusal convergence and strain distribution on the axial surface of cemented gold crowns.

The overall thrust of this work is concerned with the performance of the adhesives used to simulate cementation of gold crowns onto nickel chromium dies under static and dynamic compression. A measurement system, based on the mounting of strain gauges on the outer surface of the crowns, has been developed allowing an indirect semi-quantitative estimate of the state of adhesion. This paper reports an investigation of the effect of increased total occlusal convergence (TOC) of the nickel chromium dies from 12 degrees to 24 degrees with different degrees of cementation, a) un-cemented, b) partially cemented and c) fully cemented. Four nickel chromium dies (12 degrees TOC) and five nickel chromium dies (24 degrees TOC) for each convergence were fabricated using the lost wax technique. The axial height of all dies was 6mm. Two miniature gauges were installed on opposing axial surfaces of each gold crown 1 mm above the crown margin. Axial loading and unloading of the crowns was repeated five times for each crown and the values for strain recorded. The results showed an increase in strain at the axial surfaces with increasing TOC, providing useful design information for the durability of restorative crowns. These findings, along with the findings of earlier work are consistent with a simple model of load transfer between the crown and the die.

Compressive stress-strain behaviour of cast dental restorations in relation to luting cement distribution.

This work is concerned with the performance, under dynamic compression, of the adhesives used to simulate cementation of gold crowns onto nickel chromium dies. A measurement system, based on the mounting of strain gauges onto the outer surface of the crowns, has been developed, which allows a semi-quantitative estimate of the state of adhesion. A preliminary study was carried out where miniature gauges were bonded onto the buccal, lingual, mesial and distal surfaces of gold crowns cast to fit onto a chromium-cobalt die, as a precursor to the final design of the instrumented crown. The crowns were then loaded in compression periodically measuring the strain at all four gauges. The results showed the load-strain relationship in fully, partially and uncemented crowns, along with repeated load-unload behaviour to close to the ultimate strength of the bond. The load-strain relationship is linear and repeatable and the slope varies over a factor of about 30 between the un-cemented and fully cemented crown, demonstrating that there is significant sensitivity to bond integrity. These results were used to determine the parameters for a systematic study of S-N relationships in restorative dentistry crowns.