Influence of powder/liquid mixing ratio on the performance of a restorative glass-ionomer dental cement.

The influence of powder/liquid mixing regime on the performance of a hand-mixed restorative glass-ionomer cement (GIC) was evaluated in terms of compressive strength, working characteristics and the porosity distribution. Mean compressive fracture strengths, standard deviations and associated Weibull moduli (m) were determined from series of 20 cylindrical specimens (6mm height, 4mm diameter) prepared by hand-mixing the relative proportions of the powder and liquid constituents. Working characteristics were assessed using an oscillating rheometer whilst scanning electron microscopy and image analysis were used to investigate the influence of the mixing regime on pore distribution. For a constant volume of liquid (1ml) the mean compressive strength decreased from 102.1+/-23.1MPa for 7.4g of powder, to 93.8+/-22.9, 82.6+/-18.5 and 55.7+/-17.2MPa for 6.66, 5.94 and 3.7g of powder, respectively. A concomitant increase in both the working and setting times was also observed.GICs manipulated to a powder/liquid mixing consistency below the manufacturers' recommend ratio, for a constant volume of liquid, resulted in reduced porosity levels in the cement mass and extended working and setting times. Unfortunately, a reduction in the concentration of reinforcing glass particles in the set material below that specified by the manufacturers decreases the cements' load bearing capacity so that they fail at lower compressive stress levels in the posterior region of the mouth.