Microhardness as a predictor of sound and carious dentine removal using alumina air abrasion.

Clinical air abrasion is known to remove dental hard tissues effectively. The aim of this study was to quantify and compare the efficiency of alumina air abrasion in the removal of sound and carious dentine. Twelve non-cavitated carious lesions in freshly extracted molars were bisected mesiodistally. The Knoop hardness number (KHN) was measured on the cut sample surface at 250-microm intervals along a line running axially through the centre of the lesion, from the enamel-dentine junction to the pulp including both sound and carious tissue. The cut surface was then abraded over the same path using alumina particles (27 microm). Powder flow rates, pressure, distance and the angle of the nozzle to the tooth were kept constant throughout the experiment. Non-contact surface profilometry and surface analysis software were used to calculate the rate of dentine removal at the site of each KHN measurement. The results showed that dentine Knoop hardness can be used as a predictor of the rate of dentine removal, with a Somers' D value of 0.826. Carious dentine with a low KHN was removed less efficiently than the hard sound dentine with a higher KHN. Thus, dentists should be aware that air abrasion systems using alumina particles remove healthy dentine more efficiently than carious dentine, with the associated implications for clinical caries excavation and cavity preparation.