Erosion and attrition of human enamel in vitro Part II: Influence of time and loading

Objectives. To determine the influence of load and time on enamel wear by attrition under neutral and acid environments in vitro. Methods. Groups of 10 enamel cusp and smooth surface specimens obtained from unerupted third molars were opposed in a wear machine under loads of 200, 400 and 600 g in citric acid at pH 3.2 and in physiological saline at 35 °C for 30 min (2280 wear cycles). The experiment was repeated with a load of 600 g for 10 and 20 min accompanied by 10 taped specimens to measure erosion only. Erosion and attrition of smooth surface specimens was determined by profilometry. Results. At a load of 600 g attrition in saline increased with time from 7.9 μm (SD 3.7 μm) at 10 min, 10.2 μm (SD 4.8 μm) at 20–25.5 μm (SD 9.1 μm) after 30 min and combined attrition/erosion in citric acid from 1.5 μm (SD 0.4 μm) to 2.5 μm (SD 0.9 μm) to 6.0 μm (SD 3.7 μm). At a fixed time of 30 min, attrition in saline was 5.2 μm (SD 3.6 μm) at 200 g and 8.1 μm (SD 1.5 μm) at 400 g and combined attrition/erosion in citric acid 2.8 μm (SD 1.2 μm) and 3.6 μm (SD 0.7 μm). Comparison between both environments revealed significant differences at fixed load at all time points (p<0.01) and at fixed time between 400 and 600 g (p<0.01). Conclusions. Load and time significantly influence enamel wear both in acid and neutral conditions but the effects were not linear.