Objectives: To develop a robust in vitro method for assessing toothpaste abrasivity using Perspex as the test substrate. Perspex was chosen because it has similar hardness to human dentine, is easily available in sheet form and behaves consistently in abrasion and hardness tests. Method: Perspex (Polymethymethacrylate) samples (55x55x5mm) were indented four times using a Knoop diamond indenter at an applied load of 1000g. The indent lengths were measured optically and left for 24 hours to equilibrate. The Perspex samples were then brushed with a slurry of test toothpaste using a Goodbrand Jeffries brushing simulator. The test conditions were: applied load 375g, 400 brushing cycles, slurry produced by mixing 61.5g purified water with 38.5g toothpaste. Following the brushing, the samples were cleaned using water and dried before the indents were remeasured. The amount of material removed was calculated from the length change of the indents and the known geometry of the Knoop indents. A series of toothpastes were tested using four Perspex tiles per product and the results were compared to the results obtained for the same products using the standard radiotracer technique, relative dentine abrasion (RDA).
Results: A series of fluoride-containing toothpaste formulations labelled A – F were tested. The Perspex abrasion depth results (expressed in microns removed) were as follows (standard deviations in brackets): A 0.17(0.05), B 0.65(0.08), C 0.82(0.09), D 1.25(0.16), E 1.34(0.29), F 2.05(0.33). For the same products, the RDA results were as follows (standard deviations in brackets): A 4.17(1.26), B 74.22(8.79), C 109.08(11.34), D 110.46(18.79), E 167.58(8.61), F 208.58(11.37). The results of the two techniques correlate well, with a linear regression coefficient R2 of 0.92. Conclusions: The study demonstrates that the Perspex abrasion test using Knoop indents is able to discriminate between the abrasivity of toothpaste formulations and correlates well with results from the RDA test.