MEASUREMENT AND INTERPRETATION OF DENTIFRICE ABRASIVENPZSS 389 atom layers of the tissue and can be considered a perfectly safe level of wear. However, the fact that most commercially available dentifrices seem to be capable of removing 0.5-1.0 mm of dentine in 100,000 brush strokes implies that these dentifrices have been designed to cope with the heavy staining proportion of the population. If this is so, then the degree of dentine abrasion is possibly too high for many people, especially those showing advanced cervical exposure. The increasing use of motorized toothbrushes, although beneficial to gingival health, necessitates greater care in selecting a dentifrice which avoids excessive wear of dental tissues. The measurement of dentifrice abrasiveness has always proved dif•cult in practice, especially with enamel surfaces for which prolonged brushing is often necessary to obtain an accurately measurable amount of wear. This is aggravated by the fact that dental tissues absorb and release water so rapidly that precise weight loss measurements are not easy to make. In addition the tissues vary considerably in composition, not only from one tooth to another, but also over the surface of a single tooth. For these reasons numerous attempts have been made to find alternative test mater- ials (3), but the results have not proved encouraging. Some experiments, made by the present authors with glass surfaces of similar hardness (370 ]i-IV 2.5) to enamel, indicated that some proprietary brands of dentifrice chemically attacked the glass and yielded results which correlated badly with those obtained with human enamel. The particular glass employed was a medium barium crown glass containing 6% Tin20 3 for radiotracer purposes. Dentine has usually been the popular choice of test material, but its use in isolation can, as will be seen later, lead to gross errors in the assessment of the abrasiveness of the dentifrice to harder tissues. For the same reason, ivory has little to recommend it, even though it can be obtained in large and convenient pieces. Most investigators prefer a brushing test for evaluation purposes, since this clearly reproduces the normal oral cleaning action, but others have utilized polishing machines, based upon a wax lapping plate to retain the dentifrice (4). The latter method was favoured some years ago as it avoided the variability experienced with natural fibre brushes. However, the intro- duction of synthetic fibres has largely overcome this objection, and lapping plate machines are now rarely employed. Tissue wear may be recorded by measuring either weight loss or changes in dimensions. In spite of the problem presented by a non-uniformly

390 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS wearing surface, many workers have preferred to measure dimensional changes, either directly with the aid of a micrometer or by contour measurement. For example, the examination of a dentine/enamel junction profile can give a quick assessment of dentine wear if one assumes that the enamel wear is negligible by comparison. Any test of abrasiveness should be made on human dental tissue in view of possible chemical interaction between the testpiece and the dentifrice. Also a brushing action should be employed to reproduce those changes in dentifrice concentration normally associated with oral tooth cleansing.The ideal measurement technique avoids the problem of water absorption, but is sufficiently sensitive for a test to be completed after a short period of brushing the natural surface of an extracted tooth. A high wear sensitivity is particularly important as it permits an accurate comparison of two dentifrices to be made without altering the surface geometry of the speci- men. This approach is nearly always more desirable than the employment of an accelerated test. A radiotracer technique appears specially attractive in studying the wear of dental tissues, since phosphorus is available as one of the elements of hydroxyapatite (5). The radionuclide P 32 is a high energy [• emitter having a half life of 14.3 days, whilst its large neutron cross-section enables one to obtain a convenient level of radioactivity after a few hours irradiation in the reactor. A specific activity of about 1 millicurie per gram (mCi/g) of tissue can be obtained for instance after five hours' irradiation in a neutron flux of 1012 neutrons/cm2/s. Using a mica end-window Geiger-Miiller counter for which the background estimated activity of a dried P 32 source is 10-4 gCi, it is possible to determine 10-? g of worn dental tissue. In the case of dentine, this is approximately the amount worn away by one stroke of a brush loaded with a conventional dentifrice. The wear experiments described in this paper have employed this radio- tracer technique in conjunction with a laboratory test rig that closely simulates an oral brushing action. To solve specific problems in the accurate measurement of dentifrice abrasiveness the investigation was divided into two parts: (i) A study of the relationships that exist between tissue wear and the hardness of the abrasive, and (ii) the effect of changes in dentifrice concentration on the wear rate produced by brushing. An understanding of (i) is essential if a standard test matehal is to be chosen, whilst a knowledge of (ii) will assist in the selection of test con- ditions for standardizing purposes.