J. Soc. Cosmetic Chemists •.8 387-411 (1967) ¸ J967 Society of Cosmetic Chemists of Great Britain The measurement and interpretation of dentifrice abrasiveness K. H. R. WRIGHT and J. I. STEVENSON* Presented at the Symposium on "Product Testing", organised by the Society of Cosmetic Chemists of Great Britain at Eastbourne, Sussex, on 14th November 1966. Synopsls--A radioactive tracer technique has been used to assess the abrasivehess of denti- frices with the specific object of acquiring knowledge for the establishment of a standard dentifrice test procedure. A detailed study has been made of the relationships that exist between the wear resistance of dental tissues and their indentation hardness when using abrasives of different hardnesses. The influence on the wear rate of initial surface preparation of the tissues has been investigated together with the effect of decalcification brought about by exposure to lactic acid. It has been shown theoretically that the relationship between wear rate, dW/dS, and dentifrice concentration, C, is of the form dW/dS=a(1-e-•c), where a represents the saturation wear rate obtained when every brush fibre has trapped one or more abrasive particles, and (1-e -•c) is the trapping probability. The experimental values of the ratio of volume of the 'region of influence' surrounding each fibre tip to that of an abrasive particle, •, are in good agreement with the theoretical values based upon spherically ended fibre tip geometry. In comparative testing, the need for an integrated wear curve is stressed in order to take account of variations in dentifrice abrasiveheSS arising from changes in concentration during oral brushing. INTRODUCTION The basic requirements of an abrasive type dentifrice are, firstly, that the abrasives present shall assist in deansing the teeth without causing injury to them or irritation to the mucous membrane of the mouth, and, secondly, that the dentifrice shall be free of particulate matter that might impart an unpleasant feeling in the mouth. Although the latter refers mainly to gritty particles which might scratch the surfaces of exposed dental *National Engineering Laboratory, East Ifilbride, Glasgow. 387

388 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS tissues and perhaps impair the cosmetic properties of the product, it also includes lumps of material which the sensory tissues of the mouth dis- tinguish as discrete matter and which destroy the sensation of smoothness when the dentifrice is in use. A satisfactory test for dentifrice abrasiveness must therefore assess the overall wear likely to be caused to the main dental tissues and at the same time sense the presence of 'gritty' matter. The former is simply a quanti- tative test of the total amount of wear, whereas the latter is a form of quality control and rejects the dentifrice unless it achieves the measured level of wear without recourse to visible scratching of the surface. Two tests are therefore required. First the product should be screened to find whether it contains gritty particles, and second, if it does not contain those, its overall abrasiveness towards specific dental tissues should be measured, either absolutely, or by comparison with some reference dentifrice. The problem of grittiness is not discussed further in this paper, since the well known metal disc (coin)/glass-slide technique (1) provides a sufficiently precise assessment of its more damaging aspects. Such a test can readily detect coarse gritty matter in concentrations of 0.1% w/w, although the presence of such particles may have little effect on the overall wear. On the other hand, it is also perfectly feasible to formulate a dentifrice that will pass the glass-slide test, but exhibit a very high degree of tissue wear. Despite this possibility many National Standards Specifications still only specify the glass-slide test as the abrasiveness' control. The establishment of acceptable limits of overall abrasiveness presents many problems since the actual cleansing requirements will vary widely from one individual to another. Kitchen and Robinson (2) made an attempt to assess this variability by visually examining a large number of students' teeth which were cleaned regularly with certain proprietary dentifrices. After close examination it was found that the tooth enamel of 20% of the students was susceptible to heavy staining, but that these stains could be eliminated in 95% of this heavy-staining group by the regular use of a dentifrice capable of removing 1 mm of dentine in 100,000 brush strokes. Unfortunately, the equivalent enamel wear was not measured but judging from the type of dentifrices employed one can assume that it would be about one hundredth part of the dentine wear. It follows that during the course of a daily brushing of 15 strokes with a dentifrice of the above abrasiveness, the maximum amount of material worn away would be 1.5 x 10-6 mm of enamel or 1.5 x 10-4mm of dentine. Although the dentine wear is rather high, 1.5 x 10-6mm of enamel only corresponds to about four