482 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS scanned. The peaks on the trace indicate the presence of tin on the tooth surface and it can be seen that greatest uptake occurs in the white spot area. This may well be due to the increased surface available on this etched region rather than to any specific affinity of white spot enamel for tin. IN vivo METHODS All the tests mentioned are test-tube experiments and however much the conditions are varied in an effort to test the permanence of the treat- ment, they give no unequivocal indication of what will happen in the mouth. Waiter {18) was the first to describe a feasible technique which went some way to answering this criticism, in that during the period of the trial, the treated teeth were exposed to all the dynamic conditions normally present in the mouth. His test was again dependent upon solubility changes of tooth enamel but on humans it was conducted as follows: A 2mm diameter filter paper disc impregnated with an acid base indica- tor and with 2[d of 0.3N HC1, was placed on the selected tooth surface after it had been dried with a tissue. The time taken by the indicator to change colour {Colour Reaction Time) is a function of the tooth's 'resist- ance' to the acid attack. In a test using a panel of school children Mtihle- mann (19), using this method, reported the effectiveness of certain amine hydrofluorides. In our hands this test gave a positive result for a topical stannous fluoride treatment but even with half the amount of acid used by Mfihle- mann it had a (literally) marked effect on the subjects' teeth and had to be abandoned. An alternative method was then developed in which a weak acid was used in place of hydrochloric acid and the change in pH was recorded after a predetermined time. The technique is described by Middleton and Holmes (20) and Morley and Holmes (21), and depends upon clamping a well to an upper central incisor so that leakage cannot occur (Fig. 15). With the •vell clamped in position, 0.1 ml acetic acid (pH=3.4) is added from a syringe and is stirred by a stream of bubbles from the air line during the 3 min of the test. At the end of this time the air stream is stopped and a sample of the acid is sucked into the microcapillary glass electrode for pH measurement. This is a rather elaborate micro-electrode supplied by Messrs. Pye of Cambridge.

THE LABORATORY EVALUATION OF PROPHYLAoe TIC DENTIFRICES PERSPEX BLOCK POLYTHENE TUBI STAINLESS TUBE GLASS WELL METAL CLAMP• I TOO'I'H • TO AIR Pump Figure 1,•. Apparatus used to maintain a weak acid solution in contact with a tooth surface The results obtained from a panel of about 50 people, who used a proprietary stannous fluoride toothpaste and a control, are illustrated in Figs. 16-19. Fig. 16 shows the design of the panel test. All used the control paste first and, after 5 weeks, base line measurements were made. Half the subjects were then given the test paste while the remainder con- tinued with the control paste and measurements were taken after 4 to 8 weeks. The pastes given to the two groups were then interchanged and after a further 4-6 weeks, final measurements were made. The results for the subjects on the sequence-control, fluoride, control are given in Fig. 17 and it is evident that the rise in pH is much less after the use of the test paste and that after a further spell on the control paste, there is a trend towards the original values. For those on the sequence- control, control, fluoride (Fig. 18) the trend continues towards small decreases in rise of pH at each stage. The overall effects can be summarised in graphs of the population distribution of pH values (Fig. 19). The shift in the peak populations towards lower pH values denotes a reduction in apparent enamel solubility. Statistical analysis proved these differences to be significant. Herd and Overell (22) report results obtained for a monofluorphosphate paste using a somewhat similar well to maintain acid in contact with a tooth but their estimate of the amount of enamel dissolved was based on phosphorus determinations. They claimed a significant reduction in enamel solubility after one brushing with the test paste.