The principles of adhesion.

A MAJOR part of dental practice is the treatment of localised lesions in teeth and the replacement of lost tissue with' restorative materials. As stated by Skinner and Phillips (1967), the lack of adhesion of most of them to the structure of the prepared tooth presents a serious problem; it causes seepage of deleterious agents along the interface between the restoration and the tooth (Nelscn et al, 1952). Fig. 1. Micro-leakage of saliva, acid and bac­ teria has been demonstrated by various techniques: radioactive tracers, (Going et al, 1960; Petersen et al, 1966), dye pene­ tration (Holliger, 1967; McLcaji and Kramer. 1952), and penetration of bac­ teria (Rose et al. 1961; Kraus and Kraus, 1951). It leads to the breakdown of the mar­ ginal areas of the filling with resultant secondary caries along the interface be­ tween the restoration and the tooth struc­ ture. It is likely that certain phenomena such as post-operative sensitivity and pulp pathology are associated with the leakage pattern (Phillips, 1967). In many instances