The retaining force of inlays cemented with self-curing resin and zinc phosphate cement was studied, testing itself and the surface adhesion as an element of it. The following results and decisions were obtained.
1. The surface adhesions of resin and cement were respectively 1.6 and 7.2 kg/cm2 to dentin, and 8.4 and 18. 8 kg/cm2 to enamel.
2. The retaining force of resin was about one third that of cement. However, that of resin will be kept longer because of its excellent chemical stability. The clinical usefulness of the selfcuring resin as a cementing medium for inlays will be decided in further studies.
3.The adhesion to dentin was considerably weakened by water, though that of resin to enamel was less weakened and that of cement to enamel was reversely stronger in the wet state. Enamel walls are considered as excellent cementing bases.
4. The adhesion to coarser walls was stronger than that to smoother walls. Such a difference was great on enamel but slight on dentin.
5. Upon analysing the retaining force of the simple column inlays tested here, the interlocking force took 83% in the case of resin and 87% in that of cement. The surface adhesion took 17% and 13% respectively
6. The adhesion of resin is very strong in dry state but very week in wet state. The direct resin filling therefore must be kept perfectly dry during its setting to prevent it from separating from the cavity wall by its curing shrinkage. Besides, the cavity for resin filling must be given round undercuts with definite jaws for mechanical retension.