Cavity convergence angles for direct composite inlays

Direct composite inlays may sometimes be difficult to remove from the cavity following primary polymerization. The aim of this work was to find the lowest cavity convergence angle (taper) which would allow reliable inlay removal. Cavity finish and cavity size were also taken into account. Standardized mesio-occlusal-distal cavities of 6 degrees taper were cut into 10 teeth of varying mesiodistal width and finished with either 12-bladed tungsten carbide or 25 microns diamond burs. After separating medium had been applied. Coltene Brilliant Dentin composite resin was packed into the cavity and light cured. The force required to remove the inlay was measured with an Instron Universal Testing Machine. The cavities were then refinished with the alternative finish and the experiment repeated. The same sequence was used for cavities of 12 degrees and finally 18 degrees convergence angle which were cut consecutively in the same 10 teeth. Force levels related to cavity convergence angle, cavity finish and tooth size were tested by ANOVA and regression analysis. The forces required to remove some of the inlays from the 6 degrees, and to a lesser extent 12 degrees angled cavities, proved unacceptably high, while an 18 degrees convergence angle resulted in statistically significant lower forces which were unlikely to damage tooth or inlay. Cavity finish and tooth size did not influence inlay withdrawal force.