Effects of tooth preparation burs and luting cement types on the marginal fit of extracoronal restorations.

PURPOSE Although surface roughness of axial walls could contribute to precision of a cast restoration, it is unclear how the roughness of tooth preparation affects marginal fit of the restoration in clinical practice. The purpose of this study was to describe the morphologic features of dentin surfaces prepared by common rotary instruments of similar shapes and to determine their effects on the marginal fit for complete cast crowns. MATERIALS AND METHODS Ninety crowns were cast for standardized complete crown tooth preparations. Diamond, tungsten carbide finishing, and crosscut carbide burs of similar shape were used (N = 30). The crowns in each group were subdivided into three groups (n = 10) for use with different luting cements: zinc phosphate cement (Fleck's), glass ionomer cement (Ketac-Cem), and adhesive resin cement (Panavia 21). Marginal fit was measured with a light microscope in a plane parallel to the tooth surface before and after cementation between four pairs of index indentations placed at equal distances around the circumference of each specimen. Difference among groups was tested for statistical significance with analysis of variance (ANOVA) followed by Ryan-Einot-Gabriel-Welsch Multiple Range Test (alpha= 0.05). RESULTS Analysis of measurements disclosed a statistically significant difference for burs used to finish tooth preparations (p < 0.001); however, luting cement measurements were not significantly different (p= 0.152). Also, the interaction effect was not significantly different (p= 0.685). For zinc phosphate cement, the highest marginal discrepancy value (100 +/- 106 microm) was for tooth preparations refined with carbide burs, and the lowest discrepancy value (36 +/- 30 microm) was for tooth preparations refined with finishing burs. For glass ionomer cement, the highest marginal discrepancy value (61 +/- 47 microm) was for tooth preparations refined with carbide burs, and the lowest discrepancy value (33 +/- 40 microm) was for tooth preparations refined with finishing burs. For adhesive resin cement, the highest marginal discrepancy value (88 +/- 81 microm) was for tooth preparations refined with carbide burs, and the lowest discrepancy value (19 +/- 17 microm) was for tooth preparations refined with finishing burs. CONCLUSIONS Marginal fit of complete cast crowns is influenced by tooth preparation surface characteristics, regardless of the type of luting agent used for cementation. Tooth preparations refined with finishing burs may favor the placement of restorations with the smallest marginal discrepancies, regardless of the type of cement used.