Marginal and internal fit of curved anterior CAD/CAM-milled zirconia fixed dental prostheses: An in-vitro study

Objective: To test whether or not different types of CAD/CAM systems, processing zirconia in the densely and in the pre-sintered stage, lead to differences in the accuracy of 4-unit anterior fixed dental prosthesis (FDP) frameworks, and to evaluate the efficiency. Method and Materials: 40 curved anterior 4-unit FDP frameworks were manufactured with four different CAD/CAM systems: DCS Precident (DCS) (control group), Cercon (DeguDent) (test group 1), Cerec InLab (Sirona) (test group 2), Kavo Everest (Kavo) (test group 3). The DCS System was chosen as the control group because the zirconia frameworks are processed in its densely sintered stage and there is no shrinkage of the zirconia during the manufacturing process. The initial fit of the frameworks was checked and adjusted to a subjectively similar level of accuracy by one dental technician, and the time taken for this was recorded. After cementation, the frameworks were embedded into resin and the abutment teeth were cut in mesiodistal and orobuccal directions in four specimens. The thickness of the cement gap was measured at 50× (internal adaptation) and 200× (marginal adaptation) magnification. The measurement of the accuracy was performed at four sites. Site 1: marginal adaptation, the marginal opening at the point of closest perpendicular approximation between the die and framework margin. Site 2: Internal adaptation at the chamfer. Site 3: Internal adaptation at the axial wall. Site 4: Internal adaptation in the occlusal area. The data were analyzed descriptively using the ANOVA and Bonferroni/ Dunn tests. Results: The mean marginal adaptation (site 1) of the control group was 107 ± 26 μm; test group 1, 140 ± 26 μm; test group 2, 104 ± 40 μm; and test group 3, 95 ± 31 μm. Test group 1 showed a tendency to exhibit larger marginal gaps than the other groups, however, this difference was only significant when test groups 1 and 3 were compared (P = .0022; Bonferroni/Dunn test). Significantly more time was needed for the adjustment of the frameworks of test group 1 compared to the other test groups and the control group (21.1 min vs 3.8 min) (P .0001; Bonferroni/Dunn test). For the adjustment of the frameworks of test groups 2 and 3, the same time was needed as for the frameworks of the control group. Conclusions: No differences of the framework accuracy resulting from the different CAM and CAD/CAM procedures were found; however, only after adjustment of the fit by an experienced dental technician. Hence, the influence of a manual correction of the fit was crucial, and the efforts differed for the tested systems. The CAM system led to lower initial accuracy of the frameworks than the CAD/CAM systems, which may be crucial for the dental laboratory. The stage of the zirconia materials used for the different CAD/CAM procedures, ie presintered or densely sintered, exhibited no influence. Keywords: fixed dental prosthesis, internal fit, marginal fit, zirconia