Effect of the Preparation Angle on the Precision of Zirconia-Crown-Copings

Objectives: The aim of this in vitro study was to evaluate the effect of different preparation angles on the precision (marginal and internal fit) of zirconia crown frameworks. The effect of the milling-unit should also be examined.

Methods: Three ivorine maxillary first molars were prepared with three different preparation angles: 4°, 8° and 12° total taper. Impressions were made and 20 dies were fabricated per group of angle type. Copings were fabricated on each die by a dental laboratory (Cercon, DeguDent, Hanau, Germany) and a milling-center (Compartis, DeguDent) CAD/CAM system. All frameworks were adapted by the technician according to clinical relevant standards. After cementation with glass-ionomer, cross-sections were made and the cement gap was measured under stereo-microscope (Axioskop 2, Zeiss, Oberkochen, Germany) using a special computer evaluation program. Four different measurement locations were used to determine the precision of fit between the frameworks and the dies. Descriptive analysis was carried out to detect statistical differences between the experimental groups a two-way ANOVA and a post-hoc-test (Student-Newman-Keuls).

Results: The preparation angle and the area of measurement showed significant (ANOVA, p<0.01) influence on the precision of fit. The precision of the 12°-group was significantly higher compared to the 4°- and the 8°-group. No statistical influence was detected between the laboratory and the milling-center system (ANOVA, p=0.92). All groups showed marginal openings in the range of 37.6 to 45.5 µm. The internal adaptation was in the range of 62.1 to 106.7 µm depending on the measurement location while the largest cement gap was detected at the occlusal adaptation.

Conclusion: As a result of this study a preparation angle of 12 degrees enabled the highest precision. However, crowns with a steeper preparation design of 4 or 8 degrees were clinically acceptable, since the marginal gap was consistently less than 50 µm.