Internal and marginal fit Of pressed and cad lithium disilicate crowns made from digital and conventional impressions

Objectives: The aim of the study was to evaluate in vitro the 3D and 2D marginal fit and 2D internal fit of CAD and Press all-ceramic crowns made from digital and conventional impressions.

Methods: A dentoform replica tooth (#30) was prepared for an all-ceramic crown (Master Die). 30 impressions were made with PVS material; 30 definitive casts were poured in type IV gypsum. 30 Lava^TM C.O.S. impressions were made; 30 resin models were produced. 30 crowns were waxed and pressed in lithium disilicate (IPS e.max Press) (15 from each impression technique) and 30 crowns were milled from lithium disilicate blocks (IPS e.max CAD) (15 from each impression technique) utilizing the E4D scanner and milling engine. The Master Die and the intaglio of the 60 crowns were digitized using a 3D laser coordinate measurement machine (CMM). For each specimen a separate data set was created for the Qualify 2012 software. The two data sets, digital master die and digital intaglio of the crown, were merged using Best-Fit alignment. An area above the cavosurface margin with 0.75mm occlusal-gingival width circumferentially was defined. The 3D marginal fit of each specimen was an average of all 3D measurement values on that specified area for all the crowns, and it was used for the statistical analysis. For the 2D measurements, two sections, one facial-lingual and one mesial-distal, were made through the grooves on the standardized metal base of the tooth. The distance between the die and the intaglio surface of the crown were measured at 7 standardize points (2 on the margins, 2 at 0.75mm above the margin, 2 on the axial walls and 1 on the occlusal surface). For the 3D measurements one-way ANOVA with post-hoc Tukey's HSD test was used to determine whether there were significant differences in mean marginal fit values among four experimental groups (alpha=0.05). For the 2D data, a two-way ANOVA was performed to detect a significant interaction between the type of impressions and the type of crowns on the marginal and internal fit (alpha=0.05).

Results: One-way ANOVA revealed that the 3D mean marginal fit for Group A (0.048mm±SD 0.009) was significantly lower than those obtained from other three experimental groups Group B (0.088mm±SD 0.024), Group C (0.089mm±SD 0.020) and Group D (0.084mm±SD 0.021), while no significant differences were found among Group B, C and D. Similar results were found regarding the 2D marginal fit Group A (0.040mm±SD 0.008), Group B (0.076mm±SD 0.0234), Group C (0.075mm±SD 0.0148) and Group D (0.073mm±SD 0.0258). For the 2D internal fit, Group C (0.2109mm±SD 0.0410) had statistically significant poorer internal fit than the other three groups Group A (0.1105mm±SD 0.0474), Group B (0.1158±SD 0.02) and Group D (0.1454MM±SD 0.0245), while no significant difference was found among those three groups.

Conclusions: The combination of PVS impression method and Press fabrication technique produced the most accurate 3D and 2D marginal adaptation. The combination of Lava^TM C.O.S. impression and Press fabrication technique produced the poorer 2D internal fit.