Influence of Impression/model Fabrication on CAD/CAM Zirconia Crowns Internal Fit

Maalhagh-Fard, Ahmad, Jeremy Zaluski, Janine Matos, and Rafael Rocha Pacheco

The aim of this study was to evaluate the internal fit of milled zirconia crowns obtained by different impression/model fabrication techniques. Dental students from University of Detroit Mercy prepared 94 resin #8 teeth (Typodont, Columbia Dentiform Inc.) according to full ceramic crown preparation criteria and divided in two groups: G1 or G2. G1 students (n=47) used conventional polyvinylsiloxane [PVS] (Imprint 4, 3M ESPE) associated to acrylic resin custom trays for impression of #8 tooth preparation, followed by optical scanning (Lava-Cos Optical Scanner, 3M ESPE). G2 students (n=47) performed optical scanning of preparation, followed by PVS impression with custom trays. For G1, type IV gypsum (ApexTM, RZ10, Garreco) models were fabricated from PVS molds, while for G2, stereolithographic models (3M ESPE) were fabricated from optical scanning. For both groups, full zirconia crowns were milled using CAD/CAM technology (LAVA, 3M ESPE). In order to determine internal fit (mm), light body PVS material was used as luting agent. Crowns were removed from preparations and heavy PVS was inserted into crowns and over the light PVS (simulating tooth preparation). Light body PVS would represent luting agent thickness (mm). Samples were sectioned and evaluated under stereomicroscope (MOD CM-700AT, SER#CM70324, Clark) with a magnification of 50X. Three measurements were taken on each internal surface, in a 2mm-interval. Mean (SD) values were calculated and data was subjected to statistical analysis by using analysis of variance and Tukey test (p<0.01). For G1 and G2, mean values for occlusal were statistically higher than for other surfaces. Mesial and distal surfaces values were significantly lower for G1 when compared to G2. There was no statistical difference between G1 and G2 for occlusal, lingual and buccal surfaces. Different impression/casting techniques used by second-year dental students resulted in similar values for milled zirconia crowns internal fit, except for mesial and distal surfaces.