[Fitness of onlays fabricated with direct and indirect CAD/CAM technology in vitro]

To analyze the fitness of zirconia and lithium disilicate glass-ceramic onlays fabricated with direct and indirect computer-aided design and computer-aided manufacturing (CAD/CAM) technology in vitro.

In the study, 48 standardised typodont left mandibular first molars received standardised onlay preparation. Then, all the specimens were randomly divided into 4 groups. There were 12 specimens in each group. The preparation quality was checked under the stereomicroscope. All the specimens were fixed in typodonts. Subsequently, the typodonts were fixed in the dental simulators to simulate the oral conditions. In groups A and B, the digital impressions were obtained by using the intraoral scanner. In groups C and D, conventional impressions of polyether impression material were obtained according to the instructions of the manufacturer using individual trays. The stone casts were made with type Ⅳ gypsum later. Then, all casts were digitized with the model scanner. Based on the data obtained from the scan, onlay restorations of all the groups were designed using the corresponding software, the simulated cement thickness was set to 50 μm. Then, the final onlays restorations of all the groups were machined with the milling machines in lab. The fabrication materials were different in groups. The specimens of groups A and C were fabricated with zirconia. While, the specimens of groups B and D were fabricated with lithium disilicate glass-ceramic. The marginal gap and internal gap of all restorations were analyzed by 3D replica technique, for each measurement, the specimen was digitised using the model scanner.

The marginal gap of the onlays fabricated with indirect digital impressions were smaller than that with direct digital impressions (P < 0.05). At the same time, the internal gap of the onlays fabricated with indirect digital impressions were smaller than that with direct digi-tal impressions (P < 0.05). The marginal gap was larger in distal gingival than that in the other regions in all the groups (P < 0.05). Different fabrication materials, zirconia or lithium disilicate reinforced glass-ceramic, had no effect on onlay marginal and internal fit (P>0.05).

The marginal and internal adaptation of the onlays fabricated with indirect digital impressions was better than with direct digital impressions. Zirconia and lithium disilicate reinforced glass-ceramic had no effect on the onlay adaptation.