Effect of CNC-milling on the marginal and internal fit of dental ceramics: A pilot study

Abstract Objectives Machined restorations have been investigated for their preciseness before, while detailed information on the milling-step itself are lacking. Therefore, the aim of this laboratory study was to quantify the effect of a novel milling-procedure on the marginal and internal fit of ceramic restorations. Methods An acrylic model of a lower left first molar was prepared to receive a ceramic partial crown and was duplicated by one step dual viscosity impressions. Gypsum casts were formed and laser-scanned to realize virtual datasets, before restorations were designed, exported (PRE) and machined from lithium disilicate blanks. Crowns were digitized by a structure-light-scanner to obtain post-milling-data (POST). PRE and POST were virtually superimposed on the reference tooth and subjected to computer-aided-inspection. Visual fit-discrepancies were displayed with colors, while root mean square deviations (RMSD) and degrees of similarity (DS) were computed and analysed by t -tests for paired samples ( n = 5, α = 0.05). Results The milling procedure resulted in a small increase of the marginal and internal fit discrepancies (RMSD mean: 3 μm and 6 μm, respectively). RMSD differences were not statistically significant ( p = 0.495 and p = 0.160 for marginal and internal fit, respectively). These results were supported by the DS data. Significance The products of digital dental workflows are prone to imprecisions. However, the present findings suggest that differences between computer-aided designed and actually milled restorations are small, especially when compared to typical fit discrepancies observed clinically. Imprecisions introduced by digital design or production processes are small.