Spatial Accuracy of CNC Milling

Objectives: CNC-milled restorations have been investigated for their preciseness before, while detailed information on the milling-step itself are lacking. The authors conducted a study to investigate the influence of CNC-milling on spatial adaption 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-milling-data=PRE) and CNC-machined from lithiumdisilicate-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, mean quadratic deviations(RMS) were computed and analyzed by a Student t-test(n=5, α=0.05).

Results: Mean(SD) RMS-values were: PRE 32(2)µm, POST 38(7)µm. Differences were not statistically significant(P=0.206). Qualitatively, PRE showed oversized dimensions at the preparation finish line and the central occlusal box, while the buccal wall and the occluso-oral junction of the occlusal box were partially undersized. POST showed comparably distributed deviations, with buccal and oral cusps as well as approximal boxes being further enlarged.

Conclusions: Digital dental workflows are known to incur inaccuracies. However, present findings indicate that fit-discrepancies between virtually planned and actually milled restorations are negligibly small, when measured against clinical standards.