Digitization of simulated clinical dental impressions: virtual three-dimensional analysis of exactness

To compare the exactness of simulated clinical impressions and stone replicas of crown preparations, using digitization and virtual three-dimensional analysis.

Three master dies (mandibular incisor, canine and molar) were prepared for full crowns, mounted in full dental arches in a plane line articulator. Eight impressions were taken using an experimental monophase vinyl polysiloxane-based material. Stone replicas were poured in type IV stone (Vel-Mix Stone; Kerr). The master dies and the stone replicas were digitized in a touch-probe scanner (Procera) Forte; Nobel Biocare AB) and the impressions in a laser scanner (D250, 3Shape A/S), to create virtual models. The resulting point-clouds from the digitization of the master dies were used as CAD-Reference-Models (CRM). Discrepancies between the points in the pointclouds and the corresponding CRM were measured by a matching-software (CopyCAD 6.504 SP2; Delcam Plc). The distribution of the discrepancies was analyzed and depicted on color-difference maps.

The discrepancies of the digitized impressions and the stone replicas compared to the CRM were of similar size with a mean+/-SD within 40microm, with the exception of two of the digitized molar impressions. The precision of the digitized impressions and stone replicas did not differ significantly (F=4.2; p=0.053). However, the shape affected the digitization (F=5.4; p=0.013) and the interaction effect of shape and digitization source (impression or stone replica) was pronounced (F=28; p<0.0001). The reliability was high for both digitization methods, evaluated by repeated digitizations.

The exactness of the digitized impressions varied with shape. Both impressions and stone replicas can be digitized repeatedly with a high reliability.