Accuracy of single crowns fabricated from ultrasound digital impressions.

OBJECTIVE: This in vitro study aimed to evaluate marginal and internal fit of single crowns produced from high-frequency ultrasound based digital impressions of teeth prepared with finish lines covered by porcine gingiva, in comparison with those obtained by optical scanners with uncovered finish lines. METHODS: Ten human teeth were prepared and forty zirconia crowns were fabricated from STL-datasets obtained from four dental scanners (n=10): extraoral CS2 (Straumann), intraoral Lava COS (3M), intraoral Trios (3Shape) and extraoral ultrasound scanner. The accuracy of the crowns was compared evaluating marginal and internal fit by means of the replica technique with measurements in four areas; P1: occlusal surface; P2: transition between occlusal and axial surfaces; P3: middle of axial wall; and P4: marginal gap. Restoration margins were classified according to their mismatch as regular, underextended or overextended. Kruskal-Wallis one-way ANOVA and Mann-Whitney U test were used to evaluate the differences between groups at p<0.05. RESULTS: The median value of marginal gap (P4) for Ultrasound (113.87µm) differed statistically from that of CS2 (39.74µm), Lava COS (41.98µm) and Trios (42.07µm). There were no statistical differences between ultrasound and Lava COS for internal misfit (P1-P3), however there were statistical differences when compared with the other two scanners (Trios and CS2) at P1 and P2. SIGNIFICANCE: The ultrasound scanner was able to make digital impressions of prepared teeth through porcine gingiva (P4), however with less accuracy of fit than that of conventional optical scanners without coverage of the finish lines. Where no gingiva was available (P1-P3), the ultrasound accuracy of fit was similar to that of at least one optical scanner (Lava COS).

Soft tissue-preserving computer-aided impression: a novel concept using ultrasonic 3D-scanning.

Subgingival preparations are often affected by blood and saliva during impression taking, regardless of whether one is using compound impression techniques or intraoral digital scanning methods. The latter are currently based on optical principles and therefore also need clean and dry surfaces. In contrast, ultrasonic waves are able to non-invasively penetrate gingiva, saliva, and blood, leading to decisive advantages, as cleaning and drying of the oral cavity becomes unnecessary. In addition, the application of ultrasound may facilitate the detection of subgingival structures without invasive manipulation, thereby reducing the risk of secondary infection and treatment time, and increasing patient comfort. Ultrasound devices commonly available for medical application and for the testing of materials are only suitable to a limited extent, as their resolution, precision, and design do not fulfill the requirements for intraoral scanning. The aim of this article is to describe the development of a novel ultrasound technology that enables soft tissue-preserving digital impressions of preparations for the CAD/CAM-based production of dental prostheses. The concept and development of the high-resolution ultrasound technique and the corresponding intraoral scanning system, as well as the integration into the CAD/CAM process chain, is presented.