Distortion reduction for a dental HFUS microscanning device

Silicone impression-taking of teeth is an established but inefficient technique for computer-aided manufacturing of dental prosthetics. Hence, intra-oral scanners based on optical technologies have been developed to optimize the process of impressioning and to enable a complete chairside workflow. However, during the scan, the presence of blood or saliva can cause critical model defects and the need to invasively expose subgingival areas remains essential for scanning. We introduced high frequency ultrasound (HFUS) as a new method for scanning dental structures. Ultrasound is less sensitive to oral fluids and is inherently able to penetrate gingiva non-invasively. HFUS-based intra-oral microscanning (USM) of teeth requires both a high spatial resolution and a mechanical precision to finally achieve an optimal impression detail. We designed a microscanner based on a direct drive mechanism with 2 degrees of freedom. In a preliminary study we tested the system´s 2D-distortion by scanning a ball grid array as a reference body (RB) in degassed and tempered water. The ball grid´s accuracy was measured by a commercial optical reference scanner (OS). The Euclidean distance (ED) between ball centers of the RB and the USM were calculated. Afterwards, we implemented a 2D-distortion reduction method (DRM). The scaling factors (coefficients of the compensation matrix) were calculated by a linear regression analysis on the EDs. Finally, the system´s precision was evaluated by scanning the occlusal surface of a molar tooth with the USM and applying the DRM. The acquired model was aligned and compared to a scan obtained with the OS by using a best-fit algorithm. Without using the DRM we measured a mean deviation of 14.8 microns (positive) and 21.2 microns (negative) (SD 22.7 microns). After the application of the DRM we achieved a mean deviation of 7.6 microns (positive) and 19.9 microns (negative) (SD 16.9 microns). The scan quality of the USM was improved by the developed DRM and thus reached the anticipated accuracy range for intra-oral impressioning.