Virtual three-dimensional analysis of digitized dental impressions and stone replicas

University dissertation from Stockholm : Karolinska Institutet, Department of Odontology

Abstract: In response to increasing patient awareness of esthetic dental rehabilitation, the preference today is for all-ceramic restorations, which show superior biocompatibility and similar light characteristics to the natural tooth. The fit of a dental restoration depends on quality throughout the entire manufacturing process. The final result is affected by multiple factors, such as preparation of the tooth, the impression, production of a dental cast, fabrication of the restoration, chairside adjustment of the restoration and, finally, when the restoration is complete, the material and method used for cementation. By the use of CAD/CAM systems, the automation of the manufacturing process improves efficiency and enhances quality control. In this thesis, a methodology for virtual three-dimensional analysis was developed and applied to stages involved in the production of fixed dental prostheses. The general aim was to measure the effect of different steps in the manufacturing process on the exactness of the CAD-model. The main reason for undertaking in vitro studies was the need for a physical reference, a master model, and the opportunity to digitize it. Three different dental surface digitization devices were used in the studies, a laser scanner and two touch-probe scanners. The point-clouds resulting from the digitizations of the master dies of prepared teeth yielded virtual CAD-Reference-Models (CRMs) of each tooth shape. Best-fit alignment of the virtual point-clouds from the digitizations of the replicas to the corresponding CRM was performed. The quality of data acquired by the dental surface digitization devices was evaluated by repeated digitizations of the master models in the laser scanner and the touch-probe scanners. The reliability was estimated by comparison of the digitizations within each scanner. The potential to obtain high quality data directly from the impression, eliminating the stone replica stage from the manufacturing process was investigated. The potential geometric difference in the resultant stone replica was also analyzed. Clinical conditions were simulated by the presence of neighboring and antagonistic teeth. High quality data can be achieved directly from the digitized impressions with the optical digitization device used in the thesis. The results indicated that the discrepancies were of the same size as when digitizing stone replicas. Hence digitization of the impression may be a successful means of improving efficiency without affecting the quality of the final restoration. The thesis brings a deeper understanding of the specific problems in the evaluation of CAD/CAM technology in dentistry. The methodology developed to evaluate exactness of digitizations will be of benefit in the ongoing process of validation of dental CAD/CAM systems. Virtual three-dimensional analysis allows for differentiated quality analysis of the manufacturing process as well as comparison of different digitization methods.

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