What's the different between Intra-oral Scanning and Traditional Extra-oral Scanning

In recent years, with the development of digital technology, the three-dimensional occlusion analysis method based on digital dental models has gradually emerged and attracted attention due to its simplicity, intuitiveness, and independence from media. However, most digital dental models require traditional impression-making and plaster casting, followed by three-dimensional laser scanning of the impression or plaster model, which is usually done using an extra-oral scanning method. The emerging intraoral scanning method involves placing the scanning device directly into the patient's mouth to scan the oral soft and hard tissues in real-time to obtain a digital model, avoiding the unpredictable error associated with impression-making, plaster casting, and extra oral tooth reconstruction, and more accurately reflecting the patient's dental morphology and occlusion status. However, intra-oral scanning is limited by the restricted movements and angles of the scanning head in the limited oral cavity, brightness limitations, and the influence of various contaminants on the tooth surface, and insufficient research data on the accuracy of clinical operability and the impact of the number of teeth on the scanning accuracy compared to the data obtained by model scanning method.

The dental digitization technology gradually incorporates advanced engineering design and manufacturing technology into the entire process of diagnosis and treatment of oral diseases, making it more accurate, precise, and efficient. Digital impression technology is the premise and foundation of digital diagnosis and treatment, including the application of mature dental plaster models, 3D scanning technology, and the rapidly developing intraoral scanning technology in recent years. Although the scanning accuracy of dental models can reach as high as 10μm, the clinical operation error in making and replicating the plaster models is difficult to control due to the potential deformations and defects of the impression caused by factors such as temperature, humidity, gypsum powder ratio, and air bubbles. In contrast, the simplified workflow of intraoral scanning significantly improves the controllability of error sources, and the average accuracy of intraoral scanning is currently around 20μm. Research proves that the precision of intraoral digital impression within half of an arch range meets clinical requirements and has a certain degree of reliability and validity for diagnosing tooth form.

Scanning the narrow space inside the oral cavity requires a scanning head with a slender structure for its optical system often has a small single field of view. To obtain relatively complete dental data, a large amount of multi-view and three-dimensional data stitching processing is required, and the more times the stitching is done, the lower the data accuracy. In addition, in order to obtain more data, the scanning head needs to stay in the oral cavity for a longer time, which may increase the impact of factors such as saliva and decrease the patient's cooperation. The occlusal state may also change due to the scanning head staying in the vestibule of the oral cavity for a long time. In the preliminary experiments, when scanning more than three teeth, including adjacent teeth in the near, middle, and far range, the scanning range was large and easily affected by the patient's degree of mouth opening and operational accuracy, resulting in significant deviations. Therefore, it is not advisable to have a too large scanning range inside the oral cavity.