Study models are an essential part of an orthodontic record. Digital models are now available. One option for generating a digital model is cone-beam computed tomography (CBCT) scanning of orthodontic impressions and bite registrations. However, the accuracy of digital measurements from models generated by this method has yet to be thoroughly evaluated.
A plastic typodont was modified with reference points for standardized intra-arch and interarch measurements, and 16 sets of maxillary and mandibular vinylpolysiloxane and alginate impressions were made. A copper wax-bite registration was made with the typodont in maximum intercuspal position to accompany each set of impressions. The impressions were shipped to OrthoProofUSA (Albuquerque, NM), where digital orthodontic models were generated via CBCT. Intra-arch and interarch measurements were made directly on the typodont with electronic digital calipers and on the digital models by using OrthoProofUSA’s proprietary DigiModel software.
Percentage differences from the typodont of all intra-arch measurements in the alginate and vinylpolysiloxane groups were low, from 0.1% to 0.7%. Statistical analysis of the intra-arch percentage differences from the typodont of the alginate and vinylpolysiloxane groups had a statistically significant difference between the groups only for maxillary intermolar width. However, because of the small percentage differences, this was not considered clinically significant for orthodontic measurements. Percentage differences from the typodont of all interarch measurements in the alginate and vinylpolysiloxane groups were much higher, from 3.3% to 10.7%. Statistical analysis of the interarch percentage differences from the typodont of the alginate and vinylpolysiloxane groups showed statistically significant differences between the groups in both the maxillary right canine to mandibular right canine (alginate with a lower percentage difference than vinylpolysiloxane) and the maxillary left second molar to mandibular left second molar (alginate with a greater percentage difference than vinylpolysiloxane) segments. This difference, ranging from 0.24 to 0.72 mm, is clinically significant.
In this study, digital orthodontic models from CBCT scans of alginate and vinylpolysiloxane impressions provided a dimensionally accurate representation of intra-arch relationships for orthodontic evaluation. However, the use of copper wax-bite registrations in this CBCT-based process did not result in an accurate digital representation of interarch relationships.
Our dependence on stone models for the storage of 3-dimensional information has come under attack on several fronts involving many new technologies. As orthodontists adjust office procedures to be more efficient, answers to the question of what technology to rely on in the long term require high-level evidence. This article has answers to some of these concerns. Although laser surface scanning of plaster models generated from alginate and vinylpolysiloxane impressions has been studied extensively and has demonstrated a comparable level of accuracy to plaster models, the more recent method of CBCT scanning of alginate impressions has not received an extensive examination. Whereas this method is comparable with the scanning process used by Invisalign for vinylpolysiloxane impressions, because of the costs, most practitioners prefer to use a material with less dimensional stability over time. Therefore, although this process is promising, requiring no intermediate plaster models, the accuracy of the entire process from alginate impression to final digital model set has not been thoroughly tested.
With this in mind, this study was undertaken to answer 2 questions. (1) How do intra-arch measurements of digital models derived from CBCT scanning of 100-hour stable alginate impressions compare with those from vinylpolysiloxane impressions when shipped to OrthoProofUSA under the same conditions? (2) Does the use of a copper wax-bite registration result in an accurate reproduction of interarch dimensions, when used to generate digital model sets in this CBCT-based system?
This lengthy and complex study makes for an interesting read. The conclusions will help clinicians plan ahead. For the generation of digital models, a 100-hour dimensionally stable alginate used as the manufacturer directs and shipped within 48 hours as directed by OrthoProofUSA has comparable accuracy for intra-arch linear measurements as vinylpolysiloxane in a CBCT-based digital modeling system. The use of a copper wax-bite registration in the CBCT-based modeling system of OrthoProofUSA does not provide a consistently accurate digital representation of interarch relationships. Therefore, use of a Collision Mapping Tool, available in the DigiModel software, is often required to correct inaccuracies in occlusal relationships.