The development of modern computer graphics and 3D detection and analysis technology has facilitated the comparative study of dental models in stomatology. With the advancement of digital technology, the digitalization of dental models through intraoral scanning (IOS) or model scanning has been widely clinically confirmed in the field of fixed partial dentures and implant-supported dentures. However, in the field of complete dentures and removable partial dentures, the digitalization of dental models has been a hot topic in recent years. Due to the deformability and viscoelasticity of the oral mucosa and the influence of the movable mucosa at the margins, the IOS still cannot fully meet clinical requirements. Traditional impressions taken followed by model scanning are still widely recognized by experts as digitalization methods for edentulous models [3, 4, 5, 7]. In this study, two traditional edentulous impressions, open and closed-mouth impressions, were scanned in a Lab model scanner to realize the digitalization of edentulous impressions.
In recent years, many scholars have performed extensive work on three-dimensional comparisons of edentulous jaw models generated via various impression methods, including intraoral scanning. A prerequisite for a 3D comparative study was the reliable registration method of 3D models (also named alignment, registration, superimposed, or overlapped) [3, 4, 5, 9, 10]. Registration is also an indispensable tool for digital clinical dentistry of edentulous jaws[11, 12], such as transferring jaw relations in digital complete dentures, aligning jaw relation records with impressions, and aligning radiographic templates with CBCT in preoperative implant design[13, 14, 15, 16]. There are three main recorded model registration methods in the literature: ①Scanner's own software, such as 3Shape, which performs alignment by manually selecting three points and calculating internally[5]; ②Reverse engineering software, in which the most widely used registration method is the best fit alignment tool[ 2,3,6,7], which is also recommended for model alignment in reverse engineering software for product quality analysis; and ③When Chen Sihan et al. studied the accuracy of intraoral scanning of complete dentition, they used the manual registration command first and then the global registration command for model alignment in reverse engineering software[8]. This method is also commonly used for the registration of multiple model scanning data with overlapping areas, point cloud stitching and alignment. However, few relevant comparative studies on which method was better for model registration of edentulous jaws with less obvious curvature changes exist. In this study, the reverse engineering software Geomagic Control 2014 was used to compare the registration results of the best fit alignment method and the manual registration + global registration method, providing a reference for three-dimensional registration methods used for clinical edentulous jaw impressions.
To facilitate the comparison of the two registration methods in this study, the 3D deviation colour order scale of manual + global registration was corrected with reference to the best fit alignment. The mean nominal value threshold ranged from 0.150 to 0.196 mm. The nominal value threshold proportion of the Manual + Global Registration Method (nearly 70%) was significantly greater than that of the Best Fit Alignment Method (less than 50%), with P < 0.05. Manual + Global Registration achieved better registration quality than did the best fit alignment. Additionally, the green area of the nominal value threshold in the Manual + Global registration method was mostly distributed in the main stress support area and the secondary stress support area where there was less mucosal deformation. In contrast, the best fit alignment method showed an asymmetric deviation distribution in the main stress support area and the secondary stress support area, resulting in model misalignment. The Manual + Global registration method obtained significantly smaller average positive deviation, average negative deviation, and root mean square (RMS) values than did the Best Fit Alignment method for the maxilla but not for the mandible. The Manual + Global registration is better than the Best Fitting Alignment in terms of a smaller deviation between models, especially in the maxilla.
The reasons for the difference in the results of the two registration methods used for edentulous impressions may be as follows: ① The anatomical structure of the edentulous jaw models was affected. The edentulous jaw alveolar ridge is entirely covered by oral mucosa and is deformable and viscoelastic. In addition to the influence of muscle activity around the working area, different impression techniques can cause mucosal deformation. This deviation is much larger than that between models of teeth. ② The best fit alignment method relies solely on software calculations and may focus on minimizing overall deviation, but it may ignore clear correspondence relationships, such as anatomical marker features on the primary stress-bearing area and secondary stress-bearing area, where less mucosal mobility occurs. In a 2020 study by Professor Lucio Lo Russo et al. comparing differences between intraoral scanning and traditional impression model scanning of edentulous jaws[3], the best fit alignment method was used. One group had complete models without trimming the mobile mucosal border area, and the other group had partially trimmed models without the mobile mucosal border area. The alignment results showed significant differences; that is, the average distance between the aligned models decreased from 0.19 mm before trimming to 0.02 mm after trimming. The deviation of the models significantly decreased after trimming. The best fit alignment method may result in alignment offset when edentulous jaw models include the mobile mucosal border area. Manual multipoint alignment of edentulous jaw models is beneficial for obtaining artificial constraints on the correspondence between models. The combination of manual multi-point alignment and global alignment can achieve better alignment results for edentulous jaw models, including larger deviated marginal areas. The two registration methods have different application situations. Manual + global registration, also known as scan alignment, is the process of placing two or more objects (point or polygon) that constitute a single object in the proper position with respect to each other. The common areas between models are selected for iterative calculation of the minimum deviation without considering non overlapping areas. In this way, the main stress-bearing areas, secondary stress-bearing areas, and other areas with less mucosal mobility can be taken into account for alignment while reducing the influence of the border mobile mucosal area.
We also attempted to use N-point alignment combined with the best fit alignment command. First, the N-Point Alignment command allows you to align objects by selecting at least three (but as many as nine) corresponding points on reference and test models. After N-point alignment, when using the best fit alignment command, only the "fine adjustments only" and "automatic deviator elimination" options were selected, and the "high-precision fitting" option was not selected. The software defaults to 1500 sampling points, which can achieve alignment results similar to Manual N-point Alignment combined with Global registration. However, if the "High-Precision Fitting" option is selected, there will be a significant shift between the models.