Enhancing Accuracy of Intraoral Scanning in Pediatric Dentistry
Article information
Abstract
Digital impressions with intraoral scanner (IOS) systems have gained popularity in dentistry following the widespread availability of digital technology. These systems can simplify laboratory procedures, enhance communications among dental professionals, transform dental practices, and open up new possibilities beyond the capabilities of traditional analog-based dental treatment. This paper reviews current IOS systems by providing information on their imaging technology and accuracy. The key factors influencing the accuracy of IOS systems are also discussed. This review aimed to provide pediatric dentists with insights into optimizing IOS utilization and increasing accuracy in their clinical practice.
Introduction
The integration of digital technology into dentistry has significantly changed treatment workflows. Digital dentistry has proven its versatile use in the field of dentistry, such as in surgical guide fabrication, dental restoration, diagnosis, and treatment planning of orthodontics[1]. Specifically, digital dentistry applications are increasingly expanding the precision and scope of pediatric dentistry[2-7].
Generally, digital dentistry workflows involve digital impression, computer-aided design, and computer-aided manufacturing. The initial step in digital dentistry is creating digital impressions using intraoral scanner (IOS) systems, which is essential for ensuring the reliability of clinical procedures[8]. Digital impressions facilitate the rapid production of accurate and reproducible three-dimensional (3D) digital casts, data storage, and documentation[9]. Digital impressions can be an alternative to traditional impression methods in pediatric patients. Digital impressions enhance the experience of children by eliminating the need for impression materials[10]. A questionnaire study indicated that pediatric patients prefer digital impressions over alginate impressions[11].
For the scanned image acquisition, IOS systems generate a light projection, and optical sensors capture the reflected light, converting it into images or videos when they run in the oral tissues. Software algorithms then assemble these images or videos after recognizing the points of interest[12]. The resulting multiple sets of points are aligned and converted into a surface model made up of triangular meshes[13]. These meshes contain vertices and triangles, including data on how the vertices form the triangles and how the triangles connect[14]. The software then uses these meshes to render the surface of oral tissues as computer graphics. Consequently, the density of meshes influences the quality of surface reproduction[14].
Since IOS systems must be in close contact with oral tissues, they have a relatively small measuring area. Consequently, to create 3D digital casts, these systems require a large number of images that the software stitches together following the algorithms. This stitching process can be an error source in the final product of digital impressions[15].
Although IOS systems offer numerous advantages, it is critical to ensure the accuracy of digital impressions, particularly in pediatric patients, where their distinct anatomy in the oral cavity and limited cooperation may pose challenges. Therefore, this review aimed to offer insights on optimizing their use of IOS systems by providing an overview of these systems and the factors that affect their accuracy.
IOS technology
IOS systems include optics, imaging sensors, and software algorithms. Most IOS systems utilize structured light technology to capture the geometry of a 3D object. This involves projecting a light pattern, such as strips, grids, or dots, onto the surface of an object. The deformation of this pattern by the object’s surface allows capturing its geometry, facilitating the creation of a 3D scanned image[16]. This technology is believed to capture the object’s shape accurately and create a simulated image with a good balance of other properties. However, an ambient light environment can influence this technology. Studies have shown that ambient illumination influences the geometry of the constructed mesh structure and the accuracy of the color of the scanned images produced using structured light[17,18].
Further, image acquisition principles of intraoral scanners encompass triangular scanning and confocal laser scanning (Fig. 1). Triangulation scanning was previously implemented in non-contact methods, such as laboratory scanners. This scanning technique generates 3D digital images by calculating the angles and distances between the light source and the sensor[16]. At a fixed angle, a light source is projected onto the surface of an object, and the sensors capture the reflected light from the surface. Given the constant distance between the sensors and the emitter and the fixed angle of incidence of the light on the object, the distance to the illuminated point can be calculated using the law of sines[12]. The Medit i700 scanner (Medit, Seoul, Korea) adopts this method[19]. Confocal laser scanning is the method employed in iTero Element 5D Plus (Align Technology Inc., Arizona, USA) and TRIOS 4 (3Shape, Copenhagen, Denmark) scanners[20]. This method employs a spatial filter to focus a small beam of light at a specific depth, capturing two-dimensional (2D) images from various focal planes of the scanned object, thereby generating point illumination that eliminates out-of-focus signals. Subsequently, the object’s 3D structure is reconstructed by merging these 2D images[21].
Schematic designs of image acquisition mechanisms of intraoral scanners. (A) Triangulation scanning. Three-dimensional digital images are created by calculating the angles and distances between the light source and the sensor. (B) Confocal laser scanning. A series of two-dimensional images at different focal planes is created and stitched to reconstruct three-dimensional images.
Once IOS systems have captured the images, the software analyzes the images by identifying and registering common landmarks and recognizable features, such as dental contours. Then, to create a complete 3D model, software algorithms align the overlapping regions of these images using the identified landmarks and connect a real-time series of scans, a process known as stitching[22]. In the stitching process, the field of view (FOV) is closely related to the presence of errors (Fig. 2). If the FOV of a certain IOS system is relatively small, the software requires more image files to reconstruct a 3D scanned image, which can increase the risk of errors[23].
Stitching process to reconstruct three-dimensional images. Note that a small field of view requires more stitched images, which can increase errors.
IOS systems
Table 1 lists currently available commercial IOS systems. These systems employ different scanner technologies, image acquisition technologies, and light sources. Therefore, to obtain scanned images with high accuracy, the scanning method recommended by the manufacturer of each scanner should be followed[24].
Specifications of the intraoral scanners
Generally, IOS systems consist of a scanner head, sleeve, optical lens, light source, and software (Fig. 3). Accordingly, optical and image sensors capture high-resolution images, and software algorithms connect individual images to generate a single digital model[25].
Components of intraoral scanner systems. (A) iTero Element 5D. (B) TRIOS 4.
Intraoral scanning accuracy
Prior to the adoption of intraoral scanning in pediatric dental settings, its accuracy should be verified. Assessing IOS accuracy typically focuses on (i) trueness and (ii) precision. Trueness refers to the closeness of the scanned dimension to the actual dimension of an object. Precision refers to the consistency of the scans under the same scanning conditions[26]. Pediatric dentists should choose IOS systems with high trueness and precision. To calculate trueness and precision, discrepancies are derived from the comparison of the original and the estimated values. As these discrepancies can be given as signed values, calculating their average may result in the cancellation of the values[27]. Therefore, root mean square (RMS) values are used to assess both trueness and precision (Fig. 4), as shown below:
An example of calculating root mean square value.
Note that the discrepancies cancel each other when calculating the average. RMS: root mean square.
xn = [original value at point n] - [estimated value at point n]
IOS accuracy is widely accepted in the literature. A recent systematic review of in vivo studies concluded that the trueness of IOS systems is comparable to that of alginate full arch impressions, and the precision is high[28]. The reported average discrepancy between IOS and alginate impressions was < 100 μm[29]. A previous study evaluating complete-arch impressions from five participants found that the precision of seven IOS systems ranged within 100 μm (TRIOS 4 (3Shape), 47.5 μm; iTero Element 5D Plus (Align Technology Inc.), 68.1 μm); however, their precision was equal to or lower than that of polyether and vinylsiloxanether impressions[30]. Another study that compared two digital models created from human skulls by cone-beam computed tomography and intraoral scanning did not find significant differences in intercanine and intermolar widths between the two models[31].
IOS accuracy in pediatric populations is also well-documented. A recent prospective clinical study that compared dental arches with mixed dentition found negligible dimensional discrepancies between dental plaster casts made from alginate impressions and intra-oral scans[32]. Another in vitro study demonstrated that accurate impressions can be obtained using IOS systems even in young children, confirming that both iTero Element 5D Plus (Align Technology Inc.) and TRIOS 4 (3Shape) scanners achieved high accuracy (trueness of < 70 μm and precision of < 40 μm) under a limited mouth opening environment that simulated primary dentition [20]. This finding was also confirmed by a previous study that compared the accuracy of the TRIOS 5 (3Shape) and Helios 600 (Changzhou Sifary Medical Technology Co., Ltd, Changzhou, China) scanners[33].
Factors affecting IOS accuracy
Despite the acceptability of IOS accuracy, it can be influenced by certain factors[13]. Pediatric dentists can use IOS systems with confidence to take digital impressions if they are aware of these influencing factors. Basically, IOS accuracy is affected by the IOS technology, operator, or patient factors. First of all, pediatric dentists should choose an IOS system with high accuracy. For its daily use, IOS systems should be calibrated, except for iTero Element 5D Plus (Align Technology Inc.) Element and TRIOS 5 (3Shape)[14]. Most IOS systems should be calibrated every 20 scans and can automatically note when recalibration is necessary. However, current literature recommends daily calibration before starting intraoral scanning[34]. Next, pediatric dentists should be accustomed to running IOS systems. Although recent technical advances in IOS systems can reduce this influence, operator experience in intraoral scanning may affect the accuracy[35-37]. Furthermore, greater experience is related to shorter scanning times[35].
IOS hardware can also affect IOS accuracy. Selection of appropriate scanner head size is important. A larger scanner head offers higher IOS accuracy than a smaller head[38,39]. This is due to the stitching technique, which is mainly employed in constructing 3D scanned images in IOS systems. When scanning an object of the same volume, a small scanner head requires more image acquisition than a larger one. This increases the number of stitching processes needed, which may result in greater distortion in the final 3D scanned images if any distortion occurs during acquisition. However, a large scanner head can hinder access to anatomical structures in the oral environment[20]. Additionally, the distance between the scanning tip and the surface of an object being scanned can also affect IOS accuracy. A previous in vitro study revealed that the accuracy decreased when the scanning tip made direct contact with the object’s surface across all IOS systems evaluated (TRIOS 3 (3Shape), CS3600 (Carestream Health), PlanScan (Planmeca)). The study also found that the required scanning distance for ensuring high accuracy varied by IOS systems[40]. Another study indicated that the accuracy of the Medit i700 (Medit) scanner significantly decreased when the scanning distance was greater than 15 mm[41].
The scanning environment can also affect IOS accuracy. Evidence indicates that changes in ambient temperature can decalibrate the IOS system and thereby compromise its scanning accuracy. A previous study recommended an operating temperature between 15 and 30°C for intraoral scanning[34]. To minimize the influence of ambient temperature changes, the IOS systems require calibration before starting each workday[13]. Moreover, the ambient lighting environment can affect IOS accuracy. Studies have suggested an ambient illumination of <100 Lux for intraoral scanning[42-44]. For these conditions, intraoral scanning should be conducted with the dental chair light turned off and the ceiling lights on[13]. As the scanning pattern is also related to IOS accuracy[45], following the pattern recommended by the manufacturer is advised. In the literature, continuous scanning patterns are preferred for full arch impressions, with the sequence starting from the right posterior molar, scanning the occlusal, buccal, and palatal surfaces in that order[24,46]. However, some controversies exist regarding the sequence of surface scanning. A previous study evaluating the accuracy in pediatric models demonstrated that a sequence starting from the right posterior molar and proceeding in the occlusal, palatal, and buccal direction was more accurate than the preferred pattern when scanning the maxillary arch[47].
Regarding patient factors affecting IOS accuracy, studies have shown that the tooth type, presence of interdental spaces/reduced space, and existing restorations can affect accuracy[48-52]. A previous study demonstrated that the accuracy was lower in posterior teeth than in anterior teeth[48], likely due to light transmission interference when scanning the posterior teeth[20]. The presence of diastemas and reduced space between adjacent teeth is related to reduced accuracy, as these affect IOS accessibility, scanning angle, and data acquisition[36,49,51]. Additionally, a previous study indicated that IOS systems can more accurately capture the maxillary arch when the palate is not included in the scanning scope[53]. Moreover, existing tooth restorations may influence accuracy[54]. A related study revealed that IOS accuracy in gold crowns was lower than those in zirconia and 3D-printed resin crowns[52]. Another study showed that the accuracy was unaffected in an adjacent tooth next to a band and loop space maintainer[55]. Additionally, studies have reported that greater arch width is related to lower accuracy[54,56,57]. This may be due to the stitching technique, as mentioned previously. Humidity on an object’s surface can compromise accuracy, as reflected light from a wet surface is refracted by the water, potentially worsening the performance of IOS systems[58,59]. A previous in vitro study evaluating full mandibular arch impressions revealed that the trueness of TRIOS 3 (3Shape) in wet conditions with artificial saliva (166.94 ± 20.75 μm) was significantly lower than that in dry conditions (108.78 ± 23.05 μm)[59]. However, its trueness significantly improved after being blow-dried using a three-way syringe for 10 s (127.98 ± 11.04 μm). The results of this study indicate that saliva control can improve scanning accuracy. However, all findings presented are primarily based on individual characteristics, except wetness, which can be challenging for pediatric dentists to control. Therefore, pediatric dentists should keep in mind the importance of controlling oral humidity during intraoral scanning.
Conclusion
In conclusion, IOS systems are highly accurate, and several factors can affect their performance. These can be broadly categorized as technology-related, operator-related, and patient-related factors. Understanding these factors is essential for pediatric dentists in order to optimize the use of IOS systems and improve clinical outcomes in pediatric patients.
Notes
Conflicts of Interest
The authors have no potential conflicts of interest to disclose.
CRediT authorship contribution statement
Ju Ri Ye: Formal analysis, Investigation, Visularization, Writing ‒ original draft. Ok Hyung Nam: Conceptualization, Formal analysis, Investigation, Methodology, Supervision, Writing ‒ original draft, Writing ‒ review & editing.