The goal of orthodontic treatment is to establish ideal three-dimensional crown and root position in a functional, stable, and esthetic occlusion. Andrews reported six keys to normal occlusion, 1) molar relationship, 2) crown angulation (mesiodistal tip), 3) crown inclination (labiolingual or buccolingual inclination), 4) rotations, and 5) occlusal plane and 6) occlusion based on crown information from study models. Proper root position and parallelism are imperative for adequate occlusal function, periodontal health, implant placement, and restorative treatment.[2–5] Root position and parallelism are important factors for achieving even distribution of occlusal forces to create ideal function and for establishing proper contours and emergence profiles of restorations.[2–5] Root proximity may lead to rapid periodontal breakdown and horizontal bone loss instead of intrabony defects that are amenable to regeneration.[6–11] The consequences of roots moving out of the alveolar housing include clinical attachment loss, recession, bone dehiscence, mobility, and even tooth loss.[12, 13] Therefore, predicting root position during orthodontic treatment is a critical factor for successful outcomes.
Two-dimensional (2D) images such as cephalometric and panoramic radiographs cannot evaluate the three-dimensional (3D) position of the teeth and roots relative to the maxillofacial region and alveolar bone during orthodontic treatments. With its increasing availability, cone-beam computed tomography (CBCT) has been used to aid in diagnosis and assessment during orthodontic treatment. The advantage of CBCT is that it provides the exact 3D location of the crown and roots of teeth and their relationship with both neighboring teeth and alveolar bone.[14, 15] Therefore, visual and timely evaluation of root positioning using 3D imaging by CBCT is crucial in orthodontic treatments. However, CBCT is not without its disadvantages.
Since the effective radiation dose from CBCT is significantly higher compared to conventional radiographs, routine usage of CBCT is not recommended during orthodontic treatments as a substitute for conventional radiographs, especially when the population for orthodontic treatment is relatively young.[16, 17] Furthermore, CBCT rendering of teeth lacks precise occlusal surface and accurate interdigitation. Artifacts from metal restorations and orthodontic brackets can also result in discrepancies. On the other hand, digital scanning can provide precise tooth morphology and register accurate interocclusal relationship. Studies have also shown that brackets did not affect the accuracy of digital scans.[20, 21] Therefore, registration and subsequent superimposition of CBCT and digital scans obtained with either an intraoral scanner or an extraoral lab scanner have become a standard workflow in many orthodontic appliance systems to assess tooth alignment. While this method is generally clinically acceptable, the registration of CBCT to digital scans is less accurate compared to registration of digital scans to digital scans.
To overcome these shortcomings, Lee et al. in 2014 introduced a monitoring method by creating “teeth composites” whereby the 3D digital model was composed of crown extracted from digital scans and root portion extracted from CBCT. The author demonstrated in an ex-vivo typodont study that this method was reliable to track the 3D position of whole teeth including roots. This method was not further verified in clinical studies due to impracticality related to technique-sensitive, complex and time-consuming process of threshold segmentation of teeth in real patients.
With the rapid development of imaging and digital technology in recent years, much improvement has been made in both hardware and software. Many commercial companies now provide service to clinicians to process CBCT, digital scan data and produce 3D models with 3D printing. The cost of these services has also become significantly lower. Creating 3D digital crown/root models (3DCRM) by integrating digital scan crowns and CBCT root with patient data become a feasible approach. Therefore, this method first introduced by Lee et al. in 2014 will be an ideal modality.
This is a retrospective study using clinical cases and the aims of this study were 1) to assess the accuracy of this new method of predicting post-movement root position compared to the current method of using post-movement CBCT and 2) to analyze the association of accurately predicting root position in each of the three tooth types, and 3) to analyze the association between accuracy of test method and the amount of actual root apex movement. We hypothesize that the 3DCRM method will be as accurate as the use of post-movement CBCT in evaluating root positioning and thus eliminate the use of a second or multiple CBCTs during or at the completion of orthodontic treatment.