DOI: 10.25549/usctheses-c40-346820
title: 3D superimpositions in orthodontics: a review of current techniques and applications

description: As the technology continues to improve, the application of three-dimensional imaging is becoming more accessible and practical.  The various applications of 3D technology in the field of orthodontics have allowed for advances in diagnosis, treatment planning, and orthodontic techniques. Superimpositions of these images provide a comparison to established norms and templates, the determination of changes resulting from treatment or growth, as well as a prediction of potential treatment outcomes. It has been applied to the registration of dental models, dental hard tissue, skeletal hard tissue, including the temporomandibular joint, and soft tissue, including facial soft tissue and airway. ❧ The superimposition of 3D images can be accomplished using three distinct methods: 1. Point-based, 2. Surface-based, 3. Voxel-based. Each of these techniques has certain criteria required of the images and use a specific algorithm for proper alignment and registration. Because of this, not all may be suitable for the desired objective. There are also a variety of software programs available to perform 3D superimpositions. Each of these different methods, techniques, and software have their own advantages and disadvantages and differ in their ease of use, efficiency, accuracy, time required, and cost. The purpose of this review is to provide some insight into the different superimposition procedures and programs that will help guide its effective use to further our understanding of orthodontics. ❧ A review of the literature has revealed success in using certain superimposition techniques for the registration of particular structures and limitations for others. The use of 3D superimpositions for specific applications and a discussion of its accuracy is presented in this review. It was found that surface-based methods are the most accurate and reliable for the superimposition of digital dental models. Although registrations to compare planned setups and actual outcomes are valid and reproducible, using them to evaluate tooth movements are not as reliable. Meanwhile, point-based methods are the most accurate and reliable for the superimposition of the TMJ to assess changes in morphology. For the superimposition of 3D cephalometrics derived from CBCT scans, voxel-based methods are the most robust and avoids errors resulting from landmark identification or segmentation procedures. The evaluation of soft tissue changes is an area that could be much improved. Some studies have shown point-based and surface-based methods to be accurate, but should be held with some reservations due to the dynamic nature of facial soft tissue. Voxel-based registration on cranial base has been shown to be the more reliable method, but the main issue with this technique is the inferior image quality for soft tissue. ❧ With the use of 3D superimpositions being relatively new to the field of orthodontics, its potential for use in the research and clinical settings is still in its early stages. As can be seen through some of its limitations, much can still be improved as to how 3D registrations can be performed and applied. Nevertheless, as technology and techniques improve, registration of 3D images will become faster, easier, and more efficient to help transform and advance the way orthodontists diagnose and treat patients.