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UCSF School of Dentistry uses Avizo Software to Extract Changes in Periodontal Ligament Space and Tooth Movement Due to Orthodontic Therapy
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By Kyle H.-Y. Chan, fourth-year undergraduate student in Molecular and Cell Biology, and Public Health at UC Berkeley, FeiFei Yang, Ph.D., postdoctoral scholar in the Laboratory of Multiscale Biomechanics and Biomineralization, School of Dentistry, UCSF, and Sunita P. Ho, Ph.D., Division of Biomaterials and Bioengineering, Department of Preventive and Restorative Dental Sciences, School of Dentistry, University of California San Francisco.
During the acts of biting and chewing, the muscles of the jaw (consisting of the masseter, temporalis, and medial pterygoid in the elevator group, and lateral pterygoid as the main depressor) generate forces that dictate jaw kinematics 1 . The movement of jaws hinges about the temporomandibular joint and are brought together by the muscles attached to respective bones through bone-tendon interfaces known as entheses 2 . Thus, chewing forces affect aspects of craniofacial structure as well as bone formation and resorption.
The insertion of orthodontic braces changes the forces acting at the cementum-ligament and bone-ligament entheses, and induces orthodontic tooth movement. In this article, we analyze physical movement of teeth due to orthodontic braces commonly placed in humans to facilitate favorable occlusion.
Comparing before and after orthodontic therapy
Articulation of mandibular and maxillary jaws and interdigitation of respective teeth of a teenage patient before and after orthodontic intervention for a year was visualized using cone beam computed tomography (CBCT) at the UCSF School of Dentistry. Prior to analysis of the CBCTs, voxel size was calibrated (0.25 μm 3 per voxel). Centers of rotation of CBCT data sets from before and after intervention were registered using the Register Image function in Avizo software to identify gross tooth movement in the mandibular and maxillary jaws of the patient.
Figure 1: CT scans of a patient's head before (a) and after (b) orthodontic intervention were overlaid and image registration was performed. (c) Front view of the result of image registration. The colored portions represent the final CT scan following orthodontic intervention, while the grey scale portions represent the initial scan. Blue regions indicate areas with less displacement (400 μm or less), and green with more displacement (3000 μm or more). Angled (d) and top (e) views reveal the extent of tooth movement when compared to initial conditions (grey scale image). In the top view, the jaw was designated as the region of interest using the Extract Subvolume function for the dataset.
Mapping Surface Distance
Mapping the distance between the surfaces of the two data sets allowed visualization of the extent to which teeth were translated due to orthodontic intervention. The center of rotation of each respective CBCT scans was used to perform image registration (specifically, of the data sets below the nasal cavity). The Surface Distance function was used to calculate both magnitude and direction of displacement of jaws and teeth within the jaws due to orthodontic braces relative to the initial conditions. The colormap can be seen using the Surface View module.
Figure 2: Colormap of surface distance which can be correlated to the degree of orthodontic tooth movement (a, b). In the top view (c), the jaw was designated as the region of interest by setting an ROI (region of interest) Box as a parameter for Surface Distance.
Tethers
Tethers are another way to visualize movement. Each line, or tether, is generated and represents the distance between closest corresponding points of surfaces in each data set. Tethers can be seen by using the Surface Vectors module.
Figure 3: In this example, tethers were drawn if the distance between the points was at least 500 μm, but no more than 4000 μm. Distances shorter than 500 μm were omitted as they are below the voxel resolution, while distances longer than 4000 μm may no longer link relevant corresponding points.
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Images and text are courtesy of UCSF School of Dentistry
Acknowledgements:
The authors thank the Biomaterials and Bioengineering Micro-CT Imaging Facility, UCSF for the use of MicroXCT-200. The authors also thank Dr. Andrew Jheon for in-depth discussions related to orthodontic intervention. Support was provided by NIH/NIDCR R01DE022032 (SPH), NIH/NCRR S10RR026645 (SPH), and Departments of Preventive and Restorative Dental Sciences, and Orofacial Sciences, School of Dentistry, UCSF
References:
1 Grünheid, T. The masticatory system under varying functional load. (s.n.] ; Universiteit van Amsterdam [Host, 2010).
2 Benjamin, M. et al. Where tendons and ligaments meet bone: attachment sites ('entheses') in relation to exercise and/or mechanical load. J. Anat. 208, 471-490 (2006).