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.
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.
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.
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Images and text are courtesy of UCSF School of Dentistry
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
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).