Accelerometry-Enhanced Magnetic Sensor for Intra-Oral Continuous Jaw Motion Tracking

Abstract

Currently available jaw motion tracking methods require large accessories mounted on a patient and are utilized in controlled environments, for short-time examinations only. In some cases, especially in the evaluation of bruxism, a non-restrictive, 24-h jaw tracking method is needed. Bruxism oriented, electromyography (EMG)-based devices and sensor-enhanced occlusal splints are able to continuously detect masticatory activity but are uninformative in regards to movement trajectories and kinematics. This study explores a possibility to use a permanent magnet and a 3-axial magnetometer to track the mandible’s spatial position in relation to the maxilla. An algorithm for determining the sensor’s coordinates from magnetic field values was developed, and it was verified via analytical and finite element modeling and by using a 3D positioning system. Coordinates of the cubic test trajectory (a = 10 mm) were determined with root-mean-square error (RMSE) of 0.328±0.005 mm. Possibility for teeth impact detection by accelerometry was verified. Test on a 6 degrees-of-freedom (DOF), hexapod-based jaw motion simulator moving at natural speed confirmed the system’s ability to simultaneously detect jaw position and the impacts of teeth. Small size of MEMS sensors is suitable for a wearable intra-oral system that could allow visualization of continuous jaw movement in 3D models and could enable new research on parafunctional jaw activities.

Permanent Magnet Tracking Method Resistant to Background Magnetic Field for Assessing Jaw Movement in Wearable Devices

“…A thorough description of the magnetic position determination method can be found in our previous work [8]. However, for readability, the model and basic position calculation principle are briefly explained here.…”

“…Simply subtracting external reference B values from the main B values [8] is a primitive approach, which requires a reference sensor to be outside of the magnetic field of the permanent magnet. We raised the hypothesis that two adjacent magnetometers could be used for both position and BMF calculation.…”

“…However, they are unable to provide any information about jaw movements and trajectories. To fill the gap between primitive mastication detectors and sophisticated clinical appliances, a magnetic method for intra-oral jaw position determination was proposed in our previous research [8]. The method was able to determine the positions of the test trajectory with an average RMSE of 0.260 mm in laboratory experiments [8].…”

“…To fill the gap between primitive mastication detectors and sophisticated clinical appliances, a magnetic method for intra-oral jaw position determination was proposed in our previous research [8]. The method was able to determine the positions of the test trajectory with an average RMSE of 0.260 mm in laboratory experiments [8]. However, it can only determine linear position displacements on three axes, resulting in a three-degree-of-freedom (DOF) solution.…”

“…In our previous work, for the case of a (2 × 2 mm) cylindrical magnet with 1.4 T residual magnetism, we chose to place the reference sensor at 35 mm away. This was chosen as the minimal distance for the subtraction method to remain useful and fit intra-orally, with a 7 µT offset remaining of the average 35 µT ambient field [8]. However, for full subtractive compensation, the reference magnetometer should be completely outside of the field of the permanent magnet.…”