Accelerometry-enhanced Magnetic Sensor for Intra-oral Continuous Jaw Motion Tracking and Bruxism Detection

Currently available jaw motion tracking methods require large accessories mounted on a patient and thus are utilized in controlled environments, for short-time examinations only. In some cases, especially in the evaluation of bruxism, a non-restrictive, 24-hour jaw tracking method is needed. Bruxism oriented EMG devices and sensor-enhanced occlusal splints are able to continuously detect masticatory activity, but are uninformative in regard of movement trajectories and kinematics. This study explores a possibility to use a permanent magnet and a 3-axial magnetometer to track mandible’s spatial position in relation to maxilla. An algorithm for determining the sensor’s coordinates from magnetic field values was developed and verified via analytical and finite element modelling and by using a 3D positioning system. Errors due to background magnetic fields (BMF) were estimated and were shown to increase exponentially along magnet–sensor distance. BMF compensation techniques and sensor positioning possibilities were discussed, and BMF compensation by a reference magnetometer was implemented. The trajectory of natural masticatory movement (10 x 7 x 5 mm) was replicated, and coordinates were estimated from sensor data with RMSE of 0.260±0.004 mm. Estimation of the coordinates of cubic (a = 10 mm) trajectory resulted in RMSE of 0.328±0.005 mm. Moreover, by enhancing the method with accelerometry, bruxism events could be detected. By using a one-hinge articulator it was proved, that it is possible to detect impacts of teeth from accelerometer signals. General system test was conducted on a 6 degree-of-freedom (DOF) hexapod-based jaw motion simulator moving at natural speed, confirming system's ability to simultaneously detect jaw position and the impacts of teeth. Despite the limited working range and large uncertainties in the periphery, the method is increasingly accurate and robust when approaching occlusion. Such characteristics should be suitable to develop a wearable system, able to determine jaw trajectories and to detect bruxism. Also, the method could allow visualisation of continuous jaw movement in 3D models and could enable new research of parafunctional jaw activities.