Study on Occlusal Adjustment for Complete Denture Using Magnet

Repulsive Force-Finite Element Analysis of the Influence of Magnet Positioning on the Repulsive Force

Abstract

In severe edentulous cases, it is often very difficult to detect premature contact areas by conventional methods in occlusal adjustment because of easy denture displacements and movements. We have developed an occlusal adjustment device utilizing the magnet repulsive force based on Coble intra-oral balancer. This device can stabilize denture bases without displacement and movement and enables us to detect premature contact areas exactly at centric occlusion, because the magnet repulsive force is maximal near the centric occlusion, by which the upper and lower denture bases can be pressed firmly to residual ridge mucosae inside the oral cavity. However, how the magnet repulsive force affects the mandibular position or displacement of denture is not clear, and it is also possible that the repulsive force inverts to be attractive. To clarify these matters, two magnets were placed pole-to-pole, and how the repulsive force is changed in strength and direction by the vertical and horizontal movements of the two magnets and by simulated mandibular movements was analyzed using the finite element method. When two magnets were vertically separated keeping in parallel, the repulsive force decreased in inverse proportion to the square of inter-magnet distance. When each magnet was placed at the center of upper and lower dentures, it was necessary to keep the inter-magnet distance to be 5 mm minimum for mandibular movements without magnet contact, and the vertical repulsive force was 974 gf at this minimum inter-magnet distance. When one of the magnets was moved keeping the vertical inter-magnet distance to be 5 mm, the horizontal repulsive force became greater than the vertical repulsive force at 10 mm horizontal movement.
In simulated mandibular movements, the larger jaw opening was, the smaller was the vertical repulsive force and the greater was the horizontal repulsive force. At 40 mm jaw opening, the vertical repulsive force was smaller than the horizontal repulsive force. In protrusive and lateral magnet movements, the horizontal repulsive force increased and the vertical repulsive force decreased as the distance moved increased, and this decrease became greater as the angle of horizontal condylar indication increased. It was found finally that the jaw opening should be kept within 30 mm, protrusive movements within 8 mm and lateral movements within 16 mm taking the displacement and movement of dentures into consideration for achieving stable occlusal adjustment. With these movement ranges, this method can practically be applied to clinical occlusal adjustments.