Magnetic displacement force and torque on dental keepers in the static magnetic field of an MR scanner

Abstract

Purpose: To evaluate the effect of the static magnetic field of magnetic resonance (MR) scanners on keepers (ie, ferromagnetic stainless steel plate adhered to the abutment tooth of dental magnetic attachments). Materials and Methods: Magnetically induced displacement force and torque on keepers were measured using 1.5 Tesla (T) and 3.0 T MR scanners and a method outlined by American Society for Testing and Materials (ASTM). Changes in magnetic flux density before and after exposure to scanner static magnetic field were examined. Results: The maximum magnetically induced displacement forces were calculated to be 10.3 × 10∼2 N at 1.5 T and 13.9 × 10∼2 N at 3.0 T on the cover surface. The maximum torques exerted on the keeper (4 mm in diameter) were 0.83 N × 4 mm at 1.5 T and 0.85 N × 4 mm at 3.0 T These forces were considerably higher than the gravitational force (7.7 × 10∼4 N) of the keeper but considerably lower than the keeper-root cap proper adhesive force. The keepers' magnetic flux density remained less than that of the Earth. Conclusion: Magnetically induced displacement force and torque on the keeper in the MR scanner do not influence the keeper-root cap proper adhesive force.