Development of an Apparatus for Digital Measurement of Magnetically Induced Torque on Medical Implants to Facilitate the Application of the ASTM F2213 Standard

Objective: The ASTM F2213 standard describes the measurement of the magnetically induced torques by means of a torsional spring method. As this method is highly complex, it has rarely ever been employed. The purpose was to develop an easy-to-handle measuring setup for the reproducible, fast, and precise measurement of magnetically induced torques, following ASTM F2213, at a static magnetic field (B0) of 1.5 and 3 T. Methods: An magnetic resonance (MR)-save measuring platform was developed according to the ASTM F2213 standard and combined with a precision balance. The evaluation was performed with ten test objects, e.g., a steel screw (length 60 mm) and a neurostimulator, at 1.5 and 3 T. The torque was measured at 10° increments as the test object was rotated relative to B0. Results: The measurement setup allows fast and reproducible measurements. The maximum magnetically induced torque measured was 397 ± 2 N·mm for the steel screw and 63 ± 1 N·mm for the neurostimulator. The maximum torque on a test object is insensitive to B0 if the magnetization of the object has reached saturation. Conclusion: Ferromagnetic implants may exhibit strong magnetically induced torques during MRI examination. For analyzing these magnetically induced torques, we developed a digital measuring device that allows simplified and accelerated application of the ASTM F2213 standard test method. Significance: In a B0, ferromagnetic implants may exert considerable torques on the surrounding tissue. These forces may affect the functionality of the implant and inflict severe injuries on a patient.