Optimal Retention Force of Audio Processor Magnets

Hypothesis: Audio processor magnets used in transcutaneous active hearing implants vary in retention force. We hypothesized that a range of optimal magnetic retention forces could be derived, based on objective in-situ measurements of individual retention forces. Background: The magnetic force required to retain the proper placement of transcutaneous active hearing implants varies among patients. Currently, audiologists rely on personal experience in deciding which magnet to use. Insufficient force causes frequent loss of the single-unit processors or external coils of behind-the-ear devices, and excessive force causes pain and skin irritations. Methods: We experimentally determined magnetic retention force as a function of the distance between two magnets for different processor-implant magnet combinations. In addition, we studied individual in situ retention forces in 100 patients with hearing devices. Skin status was evaluated by a physician and assessed by patient self-reporting and a questionnaire. Results: Force-distance functions showed that different magnet strengths had differential effects only at distances less than 6 mm. Examiner and patient skin status assessments and comfort scores showed that optimal retention force range was 0.23 to 0.4 N. Conclusion: We found that the currently available magnet types restrict the range of feasible skin flap thicknesses to a maximum of 6 mm. Further we conclude that retention forces between 0.23 and 0.4 N could provide the appropriate trade-off between the risk of losing device fixation and of causing skin irritations. We recommend measuring the retention force in all patients.