Chapter 1 20 A slow increase in magnet strength from the point of fitting is advised to improve wearing time and acclimatise the skin to the processor. This irritation can be compounded by the vibration caused by passive devices, which also need to overcome the attenuation of the soft tissue and skin: As with non-surgical BCHDs, this can result in paraesthesia or numbness [75]. Magnets are available in a variety of strengths which can be tailored to the patients’ needs. Over the initial 12 months of use, it has been observed that the overlying soft tissue becomes thinner due to the compression forces effect of the magnets, and this has the additional benefit of thus reducing the required magnet retention strength [76]. Surgical approaches to these devices are more invasive than with percutaneous systems, requiring either a larger subcutaneous pocket to be created or a bony well to be drilled into the temporal bone to secure the transducer. The position of these devices can be limited by anatomy, and this needs special consideration in children who have a small skull and small mastoid, those individuals who had previous mastoid surgery and also in children with microtia. Another consideration is the compatibility for MRI. Although most systems are compatible up to 1.5 Tesla as with cochlear implants, the presence of the magnet can also interfere with Magnetic Resonant Imaging creating large imaging shadows and this may necessitate removal of the internal magnets if an MRI of the head is be undertaken. Currently Available Transcutaneous BAHI Subcutaneous transducer (active system) [77] • Osia® System Cochlear Bone-Anchored Solutions AB, Mölnlycke, [78] Implant OSI200 system fixed to BI300 Osseointegrated screw Surgical consideration Bone polishing may be required. MRI compatible: OSI200 1.5 Tesla, OSI300 3 Tesla Processor Osia® 2 (55 dB HL) • BONEBRIDGE TM MED-EL, Innsbruck, Austria [79] Implant BCI602 floating mass transducer. Surgical consideration Drilling of a Pre sigmoid bone bed required. Cortical fixation screws. (No osseointegration) MRI compatible: 1.5 Tesla Processor SAMBA 2 (45 dB HL)
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