Introduction 15 1 Limitation of non-surgical options Whatever the mounting option applied, transmission relies on the vibration signals from each processor through intact skin and soft tissues that overlie the skull, resulting in two limitations which decrease the effective amplification: 1. Signal attenuation – especially at high frequency 2. Limited wearing time due to the external pressure effect of the mounting option Although taking into account the acoustic limitations of non-surgical mounts, their use in early rehabilitation for conductive hearing losses has distinct advantages. As a temporary solution, they can be used on an extended trial basis to introduce patients into the concept of bone conducting hearing aids, as often patients have limited knowledge or experience of this technology and may be skeptical of its application. With the advent of a trial period, patients are encouraged to gain experience with a bone conducting processor and their various mounting options. In the adult population, trials of non-surgical mounts have the advantage of preoperatively demonstrating the hearing benefit a patient may expect and thus improve engagement with the overall process in a step toward an ultimate surgically mounted option. In children there is the further advantage of being an ongoing rehabilitation option for those who may be too young to undergo surgery and those who are non-compliant with standard hearing aid options. For many children, especially those with additional care needs or cognitive impairment, the trial of a non-surgical mount provides excellent predictions of audiological results rather than conventional pure tone and speech audiology. Additionally in those children with microtia or anotia, it preserves the post auricular skin envelope until the child can engage autonomously with decisions regarding future pinna reconstruction options. Surgically Implanted Hearing Systems Surgically implanted Bone anchored hearing implants (BAHI), utilise the same transduction processors whereby sound is converted to a digital signal and ultimately mechanical vibration which is transmitted directly to the skull. Classically these are divided into Percutaneous (skin penetrating) and Transcutaneous (skin preserving). Percutaneous Processors Percutaneous devices were introduced in 1977 by Tjellström [23] and became commercially available in 1987. A skin penetrating abutment is attached to an anchoring fixture screw in the skull to which a sound processor is attached, bypassing the attenuation caused by skin and overlying soft tissue [Figure 2]. Originally described surgical techniques involved complete
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