Introduction 11 1 Introduction History The concept of augmenting hearing via bone conduction was first introduced over 300 years ago [1], and as technologies and understanding have developed so too has the application of bone conduction hearing devices (BCHD). The fundamental concepts have remained the same, whereby sound is transmitted to the inner ear by the vibration of a processor in contact with the skull, bypassing the normal auditory canal in those individuals with congenital or acquired absence of the ear canal or those with pathology preventing effective sound conduction. Physiology of Hearing Air conduction hearing relies on the mechanism of effective vibration of the tympanic membrane caused by changes in air pressure generated by sound. This enters the external auditory canal and is converted to mechanical movement of the ossicular chain and ultimately the stapes footplate. This mechanical movement is then transduced into the movement of fluid within the cochlea causing deflection of the basilar membrane and stimulation in the Organ of Corti with generation of action potentials which in turn are transmitted to the auditory processing centre leading to the perception of sound. The maximal point of deflection is determined by the frequency of the tone as the sound wave travels towards the apex of the basilar membrane [2]. Bone conduction hearing relies on the same stimulation of the basilar membrane by generating a pressure gradient around a specific point along it. As with air conduction, the maximal point of deflection and its subsequent propagation is determined by the frequency of the tone and there is no physiological difference between these points in either air or bone conduction [3,4]. The stimulation occurs due to multiple alternative mechanisms which generate the pressure gradient across the basement membrane [5]. The physiological principal of the generation of the pressure gradients is grounded by the asymmetry in the movement of inner ear fluid within the scala vestibuli (SV) and scala tympani (ST) due to both volume and impedance, which are both higher in the SV [6]. Physiological Principles In 2005 Stenfelt and Goode proposed five contributing physiological factors influencing bone conduction hearing [7]. 1. Cochlear fluid inertia – proposed as the most influential factor in bone conduction hearing. The pressure gradient is created by the vibration of the cochlea and resulting movement of the round window and oval window in a compensatory fashion. This movement is larger toward the round window due to lower impedance [8].
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