K.-B. Hüttenbrink,

HNO Universitatsklinik, Dresden

The ear is a highly specialised pressure receptor of an incredible sensitivity: it reacts to sound pressure-induced vibration amplitudes which attain dimensions smaller than the diameter of a hydrogen molecule at the hearing threshold. On the other hand, this delicate construction is sturdy enough to withstand the pressure differences of ambient static air pressure, which are a million times larger, as they occur in sneezing, swallowing, diving, flying etc.. These two different requirements are answered by an intricate construction within the middle ear. The tympanic membrane and the three ossicles transmit the sound amplified and without distortion, in high fidelity. The dynamic range of 120 dB linearity is still unrivalled by any modern electric sound transmission system. The second demand, the behaviour in ambient air pressure changes is met by the construction of the articulated ossicular chain with its delicate joins: The changes of static air pressure induce huge displacements of the tympanic membrane, which cause a gliding movement of the ossicular joints. This motion generates a peculiar micromechanism with a change of direction of motion within the chain, which decouples the inner ear from the extensive displacements of the tympanic membrane and malleus. This micro-mechanism explains some anatomical details of the ossicular chain, its ligamentous structures and the function of the middle ear muscles. The understanding of these construction principles and biomechanics of the middle ear is helpful, if we want to unravel the causes for the decreased sound conduction in a pathologic middle ear or if we try to improve our surgical techniques in the reconstruction of a perforated tympanic membrane or in the replacement of a defect ossicular chain by different types of prostheses.