Middle-ear dynamics before and after ossicular replacement

The mechanism of hearing involves conduction of mechanical vibrations along the ossicular chain to the inner ear. An acoustic wave is collected and tr ansformed as it passes down the ear canal and impacts on the: tympanic memb rane tear drum). The drum is connected to the inner-ear by three ossicle bo nes (malleus, incus, and stapes) in a complex arrangement, which serves to further transform the mechanical vibration before it reaches the cochlea of the inner ear. What is the mechanical function of the ossicular chain, and what are the biomechanical consequences of surgical reconstruction with pr ostheses? To answer these questions, a three-dimensional finite element mod el of the outer ear canal and middle ear was generated. The dynamical behav iour was predicted for the normal ear, and an ear reconstructed with partia l and total ossicular replacement prostheses. For the normal ear, stapes am plitudes of 1 x 10(-8) m at low frequencies decrease to 4 x 10(-10) m at ap proximately 3 kHz with several resonance peeks in between, most significant ly at approximately 1 kHz. Thereafter a further resonance is predicted at 4 kHz associated with the ear canal. The behaviour is changed fundamentally by adding a prosthesis: the partial replacement increases the vibratory cou pling of the drum and the stapes compared to the normal ear whereas the tot al replacement does the opposite, and is predicted to have the disadvantage of bringing several new resonances of the ossicular chain into the hearing range. It is hypothesised that the function of the malleus-incus-stapes ar rangement is to link the drum to the oval window with the flexibility requi red for impedance matching but the rigidity to prevent unconstrainable reso nances from occurring in the hearing range. If this is true, then the struc tural stiffness of ossicular chain is the critical design variable for midd le-ear replacement prostheses. (C) 2000 Elsevier Science Ltd. All rights re served.