A DYNAMIC AND HARMONIC DAMPED FINITE-ELEMENT ANALYSIS MODEL OF STAPEDOTOMY

This study was undertaken in an attempt to better understand the mecha nics of sound transmission at the footplate following stapedotomy. The insertion of a Teflon (polytetrafluoroethylene) stapes prosthesis int roduces new constraints within the reconstructed ossicular chain which have an effect on the normal vibration patterns of the tympanic membr ane. In a finite element model of the ear, constraints have been repro duced as a series of spring constants in the incus/prosthesis/footplat e interfaces incorporating damping to simulate the impedance of the in ner ear. At zero damping, the frequency response at the pseudo stapes footplate exhibit several maxima and minima between 800 Hz and 2.5 Hz. At higher damping values, these maxima and minima become smoothened o ut with two or three naturals occurring over the same frequency range. Severe ankylosis of a diseased footplate is reproduced by over-damped conditions. The umbo, incus and stapes footplate vibrate in phase wit h similar frequencies at light damping levels. The movement of the pro sthesis at the pseudo-footplate can be large in the out of plane axis of the ossicular chain, unless sufficient support is provided at the r econstructed footplate. Clinically, this would suggest the vein graft interposed between the piston and stapedotomy hole should endow resist ance and elasticity to the system.