Pressure Regulation due to Displacement of the Pars flaccida and Pars tensa of the Tympanic Membrane

Abstract

Static pressure differences across the tympanic membrane occur daily in normally encountered circumstances. This causes displacement of the tympanic membrane and changes in the configuration of the middle ear ossicles. As a consequence, the physiological functioning of the middle ear is altered and the sensitivity of hearing is decreased. Smaller pressure changes are thought to be compensated for thanks to a rapid deflection of the pars flaccida of the tympanic membrane – which, as the name indicates, can be easily deflected – and a resulting change in the middle ear air pressure. Larger pressure changes are balanced when the pressure difference exceeds a certain threshold and the eustachian tube opens for a short time. With a moiré technique we have measured the displacement of the tympanic membrane as a function of transtympanic pressure. These measurements were used to calculate the volume displacement of the two distinct parts of the tympanic membrane, the pars tensa and the pars flaccida. Our full-field method revolutionizes the displacement calculations of the tympanic membrane. Because the technique provides displacement data at all image points, it allows to calculate the volume displacement of the pars tensa and pars flaccida separately. As the middle ear cavity can be modeled as a closed, air-filled cavity the displacement of the tympanic membrane will cause some pressure variation in the middle ear and this will give rise to a partial compensation of the applied transtympanic pressure. We calulated the magnitude of the pressure compensation provided by the pars flaccida on the basis of the experimental volume displacement versus pressure relation. The pressure compensation provided by the pars flaccida is so small and confined to a very small range of excess pressures that this can hardly be the role to be attributed to the pars flaccida. The pressure compensation due to displacement of the entire tympanic membrane within the same pressure range is about 10 times larger, but still quite small (<25%).