DISPLACEMENT OF THE GERBIL TYMPANIC MEMBRANE UNDER STATIC PRESSURE VARIATIONS MEASURED WITH A REAL-TIME DIFFERENTIAL MOIRE INTERFEROMETER

It is thought that chronic middle ear disease ultimately causes change s in the stiffness and elasticity of the tympanic membrane, but it is unknown whether such changes occur early in the course of the disease. In order to analyze mechanical changes in different parts of the tymp anic membrane, a full field moire interferometry technique was utilize d to measure the shape and real-time displacement in response to posit ive and negative pressure gradients applied across the tympanic membra ne. The measurements were performed on fresh isolated temporal bones f rom the Mongolian gerbil. In order to gain sufficient visual access to the pars tensa for the moire measurements, the tympanic bulla was ope ned, the tenser tympani muscle and the incudo-stapedial joint were cut , and part of the medial wall of the tympanic cavity was removed. The malleus and incus and their ligaments were kept intact. The specimens were kept continuously humidified with an evaporator or in a humid cha mber, since otherwise the tympanic membrane dries out in a few minutes when its medial surface is exposed. This desiccation reduces the elas ticity and cause shrinkage which results in a reduction of the height of the cone constituted by the pars tensa. Profiles of the tympanic me mbrane at rest and under different pressure conditions were extracted from the moire interferograms. The tympanic membrane and ossicular com plex exhibit a hysteresis effect as differences in the displacement pa tterns under identical pressure gradients during the loading and the u nloading phase; a residual displacement of the pars tensa was for inst ance seen after the pressure gradient across the tympanic membrane was eliminated.