Displacement pattern of the normal pars flaccida in the gerbil

Objective: The aim of the current study was to assess the mechanical stiffness properties of the normal pars flaccida and to compare the results with those obtained in earlier studies on the pars tensa. Background: Postinflammatory changes such as retraction pockets and cholesteatoma develop in the pars flaccida as well as in the pars tensa of the tympanic membrane, in these authors' previous experimental studies, stiffness changes are shown to develop early in the pars tense in response to purulent otitis media and otitis media with effusion. These changes are suggested to be precursors to a later development of retraction pockets and cholesteatoma. In the clinical situation, retraction pockets are often found in the pars flaccida only. This study will establish the stiffness properties of the normal pars flaccida and form a base for forthcoming studies of the pars flaccida in response to otitis media with effusion and purulent otitis media, as well as retraction pocket formation and cholesteatoma. Methods: A measure for the mechanical stiffness properties of the normal pars flaccida in the gerbil was assessed as its displacement for a given transtympanic pressure. The method used was moire interferometry, which is a noncontacting optical technique to measure the shape of the surface of an object. Results: The displacement of the pars flaccida was a nonlinear and asymmetric function of pressure. The displacement per pressure unit rose steeply at low middle ear pressures to level out and reach a steady state at higher pressures. The displacement Versus pressure characteristics for the pars flaccida strongly differed from those of the pars tensa. The pars tensa seemed more elastic. Conclusion: Reference values for displacement versus pressure characteristics of the normal gerbil pars flaccida were obtained using a moire interferometry method. The mechanical stiffness properties of the normal pars flaccida were strongly different from those of the pars tensa.