Displacement pattern of the normal pars flaccida in the gerbil

Objective: The aim of the current study was to assess the mechanical stiffn ess 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 cholest eatoma develop in the pars flaccida as well as in the pars tensa of the tym panic membrane, in these authors' previous experimental studies, stiffness changes are shown to develop early in the pars tense in response to purulen t otitis media and otitis media with effusion. These changes are suggested to be precursors to a later development of retraction pockets and cholestea toma. In the clinical situation, retraction pockets are often found in the pars flaccida only. This study will establish the stiffness properties of t he normal pars flaccida and form a base for forthcoming studies of the pars flaccida in response to otitis media with effusion and purulent otitis med ia, as well as retraction pocket formation and cholesteatoma. Methods: A measure for the mechanical stiffness properties of the normal pa rs flaccida in the gerbil was assessed as its displacement for a given tran stympanic pressure. The method used was moire interferometry, which is a no ncontacting optical technique to measure the shape of the surface of an obj ect. Results: The displacement of the pars flaccida was a nonlinear and asymmetr ic 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 p ressures. The displacement Versus pressure characteristics for the pars fla ccida strongly differed from those of the pars tensa. The pars tensa seemed more elastic. Conclusion: Reference values for displacement versus pressure characteristi cs of the normal gerbil pars flaccida were obtained using a moire interfero metry method. The mechanical stiffness properties of the normal pars flacci da were strongly different from those of the pars tensa.