Stiffness changes of the tympanic membrane in otitis media

Abstract

Acute purulent otitis media and otitis media with effusion, are common diseases in childhood. Years later some of the children will get "complications", such as retraction pockets and cholesteatoma, that often need advanced surgical treatment. Our hypothesis is that already a short period of inflammatory middle ear disease can cause a loss of stiffness in the tympanic membrane. Such a stiffness loss may predispose for the late complications. In previous experimental studies, stiffness changes were shown to develop in the pars tensa of the tympanic membrane during otitis media. Retraction pockets are, however, often found in the pars flaccida, which differs greatly from the pars tensa regarding morphologic structure.

The acoustic stiffness properties of the tympanic membrane were studied in early stages of experimental cholesteatoma in the gerbil. The measurements were performed with an invitro tympanometry model. The results showed an increased acoustic stiffness. This acoustic stiffness change was correlated to a pathological thickness increase of the tympanic membrane. The increase of acoustic stiffness contrasts to previous results showing a loss of mechanical stiffness in the same disease model. This difference emphasizes that acoustic and mechanical stiffness are governed by different structural properties. Awareness of this may be important searching for better surgical methods and materials in middle ear reconstruction.

A projection moire interferometry model was designed in order to enable measurements of the mechanical stiffness of the pars flaccida in the gerbil. Moire interferometry is a noncontacting optical method with which the shape and displacement of an object can be measured. Peak displacement versus pressure was used as a measure of mechanical stiffness. The mechanical stiffness properties of the healthy pars flaccida as well as of the pars flaccida during purulent otitis media and otitis media with effusion were assessed. The border of the pars flaccida was almost circular and the displaced pars flaccida had the shape of a sphere cap. Therefore the volume displacement could be calculated as well. In the peak displacement measurements a loss of mechanical stiffness was assessed during purulent otitis media and otitis media with effusion already within a few days. This loss of stiffness may be persistent and may predispose for future retraction pocket formation and cholesteatoma development.