[Mechanics of the ear ossicle chain in static pressures. I. The normal middle ear].

The mechanics of the ossicle chain, induced by variations of static air pressure ranging from 0 to +/- 400 H2O, were analysed in an experimental study of 25 temporal bone preparations. The three-dimensional movements of the umbo, the proc. lenticularis and stapes at these pressures, applied at the external ear canal, were measured microscopically. Direct high-grade radiographic magnification technique demonstrated these displacements dynamically. In the normal ossicle chain, the air pressure-induced inward-outward excursions of the malleus are transformed by the gliding incudo-malleal joint, resulting predominantly in an upward and downward movement of the lenticular process. Thus, the incudo-stapedial joint can glide, and the stapes is uncoupled from the extensive displacements of the drum membrane and malleus. This mode of motion accounts for some anatomical details of the middle ear and explains results of former studies of ossicle vibrations produced by unphysiologically high sound pressures.